Revealing the dominant factor of domain boundary resistance on bulk conductivity in lanthanum lithium titanates

Perovskite-type lithium lanthanum titanates (LLTO) display a high bulk ionic conductivity and are considered a promising electrolyte for building up to advanced solid-state Li-ion batteries. The LLTO crystals contain a high concentration of intrinsically formed 90°-rotated domain boundaries (DBs) serving as barriers to bulk Li-ion conduction. However, the mechanism of how the DB concentration and DB resistance can compete with each other to determine the bulk conductivity of LLTO is still unknown. Here we report a comprehensive study of LLTO compounds, aimed to unravel the mechanism and hence explore new path(s) for further improving the conductivity of this material. Our results show that both the sintering temperature and chemical composition can affect significantly the domain structures in LLTO. It is found that a decrease in the DB concentration is always accompanied by increased DB resistance due to the increased lattice mismatch at DBs, and vice versa. By unifying the electrochemical impedance spectroscopy and transmission electron microscopy analysis, it is clearly shown that the high DB resistance, instead of DB concentration, acts as the dominant factor governing the bulk conductivity of LLTO. The results thus renew the conventional understanding of the bulk Li-ion conduction in LLTO and shed light on developing novel LLTO electrolyte materials with improved ionic conductivity.