Halide Solid‐State Electrolytes for all‐Solid‐State Lithium Batteries: Structure, Transport, Interface Relationships and Design Principles

ABSTRACT Halide solid‐state electrolytes (HSEs) have recently emerged as a highly promising class of ionic conductors for all‐solid‐state lithium batteries, owing to their high ionic conductivity, favorable electrochemical stability, and superior compatibility with high‐voltage cathodes. Despite rapid progress, a fundamental understanding of the structure‐transport relationships and interfacial behaviors in halide systems remains incomplete, limiting their rational design and practical deployment. In this review, we systematically summarize the recent advances in halide electrolytes by correlating crystal chemistry, defect chemistry, and lattice dynamics with Li + transport properties. We critically compare trivalent, divalent, and mixed‐valence halide systems, highlighting the roles of structural disorder, anion framework flexibility, and cation substitution in governing ionic conductivity. Particular emphasis is placed on emerging strategies including aliovalent doping, amorphization, and lattice softening to achieve fast ion conduction. Furthermore, we analyze interfacial compatibility between halide electrolytes and electrodes, focusing on electrochemical stability, interphase formation, and chemo‐mechanical degradation mechanisms. Finally, we propose design principles and future directions toward scalable synthesis, moisture stability, and integration into practical solid‐state battery architectures. This work provides a comprehensive and critical perspective on HSEs and offers guidance for the rational design of next‐generation solid‐state ionic conductors.