Degradation Analysis by X-ray Absorption Spectroscopy for LiNbO3 Coating of Sulfide-Based All-Solid-State Battery Cathode

The surface coating of cathode active material in all-solid-state batteries using sulfide-based solid electrolytes is well-known to be a fundamental technology, and LiNbO3 is one of the most representative materials. The half cells using the cathode mixture of Li6PS5Cl/LiNbO3-coated LiNi0.5Co0.2Mn0.3O2 were exposed in harsh conditions at 60 °C and 4.25-4.55 V vs Li/Li+ and analyzed by transmission electron microscope/energy dispersive X-ray spectroscopy (TEM/EDS) and X-ray absorption spectroscopy (XAS). TEM/EDS observation shows that Nb element derived from LiNbO3 coating had remained at the interface, which means that Nb element had not migrated to the solid electrolyte and active material. On the other hand, the XAS spectra of Nb L3-edge changed corresponding to cell performance degradation. From the comparison with the spectra of the reference materials of the Li-Nb-O system, the XAS spectral changes were assigned to the decomposition reaction which released Li and O from the LiNbO3 coating. The side reaction is presumed to cause to the oxidization deterioration of sulfide electrolyte at the interface of Li6PS5Cl/LiNbO3-coated LiNi0.5Co0.2Mn0.3O2.