Diffraction‐Enabled Operando Nanoscale Tracking of Li‐ion Dynamics of Solid Electrolyte and Inhomogeneous Diffusion in Composite Cathode

Abstract Li + diffusion has mostly been studied in cathode active materials (CAMs) in liquid batteries, whereas it remains rarely explored in solid‐state electrolytes (SSEs) and all‐solid‐state batteries. Herein, diffraction was established as an effective method for all‐solid‐state lithium batteries (ASSLBs) by focusing on SSE in composite cathodes. Operando synchrotron x‐ray diffraction presented diffraction angle shifts of certain Li 3 InCl 6 Bragg planes during the first ASSLB cycle due to lithiation/delithiation into its lattice, whose preferred Li + migration pathways were suggested by the partiality of these shifts, and the three‐phase evolution indicated the fundamental Li + diffusion kinetics factors. X‐ray nanodiffraction (XND) mapped nanoscale inhomogeneous Li + distributions and diffusion within individual Li 3 InCl 6 particles, revealing facilitated Li + conduction in high‐crystallinity regions and their role as pathways across SSE/CAM interfaces, and the most negatively strained regions were shown to be less susceptible to Li + insertion. XND from various electrochemical techniques inferred high transient charging rates to be the culprit of irreversible Li + diffusion instead of the overall charging depth. This study proved diffraction to be a potent tool to probe intricate Li + dynamics in ASSLBs and provide microscopic insights for optimal battery designs.