In situ DRIFTS analysis of the evolution of surface species over Li6PS5Cl solid state electrolyte during moisture-induced degradation and during heat treatment

Sulfide SSEs (solid-state electrolytes) with high ionic conductivity are attractive for developing high-safety all-solid-state batteries (ASSBs). However, the poor chemical stability of sulfide SSEs toward moisture is a significant problem. Though the decomposition products when exposed to moisture and a possible property recovery by heat treatment are reported, the evolution of surface species in related to moisture exposure and to heat treatment is not clearly identified. This study applies in situ DRIFTS (diffuse reflectance infrared Fourier-transformed spectroscopy) analysis to examine the evolution of the surface species over pristine Li₆PS₅Cl (LPSC), during moisture exposure, and during heat treatment, complementary with tools like XRD, Raman, etc. The observed surface impurities over pristine LPSC include LiCl·H₂O, LiOH·H₂O, S₃P-SH, PS_4-xO_x, SO_x and carbonate species. Moisture exposure leads to increasing accumulation of these species over LPSC and evolving hydrogen sulfide. A stepwise heat treatment up to 480 °C illustrates the sequential removal of hydrated water, the decomposition of carbonate, LiOH, and PS_4-xO_x, leaving species like LiCl, Li₂O, and PO₄ on the surface. The EIS results shows a gradual increase in the ionic conductivity of LPSC with increasing heating temperature, mainly owing to the decreasing surface layer impedance. This strongly suggests that the surface species govern the properties of LPSC. When using the pristine LPSC after heat treatment at 480 °C, the Li||pristine LPSCHT||Li symmetric cell demonstrates a decreased polarization and a much-enhanced cycle stability comparing to the Li||pristine LPSC||Li symmetric cell.