Visualizing a Li‐Depleted Amorphous Cathode–Electrolyte Interphase in Sulfide Solid‐State Batteries Using In Situ Cryogenic Electron Microscopy

ABSTRACT Electrochemical degradation at the cathode/solid‐electrolyte interface critically limits the performance of sulfide‐based solid‐state batteries; however, its nanoscale origin remains unclear. Here, we directly visualize the evolution of Li distribution and crystal structure at the LiNi 0.5 Co 0.2 Mn 0.3 O 2 /Li 6 PS 5 Cl interface during charging using in situ scanning transmission electron microscopy, combined with electron energy‐loss spectroscopy and energy‐filtered nanobeam electron diffraction. A potential‐controlled sample‐preparation protocol is developed to preserve the native interphase structure during sample preparation, and low‐dose‐rate cryogenic electron microscopy minimizes electron‐beam‐induced damage. At the uncoated interface, charging induced the formation of a ∼50‐nm‐thick Li‐depleted interphase. Within ∼20 nm of the interface, the Li concentration decreased to below x = 4 in Li x PS 5 Cl, accompanied by amorphization of the solid electrolyte. Machine‐learning‐potential molecular dynamics simulations reveal that Li depletion destabilizes the argyrodite Li 6 PS 5 Cl framework and increases the Li‐ion migration barrier, providing microscopic insight into the increase in interfacial resistance. Conversely, LiNbO y ‐coated interfaces exhibited neither pronounced Li depletion nor significant amorphization, even with coating thicknesses as small as 5 nm, thereby elucidating the buffering mechanism of LiNbO y at the cathode/sulfide‐solid‐electrolyte interface. These results establish Li‐depletion‐induced structural disorder as an important transport‐limiting mechanism at cathode interfaces employing sulfide solid electrolytes, providing a framework for interface engineering.