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 LiNbO 3 . However, cell performance reportedly degrades under harsh environments even with a LiNbO 3 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 Li 6 PS 5 Cl/LiNbO 3 -coated LiNi 0.5 Co 0.2 Mn 0.3 O 2 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 Li 2 S appeared simultaneously. Finally, XAS analysis confirmed the decrease in the PS 4 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.