A rechargeable calcium-oxygen battery that operates at room temperature.

Calcium-oxygen (Ca-O 2 ) batteries can theoretically afford high capacity by the reduction of O 2 to calcium oxide compounds (CaO x ) at low cost 1-5 . Yet, a rechargeable Ca-O 2 battery that operates at room temperature has not been achieved because the CaO x /O 2 chemistry typically involves inert discharge products and few electrolytes can accommodate both a highly reductive Ca metal anode and O 2 . Here we report a Ca-O 2 battery that is rechargeable for 700 cycles at room temperature. Our battery relies on a highly reversible two-electron redox to form chemically reactive calcium peroxide (CaO 2 ) as the discharge product. Using a durable ionic liquid-based electrolyte, this two-electron reaction is enabled by the facilitated Ca plating-stripping in the Ca metal anode at room temperature and improved CaO 2 /O 2 redox in the air cathode. We show the proposed Ca-O 2 battery is stable in air and can be made into flexible fibres that are weaved into textile batteries for next-generation wearable systems.