Microscopic Degradation Mechanism of Argyrodite-Type Sulfide at the Solid Electrolyte–Cathode Interface

Interfacial engineering of sulfide-based solid electrolyte/lithium-transition-metal oxide active materials in all-solid-state battery cathodes is vital for cell performance parameters, such as high-rate charge/discharge, long lifetime, and wide temperature range. A typical interfacial engineering method is the surface coating of the cathode active material with a buffer layer, such as LiNbO3. However, cell performance reportedly degrades under harsh environments even with a LiNbO3 coating, such as high temperatures and high cathode potentials. Therefore, we investigated the interfacial degradation mechanism focusing on the solid electrolyte side for half cells employing the cathode mixture of argyrodite-type Li6PS5Cl/LiNbO3-coated LiNi0.5Co0.2Mn0.3O2 exposed at 60 °C and 4.25 and 4.55 V vs Li/Li+ using transmission electron microscopy/electron diffraction (TEM/ED) and X-ray absorption spectroscopy (XAS). The TEM/ED results indicated that the ED pattern of the argyrodite structure disappeared and changed to an amorphous phase as the cells degraded. Moreover, the crystal phases of LiCl and Li2S appeared simultaneously. Finally, XAS analysis confirmed the decrease in the PS4 units of the argyrodite structure and the increase in local P-S-P domains with delithiation from the interfacial solid electrolyte, corresponding to the TEM/ED results. In addition, the formation of P-O bonds was confirmed during degradation at higher cathode potentials, such as 4.55 V vs Li/Li+. These results indicate that the degradation of this interfacial region determines the cell performance.