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△ [14a-419-9] Strain effect on Li+ ionic conduction in Li3xLa2/3-xTiO3: molecular dynamics simulation
Keywords:ionic conduction, MD simulation, strain
Li ion battery (LiB) is used for almost all modern rechargeable electric devices. To improve risk of firing, the use of solid-state ionic conductor is eagerly pursued to substitute the conventional liquid electrolyte. Li3xLa2/3-xTiO3 (LLTO) is one of the most promising materials among the known solid-state Li+ conductors with high conductivity (~10-3 S/cm) at room temperature and high stability at elevated temperature. In 2015, there is a report about epitaxial-strain effects on the ionic conductivity in LLTO. In the research, Li ion mobility with perpendicular direction to extended lattice shows higher value than that perpendicular to smaller lattice. The research considered that the Li ion migration going through an enlarged bottleneck is more advantageous than that through squeezed one. In order to reveal the detail mechanism of this phenomenon, I conducted MD simulation in strained conditions.
To considering LLTO, the distribution of the elements at perovskite A sites (La3+/Li+/v’) is important factor to the Li ion migration. The distribution in strained condition was generated by Genetic Algorithm (GA) with the optimization directed to find energetically stable configuration. Four combinations of lattice constants were considered in this research: (i) experimental values of bulk, (ii) 3% expanded along a- and b-axis simultaneously, (iii) 3% compressed as a- and b-axis, (iv) 3% compressed along a-axis and 3% expanded b-axis. After generation of the distribution by GA, molecular dynamics simulation was conducted at 300 K and the mean square displacement (MSD) of the Li+ ions,∑|ri(t) – ri(0)|2, was calculated from the trajectories. As a result, the ionic mobility calculated from MSD was in the order of 10-2 - 10-3 S/cm in all cases. These values are in good agreement with the experimental data, e.g. 1.5×10-3 S/cm (300K). To see the ionic mobilityσin (i) unstrained, (ii) expanded, and (iii) compressed lattice constants, a tendency consistent to the experimental result; the ionic mobility in expanded system becomes larger while the mobility in compressed cell becomes smaller was seen. In case of (iv), the ionic migration perpendicular to expanded lattice is larger than that of compressed one like as experimental result and layered structure perpendicular to elongated lattice direction was also observed. These results implies that there is a great possibility that the added lattice change affects the Li+ ionic mobility by forming layered La3+ distributions other than the size of bottleneck.
To considering LLTO, the distribution of the elements at perovskite A sites (La3+/Li+/v’) is important factor to the Li ion migration. The distribution in strained condition was generated by Genetic Algorithm (GA) with the optimization directed to find energetically stable configuration. Four combinations of lattice constants were considered in this research: (i) experimental values of bulk, (ii) 3% expanded along a- and b-axis simultaneously, (iii) 3% compressed as a- and b-axis, (iv) 3% compressed along a-axis and 3% expanded b-axis. After generation of the distribution by GA, molecular dynamics simulation was conducted at 300 K and the mean square displacement (MSD) of the Li+ ions,∑|ri(t) – ri(0)|2, was calculated from the trajectories. As a result, the ionic mobility calculated from MSD was in the order of 10-2 - 10-3 S/cm in all cases. These values are in good agreement with the experimental data, e.g. 1.5×10-3 S/cm (300K). To see the ionic mobilityσin (i) unstrained, (ii) expanded, and (iii) compressed lattice constants, a tendency consistent to the experimental result; the ionic mobility in expanded system becomes larger while the mobility in compressed cell becomes smaller was seen. In case of (iv), the ionic migration perpendicular to expanded lattice is larger than that of compressed one like as experimental result and layered structure perpendicular to elongated lattice direction was also observed. These results implies that there is a great possibility that the added lattice change affects the Li+ ionic mobility by forming layered La3+ distributions other than the size of bottleneck.