Interfacial Chemistry-Driven Reaction Dynamics and Resultant Microstructural Evolution in All-Solid-State Batteries

Achieving a comprehensive understanding of battery systems necessitates multi-length scale analysis, spanning from the atomic to macro scale, to grasp the complex interplay of phenomena influencing performance. However, studies to understand these phenomena in all-solid-state batteries (ASSBs) poses significant challenges due to the complex microstructural evolution involved, such as the pore formation and contact loss resulting from cathode material breathing, chemical degradation at interfaces, and their interplay. Herein, we investigate the impact of chemical degradation at the cathode/solid-electrolyte interface on the cathode particle reaction behavior and microstructural evolution in composite cathodes of sulfide-based ASSBs, using a well-defined model system incorporating a non-decomposable coating layer that solely alters the interfacial chemical reactivity. By using lithium difluorophosphate (LiDFP) to suppress chemical degradation, we observed that this suppression enhances the reaction uniformity among particles and homogenizes mechanical degradation, albeit increasing pore formation and tortuosity. In addition, unbridled chemical degradation induces significant reaction heterogeneity and non-uniform mechanical degradation, with fewer pores and lower tortuosity. These findings complement the understanding of mechanical degradation, which is traditionally described using the metrics of contact loss and tortuosity, and underscore the critical role of coating layers in promoting lithium conduction by maintaining contact with the cathode surface. This research not only offers novel insights into the intertwined chemo-mechanical degradation and the functionality of the coating material but also paves the way for the multi-length scale understanding required for the development of advanced ASSBs.