Structural origin of low Li-ion conductivity in perovskite solid-state electrolyte

Perovskite solid-state electrolyte, Lithium Lanthanum Titanium Oxide, has long been considered as a promising candidate enabling all-solid-state Li batteries due to its high bulk Li-ion conductivity and wide electrochemical window. However, the Li-ion conductivity at so called “grain boundaries” (GBs) off LLTO pellets is much lower than that of its bulk phase. Thus, understanding the structural origin of the low Li-ion conductivity of LLTO is critical yet remains largely elusive. Here, we identify two types of GBs at different length scales, namely “macro” and “micro” GBs in LLTO pellets and originate them to the processing of LLTO. We also find a nanosized Li-poor orthogonal phase at micro-GBs, which largely deteriorate the Li conduction. Corroborated by electrochemical measurements, we revealed the phase transformation from tetragonal Li0.33La0.56TiO3 to orthogonal Li0.18La0.6TiO3 during the heat-treatment process is responsible for this deterioration. During phase transformation process, La atoms move to the original Li-rich layer and block the diffusion path of Li-ions in LLTO. These results reveal the structural origin of low Li-ion conductivity in LLTO pellets and provide insights to prevent Li-loss induced phase transformation for commercial application of LLTO.