Eldfellite, NaFe(SO₄)₂: an intercalation cathode host for low-cost Na-ion batteries

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%.