(Invited) Origin of the Low Grain Boundary Conductivity in Lithium Ion Conducting Perovskites: Li3xLa0.67-XTiO3

Although the bulk conductivity of lithium ion conducting Li 3x La 0.67-x TiO 3 ceramics reaches the level of 10 -3 S/cm, the grain boundary conductivity is orders of magnitude lower; the origin of the low grain boundary conductivity should be thoroughly understood as a prerequisite to improve the overall conductivity. According to SEM and TEM investigations, the grain boundaries are free of any second phase; however, the specific grain boundary conductivity is still ~4 orders of magnitude lower than the bulk conductivity. The grain-size-dependent specific grain boundary conductivity indicates that the crystallographic grain boundary does not determine the low grain boundary conductivity. The Li + ions across the grain boundaries are then determined by the space-charge layer. The grain boundary core in the Li 3x La 0.67-x TiO 3 is positively charged, causing the depletion of Li + ions in the adjacent space-charge layers. The very low grain boundary conductivity can be accounted for by the Li + depletion in the space-charge layers.