Investigation of Li Ion Dynamics in Garnet Oxide Li6.5La3Zr1.5Ta0.5O12 Using Molecular Dynamics Simulation

Abstract

To overcome the safety issue of liquid electrolytes in conventional lithium-ion batteries, various solid-state inorganic electrolytes have been studied. Among them, lithium garnet oxides have been attracting research interests due to their high ionic conductivity, Li7-xLa3Zr2-xTaxO12 (LLZTO) with have a at room temperature with a maximum value around x = 0.51,2. There are two different structures of lithium garnet oxides, a cubic structure with space group Iad and a tetragonal phase with space group I41/acd. The cubic structure has two types of Li sites, i.e., tetrahedral (Td) 24d sites and octahedral (Oh) 48g sites. The tetragonal structure has four types of Li sites, i.e., Td-8a, Td-16e, Oh-16f, and Oh-32g sites. The fully empty 16e sites and fully occupied the other sites in tetragonal structure makes its conductivity about 2 orders lower than the cubic phase. However, the complexity of structure hindered the understanding of the mechanism of lithium ion conduction. In this paper, we performed molecular dynamics (MD) simulation on Li6.5La3ZrTaO12 based on classic MD simulation from both the interatomic potential and DFTB methods to study its structure and properties including lattice parameters, diffusion parameters, ionic conductivities, dielectric constants, and elastic moduli. The results are extracted through correlation function that was used for simple liquids3. The consistency of our results with the reported experimental ones helped us to understand the Li diffusion mechanism. References (1) Y. X. Wang and W. Lai "High Ionic Conductivity Lithium Garnet Oxides of Li7-xLa3Zr2-xTaxO12 Compositions," Electrochem. Solid State Lett. 2012, 15, A68-A71. http://dx.doi.org/10.1149/2.024205esl(2) Y. T. Li, J. T. Han, C. A. Wang, H. Xie, and J. B. Goodenough "Optimizing Li+ conductivity in a garnet framework," J. Mater. Chem. 2012, 22, 15357-15361. http://dx.doi.org/10.1039/c2jm31413d(3) In Theory of Simple Liquids (Fourth Edition); J.-P. Hansen and I. R. McDonald, Eds.; Academic Press: Oxford, 2013, p i.