Mechanochemical Synthesis and Structure of Lithium Tetrahaloaluminates, LiAlX(4) (X = Cl, Br, I): A Family of Li-Ion Conducting Ternary Halides

[Image: see text] State-of-the-art oxides and sulfides with high Li-ion conductivity and good electrochemical stability are among the most promising candidates for solid-state electrolytes in secondary batteries. Yet emerging halides offer promising alternatives because of their intrinsic low Li(+) migration energy barriers, high electrochemical oxidative stability, and beneficial mechanical properties. Mechanochemical synthesis has enabled the characterization of LiAlX(4) compounds to be extended and the iodide, LiAlI(4), to be synthesized for the first time (monoclinic P2(1)/c, Z = 4; a = 8.0846(1) Å; b = 7.4369(1) Å; c = 14.8890(2) Å; β = 93.0457(8)°). Of the tetrahaloaluminates, LiAlBr(4) exhibited the highest ionic conductivity at room temperature (0.033 mS cm(–1)), while LiAlCl(4) showed a conductivity of 0.17 mS cm(–1) at 333 K, coupled with the highest thermal and oxidative stability. Modeling of the diffusion pathways suggests that the Li-ion transport mechanism in each tetrahaloaluminate is closely related and mediated by both halide polarizability and concerted complex anion motions.