Origin of the Phase Transition in Lithium Garnets

Molecular dynamic and density functional theory based simulations were performed to obtain a better understanding of the origin of phase transition in garnet-type Li7La3Zr2O12 solid electrolyte. The phase transition coincided with the lithium redistribution among all sites. With the investigation of lithium distribution and dynamics, we found one temperature-dependent lithium migration pathway in lithium garnets. Lithium ions exhibited uniformly 3-dimensional diffusion in cubic LLZO, while the lithium diffusion in tetragonal LLZO was mainly in the a and b direction. The constrained diffusion in the c direction in tetragonal LLZO could be ascribed to the blocking effect of 16f sites which were found to be thermodynamically more stable than tetragonal 8a and 32g sites through density functional theory based calculations. Besides, the stabilizing effect of supervalent doping on cubic phase was also studied through Ta-doped LLZO. Further site occupancy investigations indicated that supervalent doping introduced lithium vacancies and reduced octahedral sites (tetragonal 16f and 32g sites) occupancy due to site energy preference. The reduced octahedral sites occupancy weakened the blocking effect of tetragonal 16f sites, promoted the lithium redistribution, and eventually lowered the phase transition temperature.