Advancements in the emerging rare-earth halide solid electrolytes for next-generation all-solid-state lithium batteries

All-solid-state lithium batteries (ASSLBs) utilizing inorganic solid-state electrolytes (SEs) are widely regarded as one of the most promising next-generation energy storage technologies due to their superior energy density, enhanced safety, and extended cycle life. The successful commercialization of ASSLBs hinges on the development of SEs that exhibit high ionic conductivity, good chemical stability, and robust mechanical properties. The rare-earth-based halide solid electrolytes (REHSEs) have emerged as particularly promising candidates for ASSLBs, offering several key advantages, including high room-temperature ionic conductivity, outstanding reduction stability, excellent mechanical flexibility, and enhanced compatibility with high-voltage cathodes. Here we examine the recent progress in REHSEs to facilitate the research community's understanding of this rapidly evolving field. We begin by outlining the fundamental principles and current state of research on REHSEs. This is followed by an in-depth discussion of recent research, covering aspects such as preparation methods, phase and structural engineering, ionic conduction mechanisms, and strategies for performance optimization. Finally, we address the major challenges and propose future research directions to enable the practical application of REHSEs in ASSLBs. This review aims to provide valuable insights into the rational design of advanced REHSEs, paving the way for the development of high-performance ASSLBs. [Display omitted] •Discusses the crystallographic structures and Li+ conduction mechanisms of rare-earth-based halide solid-state electrolytes.•Summarizes various synthesis methods and highlights how they particularly influence crystal structure and ionic conductivity.•Classifies the electrochemical properties and corresponding modification strategies.•Proposes future directions for the practical application of rare-earth-based halide solid-state electrolytes.