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

Halide-based solid electrolytes have recently attracted considerable attention 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. Their ionic conductivities were evaluated by electrochemical impedance spectroscopy, while the local environments of Li and Al were investigated using solid-state NMR in combination with powder X-ray diffraction (XRD). A conductivity map was constructed from the experimental data using a ternary diagram. The highest conductivity is observed near the center of the Cl-Br-I compositional triangle, where the configurational entropy is maximized. XRD patterns indicate that both binary and ternary mixed-halide samples retain the overall framework structure. 27Al magic-angle spinning (MAS) NMR reveals multiple X combinations of AlX4- units in the binary and ternary systems, consistent with random anion mixing. 7Li MAS NMR spectra exhibit motional narrowing that correlates with enhanced ionic conductivity, particularly in the ternary compositions. The activation energy and pre-exponential factor extracted from Arrhenius plots of the conductivity follow the Meyer-Neldel rule. These results suggest that the Li+ migration barrier can be overcome through the annihilation of multi-phonon excitation modes. The enhanced ionic conductivity achieved via increased compositional complexity highlights the potential of entropy-driven design strategies for next-generation solid-state batteries.