Fluorinated Garnet: Benchmarking Li-Ion Conductivity Through Structure-Transport Correlations and Trade-Offs

Abstract

Garnet-type oxide solid electrolytes such as Li 7-x La 3 Zr 2 O 12 (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) has been widely used to enhance cubic phase stability and Li-ion conductivity, limitations persist due to structural distortions and poor interface stability, especially in Ga-doped LLZO. Anion doping-particularly fluorine-has shown potential in 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 6.25 La 3 Zr 2 O 11.25 F 0.75 as a high-performing fluorinated composition, exhibiting an outstanding Li-ion conductivity of 16 mS cm -1 and a low activation energy of 0.20 eV, surpassing the state-of-the-art Ga-doped LLZO. Our findings reveal a key inverse relationship between the diffusivity prefactor and migration 1 barrier and demonstrate how F-doping can outperform cation strategies while maintaining equivalent amount of Li-vacancy concentrations. This study establishes fundamental design rules for dopant engineering and offers a multi-property optimization framework toward next-generation garnet solid electrolytes.