Off-Stoichiometry in Alluaudite-Type Sodium Iron Sulfate Na_2+2xFe_2-x(SO₄)₃ as an Advanced Sodium Battery Cathode Material

An alluaudite-type sodium iron sulfate has recently been discovered as a 3.8V 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 Na₂SO₄-FeSO₄ binary system. Solid-state reactions at a moderate temperature of 623K produce two stable phases: 1)vanthoffite-structured Na₆Fe(SO₄)₄ and 2)alluaudite-type Na_2+2xFe_2-x(SO₄)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.