Early Onset Degradation Mechanism in All Solid-State Batteries Revealed by Operando Photoelectron Spectroscopy

Solid-state batteries (SSBs) promise enhanced safety and energy density, yet their long-term stability is limited by poorly understood interfacial degradation. Here, we perform operando hard X-ray photoelectron spectroscopy (HAXPES) on device-relevant structures, enabling depth-resolved probing of electrodes during cycling to follow lithiation and interfacial degradation processes in TiS2|Li3YCl6 half cells. We identify a reduction process for Li3YCl6. Concurrently, oxygen-containing species are found to migrate toward the cathode current collector, where they react with TiS2 to form an amorphous TiOx layer that contributes to capacity fade during extended cycling. Depth-dependent measurements confirm that oxygen enrichment is localized near the surface, establishing a kinetically driven degradation pathway linked to electron injection during discharge. These insights reveal how electrochemically induced migration of oxygen-containing species and interfacial redox can drive degradation in SSBs, providing a mechanistic basis for strategies to improve interfacial stability and cycling performance.