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

Abstract

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 TiS₂F|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.