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. In situ X-ray diffraction and ex situ57Fe Mössbauer spectroscopy clarify reversible (de)sodiation associated with the Fe3+/Fe2+ redox reaction.