In situ observation of lithium metal plating in a 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 to overcome the limitations of conventional lithium ion batteries (LIBs), particularly safety problems. Development of operando or in situ microscopy and spectroscopy measurement techniques is crucial for real-time observations of the morphological and chemical phenomena occurring in ASSBs during charge/discharge cycles. In this study, we develop in situ Auger electron microscopy and spectroscopy techniques for real-time investigations of the various phenomena occurring in ASSBs based on 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. 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 irreversible growth of Li metal is observed. In contrast, the volume expansion of the Li metal anode is negligible at the unpressurised sample position, resulting 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 industry, and the novel in situ measurement technique can be a useful tool for the real-time analysis of various phenomena in ASSBs.