A Battery That Rusts to Store 100 Hours of Power: Form Energy Raises $750M

Solar and wind have one crippling weakness: the stretch of days when the wind dies and the sun doesn’t show. Lithium-ion batteries, typically sized for four hours, can’t cover that gap. The answer from Form Energy, based in Weirton, West Virginia, is an iron-air battery that delivers power for more than 100 hours.

The mechanism is rust. Iron oxidizes into rust as the battery discharges, and that rust turns back into iron when it charges. No expensive minerals like lithium or cobalt are needed, and there is no risk of thermal runaway. The company says 80% of its battery materials are sourced in the U.S., with the rest coming from Europe and Asia — China is absent from the supply chain.

Form Energy announced a $750 million Series G led by T. Rowe Price, which also led its $405 million Series F. Total equity raised now exceeds $2 billion.

New investors include Sequoia Capital, Janus Henderson, Franklin Templeton, and PEAK6 Investments. Existing backers returned as well: Prelude Ventures, Engine Ventures, TPG Rise Climate, Capricorn’s Technology Impact Funds, Breakthrough Energy Ventures, Dustin Moskovitz and Cari Tuna, Gigascale Capital, Coatue, Energy Impact Partners, NGP, GE Vernova, Blindspot Ventures, and M&G Catalyst Fund. Morgan Stanley served as sole placement agent.

Backlog up fourfold — and Google ordered 30 GWh

The capital followed the orders. Form Energy’s backlog of iron-air battery projects grew from roughly 20 GWh earlier this year to 80 GWh. Its largest customer is Google, which ordered a 30 GWh installation for a Minnesota data center — a deal the industry estimates at around $1 billion. AI infrastructure firm Crusoe committed to 12 GWh, and Xcel Energy and FuturEnergy Ireland are customers too.

The demand traces back to AI data centers. U.S. data center electricity use is projected to quadruple by 2035, reaching roughly 20% of all power generated. Carrying that load with renewables requires storage that can fill multi-day gaps when generation stops. The same problem drove Google to acquire Intersect Power for $4.75 billion to secure power directly; this time it shows up as a large storage order.

Proceeds will expand production at the Weirton plant and fund commercial deployments. Form also strengthened its leadership team. New CFO Navneet Govil previously served as CFO and investment committee member at SoftBank Investment Advisers, overseeing finance across the Vision Funds’ portfolio. New COO Wes Sloan was EVP at Panasonic Energy North America, directing production in Nevada and leading large-scale battery manufacturing, after running engine assembly and testing at Rolls-Royce — 25 years spanning aerospace, automotive, and energy.

Multi-day storage becomes AI’s next power battleground

The power bottleneck AI created is pulling capital across storage technologies. Thermal-battery maker Antora Energy raised a $550 million Series C, and Base Power, which bundles home batteries into a grid resource, raised $1 billion at a $13 billion valuation. Software approaches have emerged too, like GridCare, which uses data to find spare capacity on the grid.

What sets Form Energy apart is duration. Four-hour lithium-ion handles peaks within a single day and Antora’s thermal batteries target industrial heat; Form aims at the windless, overcast stretches that last for days. Using cheap, abundant iron lets it drive down cost per unit of stored energy. But the systems are bulky, heavy, and slow to respond, making them a complement to lithium-ion across different time horizons rather than a replacement. The next hurdle isn’t the technology but manufacturing: whether an 80 GWh backlog can be turned into operating capacity.