All-solid-state sodium batteries (ASSSBs) emerge as a highly promising next-generation energy storage technology due to their inherent high safety, abundant sodium resources, and potential for cost-effective production. A central to the performance of these batteries is solid electrolyte (SSE) that has a pivotal role in determining the overall electrochemical performance. Among various SSE candidates, halide solid electrolytes (HSSEs) have attracted much attention due to their high ionic conductivity, wide electrochemical window and good deformability. These characteristics make HSSEs particularly suitable for high-energy-density applications, where conventional liquid electrolytes often fall short due to safety concerns and limited voltage windows.The crystal structures of halide SSEs are intricately linked to their ion transport mechanisms and electrochemical performance. These structures can be broadly classified based on the metal elements incorporated and the arrangement of halide ions. For instance, sodium-based halide SSEs can be categorized into three main types, i.e., those with subgroup 3 and 4 elements (i.e., Sc, Y, La–Lu, Zr, Hf), subgroup 5 elements (i.e., Nb, Ta), and subgroup 3 main group elements (i.e., Al, Ga, In). Each category exhibits distinct structural characteristics and corresponding ionic conductivities. For instance, Na₃YCl₆, which belongs to the subgroup 3 and 4 elements category, is reported to have an ionic conductivity of 1.4×10⁻⁷ S/cm at room temperature and a monoclinic crystal structure. In contrast, NaTaCl₆ as a member of the subgroup 5 elements category has an ionic conductivity of 6.2×10⁻⁵ S/cm and a monoclinic structure. These structural differences can affect the ion transport mechanisms and overall electrochemical performance of SSEs.The synthesis methods of halide SSEs play a crucial role in determining their structural and electrochemical properties. Common synthesis methods include mechanical milling, solid-state annealing, and wet chemical synthesis. Mechanical milling, especially high-energy ball milling, can introduce structural disorder and defects, which enhance the ionic conductivity of SSEs. For instance, ball-milled NaTaCl₆ (NTC) exhibits a higher ionic conductivity of 4×10⁻³ S/m, compared to its as-synthesized form. Solid-state annealing can improve the crystallinity and phase purity of SSEs, but it may also lead to a decrease in ionic conductivity due to the reduction of structural defects. Wet chemical synthesis offers a more scalable and energy-efficient approach, but it requires careful control of the synthesis parameters to achieve the desired crystal structure and ionic conductivity.Interface stability between SSEs and the electrodes is another critical issue that needs to be addressed for the practical application of ASSSBs. Halide SSEs generally exhibit good chemical stability and compatibility with cathode materials due to their wide electrochemical window and high oxidation resistance. However, the interface between SSEs and sodium metal anode remains a challenge. The poor electrochemical reduction stability of halide SSEs can lead to severe interfacial reactions and degradation when in direct contact with sodium metal. To mitigate this issue, various strategies are proposed, such as alloying sodium with other metals (i.e., Sn) to form a more stable interface, using protective coatings to prevent direct contact between SSEs and sodium, and employing composite electrolytes to enhance the overall stability of the interface.In addition, the scalability and reproducibility of halide SSEs are also critical factors for their practical application. The existing synthesis methods often involve complex procedures and high-energy inputs, which limit the large-scale production of these materials. It is essential for the commercialization of halide SSEs to develop cost-effective and scalable synthesis routes. Moreover, the long-term stability and reliability of ASSSBs incorporating halide SSEs need to be thoroughly evaluated under various operating conditions (i.e., temperature, cycling rate and environmental factor).Summary and ProspectsHalide solid-state electrolytes (HSSEs) are poised to revolutionize all-solid-state sodium batteries (ASSSBs) due to their exceptional ionic conductivity (i.e., often surpassing 10^–3 S/cm), broad electrochemical stability windows (i.e., up to 6 V vs. Na⁺/Na), and mechanical flexibility, which enable dense electrode-electrolyte integration and mitigate dendrite growth. However, their practical implementation faces multifaceted challenges. Crystal structure optimization is required to balance ionic transport and thermodynamic stability, particularly in systems like Na₃YCl₆ or Na₂ZrCl₆, where lattice defects and anion/cation disorder impede performance. Synthesis methods such as mechanochemical milling or solvent-based routes need refinement to reduce impurities and ensure reproducibility. Electrode-electrolyte interface instability, driven by chemical incompatibility or volumetric changes during cycling, remains a critical bottleneck for long-term cycling. Scalable production techniques should bridge a gap between lab-scale innovations and industrial manufacturing. Future research should prioritize structure-property relationship studies using computational tools like density functional theory (DFT) to design HSSEs with tailored ion migration pathways, coupled with advanced in-situ characterization (i.e., synchrotron X-ray tomography or cryo-electron microscopy) to probe dynamic interfacial degradation mechanisms. Simultaneously, innovative material engineering strategies, such as inorganic-polymer composites (i.e., HSSE-PEO hybrids) to enhance interfacial adhesion or novel dual-phase electrolytes to suppress side reactions can address stability and conductivity trade-offs. The development of low-cost synthesis routes (i.e., aqueous precursor processing or scalable sintering) and rigorous evaluation of HSSEs under extreme temperatures, high current densities, and a prolonged cycling will be pivotal for commercialization. In addition, interface optimization via atomic-layer-deposited protective coatings or 3D nanostructured electrodes can also minimize interfacial resistance and improve charge transfer kinetics. With rapid advancements in material discovery, machine learning-driven design, and interdisciplinary collaboration, HSSEs become a threshold of practical application, having ASSSBs with unparalleled energy density (i.e., >400 Wh/kg), inherent safety, and cycle lifetimes exceeding 5000 cycles. The field trajectory indicates that resolving these challenges could have sodium-based solid-state batteries as mainstream solutions for grid storage and electric vehicles, marking a paradigm shift in sustainable energy storage.