Reduction Resistible Halide Solid Electrolytes Enabled by Orbital Gap Modulation

Abstract Lithium‐metal‐chloride (Li‐M‐Cl) ionic conductors, with the high Li + conductivity, excellent cathode compatibility, and favorable mechanical properties, have emerged as a promising solid electrolyte (SE) candidate for all‐solid‐state batteries (ASSBs). However, their poor compatibility with lithium anodes, due to the presence of high‐valence M species that are highly susceptible to reduction reaction, remains a major challenge. In this study, a strategic modification of the central metal's electronic structure is presented to enable controlled modulation of the orbital gap, thereby tuning its resistance to reduction. By incorporating lanthanide elements (Ho and Lu) at the M sites, which can effectively increase the orbital gap of the metal‐centered polyhedra, improving the reduction stability of the solid electrolytes. As proof of concept, a series of new Li‐M‐Cl halide superionic conductors is synthesized that exhibit excellent compatibility with Li anodes. ASSBs utilizing these newly developed SEs as a single electrolyte layer achieve outstanding cycling stability, retaining up to 80.1% of their capacity after 500 cycles at 1 C. This study offers an effective strategy to address the challenge of anode incompatibility in halide SEs, advancing the development of high‐energy‐density ASSBs.