# Metal-air batteries

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

Source page: https://www.materialinsights.org/energy/e_metal_air/

Storage · Electrochemical · No. 76

- Storage
- Duration: Days
 Storage · Electrochemical · Foundation · Energy

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 pellets**Cheap, abundant and fully recyclable metal
2. 2 **Discharge**Iron rusts, drawing oxygen from air
3. 3 **Charge**Electricity turns rust back into iron
4. 4 **100-hour block**15 MW plant holds 1,500 MWh
5. 5 **Grid backstop**Covers multi-day wind and solar lulls

##### Go deeper pick a tab to open it in full

## Other sections

- [Overview](https://www.materialinsights.org/energy/e_metal_air/overview/index.md)
- [Correlation](https://www.materialinsights.org/energy/e_metal_air/correlation/index.md)
- [Science and technology](https://www.materialinsights.org/energy/e_metal_air/sg-sci/index.md)
- [Engineering and performance](https://www.materialinsights.org/energy/e_metal_air/sg-eng/index.md)
- [Economics](https://www.materialinsights.org/energy/e_metal_air/sg-econ/index.md)
- [Impact and additionality](https://www.materialinsights.org/energy/e_metal_air/sg-impact/index.md)
- [What to track](https://www.materialinsights.org/energy/e_metal_air/tracking/index.md)
- [Relationships](https://www.materialinsights.org/energy/e_metal_air/links/index.md)