Iron-air batteries that store power for 100 hours head to the grid

As wind and solar grow, grids increasingly need storage that lasts far longer than the few hours lithium batteries typically provide. Form Energy is tackling that gap with an iron-air battery designed to discharge at full power for 100 hours, enough to ride out multi-day lulls in renewable generation. The chemistry is deliberately humble: it stores and releases energy by oxidizing and de-oxidizing iron particles in a water-based electrolyte, a process the company calls reversible rusting.

The appeal is cost and abundance. Iron is one of the cheapest and most common materials on Earth, avoiding the supply pressures of lithium and cobalt. In October 2024, Form Energy detailed a $405 million Series F round and a collaboration with GE Vernova covering manufacturing, supply chain and sourcing. Its first factory, in Weirton, West Virginia, sits on the site of a former steel mill and was expanding toward roughly 850,000 square feet by the end of 2025.

“Form Energy is tackling that gap with an iron-air battery designed to discharge at full power for 100 hours, enough to ride out multi-day lulls in renewable generation.”

The technology is moving from prototype to real projects. Form broke ground on its first commercial pilot, a 1.5 megawatt system rated for 100 hours of discharge, with Minnesota cooperative Great River Energy, expected to begin operating by the end of 2025. The company has announced a broader pipeline, including projects with Xcel Energy in Minnesota and self-developed projects in Maine and California backed by federal and state grants, totaling roughly 14 gigawatt-hours of long-duration storage announced to date.

The caveats matter. Iron-air batteries are heavy and slow to charge and discharge, so they are not a replacement for lithium in cars or short-duration grid services; they are a complement aimed squarely at long-duration storage. Multi-day pilots still need to prove durability and cost over years of real operation, and announced project pipelines do not always become built capacity. But if reversible rusting works at scale, it could give grids an inexpensive way to store clean energy for the long stretches when the sun and wind go quiet.