ESS Tech, Inc.

ESS Tech, Inc. (“ESS” or the “Company”, f/k/a ACON S2 Acquisition Corp.) has determined that it is unable, without unreasonable effort or expense, to file its Quarterly Report on Form 10-Q for the fiscal quarter ended September 30, 2021 (the “Q3 2021 Form 10-Q”) by the prescribed due date for the reasons described below. As a result of recent guidance to Special Purpose Acquisition Companies by the U.S. Securities and Exchange Commission regarding redeemable equity instruments, the Company revisited its application of ASC 480-10-S99 on its financial statements. The Company had previously classified a portion of the Class A ordinary shares (the “Class A Ordinary Shares”) of ACON S2 Acquisition Corp, which consummated a business combination with the Company on October 8, 2021, in permanent equity. In consultation with the Company’s management and independent registered public accounting firm, the Company’s audit committee is currently evaluating the appropriate classification of all of the Class A Ordinary Shares in accordance with ASC 480-10-S99. The Company is working diligently to complete the Q3 2021 Form 10-Q as soon as possible; however, given the scope of the process for determining the appropriate treatment of the redeemable equity instruments in accordance with ASC 480-10-S99 and appropriately reporting such treatment in the Q3 2021 Form 10-Q, the Company is unable to complete and file the Q3 2021 Form 10-Q by the required due date of November 15, 2021 without unreasonable effort and expense. The Company does, however, expect to file such report within five calendar days thereof

ACON S2 Acquisition Corp. (the “Company”) is filing this Notification of Late Filing on Form 12b-25 with respect its Quarterly Report on Form 10-Q for the period ended June 30, 2021 (the “Quarterly Report”). The Company has recently experienced unanticipated delays in its third-party services providers’ clearance of internal procedures necessary to prepare a complete filing of the Quarterly Report in a timely manner. The Company is working diligently with its third-party service providers to complete the Quarterly Report as soon as possible; however, given extraordinary circumstances impacting the availability of third-party service providers involved with the review of the Company’s financial statements, the Company is unable to complete and file the Quarterly Report by the required due date of August 16, 2021 without unreasonable effort and expense. The Company does, however, expect to file such report within five calendar days thereof. The Company intends to file the Quarterly Report with the SEC as soon as practicable and currently expects to do so within the five-day extension period afforded by Rule 12b-25 under the Securities Exchange Act of 1934, as amended

ACON S2 Acquisition Corp. (the “Company”) is filing this Notification of Late Filing on Form 12b-25 with respect its Quarterly Report on Form 10-Q for the period ended March 31, 2021 (the “Quarterly Report”). The Company has determined that it is unable to file its Quarterly Report within the prescribed time period without unreasonable effort or expense as Marcum LLP (“Marcum”) needs additional time to complete its review of the financial statements for the period ended March 31, 2021. Marcum has informed the Company that the Staff of the U.S. Securities and Exchange Commission (the “SEC”) issued the “Staff Statement on Accounting and Reporting Considerations for Warrants Issued by Special Purpose Acquisition Companies (“SPACs”)” (the “SEC Statement”), which clarified guidance for all SPAC-related companies regarding the accounting and reporting for their warrants. Following review of the SEC Statement, the Company reevaluated the accounting treatment of its public warrants and private placement warrants as equity, and concluded that, based on the SEC Statement, the public warrants and private placement warrants should be, and should previously have been, classified as a liability measured at fair value, with non-cash fair value adjustments recorded in earnings at each reporting period. Marcum has faced significant delays in reviewing the Company’s financial statements following the Company’s reevaluation of the accounting treatment of its public warrants and private placement warrants. The Company intends to file the Quarterly Report with the SEC as soon as practicable and currently expects to do so within the five-day extension period afforded by Rule 12b-25 under the Securities Exchange Act of 1934, as amended