In situ Observation of Lithium Metal Plating into Sulfur-based Solid Electrolyte for All-solid-state Batteries
All-solid-state batteries (ASSBs) based on solid-state electrolytes (SSEs) are receiving significant interest as promising candidates for overcoming the limitations of conventional lithium ion batteries (LIBs), particularly safety problems. For real-time observations of the morphological and chemical phenomena that occur in ASSBs during charge/discharge cycles, the development of operando or in situ microscopic and spectroscopic measurement techniques is of significance. In this study, we develop in situ Auger electron microscopic and spectroscopic techniques for real-time investigations of the various phenomena occurring in ASSBs, including pelletized argyrodite (Li6PS5Cl; LiPS) SSEs. Volume expansion/contraction of the Li metal anode owing to the injection/extraction of Li is observed during the charge/discharge process. The Li migration to the Li metal anode is strongly dependent on the pressing force applied to the sample during the charge/discharge cycles. At the sample position under excessively high local pressure, a superabundant amount of Li metal migrates to the anode, and the irreversible growth of Li metal is observed. In contrast, the volume expansion of the Li metal anode is negligible at the unpressurised sample position, which can result in poor capacity of the ASSBs. This result demonstrates that homogeneous pressurization of the sample is a critical factor for a stable charge/discharge process. The results provided herein can guide the development of practical ASSBs in the industries and the novel in situ measurement technique can be a useful tool for the real-time analysis of various phenomena in ASSBs.