Correlative Operando X‐Ray Imaging and Diffraction Revealing Mechanical Failure Pathways in Lithium Metal Sulfide‐Based All‐Solid‐State Batteries

All‐solid‐state batteries (ASSBs) employing sulfide solid electrolytes and lithium metal anodes offer a promising route toward high energy density and improved safety. However, their practical implementation is hindered by coupled electro‐chemo‐mechanical interfacial instabilities, which evolve across multiple length scales and remain poorly understood under operating conditions. Here, we present a multimodal operando characterization approach combining X‐ray computed tomography (XCT), X‐ray diffraction (XRD), and stack pressure monitoring to elucidate degradation mechanisms in LiNi 0.8 Co 0.1 Mn 0.1 O 2 (NCM811) | Li 6 PS 5 Cl (LPSC) | Li cells. By correlating particle‐level morphology with lattice‐scale structural and strain evolution, this approach provides a unified multiscale description of interfacial degradation. Simultaneous operando XCT and XRD measurements reveal reversible ∼6% volume changes in NCM811 particles during (de‐)lithiation, while spatially resolved XRD uncovers heterogeneous lithiation and depth‐dependent electrochemical activity across the cathode. Partial relaxation of interfacial microstrain is linked to oxidative decomposition of LPSC. At the LPSC|Li interface, operando XCT visualizes crack formation, lithium infiltration, and progressive contact loss caused by plating‐induced stress, ultimately leading to mechanical disconnection and short‐circuiting at higher current densities. These mechanically induced degradation processes are partially reversible during lithium stripping. This multimodal operando framework provides critical insights into degradation pathways and design principles for mechanically robust sulfide‐based ASSBs.