Off‐Stoichiometry in Alluaudite‐Type Sodium Iron Sulfate Na2+2xFe2−x(SO4)3 as an Advanced Sodium Battery Cathode Material

Abstract An alluaudite‐type sodium iron sulfate has recently been discovered as a 3.8 V cathode material for low‐cost, high‐power, and efficient sodium‐ion batteries. To optimize the composition of the alluaudite phase and to explore further compounds, we have carefully surveyed the Na 2 SO 4 ‐FeSO 4 binary system. Solid‐state reactions at a moderate temperature of 623 K produce two stable phases: 1) vanthoffite‐structured Na 6 Fe(SO 4 ) 4 and 2) alluaudite‐type Na 2+2 x Fe 2− x (SO 4 ) 3 with a certain non‐stoichiometry. The possible compositional and structural flexibilities demonstrated in this work inspire further improvement of the alluaudite‐type sodium metal sulfates for advanced sodium‐ion batteries.