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 Na₂FeS₂ 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 Na₂FeS₂ 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 Na_xFeS₂ (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 Na₂FeS₂, which combines low-cost elements, is one of the candidates that can meet the high requirements of practical applications.