Impact of Lithium Nonstoichiometry on Ionic Diffusion in Tetragonal Garnet-Type Li₇La₃Zr₂O₁₂

Understanding ion transport mechanisms on the atomistic scale in solid-state electrolytes is crucial for the development of all-solid-state batteries. Li7La3Zr2O12 (LLZO) is a promising oxide solid electrolyte material, whose phase transition behavior and ion transport mechanisms have attracted significant research attention. Previous studies have primarily focused on ion transport in the cubic phase (intrinsic high-temperature phase or doped variants). In contrast, the tetragonal phase of LLZO, despite its close relationship with the cubic phase, has received less attention due to its relatively low ionic conductivity and high computational cost. A few recent computational studies have shown significant discrepancies in conductivity and activation energy between calculated and experimental values. Therefore, the unclear ion transport mechanisms in the tetragonal phase of LLZO are critical to understanding and designing oxide solid electrolytes. In this study, we employ state-of-the-art machine-learning-based neuroevolution potential molecular dynamics simulations to investigate the effects of lithium nonstoichiometry on the ionic conductivity and phase stability of LLZO. We demonstrate that small deviations from stoichiometry, particularly lithium deficiency, dramatically reduce the activation energy for Li+ diffusion in tetragonal LLZO from 1.227 to 0.425 eV, increasing room-temperature ionic conductivity by 10 orders of magnitude. The slight lithium nonstoichiometry, which commonly occurs during high-temperature synthesis, has a significant effect on ion transport in the tetragonal phase. Our findings highlight the crucial role of lithium nonstoichiometry and defect chemistry in enhancing LLZO performance and provide insights for the rational design of high-performance solid electrolytes through defect engineering.