Advanced Characterization Techniques for Sulfide‐Based Solid‐State Lithium Batteries

Abstract Solid‐state lithium batteries with sulfide solid electrolytes have attracted extensive attention as next‐generation secondary batteries with high energy and power densities because sulfide solid electrolytes possess several advantages over liquid electrolytes, such as their nonfluidity, single‐ion conductivity, and low flammability. However, they still face barriers that limit their practical application, such as the (electro)chemical decomposition of solid electrolytes, mechanical degradation at interfaces, dendrite growth of lithium metal, and slow lithium diffusion in active materials. These limitations are dynamic phenomena that occur during charge and discharge reactions. The dynamic behavior inside a battery must be understood to rationally design high‐performance solid‐state lithium batteries. For this purpose, operando and in situ analyses, which analyze devices under working conditions, are promising characterization techniques. This review focuses primarily on the four issues mentioned above for bulk‐type solid‐state lithium batteries with sulfide solid electrolytes. The current status and future prospects of advanced characterization techniques, such as X‐ray photoelectron spectroscopy, time‐of‐flight secondary ion mass spectrometry, transmission electron microscopy, and X‐ray computed tomography, for addressing these issues are reviewed.