Halide Electrolytes for All‐Solid‐State Sodium Batteries: From Fundamental Chemistry to Interface Engineering

All-solid-state sodium batteries (ASSSBs) stand out as a transformative energy storage technology, combining sodium's natural abundance with enhanced safety and competitive energy density. Solid electrolytes are pivotal to this innovation, with halide electrolytes emerging as prominent candidates due to their unique strengths-superior deformability for intimate electrode contact, strong cathode compatibility, and promising Na-ion conductivity. Despite recent progress, significant challenges persist in scalable synthesis, performance optimization, and mechanistic understanding of ion transport and interfacial interactions. This review comprehensively covers sodium-based halide electrolytes, including their structural chemistry, ion transport, synthesis, modification, electrochemical stability, interfacial behavior, and computational insights. We further integrate a systematic framework to elucidate intricate synthesis-structure-property relationships, enabling a holistic understanding for rational material design. Crucially, this work distinguishes itself by distilling concrete design principles for Na-halide conductors, providing quantitative insights into humidity stability, and establishing in-depth correlations between interphases/degradation modes and full-cell metrics. Moreover, a practical assessment of key performance metrics (energy density, power density, cycle life) and design guidance is presented. Finally, we pinpoint critical barriers (moisture sensitivity, anode incompatibility, and conductivity limitations) and outline a roadmap emphasizing compositional design, interface engineering, manufacturing scalability, machine learning, operando characterization, and standardized metrics to accelerate commercialization.