Light Ga/Ta co-doping enhances cubic LLZO stability and improves Li-ion conductivity

Abstract

Garnet-type Li₇La₃Zr₂O₁₂ (LLZO) is a promising solid electrolyte for next-generation high-energy-density lithium batteries, but stabilizing the cubic phase while maintaining fast Li-ion transport remains challenging. Here, we combine density functional theory (DFT), machine-learned-force-field-assisted ab initio molecular dynamics (AIMD), nudged elastic band (NEB) calculations, and experimental synthesis to investigate Ga/Ta co-doping in LLZO. The optimized composition, Li₆.₃₇₅Ga₀.₁₂₅La₃Zr₁.₇₅Ta₀.₂₅O₁₂, exhibits an experimentally extracted room-temperature bulk ionic conductivity of 9.18×10⁻³ S cm⁻¹, in close agreement with the ideal bulk AIMD prediction of 1.016×10⁻² S cm⁻¹. Structural descriptors, including radial distribution functions, bond-length distributions, bond-angle distributions, and coordination-number trends, show that light co-doping balances Li-vacancy formation and local disorder, widening migration bottlenecks without disrupting long-range percolation pathways. The revised analysis also clarifies the assumptions behind dopant-site occupation, high-temperature AIMD extrapolation, and separation of ionic and possible electronic contributions. These results provide design guidelines for LLZO-based solid electrolytes: stabilize the cubic framework through controlled aliovalent substitution, tune rather than maximize Li-sublattice disorder, and preserve framework connectivity while improving Li-ion mobility.