A rechargeable iron-air battery capable of delivering electricity for 100 hours at system costs competitive with conventional power plants and at less than one-tenth the cost of lithium-ion has been developed by Massachusetts-based startup Form Energy.

In discharge mode, thousands of tiny iron pellets are exposed to the air, which makes them rust and form iron oxide. When the system is charged with an electric current, the oxygen is removed, and the material reverts back to its initial iron state.

Each battery module is filled with a water-based, non-flammable electrolyte, similar to the electrolyte used in AA batteries. Inside of the liquid electrolyte are stacks of 10 m to 20 m scale cells incorporated in the iron electrodes and air electrodes that facilitate the storage and discharge of electricity.

The technology is suitable for complementing large-scale lithium battery system performance alongside big lithium batteries. The iron-air battery is capable of mass deployment at very low cost, using extremely common materials that can be recycled at the end of its service life.

A commercial-scale 1 MW, grid-connected storage system capable of delivering its rated power continuously for 150 hours is planned for construction by 2023 in Minnesota.