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

Abstract

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 <i>in situ</i> 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 LiNi_0.8Co_0.15Al_0.05O₂ (NCA) and 75Li₂S·25P₂S₅ (LPS) during cell operation using <i>operando</i> 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 PO_<i>x</i>^- and SO_<i>x</i>^- 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.