Metal-air batteries

Batteries that breathe oxygen from air; iron-air targets multi-day grid storage.

Storage · Electrochemical · No. 76

• Storage
• Duration: Days

One start-up stores electricity by letting iron rust and un-rust, aiming to power grids for 100 hours straight.

Chart: Illustration: Metal-air batteries

Where it comes from

Chart: Map of where it comes from

Half the reactant is oxygen from the air, free and weightless, while the metal is iron, zinc or aluminium; iron-air's theoretical limit is about 1,200 Wh/kg.

Three things to know

1. Iron-air returns only about 35 to 40% of the electricity put in, against 85 to 90% for lithium-ion.
2. Aluminium-air has a theoretical 8,100 Wh/kg, but its plates are swapped and sent back to the smelter rather than recharged.
3. In February 2026 Xcel Energy and Google announced a 30 GWh iron-air system for a Minnesota data centre.

The journey

1. 1 Iron pelletsCheap, abundant and fully recyclable metal
2. 2 DischargeIron rusts, drawing oxygen from air
3. 3 ChargeElectricity turns rust back into iron
4. 4 100-hour block15 MW plant holds 1,500 MWh
5. 5 Grid backstopCovers multi-day wind and solar lulls

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