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

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 LiNi0.5Co0.2Mn0.3O2/Li6PS5Cl 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 LixPS5Cl, accompanied by amorphization of the solid electrolyte. Machine-learning-potential molecular dynamics simulations reveal that Li depletion destabilizes the argyrodite Li6PS5Cl framework and increases the Li-ion migration barrier, providing microscopic insight into the increase in interfacial resistance. Conversely, LiNbOy-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 LiNbOy 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.