First-principles study of the structure and ionic diffusion mechanism in lanthanum lithium titanate (La2/3-xLi3xTiO3)

Solid-state electrolytes are used in all solid-state lithium-ion batteries instead of traditional organic liquid electrolytes. Solid-state electrolytes are expected to solve the battery safety problem from the root and is an ideal kind of chemical power source for electric vehicles and large-scale energy storage. Lanthanum lithium titanate (LLTO) is a high-quality electrolyte material, but its internal resistance is high and its conductivity is not good enough. There is still a certain gap with the traditional liquid electrolyte. In this project, a series of simulation methods have been conducted to explore the ionic conductivity mechanism of LLTO and try to explain the relationship between its conductivity and its structure and ionic diffusion mechanism. In this project, the structure of the LLTO supercell was optimized firstly. The optimized supercell was used as a benchmark model in subsequent calculations. Also, A-site vacancies and oxygen vacancies were manufactured in the supercell. According to the calculation, the formation energy of the A-site vacancy is 0.1947eV, and the formation energy of the oxygen vacancy is 5.6723eV. This shows that A-site vacancy is easier to be introduced. Next, multiple LLTO supercells were constructed which were doped with different elements and calculated the energy barriers for their transitions. It is acknowledged that the transition energy barrier of the initial LLTO model is 0.924eV. This means the transition energy barriers after doping are reduced. On the basis of the Li-ion transition landscape, it is claimed that the partially interrupted pathway is reconnected after doping. Finally, for the sake of understanding the mechanism of Li-ion migration, the Ab-initio molecular dynamics (AIMD) simulation process was carried out. It was surprisingly found that not all Li ions jumped to other vacancies during the AIMD process. And some nearby Li ions show some short-range diffusion mechanisms. When one of the two adjacent Li ions jumps to the surrounding vacancy first, the other one will follow the previous Li ion and move to its position; and when two Li ions are competing for a vacancy, if one of them has moved to this vacancy, the other Li ion will be pushed to another nearby vacancy