In situ transmission electron microscopy for understanding materials and interfaces challenges in all-solid-state lithium batteries

All-solid-state lithium batteries (ASSLBs) using solid electrolytes have been identified as promising alternatives to conventional organic liquid electrolyte-dominated lithium-ion batteries (LIBs) owing to their high energy density, high safety, and wide temperature tolerance. To enhance their electrochemical performance, it is critical to fully understand the dynamic electrochemical processes and degradation mechanisms in electrodes, solid electrolytes and within the electrode/solid electrolyte interfaces of ASSLBs. The invention of in situ transmission electron microscopy (TEM) technique offers exceptional opportunities for understanding and directly tracking the dynamic processes of electrochemical reactions in ASSLBs using real-time imaging, diffraction, and spectroscopy techniques with high spatial resolution down to the atomic scale. To this end, the latest significant advances in the utilization of in situ TEM for a fundamental understanding of the materials and interface challenges in ASSLBs with a specific focus on real-time observations of reaction and degradation occurring in electrodes, solid electrolytes, and their interfaces are systematically summarized. The invaluable real-time information and scientific findings enabled by in situ TEM techniques are highlighted. Finally, the grand challenges and opportunities in using in situ TEM to reveal the degradation mechanisms of ASSLBs are discussed. This review is expected to provide guiding principles to address the critical challenges with existing electrode materials, solid electrolytes, and their interfaces in ASSLBs.