Iron Sulfide Na2FeS2 as Positive Electrode Material with High Capacity and Reversibility Derived from Anion–Cation Redox in All‐Solid‐State Sodium Batteries

It is desirable for secondary batteries to have high capacities and long lifetimes. This paper reports the use of Na2 FeS2 with a specific structure consisting of edge-shared and chained FeS4 as the host structure and as a high-capacity active electrode material. An all-solid-state sodium cell that uses Na2 FeS2 exhibits a high capacity of 320 mAh g-1 , which is close to the theoretical two-electron reaction capacity of 323 mAh g-1 , and operates reversibly for 300 cycles. The excellent electrochemical properties of all-solid-state sodium cells are derived from the anion-cation redox and rigid host structure during charging/discharging. In addition to the initial one-electron reaction of Nax FeS2 (1 ≤ x ≤ 2) activated Fe2+ /Fe3+ redox as the main redox center, the reversible sulfur redox further contributes to the high capacity. Although the additional sulfur redox affects the irreversible crystallographic changes, stable and reversible redox reactions are observed without capacity fading, owing to the local maintenance of the chained FeS4 in the host structure. Sodium iron sulfide Na2 FeS2 , which combines low-cost elements, is one of the candidates that can meet the high requirements of practical applications.