Investigation of Moisture‐Induced Degradation Mechanisms and a Reviving Strategy for Sulfide‐Based Solid Electrolytes in All‐Solid‐State Composite Cathode

ABSTRACT Sulfide‐based argyrodite Li 6 PS 5 Cl (LPSC) is one of the most promising solid electrolytes for high‐energy all‐solid‐state batteries (ASSBs) owing to its high ionic conductivity and favorable mechanical deformability. However, even trace moisture exposure under dry room conditions induces rapid and irreversible degradation, severely limiting scalable manufacturing. Beyond the widely recognized loss of ionic conductivity, the chemical consequences of moisture exposure on cathode remain poorly understood. Here, using synchrotron‐based transmission X‐ray microscopy combined with hard and tender X‐ray absorption spectroscopy, and high‐resolution X‐ray diffraction, we reveal that moisture exposure transforms LPSC into chemically reductive phases. These degradation products actively extract lattice oxygen from Ni‐rich NMC cathodes, inducing Ni reduction and cation disorder, and trigger irreversible surface reconstruction into spinel‐like phases. Importantly, we demonstrate a simple and process‐compatible regeneration strategy: a brief annealing treatment followed by controlled oxygen exposure selectively quenching reductive surface phases. This regenerated LPSC recovers 81% of its original ionic conductivity and restores cathode interfacial stability, enabling ASSBs to achieve 97% of the discharge capacity of the pristine LPSC and 95% capacity retention over 200 cycles. These results highlight moisture degradation of LPSC as a chemically driven interfacial failure mechanism and provide practical guidelines for mitigating degradation during manufacturing.