Lithium deposition in solid-state electrolytes: Fundamental mechanisms, advanced characterization, and mitigation strategies

The deposition of metallic lithium inside inorganic solid-state electrolytes (SSEs) is a challenge that poses a concern for the development of all-solid-state lithium batteries. Unlike lithium dendrites that form and grow from the surface of Li-metal anode, metallic lithium can nucleate and propagate directly inside the bulk SSE, which can cause an instant failure of SSE. This aim of this Review was to provide a timely report on the progress of relevant research from both theoretical and experimental aspects. We first discuss the formation mechanism of metallic lithium inside SSEs from thermodynamic, kinetic, and electrochemical points of view. Then, recent research efforts toward a direct observation of metallic Li inside SSEs are summarized. Mechanisms proposed regarding intergranular and intragranular deposition of Li, direct nucleation of Li inside SSEs driven by the electronic conductivity of SSE, and reduction of Li caused by excess electrons or negatively charged species are discussed in detail. On the basis of theoretical and experimental discussions, we propose future research directions. With this Review, we hope to stimulate the researchers in the field of all-solid-state batteries toward a deep understanding of the Li deposition behavior inside SSEs and formulating proper strategies to address the issue.