Anion-Sublattice-Modulated Oxyfluoride Garnet Electrolytes for Enhanced Li-Ion Transport in Solid-State Lithium Batteries

Garnet-type solid electrolytes are promising for solid-state lithium batteries owing to their chemical robustness, a wide electrochemical stability window, and compatibility with lithium metal. However, limited Li-sublattice connectivity and sluggish interfacial kinetics still restrict practical implementation. Here, lithium aluminum lanthanum zirconium oxyfluoride garnets are designed through the systematic tuning of the Li+, Al3+, and F– contents to modulate the anion sublattice and enhance Li-ion transport. Synchrotron X-ray diffraction and Rietveld refinement reveal that partial fluorination induces slight lattice contraction, reduces La–O/F bond-length disparity, increases Li occupancy at the 96h site, and shortens Li–Li separations, thereby improving Li-sublattice connectivity. The lower formal charge of F– relative to O2– weakens Li-anion interactions and softens the Li+ migration landscape. The optimized oxyfluoride garnet exhibits a bulk ionic conductivity of 2.51 × 10–4 S/cm, a low activation energy of 0.25 eV, and a low migration barrier of 0.297 eV. BVSE-derived calculations and crystallographic-plane analysis support improved Li-ion migration connectivity between the 24 d and 96 h sites in the fluorinated garnet. Electrochemical analyses further demonstrate reduced area-specific resistance, increased exchange current density, and enhanced critical current density. In Li/LiFePO4 cells with liquid-wetted interfaces, the optimized electrolyte delivers approximately 155 mAh/g at 0.1 C and retains approximately 92% of its capacity after 100 cycles at 1 C and 25 oC.