Enhancing Ionic Conductivity in Lithium Tetrahaloaluminates via a Mixed-Halide Strategy

Halide-based solid electrolytes attract interest owing to their wide electrochemical windows and moderate ionic conductivities. Here, we demonstrate a mixed-halide strategy to enhance the ionic conductivity of lithium tetrahaloaluminates, LiAlX₄ (X = Cl, Br, I). Twenty compositions, including single-, binary-, and ternary-halide systems, were synthesized via a mechanochemical route. Ionic conductivities were measured by electrochemical impedance spectroscopy, and the local environments of Li and Al were probed using solid-state NMR and powder X-ray diffraction (XRD). A conductivity map based on a ternary diagram shows the highest conductivity near the center of the Cl-Br-I triangle, where configurational entropy is maximized. ²⁷Al magic-angle spinning (MAS) NMR reveals multiple AlX₄^- environments in the mixed-halide systems, consistent with random anion mixing. ⁷Li MAS NMR spectra exhibit motional narrowing that correlates with enhanced ionic conductivity, especially in ternary compositions. Activation energies and pre-exponential factors from Arrhenius plots follow the Meyer-Neldel rule, suggesting that Li⁺ migration barriers are overcome via multiphonon excitations. These results demonstrate that increased compositional complexity can enhance ionic conductivity, highlighting entropy-driven design as a promising strategy for next-generation solid-state batteries.