HPO32− as a building unit for sodium-ion battery cathodes: 3.1 V operation of Na2−xFe(HPO3)2 (0 < x < 1)

We report Na₂Fe(HPO₃)₂ as the first HPO₃²⁻-based iron(ii) cathode material for sodium-ion batteries, which delivers a reversible capacity of approximately 100 mA h g⁻¹ at an average reaction voltage of 3.1 V.