The Working Principle of a Li2CO3/LiNbO3 Coating on NCM for Thiophosphate-Based All-Solid-State Batteries

Large-scale industrial application of all-solid-state-batteries (ASSBs) is currently hindered by numerous problems. Regarding thiophosphate-based ASSBs, interfacial reactions with the solid electrolyte are considered a major reason for capacity fading. On the positive electrode side, cathode active material coating addresses these issues and improves the ASSB performance. Yet, the working principle of the coating often remains unclear, and protection concepts on the way to long-term stable ASSBs remain empirical. In this work, we characterize the influence of a Li2CO3/LiNbO3 cathode active material coating on the battery performance and cathode degradation reactions of a Li4Ti5O12/Li6PS5Cl/Super C65|Li6PS5Cl|LiNi0.6Co0.2Mn0.2O2/Li6PS5Cl/Super C65 cell. The coating microstructure is characterized comprehensively using a combination of focused ion beam scanning electron microscopy (FIB-SEM), X-ray photoelectron spectroscopy (XPS), and time-of-flight secondary ion mass spectrometry (ToF-SIMS). Based on this knowledge, we demonstrate and discuss the positive effect of the coating on the ASSB performance. Finally, we present an in-depth post-mortem analysis of composite cathodes by combining XPS depth profiling with ToF-SIMS. The Li2CO3/LiNbO3 coating suppresses the interfacial reaction at the cathode active material/solid electrolyte interface, in particular, the formation of oxygenated phosphorous and sulfur compounds such as phosphates and sulfates/sulfites, leading to a significantly enhanced ASSB performance.