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. However, sustainable solid-state electrolytes (SE) with high ionic conductivity need to be developed. Halide solid electrolytes have attracted significant attention, especially for use in catholyte composites, due to their sufficient ionic conductivity, stability in contact with cathodes and adequate mechanical properties. Here we investigate novel SE based on NaAlCl4 and Na2ZnCl4, nominally Na1+xZnxAl1-xCl4. Compounds synthesized by ball milling and investigated by XRD revealed a two-phase system, with however a solid solution in the Na2ZnCl4-type structure extending to 30% Al substitution. EIS results demonstrate that the ionic conductivity is optimal near the edge of the miscibility gap (x = 0.69), where is increased by several orders of magnitude as compared to NZC and reaches 1.5×10⁻5 S/cm (35 MPa, 25 °C), above the values of NaAlCl4. The combined use of molecular dynamics simulations and nuclear magnetic resonance distinctly elucidate the importance of achieving enough Na⁺ vacancies in both Na sublattices in NZC-type structures. This work introduces a novel class of SE based on the NZC structure and provides clear insights into the correlation between composition, crystalline structure, and ionic conduction pathways.