Grain boundary Li-ion conductivity in (Li0.33La0.56)TiO3 polycrystal

Lithium lanthanum titanate (LLTO) is one of the promising solid-state Li-ion electrolytes for an all-solid-state Li-ion battery system. Although LLTO shows a significantly high Li-ion conductivity of 2.2 x 10(-3) S cm(-1) in the bulk, the Li-ion conductivity at the grain boundary is largely reduced to 4.2 x 10(-5) S cm(-1), which prevents the practical application of solid-state Li-ion electrolytes. To solve this problem, the origin of such a low Li-ion conductivity at the grain boundary should be clarified. In this study, we investigated the relationship between the Li-ion conductivity and the geometric structure of the grain boundary in the (Li0.33La0.56)TiO3 polycrystal by using electrochemical strain microscopy in atomic force microscopy combined with electron backscatter diffraction in scanning electron microscopy. The experimental data suggest that the Li-ion conductivity is significantly reduced at the random grain boundaries but not at the coincidence-site-lattice (CSL) grain boundaries. Such a small reduction of the Li-ion conductivity at the CSL grain boundaries may originate from the smaller increment of the activation energy, owing to less composition deviations and less structural distortions at the CSL grain boundaries. These results suggest that it is effective to control the geometries of the grain boundaries for further improvement of the Li-ion conductivity in LLTO.