Formation Process of Na₃SbS₄ Solid Electrolyte via a Liquid-Phase Reaction Using Acetonitrile

Antimony-based sulfide solid electrolyte Na₃SbS₄ with good moisture stability is attractive for practical application in all-solid-state sodium batteries. Liquid-phase reaction is a facile and widely-used strategy, but the specific reactions in synthetic process and related mechanisms are still unclear. Herein, the reaction processes of synthesizing Na₃SbS₄ by liquid-phase reaction of Na₂S, Sb₂S₃ and S in acetonitrile (ACN) are extensively investigated. A colorless supernatant and dark green precipitate are obtained after reacting for 24 h, the raw materials are negligible in supernatant. More importantly, the post-treatment temperature is investigated and the tetragonal Na₃SbS₄ (t-Na₃SbS₄) with high ionic conductivity (0.46 mS cm⁻¹ at 25 °C) is obtained after heating at 450 °C for 3 h. Remarkably, the direct stacking of electrolyte is developed and t-Na₃SbS₄ is further prepared into a free-standing sheet-type solid electrolyte membrane using polytetrafluoroethylene (PTFE) fibrillation, then an ionic conductivity is achieved (0.17 mS cm⁻¹ at 25 °C).