The mineral eldfellite, NaFe(SO4)2, is characterized as a potential cathode for a Na-ion battery that is fabricated in charged state; its 3 V discharge versus sodium for reversible Na+ intercalation is shown to have a better capacity, but lower insertion rate than Li+ intercalation. The theoretical specific capacity for Na+ insertion is 99 mA h g−1. After 80 cycles at 0.1C versus a Na anode, the specific capacity was 78 mA h g−1 with a coulomb efficiency approaching 100%.
