Toward an improved solid-state Li electrolyte: A first-principles investigation of the structure, Li-ion migration pathways, and ionic conductivity of Li7La3Zr2O12

Abstract

Among solid state electrolytes, garnet-type Li₇La₃Zr₂O₁₂ (LLZO) has attracted considerable attention due to its high electrochemical stability, safety, and compatibility with lithium metal anodes. However, its lithium-ion conductivity strongly depends on the crystal structure: the tetragonal phase exhibits a significantly lower ionic conductivity than the cubic phase. In this work, first-principles density functional theory (DFT) and ab initio molecular dynamics (AIMD) are employed to systematically investigate the crystallographic structure, lithium-ion migration pathways, and ionic conductivity of both tetragonal and cubic LLZO. Lithium ion trajectories were analysed to determine diffusion coefficients over a wide temperature range. Activation energies are extracted from Arrhenius behaviour, and room-temperature ionic conductivities are extrapolated from high-temperature simulations. The results reveal that cubic LLZO possesses an intrinsically disordered lithium sublattice with abundant vacant sites and shorter migration pathways, which significantly enhance lithium-ion mobility. Consequently, the extrapolated room temperature ionic conductivity of cubic LLZO reaches the order of ~10^-3 S/cm, in good agreement with experimental reports, while tetragonal LLZO exhibits much lower conductivity. This study provides atomistic-level insight into lithium diffusion mechanisms in LLZO and offers guidance for designing high-performance garnet-type solid electrolytes through structural disorder and vacancy engineering.