Na ₅ FeS ₄ as High‐Capacity Positive Electrode Active Material for All‐Solid‐State Sodium Batteries

Meeting the growing demand for energy storage systems requires the development of batteries that satisfy the requirements of high safety, low cost, and high energy density. All‐solid‐state sodium batteries with sulfide‐based active materials are a promising energy storage technology because they satisfy these requirements. In particular, sodium‐containing iron sulfides are attractive for positive electrode active materials because of their high capacities and ionic conductivities. In this study, Na ₅ FeS ₄ is investigated as a positive electrode active material for all‐solid‐state sodium batteries. Na ₅ FeS ₄ is synthesized by ambient‐pressure heat treatment using sodium polysulfides. All‐solid‐state cells using Na ₅ FeS ₄ as a positive electrode active material exhibit a high reversible capacity of 480 mAh g ^–1 at ≈25 °C. The active material mainly became amorphous during the initial charge, and this amorphous state contribute to reversible charge–discharge with high capacity. This study advances the development of low‐cost and high‐capacity sulfide positive electrode active materials for all‐solid‐state sodium batteries suitable for large‐scale energy storage.