Structure and Ionic Conductivity of Halide Solid Electrolytes Based on NaAlCl₄ and Na₂ZnCl₄

Sodium-based solid-state batteries may represent safe and cost-effective energy storage devices, complementing Li-ion for specific applications such as grid storage. Thus, sustainable solid-state electrolytes (SSE) with high ionic conductivity need to be developed. Sodium metal halide SSEs have attracted significant attention due to their ionic conductivity, electrochemical stability, and adequate processibility. Here, SSE based on NaAlCl₄ (NAC) and Na₂ZnCl₄ (NZC) are investigated, nominally Na_1+xZn_xAl_1-xCl₄. Compounds synthesized by ball-milling and investigated by X-ray diffraction revealed a two-phase system, with a solid solution in the Na₂ZnCl₄-type structure extending to ≈34(3)% Al substitution. EIS results demonstrate the highest ionic conductivity is near the miscibility gap edge (x = 0.625), where σ is increased by several orders of magnitude as compared to NZC and reaches 1.5×10⁻⁵ S cm^-1 at 25 °C, above the values of Na₂ZnCl₄/NaAlCl₄. The combined use of molecular dynamics simulations and nuclear magnetic resonance distinctly elucidates the importance of achieving enough Na⁺ vacancies in both Na sublattices in NZC-type structures. This work introduces a novel class of SSE based on the NZC olivine structure, demonstrates that they can be used as catholytes to assemble working solid-state sodium batteries, and provides insights into the correlation between composition, crystalline structure, and ionic conduction pathways.