Formation Mechanism of Thiophosphate Anions in the Liquid-Phase Synthesis of Sulfide Solid Electrolytes Using Polar Aprotic Solvents

Preparation of sulfide-based Li-ion conductive solid electrolytes by a liquid-phase process has been proposed as an effective route for industrial scaling-up. However, reaction mechanisms in the liquid-phase synthesis or the role of solvents in the reactions is not yet well understood. Here, the reaction between Li2S and P2S5 in the composition of 50:50 mol % mediated by a polar aprotic solvent (acetonitrile) was investigated. The study of the crystal and local structure of the 50Li2S·50P2S5 sample revealed large chemical changes upon crystallization, highlighting the initial formation of a polymer-like (PS3–)n intermediate. On the basis of the (PS3–)n intermediate, the reaction pathways for the formation of P2S62–, PS43–, and P2S74– anions are elucidated. These findings give a deeper insight into the reaction mechanisms governing the liquid-phase synthesis of sulfide solid electrolytes and provide more specific criteria for designing novel materials with superior characteristics through this liquid-phase approach.