EX-5.1

Exhibit 5.1

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Wilson Sonsini Goodrich & Rosati

Professional Corporation

650 Page Mill Road

Palo Alto, California 94304-1050

O: 650.493.9300

F: 650.493.6811

December 27, 2021

ESS Tech, Inc.

132426440 SW Parkway Ave. Bldg. 83

Wilsonville, OR 97070

Re:

Registration Statement on Form S-1

Ladies and Gentlemen:

This opinion is furnished to you in connection with the Registration Statement on Form S-1, as may be amended (the “Registration Statement”), filed by ESS Tech, Inc. (f/k/a ACON S2 Acquisition Corp.), a Delaware corporation (the “Company”), with the Securities and Exchange Commission (the “Commission”) in connection with the registration under the Securities Act of 1933, as amended (the “Securities Act”), of up to 6,500,000 shares (including 1,500,000 shares issuable upon exercise of an option to purchase additional shares granted to the underwriters by the Company) of the Company’s Common Stock, par value $0.0001 per share (the “Company Securities”), which will be issued and sold by the Company.

Pursuant to Rule 429, the prospectus contained in the Registration Statement (the “Prospectus”) also covers 5,000,000 shares of the Company’s Common Stock (the “Stockholder Securities” and together with the Company Securities, the “Securities”) previously registered under the Company’s registration statement on Form S-1 filed by the Company with the Commission on November 3, 2021 (File No. 333-260693), as amended by Post-Effective Amendment No. 1 filed by the Company with the Commission on December 9, 2021 (the “Prior Registration Statement”), which will be sold by a certain selling stockholder identified in the Registration Statement under the caption “Principal and Selling Stockholders” (the “Selling Stockholder”).

We understand that the Securities are to be sold to the underwriters for resale to the public as described in the Registration Statement and pursuant to an underwriting agreement, substantially in the form to be filed as an exhibit to the Registration Statement, to be entered into by and among the Company, the Selling Stockholder and the underwriters (the “Underwriting Agreement”).

We are acting as counsel for the Company in connection with the registration of the Securities. As such counsel, we have made such legal and factual examinations and inquiries as we have deemed necessary or advisable for the purpose of rendering the opinions and statements set forth below. In rendering the opinions and statements expressed below, we have examined originals or copies, certified or otherwise identified to our satisfaction, of such documents, corporate records, certificates of public officials and other instruments as we have deemed necessary for the purposes of rendering this opinion.

AUSTIN BEIJING BOSTON BRUSSELS HONG KONG LONDON LOS ANGELES NEW YORK PALO ALTO

SAN DIEGO SAN FRANCISCO SEATTLE SHANGHAI WASHINGTON, DC WILMINGTON, DE

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ESS Tech, Inc.

December 27, 2021

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In addition, we have reviewed originals or copies of such corporate records of the Company, certificates of public officials, a certificate of an officer of the Company as to factual matters, and such other documents which we consider necessary or advisable for the purpose of rendering the opinions set forth below. We have not independently established the facts stated therein.

In our examination, we have assumed the genuineness of all signatures, the authenticity and completeness of all documents submitted to us as originals, the conformity with the originals of all documents submitted to us as copies, the authenticity of the originals of such documents and the legal competence of all signatories to such documents. We have also assumed the authority of such persons signing on behalf of the parties thereto other than the Company and the due authorization, execution and delivery of all documents by the parties thereto other than the Company. We have assumed that the certificates representing the Securities have been properly authenticated by the signature of an authorized officer of the Company’s transfer agent. We have also assumed the conformity of the documents filed with the Commission via the Electronic Data Gathering, Analysis and Retrieval System (“EDGAR”), except for required EDGAR formatting changes, to physical copies submitted for our examination and the absence of any evidence extrinsic to the provisions of the written agreements between the parties that the parties intended a meaning contrary to that expressed by those provisions.

We express no opinion as to any matter relating to the laws of any jurisdiction other than the federal laws of the United States of America and the General Corporation Law of the State of Delaware.

Based upon and subject to the foregoing qualifications, assumptions and limitations and the further limitations set out below, we are of the opinion that:

With respect to the Company Securities to be issued and sold by the Company pursuant to the Registration Statement, such shares have been duly authorized by all necessary corporate action on the part of the Company and, upon issuance, delivery and payment therefor in the manner contemplated by the Underwriting Agreement and the Registration Statement, such shares will be validly issued, fully paid and non-assessable.

2.	With respect to the Stockholder Securities to be sold by the Selling Stockholder pursuant to the Registration Statement, such shares have been duly authorized and are validly issued, fully paid and non-assessable.

Our opinion that any document is legal, valid and binding is qualified as to:

(a)	limitations imposed by bankruptcy, insolvency, reorganization, arrangement, fraudulent transfer, moratorium or other similar laws relating to or affecting the rights of creditors generally;

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(b)	rights to indemnification and contribution, which may be limited by applicable law or equitable principles; and

(c)	the effect of general principles of equity, including without limitation concepts of materiality, reasonableness, good faith and fair dealing, and the possible unavailability of specific performance or injunctive relief, whether considered in a proceeding in equity or at law.

This opinion is furnished to you in connection with the filing of the Registration Statement, and is not to be used, circulated, quoted or otherwise relied upon for any other purpose.

We hereby consent to the filing of this opinion as an exhibit to the above-referenced Registration Statement and to the use of our name wherever it appears in the Registration Statement, the Prospectus, any Prospectus Supplement, and in any amendment or supplement thereto. In giving such consent, we do not thereby admit that we are in the category of persons whose consent is required under Section 7 of the Securities Act or the rules and regulations of the Commission thereunder.

Very truly yours,

/s/ Wilson Sonsini Goodrich & Rosati

WILSON SONSINI GOODRICH & ROSATI

Professional Corporation