A high-performance sodium halide solid electrolyte, Na₀.₇La₀.₇Zr₀.₃Cl₄, was developed for all-solid-state sodium-ion batteries (ASSBs). This material exhibits a high ionic conductivity of 2.9 × 10⁻⁴ S cm⁻¹ at 30°C, good compressibility, and high oxidative stability. It was synthesized using a solid-state reaction combined with mechanochemical methods. X-ray diffraction reveals a hexagonal structure (P6₃/m) with Na⁺ ions forming a one-dimensional diffusion channel along the c-axis. First-principle calculations and X-ray absorption fine structure analysis show that the ionic conductivity is mainly determined by the size of Na⁺ channels and the Na⁺/La³⁺ mixing in the diffusion channels. When used as a catholyte, the NaCrO₂||Na₀.₇Zr₀.₃La₀.₇Cl₄||Na₃PS₄||Na₂Sn all-solid-state battery demonstrates an initial capacity of 114 mA h g⁻¹ and 88% retention after 70 cycles at 0.3 C. At 1 C current density, it maintains a high capacity of 94 mA h g⁻¹. Sodium-ion all-solid-state batteries have gained attention due to their safety and cost advantages over lithium-ion batteries. Sodium solid-state electrolytes (SSEs) are critical components that determine the electrochemical performance of ASSBs. To achieve high power density, energy density, cycle life, and low cost, SSEs must exhibit high ionic conductivity, broad electrochemical window, excellent compatibility with electrode materials, and low cost. Various types of sodium SSEs have been developed, including organic polymers, inorganic sulfides, oxides, halides, and borohydrides. Polymer-based SSEs are cheap and easy to process but have low ionic conductivity. Oxides show high ionic conductivity but are incompatible with cathode materials. Sulfides have the highest ionic conductivity but poor electrochemical stability. Borohydrides have high ionic conductivity but low oxidation potential. Halides are considered promising SSEs due to their high ionic conductivity, deformability, chemical stability, and good oxidative stability. Many lithium-ion halide SSEs with high ionic conductivity have been developed. Recent studies have developed several sodium halide SSEs, such as Na₂ZrCl₆, Na₃YCl₆, Na₃ErCl₆, and NaAlCl₄. Although these sodium halides exhibit oxidation stability, their sodium ionic conductivity is much lower. Therefore, developing sodium halide SSEs with high ionic conductivity is urgent. A typical halide electrolyte is composed of MX₆ (M = Y³⁺, Zr⁴⁺, ScA high-performance sodium halide solid electrolyte, Na₀.₇La₀.₇Zr₀.₃Cl₄, was developed for all-solid-state sodium-ion batteries (ASSBs). This material exhibits a high ionic conductivity of 2.9 × 10⁻⁴ S cm⁻¹ at 30°C, good compressibility, and high oxidative stability. It was synthesized using a solid-state reaction combined with mechanochemical methods. X-ray diffraction reveals a hexagonal structure (P6₃/m) with Na⁺ ions forming a one-dimensional diffusion channel along the c-axis. First-principle calculations and X-ray absorption fine structure analysis show that the ionic conductivity is mainly determined by the size of Na⁺ channels and the Na⁺/La³⁺ mixing in the diffusion channels. When used as a catholyte, the NaCrO₂||Na₀.₇Zr₀.₃La₀.₇Cl₄||Na₃PS₄||Na₂Sn all-solid-state battery demonstrates an initial capacity of 114 mA h g⁻¹ and 88% retention after 70 cycles at 0.3 C. At 1 C current density, it maintains a high capacity of 94 mA h g⁻¹. Sodium-ion all-solid-state batteries have gained attention due to their safety and cost advantages over lithium-ion batteries. Sodium solid-state electrolytes (SSEs) are critical components that determine the electrochemical performance of ASSBs. To achieve high power density, energy density, cycle life, and low cost, SSEs must exhibit high ionic conductivity, broad electrochemical window, excellent compatibility with electrode materials, and low cost. Various types of sodium SSEs have been developed, including organic polymers, inorganic sulfides, oxides, halides, and borohydrides. Polymer-based SSEs are cheap and easy to process but have low ionic conductivity. Oxides show high ionic conductivity but are incompatible with cathode materials. Sulfides have the highest ionic conductivity but poor electrochemical stability. Borohydrides have high ionic conductivity but low oxidation potential. Halides are considered promising SSEs due to their high ionic conductivity, deformability, chemical stability, and good oxidative stability. Many lithium-ion halide SSEs with high ionic conductivity have been developed. Recent studies have developed several sodium halide SSEs, such as Na₂ZrCl₆, Na₃YCl₆, Na₃ErCl₆, and NaAlCl₄. Although these sodium halides exhibit oxidation stability, their sodium ionic conductivity is much lower. Therefore, developing sodium halide SSEs with high ionic conductivity is urgent. A typical halide electrolyte is composed of MX₆ (M = Y³⁺, Zr⁴⁺, Sc