Form Energy Begins Manufacturing Iron-Air Batteries That Store Electricity for 100 Hours Using Rust

One of the biggest challenges facing the renewable energy transition has been the question of storage: what happens when the sun doesn't shine and the wind doesn't blow for days at a time? Form Energy believes it has cracked the problem with an elegantly simple solution — batteries made of iron and air.

In 2025, Form Energy began manufacturing its iron-air batteries at scale, marking a breakthrough for multi-day energy storage. The batteries can store electricity for up to 100 hours — enough to keep the grid powered through extended periods of calm, cloudy weather that would drain conventional lithium-ion batteries in just four hours.

“Form Energy believes it has cracked the problem with an elegantly simple solution — batteries made of iron and air.”

The technology works through a reversible rusting process. During charging, electrical energy converts iron oxide (rust) back into metallic iron. During discharging, the iron is exposed to air and oxidizes — literally rusting — releasing electrons that generate electricity. The process is remarkably efficient and uses materials that are abundant, inexpensive, and non-toxic.

Iron is the fourth most common element in Earth's crust and costs a tiny fraction of the lithium, cobalt, and nickel used in conventional batteries. This means iron-air batteries can be manufactured at roughly one-tenth the cost of lithium-ion systems, making large-scale grid storage economically viable for the first time.

Form Energy's manufacturing facility produces battery modules that can be assembled into systems ranging from neighborhood-scale to utility-scale installations. Several major U.S. utilities have already signed contracts for deployments that will begin providing multi-day backup power to their grids.

The implications for the clean energy transition are profound. Multi-day storage has been called the "missing piece" of a fully renewable grid. With iron-air batteries, utilities can now build renewable energy systems confident that they can maintain reliability even during extended weather events — removing one of the last major arguments against transitioning away from fossil fuel power plants.