US-based ESS Tech has signed a strategic partnership with Alsym Energy to supply 8.5 GWh of non-lithium battery storage systems, marking a significant move beyond its core long-duration iron flow technology.
The agreement, structured as a letter of intent, will see ESS integrate Alsym’s sodium-ion cells and modules into its portfolio, targeting short- and medium-duration battery energy storage system (BESS) applications traditionally dominated by lithium-ion technologies.
The companies said the partnership is designed to address both conventional lithium-ion use cases and applications where lithium chemistries are less suitable, particularly due to safety or cost constraints.
“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.”
Alsym’s sodium-ion cells and ESS’s non-lithium strategy
The move positions ESS to offer a full-spectrum, non-lithium storage platform spanning durations from a few hours to more than 24 hours. Its existing Energy Base iron flow system is designed for long-duration applications (8–24 hours), while sodium-ion technology will address shorter-duration, high-power use cases.
Randall Selesky, chief commercial officer at ESS, described the agreement as “a major milestone” in the company’s strategy to become a full-service provider of non-lithium energy storage solutions.
Safety and cost drivers
Alsym’s sodium-ion batteries are positioned as a safer alternative to lithium-ion systems, with the companies highlighting the absence of thermal runaway risk and the use of non-flammable materials. The technology is also said to reduce system complexity by removing the need for extensive HVAC requirements, while maintaining high round-trip efficiency and fast charge/discharge capability.
The chemistry is designed to lower total cost of ownership and avoid reliance on materials sourced from “foreign entities of concern”, aligning with growing supply-chain security considerations in the battery sector.
Market implications
The partnership reflects a broader shift in the stationary storage market towards diversification beyond lithium-ion, particularly as utilities and developers seek safer, lower-cost and more sustainable alternatives across different duration requirements.
By combining sodium-ion and iron flow technologies, ESS aims to compete across a wider range of grid-scale and commercial applications, including renewable integration, energy shifting and hybrid storage projects.
The deal also represents one of the largest announced deployments of sodium-ion technology to date, signalling growing commercial traction for non-lithium chemistries in stationary energy storage.
