Visualizing Lithium Distribution and Degradation of Composite Electrodes in Sulfide-based All-Solid-State Batteries Using Operando Time-of-Flight Secondary Ion Mass Spectrometry.

Understanding the electrochemical reactions taking place in composite electrodes during cell cycling is essential for improving the performance of all-solid-state batteries. However, comprehensive in situ monitoring of Li distribution, along with measurement of the evolution of degradation, is challenging because of the limitations of the characterization techniques commonly used. This study demonstrates the observation of Li distribution and degradation in composite cathodes consisting of LiNi0.8Co0.15Al0.05O2 (NCA) and 75Li2S·25P2S5 (LPS) during cell operation using operando time-of-flight secondary ion mass spectrometry. The evolution of the nonuniform reaction of NCA particles during charge and discharge cycles was successfully visualized by mapping fragments containing Li. Furthermore, degradation of the NCA/LPS interface was investigated by mapping POx- and SOx- fragments, which are related to the solid electrolyte interphase. We found that during the charge-discharge cycle and application of a high-voltage stress to the composite electrodes, the PO2- and PO3- fragments increased monotonically, whereas the SO3- fragment exhibited a reversible increase-decrease behavior, implying the existence of a redox-active component at the NCA/LPS interface. The demonstrated technique provides insights into both the optimized structures of composite electrodes and the underlying mechanisms of interfacial degradation at active material/solid electrolyte interfaces.