Origin of fast ionic diffusion in superionic tetragonal LGPO: atomistic insights

We address Li-ion diffusivity in hypothetical tetragonal Li₁₀GeP₂O₁₂ (t-LGPO) from ab initio molecular dynamics, confirming it to be a prospective fast-ion conductor. We further confirm these results with a machine-learning molecular dynamics study, exploiting a pre-trained graph neural network universal potential, and with ab initio nudge elastic band (NEB) calculations. In order to shed light onto the diffusion mechanism of this promising oxide, we perform NEB calculations also for the existing isostoichiometric orthorhombic oxide (o-LGPO), notably less conductive than t-LGPO, and provide minimum energy paths for Li-ion diffusion to compare with t-LGPO. The analysis of the lithium coordination environments along the diffusion paths, based on the continuous symmetry measure, reveals that the superior ionic conductivity of t-LGPO is due to the remarkably higher level of distortion of the lithium polyhedra in the tetragonal phase. These results pave the way to novel synthesis efforts, aimed at achieving attractive oxide fast Li-ion conductors for all-solid-state batteries.