Preparation and characterization of Na6FeS4 and Na7Fe2S6 as positive electrode active materials for all-solid-state sodium batteries

All-solid-state sodium batteries are gaining attention because they are safe, use abundant sodium resources, and are promising candidates for future energy storage applications.Sodium iron sulfides are suitable positive electrode active materials because of their high theoretical capacity and use of low-cost elements.In this study, the electrode properties of Na 6 FeS 4 and Na 7 Fe 2 S 6 in all-solid-state cells were evaluated in comparison with those of Na 2 FeS 2 and Na 5 FeS 4 .Na 6 FeS 4 , Na 5 FeS 4 , Na 7 Fe 2 S 6 , and Na 2 FeS 2 were synthesized by ambient-pressure heat treatment using sodium polysulfides.The all-solid-state cells using Na 6 FeS 4 and Na 7 Fe 2 S 6 as positive electrode active materials exhibited initial charge capacities close to their theoretical capacities and operated reversibly with high capacities of over 400 mAh g -1 , exceeding the capacity of Na 2 FeS 2 .This study contributes to the development of low-cost and highcapacity positive electrode active materials for all-solid-state sodium batteries.