<i>Operando</i> Micro- and Nano-Computed Tomography Reveals Silicon–Electrolyte Interface Dynamics and Anisotropic Contact Loss in All-Solid-State Batteries

All-solid-state batteries (ASSBs) with silicon anodes offer high energy density and mitigate issues such as continuous solid-electrolyte interphase (SEI) formation in lithium-ion batteries with liquid electrolytes. However, the evolution of the mechanical contact interface between silicon (Si) and the rigid solid electrolyte during cycling remains poorly understood. This study utilized <i>operando</i> synchrotron X-ray micro-computed tomography (micro-CT) and nano-computed tomography (nano-CT) to achieve high-resolution, 3D visualization of the silicon-electrolyte interface during lithiation and delithiation. Micro-CT revealed that silicon particles retain partial contact with the solid electrolyte as they delithiate and shrink to form shell voids, preserving ionic conduction pathways. High-resolution nano-CT further revealed a thin, previously undetectable solid electrolyte layer that adheres to the surfaces of the silicon particles and helps maintain these contact points. Additionally, interfacial delamination of the silicon was found to be highly anisotropic, initiating from sides that were laterally unconstrained due to uneven mechanical pressure and reaction inhomogeneity. Meanwhile, the vertically compressed interface remained largely intact. These findings elucidate the morphological evolution of the Si/electrolyte interface in ASSBs and demonstrate that continuous ion transport can be partially maintained despite significant volume changes.