Self‐Closing of Cracks Generated in Microstructure‐Controlled 400 μm‐Thick Composite Cathodes for All‐Solid‐State Batteries: Observed by <i>In Situ</i> Scanning Electron Microscopy with Energy‐Dispersive X‐Ray Spectroscopy

All‐solid‐state batteries (ASSBs) are a promising next‐generation secondary battery technology. To achieve high energy and power densities, the thickness of composite electrodes must be increased. The microstructure and mechanical properties of the composites must be carefully controlled to ensure sufficient contact between particles. The 400 μm‐thick LiCoO 2 ‐Li 10.35 Ge 1.35 P 1.65 S 12 (LCO–LGPS) composites are fabricated using LGPS with various particle sizes. The composites using small‐sized LGPS particles exhibited higher capacity and retention than those using large‐sized particles. In situ cross‐sectional scanning electron microscopy with energy‐dispersive X‐ray spectroscopy reveals that the cracks generated during charging demonstrated self‐closing during discharge in the composite with small‐sized LGPS regardless of the generated locations, leading to high‐capacity retention. However, this self‐closing is not observed in the composite using large‐sized LGPS. The self‐closing behavior depends on the microstructure and mechanical properties of ASSBs. Furthermore, this self‐closing finding provides new strategies for designing the microstructure and mechanical properties of ASSBs.