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%.

Energy Environ. Sci., 2015,8, 3000-3005
DOI: 10.1039/C5EE02274F

http://pubs.rsc.org/en/content/articlepdf/2015/ee/c5ee02274f