Iron-Air Battery
An iron-air battery stores energy through reversible oxidation and reduction of iron — essentially controlled rusting and un-rusting. Iron is cheap, abundant, and non-flammable, but energy density is low so cells are bulky. Targeting 100+ hour discharge, it is aimed squarely at multi-day grid storage; Form Energy is the leading commercial developer.
An iron-air battery stores energy using the reversible oxidation and reduction of iron, in effect a controlled cycle of rusting and un-rusting of iron material (Fe + H2O reversibly forms FeO + H2). On discharge the iron oxidizes while oxygen from the air is reduced; on charge the process reverses, regenerating metallic iron from iron oxide. The appeal is the anode material: iron and iron oxide are abundant, non-toxic, inexpensive, and non-flammable, with existing recycling infrastructure, making the chemistry far cheaper and safer per kWh than lithium-based cells. Theoretical specific energy is high (over 1,400 Wh/kg including oxygen) but practical energy density per unit volume is poor, so cells are much larger than lithium-ion for the same stored energy. Conversion efficiency is also lower, and a known durability challenge is sintering and pulverization of the iron powder over many cycles. Those trade-offs make iron-air a poor fit for portable use but a strong candidate for long-duration energy storage on the grid, with developers targeting 100 or more hours of discharge, enough to store days of energy rather than minutes. The leading commercial developer is Form Energy. Iron-air is one of the electrochemical options in Grid Energy Storage, complementing the short-duration Lithium-Ion Battery and the Flow Battery for cases where you can fill a field and want very cheap, very long storage.