Molecular Dynamics Study on the Li+ Transport Properties in LLTO Grain Boundaries

The Perovskite Li3xLa2/3-xTiO3 (LLTO) has been investigated as a Li-ion solid electrolyte material and has attracted significant attention due to its wide operating voltage range. Polycrystalline and grain boundaries (GBs) are a common structural motif found in ceramic oxides. So, GBs can have a significant impact on the material properties. Here, we presented a molecular dynamics (MD) study that quantifies the effect of LLTO GBs on Li-ion transport. We examined six types of LLTO GBs, including P- Σ5(210), P-Σ5(310), P-Σ13(510) in the Li-poor phase and R-Σ5(210), R- Σ5(310), R-Σ13(510) in the Li-rich phase. We also consider LLTO bulk for comparison. The results show that the grain boundary formation energies of the six GBs are all below 1.30 J/m2, indicating the presence of a high concentration of GBs in polycrystalline LLTO. It is likely to find a highest concentration of Σ5(210) GB due to its lowest formation energy (1.00 J/m2 for P-Σ5(210) and 0.89 J/m2 for R-Σ5(210)). Compared with the bulk LLTO, Li+ in the six GBs exhibits a lower mean squared displacement (MSD), a smaller migration energy barrier and a lower ionic conductivity. These results confirm that LLTO GBs hinder Li+ transport. For bulk LLTO, the Li+ migration barrier is determined to be 0.30 eV (Li-poor phase) and 0.26 eV (Li-rich phase). In comparison, the migration barrier of LLTO GBs exhibits a slight decrease, ranging from 0.32 to 0.37 eV (Li-poor phase) and 0.27 to 0.31 eV (Li-rich phase). The computed Li-ion conductivities of the six GBs are 1 to 2 orders of magnitude lower than those of the corresponding bulk counterparts. Among the six GBs, P-Σ13(510) exhibits the highest Li+ conductivity of 4.76 × 10-5 S/cm in the Li-poor phase, whereas R-Σ5(310) shows the maximum Li+ conductivity of 1.31 × 10-3 S/cm in the Li-rich phase. Furthermore, the peak Li+ conductivity in the Li- rich phase is substantially higher than that in the Li-poor phase. In addition, Li+ transport perpendicular to the GB (i.e., from grain to grain) is more hindered relative to transport along the GB. Nevertheless, the Li+ diffusion can be improved by increasing the Li content within the GB region. The Li+ diffusion maps can be visualized by analyzing the Li+ trajectories of the MD simulations. We found that Li+ transport is restricted to the GB region first, then gradually turns to the bulk region, and finally forms a two-dimensional diffusion path similar to that of the LLTO bulk.Furthermore, the Li+ diffusion strongly depends on the distribution of O ions in LLTO GBs. For example, in the Li-poor-phase P-Σ5(310) GB, the number of O ions in the GB region is greater than that in the bulk region, which indicates a stronger Li-O attractive interaction in the GB region and so hinders Li+ transport towards the bulk region. Collectively, these atomic- scale insights deepen our understanding of LLTO GBs and their influence on Li+ transport.