ESS Partners with Alsym Energy to Deliver 8.5 GWh of Non-Lithium Battery Energy Storage Solutions

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ESS Partners with Alsym Energy to Deliver 8.5 GWh of Non-Lithium Battery Energy Storage Solutions

Next Generation Sodium-ion Battery Solution Enables ESS Transition to a Full- Service BESS Platform Provider for Expanded Applications

Partnership Enables ESS to Enter the Short and Medium Duration Energy Storage Segment

Wilsonville, Ore. and Malden, MA - April 29, 2026 - ESS Tech, Inc., a leading manufacturer of sustainable, long-duration energy storage systems ("LDES"), today announced the signing of a letter of intent for a strategic partnership with Alsym Energy, a pioneer in non-flammable, high-performance
sodium-ion batteries, to add 8.5 GWh of sodium-ion cells and modules to its portfolio.

This next-generation battery solution is designed to address use cases traditionally served by lithium-ion systems - and those where lithium cannot go - but without the inherent thermal run-away risks associated with lithium chemistries.

This partnership marks ESS's entry into the short- and medium-duration BESS ("Battery Energy Storage System") segment, a market historically dominated by lithium-ion. It meaningfully expands the Company's addressable market beyond its established position in long-duration storage. The Alsym sodium-ion technology virtually eliminates thermal runaway risk and lowers total cost of ownership. In addition, the solution does not require complex HVAC systems, demonstrates high round trip efficiency, employs fast charge and discharge capabilities and offers a simpler, safer deployment profile for customers seeking superior stationary storage solutions.

"Sodium-ion and iron flow are complementary technologies," said Drew Buckley, Chief Executive Officer of ESS. "Alsym's sodium-ion Na-Series is an ideal solution for ESS's short- and medium-duration applications where high power, fast cycling, and rapid response are paramount. ESS's existing Energy Base® iron flow platform is engineered for the 8-24 hour long-duration segment, where deep daily cycling,25-year asset life, and zero capacity degradation deliver the lowest levelized cost of storage. Together, the two chemistries form a unified, non-lithium platform that enables ESS to meet customers' full storage needs from a single trusted provider, whether the application calls for firming renewables over a few hours, shifting energy across a full day, or pairing both within a single project to optimize economics across the full duration curve."

Randall Selesky, Chief Commercial Officer at ESS, added, "This partnership represents a major milestone in our strategy to become a full-spectrum, non-lithium solutions provider for the entire energy storage market with safer, more sustainable technologies. By combining Alsym's high performance, nonflammable sodium-ion technology with ESS' systems expertise and Energy Base® long-duration solutions, we are giving customers a clear pathway beyond lithium-ion - without compromising performance or economics.

"Unlike lithium-ion batteries and many other sodium-ion batteries, Alsym's Na-Series batteries are non-combustible and thermally stable, reducing system complexity, improving safety, and lowering total cost of ownership by reducing the need for extensive fire suppression and HVAC infrastructure. Alsym's Na-Series has been developed using a proprietary, physics-informed AI platform for battery development that dramatically shortens the time to bring innovation to the market. The batteries utilize non-foreign entity of concern ("FEOC") sourced materials and provide integrators and OEMs with a safe, cost-effective, supply-secure battery solution," Selesky concluded.

Mukesh Chatter, Chief Executive Officer for Alsym Energy, commented, "ESS is a leading innovator in stationary storage, and we are very pleased to be partnering with them. As demand grows, it is increasingly clear that the industry needs solutions beyond lithium-ion to meet the speed and scale projections. By combining high performance, inherent safety, and supply chain resilience, Alsym's Na-Series delivers that capability and ESS brings deep experience delivering grid-scale systems that maximize the value of renewable energy. Together, we are enabling a better path forward for energy storage."

With the combined sodium-ion and iron-flow platform, ESS is positioned to support utilities, IPPs, data centers, and C&I customers seeking American-made, flexible, and future-proof energy storage solutions across a wide range of applications.

About ESS Tech, Inc.
ESS (NYSE: GWH) is the leading manufacturer of long-duration iron flow energy storage solutions. ESS was established in 2011 with a mission to accelerate decarbonization safely and sustainably through longer lasting energy storage. Using easy-to-source iron, salt, and water, ESS iron flow technology enables energy security, reliability and resilience. We build flexible storage solutions that allow our customers to meet increasing energy demand without power disruptions and maximize the value potential of excess energy. For more information visit www.essinc.com.

About Alsym EnergyAlsym Energy is enabling a safer, scalable energy future by rethinking battery chemistry. The company's flagship Na-Series are non-flammable, high-performance, low cost sodium-ion batteries made with earth abundant materials. They are designed using a proprietary, physics-informed AI platform that enables the discovery of materials and commercially viable chemistries 10x faster than traditional, trial and error experiment-only methods. By combining DeepTech expertise in batteries with physics-informed AI, the platform is a closed-loop system that accelerates the entire battery development process, from ideation to manufacturing. Alysm's Na-Series technology eliminates thermal runaway and allows energy storage to be deployed safely, and at scale, anywhere energy storage is needed - from data centers and industrial facilities to residential buildings, commercial real estate, mining, military installations or utility grids. Its wide operating temperature range avoids the need for HVAC systems for safety or performance, and fast charge and discharge rates allow multiple cycles per day, creating a powerful economic model for energy storage systems. Alsym Na-Series: A better battery for energy storage. To learn more, visit https://www.alysm.com.

Forward-Looking Statements
This communication contains certain forward-looking statements, including statements regarding ESS and its management team's expectations, hopes, beliefs, intentions or strategies regarding the future. The words "anticipate," "believe," "continue," "could," "estimate," "expect," "intends," "may," "might," "plan," "possible," "potential," "predict," "project," "should," "will," "would" and similar expressions may identify forward-looking statements, but the absence of these words does not mean that a statement is not forward-looking. Examples of forward-looking statements include, among others, statements regarding the Company's productization, manufacturing and delivery of the Energy Base and relationships with customers, including Salt River Project. These forward-looking statements are based on ESS' current expectations and beliefs concerning future developments and their potential effects on ESS. Many factors could cause actual future events to differ materially from the forward-looking statements in this communication. There can be no assurance that the future developments affecting ESS will be those that we have anticipated. These forward-looking statements involve a number of risks, uncertainties (some of which are beyond ESS control) or other assumptions that may cause actual results or performance to be materially different from those expressed or implied by these forward-looking statements, which include, but are not limited to, general economic and market conditions as well as geopolitical developments and other risks and uncertainties described more fully in the section titled "Risk Factors" in the Company's Quarterly Report on Form 10-Q filed on November 13, 2025, and the Company's other filings with the U.S. Securities and Exchange Commission. Except as required by law, ESS is not undertaking any obligation to update or revise any forward-looking statements whether as a result of new information, future events or otherwise.

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