Mechanochemical Synthesis of Pyrite Ni_1−xFe_xS₂ Electrode for All-solid-state Sodium Battery (Supporting Information)

All-solid-state sodium secondary batteries are expected to be low-cost, next-generation batteries. NiS₂ and FeS₂ are potential candidates as positive electrode materials owing to their high theoretical capacities. However, it is difficult to achieve sufficient capacity with bulk FeS₂. In this study, pyrite Ni_1−xFe_xS₂ (x = 0, 0.3, 0.5, 0.7, 0.9, and 1) electrodes are prepared by a mechanochemical process. The all-solid-state sodium cells with Ni_1−xFe_xS₂ show higher discharge–charge potentials than those with NiS₂, and higher capacities than those with FeS₂. In addition, Ni_1−xFe_xS₂ exhibits a higher rate performance than those of NiS₂ and FeS₂. The all-solid-state cells using Ni_1−xFe_xS₂ (x = 0.3, 0.5, and 0.7) are discharged and charged with a high capacity of approximately 390 mAh g⁻¹, without significant capacity fading for at least 30 cycles. The solid-solution formation of NiS₂ and FeS₂ results in lower material cost, higher rate performance, higher discharge–charge potential than those of NiS₂, and higher capacity than that of FeS₂. Pyrite Ni_1−xFe_xS₂ is a promising positive electrode material for all-solid-state sodium secondary batteries.