Hyliion (HYLN) POS AM 9 Jul 21 Prospectus update (post-effective amendment) Financial data

Document And Entity Information

Document And Entity Information	Mar. 31, 2021
Document Information Line Items
Entity Registrant Name	Hyliion Holdings Corp.
Document Type	POS AM
Amendment Flag	true
Amendment Description	On October 23, 2020, Hyliion Holdings Corp (the “Company”) filed a registration statement with the Securities and Exchange Commission (the “SEC”) on Form S-1 (File No. 333-249649) (the “Registration Statement”). The Registration Statement, as amended, was initially declared effective by the SEC on November 27, 2020 and initially registered the issuance by the Company of up to an aggregate of up to 19,185,637 shares of common stock, $0.0001 par value per share (“Common Stock”), which consisted of (i) up to 6,660,183 shares of Common Stock issuable upon the exercise of 6,660,183 warrants (the “Private Placement Warrants”) originally issued in a private placement in connection with the initial public offering of Tortoise Acquisition Corp. (“TortoiseCorp”) by the holders thereof, (ii) up to 875,000 shares of Common Stock that are issuable upon the exercise of 875,000 warrants (the “Forward Purchase Warrants” and together with the Private Placement Warrants, the “Private Warrants”) originally issued in a private placement at the closing of the Business Combination (as defined below) by the holders thereof other than the initial holder and (iii) up to 11,650,454 shares of Common Stock issuable upon the exercise of 11,650,454 warrants (the “Public Warrants” and, together with the Private Warrants, the “Warrants”) originally issued in the initial public offering of TortoiseCorp by the holders thereof. The Registration Statement also initially registered the offer and sale from time to time by the selling stockholders identified in the prospectus of (i) up to 132,637,517 shares of Common Stock (including up to 7,535,183 shares of Common Stock that may be issued upon exercise of the Private Warrants and 20,000 shares of Common Stock that may be issued upon exercise of 20,000 Public Warrants) and (ii) up to 7,555,183 Warrants, which consists of up to 7,535,183 Private Warrants and up to 20,000 Public Warrants. This Post-Effective Amendment No. 1 to Form S-1 (“Post-Effective Amendment No. 1”) is being filed by the Company to update the Registration Statement to (i) remove references to the registration of the Warrants and the shares of Common Stock to be issued upon exercise thereof to reflect the redemption or exercise of all Warrants on or before December 31, 2020; (ii) include updated information regarding the selling stockholders named in the prospectus, including a reduction in the number of shares of Common Stock being offered by the selling stockholders to 91,394,533 shares of Common Stock and (iii) include the information from the Company’s filings with the SEC. No additional securities are being registered under this Post-Effective Amendment No. 1. All applicable registration fees were paid at the time of the original filing of the Registration Statement.
Entity Central Index Key	0001759631
Entity Filer Category	Non-accelerated Filer
Document Period End Date	Mar. 31, 2021
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	false
Entity Incorporation, State or Country Code	DE

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Assets
Cash and cash equivalents	$ 334,718	$ 389,705	$ 6,285
Accounts receivable	80	92	145
Prepaid expenses and other current assets	3,616	20,690	414
Short-term investments	144,829	201,881
Total current assets	483,243	612,368	6,844
Property and equipment, net	1,350	1,171	1,635
Operating lease right-of-use assets	4,833	5,055	4,976
Intangible assets, net	308	332	429
Other assets	193	193	212
Long-term investments	152,481	35,970
Total assets	642,408	655,089	14,096
Current liabilities:
Accounts payable	2,022	1,890	1,156
Convertible notes payable derivative liabilities			3,029
Current portion of operating lease liabilities	790	734	953
Current portion of debt		49	6,720
Accrued expenses and other current liabilities	5,073	6,313	500
Total current liabilities	7,885	8,937	12,358
Operating lease liabilities, net of current portion	4,838	5,076	4,803
Convertible notes payable derivative liabilities, net of current portion			5,322
Debt, net of current portion		908	9,682
Total liabilities	12,723	14,921	32,165
Commitments and contingencies (Note 15)
Stockholders’ Equity:
Common stock, $0.0001 par value; 250,000,000 shares authorized; 171,519,811 and 169,316,421 shares issued and outstanding at March 31, 2021 and December 31, 2020, respectively	19	19	9
Additional paid-in capital	371,077	364,998	30,888
Accumulated deficit	258,589	275,151	(48,966)
Total stockholders’ equity	629,685	640,168	(18,069)
Total liabilities and stockholders’ equity	$ 642,408	$ 655,089	$ 14,096

Condensed Consolidated Balanc_2

Condensed Consolidated Balance Sheets (Parentheticals) - $ / shares	Mar. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Statement of Financial Position [Abstract]
Common stock, par value (usd per share) (in Dollars per share)	$ 0.0001	$ 0.0001	$ 0.0001
Common stock, shares authorized	250,000,000	250,000,000	250,000,000
Common stock, shares issued	171,519,811	169,316,421	86,762,463
Common stock, shares outstanding	171,519,811	169,316,421	86,762,463

Unaudited Condensed Consolidate

Unaudited Condensed Consolidated Statements of Operations - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020	Dec. 31, 2019
Operating expenses:
Research and development	$ (9,332)	$ (2,671)	$ (12,598)	$ (9,269)
Selling, general and administrative expenses	(7,399)	(691)	(9,585)	(2,730)
Loss from operations	(16,731)	(3,362)	(22,183)	(11,999)
Other income (expense)
Interest expense		(1,565)	(5,459)	(3,260)
Interest income	169
Change in fair value of convertible notes payable derivative liabilities		(635)	(1,358)	1,119
Change in fair value of warrant liabilities			363,299
Total other income (expense)	169	(2,200)	346,300	(2,114)
Other income (expense)			(12)	27
Loss on extinguishment of debt			(10,170)
Net income (loss)	$ (16,562)	$ (5,562)	$ 324,117	$ (14,113)
Net income (loss) per share, basic (in Dollars per share)			$ 3.11	$ (0.16)
Net loss per share, diluted (in Dollars per share)			$ (0.35)	$ (0.16)
Weighted-average shares outstanding, basic (in Shares)			104,324,059	86,643,714
Weighted-average shares outstanding, diluted (in Shares)	170,249,708	86,762,463	112,570,960	86,643,714
Weighted-average shares outstanding, basic and diluted (in Shares)	170,249,708	86,762,463
Net loss per share, basic and diluted (in Dollars per share)	$ (0.10)	$ (0.06)

Unaudited Condensed Consolida_2

Unaudited Condensed Consolidated Statements of Changes in Stockholders’ Equity - USD ($) $ in Thousands	Series A-1 RedeemableConvertible Preferred Stock	Series A-2 RedeemableConvertible Preferred Stock	Series A-3 RedeemableConvertible Preferred Stock	Common Stock	Additional Paid-In Capital	Accumulated Deficit	Total
Balance at Dec. 31, 2018	$ 20,750	$ 3,893	$ 2,026	$ 24	$ 4,072	$ (34,853)	$ (30,757)
Balance (in Shares) at Dec. 31, 2018	23,460,903	8,793,755	2,545,155	24,453,750
Retroactive application of recapitalization (See Note 4)	$ (20,750)	$ (3,893)	$ (2,026)	$ (15)	26,684		26,669
Retroactive application of recapitalization (See Note 4) (in Shares)	(23,460,903)	(8,793,755)	(2,545,155)	61,890,680
Adjusted balance, beginning of period				$ 9	30,756	(34,853)	(4,088)
Adjusted balance, beginning of period (in Shares)				86,344,430
Exercise of common stock options					7		7
Exercise of common stock options (in Shares)				418,033
Share-based compensation					125		125
Share-based compensation, shares (in Shares)
Net loss						(14,113)	(14,113)
Balance at ending at Dec. 31, 2019				$ 9	30,888	(48,966)	(18,069)
Balance at ending (in Shares) at Dec. 31, 2019				86,762,463
Share-based compensation					57		57
Net loss						(5,562)	(5,562)
Balance at ending at Mar. 31, 2020				$ 9	30,945	(54,528)	(23,574)
Balance at ending (in Shares) at Mar. 31, 2020				86,762,463
Balance at Dec. 31, 2019				$ 9	30,888	(48,966)	(18,069)
Balance (in Shares) at Dec. 31, 2019				86,762,463
Common stock issued for warrants exercised, net of issuance costs				$ 4	136,512		136,516
Common stock issued for warrants exercised, net of issuance costs (in Shares)				15,414,592
Redemption of unexercised warrants					(3)		(3)
Redemption of unexercised warrants (in Shares)
Exercise of common stock options					121		121
Exercise of common stock options (in Shares)				1,112,160
Conversion of convertible notes payable to common stock					44,039		$ 44,039
Conversion of convertible notes payable to common stock (in Shares)				4,404,367			15,414,592
Business Combination and PIPE financing				$ 6	153,147		$ 153,153
Business Combination and PIPE financing (in Shares)				61,622,839
Share-based compensation					294		294
Share-based compensation, shares (in Shares)
Net loss						324,117	324,117
Balance at ending at Dec. 31, 2020				$ 19	364,998	275,151	640,168
Balance at ending (in Shares) at Dec. 31, 2020				169,316,421
Common stock issued for warrants exercised, net of issuance costs					4,282		4,282
Common stock issued for warrants exercised, net of issuance costs (in Shares)				371,535
Exercise of common stock options					287		287
Exercise of common stock options (in Shares)				1,831,855
Share-based compensation					1,510		1,510
Net loss						(16,562)	(16,562)
Balance at ending at Mar. 31, 2021				$ 19	$ 371,077	$ 258,589	$ 629,685
Balance at ending (in Shares) at Mar. 31, 2021				171,519,811

Unaudited Condensed Consolida_3

Unaudited Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020	Dec. 31, 2019
Cash Flows from Operating Activities:
Net income (loss)	$ (16,562)	$ (5,562)	$ 324,117	$ (14,113)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	203	251	850	1,028
Loss on extinguishment of debt			10,170
Noncash lease expense	222	276	928	1,312
Paid-in-kind interest on convertible notes payable		306	1,085	723
Amortization of debt discount		1,250	4,237	2,485
Share-based compensation	1,510	57	294	125
Change in fair value of convertible notes payable derivative liabilities		635	1,358	(1,118)
Change in fair value of contingent consideration liability				(27)
Change in fair value of warrant liability			(363,299)
Changes in operating assets and liabilities:
Accounts receivable	12	107	53	(28)
Prepaid expenses and other current assets	817	28	(8,301)	(62)
Other assets			19	106
Accounts payable	132	(270)	734	(684)
Accrued expenses and other current liabilities	3,091	(120)	5,764	(21)
Operating lease liabilities	(182)	(283)	(953)	(798)
Net cash used in operating activities	(10,757)	(3,325)	(22,944)	(11,072)
Cash Flows from Investing Activities:
Purchase of property and equipment	(358)	(80)	(311)	(349)
Purchase of investments	(219,460)		(237,851)
Proceeds from sale of property and equipment			22
Proceeds from sale of investments	160,001
Net cash used in investing activities	(59,817)	(80)	(238,140)	(349)
Cash Flows from Financing Activities:
Proceeds from exercise of stock warrants, net of issuance costs	16,257		140,800
Payments for Paycheck Protection Program loan	(908)
Proceeds from exercise of common stock options	287		121	7
Proceeds from convertible notes payable issuance and derivative liability		3,200
Business Combination and PIPE financing, net of issuance costs paid			516,454
Proceeds from the exercise of stock warrants			124,536
Proceeds from convertible notes payable issuance and derivative liabilities			3,200	16,803
Proceeds from Paycheck Protection Program loan			908
Payments for deferred financing costs			(468)
Repayments on finance lease obligations	(49)	(54)	(247)	(201)
Net cash provided by financing activities	15,587	3,146	644,504	16,609
Net decrease in cash and cash equivalents	(54,987)	(259)	383,420	5,188
Cash and cash equivalents- beginning of the period	389,705	6,285	6,285	1,097
Cash and cash equivalents - end of the period	$ 334,718	$ 6,026	$ 389,705	$ 6,285

Description of Organization and

Description of Organization and Business Operations	3 Months Ended	12 Months Ended
	Mar. 31, 2021	Dec. 31, 2020
Accounting Policies [Abstract]
Description of Organization and Business Operations	1. Description of Organization and Business Operations On October 1, 2020, our predecessor company, Tortoise Acquisition Corp. (“Tortoise”), consummated a business combination (the “Business Combination”) with Hyliion Inc., a Delaware corporation (“Legacy Hyliion”) pursuant to which Legacy Hyliion merged with and into SHLL Merger Sub, Inc., a Delaware corporation and a wholly-owned subsidiary of Tortoise, with Legacy Hyliion surviving the merger (together with the related transactions, the “Business Combination”). Upon consummation of the Business Combination, Legacy Hyliion became a direct wholly-owned subsidiary of Tortoise, and Tortoise was renamed Hyliion Holdings Corp. References to the “Company” in this report refer to Tortoise before the consummation of the Business Combination or Hyliion Holdings Corp. and its wholly owned subsidiary (“Hyliion”, “we” or “us”) after the Business Combination, unless expressly indicated or the context otherwise requires. Hyliion designs and develops hybrid and electrified powertrain systems for long haul “Class 8” semi-trucks which modify semi-tractors into Hybrid and fully electric range extender vehicles, respectively. The Company’s Hybrid systems utilize intelligent electric drive axles with advanced algorithms and battery technology to optimize fuel savings and vehicle performance with reduced emissions, enabling fleets to access an easy, efficient way to decrease fuel expenses, lower emissions and/or improve vehicle performance. The Company’s fully electric range extender systems utilize an intelligent electric powertrain with advanced algorithms to optimize emissions performance and efficiency with no new infrastructure required. The Hypertruck ERX system enables fleets to reduce the cost of ownership while providing the ability to deliver net-negative carbon emissions and operate fully electric when needed. The Company is in a pre-commercialization stage of development in which its electric Hybrid system is in the testing phase and the Hypertruck ERX system is in the prototype phase. On October 1, 2020, the Company consummated a business combination which was accounted for as a reverse recapitalization. For more details on the reverse recapitalization, see Note 3 to the Company's Consolidated Financial Statements as presented in its Annual Report, as amended on Form 10-K/A for the year ended December 31, 2020. As a result of the reverse recapitalization, all references to numbers of common shares and per common share data for 2020 in these condensed consolidated financial statements and related notes have been retroactively adjusted to account for the effect of the reverse recapitalization. These condensed consolidated financial statements have been prepared on a going concern basis, which contemplates the realization of assets and settlement of liabilities in the normal course of business. The Company is an early-stage growth company in the pre-commercialization stage of development and has generated negative cash flows from operating activities since inception. As of March 31, 2021, the Company had a cash and cash equivalents balance of $334.7 million and total investments of $297.3 million. Based on this, the Company has sufficient funds to continue to execute its business strategy for the next twelve months.	Note 1. Description of business and basis of presentation Hyliion Holdings Corp. and its wholly owned subsidiary, designs and develops hybrid and electrified powertrain systems for long haul “Class 8” semi-tractors which modify semi-tractors into Hybrid and fully electric range extender vehicles, respectively. Hyliion Holdings Corp.’s Hybrid systems utilize intelligent electric drive axles with advanced algorithms and battery technology to optimize fuel savings and vehicle performance with reduced emissions, enabling fleets to access an easy, efficient way to decrease fuel expenses, lower emissions and/or improve vehicle performance. Hyliion Holdings Corp.’s fully electric range extender systems utilize an intelligent electric powertrain with advanced algorithms to optimize emissions performance and efficiency with no new infrastructure required. The Hypertruck ERX system enables fleets to reduce the cost of ownership while providing the ability to deliver net-negative carbon emissions and operate fully electric when needed. Hyliion Holdings Corp. is in a pre-commercialization stage of development in which its electric Hybrid system is in the testing phase and the Hypertruck ERX system is in the prototype phase. Basis of Presentation and Principles of Consolidation: On the Closing Date, and in connection with the closing of the Business Combination, TortoiseCorp changed its name to Hyliion Holdings Corp. (the “Company” or “Hyliion”) and the Company’s common stock began trading on the New York Stock Exchange under the ticker symbol HYLN. Legacy Hyliion was deemed the accounting acquirer in the Business Combination based on an analysis of the criteria outlined in Accounting Standards Codification (“ASC”) 805. The determination was primarily based on Legacy Hyliion’s shareholders prior to the Business Combination having a majority of the voting interests in the combined company, Legacy Hyliion’s board of directors comprising a majority of the board of directors of the combined company, Legacy Hyliion’s existing shareholders’ control over decisions regarding the election and removal of directors and officers of the combined company’s board of directors, and Legacy Hyliion’s senior management comprising the senior management of the combined company. Accordingly, for accounting purposes, the Business Combination was treated as the equivalent of Legacy Hyliion issuing stock for the net assets of TortoiseCorp, accompanied by a recapitalization. The net assets of TortoiseCorp are stated at historical cost, with no goodwill or other intangible assets recorded. While TortoiseCorp was the legal acquirer in the Business Combination, because Legacy Hyliion was deemed the accounting acquirer, the historical financial statements of Legacy Hyliion became the historical financial statements of the combined company, upon the consummation of the Business Combination. As a result, the financial statements included in this report reflect (i) the historical operating results of Legacy Hyliion prior to the Business Combination; (ii) the combined results of TortoiseCorp and Legacy Hyliion following the closing of the Business Combination; (iii) the assets and liabilities of Legacy Hyliion at their historical cost; and (iv) the Company’s equity structure for all periods presented. In accordance with guidance applicable to these circumstances, the equity structure has been restated in all comparative periods up to the Closing Date, to reflect the number of shares of the Company’s common stock, $0.0001 par value per share, issued to Legacy Hyliion shareholders and Legacy Hyliion convertible noteholders in connection with the recapitalization transaction. As such, the shares and corresponding capital amounts and earnings per share related to Legacy Hyliion redeemable convertible preferred stock and Legacy Hyliion common stock prior to the Business Combination have been retroactively restated as shares reflecting the exchange ratio established in the Business Combination. The accompanying consolidated financial statements include the accounts of Hyliion Holdings Corp. and its wholly-owned subsidiary. Intercompany transactions and balances have been eliminated upon consolidation. The consolidated financial statements and accompanying notes have been prepared in accordance with generally accounting principles in the United States of America (“U.S. GAAP”) and in accordance with the rules and regulations of the Unites States Securities and Exchange Commission (“SEC”). Any reference in these footnotes to the applicable guidance is meant to refer to the authoritative U.S. GAAP as found in the Accounting Standards Codification and Accounting Standards Updates (“ASU”) of the Financial Accounting Standards Board (“FASB”). Liquidity: On October 1, 2020, the Company consummated the Business Combination and raised net proceeds of $516.5 million net of transaction costs and expenses. As of December 31, 2020, all outstanding warrants were either exercised or redeemed, with gross proceeds of $140.8 million raised, of which $16.3 million was collected during the first quarter of 2021 (see Note 7). As of December 31, 2020, the Company had a cash and cash equivalents balance of $389.7 million and total investments of $237.9 million. Based on this, the Company has sufficient funds to continue to execute its business strategy for the next twelve months.

Significant Accounting Policies

Significant Accounting Policies	3 Months Ended	12 Months Ended
	Mar. 31, 2021	Dec. 31, 2020
Accounting Policies [Abstract]
Significant Accounting Policies	2. Significant Accounting Policies Basis of Presentation: These condensed consolidated financial statements include the accounts of the Company and its wholly owned subsidiary. Intercompany accounts and transactions have been eliminated in consolidation. These interim consolidated statements have been prepared pursuant to the rules and regulations of the SEC, which permit reduced disclosure for interim periods. The Consolidated Balance Sheet as of December 31, 2020 was derived from audited financial statements for the fiscal year then ended, but does not include all necessary disclosures required by accounting principles generally accepted in the United States of America (“GAAP”) with respect to annual financial statements. In the opinion of management, these unaudited condensed consolidated financial statements include all recurring adjustments and normal accruals necessary for a fair presentation of the Company’s financial position, results of operations and cash flows for the dates and periods presented. These condensed consolidated financial statements and accompanying notes should be read in conjunction with the Company’s 2020 Amended Annual Report. Results for interim periods are not necessarily indicative of the results to be expected for a full fiscal year or for any future period. Emerging Growth Company: Use of estimates and uncertainty of the coronavirus pandemic: On January 30, 2020, the World Health Organization declared the coronavirus outbreak a “Public Health Emergency of International Concern” and on March 11, 2020, declared the coronavirus outbreak a pandemic. In mid-March 2020, U.S. state governors, local officials and leaders outside of the U.S. began ordering various “shelter-in-place” orders, which have had various impacts on the U.S. and global economies. This has required greater use of estimates and assumptions in the preparation of the unaudited condensed consolidated financial statements. As the coronavirus pandemic continues to evolve, the Company believes the extent of the impact to its businesses, operating results, cash flows, liquidity and financial condition will be primarily driven by the severity and duration of the coronavirus pandemic, the pandemic’s impact on the U.S. and global economies and the timing, scope, and effectiveness of federal, state and local governmental responses to the pandemic. Those primary drivers are beyond the Company’s knowledge and control, and as a result, at this time the Company is unable to predict the cumulative impact, both in terms of severity and duration, that the coronavirus pandemic will have on its business, operating results, cash flows and financial condition, but it could be material if the current circumstances continue to exist for a prolonged period. Although the Company has made its best estimates based upon current information, actual results could materially differ from the estimates and assumptions developed by management. If so, the Company may be subject to future impairment charges as well as changes to recorded reserves and valuations. Recently Adopted Accounting Pronouncements: In December 2019, the FASB issued ASU 2019-12, Income Taxes (Topic 740): Simplifying the Accounting for Income Taxes, which is intended to simplify various aspects related to accounting for income taxes. The pronouncement is effective for fiscal years, and for interim periods within those fiscal years, beginning after December 15, 2020. The Company adopted ASU 2019-12 on January 1, 2021. However, there is no impact to the Company as a result of the adoption in the current quarter, nor is there an expected impact to the Company for the remainder of the year.	Note 3. Summary of significant accounting policies Emerging Growth Company Use of estimates and uncertainty of the coronavirus pandemic On January 30, 2020, the World Health Organization declared the coronavirus outbreak a “Public Health Emergency of International Concern” and on March 11, 2020, declared the coronavirus outbreak a pandemic. In mid-March 2020, U.S. State Governors, local officials and leaders outside of the U.S. began ordering various “shelter-in-place” orders, which have had various impacts on the U.S. and global economies. This has required greater use of estimates and assumptions in the preparation of the unaudited consolidated financial statements. As the coronavirus pandemic continues to evolve, the Company believes the extent of the impact to its businesses, operating results, cash flows, liquidity and financial condition will be primarily driven by the severity and duration of the coronavirus pandemic, the pandemic’s impact on the U.S. and global economies and the timing, scope and effectiveness of federal, state and local governmental responses to the pandemic. Those primary drivers are beyond the Company’s knowledge and control, and as a result, at this time the Company is unable to predict the cumulative impact, both in terms of severity and duration, that the coronavirus pandemic will have on its business, operating results, cash flows and financial condition, but it could be material if the current circumstances continue to exist for a prolonged period. Although the Company has made its best estimates based upon current information, actual results could materially differ from the estimates and assumptions developed by management. If so, the Company may be subject to future impairment charges as well as changes to recorded reserves and valuations. Segment information: Segment Reporting Concentration of supplier risk: Cash and cash equivalents: The Company maintains cash in excess of federally insured limits at financial institutions. The Company makes such deposits with entities it believes are of high credit quality and has not incurred any losses related to these balances to date. Management believes its credit risk, with respect to the financial institutions to be minimal. Accounts receivable: Investments: The Company uses the specific identification method to determine the cost basis of securities sold. Investments are impaired when a decline in fair value is judged to be other-than-temporary. The Company evaluates an investment for impairment by considering the length of time and extent to which market value has been less than cost or amortized cost, the financial condition and near-term prospects of the issuer as well as specific events or circumstances that may influence the operations of the issuer and the Company’s intent to sell the security or the likelihood that it will be required to sell the security before recovery of the entire amortized cost. Once a decline in fair value is determined to be other-than-temporary, an impairment charge is recorded to other income (expense) and a new costs basis in the investment is established. Fair value measurements: Fair Value Measurements Level I Level II Level III An asset’s or liability’s fair value measurement level within the fair value hierarchy is based on the lowest level of any input that is significant to the fair value measurement. Valuation techniques used need to maximize the use of observable inputs and minimize the use of unobservable inputs. Assets and liabilities measured at fair value are based on one or more of the following three valuation techniques noted in ASC 820: ● Market approach: Prices and other relevant information generated by market transactions involving identical or comparable assets or liabilities. ● Cost approach: Amount that would be required to replace the service capacity of an asset (replacement cost). ● Income approach: Techniques to convert future amounts to a single present value amount based upon market expectations (including present value techniques, option pricing and excess earnings models) The Company believes its valuation methods are appropriate and consistent with other market participants, however the use of different methodologies or assumptions to determine the fair value of certain financial instruments could result in a different fair value measurement at the reporting date. The Company’s financial instruments consist of cash and cash equivalents, accounts receivable, investments, accounts payable, accrued expenses, contingent consideration liability, convertible notes payable derivative liability, warrant liabilities, and convertible notes payable. The carrying value of cash and cash equivalents, accounts receivable, accounts payable, and accrued expenses approximates fair value because of the short-term nature of those instruments. We estimate the fair value of our convertible notes payable using Level II and Level III inputs by discounting the future cash flows using current interest rates at which we could obtain similar borrowings in consideration of the estimated enterprise value of the Company. The fair value of corporate bonds, treasury securities and commercial paper are based on quoted prices for identical or similar instruments in markets that are not active. As a result, corporate bonds, treasury securities and commercial paper are classified within Level II of the fair value hierarchy. The fair value of the Company’s public warrant liabilities are based on Level I inputs, while the fair value of the private warrants is determined using the trading price of the public warrants, a Level II input. The Company’s assets and liabilities that are measured at fair value on a recurring basis include the Company’s contingent consideration liability, warrant liabilities, and convertible notes payable derivative liabilities (See Note 5). Prepaid expenses and other current assets: Property and equipment, net: Production machinery and equipment 2 to 7 years Vehicles 3 to 7 years Leasehold improvements shorter of lease term or 7 years Demo fleet systems 2 to 3 years Furniture and fixtures 3 years Computers and related equipment 3 to 7 years Major renewals and improvements are capitalized, while replacements, maintenance and repairs, which do not improve or extend the lives of the respective assets, are expensed as incurred. When property and equipment is retired or otherwise disposed of, the related cost and accumulated depreciation are removed from the accounts, and any gain or loss on the disposition is recorded in the consolidated statement of operations as a component of other (expense) income. Intangible assets, net: Impairment of long-lived assets Impairment or Disposal of Long-Lived Assets Revenue: Revenue from Contracts with Customers ● Step 1: Identify the contract(s) with a customer ● Step 2: Identify the performance obligations in the contract ● Step 3: Determine the transaction price ● Step 4: Allocate the transaction price to the performance obligations in the contract ● Step 5: Recognize revenue when (or as) a performance obligation is satisfied The Company intends to generate revenue from the sale of its hybrid and electrified drive systems for the long haul “Class 8” semi-tractors. However, since the Company is still in the pre-commercialization stage, it has not generated revenue from the sale of the products. The Company did not enter into any agreement that meets the definition of a contract with a customer that would be accounted for under ASC 606 through December 31, 2020. Leases: Lessee ROU assets represent the Company’s right to use underlying assets for the lease term, and lease liabilities represent the Company’s obligation to make lease payments arising from the leases. ROU assets and lease liabilities are recognized at the commencement date based on the present value of lease payments over the lease term. The discount rate used to calculate the present value for lease payments is the Company’s incremental borrowing rate, which is determined based on information available at lease commencement and is equal to the rate of interest that the Company would have to pay to borrow on a collateralized basis over a similar term in an amount equal to the lease payments in a similar economic environment. The Company uses the implicit rate when readily determinable. The Company has entered into operating leases for corporate offices having initial lease terms of one to eight years. The Company has entered into finance leases primarily for vehicles and equipment, having initial terms of three years. The Company’s real estate leases may include one or more options to renew, with the renewal extending the lease term for an additional one to five years. The exercise of lease renewal option is at the Company’s sole discretion. In general, the Company does not consider renewal option to be reasonably likely to be exercised, therefore renewal option are generally not recognized as part of the ROU assets and lease liabilities. Lease costs for lease payments are recognized on a straight-line basis over the lease term, unless there is a transfer of title or purchase option reasonably certain to be exercised. The Company does not record operating leases with an initial term of twelve months or less (“short-term leases”) in the consolidated balance sheets. The Company’s vehicle and equipment leases may include transfer rights or options to purchase at the end of the lease that the Company is reasonably certain to exercise. Interest expense is recognized using the effective interest rate method, and the ROU asset is amortized over the useful life of the underlying asset. Lessor The Company has entered into various trial and evaluation agreements that contain an operating lease component that is within the scope of ASC 842, Leases The trial and evaluation agreements contain only variable payments not based on an index or rate as a result of refund provisions within those contracts. The Company records accounts receivable when the Company meets the criteria within the trial and evaluation agreements to invoice the lessee. In accordance with ASC 842, the Company recognizes variable lease payments as profit or loss in the period in which the changes in facts and circumstances on which the variable lease payments are based occur, which will generally be the end of the trial period when the customer refund rights lapse. During the years ended December 31, 2020 and 2019, the Company has not recognized any lease income related to these trial and evaluation agreements either because the Company has not received any consideration from the lease contracts, or the uncertainty related to the consideration received has not been resolved. Certain of the Company’s lessee and lessor lease agreements contain both lease and non-lease components, which are generally accounted for as a single lease component. Additionally, for certain vehicle leases, we apply a portfolio approach to effectively account for the finance lease ROU assets and liabilities. Income taxes: Income Taxes Due to the Company’s history of losses since inception, the net deferred tax assets have been fully offset by a valuation allowance as of December 31, 2020 and 2019. Uncertain tax positions taken or expected to be taken in a tax return are accounted for using the more likely than not threshold for financial statement recognition and measurement. For the years ended December 31, 2020 and 2019, there were no uncertain tax positions taken or expected to be taken in the Company’s tax returns. Share-based compensation: Compensation – Stock Compensation The Company utilizes the Black-Scholes model to determine the fair value of the stock option awards, which requires the input of subjective assumptions. These assumptions include estimating (a) the length of time grantees will retain their vested stock options before exercising them for employees and the contractual term of the option for nonemployees (“expected term”), (b) the volatility of the Company’s common stock price over the expected term, (c) expected dividends, and (d) the fair value of a share of common stock prior to the Business Combination. After the closing of the Business Combination, the Company’s board of directors determined the fair value of each share of common stock underlying stock-based awards based on the closing price of the Company’s common stock as reported by the NYSE on the date of grant. The Company has elected to recognize the adjustment to share-based compensation expense in the period in which forfeitures occur. The assumptions used in the Black-Scholes model are management’s best estimates, but the estimates involve inherent uncertainties and the application of management judgment (see Note 9). As a result, if other assumptions had been used, the recorded share-based compensation expense could have been materially different from that depicted in the financial statements. Research and development expense: Net income (loss) per share: Recent accounting pronouncements issued, not yet adopted: In June 2016, the FASB issued ASU 2016-13, Financial Instruments—Credit Losses (Topic 326): Measurement of Credit Losses of Financial Instruments In December 2019, the FASB issued ASU 2019-12, Income Taxes (Topic 740): Simplifying the Accounting for Income Taxes

Debt

Debt	3 Months Ended	12 Months Ended
	Mar. 31, 2021	Dec. 31, 2020
Debt Disclosure [Abstract]
Debt	3. Debt Convertible Notes Payable: (a) Optional conversion upon the next equity financing of at least $15.0 million in proceeds. The conversion price will be based on the per share price of the next equity financing, with a 50% discount. (b) Optional conversion upon a subsequent equity financing of at least $15.0 million if the holder did not elect to convert upon the next equity financing, at the price that is set by the subsequent equity financing (no discount). (c) Optional conversion upon a change in control. In the event of a change in control, the holder can elect to convert the January 2020 Note into shares of common stock at a conversion price equal to (i) the product of the change in control purchase price multiplied by 50%, divided by (ii) the total number of outstanding shares of capital stock of the Company (on a fully diluted basis). (d) Optional redemption upon a change in control. In the event of a change in control, the holder can elect to request payment of all outstanding principal (with no penalty) and unpaid accrued interest. (e) Optional redemption upon the Company obtaining at least $10.0 million in commercial debt which would result in the January 2020 Note having the same priority or being treated as subordinate to the commercial debt. In such scenario, the holder can elect to request payment of all outstanding principal (with no penalty) and unpaid accrued interest. (f) Automatic or optional redemption upon an event of default. Upon the occurrence of an event of default, the January 2020 Note will either automatically become due and payable or can become due and payable at the holder’s option (based on the nature of the event of default). Upon such acceleration, all outstanding principal (with no penalty) and unpaid accrued interest will become payable. (g) Additional interest of 3% (or a total of 9%) upon an event of default. In addition, in the event the holder did not convert upon an equity financing or change in control event, the noteholder may extend the maturity date of the January 2020 Note by five years beyond the original maturity date. In addition, in the event the holder did not convert upon an equity financing, the interest rate on the January 2020 Note will automatically be adjusted to a rate of 4% per annum. The Company assessed the embedded features within the January 2020 Note and determined that the automatic and optional conversion features upon the next equity financing (share-settled redemption features), the additional interest feature and the term extension feature met the definition of a derivative and were not clearly and closely related to the host contract and required separate accounting. The Company also concluded that the conversion features did not represent beneficial conversion features. At issuance, the Company estimated the fair value of the automatic and optional conversion features to be approximately $2.7 million. At issuance, the Company concluded the fair value of the additional interest and term extension features was de minimis. The terms of the convertible notes payable include certain restrictive covenants related to the Company’s ability to enter into certain transactions or agreements, pay dividends, or take other similar corporate actions. In connection with the reverse recapitalization, immediately prior to the closing of the Business Combination, these convertible notes, plus accrued paid-in-kind interest, were converted into the Company’s common stock on the closing date. Payroll Protection Program loan:	Note 5. Debt At December 31, 2020 and 2019, the carrying value of debt was as follows: December 31, 2020 2019 (in thousands) Convertible notes payable, net of unamortized discount at December 31, 2020 and 2019 of $0 and $6,451, respectively $ - $ 16,113 Paycheck Protection Program loan 908 - Finance lease obligations 49 289 957 16,402 Less current portion 49 6,720 Debt, net of current portion $ 908 $ 9,682 During 2018, the Company issued a convertible note payable in exchange for cash totaling $5.0 million (the “2018 Note”). The 2018 Note bears interest at 6% per annum and matures in September 2020 (two years subsequent to its issuance date). The 2018 Note includes the following embedded features: (a) Automatic conversion upon the next equity financing of at least $5.0 million in proceeds. The conversion price is dependent upon the pre-money valuation of the Company in connection with the next equity financing, with the conversion price set at a 35% discount on the next equity financing price if the pre-money valuation is $100.0 million or less, or 35% multiplied by the quotient of $100.0 million divided by the pre-money valuation if it is greater than $100.0 million. (b) Optional conversion upon a change in control. In the event of a change in control, the holder can elect to convert the 2018 Note into shares of common stock at a conversion price equal to (i) the product of the change in control purchase price multiplied by 65%, divided by (ii) the total number of outstanding shares of capital stock of the Company (on a fully diluted basis). (c) Optional redemption upon a change in control. In the event of a change in control, the holder can elect to request payment of all outstanding principal (with no penalty) and unpaid accrued interest. (d) Automatic or optional redemption upon an event of default. Upon the occurrence of an event of default, the 2018 Note will either automatically become due and payable or can become due and payable at the holder’s option (based on the nature of the event of default). Upon such acceleration, all outstanding principal (with no penalty) and unpaid accrued interest will become payable. (e) Additional interest of 3% (or a total of 9%) upon an event of default. In addition to the above embedded features, the Company agreed that the holder of the 2018 Note would be the Company’s preferred supplier for certain components or products that the holder sells. See Note 15 for further details on this related party agreement. The Company assessed the embedded features within the 2018 Note and determined that the automatic conversion feature upon next equity financing and optional conversion feature upon change in control (share-settled redemption features) and the additional interest feature met the definition of a derivative and were not clearly and closely related to the host contract and required separate accounting. At issuance, the Company estimated the fair value of the automatic and optional conversion features to be approximately $1.8 million. The Company’s fair value measurements are more fully described in (Note 7). At issuance, the Company concluded the fair value of the additional interest feature was de minimis. Between February and July 2019, the Company issued a series of convertible notes payable in exchange for cash totaling $13.6 million (the “Initial 2019 Notes”). The Initial 2019 Notes bear interest at 6% per annum and mature two to five years after their respective issuance dates. The Initial 2019 Notes are only prepayable with the consent of the holders. One of the Initial 2019 Notes (totaling $1.8 million) is secured by substantially all of the assets of the Company, subordinate to the first priority, senior secured interest held by a note holder of a convertible note issued in January 2020. The holder of this note has first priority secured interest in these assets. The Initial 2019 Notes include the following embedded features: (a) Automatic or optional (for one of the Initial 2019 Notes) conversion upon the next equity financing of at least $15.0 million in proceeds (the “Next Equity Financing”). The conversion price is dependent upon the pre-money valuation of the Company in connection with the next equity financing, with the conversion price set at a 25% discount on the next equity financing price if the pre-money valuation is $100.0 million or less, or 25% multiplied by the quotient of $100.0 million divided by the pre-money valuation if it is greater than $100.0 million. (b) Optional conversion (for one of the Initial 2019 Notes) upon a subsequent equity financing if the holder did not elect to convert upon the Next Equity Financing, at the price that is set by the subsequent equity financing (no discount). (c) Optional conversion upon a change in control. In the event of a change in control, the holder can elect to convert the Initial 2019 Notes into shares of common stock at a conversion price equal to (i) the product of the change in control purchase price multiplied by 75%, divided by (ii) the total number of outstanding shares of capital stock of the Company (on a fully diluted basis). (d) Optional redemption upon a change in control. In the event of a change in control, the holder can elect to request payment of all outstanding principal (with no penalty) and unpaid accrued interest. (e) Automatic or optional redemption upon an event of default. Upon the occurrence of an event of default, the Initial 2019 Notes will either automatically become due and payable or can become due and payable at the holder’s option (based on the nature of the event of default). Upon such acceleration, all outstanding principal (with no penalty) and unpaid accrued interest will become payable. (f) Additional interest of 3% (or a total of 9%) upon an event of default. In addition, the Company has the right to modify one of the Initial 2019 Notes (totaling $1.8 million) in the event the holder does not convert upon next equity financing to adjust the interest rate to 4% per annum. The Company assessed the embedded features within the Initial 2019 Notes and determined that the automatic or optional conversion feature upon next equity financing and the optional conversion feature upon change in control (share-settled redemption features), the additional interest feature, and the interest rate adjustment feature met the definition of a derivative and were not clearly and closely related to the host contract and required separate accounting. At issuance, the Company estimated the fair value of the automatic and optional conversion features to be approximately $6.0 million. The Company’s fair value measurements are more fully described in (Note 7). At issuance, the Company concluded the fair value of the additional interest feature and the interest rate adjustment feature was de minimis. In December 2019, the Company issued a convertible note payable in exchange for cash totaling $3.2 million (the “December 2019 Note”). The December 2019 Note bears interest at 6% per annum and matures in December 2020 (one year subsequent to its issuance date). The December 2019 Note is only prepayable with the consent of the holder. The December 2019 Note is secured by substantially all of the assets of the Company, subordinate to the security interest held by one of the Initial 2019 Note holders. The December 2019 Note includes the following embedded features: (a) Automatic conversion upon the next equity financing of at least $35.0 million in proceeds. The conversion price will be based on the next equity financing per share price, with a 50% discount. (b) Optional conversion upon the next equity financing of at least $15.0 million in proceeds. The conversion price will be based on the next equity financing per share price, with a 50% discount. (c) Automatic conversion upon a subsequent equity financing of at least $35.0 million if the holder did not elect to convert upon any previous equity financing, at the price that is set by the subsequent equity financing (no discount). (d) Optional conversion upon a change in control. In the event of a change in control, the holder can elect to convert the December 2019 Note into shares of common stock at a conversion price equal to (i) the product of the change in control purchase price multiplied by 50%, divided by (ii) the total number of outstanding shares of capital stock of the Company (on a fully diluted basis). (e) Optional redemption upon a change in control. In the event of a change in control, the holder can elect to request payment of all outstanding principal (with no penalty) and unpaid accrued interest. (f) Automatic or optional redemption upon an event of default. Upon the occurrence of an event of default, the December 2019 Note will either automatically become due and payable or can become due and payable at the holder’s option (based on the nature of the event of default). Upon such acceleration, all outstanding principal (with no penalty) and unpaid accrued interest will become payable. (g) Additional interest of 3% (or a total of 9%) upon an event of default. In addition, in the event the holder does not convert upon an equity financing, the maturity date of the December 2019 Note will automatically extend by one year. In such situation, the holder also has the right to extend the maturity date for an additional two years beyond the modified maturity date. The Company assessed the embedded features within the December 2019 Note and determined that the automatic and optional conversion features upon next equity financing (share-settled redemption features), the additional interest feature and the term extension feature met the definition of a derivative and were not clearly and closely related to the host contract and required separate accounting. The Company also concluded that the conversion features did not represent beneficial conversion features. At issuance and at December 31, 2019, the Company estimated the fair value of the automatic and optional conversion features to be approximately $1.4 million. The Company’s fair value measurements are more fully described in (Note 7). At issuance, the Company concluded the fair value of the additional interest and term extension features was de minimis. During January 2020, the Company issued a convertible note payable in exchange for cash totaling $3.2 million (the “January 2020 Note”). The January 2020 Note bears interest at 6% per annum and matures in January 2025 (five years subsequent to its issuance date). The January 2020 Note is only prepayable with the consent of the holder. The January 2020 Note is secured by a first priority, senior secured interest in substantially all of the assets of the Company. The January 2020 Note includes the following embedded features: (a) Optional conversion upon the next equity financing of at least $15.0 million in proceeds. The conversion price will be based on the next equity financing per share price, with a 50% discount. (b) Optional conversion upon a subsequent equity financing of at least $15.0 million if the holder did not elect to convert upon the next equity financing, at the price that is set by the subsequent equity financing (no discount). (c) Optional conversion upon a change in control. In the event of a change in control, the holder can elect to convert the January 2020 Note into shares of common stock at a conversion price equal to (i) the product of the change in control purchase price multiplied by 50%, divided by (ii) the total number of outstanding shares of capital stock of the Company (on a fully diluted basis). (d) Optional redemption upon a change in control. In the event of a change in control, the holder can elect to request payment of all outstanding principal (with no penalty) and unpaid accrued interest. (e) Optional redemption upon the Company obtaining at least $10.0 million in commercial debt which would result in the January 2020 Note having the same priority or being treated as subordinate to the commercial debt. In such scenario, the holder can elect to request payment of all outstanding principal (with no penalty) and unpaid accrued interest. (f) Automatic or optional redemption upon an event of default. Upon the occurrence of an event of default, the January 2020 Note will either automatically become due and payable or can become due and payable at the holder’s option (based on the nature of the event of default). Upon such acceleration, all outstanding principal (with no penalty) and unpaid accrued interest will become payable. (g) Additional interest of 3% (or a total of 9%) upon an event of default. In addition, in the event the holder does not convert upon an equity financing or change in control event, the noteholder may extend the maturity date of the January 2020 Note by five years beyond the original maturity date. In addition, in the event the holder does not convert upon an equity financing, the interest rate on the January 2020 Note will automatically be adjusted to a rate of 4% per annum. The Company assessed the embedded features within the January 2020 Note and determined that the automatic and optional conversion features upon next equity financing (share-settled redemption features), the additional interest feature and the term extension feature met the definition of a derivative and were not clearly and closely related to the host contract and required separate accounting. The Company also concluded that the conversion features did not represent beneficial conversion features. At issuance, the Company estimated the fair value of the automatic and optional conversion features to be approximately $2.7 million. The Company’s fair value measurements are more fully described in (Note 7). At issuance, the Company has concluded the fair value of the additional interest and term extension features was de minimis. The terms of the convertible notes payable include certain restrictive covenants related to the Company’s ability to enter into certain transactions or agreements, pay dividends, or take other similar corporate actions. During June 2020, the holders of the convertible notes executed amendments (the “Note Amendments”) to their respective convertible notes clarifying the planned Business Combination would qualify as a next financing, as defined in the respective convertible notes. The convertible notes would either automatically convert or convert at the holder’s option (the election of which was evidenced by entering into the Note Amendments) in connection with such next financing (in this case the Business Combination). The convertible notes would convert into shares of common stock at a conversion price equal to (i) the valuation of the Company established in connection with such next financing, divided by (ii) the total number of shares of capital stock of the Company (on a fully diluted and as-converted basis), as established in the original respective convertible notes. This conversion price would then be discounted based on the negotiated conversion discounts that were established in the noteholders’ original convertible notes. The amended terms of the Note Amendments were determined to be clarifications of the existing terms and did not result in substantially different terms. Accordingly, the Note Amendments were accounted for as modifications. In connection with the reverse recapitalization discussed in Note 4, immediately prior to the closing of the Business Combination, the convertible notes, plus accrued paid-in-kind interest, totaling $26.8 million were converted into an aggregate of 2,336,235 shares of Legacy Hyliion common stock, which were then exchanged for an aggregate of 3,404,367 shares of the Company’s common stock on the Closing Date (see Note 4). In addition, the Company issued 1,000,000 shares of Legacy Hyliion common stock to a noteholder of the 2018 Note, Initial 2019 Notes, and January 2020 Note, with a grant date fair value of $10.00 per share in accordance with the Commercial Matters Agreement (see Note 15). In connection with this conversion of the convertible notes, the Company recorded a loss on extinguishment of $10.2 million included within other income (expense) on the accompanying consolidated statements of operations. Term Loan : D Payroll Protection Program loan Finance Lease Obligations

Investments

Investments	3 Months Ended	12 Months Ended
	Mar. 31, 2021	Dec. 31, 2020
Investments, Debt and Equity Securities [Abstract]
Investments	4. Investments The amortized cost, unrealized gains and losses, and fair value of our held-to-maturity investments at March 31, 2021 and December 31, 2020 are summarized as follows: Fair Value Measurements as of Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value (in thousands) Held-to-maturity investments Commercial paper $ 116,658 $ — $ (24 ) $ 116,634 State and municipal bonds 15,805 — (21 ) 15,784 Corporate bonds and notes 164,847 — (644 ) 164,203 Total held-to-maturity investments $ 297,310 $ — $ (689 ) $ 296,621 As of March 31, 2021, the Company has determined that the unrealized losses totaling $0.7 million is temporary and fully expects to recover the cost basis. Fair Value Measurements as of Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value (in thousands) Held-to-maturity investments Treasury securities $ 149,996 $ — $ (1 ) $ 149,995 Commercial paper 37,963 — (15 ) 37,948 Corporate bonds and notes 49,892 — (63 ) 49,829 Total held-to-maturity investments $ 237,851 $ — $ (79 ) $ 237,772 March 31, December 31, Amortized Cost Fair Value Amortized Cost Fair Value (in thousands) (in thousands) Due in one year or less $ 144,829 $ 144,788 $ 201,881 $ 201,864 Due after one year through five years 152,481 151,833 35,970 35,908 Total held-to-maturity securities $ 297,310 $ 296,621 $ 237,851 $ 237,772	Note 6. Investments The amortized cost, unrealized gains and losses, and fair value of our investments at December 31, 2020 are summarized as follows: Fair Value Measurements as of Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value (in thousands) Held-to-maturity investments Treasury securities $ 149,996 $ - $ (1 ) $ 149,995 Commercial paper 37,963 - (15 ) 37,948 Corporate bonds and notes 49,892 - (63 ) 49,829 Total held-to-maturity investments $ 237,851 $ - $ (79 ) $ 237,772 December 31, 2020 Amortized Cost Fair Value (in thousands) Due in one year or less $ 201,881 $ 201,864 Due after one year through five years 35,970 35,908 Total held-to-maturity securities $ 237,851 $ 237,772 The Company did not have any investments at December 31, 2019.

Fair Value Measurements

Fair Value Measurements	3 Months Ended	12 Months Ended
	Mar. 31, 2021	Dec. 31, 2020
Fair Value Disclosures [Abstract]
Fair Value Measurements	5. Fair Value Measurements ASC Topic 820, Fair Value Measurements and Disclosures, establishes a framework for measuring fair value and the corresponding disclosure requirements around fair value measurements. This topic applies to all financial instruments that are being measured and reported on a fair value basis. The following table shows the fair value measurements of the Company's assets that are measured at fair value on a recurring basis as of March 31, 2021 and 2020. Fair Value Measurements as of March 31, 2021 Level I Level II Level III Total (in thousands) Assets Cash and cash equivalents $ 334,718 $ — $ — $ 334,718 Held-to-maturity investments: Commercial paper — 116,634 — 116,634 State and municipal bonds — 15,784 — 15,784 Corporate bonds and notes — 164,203 — 164,203 Total Assets $ 334,718 $ 296,621 $ — $ 631,339 Fair Value Measurements as of December 31, 2020 Level I Level II Level III Total (in thousands) Assets Cash and cash equivalents $ 389,705 $ — $ — $ 389,705 Held-to-maturity investments: Treasury securities — 149,995 — 149,995 Commercial paper — 37,948 — 37,948 Corporate bonds and notes — 49,829 — 49,829 Total Assets $ 389,705 $ 237,772 $ — $ 627,477	Note 7. Fair Value Measurements – As Restated The convertible notes payable derivative liabilities are considered a Level III measurement due to the utilization of significant unobservable inputs in the valuation. The Company utilized a scenario-based with and without valuation model to estimate the fair value of the embedded derivative features requiring bifurcation associated with the convertible notes payable at issuance, as of the December 31, 2019 reporting date, and upon the settlement of the convertible notes payable derivative liabilities in connection with the extinguishment accounting applied to the convertible notes payable (see Note 5). This valuation model is designed to utilize the Company’s best estimates of the timing and likelihood of the settlement events that are related to the embedded derivative features in order to estimate the fair value of the respective convertible notes with these embedded derivative features. The fair value of the convertible notes with the derivative features is compared to the fair value of a plain vanilla note (excluding the derivative features), which is calculated based on the present value of the future cash flows. The difference between the two values represents the fair value of the bifurcated derivative features as of each respective valuation date. The key inputs to the valuation models that were utilized to estimate the fair value of the convertible debt derivative liabilities include: Input October 1, 2020 Issuance of January 2020 Note Issuance of December 2019 Note and December 31, 2019 Issuances of Initial 2019 Notes Issuances of Initial 2019 Notes Issuances of Initial 2019 Notes Probability-weighted conversion discount 2.5 - 50.0% 50.0% 23.9 - 50.0% 24.1% 24.4% 24.4% Remaining term (years) 0.0 - 4.3 5.0 0.7 - 4.5 5.0 2.0 2.0 Equity volatility NA NA 63.0 - 71.0% 74.0% 78.0% 75.0% Risk rate 1 19.6 - 57.7% 50.0% 27.2 - 50.0% 29.0% 26.6% 34.2% Probability of next financing event 1 100.0% 70.0% 70.0% 50.0% 50.0% 50.0% Timing of next financing event 1 10/1/2020 9/30/2020 9/30/2020 3/31/2020 3/31/2020 9/30/2019 Probability of default event 1 0.0% 30.0% 25.0 - 30.0% 50.0% 50.0% 50.0% Timing of default event 1 NA 9/30/2020 9/30/2020 3/31/2020 3/31/2020 9/30/2019 Probability of sale event 1 0.0% 0.0% 0.0 - 5.0% 0.0% 0.0% 0.0% Timing of sale event 1 NA NA 9/30/2020 NA NA NA Negotiation discount 1 2 0.0 - 0.1% 24.2% 21.7% 0.0% 0.0% 0.0% 1 Represents a Level III unobservable input 2 Based on the terms and provisions of the December 2019 and January 2020 Notes, the valuation model incorporated this additional assumption The key inputs to the valuation models are defined as follows: ● The probability-weighted conversion discount is based on the contractual terms of the convertible note agreement and the expectation of the pre-money valuation of the Company as of the estimated date that the next equity financing event occurs. ● The remaining term was determined based on the remaining time period to maturity of the related convertible note with embedded features subject to valuation (as of the respective valuation date). ● The Company’s equity volatility estimate was based on the re-levered historical equity volatility of a selection of the Company’s comparable guideline public companies, based on the remaining term of the respective convertible notes. ● The risk rate was the discount rate utilized in the valuation and was determined based on reference to market yields for debt instruments with similar credit ratings and terms. ● The probabilities and timing of the next financing event and default event are based on management’s best estimate of the future settlement of the respective convertible notes. ● The negotiation discount utilized was calculated in order to further discount the specified instruments in order to agree to the principal value of the convertible notes at issuance. The utilization of the negotiation discount reflects the fact that there was a significant need for new investment and limited availability of market participants who have interest in making investments in such companies. The presence of the additional discount reflects the higher rate of return that these investors would seek in making such investments. The convertible notes payable derivative liabilities were settled upon the conversion of the related convertible notes during the year ended December 31, 2020 (see Note 5). The following table shows the fair value measurements of the Company’s assets and liabilities that are measured at fair value on a recurring basis at December 31, 2020 and 2019: Fair Value Measurements as of December 31, 2020 Level I Level II Level III Total Assets (in thousands) Cash and cash equivalents $ 389,705 $ - $ - $ 389,705 Held-to-maturity investments: Treasury securities - 149,995 - 149,995 Commercial paper - 37,948 - 37,948 Corporate bonds and notes - 49,829 - 49,829 Total Assets $ 389,705 $ 237,772 $ - $ 627,477 Fair Value Measurements as of December 31, 2019 Level I Level II Level III Total Liabilities (in thousands) Convertible notes payable derivative liabilities $ - $ - $ 8,351 $ 8,351 Total Liabilities $ - $ - $ 8,351 $ 8,351 The following is a rollforward of the Company’s Level III instruments (in thousands): Balance, December 31, 2018 $ 2,068 Issuance of convertible notes payable derivative liabilities 7,428 Fair value adjustments (1,145 ) Balance, December 31, 2019 8,351 Issuance of convertible note payable derivative liability 2,656 Fair value adjustments 1,358 Settlement of convertible notes payable derivative liabilities (12,365 ) Balance, December 31, 2020 $ -

Commitments and Contingencies

Commitments and Contingencies	3 Months Ended	12 Months Ended
	Mar. 31, 2021	Dec. 31, 2020
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	6. Commitments and Contingencies Legal Proceedings:	Note 15. Commitments and Contingencies Economic Incentive Agreement Should the Company fail to meet and maintain any performance requirements, all amounts received from the EDC are subject to refund. During 2018, the Company achieved the first performance requirement and received a payment of $0.2 million. During 2019, the Company continued maintaining the employment level of the first performance requirement but failed to meet the second performance requirement. As a result, the Company did not receive any additional grant funding in 2019, the agreement is subject to termination by the EDC and all amounts received are subject to refund. As the terms of the EDC grant agreement require the Company to meet and maintain all of the performance requirements throughout the term of the agreement, the Company has not substantially met all the conditions for the grant funding received. Accordingly, the grant funding of $0.2 million received in 2018 is recorded as part of accrued expenses and other current liabilities as of December 31, 2020 and 2019 and will continue to be reflected as a currently liability until all related performance requirements have been met through the end of the agreement on December 31, 2024. Under the agreement, the EDC has the right to file a security interest to all assets of the Company. This security interest is subordinate to the holders of the convertible notes payable with security interests. Preferred Sourcing Arrangement and Commercial Matters Agreement: Under the PSA, the Company is allowed to purchase competing products upon the request of any customer. In June 2020 and in conjunction with the Business Combination, the Company entered into a Commercial Matters Agreement with the PSA Partner pursuant to which, among other things, contingent and effective upon the execution of the Business Combination, the Company issued to the PSA Partner $10.0 million worth of Legacy Hyliion’s Common Stock, immediately prior to the effective time of the merger in consideration for the Note Amendments and for any future services to be provided pursuant to the terms of a services agreement to provide engineering or operational services to the Company that was entered into in June 2020. The terms of the services agreement are yet to, and may ultimately not, be negotiated and the PSA Partner is under no obligation to enter into such services agreement. As a result, immediately prior to the consummation of the Business Combination discussed in Note 4, the Company issued 1,000,000 shares of Legacy Hyliion common stock with a fair value of $10.00 per share in exchange for future services to the Company. Legal Proceedings

Warrants

Warrants	3 Months Ended
	Mar. 31, 2021
Stockholders' Equity Note [Abstract]
Warrants	7. Warrants On November 30, 2020, the Company issued a notice of redemption of all its outstanding Public Warrants and Forward Purchase Warrants which was completed in December 2020. However, the Private Warrants held by the initial holders thereof or permitted transferees of the initial holders were not subject to this redemption. As of December 31, 2020, all outstanding Public Warrants and Forward Purchase Warrants were either exercised or redeemed by the holder. As of December 31, 2020, the Company’s transfer agent received gross proceeds of $140.8 million corresponding to the exercise of 15,786,127 warrants. However, due to the timing of the receipt of the warrant exercise and the cash, the Company’s transfer agent issued 15,414,592 shares of common stock as of December 31, 2020. The remaining 371,535 shares of common stock were issued in January 2021. Additionally, as of December 31, 2020, the Company’s transfer agent had not yet remitted $12.0 million of the gross proceeds associated with the shares of issued common stock to the Company and is included within prepaid expenses and other current assets on the accompanying consolidated balance sheets as of December 31, 2020. There were 281,065 warrants not exercised by the end of the redemption period that were redeemed for a price of $0.01 per warrant, and subsequently cancelled by the Company. The Company made the redemption payment on these cancelled warrants in January 2021. Certain holders of the warrants elected a cashless exercise, resulting in the forfeiture of 3,118,445 shares. The accrued liability totaling $4.3 million for warrants exercised but not settled represents all warrants that were exercised as of December 31, 2020 under broker protects resulting in cash collection and share issuance being delayed until January 4, 2021.

Share-based Compensation

Share-based Compensation	3 Months Ended	12 Months Ended
	Mar. 31, 2021	Dec. 31, 2020
Share-based Payment Arrangement [Abstract]
Share-based Compensation	8. Share-based Compensation During the three months ended March 31, 2021, the Company granted 3,174,341 restricted stock units to certain employees, some of which vested at issuance, some of which will vest over a period of three or four years, and some of which will vest based on achievement of performance criteria. The criteria for the awards is based on the achievement of key milestones based on the Company's performance. During the three months ended March 31, 2020, the Company awarded 1,920,000 options to certain employees and non-employees, which will vest over a period that ranges from four to ten years. The estimated grant date fair value of the options granted during the three months ended March 31, 2020 totaled $0.4 million. Share-based compensation expense for the three months ended March 31, 2021 and 2020 was $1.5 million and $0.1 million, respectively.	Note 9. Share-based Compensation 2016 Equity Incentive Plan For periods prior to the reverse recapitalization (See Note 4), the Hyliion Inc. 2016 Equity Incentive Plan (the “2016 Plan”), as amended in August 2017 and approved by the board of directors (the “Board”), permitted the granting of various awards including stock options (including both nonqualified options and incentive options), stock appreciation rights (“SARs”), stock awards, phantom stock units, performance awards, and other share-based awards to employees, outside directors and consultants and advisors of the Company. Only stock options have been awarded to employees, consultants and advisors under the 2016 Plan. Legacy Options converted into an option to purchase a number of shares of common stock equal to the product of the number of shares of Legacy Hyliion common stock and the Exchange Ratio at an exercise price per share equal to the exercise price of the Legacy Option divided by the Exchange Ratio. Each exchanged option is governed by the same terms and conditions applicable to the Legacy Option prior to the Business Combination. No further grants can be made under the 2016 Plan. The option exercise price for all grantees equals the stock’s estimated fair value on the date of the grant, after giving effect to the Exchange Ratio. The Board determined the fair value of common stock at the time of grant by considering a number of objective and subjective factors, including independent third-party valuations of the Company’s common stock, operating and financial performance, the lack of liquidity of capital stock, and general and industry-specific economic outlook, amongst other factors. The Company believes the fair value of the stock options granted to nonemployees is more readily determinable than the fair value of the services received. The fair value of each option is estimated on the date of the grant using the Black-Scholes option-pricing model in order to measure the compensation cost associated with the award. This model incorporates certain assumptions for inputs including an expected volatility in the market value of the underlying common stock, expected term, a risk-free interest rate, and the expected dividend yield of the underlying common stock. The following assumptions were used for options issued in the following periods: Years Ended December 31, 2020 2019 Expected volatility 70.0% 70.0% Expected term (in years) 6.1 6.1 - 10 Risk-free interest rate 1.7% 1.4 - 3.0% Expected dividend yield 0.0% 0.0% ● Expected volatility ● Expected term ● Risk-free interest rate ● Expected dividend yield Employee and nonemployee stock options generally vest over four years, with a maximum term of ten years from the date of grant. These awards become available to the recipient upon the satisfaction of a vesting condition based on a period of service, which may be accelerated at the discretion of the Board. Share-based compensation expense is recognized on a straight-line basis over the applicable vesting period. A summary of the status of the 2016 Plan at December 31, 2020 and 2019, and changes during the same periods is presented below: Options Shares Weighted Average Exercise Price Weighted Average Remaining Contractual Term Outstanding at December 31, 2018 5,508,031 $ 0.11 8.7 Granted 3,213,131 0.16 Exercised (418,033 ) 0.14 Cancelled or forfeited (1,715,847 ) 0.13 Outstanding at December 31, 2019 6,587,282 0.13 8.2 Granted 2,797,828 0.23 Exercised (1,112,960 ) 0.11 Cancelled or forfeited (1,289,653 ) 0.19 Outstanding at December 31, 2020 6,982,497 $ 0.16 7.8 Exercisable at December 31, 2019 2,482,987 $ 0.10 7.2 Exercisable at December 31, 2020 3,851,486 $ 0.13 7.1 As of December 31, 2020, the options outstanding and exercisable have an intrinsic value of $113.8 million and $62.8 million, respectively. There were no options with an exercise price greater than the market price on December 31, 2020 to exclude from the intrinsic value computation. The intrinsic value of options exercised during the years ended December 31, 2020 and 2019 was $18.4 million and less than $0.1 million, respectively. Share-based compensation expense for the years ended December 31, 2020 and 2019 was $0.3 million and $0.1 million, respectively. As of December 31, 2020, there was $0.4 million of unrecognized compensation cost related to share-based payments, which is expected to be recognized over the remaining vesting periods, with a weighted-average period of 2.6 years. 2020 Equity Incentive Plan On October 1, 2020, the Company’s shareholders approved a new long-term incentive award plan (the “2020 Plan”) in connection with the Business Combination. The 2020 Plan is administered by the Board and the compensation committee. The selection of participants, allotment of shares, determination of price and other conditions are approved by the Board and the compensation committee at its sole discretion in order to attract and retain personnel instrumental to the success of the Company. Under the 2020 Plan, the Company may grant an aggregate of 12,937,713 shares of common stock in the form of nonstatutory stock options, incentive stock options, SARs, restricted stock awards, performance awards, and other awards. No grants have been authorized to date by the Company’s Board and the compensation committee under the 2020 Plan.

Net Loss Per Share

Net Loss Per Share	3 Months Ended	12 Months Ended
	Mar. 31, 2021	Dec. 31, 2020
Earnings Per Share [Abstract]
Net Loss Per Share	9. Net Loss Per Share The following table sets forth the computation of basic and diluted net loss per share of common stock for the three months ended March 31, 2021 and 2020: Three Months Ended 2021 2020 (Dollar amounts in thousands, except for shares and per share data) Numerator: Net loss attributable to common stockholders $ (16,562 ) $ (5,562 ) Denominator: Weighted average shares outstanding, basic and diluted 170,249,708 86,762,463 Net loss per share, basic and diluted $ (0.10 ) $ (0.06 ) The Company excluded the following weighted average potential common shares from the computation of diluted net loss per share for the three months ended March 31, 2021 and 2020 because including them would have had an anti-dilutive effect: Three Months Ended 2021 2020 Stock options, including incentive stock options and non-qualified 4,360,010 4,559,583 Unvested restricted stock units 3,174,341 — Total 7,534,351 4,559,583	Note 16. Net Income (Loss) Per Share – As Restated As a result of the reverse recapitalization (see Note 4), the Company has retroactively adjusted the weighted average shares outstanding prior to October 1, 2020 to give effect to the Exchange Ratio used to determine the number of shares of common stock into which they were converted. The following table sets forth the computation of basic and diluted net income (loss) per share of common stock for the years ended December 31, 2020, and 2019: Years Ended December 31, 2020 as restated 2019 (in thousands, except share and per share data) Numerator: Net income (loss) attributable to common stockholders - basic $ 324,117 $ (14,113 ) Less: gain from change in fair value of warrant liabilities (363,299 ) - Net loss attributable to common shareholders – diluted $ (39,182 ) $ (14,113 ) Denominator: Weighted average shares outstanding, basic 104,324,059 86,643,714 Weighted average shares outstanding, diluted 112,570,960 86,643,714 Net income (loss) per share, basic $ 3.11 $ (0.16 ) Net loss per share, diluted $ (0.35 ) $ (0.16 ) The Company included the following weighted average potential common shares in the computation of diluted net income per share for the years ended December 31, 2020, but not for the years ended December 31, 2019 because including them would have had an anti-dilutive effect: Years Ended December 31, 2020 as restated 2019 Stock options, including incentive stock options and non-qualified 6,326,479 3,772,368 Common shares issuable from the exercise of warrants 1,920,426 - Total 8,246,905 3,772,368

Supplemental Cash Flow Informat

Supplemental Cash Flow Information	3 Months Ended	12 Months Ended
	Mar. 31, 2021	Dec. 31, 2020
Supplemental Cash Flow Elements [Abstract]
Supplemental Cash Flow Information	10. Supplemental Cash Flow Information The following table provides supplemental cash flow information for the three months ended March 31, 2021 and 2020: Three months ended 2021 2020 (in thousands) Cash paid for interest $ — $ 306 Cash paid for amounts included in the measurement of lease liabilities: Operating cash flows from operating leases $ (321 ) $ (404 ) Operating cash flows from finance leases $ (1 ) $ (9 )	Note 17. Supplemental Cash Flow Information The following table provides supplemental cash flow information for the years ended December 31, 2020 and 2019: Years Ended December 31, 2020 2019 (in thousands) Cash paid for interest $ (144 ) $ (53 ) Cash paid for taxes $ - $ - Cash paid for amounts included in the measurement of lease liabilities: Operating cash flows from operating leases $ (1,446 ) $ (1,255 ) Operating cash flows from finance leases $ (29 ) $ (50 ) Right-of-use assets obtained in exchange for lease obligations $ 1,007 $ 21 The following table provides supplemental disclosures of noncash financing activities for the year ended December 31, 2020 and 2019: Years Ended December 31, 2020 2019 (in thousands) Warrants exercised where proceeds are included within prepaid expenses and other current assets $ 11,978 $ - Settlement of convertible notes payable and convertible note payable derivative liabilities $ 44,039 $ - Redemption of unexercised warrants included within prepaid expenses and other current assets $ (3 ) $ -

Restatement of Previously Issue

Restatement of Previously Issued Financial Statements	12 Months Ended
	Dec. 31, 2020
Accounting Standards Update and Change in Accounting Principle [Abstract]
Restatement of Previously Issued Financial Statements	Note 2. Restatement of Previously Issued Financial Statements On April 12, 2021, the Acting Director of the Division of Corporation Finance and Acting Chief Accountant of the Securities and Exchange Commission together issued a statement regarding the accounting and reporting considerations for warrants issued by special purpose acquisition companies entitled “Staff Statement on Accounting and Reporting Considerations for Warrants Issued by Special Purpose Acquisition Companies (“SPACs”)” (the “SEC Statement”). Specifically, the SEC Statement focused in part on provisions in warrant agreements that provide for potential changes to the settlement amounts dependent upon the characteristics of the warrant holder and because the holder of a warrant is not an input into the pricing of a fixed-for-fixed option on equity shares, such provision would preclude the warrant from being classified in equity and thus the warrant should be classified as a liability. As a result of the SEC Statement, the Company reevaluated the accounting treatment of the Warrants issued in connection with the IPO of TortoiseCorp and recorded in equity on the Company’s consolidated balance sheet as a result of the merger and reverse recapitalization occurring on October 1, 2020. The Company concluded that the Warrants should have been recorded at fair value as a liability in the Company’s consolidated balance sheet. All public and private warrants were exercised by December 31, 2020, so the warrant liability on the Company’s consolidated balance sheet recorded on the date of the acquisition has been extinguished, and the change in the fair value of the liability as of the exercise date was recognized as a gain in the Company’s consolidated statement of operations. While all warrants were exercised as of December 31, 2020, 371,535 warrants which were exercised on December 30, 2020 were broker protected, resulting in cash collection and share issuance being delayed until January 4, 2021. Accounts receivable for the net amount due from investors of $4.3 million and an associated liability for these common shares to be issued has been recognized at the balance sheet date and included below. Immaterial Error Correction: The restatement and immaterial error correction adjustments reflect the entries to record the initial warrant liability from the Warrants, to revalue the warrant liability to the then fair value as of the exercise date, the subsequent extinguishment of the liability, and the accounts receivable and associated liability arising from the broker protected warrants exercised but not settled. The following presents a reconciliation of the consolidated balance sheet as previously reported to the restated amounts as of December 31, 2020 (in thousands): For the Year Ended December 31, 2020 As Reported Restatement Impact As Restated Consolidated Statement of Operations: Change in fair value of warrant liabilities $ — $ 363,299 $ 363,299 Net income (loss) $ (39,182 ) $ 363,299 $ 324,117 Earnings (loss) per share: Basic $ (0.38 ) $ 3.49 $ 3.11 Earnings (loss) per share, diluted $ 2.93 $ (3.28 ) $ (0.35 ) Weighted-average shares outstanding, diluted 110,696,489 1,874,471 112,570,960 As of December 31, 2020 As Reported Restatement Impact As Restated Consolidated Balance Sheets: Prepaid expenses and other current assets $ 16,408 $ 4,282 $ 20,690 Total current assets $ 608,086 $ 4,282 $ 612,368 Total assets $ 650,807 $ 4,282 $ 655,089 Warrant liabilities $ — $ — $ — Accrued expenses and other current liabilities $ 1,982 $ 4,282 $ 6,264 Total current liabilities $ 4,655 $ 4,282 $ 8,937 Total liabilities $ 10,639 $ 4,282 $ 14,921 Common Stock $ 17 $ 2 $ 19 Additional paid-in-capital $ 728,299 $ (363,301 ) $ 364,998 Accumulated earnings $ (88,148 ) $ 363,299 $ 275,151 Total equity (deficit) $ 640,168 $ — $ 640,168 As of December 31, 2020 As Reported Restatement Impact As Restated Consolidated Statement of Stockholders’ Equity (Deficit): Common Stock Par Value $ 17 $ 2 $ 19 Additional paid-in-capital $ 728,299 $ (363,301 ) $ 364,998 Accumulated earnings $ (88,148 ) $ 363,299 $ 275,151 For the Year Ended December 31, 2020 As Reported Restatement Impact As Restated Consolidated Statement of Cash Flow: Net income (loss) $ (39,182 ) $ 363,299 $ 324,117 Change in fair value of warrant liability $ — $ (363,299 ) $ (363,299 )

Reverse Recapitalization

Reverse Recapitalization	12 Months Ended
	Dec. 31, 2020
Reverse Recapitalization [Abstract]
Reverse Recapitalization	Note 4. Reverse Recapitalization On October 1, 2020, Legacy Hyliion and TortoiseCorp consummated the merger contemplated by the Business Combination, with Legacy Hyliion surviving the merger as a wholly-owned subsidiary of TortoiseCorp. Upon the closing of the Business Combination, TortoiseCorp’s certificate of incorporation was amended and restated to, among other things, increase the total number of authorized shares of capital stock to 260,000,000 shares, of which 250,000,000 shares were designated common stock, $.0001 par value per share, and of which 10,000,000 shares were designated preferred stock, $0.0001 par value per share. Immediately prior to the closing of the Business Combination, each ● issued and outstanding share of Legacy Hyliion’s redeemable, convertible preferred stock, was converted into shares Legacy Hyliion common stock based on a one-to-one ratio (see Note 8). The Business Combination is accounted for with a retrospective application of the Business Combination that results in 34,799,813 shares of redeemable, convertible preferred stock converting into the same number of shares of Legacy Hyliion common stock. ● convertible note payable, plus accrued paid-in-kind interest, was converted into an aggregate 2,336,235 shares of Legacy Hyliion common stock at the predetermined discount (see Note 4). Upon the consummation of the Business Combination, each share of Legacy Hyliion common stock issued and outstanding was cancelled and converted into the right to receive 1.45720232 shares (the “Exchange Ratio”) of the Company’s common stock (the “Per Share Merger Consideration”). Additionally, Legacy Hyliion issued 1,000,000 shares of Legacy Hyliion common stock with an estimated grant date fair value of $10.00 per share to one of the convertible noteholders in connection with the commercial matters agreement (“Commercial Matters Agreement”) that was entered into in June 2020, that was not subject to the Exchange Ratio (see Note 15). Outstanding stock options, whether vested or unvested, to purchase shares of Legacy Hyliion common stock granted under the 2016 Plan (“Legacy Options”) (see Note 9) converted into stock options for shares of the Company’s common stock upon the same terms and conditions that were in effect with respect to such stock options immediately prior to the Business Combination, after giving effect to the Exchange Ratio. Outstanding warrants to purchase shares of TortoiseCorp Class A common stock will remain outstanding at the Closing Date. The warrants will become exercisable 30 days after the completion of the Business Combination and will expire five years after the completion of the Business Combination or earlier upon redemption or liquidation. On November 30, 2020, the Company issued a notice of redemption to the warrant holders and on December 31, 2020, it redeemed all outstanding public warrants. See Note 8 “Capital Structure” for more information. In connection with the Business Combination, ● certain TortoiseCorp shareholders exercised their right to redeem certain of their outstanding shares for cash, resulting in the redemption of 3,308 shares of TortoiseCorp common stock for gross redemption payments of less than $0.1 million. ● a number of investors purchased from the Company an aggregate of 30,750,000 shares of common stock (the “PIPE Shares”), for a purchase price of $10.00 per share and an aggregate purchase price of $307.5 million pursuant to separate subscription agreements entered into effective June 18, 2020 (the “PIPE”). The PIPE investment closed simultaneously with the consummation of the Business Combination. ● an investor purchased 1,750,000 TortoiseCorp units (consisting of one share of common stock and one half of one warrant, the “Forward Purchase Units”), consisting of 1,750,000 shares of common stock (“Forward Purchase Shares”) and warrants to purchase 875,000 shares of common stock (“Forward Purchase Warrants”) for an aggregate purchase price of $17.5 million pursuant to a forward purchase agreement entered into effective February 6, 2019, as amended by the First Amendment to Amended and Restated Forward Purchase Agreement, dated June 18, 2020. The Business Combination is accounted for as a reverse recapitalization in accordance with U.S. GAAP. Under this method of accounting, TortoiseCorp was treated as the “acquired” company for financial reporting purposes. See Note 1 “Description of business and basis of presentation” for further details. Accordingly, for accounting purposes, the Business Combination was treated as the equivalent of Legacy Hyliion issuing stock for the net assets of TortoiseCorp, accompanied by a recapitalization. The net assets of TortoiseCorp are stated at historical cost, with no goodwill or intangible assets recorded. Prior to the Business Combination, Legacy Hyliion and TortoiseCorp filed separate standalone federal, state and local income tax returns. As a result of the Business Combination Legacy Hyliion will file a consolidated income tax return. Although, for legal purposes, TortoiseCorp acquired Legacy Hyliion, and the transaction represents a reverse acquisition for federal income tax purposes. TortoiseCorp will be the parent of the consolidated group with Legacy Hyliion a subsidiary, but in the year of the closing of the Business Combination, Legacy Hyliion will file a full year tax return with TortoiseCorp joining in the return the day after the Closing Date. The following table reconciles the elements of the Business Combination to the consolidated statements of cash flows and the consolidated statement of changes in stockholders’ equity (deficit) for the year ended December 31, 2020 (in thousands): Cash - TortoiseCorp’s trust and cash (net of redemption) $ 236,484 Cash - PIPE 307,500 Cash - forward purchase units 17,500 Less: transaction costs and advisory fees paid (45,030 ) Net Business Combination and PIPE financing $ 516,454 The number of shares of common stock issued immediately following the consummation of the Business Combination were: Common stock, outstanding prior to Business Combination 23,300,917 Less: redemption of TortoiseCorp shares (3,308 ) Common stock of TortoiseCorp 23,297,609 TortoiseCorp founder shares 5,825,230 Shares issued in PIPE 30,750,000 Shares issued in connection with forward purchase agreement 1,750,000 Business Combination, PIPE, and forward purchase agreement financing shares 61,622,839 Legacy Hyliion shares (1) 92,278,990 Total shares of common stock immediately after Business Combination 153,901,829 Hyliion Holdings Corp. exercise of warrants 15,414,592 Total shares of common stock at December 31, 2020 169,316,421 (1) The number of Legacy Hyliion shares was determined as follows: Legacy Hyliion Legacy Hyliion Balance at December 31, 2018 24,453,750 35,634,061 Recapitalization applied to Series A outstanding at December 31, 2018 34,799,813 50,710,369 Exercise of common stock options - 2019 286,874 418,033 Exercise of common stock options - 2020 (pre-Closing) 763,216 1,112,160 Conversion of convertible notes payable to common stock (2) 2,336,235 4,404,367 92,278,990 (2) The number of shares issued for the conversion of convertible notes payable to common stock is calculated by applying the Exchange Ratio to the Legacy Hyliion shares issued at the time of conversion and adding 1,000,000 shares issued in connection with the Commercial Matters Agreement. All fractions were rounded down. Lock-Up Arrangements Certain former stockholders of Legacy Hyliion and TortoiseCorp have agreed to lock-up restrictions regarding the future transfer shares of common stock. Such shares may not be transferred or otherwise disposed of for a period of six months through April 1, 2021, subject to certain exceptions. Transaction costs: Transaction costs incurred in connection with the Business Combination totaled approximately $45.0 million which were charged to additional paid-in capital for the year ended December 31, 2020.

Capital Structure

Capital Structure	12 Months Ended
	Dec. 31, 2020
Partners' Capital Notes [Abstract]
Capital Structure	Note 8. Capital Structure As discussed in Note 1 and Note 4, on October 1, 2020, the Company consummated the Business Combination, which has been accounted for as a reverse recapitalization. Pursuant to the Certificate of Incorporation as amended on October 1, 2020 and as a result of the reverse recapitalization, the Company has retrospectively adjusted the Legacy Hyliion preferred shares and Legacy Hyliion common shares issued and outstanding prior to October 1, 2020 to give effect to the Exchange Ratio used to determine the number of shares of common stock of the combined entity into which they were converted. Preferred Stock Common Stock The following shares of common stock are reserved for future issuance: Stock options issued and outstanding 6,982,497 Authorized for future grant under 2020 Equity Incentive Plan 12,937,713 19,920,210 Warrants Public Warrants Private Placement Warrants Forward Purchase Warrants Because the Company’s Warrants contain provisions whereby the settlement amount varies depending upon the characteristics of the warrant holder, all warrants were determined to have liability classification at issuance, and as such, were recorded at fair value as a warrant liability in the Company’s consolidated balance sheet at the date of the merger. On November 30, 2020, the Company issued a notice of redemption of all its outstanding Public Warrants and Forward Purchase Warrants which was completed in December 2020. However, the Private Warrants held by the initial holders thereof or permitted transferees of the initial holders were not subject to this redemption. As of December 31, 2020, all outstanding Public Warrants and Forward Purchase Warrants were either exercised or redeemed by the holder. As of December 31, 2020, the Company’s transfer agent received gross proceeds of $140.8 million corresponding to the exercise of 15,786,127 warrants. However, due to the timing of the receipt of the warrant exercise and the cash, the Company’s transfer agent issued 15,414,592 shares of common stock as of December 31, 2020. The remaining 371,535 shares of common stock were issued in January 2021. Additionally, as of December 31, 2020, the Company’s transfer agent had not yet remitted $12.0 million of the gross proceeds associated with the shares of issued common stock to the Company and is included within prepaid expenses and other current assets on the accompanying consolidated balance sheets as of December 31, 2020. There were 281,065 warrants not exercised by the end of the redemption period that were redeemed for a price of $0.01 per warrant, and subsequently cancelled by the Company. The Company made the redemption payment on these cancelled warrants in January 2021. Certain holders of the warrants elected a cashless exercise, resulting in the forfeiture of 3,118,445 shares.

Leases

Leases	12 Months Ended
	Dec. 31, 2020
Leases [Abstract]
Leases	Note 10. Leases The Company has operating and finance leases for its corporate office, temporary office, vehicles and equipment. In addition, the Company enters into arrangements whereby portions of the leased premises are subleased to third parties and are classified as operating leases. The following table provides a summary of the components of lease income, costs and rent, which are included within research and development and selling, general and administrative on the accompanying consolidated statements of operations: Years Ended December 31, 2020 2019 (in thousands) Operating lease costs: Operating lease cost $ 1,389 $ 1,908 Short-term lease cost 42 4 Variable lease cost (14 ) (140 ) Sublessor income (326 ) (421 ) Total operating lease costs $ 1,091 $ 1,351 Finance lease costs: Amortization of right-of-use assets $ 112 $ 112 Interest on lease liabilities 21 50 Total finance lease costs $ 133 $ 162 Finance lease ROU assets were $0.3 million and $0.7 million as of December 31, 2020 and 2019 and accumulated amortization was $0.1 million and $0.2 million as of December 31, 2020 and 2019, respectively. The following table provides the weighted-average lease terms and discount rates used for the Company’s operating and finance leases: December 31, Weighted-average remaining lease term (in years): Operating leases 5.0 Finance leases 0.3 Weighted-average discount rate: Operating leases 9.9 % Finance leases 14.2 % The following table provides a summary of lease liability maturities for the next five years and thereafter: Operating Finance Leases Leases (in thousands) 2021 $ 1,269 $ 49 2022 1,441 - 2023 1,484 - 2024 1,529 - 2025 1,575 - Thereafter 133 - Total lease payments 7,431 49 Less: Imputed interest (1,621 ) - Total lease obligations $ 5,810 $ 49

Property and Equipment, Net

Property and Equipment, Net	12 Months Ended
	Dec. 31, 2020
Property, Plant and Equipment [Abstract]
Property and Equipment, net	Note 11. Property and Equipment, net Property and equipment, net consisted of the following at December 31, 2020 and 2019: December 31, 2020 2019 (in thousands) Production machinery and equipment $ 1,751 $ 1,751 Vehicles 712 727 Leasehold improvements 749 670 Demo fleet systems 263 263 Office furniture and fixtures 64 28 Computers and related equipment 195 24 3,734 3,463 Less accumulated depreciation (2,563 ) (1,828 ) Property and equipment, net $ 1,171 $ 1,635 Depreciation expense for the years ended December 31, 2020 and 2019 totaled approximately $0.8 million and $0.9 million, respectively. For the year ended December 31, 2020, less than $0.1 million and $0.7 million is included within selling, general and administrative expenses and research and development expenses on the accompanying consolidated statements of operations, respectively. For the year ended December 31, 2019, $0.1 million and $0.8 million is included within selling, general and administrative expenses and research and development expenses on the accompanying consolidated statements of operations, respectively.

Intangible Assets, Net

Intangible Assets, Net	12 Months Ended
	Dec. 31, 2020
Goodwill and Intangible Assets Disclosure [Abstract]
Intangible assets, net	Note 12. Intangible assets, net The gross carrying amount and accumulated amortization of separately identifiable intangible assets at December 31, 2020 and 2019 are as follows: December 31, 2020 Intangible Asset Useful Life Weighted Average Remaining Life Gross Carrying Value Accumulated Amortization Net (in thousands) Developed technology 6 years 3.4 years $ 578 $ (247 ) $ 331 Non-compete 3 years 0.4 years 5 (4 ) 1 $ 583 $ (251 ) $ 332 December 31, 2019 Intangible Asset Gross Carrying Value Accumulated Amortization Net (in thousands) Developed technology $ 578 $ (151 ) $ 427 Non-compete 5 (3 ) 2 $ 583 $ (154 ) $ 429 Total amortization expense was $0.1 million for each of the years ended December 31, 2020 and 2019 and is included within selling, general and administrative expenses on the accompanying consolidated statements of operations. Total future amortization expense for the finite-lived intangible assets is estimated as follows (in thousands): 2021 $ 97 2022 97 2023 97 2024 41 $ 332

Accrued Expenses and Other Curr

Accrued Expenses and Other Current Liabilities – As Restated	12 Months Ended
	Dec. 31, 2020
Accrued Liabilities and Other Liabilities [Abstract]
Accrued Expenses and Other Current Liabilities – As Restated	Note 13. Accrued Expenses and Other Current Liabilities – As Restated Accrued expenses and other current liabilities consisted of the following at December 31, 2020 and 2019: December 31, 2020 as restated 2019 (in thousands) Accrued professional services $ 1,032 $ 120 Accrued compensation and related benefits 615 - Refundable grant 175 175 Accrued liability for warrants exercised but not settled 4,282 - Other accrued liabilities 160 205 $ 6,264 $ 500 The accrued liability totaling $4.3 million for warrants exercised but not settled represents all warrants that were exercised as of December 31, 2020 under broker protects resulting in cash collection and share issuance being delayed until January 4, 2021.

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2020
Income Tax Disclosure [Abstract]
Income Taxes	Note 14. Income Taxes – As Restated The income tax provision consists of the following: Years Ended December 31, 2020 2019 (in thousands) Current tax expense (benefit): Federal $ - $ - State - - Total current tax expense $ - $ - Deferred tax expense (benefit): Federal $ (8,952 ) $ (2,788 ) State (291 ) - Valuation allowance 9,243 2,788 Total deferred tax expense (benefit) $ - $ - The components of deferred taxes as of December 31, 2020 and 2019 are as follows: Years Ended December 31, 2020 2019 (in thousands) Deferred tax assets: Federal net operating loss carryforwards $ 17,265 $ 9,083 State net operating loss carryforwards 984 825 Operating lease obligation 1,009 1,209 R&D tax credit 481 - Other 224 - Property and equipment, net 29 - Total deferred tax assets 19,992 11,117 Deferred tax liabilities: Operating lease right of use asset, net 854 1,045 Intangible assets, net 70 90 Property and equipment, net - 18 Other - 139 Total deferred tax liabilities 924 1,292 Total net deferred tax assets (liabilities) 19,068 9,825 Less valuation allowance (19,068 ) (9,825 ) Net deferred tax assets (liabilities) $ - $ - The reconciliation of taxes at the federal statutory rate to the Company’s provision for income taxes for the years ended December 31, 2020, and 2019 was as follows: Years Ended December 31, 2020 as restated 2019 (in thousands) Provision at statutory rate of 21% $ 68,069 $ (2,964 ) Non-deductible convertible debt interest expense 227 152 Non-deductible gain related to warrant conversions (76,293 ) State tax expense (158 ) Stock options 54 15 Transaction costs (2,947 ) - Shares issued in connection with Commercial Matters Agreement (see Notes 4, 5, and 15) 2,100 - Other (102 ) 9 R&D tax credit (193 ) - Change in valuation allowance 9,243 2,788 $ - $ - The net change in the total valuation allowance for the year ended December 31, 2020, was an increase of $9.2 million, (compared to an increase of $2.8 million in 2019). In assessing the realizability of deferred tax assets, management considered whether it is more likely than not that some portion or all of the deferred tax assets will not be realized. The ultimate realization of deferred tax assets is dependent upon the generation of future taxable income during the periods in which those temporary differences become deductible. Management considered the scheduled reversal of deferred tax liabilities, projected future taxable income and tax planning strategies in making this assessment. Based upon the level of historical taxable income and projections for future taxable income over the periods in which the deferred tax assets are deductible, management believes it is more likely than not that the Company will not realize the benefits of these deductible differences at December 31, 2020. The Company has federal net operating loss carryforwards of approximately $82.2 million and $43.3 million at December 31, 2020 and 2019, respectively. $10.5 million of this amount will begin to expire in 2036. The remaining $71.7 million has an indefinite carryforward period. The Company also has state net operating loss carryforwards of approximately $12.5 million and $10.5 million at December 31, 2020 and 2019. They will expire beginning in 2036. The Company also has R&D credits of $0.3 million that begin to expire in 2037. The Company’s ability to utilize a portion of its net operating loss carryforwards and credits to offset future taxable income, and tax, respectively, is subject to certain limitations under section 382 of the Internal Revenue Code upon changes in equity ownership of the Company. Due to such limitation, $2.0 million of the Company’s net operating loss and less than $0.1 million of the Company’s R&D credits will expire unused, regardless of taxable income in future years. The Company files a United States federal income tax return, as well as income tax returns in various states. The tax returns for years 2016 and thereafter remain open for examination.

Retirement Plan

Retirement Plan	12 Months Ended
	Dec. 31, 2020
Retirement Benefits [Abstract]
Retirement Plan	Note 18. Retirement Plan The Company has adopted a 401(k) plan to provide all eligible employees a means to accumulate retirement savings on a tax-advantaged basis. The 401(k) plan requires participants to be at least 20 years old. Plan participants may make before tax elective contributions up to the maximum percentage of compensation and dollar amount allowed under the Internal Revenue Code and are always 100% vested in their elective contributions. The Company makes discretionary employer contributions at its election. Plan participants must be employed on the last day of the year to be eligible for the employer match. Participants may defer specified portions of their compensation. The Company did not provide a match of the employee’s contribution for the years ended December 31, 2020 and 2019.

Accounting Policies, by Policy

Accounting Policies, by Policy (Policies)	3 Months Ended	12 Months Ended
	Mar. 31, 2021	Dec. 31, 2020
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation: These condensed consolidated financial statements include the accounts of the Company and its wholly owned subsidiary. Intercompany accounts and transactions have been eliminated in consolidation. These interim consolidated statements have been prepared pursuant to the rules and regulations of the SEC, which permit reduced disclosure for interim periods. The Consolidated Balance Sheet as of December 31, 2020 was derived from audited financial statements for the fiscal year then ended, but does not include all necessary disclosures required by accounting principles generally accepted in the United States of America (“GAAP”) with respect to annual financial statements. In the opinion of management, these unaudited condensed consolidated financial statements include all recurring adjustments and normal accruals necessary for a fair presentation of the Company’s financial position, results of operations and cash flows for the dates and periods presented. These condensed consolidated financial statements and accompanying notes should be read in conjunction with the Company’s 2020 Amended Annual Report. Results for interim periods are not necessarily indicative of the results to be expected for a full fiscal year or for any future period.
Emerging Growth Company	Emerging Growth Company:	Emerging Growth Company
Use of estimates and uncertainty of the coronavirus pandemic	Use of estimates and uncertainty of the coronavirus pandemic: On January 30, 2020, the World Health Organization declared the coronavirus outbreak a “Public Health Emergency of International Concern” and on March 11, 2020, declared the coronavirus outbreak a pandemic. In mid-March 2020, U.S. state governors, local officials and leaders outside of the U.S. began ordering various “shelter-in-place” orders, which have had various impacts on the U.S. and global economies. This has required greater use of estimates and assumptions in the preparation of the unaudited condensed consolidated financial statements. As the coronavirus pandemic continues to evolve, the Company believes the extent of the impact to its businesses, operating results, cash flows, liquidity and financial condition will be primarily driven by the severity and duration of the coronavirus pandemic, the pandemic’s impact on the U.S. and global economies and the timing, scope, and effectiveness of federal, state and local governmental responses to the pandemic. Those primary drivers are beyond the Company’s knowledge and control, and as a result, at this time the Company is unable to predict the cumulative impact, both in terms of severity and duration, that the coronavirus pandemic will have on its business, operating results, cash flows and financial condition, but it could be material if the current circumstances continue to exist for a prolonged period. Although the Company has made its best estimates based upon current information, actual results could materially differ from the estimates and assumptions developed by management. If so, the Company may be subject to future impairment charges as well as changes to recorded reserves and valuations.	Use of estimates and uncertainty of the coronavirus pandemic On January 30, 2020, the World Health Organization declared the coronavirus outbreak a “Public Health Emergency of International Concern” and on March 11, 2020, declared the coronavirus outbreak a pandemic. In mid-March 2020, U.S. State Governors, local officials and leaders outside of the U.S. began ordering various “shelter-in-place” orders, which have had various impacts on the U.S. and global economies. This has required greater use of estimates and assumptions in the preparation of the unaudited consolidated financial statements. As the coronavirus pandemic continues to evolve, the Company believes the extent of the impact to its businesses, operating results, cash flows, liquidity and financial condition will be primarily driven by the severity and duration of the coronavirus pandemic, the pandemic’s impact on the U.S. and global economies and the timing, scope and effectiveness of federal, state and local governmental responses to the pandemic. Those primary drivers are beyond the Company’s knowledge and control, and as a result, at this time the Company is unable to predict the cumulative impact, both in terms of severity and duration, that the coronavirus pandemic will have on its business, operating results, cash flows and financial condition, but it could be material if the current circumstances continue to exist for a prolonged period. Although the Company has made its best estimates based upon current information, actual results could materially differ from the estimates and assumptions developed by management. If so, the Company may be subject to future impairment charges as well as changes to recorded reserves and valuations.
Recent accounting pronouncements issued, not yet adopted	Recently Adopted Accounting Pronouncements: In December 2019, the FASB issued ASU 2019-12, Income Taxes (Topic 740): Simplifying the Accounting for Income Taxes, which is intended to simplify various aspects related to accounting for income taxes. The pronouncement is effective for fiscal years, and for interim periods within those fiscal years, beginning after December 15, 2020. The Company adopted ASU 2019-12 on January 1, 2021. However, there is no impact to the Company as a result of the adoption in the current quarter, nor is there an expected impact to the Company for the remainder of the year.	Recent accounting pronouncements issued, not yet adopted: In June 2016, the FASB issued ASU 2016-13, Financial Instruments—Credit Losses (Topic 326): Measurement of Credit Losses of Financial Instruments In December 2019, the FASB issued ASU 2019-12, Income Taxes (Topic 740): Simplifying the Accounting for Income Taxes
Segment information		Segment information: Segment Reporting
Concentration of supplier risk		Concentration of supplier risk:
Cash and cash equivalents		Cash and cash equivalents: The Company maintains cash in excess of federally insured limits at financial institutions. The Company makes such deposits with entities it believes are of high credit quality and has not incurred any losses related to these balances to date. Management believes its credit risk, with respect to the financial institutions to be minimal.
Accounts receivable		Accounts receivable:
Investments		Investments: The Company uses the specific identification method to determine the cost basis of securities sold. Investments are impaired when a decline in fair value is judged to be other-than-temporary. The Company evaluates an investment for impairment by considering the length of time and extent to which market value has been less than cost or amortized cost, the financial condition and near-term prospects of the issuer as well as specific events or circumstances that may influence the operations of the issuer and the Company’s intent to sell the security or the likelihood that it will be required to sell the security before recovery of the entire amortized cost. Once a decline in fair value is determined to be other-than-temporary, an impairment charge is recorded to other income (expense) and a new costs basis in the investment is established.
Fair value measurements		Fair value measurements: Fair Value Measurements Level I Level II Level III An asset’s or liability’s fair value measurement level within the fair value hierarchy is based on the lowest level of any input that is significant to the fair value measurement. Valuation techniques used need to maximize the use of observable inputs and minimize the use of unobservable inputs. Assets and liabilities measured at fair value are based on one or more of the following three valuation techniques noted in ASC 820: ● Market approach: Prices and other relevant information generated by market transactions involving identical or comparable assets or liabilities. ● Cost approach: Amount that would be required to replace the service capacity of an asset (replacement cost). ● Income approach: Techniques to convert future amounts to a single present value amount based upon market expectations (including present value techniques, option pricing and excess earnings models) The Company believes its valuation methods are appropriate and consistent with other market participants, however the use of different methodologies or assumptions to determine the fair value of certain financial instruments could result in a different fair value measurement at the reporting date. The Company’s financial instruments consist of cash and cash equivalents, accounts receivable, investments, accounts payable, accrued expenses, contingent consideration liability, convertible notes payable derivative liability, warrant liabilities, and convertible notes payable. The carrying value of cash and cash equivalents, accounts receivable, accounts payable, and accrued expenses approximates fair value because of the short-term nature of those instruments. We estimate the fair value of our convertible notes payable using Level II and Level III inputs by discounting the future cash flows using current interest rates at which we could obtain similar borrowings in consideration of the estimated enterprise value of the Company. The fair value of corporate bonds, treasury securities and commercial paper are based on quoted prices for identical or similar instruments in markets that are not active. As a result, corporate bonds, treasury securities and commercial paper are classified within Level II of the fair value hierarchy. The fair value of the Company’s public warrant liabilities are based on Level I inputs, while the fair value of the private warrants is determined using the trading price of the public warrants, a Level II input. The Company’s assets and liabilities that are measured at fair value on a recurring basis include the Company’s contingent consideration liability, warrant liabilities, and convertible notes payable derivative liabilities (See Note 5).
Prepaid expenses and other current assets		Prepaid expenses and other current assets:
Property and equipment, net		Property and equipment, net: Production machinery and equipment 2 to 7 years Vehicles 3 to 7 years Leasehold improvements shorter of lease term or 7 years Demo fleet systems 2 to 3 years Furniture and fixtures 3 years Computers and related equipment 3 to 7 years Major renewals and improvements are capitalized, while replacements, maintenance and repairs, which do not improve or extend the lives of the respective assets, are expensed as incurred. When property and equipment is retired or otherwise disposed of, the related cost and accumulated depreciation are removed from the accounts, and any gain or loss on the disposition is recorded in the consolidated statement of operations as a component of other (expense) income.
Intangible assets, net		Intangible assets, net:
Impairment of long-lived assets		Impairment of long-lived assets Impairment or Disposal of Long-Lived Assets
Revenue		Revenue: Revenue from Contracts with Customers ● Step 1: Identify the contract(s) with a customer ● Step 2: Identify the performance obligations in the contract ● Step 3: Determine the transaction price ● Step 4: Allocate the transaction price to the performance obligations in the contract ● Step 5: Recognize revenue when (or as) a performance obligation is satisfied The Company intends to generate revenue from the sale of its hybrid and electrified drive systems for the long haul “Class 8” semi-tractors. However, since the Company is still in the pre-commercialization stage, it has not generated revenue from the sale of the products. The Company did not enter into any agreement that meets the definition of a contract with a customer that would be accounted for under ASC 606 through December 31, 2020.
Leases		Leases: Lessee ROU assets represent the Company’s right to use underlying assets for the lease term, and lease liabilities represent the Company’s obligation to make lease payments arising from the leases. ROU assets and lease liabilities are recognized at the commencement date based on the present value of lease payments over the lease term. The discount rate used to calculate the present value for lease payments is the Company’s incremental borrowing rate, which is determined based on information available at lease commencement and is equal to the rate of interest that the Company would have to pay to borrow on a collateralized basis over a similar term in an amount equal to the lease payments in a similar economic environment. The Company uses the implicit rate when readily determinable. The Company has entered into operating leases for corporate offices having initial lease terms of one to eight years. The Company has entered into finance leases primarily for vehicles and equipment, having initial terms of three years. The Company’s real estate leases may include one or more options to renew, with the renewal extending the lease term for an additional one to five years. The exercise of lease renewal option is at the Company’s sole discretion. In general, the Company does not consider renewal option to be reasonably likely to be exercised, therefore renewal option are generally not recognized as part of the ROU assets and lease liabilities. Lease costs for lease payments are recognized on a straight-line basis over the lease term, unless there is a transfer of title or purchase option reasonably certain to be exercised. The Company does not record operating leases with an initial term of twelve months or less (“short-term leases”) in the consolidated balance sheets. The Company’s vehicle and equipment leases may include transfer rights or options to purchase at the end of the lease that the Company is reasonably certain to exercise. Interest expense is recognized using the effective interest rate method, and the ROU asset is amortized over the useful life of the underlying asset. Lessor The Company has entered into various trial and evaluation agreements that contain an operating lease component that is within the scope of ASC 842, Leases The trial and evaluation agreements contain only variable payments not based on an index or rate as a result of refund provisions within those contracts. The Company records accounts receivable when the Company meets the criteria within the trial and evaluation agreements to invoice the lessee. In accordance with ASC 842, the Company recognizes variable lease payments as profit or loss in the period in which the changes in facts and circumstances on which the variable lease payments are based occur, which will generally be the end of the trial period when the customer refund rights lapse. During the years ended December 31, 2020 and 2019, the Company has not recognized any lease income related to these trial and evaluation agreements either because the Company has not received any consideration from the lease contracts, or the uncertainty related to the consideration received has not been resolved. Certain of the Company’s lessee and lessor lease agreements contain both lease and non-lease components, which are generally accounted for as a single lease component. Additionally, for certain vehicle leases, we apply a portfolio approach to effectively account for the finance lease ROU assets and liabilities.
Income taxes		Income taxes: Income Taxes Due to the Company’s history of losses since inception, the net deferred tax assets have been fully offset by a valuation allowance as of December 31, 2020 and 2019. Uncertain tax positions taken or expected to be taken in a tax return are accounted for using the more likely than not threshold for financial statement recognition and measurement. For the years ended December 31, 2020 and 2019, there were no uncertain tax positions taken or expected to be taken in the Company’s tax returns.
Share-based compensation		Share-based compensation: Compensation – Stock Compensation The Company utilizes the Black-Scholes model to determine the fair value of the stock option awards, which requires the input of subjective assumptions. These assumptions include estimating (a) the length of time grantees will retain their vested stock options before exercising them for employees and the contractual term of the option for nonemployees (“expected term”), (b) the volatility of the Company’s common stock price over the expected term, (c) expected dividends, and (d) the fair value of a share of common stock prior to the Business Combination. After the closing of the Business Combination, the Company’s board of directors determined the fair value of each share of common stock underlying stock-based awards based on the closing price of the Company’s common stock as reported by the NYSE on the date of grant. The Company has elected to recognize the adjustment to share-based compensation expense in the period in which forfeitures occur. The assumptions used in the Black-Scholes model are management’s best estimates, but the estimates involve inherent uncertainties and the application of management judgment (see Note 9). As a result, if other assumptions had been used, the recorded share-based compensation expense could have been materially different from that depicted in the financial statements.
Research and development expense		Research and development expense:
Net income (loss) per share		Net income (loss) per share:

Investments (Tables)

Investments (Tables)	3 Months Ended	12 Months Ended
	Mar. 31, 2021	Dec. 31, 2020
Investments, Debt and Equity Securities [Abstract]
Schedule of held-to-maturity investments	Fair Value Measurements as of Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value (in thousands) Held-to-maturity investments Commercial paper $ 116,658 $ — $ (24 ) $ 116,634 State and municipal bonds 15,805 — (21 ) 15,784 Corporate bonds and notes 164,847 — (644 ) 164,203 Total held-to-maturity investments $ 297,310 $ — $ (689 ) $ 296,621 Fair Value Measurements as of Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value (in thousands) Held-to-maturity investments Treasury securities $ 149,996 $ — $ (1 ) $ 149,995 Commercial paper 37,963 — (15 ) 37,948 Corporate bonds and notes 49,892 — (63 ) 49,829 Total held-to-maturity investments $ 237,851 $ — $ (79 ) $ 237,772	Fair Value Measurements as of Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value (in thousands) Held-to-maturity investments Treasury securities $ 149,996 $ - $ (1 ) $ 149,995 Commercial paper 37,963 - (15 ) 37,948 Corporate bonds and notes 49,892 - (63 ) 49,829 Total held-to-maturity investments $ 237,851 $ - $ (79 ) $ 237,772
Schedule of investment maturity		March 31, December 31, Amortized Cost Fair Value Amortized Cost Fair Value (in thousands) (in thousands) Due in one year or less $ 144,829 $ 144,788 $ 201,881 $ 201,864 Due after one year through five years 152,481 151,833 35,970 35,908 Total held-to-maturity securities $ 297,310 $ 296,621 $ 237,851 $ 237,772
Schedule of investment maturity		December 31, 2020 Amortized Cost Fair Value (in thousands) Due in one year or less $ 201,881 $ 201,864 Due after one year through five years 35,970 35,908 Total held-to-maturity securities $ 237,851 $ 237,772

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	3 Months Ended	12 Months Ended
	Mar. 31, 2021	Dec. 31, 2020
Fair Value Disclosures [Abstract]
Schedule of fair value measurements	Fair Value Measurements as of March 31, 2021 Level I Level II Level III Total (in thousands) Assets Cash and cash equivalents $ 334,718 $ — $ — $ 334,718 Held-to-maturity investments: Commercial paper — 116,634 — 116,634 State and municipal bonds — 15,784 — 15,784 Corporate bonds and notes — 164,203 — 164,203 Total Assets $ 334,718 $ 296,621 $ — $ 631,339 Fair Value Measurements as of December 31, 2020 Level I Level II Level III Total (in thousands) Assets Cash and cash equivalents $ 389,705 $ — $ — $ 389,705 Held-to-maturity investments: Treasury securities — 149,995 — 149,995 Commercial paper — 37,948 — 37,948 Corporate bonds and notes — 49,829 — 49,829 Total Assets $ 389,705 $ 237,772 $ — $ 627,477	Fair Value Measurements as of December 31, 2020 Level I Level II Level III Total Assets (in thousands) Cash and cash equivalents $ 389,705 $ - $ - $ 389,705 Held-to-maturity investments: Treasury securities - 149,995 - 149,995 Commercial paper - 37,948 - 37,948 Corporate bonds and notes - 49,829 - 49,829 Total Assets $ 389,705 $ 237,772 $ - $ 627,477 Fair Value Measurements as of December 31, 2019 Level I Level II Level III Total Liabilities (in thousands) Convertible notes payable derivative liabilities $ - $ - $ 8,351 $ 8,351 Total Liabilities $ - $ - $ 8,351 $ 8,351
Schedule of fair value of the convertible debt derivative liabilities		Input October 1, 2020 Issuance of January 2020 Note Issuance of December 2019 Note and December 31, 2019 Issuances of Initial 2019 Notes Issuances of Initial 2019 Notes Issuances of Initial 2019 Notes Probability-weighted conversion discount 2.5 - 50.0% 50.0% 23.9 - 50.0% 24.1% 24.4% 24.4% Remaining term (years) 0.0 - 4.3 5.0 0.7 - 4.5 5.0 2.0 2.0 Equity volatility NA NA 63.0 - 71.0% 74.0% 78.0% 75.0% Risk rate 1 19.6 - 57.7% 50.0% 27.2 - 50.0% 29.0% 26.6% 34.2% Probability of next financing event 1 100.0% 70.0% 70.0% 50.0% 50.0% 50.0% Timing of next financing event 1 10/1/2020 9/30/2020 9/30/2020 3/31/2020 3/31/2020 9/30/2019 Probability of default event 1 0.0% 30.0% 25.0 - 30.0% 50.0% 50.0% 50.0% Timing of default event 1 NA 9/30/2020 9/30/2020 3/31/2020 3/31/2020 9/30/2019 Probability of sale event 1 0.0% 0.0% 0.0 - 5.0% 0.0% 0.0% 0.0% Timing of sale event 1 NA NA 9/30/2020 NA NA NA Negotiation discount 1 2 0.0 - 0.1% 24.2% 21.7% 0.0% 0.0% 0.0% 1 Represents a Level III unobservable input 2 Based on the terms and provisions of the December 2019 and January 2020 Notes, the valuation model incorporated this additional assumption
Schedule of level 3 instruments		Balance, December 31, 2018 $ 2,068 Issuance of convertible notes payable derivative liabilities 7,428 Fair value adjustments (1,145 ) Balance, December 31, 2019 8,351 Issuance of convertible note payable derivative liability 2,656 Fair value adjustments 1,358 Settlement of convertible notes payable derivative liabilities (12,365 ) Balance, December 31, 2020 $ -

Net Loss Per Share (Tables)

Net Loss Per Share (Tables)	3 Months Ended	12 Months Ended
	Mar. 31, 2021	Dec. 31, 2020
Earnings Per Share [Abstract]
Schedule of basic and diluted net loss per share of common stock	Three Months Ended 2021 2020 (Dollar amounts in thousands, except for shares and per share data) Numerator: Net loss attributable to common stockholders $ (16,562 ) $ (5,562 ) Denominator: Weighted average shares outstanding, basic and diluted 170,249,708 86,762,463 Net loss per share, basic and diluted $ (0.10 ) $ (0.06 )	Years Ended December 31, 2020 as restated 2019 (in thousands, except share and per share data) Numerator: Net income (loss) attributable to common stockholders - basic $ 324,117 $ (14,113 ) Less: gain from change in fair value of warrant liabilities (363,299 ) - Net loss attributable to common shareholders – diluted $ (39,182 ) $ (14,113 ) Denominator: Weighted average shares outstanding, basic 104,324,059 86,643,714 Weighted average shares outstanding, diluted 112,570,960 86,643,714 Net income (loss) per share, basic $ 3.11 $ (0.16 ) Net loss per share, diluted $ (0.35 ) $ (0.16 )
Schedule of weighted aaverage potential common shares	Three Months Ended 2021 2020 Stock options, including incentive stock options and non-qualified 4,360,010 4,559,583 Unvested restricted stock units 3,174,341 — Total 7,534,351 4,559,583	Years Ended December 31, 2020 as restated 2019 Stock options, including incentive stock options and non-qualified 6,326,479 3,772,368 Common shares issuable from the exercise of warrants 1,920,426 - Total 8,246,905 3,772,368

Supplemental Cash Flow Inform_2

Supplemental Cash Flow Information (Tables)	3 Months Ended	12 Months Ended
	Mar. 31, 2021	Dec. 31, 2020
Supplemental Cash Flow Elements [Abstract]
Schedule of Cash Flow, Supplemental Disclosures [Table Text Block]	Three months ended 2021 2020 (in thousands) Cash paid for interest $ — $ 306 Cash paid for amounts included in the measurement of lease liabilities: Operating cash flows from operating leases $ (321 ) $ (404 ) Operating cash flows from finance leases $ (1 ) $ (9 )	Years Ended December 31, 2020 2019 (in thousands) Cash paid for interest $ (144 ) $ (53 ) Cash paid for taxes $ - $ - Cash paid for amounts included in the measurement of lease liabilities: Operating cash flows from operating leases $ (1,446 ) $ (1,255 ) Operating cash flows from finance leases $ (29 ) $ (50 ) Right-of-use assets obtained in exchange for lease obligations $ 1,007 $ 21
Schedule of provides supplemental disclosures of noncash financing activities		Years Ended December 31, 2020 2019 (in thousands) Warrants exercised where proceeds are included within prepaid expenses and other current assets $ 11,978 $ - Settlement of convertible notes payable and convertible note payable derivative liabilities $ 44,039 $ - Redemption of unexercised warrants included within prepaid expenses and other current assets $ (3 ) $ -

Restatement of Previously Iss_2

Restatement of Previously Issued Financial Statements (Tables)	12 Months Ended
	Dec. 31, 2020
Accounting Standards Update and Change in Accounting Principle [Abstract]
Schedule of Error Corrections and Prior Period Adjustments [Table Text Block]	For the Year Ended December 31, 2020 As Reported Restatement Impact As Restated Consolidated Statement of Operations: Change in fair value of warrant liabilities $ — $ 363,299 $ 363,299 Net income (loss) $ (39,182 ) $ 363,299 $ 324,117 Earnings (loss) per share: Basic $ (0.38 ) $ 3.49 $ 3.11 Earnings (loss) per share, diluted $ 2.93 $ (3.28 ) $ (0.35 ) Weighted-average shares outstanding, diluted 110,696,489 1,874,471 112,570,960 As of December 31, 2020 As Reported Restatement Impact As Restated Consolidated Balance Sheets: Prepaid expenses and other current assets $ 16,408 $ 4,282 $ 20,690 Total current assets $ 608,086 $ 4,282 $ 612,368 Total assets $ 650,807 $ 4,282 $ 655,089 Warrant liabilities $ — $ — $ — Accrued expenses and other current liabilities $ 1,982 $ 4,282 $ 6,264 Total current liabilities $ 4,655 $ 4,282 $ 8,937 Total liabilities $ 10,639 $ 4,282 $ 14,921 Common Stock $ 17 $ 2 $ 19 Additional paid-in-capital $ 728,299 $ (363,301 ) $ 364,998 Accumulated earnings $ (88,148 ) $ 363,299 $ 275,151 Total equity (deficit) $ 640,168 $ — $ 640,168 As of December 31, 2020 As Reported Restatement Impact As Restated Consolidated Statement of Stockholders’ Equity (Deficit): Common Stock Par Value $ 17 $ 2 $ 19 Additional paid-in-capital $ 728,299 $ (363,301 ) $ 364,998 Accumulated earnings $ (88,148 ) $ 363,299 $ 275,151 For the Year Ended December 31, 2020 As Reported Restatement Impact As Restated Consolidated Statement of Cash Flow: Net income (loss) $ (39,182 ) $ 363,299 $ 324,117 Change in fair value of warrant liability $ — $ (363,299 ) $ (363,299 )

Summary of Significant Accounti

Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2020
Accounting Policies [Abstract]
Public Utility Property, Plant, and Equipment [Table Text Block]	Production machinery and equipment 2 to 7 years Vehicles 3 to 7 years Leasehold improvements shorter of lease term or 7 years Demo fleet systems 2 to 3 years Furniture and fixtures 3 years Computers and related equipment 3 to 7 years

Reverse Recapitalization (Table

Reverse Recapitalization (Tables)	12 Months Ended
	Dec. 31, 2020
Reverse Recapitalization [Abstract]
Business Combination, Separately Recognized Transactions [Table Text Block]	Cash - TortoiseCorp’s trust and cash (net of redemption) $ 236,484 Cash - PIPE 307,500 Cash - forward purchase units 17,500 Less: transaction costs and advisory fees paid (45,030 ) Net Business Combination and PIPE financing $ 516,454
Business Combination, Segment Allocation [Table Text Block]	Common stock, outstanding prior to Business Combination 23,300,917 Less: redemption of TortoiseCorp shares (3,308 ) Common stock of TortoiseCorp 23,297,609 TortoiseCorp founder shares 5,825,230 Shares issued in PIPE 30,750,000 Shares issued in connection with forward purchase agreement 1,750,000 Business Combination, PIPE, and forward purchase agreement financing shares 61,622,839 Legacy Hyliion shares (1) 92,278,990 Total shares of common stock immediately after Business Combination 153,901,829 Hyliion Holdings Corp. exercise of warrants 15,414,592 Total shares of common stock at December 31, 2020 169,316,421
Schedule of Legacy Hyliion shares	Legacy Hyliion Legacy Hyliion Balance at December 31, 2018 24,453,750 35,634,061 Recapitalization applied to Series A outstanding at December 31, 2018 34,799,813 50,710,369 Exercise of common stock options - 2019 286,874 418,033 Exercise of common stock options - 2020 (pre-Closing) 763,216 1,112,160 Conversion of convertible notes payable to common stock (2) 2,336,235 4,404,367 92,278,990

Debt (Tables)

Debt (Tables)	12 Months Ended
	Dec. 31, 2020
Debt Disclosure [Abstract]
Schedule of carrying value of debt	December 31, 2020 2019 (in thousands) Convertible notes payable, net of unamortized discount at December 31, 2020 and 2019 of $0 and $6,451, respectively $ - $ 16,113 Paycheck Protection Program loan 908 - Finance lease obligations 49 289 957 16,402 Less current portion 49 6,720 Debt, net of current portion $ 908 $ 9,682

Capital Structure (Tables)

Capital Structure (Tables)	12 Months Ended
	Dec. 31, 2020
Partners' Capital Notes [Abstract]
Schedule of common stock reserved	Stock options issued and outstanding 6,982,497 Authorized for future grant under 2020 Equity Incentive Plan 12,937,713 19,920,210

Share-based Compensation (Table

Share-based Compensation (Tables)	12 Months Ended
	Dec. 31, 2020
Share-based Payment Arrangement [Abstract]
Schedule of assumptions were used for options issued	Years Ended December 31, 2020 2019 Expected volatility 70.0% 70.0% Expected term (in years) 6.1 6.1 - 10 Risk-free interest rate 1.7% 1.4 - 3.0% Expected dividend yield 0.0% 0.0%
Schedule of share option activity	Options Shares Weighted Average Exercise Price Weighted Average Remaining Contractual Term Outstanding at December 31, 2018 5,508,031 $ 0.11 8.7 Granted 3,213,131 0.16 Exercised (418,033 ) 0.14 Cancelled or forfeited (1,715,847 ) 0.13 Outstanding at December 31, 2019 6,587,282 0.13 8.2 Granted 2,797,828 0.23 Exercised (1,112,960 ) 0.11 Cancelled or forfeited (1,289,653 ) 0.19 Outstanding at December 31, 2020 6,982,497 $ 0.16 7.8 Exercisable at December 31, 2019 2,482,987 $ 0.10 7.2 Exercisable at December 31, 2020 3,851,486 $ 0.13 7.1

Leases (Tables)

Leases (Tables)	12 Months Ended
	Dec. 31, 2020
Leases [Abstract]
Lease, Cost [Table Text Block]	Years Ended December 31, 2020 2019 (in thousands) Operating lease costs: Operating lease cost $ 1,389 $ 1,908 Short-term lease cost 42 4 Variable lease cost (14 ) (140 ) Sublessor income (326 ) (421 ) Total operating lease costs $ 1,091 $ 1,351 Finance lease costs: Amortization of right-of-use assets $ 112 $ 112 Interest on lease liabilities 21 50 Total finance lease costs $ 133 $ 162
Schedule of weighted-average lease terms and discount rates	December 31, Weighted-average remaining lease term (in years): Operating leases 5.0 Finance leases 0.3 Weighted-average discount rate: Operating leases 9.9 % Finance leases 14.2 %
Schedule of lease liability maturities for the next five years	Operating Finance Leases Leases (in thousands) 2021 $ 1,269 $ 49 2022 1,441 - 2023 1,484 - 2024 1,529 - 2025 1,575 - Thereafter 133 - Total lease payments 7,431 49 Less: Imputed interest (1,621 ) - Total lease obligations $ 5,810 $ 49

Property and Equipment, Net (Ta

Property and Equipment, Net (Tables)	12 Months Ended
	Dec. 31, 2020
Property, Plant and Equipment [Abstract]
Schedule of property and equipment, net	December 31, 2020 2019 (in thousands) Production machinery and equipment $ 1,751 $ 1,751 Vehicles 712 727 Leasehold improvements 749 670 Demo fleet systems 263 263 Office furniture and fixtures 64 28 Computers and related equipment 195 24 3,734 3,463 Less accumulated depreciation (2,563 ) (1,828 ) Property and equipment, net $ 1,171 $ 1,635

Intangible Assets, Net (Tables)

Intangible Assets, Net (Tables)	12 Months Ended
	Dec. 31, 2020
Goodwill and Intangible Assets Disclosure [Abstract]
Schedule of identifiable intangible assets	December 31, 2020 Intangible Asset Useful Life Weighted Average Remaining Life Gross Carrying Value Accumulated Amortization Net (in thousands) Developed technology 6 years 3.4 years $ 578 $ (247 ) $ 331 Non-compete 3 years 0.4 years 5 (4 ) 1 $ 583 $ (251 ) $ 332 December 31, 2019 Intangible Asset Gross Carrying Value Accumulated Amortization Net (in thousands) Developed technology $ 578 $ (151 ) $ 427 Non-compete 5 (3 ) 2 $ 583 $ (154 ) $ 429
Schedule of future amortization expenses	2021 $ 97 2022 97 2023 97 2024 41 $ 332

Accrued Expenses and Other Cu_2

Accrued Expenses and Other Current Liabilities – As Restated (Tables)	12 Months Ended
	Dec. 31, 2020
Accrued Liabilities and Other Liabilities [Abstract]
Schedule of accrued expenses and other current liabilities	December 31, 2020 as restated 2019 (in thousands) Accrued professional services $ 1,032 $ 120 Accrued compensation and related benefits 615 - Refundable grant 175 175 Accrued liability for warrants exercised but not settled 4,282 - Other accrued liabilities 160 205 $ 6,264 $ 500

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2020
Income Tax Disclosure [Abstract]
Schedule of provision for income taxes	Years Ended December 31, 2020 2019 (in thousands) Current tax expense (benefit): Federal $ - $ - State - - Total current tax expense $ - $ - Deferred tax expense (benefit): Federal $ (8,952 ) $ (2,788 ) State (291 ) - Valuation allowance 9,243 2,788 Total deferred tax expense (benefit) $ - $ -
Schedule of deferred taxes	Years Ended December 31, 2020 2019 (in thousands) Deferred tax assets: Federal net operating loss carryforwards $ 17,265 $ 9,083 State net operating loss carryforwards 984 825 Operating lease obligation 1,009 1,209 R&D tax credit 481 - Other 224 - Property and equipment, net 29 - Total deferred tax assets 19,992 11,117 Deferred tax liabilities: Operating lease right of use asset, net 854 1,045 Intangible assets, net 70 90 Property and equipment, net - 18 Other - 139 Total deferred tax liabilities 924 1,292 Total net deferred tax assets (liabilities) 19,068 9,825 Less valuation allowance (19,068 ) (9,825 ) Net deferred tax assets (liabilities) $ - $ -
Schedule of reconciliation of taxes at federal statutory rate to provision for income taxes	Years Ended December 31, 2020 as restated 2019 (in thousands) Provision at statutory rate of 21% $ 68,069 $ (2,964 ) Non-deductible convertible debt interest expense 227 152 Non-deductible gain related to warrant conversions (76,293 ) State tax expense (158 ) Stock options 54 15 Transaction costs (2,947 ) - Shares issued in connection with Commercial Matters Agreement (see Notes 4, 5, and 15) 2,100 - Other (102 ) 9 R&D tax credit (193 ) - Change in valuation allowance 9,243 2,788 $ - $ -

Description of Organization a_2

Description of Organization and Business Operations (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2020	Mar. 31, 2021	Oct. 01, 2020	Dec. 31, 2019
Accounting Policies [Abstract]
Cash and cash equivalents	$ 389.7	$ 334.7
Amount of investment	$ 237.9	$ 297.3
Common stock, par value (in Dollars per share)	$ 0.0001	$ 0.0001		$ 0.0001
Net of transaction costs and expenses			$ 516.5
Gross proceeds	$ 140.8
Proceeds to be collected	$ 16.3

Debt (Details)

Debt (Details) - USD ($) $ in Thousands	1 Months Ended			3 Months Ended		12 Months Ended
	Aug. 31, 2020	May 31, 2020	Jan. 31, 2020	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Feb. 29, 2020
Debt (Details) [Line Items]
Interest			4.00%
				$ 900
Convertible notes payable			$ 3,200
Equity financing			$ 15,000				$ 15,000
Divided							50.00%
Additional interest			3.00%			3.00%	3.00%	3.00%
Conversion features amount			$ 2,700			$ 6,000		$ 1,800
Convertible notes payable, description							The Initial 2019 Notes bear interest at 6% per annum and mature two to five years after their respective issuance dates. The Initial 2019 Notes are only prepayable with the consent of the holders. One of the Initial 2019 Notes (totaling $1.8 million) is secured by substantially all of the assets of the Company, subordinate to the first priority, senior secured interest held by a note holder of a convertible note issued in January 2020. The holder of this note has first priority secured interest in these assets.
Equity financing per share price			50.00%				50.00%
Subsequent equity financing			$ 15,000
Equity financing maturity date, description							the maturity date of the December 2019 Note will automatically extend by one year. In such situation, the holder also has the right to extend the maturity date for an additional two years beyond the modified maturity date.
Conversion price							$ 1,400
Debt, description			In connection with the reverse recapitalization discussed in Note 4, immediately prior to the closing of the Business Combination, the convertible notes, plus accrued paid-in-kind interest, totaling $26.8 million were converted into an aggregate of 2,336,235 shares of Legacy Hyliion common stock, which were then exchanged for an aggregate of 3,404,367 shares of the Company’s common stock on the Closing Date (see Note 4). In addition, the Company issued 1,000,000 shares of Legacy Hyliion common stock to a noteholder of the 2018 Note, Initial 2019 Notes, and January 2020 Note, with a grant date fair value of $10.00 per share in accordance with the Commercial Matters Agreement (see Note 15).
Commercial debt			$ 10,000
Loss on extinguishment			$ 10,200			$ (10,170)
Term loan, description	uring August 2020, the Company issued a term loan (the “Term Loan”) with a principal balance totaling $10.1 million that matured on the earlier of (i) December 15, 2020, (ii) the termination of the Business Combination or, (iii) the consummation of the Business Combination as provided in the Business Combination. In connection with the Term Loan, the Company paid $0.5 million of financing costs. The Term Loan bore interest at a rate equal to 6.5% plus the greater of (a) the Federal Funds rate plus 0.5%, (b) LIBOR Rate for a one-month interest period plus 1.0%, and (c) Prime Rate in effect on such day. While outstanding in 2020, the Term Loan bore interest at 8.5% per annum. The Term Loan plus accrued interest was repaid in full in October 2020.
Payroll protection program loan, description		During May 2020, the Company received loan proceeds in the amount of $0.9 million under the Payroll Protection Program (the “PPP”). The PPP was established as part of Coronavirus Aid, Relief, and Economic Security Act and provides for loans to qualifying businesses for amounts up to 2.5 times the average monthly payroll expenses of the business, subject to certain limitations. The loans and accrued interest are forgivable after eight weeks so long as the borrower uses the loan proceeds for eligible purposes, including payroll, benefits, rent and utilities, and so long as the borrower maintains its pre-funding employment and wage levels. Although the Company used the PPP loan proceeds for purposes consistent with the provisions of the PPP and that such usage met the criteria established for forgiveness of the loan, the Company intends to repay the PPP loan plus accrued interest. The PPP loan matures in May 2022.
Convertible Debt [Member]
Debt (Details) [Line Items]
Cash proceeds from debt				$ 3,200
Interest				6.00%
Debt term				5 years
Optional conversion, equity financing (at least)				$ 15,000
Conversion price, discount from equity financing share price				50.00%
Optional conversion,subsequent equity financing (at least)				$ 15,000
Optional conversion, change in control, conversion price, purchase price multiplier				50.00%
Optional conversion, minimum amount of new commercial debt				$ 10,000
Event of default, additional interest rate				3.00%
Event of default, interest rate				9.00%
Debt term optional extension				5 years
Interest rate if not converted				4.00%
Fair value of conversion features				$ 2,700
2018 Note [Member]
Debt (Details) [Line Items]
Interest								6.00%
Convertible notes payable								$ 5,000
Equity financing								$ 5,000
Divided								65.00%
Additional interest								9.00%
2019 Note [Member]
Debt (Details) [Line Items]
Interest						4.00%	6.00%
Convertible notes payable									$ 13,600
Equity financing						$ 15,000	$ 35,000
Divided						75.00%
Additional interest						9.00%	9.00%
Debt total						$ 1,800
Equity financing per share price							50.00%
Subsequent equity financing							$ 35,000
December 2019 Note [Member]
Debt (Details) [Line Items]
Convertible notes payable							$ 3,200
January 2020 Note [Member]
Debt (Details) [Line Items]
Additional interest			9.00%
Debt, description			The January 2020 Note bears interest at 6% per annum and matures in January 2025 (five years subsequent to its issuance date).

Investments (Details)

Investments (Details) $ in Millions	3 Months Ended
	Mar. 31, 2021USD ($)
Investments, Debt and Equity Securities [Abstract]
Unrealized loss	$ 0.7

Investments (Details) - Schedul

Investments (Details) - Schedule of held-to-maturity investments - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2021	Dec. 31, 2020
Commercial Paper [Member]
Held-to-maturity investments
Amortized Cost	$ 116,658	$ 37,963
Gross Unrealized Gains
Gross Unrealized Losses	(24)	(15)
Fair Value	116,634	37,948
State and municipal bonds [Member]
Held-to-maturity investments
Amortized Cost	15,805
Gross Unrealized Gains
Gross Unrealized Losses	(21)
Fair Value	15,784
Corporate bonds and notes [Member]
Held-to-maturity investments
Amortized Cost	164,847
Gross Unrealized Gains
Gross Unrealized Losses	(644)
Fair Value	164,203
Treasury securities [Member]
Held-to-maturity investments
Amortized Cost		149,996
Gross Unrealized Gains
Gross Unrealized Losses		(1)
Fair Value		149,995
Corporate bonds and notes [Member]
Held-to-maturity investments
Amortized Cost		49,892
Gross Unrealized Gains
Gross Unrealized Losses		(63)
Fair Value		49,829
Total held-to-maturity investments [Member]
Held-to-maturity investments
Amortized Cost	297,310	237,851
Gross Unrealized Gains
Gross Unrealized Losses	(689)	(79)
Fair Value	$ 296,621	$ 237,772

Investments (Details) - Sched_2

Investments (Details) - Schedule of investment maturity - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Schedule of investment maturity [Abstract]
Due in one year or less	$ 144,829	$ 201,881
Due in one year or less	144,788	201,864
Due after one year through five years	152,481	35,970
Due after one year through five years	151,833	35,908
Total held-to-maturity securities	297,310	237,851
Total held-to-maturity securities	$ 296,621	$ 237,772

Fair Value Measurements (Detail

Fair Value Measurements (Details) - Schedule of fair value measurements - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Assets
Cash and cash equivalents	$ 334,718	$ 389,705
Held-to-maturity investments:
Held-to-maturity investments	296,621	237,772
Total Assets	631,339	627,477
Commercial paper [Member]
Held-to-maturity investments:
Held-to-maturity investments	116,634	37,948
State and municipal bonds [Member]
Held-to-maturity investments:
Held-to-maturity investments	15,784
Corporate bonds and notes [Member]
Held-to-maturity investments:
Held-to-maturity investments	164,203	49,829
Treasury securities [Member]
Held-to-maturity investments:
Held-to-maturity investments		149,995
Level I [Member]
Assets
Cash and cash equivalents	334,718	389,705
Held-to-maturity investments:
Total Assets	334,718	389,705
Level I [Member] | Commercial paper [Member]
Held-to-maturity investments:
Held-to-maturity investments
Level I [Member] | State and municipal bonds [Member]
Held-to-maturity investments:
Held-to-maturity investments
Level I [Member] | Corporate bonds and notes [Member]
Held-to-maturity investments:
Held-to-maturity investments
Level I [Member] | Treasury securities [Member]
Held-to-maturity investments:
Held-to-maturity investments
Level II [Member]
Assets
Cash and cash equivalents
Held-to-maturity investments:
Total Assets	296,621	237,772
Level II [Member] | Commercial paper [Member]
Held-to-maturity investments:
Held-to-maturity investments	116,634	37,948
Level II [Member] | State and municipal bonds [Member]
Held-to-maturity investments:
Held-to-maturity investments	15,784
Level II [Member] | Corporate bonds and notes [Member]
Held-to-maturity investments:
Held-to-maturity investments	164,203	49,829
Level II [Member] | Treasury securities [Member]
Held-to-maturity investments:
Held-to-maturity investments		149,995
Level III [Member]
Assets
Cash and cash equivalents
Held-to-maturity investments:
Total Assets
Level III [Member] | Commercial paper [Member]
Held-to-maturity investments:
Held-to-maturity investments
Level III [Member] | State and municipal bonds [Member]
Held-to-maturity investments:
Held-to-maturity investments
Level III [Member] | Corporate bonds and notes [Member]
Held-to-maturity investments:
Held-to-maturity investments
Level III [Member] | Treasury securities [Member]
Held-to-maturity investments:
Held-to-maturity investments

Commitments and Contingencies (

Commitments and Contingencies (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	3 Months Ended	12 Months Ended
	Jun. 30, 2020	Mar. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Commitments and Contingencies Disclosure [Abstract]
Received payments					$ 200
Grant funding received					$ 200
Issuance of common stock	$ 10,000	$ 287	$ 121	$ 7
Share of common stock issued (in Shares)	1,000,000
Common stock fair value per share (in Dollars per share)	$ 10

Warrants (Details)

Warrants (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	3 Months Ended	12 Months Ended
	Jan. 31, 2021	Mar. 31, 2021	Dec. 31, 2020
Warrants (Details) [Line Items]
Proceeds from exercise of stock warrants, net of issuance costs (in Dollars)		$ 16,257	$ 140,800
Warrants exercised			15,786,127
Common stock issued for warrants exercised	371,535		15,414,592
Gross Proceeds (in Dollars)		$ 12,000
Warrants forfeited		3,118,445
Accrued Liabilities (in Dollars)		$ 4,300	$ 4,282
Public Warrants [Member]
Warrants (Details) [Line Items]
Warrants issued and outstanding		281,065
Private Placement [Member]
Warrants (Details) [Line Items]
Redemption price of warrants Per Share (in Dollars per share)		$ 0.01

Share-based Compensation (Detai

Share-based Compensation (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020	Dec. 31, 2019
Share-based Compensation (Details) [Line Items]
Shares granted (in Shares)			2,797,828	3,213,131
Share-based compensation expense	$ 1.5
Options outstanding			$ 113.8
Options exercisable			62.8
Intrinsic value of options exercised			18.4	$ 0.1
Stock based compensation			0.3	$ 0.1
Unrecognized compensation cost related to share-based payments			$ 0.4
Weighted-average period			7 years 36 days	7 years 73 days
Restricted Stock Units (RSUs) [Member]
Share-based Compensation (Details) [Line Items]
Awards granted (in Shares)	3,174,341
Share-based compensation expense		$ 0.1
Share-based Payment Arrangement, Option [Member]
Share-based Compensation (Details) [Line Items]
Shares granted (in Shares)		1,920,000
Estimated grant date fair value of options granted	$ 0.4
Minimum [Member]
Share-based Compensation (Details) [Line Items]
Vesting Period			4 years
Minimum [Member] | Restricted Stock Units (RSUs) [Member]
Share-based Compensation (Details) [Line Items]
Vesting Period	3 years
Minimum [Member] | Share-based Payment Arrangement, Option [Member]
Share-based Compensation (Details) [Line Items]
Vesting Period	4 years
Maximum [Member]
Share-based Compensation (Details) [Line Items]
Vesting Period			10 years
Maximum [Member] | Restricted Stock Units (RSUs) [Member]
Share-based Compensation (Details) [Line Items]
Vesting Period	4 years
Maximum [Member] | Share-based Payment Arrangement, Option [Member]
Share-based Compensation (Details) [Line Items]
Vesting Period	10 years
2016 Equity Incentive Plan [Member]
Share-based Compensation (Details) [Line Items]
Weighted-average period			2 years 219 days
2020 Equity Incentive Plan [Member]
Share-based Compensation (Details) [Line Items]
Shares granted (in Shares)			12,937,713

Net Loss Per Share (Details) -

Net Loss Per Share (Details) - Schedule of basic and diluted net loss per share of common stock - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020	Dec. 31, 2019
Numerator:
Net loss attributable to common stockholders	$ (16,562)	$ (5,562)
Denominator:
Weighted average shares outstanding, basic and diluted	170,249,708	86,762,463	112,570,960	86,643,714
Net loss per share, basic and diluted	$ (0.10)	$ (0.06)

Net Loss Per Share (Details) _2

Net Loss Per Share (Details) - Schedule of Weighted Average Potential Common Shares - shares	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total	7,534,351	4,559,583
Stock options, including incentive stock options and non-qualified [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total	4,360,010	4,559,583
Unvested restricted stock units [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total	3,174,341

Supplemental Cash Flow Inform_3

Supplemental Cash Flow Information (Details) - Supplemental Cash Flow Information - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Supplemental Cash Flow Information [Abstract]
Cash paid for interest		$ 306
Cash paid for amounts included in the measurement of lease liabilities:
Operating cash flows from operating leases	(321)	(404)
Operating cash flows from finance leases	$ (1)	$ (9)

Restatement of Previously Iss_3

Restatement of Previously Issued Financial Statements (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Restatement of Previously Issued Financial Statements (Details) [Line Items]
Share of warrants exercised (in Shares)	371,535
Accounts receivable due from investors (in Dollars)	$ 4.3
Earnings (loss) per share, diluted	$ (0.35)	$ (0.16)
As Restated [Member]
Restatement of Previously Issued Financial Statements (Details) [Line Items]
Earnings (loss) per share, diluted	(0.35)
As Reported [Member]
Restatement of Previously Issued Financial Statements (Details) [Line Items]
Earnings (loss) per share, diluted	$ 2.93

Restatement of Previously Iss_4

Restatement of Previously Issued Financial Statements (Details) - Schedule of the restatement on each financial statement $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2020USD ($)$ / sharesshares
As Reported [Member]
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Change in fair value of warrant liabilities
Net income (loss)	(39,182)
Change in fair value of warrant liability
Earnings (loss) per share:
Basic (in Dollars per share) | $ / shares	$ (0.38)
Earnings (loss) per share, diluted (in Dollars per share) | $ / shares	$ 2.93
Weighted-average shares outstanding, diluted (in Shares) | shares	110,696,489
Prepaid expenses and other current assets	$ 16,408
Total current assets	608,086
Total assets	650,807
Warrant liabilities
Accrued expenses and other current liabilities	1,982
Total current liabilities	4,655
Total liabilities	10,639
Common Stock	17
Additional paid-in-capital	728,299
Accumulated earnings	(88,148)
Total equity (deficit)	640,168
Common Stock Par Value	17
Restatement Impact [Member]
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Change in fair value of warrant liabilities	363,299
Net income (loss)	363,299
Change in fair value of warrant liability	$ (363,299)
Earnings (loss) per share:
Basic (in Dollars per share) | $ / shares	$ 3.49
Earnings (loss) per share, diluted (in Dollars per share) | $ / shares	$ (3.28)
Weighted-average shares outstanding, diluted (in Shares) | shares	1,874,471
Prepaid expenses and other current assets	$ 4,282
Total current assets	4,282
Total assets	4,282
Warrant liabilities
Accrued expenses and other current liabilities	4,282
Total current liabilities	4,282
Total liabilities	4,282
Common Stock	2
Additional paid-in-capital	(363,301)
Accumulated earnings	363,299
Total equity (deficit)
Common Stock Par Value	2
As Restated [Member]
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Change in fair value of warrant liabilities	363,299
Net income (loss)	324,117
Change in fair value of warrant liability	$ (363,299)
Earnings (loss) per share:
Basic (in Dollars per share) | $ / shares	$ 3.11
Earnings (loss) per share, diluted (in Dollars per share) | $ / shares	$ (0.35)
Weighted-average shares outstanding, diluted (in Shares) | shares	112,570,960
Prepaid expenses and other current assets	$ 20,690
Total current assets	612,368
Total assets	655,089
Warrant liabilities
Accrued expenses and other current liabilities	6,264
Total current liabilities	8,937
Total liabilities	14,921
Common Stock	19
Additional paid-in-capital	364,998
Accumulated earnings	275,151
Total equity (deficit)	640,168
Common Stock Par Value	$ 19

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Details) - Schedule of property and equipment	12 Months Ended
	Dec. 31, 2020
Leasehold Improvements [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Estimated useful life	7 years
Furniture and Fixtures [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Estimated useful life	3 years
Minimum [Member] | Production Machinery and Equipment [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Estimated useful life	2 years
Minimum [Member] | Vehicles [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Estimated useful life	3 years
Minimum [Member] | Demo Fleet Systems [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Estimated useful life	2 years
Minimum [Member] | Computers and Related Equipment [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Estimated useful life	3 years
Maximum [Member] | Production Machinery and Equipment [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Estimated useful life	7 years
Maximum [Member] | Vehicles [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Estimated useful life	7 years
Maximum [Member] | Demo Fleet Systems [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Estimated useful life	3 years
Maximum [Member] | Computers and Related Equipment [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Estimated useful life	7 years

Reverse Recapitalization (Detai

Reverse Recapitalization (Details) - USD ($) $ / shares in Units, $ in Millions	Oct. 01, 2020	Mar. 31, 2021	Dec. 31, 2020	Jun. 30, 2020	Dec. 31, 2019
Reverse Recapitalization (Details) [Line Items]
Convertible redeemable preferred stock	34,799,813
Aggregate number of convertible shares	2,336,235
Cancellation of common stock exchange ratio, description	Upon the consummation of the Business Combination, each share of Legacy Hyliion common stock issued and outstanding was cancelled and converted into the right to receive 1.45720232 shares (the “Exchange Ratio”) of the Company’s common stock (the “Per Share Merger Consideration”).
Redemption of common stock	3,308
Gross redemption payments for common stock (in Dollars)	$ 0.1
Price per unit (in Dollars per share)				$ 10
Purchase agreement, description	●an investor purchased 1,750,000 TortoiseCorp units (consisting of one share of common stock and one half of one warrant, the “Forward Purchase Units”), consisting of 1,750,000 shares of common stock (“Forward Purchase Shares”) and warrants to purchase 875,000 shares of common stock (“Forward Purchase Warrants”) for an aggregate purchase price of $17.5 million pursuant to a forward purchase agreement entered into effective February 6, 2019, as amended by the First Amendment to Amended and Restated Forward Purchase Agreement, dated June 18, 2020.
Issuance of common shares		171,519,811	169,316,421		86,762,463
Business combination transaction cost (in Dollars)	$ 45
Business Acquisition [Member]
Reverse Recapitalization (Details) [Line Items]
Business combination, description	Upon the closing of the Business Combination, TortoiseCorp’s certificate of incorporation was amended and restated to, among other things, increase the total number of authorized shares of capital stock to 260,000,000 shares, of which 250,000,000 shares were designated common stock, $.0001 par value per share, and of which 10,000,000 shares were designated preferred stock, $0.0001 par value per share
Legacy Hyliion [Member]
Reverse Recapitalization (Details) [Line Items]
Shares issued	1,000,000
Fair value per share (in Dollars per share)	$ 10
Issuance of common shares	1,000,000
Warrant [Member]
Reverse Recapitalization (Details) [Line Items]
Business combination expiration period	5 years
PIPE Shares [Member]
Reverse Recapitalization (Details) [Line Items]
Aggregate number of common stock purchase	30,750,000
Price per unit (in Dollars per share)	$ 10
Gross proceeds (in Dollars)	$ 307.5

Reverse Recapitalization (Det_2

Reverse Recapitalization (Details) - Schedule of business combination to the consolidated statements of cash flows and the consolidated statement of changes in stockholders’ equity (deficit) $ in Thousands	12 Months Ended
	Dec. 31, 2020USD ($)
Schedule of business combination to the consolidated statements of cash flows and the consolidated statement of changes in stockholders’ equity (deficit) [Abstract]
Cash - TortoiseCorp’s trust and cash (net of redemption)	$ 236,484
Cash - PIPE	307,500
Cash - forward purchase units	17,500
Less: transaction costs and advisory fees paid	(45,030)
Net Business Combination and PIPE financing	$ 516,454

Reverse Recapitalization (Det_3

Reverse Recapitalization (Details) - Schedule of shares of common stock issued immediately following the consummation of the Business Combination	Dec. 31, 2020shares
Schedule of shares of common stock issued immediately following the consummation of the Business Combination [Abstract]
Common stock, outstanding prior to Business Combination	23,300,917
Less: redemption of TortoiseCorp shares	(3,308)
Common stock of TortoiseCorp	23,297,609
TortoiseCorp founder shares	5,825,230
Shares issued in PIPE	30,750,000
Shares issued in connection with forward purchase agreement	1,750,000
Business Combination, PIPE, and forward purchase agreement financing shares	61,622,839
Legacy Hyliion shares	92,278,990	[1]
Total shares of common stock immediately after Business Combination	153,901,829
Hyliion Holdings Corp. exercise of warrants	15,414,592
Total shares of common stock at December 31, 2020	169,316,421
[1]	The number of Legacy Hyliion shares was determined as follows:

Reverse Recapitalization (Det_4

Reverse Recapitalization (Details) - Schedule of Legacy Hyliion shares	Dec. 31, 2020shares
Legacy Hyliion shares [Member]
Reverse Recapitalization (Details) - Schedule of Legacy Hyliion shares [Line Items]
Balance at December 31, 2018	24,453,750
Recapitalization applied to Series A outstanding at December 31, 2018	34,799,813
Exercise of common stock options - 2019	286,874
Exercise of common stock options - 2020 (pre-Closing)	763,216
Conversion of convertible notes payable to common stock	2,336,235	[1]
Legacy Hyliion shares, effected for Exchange Ratio [Member]
Reverse Recapitalization (Details) - Schedule of Legacy Hyliion shares [Line Items]
Balance at December 31, 2018	35,634,061
Recapitalization applied to Series A outstanding at December 31, 2018	50,710,369
Exercise of common stock options - 2019	418,033
Exercise of common stock options - 2020 (pre-Closing)	1,112,160
Conversion of convertible notes payable to common stock	4,404,367	[1]
Legacy Hyliion shares	92,278,990
[1]	The number of shares issued for the conversion of convertible notes payable to common stock is calculated by applying the Exchange Ratio to the Legacy Hyliion shares issued at the time of conversion and adding 1,000,000 shares issued in connection with the Commercial Matters Agreement. All fractions were rounded down.

Debt (Details) - Schedule of ca

Debt (Details) - Schedule of carrying value of debt - Debt [Member] - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Debt (Details) - Schedule of carrying value of debt [Line Items]
Convertible notes payable, net of unamortized discount at December 31, 2020 and 2019 of $0 and $6,451, respectively		$ 16,113
Paycheck Protection Program loan	908
Finance lease obligations	49	289
Total notes payable	957	16,402
Less current portion	49	6,720
Debt, net of current portion	$ 908	$ 9,682

Debt (Details) - Schedule of _2

Debt (Details) - Schedule of carrying value of debt (Parentheticals) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Debt [Member]
Debt (Details) - Schedule of carrying value of debt (Parentheticals) [Line Items]
Unamortized discount	$ 0	$ 6,451

Investments (Details) - Sched_3

Investments (Details) - Schedule of amortized cost, unrealized gains and losses, and fair value - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2021	Dec. 31, 2020
Treasury securities [Member]
Held-to-maturity investments
Amortized Cost		$ 149,996
Unrealized Gains
Gross Unrealized Losses		(1)
Fair Value		149,995
Commercial Paper [Member]
Held-to-maturity investments
Amortized Cost	$ 116,658	37,963
Unrealized Gains
Gross Unrealized Losses	(24)	(15)
Fair Value	116,634	37,948
Total held-to-maturity investments [Member]
Held-to-maturity investments
Amortized Cost	297,310	237,851
Unrealized Gains
Gross Unrealized Losses	(689)	(79)
Fair Value	$ 296,621	237,772
Corporate bonds and notes [Member]
Held-to-maturity investments
Amortized Cost		49,892
Unrealized Gains
Gross Unrealized Losses		(63)
Fair Value		$ 49,829

Investments (Details) - Sched_4

Investments (Details) - Schedule of investment maturity - Total held-to-maturity securities [Member] $ in Thousands	Dec. 31, 2020USD ($)
Investments (Details) - Schedule of investment maturity [Line Items]
Amortized Cost, Due in one year or less	$ 201,881
Fair Value, Due in one year or less	201,864
Amortized Cost, Due after one year through five years	35,970
Fair Value, Due after one year through five years	35,908
Amortized Cost, Total held-to-maturity securities	237,851
Fair Value, Total held-to-maturity securities	$ 237,772

Fair Value Measurements (Deta_2

Fair Value Measurements (Details) - Schedule of fair value of the convertible debt derivative liabilities	12 Months Ended
	Dec. 31, 2020
October 1, 2020 [Member]
Fair Value Measurements (Details) - Schedule of fair value of the convertible debt derivative liabilities [Line Items]
Equity volatility
Probability of next financing event	100.00%	[1]
Timing of next financing event	Oct. 1, 2020	[1]
Probability of default event	0.00%	[1]
Timing of default event		[1]
Probability of sale event	0.00%	[1]
Timing of sale event		[1]
October 1, 2020 [Member] | Minimum [Member]
Fair Value Measurements (Details) - Schedule of fair value of the convertible debt derivative liabilities [Line Items]
Probability-weighted conversion discount	2.50%
Remaining term (years)	0 years
Risk rate	19.60%	[1]
Negotiation discount	0.00%	[1],[2]
October 1, 2020 [Member] | Maximum [Member]
Fair Value Measurements (Details) - Schedule of fair value of the convertible debt derivative liabilities [Line Items]
Probability-weighted conversion discount	50.00%
Remaining term (years)	4 years 109 days
Risk rate	57.70%	[1]
Negotiation discount	0.10%	[1],[2]
Issuance of January 2020 Note (January 2020) [Member]
Fair Value Measurements (Details) - Schedule of fair value of the convertible debt derivative liabilities [Line Items]
Probability-weighted conversion discount	50.00%
Remaining term (years)	5 years
Equity volatility
Risk rate	50.00%	[1]
Probability of next financing event	70.00%	[1]
Timing of next financing event	Sep. 30, 2020	[1]
Probability of default event	30.00%	[1]
Timing of default event	Sep. 30, 2020	[1]
Probability of sale event	0.00%	[1]
Timing of sale event		[1]
Negotiation discount	24.20%	[1],[2]
Issuance of December 2019 Note and December 31, 2019 [Member]
Fair Value Measurements (Details) - Schedule of fair value of the convertible debt derivative liabilities [Line Items]
Probability of next financing event	70.00%	[1]
Timing of next financing event	Sep. 30, 2020	[1]
Timing of default event	Sep. 30, 2020	[1]
Timing of sale event	Sep. 30, 2020	[1]
Negotiation discount	21.70%	[1],[2]
Issuance of December 2019 Note and December 31, 2019 [Member] | Minimum [Member]
Fair Value Measurements (Details) - Schedule of fair value of the convertible debt derivative liabilities [Line Items]
Probability-weighted conversion discount	23.90%
Remaining term (years)	255 days
Equity volatility	63.00%
Risk rate	27.20%	[1]
Probability of default event	25.00%	[1]
Probability of sale event	0.00%	[1]
Issuance of December 2019 Note and December 31, 2019 [Member] | Maximum [Member]
Fair Value Measurements (Details) - Schedule of fair value of the convertible debt derivative liabilities [Line Items]
Probability-weighted conversion discount	50.00%
Remaining term (years)	4 years 6 months
Equity volatility	71.00%
Risk rate	50.00%	[1]
Probability of default event	30.00%	[1]
Probability of sale event	5.00%	[1]
Issuances of Initial 2019 Notes (July 2019) [Member]
Fair Value Measurements (Details) - Schedule of fair value of the convertible debt derivative liabilities [Line Items]
Probability-weighted conversion discount	24.10%
Remaining term (years)	5 years
Equity volatility	74.00%
Risk rate	29.00%	[1]
Probability of next financing event	50.00%	[1]
Timing of next financing event	Mar. 31, 2020	[1]
Probability of default event	50.00%	[1]
Timing of default event	Mar. 31, 2020	[1]
Probability of sale event	0.00%	[1]
Timing of sale event		[1]
Negotiation discount	0.00%	[1],[2]
Issuances of Initial 2019 Notes (June 2019) [Member]
Fair Value Measurements (Details) - Schedule of fair value of the convertible debt derivative liabilities [Line Items]
Probability-weighted conversion discount	24.40%
Remaining term (years)	2 years
Equity volatility	78.00%
Risk rate	26.60%	[1]
Probability of next financing event	50.00%	[1]
Timing of next financing event	Mar. 31, 2020	[1]
Probability of default event	50.00%	[1]
Timing of default event	Mar. 31, 2020	[1]
Probability of sale event	0.00%	[1]
Timing of sale event		[1]
Negotiation discount	0.00%	[1],[2]
Issuances of Initial 2019 Notes (February 2019) [Member]
Fair Value Measurements (Details) - Schedule of fair value of the convertible debt derivative liabilities [Line Items]
Probability-weighted conversion discount	24.40%
Remaining term (years)	2 years
Equity volatility	75.00%
Risk rate	34.20%	[1]
Probability of next financing event	50.00%	[1]
Timing of next financing event	Sep. 30, 2019	[1]
Probability of default event	50.00%	[1]
Timing of default event	Sep. 30, 2019	[1]
Probability of sale event	0.00%	[1]
Timing of sale event		[1]
Negotiation discount	0.00%	[1],[2]
[1]	Represents a Level III unobservable input
[2]	Based on the terms and provisions of the December 2019 and January 2020 Notes, the valuation model incorporated this additional assumption

Fair Value Measurements (Deta_3

Fair Value Measurements (Details) - Schedule of assets and liabilities that are measured at fair value on a recurring basis - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020	Dec. 31, 2018
Fair Value Measurements (Details) - Schedule of assets and liabilities that are measured at fair value on a recurring basis [Line Items]
Cash and cash equivalents	$ 6,285	$ 334,718	$ 389,705	$ 6,026	$ 1,097
Held-to-maturity investments:
Total Assets	627,477
Convertible notes payable derivative liabilities	8,351				$ 2,068
Total Liabilities	8,351
Treasury securities [Member]
Held-to-maturity investments:
Held-to-maturity investments			149,995
Commercial paper [Member]
Held-to-maturity investments:
Held-to-maturity investments			37,948
Corporate bonds and notes [Member]
Held-to-maturity investments:
Held-to-maturity investments			49,829
Level I [Member]
Fair Value Measurements (Details) - Schedule of assets and liabilities that are measured at fair value on a recurring basis [Line Items]
Cash and cash equivalents			389,705
Held-to-maturity investments:
Total Assets	389,705
Convertible notes payable derivative liabilities
Total Liabilities
Level I [Member] | Treasury securities [Member]
Held-to-maturity investments:
Held-to-maturity investments
Level I [Member] | Commercial paper [Member]
Held-to-maturity investments:
Held-to-maturity investments
Level I [Member] | Corporate bonds and notes [Member]
Held-to-maturity investments:
Held-to-maturity investments
Level II [Member]
Fair Value Measurements (Details) - Schedule of assets and liabilities that are measured at fair value on a recurring basis [Line Items]
Cash and cash equivalents
Held-to-maturity investments:
Total Assets	237,772
Convertible notes payable derivative liabilities
Total Liabilities
Level II [Member] | Treasury securities [Member]
Held-to-maturity investments:
Held-to-maturity investments			149,995
Level II [Member] | Commercial paper [Member]
Held-to-maturity investments:
Held-to-maturity investments			37,948
Level II [Member] | Corporate bonds and notes [Member]
Held-to-maturity investments:
Held-to-maturity investments			49,829
Level III [Member]
Fair Value Measurements (Details) - Schedule of assets and liabilities that are measured at fair value on a recurring basis [Line Items]
Cash and cash equivalents
Held-to-maturity investments:
Total Assets
Convertible notes payable derivative liabilities	8,351
Total Liabilities	$ 8,351
Level III [Member] | Treasury securities [Member]
Held-to-maturity investments:
Held-to-maturity investments
Level III [Member] | Commercial paper [Member]
Held-to-maturity investments:
Held-to-maturity investments
Level III [Member] | Corporate bonds and notes [Member]
Held-to-maturity investments:
Held-to-maturity investments

Fair Value Measurements (Deta_4

Fair Value Measurements (Details) - Schedule of level 3 instruments - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Schedule of level 3 instruments [Abstract]
Beginning balance	$ 8,351	$ 2,068
Ending balance		8,351
Issuance of convertible note payable derivative liability	2,656	7,428
Fair value adjustments	1,358	$ (1,145)
Settlement of convertible notes payable derivative liabilities	$ (12,365)

Capital Structure (Details)

Capital Structure (Details) - $ / shares	Oct. 31, 2020	Mar. 04, 2019	Dec. 31, 2020	Mar. 31, 2021	Dec. 31, 2019
Capital Structure (Details) [Line Items]
Preferred stock, shares authorized			10,000,000
Preferred stock, par value (in Dollars per share)			$ 0.0001
Common stock, shares authorized			250,000,000	250,000,000	250,000,000
Common stock, par value (in Dollars per share)			$ 0.0001	$ 0.0001	$ 0.0001
Common stock, shares issued			169,316,421	171,519,811	86,762,463
Description of public warrants		TortoiseCorp completed an initial public offering that included warrants for shares of common stock (the “Public Warrants”). Each Public Warrant entitles the holder to the right to purchase one share of common stock at an exercise price of $11.50 per share. No fractional shares will be issued upon exercise of the Public Warrants. The Company may elect to redeem the Public Warrants, in whole and not in part, at a price of $0.01 per Public Warrant if (i) 30 days’ prior written notice of redemption is provided to the holders, and (ii) the last reported sale price of the Company’s common stock equals or exceeds $18.00 per share (as adjusted for stock splits, stock dividends, reorganizations, recapitalizations and the like) for any 20 trading days within a 30-trading day period ending on the third business day prior to the date on which the Company sends the notice of redemption to the warrant holders. Upon issuance of a redemption notice by the Company, the warrant holders have a period of 30 days to exercise for cash, or on a cashless basis. On the Closing Date, there were 11,650,458 Public Warrants issued and outstanding.
Warrants to purchase	875,000
Description of warrants			As of December 31, 2020, the Company’s transfer agent received gross proceeds of $140.8 million corresponding to the exercise of 15,786,127 warrants. However, due to the timing of the receipt of the warrant exercise and the cash, the Company’s transfer agent issued 15,414,592 shares of common stock as of December 31, 2020. The remaining 371,535 shares of common stock were issued in January 2021. Additionally, as of December 31, 2020, the Company’s transfer agent had not yet remitted $12.0 million of the gross proceeds associated with the shares of issued common stock to the Company and is included within prepaid expenses and other current assets on the accompanying consolidated balance sheets as of December 31, 2020. There were 281,065 warrants not exercised by the end of the redemption period that were redeemed for a price of $0.01 per warrant, and subsequently cancelled by the Company. The Company made the redemption payment on these cancelled warrants in January 2021. Certain holders of the warrants elected a cashless exercise, resulting in the forfeiture of 3,118,445 shares.
Private Placement Warrants [Member]
Capital Structure (Details) [Line Items]
Warrant purchase price (in Dollars per share)		$ 1
Warrants issued and outstanding		6,660,183

Capital Structure (Details) - S

Capital Structure (Details) - Schedule of common stock reserved	Dec. 31, 2020shares
Schedule of common stock reserved [Abstract]
Stock options issued and outstanding	6,982,497
Authorized for future grant under 2020 Equity Incentive Plan	12,937,713
Total	19,920,210

Share-based Compensation (Det_2

Share-based Compensation (Details) - Schedule of assumptions were used for options issued	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Share-based Compensation (Details) - Schedule of assumptions were used for options issued [Line Items]
Expected volatility	70.00%	70.00%
Expected term (in years)	6 years 36 days
Risk-free interest rate	1.70%
Expected dividend yield	0.00%	0.00%
Minimum [Member]
Share-based Compensation (Details) - Schedule of assumptions were used for options issued [Line Items]
Expected term (in years)		6 years 36 days
Risk-free interest rate		1.40%
Maximum [Member]
Share-based Compensation (Details) - Schedule of assumptions were used for options issued [Line Items]
Expected term (in years)		10 years
Risk-free interest rate		3.00%

Share-based Compensation (Det_3

Share-based Compensation (Details) - Schedule of share option activity - $ / shares	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Schedule of share option activity [Abstract]
Shares, Balance	6,587,282	5,508,031
Weighted Average Exercise Price, Balance	$ 0.13	$ 0.11
Weighted Average Remaining Contractual Term, Balance		8 years 255 days
Shares, Granted	2,797,828	3,213,131
Weighted Average Exercise Price, Granted	$ 0.23	$ 0.16
Shares, Exercised	(1,112,960)	(418,033)
Weighted Average Exercise Price, Exercised	$ 0.11	$ 0.14
Shares, Cancelled or forfeited	(1,289,653)	(1,715,847)
Weighted Average Exercise Price, Cancelled or forfeited	$ 0.19	$ 0.13
Shares, Balance	6,982,497	6,587,282
Weighted Average Exercise Price, Balance	$ 0.16	$ 0.13
Weighted Average Remaining Contractual Term, Balance	7 years 292 days	8 years 73 days
Shares, Exercisable	3,851,486	2,482,987
Weighted Average Exercise Price, Exercisable	$ 0.13	$ 0.10
Weighted Average Remaining Contractual Term, Exercisable	7 years 36 days	7 years 73 days

Leases (Details)

Leases (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Leases [Abstract]
Finance lease right-of-use assets	$ 0.3	$ 0.7
Accumulated amortization	$ 0.1	$ 0.2

Leases (Details) - Schedule of

Leases (Details) - Schedule of operating lease costs and finance lease costs - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Operating lease costs:
Operating lease cost	$ 1,389	$ 1,908
Short-term lease cost	42	4
Variable lease cost	(14)	(140)
Sublessor income	(326)	(421)
Total operating lease costs	1,091	1,351
Finance lease costs:
Amortization of right-of-use assets	112	112
Interest on lease liabilities	21	50
Total finance lease costs	$ 133	$ 162

Leases (Details) - Schedule o_2

Leases (Details) - Schedule of weighted-average lease terms and discount rates	Dec. 31, 2020
Weighted-average remaining lease term (in years):
Operating leases	5 years
Finance leases	109 days
Weighted-average discount rate:
Operating leases	9.90%
Finance leases	14.20%

Leases (Details) - Schedule o_3

Leases (Details) - Schedule of lease liability maturities for the next five years $ in Thousands	Dec. 31, 2020USD ($)
Schedule of lease liability maturities for the next five years [Abstract]
Operating leases, 2021	$ 1,269
Finance Leases, 2021	49
Operating leases, 2022	1,441
Finance Leases, 2022
Operating leases, 2023	1,484
Finance Leases, 2023
Operating leases, 2024	1,529
Finance Leases, 2024
Operating leases, 2025	1,575
Finance Leases, 2025
Operating leases, Thereafter	133
Finance Leases, Thereafter
Operating leases, Total lease payments	7,431
Finance Leases, Total lease payments	49
Operating leases, Less: Imputed interest	(1,621)
Finance Leases, Less: Imputed interest
Operating leases, Total lease obligations	5,810
Finance Leases, Total lease obligations	$ 49

Property and Equipment, Net (De

Property and Equipment, Net (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Property and Equipment, Net (Details) [Line Items]
Depreciation expense	$ 0.8	$ 0.9
Property, Plant and Equipment [Member]
Property and Equipment, Net (Details) [Line Items]
Selling, general and administrative expenses	0.1	0.1
Selling, general and administrative expenses	$ 0.7	$ 0.8

Property and Equipment, Net (_2

Property and Equipment, Net (Details) - Schedule of property and equipment, net - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Property, Plant and Equipment [Line Items]
Property and equipment, gross		$ 3,734	$ 3,463
Less accumulated depreciation		(2,563)	(1,828)
Property and equipment, net	$ 1,350	1,171	1,635
Production machinery and equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross		1,751	1,751
Vehicles [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross		712	727
Leasehold improvements [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross		749	670
Demo fleet systems [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross		263	263
Office furniture and fixtures [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross		64	28
Computers and related equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross		$ 195	$ 24

Intangible Assets, Net (Details

Intangible Assets, Net (Details) $ in Millions	12 Months Ended
	Dec. 31, 2020USD ($)
Goodwill and Intangible Assets Disclosure [Abstract]
Total amortization expense	$ 0.1

Intangible Assets, Net (Detai_2

Intangible Assets, Net (Details) - Schedule of identifiable intangible assets - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Mar. 31, 2021	Dec. 31, 2019
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Value	$ 583		$ 583
Accumulated Amortization	(251)		(154)
Net	$ 332	$ 308	429
Developed technology [Member]
Finite-Lived Intangible Assets [Line Items]
Useful Life	6 years
Weighted Average Remaining Life	3 years 146 days
Gross Carrying Value	$ 578		578
Accumulated Amortization	(247)		(151)
Net	$ 331		427
Non-compete [Member]
Finite-Lived Intangible Assets [Line Items]
Useful Life	3 years
Weighted Average Remaining Life	146 days
Gross Carrying Value	$ 5		5
Accumulated Amortization	(4)		(3)
Net	$ 1		$ 2

Intangible Assets, Net (Detai_3

Intangible Assets, Net (Details) - Schedule of future amortization expenses $ in Thousands	12 Months Ended
	Dec. 31, 2020USD ($)
Schedule of future amortization expenses [Abstract]
2021	$ 97
2022	97
2023	97
2024	41
Total	$ 332

Accrued Expenses and Other Cu_3

Accrued Expenses and Other Current Liabilities – As Restated (Details) $ in Millions	Dec. 31, 2020USD ($)
Disclosure Text Block Supplement [Abstract]
Accrued liability	$ 4.3

Accrued Expenses and Other Cu_4

Accrued Expenses and Other Current Liabilities – As Restated (Details) - Schedule of accrued expenses and other current liabilities - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Accrued Liabilities and Other Liabilities [Abstract]
Accrued professional services		$ 1,032	$ 120
Accrued compensation and related benefits		615
Refundable grant		175	175
Accrued liability for warrants exercised but not settled	$ 4,300	4,282
Other accrued liabilities		160	205
Total		$ 6,264	$ 500

Income Taxes (Details)

Income Taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Income Taxes (Details) [Line Items]
Net change in total valuation allowance	$ 9,243	$ 2,788
Carryforward outstanding and expiry, description	$10.5 million of this amount will begin to expire in 2036. The remaining $71.7 million has an indefinite carryforward period. The Company also has state net operating loss carryforwards of approximately $12.5 million and $10.5 million at December 31, 2020 and 2019. They will expire beginning in 2036. The Company also has R&D credits of $0.3 million that begin to expire in 2037. The Company’s ability to utilize a portion of its net operating loss carryforwards and credits to offset future taxable income, and tax, respectively, is subject to certain limitations under section 382 of the Internal Revenue Code upon changes in equity ownership of the Company. Due to such limitation, $2.0 million of the Company’s net operating loss and less than $0.1 million of the Company’s R&D credits will expire unused, regardless of taxable income in future years.
Federal [Member]
Income Taxes (Details) [Line Items]
Net operating loss carryforwards	$ 82,200	43,300
State and Local Jurisdiction [Member]
Income Taxes (Details) [Line Items]
Net operating loss carryforwards	$ 12,500	$ 10,500

Income Taxes (Details) - Schedu

Income Taxes (Details) - Schedule of provision for income taxes - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Current tax expense (benefit):
Federal
State
Total current tax expense
Deferred tax expense (benefit):
Federal	(8,952)	(2,788)
State	(291)
Valuation allowance	9,243	2,788
Total deferred tax expense (benefit)

Income Taxes (Details) - Sche_2

Income Taxes (Details) - Schedule of deferred taxes - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Deferred tax assets:
Federal net operating loss carryforwards	$ 17,265	$ 9,083
State net operating loss carryforwards	984	825
Operating lease obligation	1,009	1,209
R&D tax credit	481
Other	224
Property and equipment, net	29
Total deferred tax assets	19,992	11,117
Deferred tax liabilities:
Operating lease right of use asset, net	854	1,045
Intangible assets, net	70	90
Property and equipment, net		18
Other		139
Total deferred tax liabilities	924	1,292
Total net deferred tax assets (liabilities)	19,068	9,825
Less valuation allowance	(19,068)	(9,825)
Net deferred tax assets (liabilities)

Income Taxes (Details) - Sche_3

Income Taxes (Details) - Schedule of reconciliation of taxes at federal statutory rate to provision for income taxes - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Schedule of reconciliation of taxes at federal statutory rate to provision for income taxes [Abstract]
Provision at statutory rate of 21%	$ 68,069	$ (2,964)
Non-deductible convertible debt interest expense	227	152
Non-deductible gain related to warrant conversions	(76,293)
State tax expense	(158)
Stock options	54	15
Transaction costs	(2,947)
Shares issued in connection with Commercial Matters Agreement (see Notes 3, 4, and 14)	2,100
Other	(102)	9
R&D tax credit	(193)
Change in valuation allowance	9,243	2,788
Total

Net Income (Loss) Per Share (D

Net Income (Loss) Per Share (Details) - Schedule of basic and diluted net loss per share - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020	Dec. 31, 2019
Numerator:
Net income (loss) attributable to common stockholders - basic	$ (16,562)	$ (5,562)	$ 324,117	$ (14,113)
Less: gain from change in fair value of warrant liabilities			(363,299)
Net loss attributable to common shareholders – diluted			$ (39,182)	$ (14,113)
Denominator:
Weighted average shares outstanding, basic (in Shares)			104,324,059	86,643,714
Weighted average shares outstanding, diluted (in Shares)			112,570,960	86,643,714
Net income (loss) per share, basic (in Dollars per share)			$ 3.11	$ (0.16)
Net loss per share, diluted (in Dollars per share)			$ (0.35)	$ (0.16)

Net Income (Loss) Per Share _2

Net Income (Loss) Per Share (Details) - Schedule of weighted average potential common shares - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Schedule of Weighted Average Potential Common Shares [Abstract]
Stock options, including incentive stock options and non-qualified	6,326,479	3,772,368
Common shares issuable from the exercise of warrants	1,920,426
Total (in Dollars)	$ 8,246,905	$ 3,772,368

Supplemental Cash Flow Inform_4

Supplemental Cash Flow Information (Details) - Schedule of provides supplemental cash flow information - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Schedule of provides supplemental cash flow information [Abstract]
Cash paid for interest	$ (144)	$ (53)
Cash paid for taxes
Cash paid for amounts included in the measurement of lease liabilities:
Operating cash flows from operating leases	(1,446)	(1,255)
Operating cash flows from finance leases	(29)	(50)
Right-of-use assets obtained in exchange for lease obligations	$ 1,007	$ 21

Supplemental Cash Flow Inform_5

Supplemental Cash Flow Information (Details) - Schedule of provides supplemental disclosures of noncash financing activities - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Schedule of provides supplemental disclosures of noncash financing activities [Abstract]
Warrants exercised where proceeds are included within prepaid expenses and other current assets	$ 11,978
Settlement of convertible notes payable and convertible note payable derivative liabilities	44,039
Redemption of unexercised warrants included within prepaid expenses and other current assets	$ (3)

Retirement Plan (Details)

Retirement Plan (Details)	12 Months Ended
	Dec. 31, 2020
Retirement Benefits [Abstract]
Retirement plan, description	The Company has adopted a 401(k) plan to provide all eligible employees a means to accumulate retirement savings on a tax-advantaged basis. The 401(k) plan requires participants to be at least 20 years old. Plan participants may make before tax elective contributions up to the maximum percentage of compensation and dollar amount allowed under the Internal Revenue Code and are always 100% vested in their elective contributions.