Influence of Halogens on PS ₄ ³– Speciation and Formation Pathways in Acetonitrile

High Resolution Image Download MS PowerPoint Slide Liquid-phase synthesis of Li 2 S–P 2 S 5 derived sulfide solid electrolytes often proceeds through suspended precursor particles and solvated thiophosphate intermediates, yet analogous intermediate formation during argyrodite Li 6 PS 5 X 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 PS 4 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 Li 6 PS 5 Cl 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.