Impact of TiO₂ at the Grain Boundaries in Lithium Lanthanum Titanate Solid Electrolytes

The enhancement of Li-ion conductivity within the perovskite Li–La–Ti-O samples (LLTO) by the addition of TiO2 remains unexplained in the literature. Herein, microscopy shows that TiO2 appears at the grain boundaries (GB) of the perovskite, prompting a comprehensive molecular dynamics investigation. In this work, we analyzed symmetric and mixed LLTO GBs, as well as LLTO/TiO2 interfaces, to understand the impact of the secondary phase on Li-ion conductivity compared to other factors, such as disorder or strain present in the samples due to the GBs. The rigid-ion Buckingham-type potential combined with a long-range Coulombic term was used to accurately model ionic interactions. The investigation of diffusion mechanisms through mean squared displacement (MSD) analysis unveiled that disordered TiO2 phases significantly enhance Li-ion mobility compared with more orderly Sigma 5 or mixed GBs. This suggests that the disorder may create additional pathways for ion diffusion not present in symmetric GBs or crystalline LLTO samples. Furthermore, the enhanced Li-ion diffusion through LLTO/TiO2 interfaces was indeed observed in the calculations and is attributed to the presence of TiO2 phases and, to a lesser extent, to the highly disordered interface formed between them. These insights into the intricate migration mechanisms of Li ions through the exceptionally complex microstructures present in LLTO could advance the development of efficient solid-state electrolytes for Li-ion battery applications.