Cubic Iodide LixYI3+x Superionic Conductors through Defect Manipulation for All-Solid-State Li Batteries

Halide solid electrolytes (SEs) have attracted significant attention due to their competitive ionic conductivity and good electrochemical stability. Among typical halide SEs (chlorides, bromides, and iodides), substantial efforts have been dedicated to chlorides or bromides, with iodide SEs receiving less attention. Nevertheless, compared with chlorides or bromides, iodides have both a softer Li sublattice and lower reduction limit, which enable iodides to possess potentially high ionic conductivity and intrinsic anti-reduction stability, respectively. Herein, we report a new series of iodide SEs: Li_xYI_3+x (x=2, 3, 4, or 9). Through synchrotron X-ray/neutron diffraction characterizations and theoretical calculations, we revealed that the Li_xYI_3+x SEs belong to the high-symmetry cubic structure, and can accommodate abundant vacancies. By manipulating the defects in the iodide structure, balanced Li-ion concentration and generated vacancies enables an optimized ionic conductivity of 1.04 × 10⁻³ S cm⁻¹ at 25 °C for Li₄YI₇. Additionally, the promising Li-metal compatibility of Li₄YI₇ is demonstrated via electrochemical characterizations (particularly all-solid-state Li-S batteries) combined with interface molecular dynamics simulations. Our study on iodide SEs provides deep insights into the relation between high-symmetry halide structures and ionic conduction, which can inspire future efforts to revitalize halide SEs.