Multi-layered electrolytes for solid-state lithium batteries

Solid-state lithium batteries are promising candidates for improving battery safety and boosting energy density. However, the application of both typical solid-state electrolytes, inorganic ceramic/glass and organic polymer electrolytes, are facing their respective inherent challenges, including large interfacial resistance and unwanted interfacial reactions of inorganic electrolytes, and narrow electrochemical stability windows of polymer electrolytes. The design of multi-layer solid-state electrolytes, such as inorganic/inorganic, inorganic/polymer and polymer/polymer structures, provides a viable solution to these issues by effectively improving the interfacial stability and widening the electrochemical window, which exhibits unparallel advantages in solid-state batteries with the help of non-diffusible characteristic of solid-state electrolytes. Given the extensive research and progress reported, this paper reviews the latest advancements in multi-layer solid-state electrolytes and categorizes them into inorganic-based, polymer-based and composite-based systems. Each is discussed separately, focusing on their design rationales, functional mechanisms, electrochemical performances, and residual challenges. In addition, to meet the requirements of practical applications, the potential strategies for further development of multi-layer solid-state electrolytes are also briefly presented.