Abstract
Argyrodite Li₆PS₅Cl (LPSC) possesses both high Li-ion conductivity ( ~ 10 mS cm⁻¹ at room temperature) and mechanical softness, positioning it as a flagship solid electrolyte for next-generation all-solid-state batteries (ASSBs). However, even trace amounts of moisture in industrial dry rooms (dew-point -60 to -70 °C) rapidly degrade its surface, diminishing ionic transport and impeding scalable processes. Here, we elucidate the moisture-triggered surface degradation mechanism of LPSC under dry-room conditions by combining first-principles calculations with depth-profiling X-ray photoelectron spectroscopy analysis. The combined analysis reveals a five-step sequence: (i) H2O adsorptions on S-rich surface, (ii) P-S bond weakening followed by thermodynamically favoured S-O substitutions, (iii) rotation of the O-substituted PS₄ tetrahedra that drives O migration into subsurface layers, (iv) formation of an O-rich Li₆PO₅Cl-like surface, and (v) volume-shrinking phase separation into LiCl, Li₃PO₄, Li₂SO₄, LiOH, and Li2CO3. The resulting porous, O-enriched layer fails to passivate the electrolyte, causing a 36% drop in ionic conductivity within three days. These mechanistic insights highlight polyhedral-rigidity tuning and moisture-blocking surface chemistries as complementary strategies for stabilizing thiophosphate electrolytes during practical cell fabrication.