Influence of Halogens on PS₄^3- Speciation and Formation Pathways in Acetonitrile

Liquid-phase synthesis of Li₂S-P₂S₅ derived sulfide solid electrolytes often proceeds through suspended precursor particles and solvated thiophosphate intermediates, yet analogous intermediate formation during argyrodite Li₆PS₅X synthesis remains poorly understood. Here, we investigate halide-modulated thiophosphate speciation in acetonitrile using minute-resolved in situ Raman spectroscopy and ex situ XRD. The PS₄^3--associated Raman feature near 420 cm^-1 develops progressively faster across the series I^- < Br^- < Cl^-, while the ACN-associated band near 390 cm^-1 evolves concurrently with thiophosphate formation, consistent with changes in the local ACN coordination environment. XRD of recovered powders shows ACN complexed LPS intermediates in the halide-free reaction, predominantly amorphous material for LiI, partial argyrodite formation for LiBr, and near-complete Li₆PS₅Cl formation for LiCl. These results suggest that halide identity influences both thiophosphate speciation and phase selection through coupled solubility, interfacial reactions, and Li⁺ coordination effects. The findings provide a mechanistic basis for using halide chemistry to modulate low-temperature liquid-phase synthesis of sulfide solid electrolytes.