Universal Solution Synthesis of Sulfide Solid Electrolytes Using Alkahest for All‐Solid‐State Batteries

Abstract The wet‐chemical processability of sulfide solid electrolytes (SEs) provides intriguing opportunities for all‐solid‐state batteries. Thus far, sulfide SEs are wet‐prepared either from solid precursors suspended in solvents (suspension synthesis) or from homogeneous solutions using SEs (solution process) with restricted composition spaces. Here, a universal solution synthesis method for preparing sulfide SEs from precursors, not only Li₂S, P₂S₅, LiCl, and Na₂S, but also metal sulfides (e.g., GeS₂ and SnS₂), fully dissolved in an alkahest: a mixture solvent of 1,2‐ethylenediamine (EDA) and 1,2‐ethanedithiol (EDT) (or ethanethiol). Raman spectroscopy and theoretical calculations reveal that the exceptional dissolving power of EDA–EDT toward GeS₂ is due to the nucleophilicity of the thiolate anions that is strong enough to dissociate the GeS bonds. Solution‐synthesized Li₁₀GeP₂S₁₂, Li₆PS₅Cl, and Na₁₁Sn₂PS₁₂ exhibit high ionic conductivities (0.74, 1.3, and 0.10 mS cm⁻¹ at 30 °C, respectively), and their application for all‐solid‐state batteries is successfully demonstrated.