Lithium ion migration pathways in Li3xLa2/3−x□1/3−2xTiO3

We studied the Li+ ion migration pathways in Li3xLa2/3−x□1/3−2xTiO3 at x = 1/24 using density functional theory. When four La atoms occupied the A-sites of the same layer (La-rich layer) and the remainder occupied the A-sites of another layer (La-poor layer), the structure was thermodynamically the most favorable. Because a Li+ ion migrates through A-site vacancies, its migration in this structure occurs on the La-poor layer. There were two cavities through which the Li+ ion could pass for long distance migration: between two La atoms (C1) and within the center of the square that was formed by the four La atoms (C2) in the La-poor layer. The Li+ ion migrated around the A-site center from an off-centered favorable position to the symmetrical position at the C1 site, and its energy barrier was 0.13 eV. Because the C2 site was energetically unfavorable for the stay of the Li+ ion, the Li+ ion rotated at a right angle toward another C1 site to avoid the C2 site with the energy barrier of 0.24 eV. The results indicated that the Li+ ion was conducted by the repetition of the transfer at the C1 site and rotation near the C2 site two-dimensionally on the La-poor layer, and the overall energy barrier for the lithium migration was 0.24 eV. We believe this value is reasonable, because the experimentally determined values in the range of 0.36–0.40 eV should consider migration through grain boundaries as well as through grains.