Filing pursuant to Rule 425

Filer: ACON S2 Acquisition Corp.

Subject Company: ACON S2 Acquisition Corp.

Registration No: 001-39525

Date: June 16, 2021

Long Duration Energy Storage Systems for a Cleaner Future JUNE 2021    

Disclaimers This is for presentation discussion purposes (this “Presentation”) only and does was not prepared constitute for an informational offer to purchase purposes nor only a solicitation to assist interested of an offer parties to sell in shares making of ACON, their own ESS evaluation or any successor of the proposed entity of the transaction Transaction, (the “Transaction”) nor shall there between be any sale Acon of S2 securities Acquisition in any Corp jurisdiction . (“ACON”, in which “we”, or such “our”) offer, and solicitation, ESS Inc.(“ESS”) or sale . This would Presentation be unlawful will be given prior by to ACON, registration ESS or or their qualification respective under affiliates the securities and advisors laws as of to any the such accuracy jurisdiction or completeness . This Presentation of the information is not intended contained to form herein, the basis or any of any other investment written or decision oral information by the recipient made available and does in not the constitute course of investment, an evaluation tax of or the legal Transaction advice. No . This representation, communication express is not or a implied, proxy is or statement or solicitation of a proxy, consent or authorization with respect to any securities or in respect of the potential transaction. This “could”, Presentation “estimate”, provided “expect”, by “intends”, ACON and “may”, ESS “might”, may contain “plan”, certain “possible”, forward “potential”, looking statements, “predict”, “project”, including “should”, statements “would” regarding and similar ACON’s, expressions ESS’s or their may management identify forward teams’ -looking expectations, statements, hopes, but the beliefs, absence intentions of these or strategies words does regarding not mean the that future a statement . The words is not “anticipate”, forward-looking “believe”, . These “continue”,    forward forward—looking looking statements statements are in this based presentation, on ACON’s including and ESS’s but current not limited expectations to: (i) the and risk beliefs that the concerning Transaction future may developments not be completed and their in a timely potential manner effects or on at all, ACON, which ESS may or adversely any successor affect entity the price of the of Transaction ACON’s securities, . Many factors (ii) the could failure cause to satisfy actual the future conditions events to to the differ consummation materially from of the the    generally, Transaction, (v) (iii) risks the that occurrence the Transaction of any event, disrupts change current or plans other and circumstance operations that of ESS, could (vi) give changes rise to in the the termination competitive of and the highly Agreement regulated and industries Plan of Merger, in which (iv) ESS the plans effect to of operate, the announcement variations in or operating pendency performance of the Transaction across competitors, on ESS’s business changes relationships, in laws and operating regulations results affecting and business ESS’s separate business and summary changes risk in factors the combined . There can capital be no structure, assurance (vii) that the the ability future to developments implement business affecting plans, ACON, forecasts ESS or and any other successor expectations entity of after the the Transaction completion will of be the those Transaction, that we have and anticipated identify and . These realize forward additional -looking opportunities, statements and involve (viii) such a number other of risks risks, and uncertainties uncertainties (some included of which in the are    beyond revise any ACON’s forward or—looking ESS’s control) statements or other whether assumptions as a result that of may new cause information, actual results future or events performance or otherwise to be . You materially should different carefully from consider those the expressed foregoing or factors implied and by the these other forward risks -and looking uncertainties statements described . Except as in required the “Risk by Factors” law, ACON section and of ACON ESS are S2’s not registration undertaking on any Form obligation S-1 (File to No update . 333- or events 248515), and the results registration to differ statement materially on from Form those S-4 expected contained to in be the filed forward in connection -looking statements with the business . Except combination, as required by and law, other ACON documents S2 and ESS filed are by not ACON undertaking S2 from time any obligation to time with to the update SEC .or These revise filings any forward identify—and looking address statements other important whether as risks a result and uncertainties of new information, that could future cause events actual or otherwise All rights to . Neither the trademarks, ACON S2 copyrights, nor ESS gives logos any and assurance other intellectual that either property the ACON listed S2 or herein ESS, belong or the combined to their respective company, owners will achieve and ACON’s its expectations or ESS’s .use thereof does not imply an affiliation with, or endorsement by the owners of such trademarks, copyrights, logos and other intellectual property. Solely for convenience, trademarks and trade names referred to in this Presentation may appear with the ® or ™ symbols, but such references are not intended to indicate, in any way, that such names and logos are trademarks or registered trademarks of ACON. accordingly, Use of Projections neither . The expresses financial an and opinion operating or other forecasts form of and assurance projections with contained respect thereto herein. represent Furthermore certain none estimates of ACON, of ESS ESS nor as of their the respective date thereof management . Neither ACON’s teamsnor can ESS’s give any independent assurance public that the accountants forecasts or have projections examined, contained reviewed herein or compiled accurately the represents forecasts or ESS’s projections future operations and, or the financial prospective conditions financial . Such information information . Accordingly, is subject to a there wide can variety be no of assurance significant that business, the prospective economic and results competitive are indicative risks of and the uncertainties, future performance including of but ACON not or limited ESS or to that those actual set forth results in the will second not differ paragraph materially above from those that could presented cause in actual the prospective results to differ financial materially information from those . Some contained of the in assumptions Inclusion of the upon prospective which the financial projections information are based in inevitably this presentation will not should materialize not be and regarded unanticipated as a representation events may occur by any that person could that affect the results results . Therefore, contained actual in the results prospective achieved financial during information the periods are covered indicative by the of future projections results may or will vary be and achieved may vary . materially from the projected results. This We have Presentation not independently contains statistical verified data, the accuracy estimates or and completeness forecasts that of the are data based that on has independent been contained industry in these publications industry or publications other publicly and available other publicly information available . This information information . Accordingly, involves many none assumptions of ACON, and ESS limitations nor their respective and you are affiliates cautioned and advisors not to give makes undue any weight representations to these estimates as to the. accuracy or completeness of these data. This Presentation contains references to ESS’s achievements compared to other companies. All of such references are based on the belief of ESS’s management based on publicly available information known to ESS’s management. Non gross -GAAP margin Financial and Adjusted Measures EBITDA . The. financial ACON and information ESS believe and that data these contained non-GAAP in this measures Presentation of financial is unaudited results and provide does useful not conform information to Regulation to management S-X promulgated and investors under regarding the Securities certain Act financial of 1933, and as business amended trends . This relating Presentation to ESS’s also financial includes condition non-GAAP and financial results of measures, operations including . ESS’s management measures to the uses most certain directly of these comparable non-GAAP GAAP measures financial to measures compare is ESS’s available performance without unreasonable to that of prior efforts periods at this for trend time. Specifically, analyses and ESS for does budgeting not provide and planning such quantitative purposes.reconciliation Not all of the    due information to the inherent necessary difficulty for a quantitative in forecasting reconciliation and quantifying of these certain forward amounts -looking that non are -necessary GAAP financial for such reconciliations. This sale or Presentation exchange would relates be to unlawful a proposed prior transaction to registration between or qualification ESS and ACON under . This the Presentation securities laws does of any not constitute such jurisdiction an offer . ACON to sell and or exchange, ESS intend or to the file solicitation a registration of an statement offer to on buy Form or exchange, S-4 with the any U securities, .S. Securities nor and shall Exchange there be any Commission sale of securities (the “SEC”), in any which jurisdiction will include in which a document such offer, that    serves voting as decision, a joint investors prospectus and and security proxy holders statement, of ESS referred and ACON to as a are proxy urged statement/prospectus to read the registration . A proxy statement, statement/prospectus the proxy statement/prospectus will be sent to all and ESS all and other ACON relevant shareholders documents . ESS filed and or ACON that will will also be filed file with other the documents SEC in connection regarding with the the proposed proposed transaction transaction with as the they SEC become . Before available making any    because and ACON they through will contain the website important maintained information by the about SEC the at www proposed .sec.gov transaction . . Investors and security holders will be able to obtain free copies of the registration statement, the proxy statement/prospectus and all other relevant documents filed or that will be filed with the SEC by ESS ACON ACON S2 S2’s and securities ESS and is set their forth respective in ACON directors S2’s filings and with officers the may SEC. be To deemed the extent to that be participants holdings of in ACON the solicitation S2’s securities of proxies have changed from ACON since S2’s the stockholders amounts printed in connection in ACON with S2’s the Registration proposed Statement transaction on . Information Form S-1, such about changes ACON have S2’s been directors or will and be executive reflected officers on Statements and their of ownership Change in of    the Ownership proposed on transaction Form 4 filed when with it the becomes SEC. Additional available information . You may obtain regarding free copies the interests of these of those documents persons as and described other in persons the preceding who may paragraph be deemed . participants in the proposed transaction may be obtained by reading the proxy statement/consent solicitation statement/prospectus regarding CONFIDENTIAL 1    

Game Changin Technology The Power Grid of the Futur Feasible Today STABLE. SECURE. CLEAN.    

Transaction Overview
Category Catalyst in Long Duration Energy Storage Solutions
ESS Founded in 2011 to enable the stable, decentralized
and decarbonized power grid of the future
Offering Size ACON S2 (NASDAQ: STWO): a special purpose
acquisition company
$250 million cash in trust
PIPE size of $250 million
Valuation $1,072 million pro forma enterprise value
Attractive value, high-growth, genuinely sustainable
business
Capital ESS shareholders rolling 100% of equity
Structure $465 million net proceeds (assuming no redemptions)
Fully funded to projected cash flow profitability
ESS’ Key Investors and Partners
Leadership
Craig Evans Eric Dresselhuys Julia Song Amir Moftakhar
President & Founder CEO CTO & Founder CFO
(March 2021)
Adam Kriger John Roush Alan Greenshields
CEO & Director CFO & Chairman ACON Advisor
Note For additional information, please refer to the Detailed Transaction Overview on p. 25 of this presentation.
CONFIDENTIAL
3

ESS: A Category Defining Investment Opportunity 1 Large and Fast-Growing TAM: ~$56bn by 2027 growing at a 33% CAGR1 2 Simple Yet Revolutionary Technology: Iron, salt and water; strong patent portfolio 3 Compelling Value Proposition: Highest performance, lowest cost2 and most sustainable 4 Low Risk Expansion Plan: Field proven3 technology with low-cost manufacturing build out 5 $7bn of Identified Opportunities4: $300m+ SoftBank Energy framework agreement through 2026 6 Premier Management Team: Founders and inventors supported by an experienced team 1 2020; Guidehouse and Navigant Insights, Research, ‘Market Data: ‘Distributed Utility-Scale Energy Energy Storage Storage Overview’, Market 4Q Update’, 2019. 3Q 2020; Guidehouse Insights, ‘Market Data: Energy Storage for Microgrids and Remote Power Systems’, 2Q 2 3 Management Based on our Generation Estimates of I levelized products,cost which of are storage no longer (LCOS) deployed. among long duration Storage Systems. 4 Pipeline of visible potential orders. CONFIDENTIAL 4    

Market Opportunity CONFIDENTIAL 5    

What Is Long Duration Storage? Shift Supply to Meet Low Cost to Enable Replacement of Alternatives Reliable Demand from 4 – 12 Hours (Peaker Plants) (Grid Stability) 6    

ESS Transforms the Value Proposition for Long Duration Storage How ESS Transforms the Grid ï,§ Can replace coal and natural gas with solar and Longer Durationï,§ Up to 12 hours wind powerï,§ Flexibility allows multiple revenue streams Greater resiliency to unexpected ï,§ events Lower ï,§ Step function improvement in economics of Low Costï,§ LCOS than other technologies in the market storage ï,§ Incremental cost of storage <$20/kWh multiple use ï,§ Enables cases ï,§ <1 second response time Power On Demandï,§ Improved grid resiliency and flexibilityï,§ >20,000 cycle life – $0 marginal cost per cycle ï,§ Can deploy in a wide range of geographies and ï,§ Non-flammable, non-toxic, no explosion risk climates Safety and Reliability ï,§ Munich RE insures technology riskï,§ Customers can be confident in a long-term solution ï,§ Easily sourced materials; recyclable componentsï,§ Environmentally sustainable Sustainability ï,§ “Plug and play” with 25-year operating lifeï,§ Accelerates clean energy transition CONFIDENTIAL

Strong and Growing Demand for Energy Storage Cumulative Additions to Global Storage Capacity (GWh) Global Total Addressable Market ($bn) Front of the meter +43% Front of the meter 534    CAGR +33% CAGR $56 397 $46 301 +44% +32% CAGR $39 $28 CAGR 287 $33 $22 226    $27 $19 199 $22 168 $18 13 $17 $15 130 +34% 26 121 $14 $28 79 233 +42% $24 CAGR 83 CAGR $8 $12 $20 171 $15 39 119 $4 $12 11 52 77 $8 6 26 47 $4 $5 5 13 27 2020 2021 2022 2023 2024 2025 2026 2027 2020 2021 2022 2023 2024 2025 2026 2027 ESS has observed even greater demand from customers than these current analyst estimates Source 2020; Guidehouse and Navigant Insights, Research, ‘Market Data: ‘Distributed Utility-Scale Energy Energy Storage Storage Overview’, Market 4Q Update’, 2019. 3Q 2020; Guidehouse Insights, ‘Market Data: Energy Storage for Microgrids and Remote Power Systems’, 2Q CONFIDENTIAL 9    

ESS Wins on Performance Operational 4-12 hrs 16+ hrs Flexibility Li-Ion <4 hrs Compelling Performance Longer Asset Life Unlimited cycles, no capacity loss Li-Ion 6,000 cycles1 Ambient Superior    Operating -10°C- 60°C Temperature Li-Ion 20-25°C Safety Non-flammable No Explosion Risk Li-Ion 1 Li-Ion cyclability from BYD energy storage system factsheets. CONFIDENTIAL 10    

ESS Wins on Cost

ESS is a Category Defining Technology for Long Duration Storage Li-Ion Li Metal Zinc Vanadium, Bromine Sodium Sulfur Compressed Air Pumped Hydro Low 4 – 12 cost hours at Field proven1 materials Earth abundant Unlimited cycling Zero capacity fade Wide temperature operational range sustainable Environmentally No explosion fire/ risk Note 1 Internally Based on developed our Generation table I based products, on company which are data no longer and publicly deployed. available information. CONFIDENTIAL 12    

ESS Technology is Proven and Insured Munich RE Aon One Insurance Beacon Investment-Grade 10-year extended Surety and Corporate Bonding Warranty warranty covering battery modules Growing project surety capacity Investment-Grade Warranty continuity Project Insurance insurance provides additional surety to customers and financiers EXIM “ The ability to ensure battery performance is a key piece of the puzzle in decarbonizing US Export-Import Bank Qualified our energy sector.” Pre-qualified financing available for overseas buyers –Peter Röder, Member of the Board of Management, Munich RE CONFIDENTIAL 13    

Technology Overview    

Technological Breakthrough, Field Proven and Shipping Now breakthrough Technological – R&D roadmap for Iron Flow first conceived But “dirty” electrolyte Field proven1; S200 additional breakthroughs in 1970s caused rapid degradation power Proton fade Pump and eliminates limits on shipping now to extend technology cycle life advantage Technological Success Proven Over Time Innovative Technology 2011 Proton Pump Company formed operating Demonstrated cycles 10,000+ in the lab launched Gen I EW product line assembly Installed S200 line automated Developed lab scale battery Energy launched Center™ product line _ + (+) High ELECTRODE: surface area carbon    electrode (-) Carbon ELECTRODE: composite substrate with 2019 polypropene spacer development Awarded ARPA-e of Iron grant based for First commercial deployment S200 commercial battery SEPARATOR: module launched Porous polyethylene separator battery Ferrous ELECTROLYTE: Chloride in aqueous solution 1 Based on our Generation I products, which are no longer deployed. CONFIDENTIAL 15    

Robust Intellectual Property Portfolio ESS Critical Technology ESS IP Portfolio (-) (+) Power Module Electrode Electrode Collector Current Proton Pump Pressure Plate Conductive Separator Separator Porous Electrolyte protected = Patent CONFIDENTIAL 16    

Business Overview CONFIDENTIAL 17    

One Technology – Two Products of Different Scale Energy Warehouse™ Energy Center™ ï,§ Behind the meter solutionï,§ Front of the meter solutionï,§ 50kW – 90kW configurable rangeï,§ Customizable configuration rangeï,§ First commercial deployment in 2015ï,§ Customer trials starting in 2021ï,§ Generation II launched in 2020 turnkeyï,§ “Battery in a Building” platformï,§ Containerized design for deliveryï,§ Fast to build and commissionï,§ Modular design for utility-class CONFIDENTIAL 18    

Validated by a Blue-Chip Customer Base Utilities IPPs/Developers Commercial & Industrial EW EC EW EC EW EC ï,§ Peaker replacementsï,§ Peaker replacementsï,§ Energy cost savings Demand ï,§ T&D upgrade deferralsï,§ Resource adequacy & grid reliabilityï,§ Operational resiliency Driversï,§ Wildfire resiliencyï,§ 24/7 power supplyï,§ RE integration ï,§ Distributed energy services productsï,§ Microgridsï,§ Carbon footprint reduction/ESG goals Engie Applied Medical Select Use Customers Cases / San Diego Gas & Electric Pacto Energia ÈEZ Group Duke ConEdison Enel Honeywell Idimax Energy Energy Select Pipeline Naturgy Grupo Starwood Marathon SAESA Energy PacifiCorp Note “EW” refers to Energy Warehouse™, “EC” refers to Energy Center™. CONFIDENTIAL    

Financial Forecast CONFIDENTIAL 20    

ESS’ Robust Revenue Growth Projected Revenue by Product Offering ($in millions) $3,562 ï,§ Growth Center deployments accelerates as start Energy in    EC EW EF Services 2023 $2,572 ï,§ Forecast driven by identified pipeline of near-term opportunities $1,645 ï,§ ESS expansion into Australia (2023) and Europe (2024) $803 supports continued growth $300 ï,§ Energy Franchise lease and $2 $37 Services revenue streams become bigger contributors 2021E 2022E 2023E 2024E 2025E 2026E 2027E as ESS expands 0% 0% 1% 2% 4% 6% Note Total addressable market value for the microgrid, distributed energy and utility industries from Guidehouse Insights. CONFIDENTIAL 21    

ESS Delivers Compelling Profitability Projected Gross Margin ($in millions) Projected EBITDA ($in millions) $1,332 $1,059 $950 $739    $508 $360 $217 $131    ($2) ($13) $47 ($20) ($43) $2    2021E 2022E 2023E 2024E 2025E 2026E 2027E 2021E 2022E 2023E 2024E 2025E 2026E 2027E 16% 27% 31% 37% 37% 1% 16% 22% 29% 30% CONFIDENTIAL 22    

Potential Upside to Business Plan decarbonization New US federal and and state national policies security on infrastructure, Emerging on decarbonization mandates and in EU storage and Asia-Pacific and Demand World impact Bank of targets USTDA, Power Africa, UNDP Further economies of scale and technology enhancements (e. Additional g., Storage revenue as a Service, streams Warranty)    23    

Valuation Overview CONFIDENTIAL 24    

Detailed Transaction Overview ($ in millions, except per share data; shares in millions) ï,§ Pro forma enterprise value of $1,072 million (0.7x 2025E Revenue) Total Enterprise Value Summary Pro forma shares outstanding 156.5 ï,§ $465 million cash proceeds inclusive of $250 million PIPE proceeds (x) ESS share price $10.00 and transaction expenses assuming zero redemptions1 Pro Forma Equity Value $1,565ï,§ Pro forma net cash of $493 million1,2 (-) Current cash (28) (-) Net proceeds (465) ï,§ Inclusive of $28 million existing net cash on balance sheet2 Pro Forma Enterprise Value $1,072ï,§ ESS shareholders are rolling 100% of equity ownership Valuation Multiples Metric Multiple EV / 2025E Revenue $1,645 0.7x EV / 2025E EBITDA $360 3.0x Sources $ % Shares Founder shares Rollover equity 1,003 64% 100.3 4% ACON S2 cash in trust 250 16% 25.0 PIPE investors PIPE investment 250 16% 25.0 16% Founder Shares 63 4% 6.3 Total sources $1,565 100% 156.5 ESS existing Uses $ % shareholders Rollover equity 1,003 64% ACON S2 public Cash to balance sheet 465 30% shareholders 64% Founder shares 63 4% 16% Estimated fees and expenses 35 2% Total uses $1,565 100% Note 1 Figures SB Energy may Global not sum Holdings due to Limited rounding and . Breakthrough Energy Ventures, LLC, existing equity investors in ESS, have indicated an interest in investing an aggregate of $51.5 million in the offering. These existing investors prior to the are closing expected of the to agree offering to. reduce In exchange the amount for this of agreement, their existing such option investors to invest would in receive the C-2 warrants raise to an to purchase aggregate an of aggregate $16 million, of which 14,364,222 amount shares would of be ESS invested Series C (if -2 such preferred option stock is exercised) at an exercise immediately price of    2 $ Pro 0.001 forma per 12/31/2020 share, which net warrants cash assumes would automatically funding of an be aggregate net-exercised of $27 immediately .5 million in the prior C —to 2 the raise, closing of which of the $11 offering .5 million or has terminate been funded unexercised and $16 if million the offering is expected does not to be close funded . by SB Energy Global Holdings 3 Additional Limited and dilutive Breakthrough securities Energy include Ventures, 8.3m ACON LLC, S2 as described public warrants, in the 4 footnote .1m founder above warrants . Net cash and also $165m includes shareholder $1.5 million earnout of restricted . cash. CONFIDENTIAL 25    Illustrative Cost Comparison Versus Li-Ion How ESS’ Technology Delivers Superior Economics1 4 hours 12 hours Competitive with Li-Ion at 4 hours; Ion No scaling ESS wins thereafter Li—$200/kWh $200/kWh Marginal of $20/kWh cost $200/kWh $80/kWh Li-Ion LCOS at 4 hours vs. 12 hours2 Li-Ion $0.07    $0.02    0 4 8 12 16 12 hours 4 hours 1 2 Figures Superior shown economics are illustrative. based on Levelized Cost of Storage (LCOS).ð¿ð¿ð¿ð¿ð¿ð¿ð¿ð¿ = â^‘.ð¿ð¿ð¶ð¶ð¶ð¶ð¶ð¶ð¶ð¶ + â^‘.â^‘ð¼ð¼ð¼ð¼ð¼ð¼ð¼ð¼ð¶ð¶ð¼ð¼ð¼ð¼ð¶ð¶ð¼ð¼ð¼ð¼ð¼ð¼ð¼ð¼ .ð´ð´ð¼ð¼ð¼ð¼ð´ð´ð¶ð¶ð¼ð¼ð‘^ð‘^ð¼ð¼ð¶ð¶ð‘^ð‘^ð¼ð¼ð‘^ð‘^ + â^‘.ð·ð·ð¼ð¼ð¼ð¼ð¶ð¶ð¼ð¼ð¼ð¼ð¶ð¶ð¼ð¼ð¾ð¾ð¾ð¾ð¾ + â^‘.ð¿ð¿&ð‘€ð‘€ CONFIDENTIAL 11    

Stabilize the Grid and Accelerate Renewables US Renewable Energy Penetration (2015-2050)1 57% 65% 47% 36% 40% Renewable intermittency creates a 31% 23% massive problem for the grid, 15% particularly >25% penetration 2015 2020 2025 2030 2035 2040 2045 2050 California Duck Curve and % Renewable Penetration1,2 2013 (10%) 2019 (27%) 25 GW 15 5 1 2 3 4 5 6 7 8 9 10 Hour 11 12 of 13 day 14 15 16 17 18 19 20 21 22 23 24 1 2 BloombergNEF. IEA, “The California Duck Curve”, December 2019. % figures represent solar and wind power penetration in each year. CONFIDENTIAL 8