ESS acquires VoltStorage GmbH to accelerate long-duration energy storage

US-based iron flow battery specialist ESS Tech, Inc. has announced the acquisition of the intellectual property and assets of VoltStorage GmbH, a German developer of iron-salt battery technology that ceased operations in mid-2025.

“This strategic move combines two complementary technologies to deliver the most flexible, cost-effective long-duration energy storage solution available on the market,” ESS said in a press release on Wednesday.

Founded in 2011, ESS manufactures iron flow batteries using widely available materials such as iron, salt, and water. Designed for long-duration storage applications applications, ESS Inc.’s iron flow battery systems are typically offering 8–12 hours of discharge at rated power in commercially available configurations, with technology capable of up to 22 hours of storage depending on system size and design.

ESS packages this technology into modular platforms tailored for different markets. The Energy Warehouse™ targets commercial and industrial users with several hours of storage. The Energy Center™ platform is designed for utility-scale deployments, while the Energy Base™ platform supports long-duration storage of 8–22+ hours at gigawatt-hour scale.

With the acquisition of VoltStorage’s portfolio of patents and technical developments, ESS is substantially expanding its intellectual property base while also gaining access to skilled personnel.

In August 2025, VoltStorage announced it was closing operations. “The reasons are complex: market adversity, investment cycles, and strategic decisions all played a role. What I want to emphasize is this: the technology and the team delivered,” wrote John Alper, principal technologist at VoltStorage, on LinkedIn at the time.

VoltStorage developed its proprietary Iron Salt Battery technology, positioning it as a cost-competitive, sustainable alternative to lithium-ion systems for long-duration grid and renewable energy storage. The Iron Salt Battery, a type of redox flow battery using iron and salt as the active electrolyte, offers over 10,000 charge cycles without capacity loss, is non-flammable, and features flexible scalability. It is designed for long-duration energy storage applications, capable of discharging over extended periods, typically 8–24+ hours, and modular scalability.

ESS believes the combined capabilities will accelerate strategic growth, expand market reach, and position the company at the forefront of long-duration energy storage, delivering the industry’s lowest Levelized Cost of Energy (LCOE).

“This acquisition accelerates our mission to provide clean, reliable, and affordable energy storage solutions worldwide,” said Drew Buckley, CEO of ESS. “By combining decades of iron flow battery expertise across two companies at ESS, we are accelerating innovation to create the most advanced iron-salt battery platform for long-duration storage.”