Decoupling Thermodynamic and Kinetic Moisture Instability Reveals a Critical Humidity Threshold for Sodium Thiophosphate Solid Electrolyte

Sodium sulfide solid electrolytes are widely regarded as highly moisture‐sensitive, requiring ultralow‐humidity (typically 15–100 ppm) processing that imposes a major cost barrier to solid‐state battery manufacturing. Yet the actual moisture‐stability limit of these materials remains poorly quantified, and the mechanistic origin of degradation, whether thermodynamic or kinetic, is unresolved. Here, using sodium thiophosphate, Na 3 PS 4 , as a model system, we establish a quantitative moisture‐stability framework that decouples thermodynamic and kinetic contributions to apparent instability. Controlled exposure studies across 13%–38% relative humidity, correlated with H 2 S evolution, mass uptake, structural evolution, and ionic conductivity recovery, reveal a critical humidity threshold near 15% relative humidity, below which Na 3 PS 4 undergoes reversible hydration with minimal hydrolysis on the surface, fully recovering its ionic conductivity at 13% relative humidity after brief heat treatment. Density functional theory shows that hydration is thermodynamically favorable (ΔG = −0.76 eV), whereas hydrolysis is kinetically gated by a 1.78 eV activation barrier, rationalizing the unexpected tolerance. By defining a practical humidity‐tolerance window and a reproducible benchmarking protocol, this work provides a transferable foundation for the rational design of moisture‐tolerant sulfide electrolytes toward substantially relaxed humidity requirements for all‐solid‐state battery manufacturing.