Solid-state lithium-ion batteries have attracted much attention due to their high safety, high energy densities and other advantages. However, solid-state lithium-ion batteries cannot realize large-scale commercial use. There are key scientific and technical issues that have not been resolved, especially interface issues, such as high resistance and instability of the interface. The X-ray photoelectron spectroscopy (XPS), as an important surface analysis method, can perform qualitative and semi-quantitative chemical analysis of the interface, which makes XPS can be widely used to study the solid-state lithium-ion battery interfaces. In this paper, we review the recent research progress of solid-state lithium-ion battery interfaces by using XPS, and summarize and review the XPS experimental principle, experimental method, experimental results and their effects on interface performance. The XPS analysis methods for solid-state lithium-ion batteries include ex-situ XPS, in-situ XPS reflecting the real-time changes of the battery interface, and operando XPS based on the actual working conditions of the battery.

The ex-situ XPS can study oxide solid electrolyte interfaces, sulfide solid electrolyte interfaces and artificial solid electrolyte interface (SEI) layers to access information about the chemical composition of the interface, predict the performance of the interface, obtain the chemical distribution in space, and evaluate the chemical structure and irregularity of the interface. With ultraviolet photoemission spectroscopy (UPS) the interface work function, energy band bending and energy structure of the full battery can be obtained. In-situ XPS can effectively study the process of chemical reactions between the electrolyte and the electrode. The key prerequisite is the controllable in-situ construction of the electrolyte/electrode interface. In-situ XPS research can directly study the electrochemical changes of the interface. In-situ XPS/UPS can study the energy level alignment of solid-state lithium-ion batteries, indicating that a space charge layer is formed at the solid electrolyte interface, and the energy band bending occurs. The degree of energy band bending is reflected in the binding energy shifts of the related elements at the interface. The change of the energy structure in the deposition process can be determined by the binding energy shifts of the related elements at the interface and the change of the interface work function. Operando XPS performs XPS characterization at the same time under the working condition of the battery. Operando XPS can be combined with electrochemical characterization to observe the effects of interface reaction and solid electrolyte decomposition products on electrochemical performance, thereby determining the main components that affect electrochemical performance. It can also be combined with the ex-situ XPS to study the interface reaction mechanism and influencing factors. The information obtained includes the chemical states of elements after the interface reaction has occurred, the evolution of interface elements in the process of real-time interface reaction, the energy structure change and interface component overpotential, thus having a better understanding of interface composition, interfacial structure change, kinetics of interface reaction, and interfacial ion migration of the solid-state lithium ion batteries.