Elastic Strain Effects on Li-Ion Transport in Garnet-Type LLZO Solid Electrolytes

Solid-state batteries offer better safety and higher energy density than liquid organic counterparts. However, the relatively low Li-ion conductivity of solid electrolytes limits battery charging rates and power density. This issue is primarily due to the complex electro-chemo-mechanical interactions at the electrode/electrolyte interfaces, where mechanical stresses can reach 1-10 GPa due to volumetric changes in the electrodes and interfacial reactions, substantially affecting the conductivity of solid electrolytes. Disentangling the impact of strain from other factors, such as space charge effects and interfacial reactions, poses a substantial experimental challenge. This work aims to quantify and elucidate the mechanisms by which elastic strains influence Li-ion transport in garnet-type LLZO (Li₇La₃Zr₂O₁₂) solid electrolytes using <i>ab initio</i> molecular dynamics (AIMD). We find that both Al- and Ta-doped LLZO exhibit higher occupancy in Li2 octahedral sites as compared to Li1 tetrahedral sites when accounting for the disorder in these sites. AIMD analysis shows that Ta-doped LLZO exhibits higher Li-ion diffusivity and conductivity than Al-doped LLZO, attributed to more accessible Li sites and higher Li⁺ concentrations. Ta doping, which replaces Zr sites, facilitates Li⁺ migration, while Al doping hinders transport by occupying Li sites and limiting migration pathways. Next, we have explored the effect of isotropic elastic strains on Li-ion diffusion, revealing that isotropic expansion (+2%) significantly increases Li⁺diffusivity, while isotropic compression (-2%) decreases it. This effect is due to increased Li-Li and Li-O distances under isotropic expansion (+2%), resulting in a larger bottleneck size and facilitating diffusion between Li1 tetrahedral and Li2 octahedral sites, with compression having the opposite effect. Furthermore, we have started applying anisotropic strain tensors, as these conditions can occur in realistic microstructures and can be strategically used to design solid electrolytes with specific strain profiles. Our results indicate that compressive biaxial strains (b-c, a-c) decrease the Li-ion diffusivity (D_Li), whereas tensile biaxial strain has a lesser impact on Ta-doped LLZO. This non-monotonic response under biaxial strain cannot be solely explained by changes in bottleneck size. Therefore, understanding the response of elastic strains is crucial for guiding the engineering of high-performance all-solid-state batteries.