Lithium-ion batteries (LIBs) have been successfully applied in electric vehicles, portable devices, and energy storage solutions. However, safety concerns, the scarcity of lithium resources, and the resulting fluctuating prices remain the Achilles' heel of LIBs. 4 Sodium solid-state batteries (SSBs) have attracted enormous attention and are regarded as a potential alternative to LIBs. Ideally, sodium SSBs offer reduced costs due to the abundance of sodium on Earth and improved safety because of the use of inflammable solid-state components. One of the perspectives of sodium SSBs is applying them in large-scale energy storage systems owing to their excellent cost-effectiveness and to the similar manufacturing process to LIBs. The key component that significantly impacts solid-state batteries' properties and performance is the solid-state electrolyte (SE). The design of SEs must balance various properties, such as superionic conductivities (>10 -3 S/cm), electrochemical stability against both electrodes and processability. Traditional inorganic Na-based SE families include oxide ceramics (NASICON-type oxides , β-Al2O3 ) and sulfides (Na3PS4, 14 Na2.9Sb0.9W0.1S4, Na11Sn2PS12-type ). However, both have drawbacks: oxide ceramics have high interfacial impedance and require high sintering temperatures. At the same time, sulfides exhibit a narrow electrochemical stability window, making them incompatible with high-voltage cathodes and sodium metal. Despite the unresolved issue of low ionic conductivity (10 -6 to 10 -5 S/cm), sodium metal halides have emerged as promising catholyte materials, offering excellent mechanical processability (via coldpressing) and compatibility with high-voltage electrodes. In 2018, Asano et al. demonstrated the feasibility of using ball-milling to create a disordered cation (Y 3+ ) arrangement in the Li3YCl6 (LYC) crystal. This boosted the ionic conductivity of LYC to ~0.5*10 -3 S/cm by opening Li + transport pathways through disordering and stacking faults. Na-based halide SEs have also been reported and can be divided into different families according to cation valence: NaM 5+ Cl6 (M = Nb, Ta), Na2Zr 4+ Cl6, 21 Na3M 3+ Cl6 (M = In, Y, Er, Cr, Sc, La, Sm), NaAl 3+ Cl4 29 and Na2M 2+ Cl4 (M = Mg, Co, Zn, Cd). Most of them exhibit however low ionic conductivity of 10 -5 S/cm or below. Tuning the concentration of vacancies via aliovalent substitution is a viable strategy to address this issue. The targeted introduction of vacancies via the mixing of two cations with different valences is widely employed to directly influence the ionic conductivity of the compound. Investigations related to the aliovalent substitution of sodium SEs have emerged in the past 4 years. Examples include Na3-xY1-xZrxCl6 and Na3-xIn1-xZrxCl6. The optimum ionic conductivity of intermediate compositions is normally 1~3 orders of magnitude larger than that of the associated end members.