Fluorinated garnet: benchmarking Li-ion conductivity through structure–transport correlations and trade-offs

Abstract

Garnet-type oxide solid electrolytes such as Li₇La₃Zr₂O₁₂ (LLZO) are promising candidates for all-solid-state lithium batteries due to their electrochemical stability and compatibility with lithium metal. While aliovalent cation doping (e.g., Al, Ga, Ta, etc.) has been widely used to enhance cubic phase stability and Li-ion conductivity, limitations persist due to unfavorable local structural distortion of the Li sublattice and poor wetting with Li metal, especially in Ga-doped LLZO. Anion doping, particularly fluorine, has shown potential for reducing interfacial resistance and promoting Li mobility, but its effectiveness remains underexplored. In this work, we systematically evaluate a wide compositional range of doped LLZO using density functional theory (DFT), ab initio molecular dynamics (AIMD), and data-driven analysis to elucidate structure–property relationships. We identify Li_6.25La₃Zr₂O_11.25F_0.75 as a high-performing fluorinated composition, exhibiting an outstanding Li-ion conductivity of 16 mS cm⁻¹ and a low Li-migration barrier of 0.20 eV, surpassing the state-of-the-art Ga-doped LLZO (Li_6.25Ga_0.25La₃Zr₂O₁₂). Our findings reveal a key inverse relationship between the diffusivity prefactor and the migration barrier F-doping outperform cation strategies by inducing localized ZrO_6−xF_x polyhedral distortions and a more spatially distributed Li-vacancy landscape. This behavior departs from Meyer–Neldel compensation while maintaining equivalent Li-vacancy concentrations. This study establishes fundamental design rules for anion-engineering and offers a multi-property optimization framework toward next-generation garnet solid electrolytes.