Compositional tuning of NaAlI4: effects of Br⁻ substitution and excess Na+ on ionic conductivity

Abstract Solid-state electrolytes are gaining attention as safer alternatives to conventional liquid electrolytes in lithium- and sodium-ion batteries, particularly for large-scale applications. Among them, sodium-based systems offer cost and resource advantages. Halide-based solid electrolytes allow compositional tuning via homovalent halide substitution, which has been shown to enhance ionic conductivity in both Li⁺ and Na⁺ systems. Recent studies on NaAlBr 4 suggest that Na + mobility can be improved through halide substitution and non-stoichiometry. However, the effects of Br⁻/I⁻ exchange in sodium aluminum halides remain largely unexplored. This study provides the effects of Br − substitution on the conduction properties of NaAlI 4 , particularly for the activation energy for ion conduction. Br⁻ substitution was systematically investigated across the full compositional range. A complete solid solution was confirmed, accompanied by lattice shrinkage with increasing Br⁻ content, reflecting the replacement of larger I⁻ ions with smaller Br⁻ ions. The activation energy for Na⁺ conduction varied with Br⁻ fraction, peaking at Br⁻/I⁻ = 1.0. From the variation in lattice parameters, it was suggested that the NaI6 prism undergoes a pincer-like deformation rather than uniform shrinkage, with the strongest deviation from ideality observed in the Br⁻ 40–70% range. This structural distortion was correlated with increased hopping barriers, likely hindering Na⁺ migration along the a and b axes. Although Br⁻ substitution alone was not favorable for Na⁺ conduction, the introduction of excess Na⁺ significantly enhanced conductivity, reaching 1.5 × 10⁻ 5 S/cm at 303 K. These findings highlight the interplay between halide composition, lattice distortion, and ionic transport, offering insights for the design of high-performance Na⁺ conductors.