Diffusion behavior of Li ions in crystalline and amorphous Li-Zr-O and Li-Nb-O phases

TL;DR

This study uses molecular dynamics simulations to analyze Li ion diffusion in crystalline and amorphous Li-Zr-O and Li-Nb-O phases, revealing different migration mechanisms and activation energies.

Contribution

It provides new insights into Li ion migration mechanisms and diffusivities in both crystalline and amorphous phases of Li-Zr-O and Li-Nb-O materials.

Findings

Crystalline Li₂ZrO₃ shows vacancy-mediated and site exchange migration mechanisms.

Amorphous phases exhibit similar mechanisms with reduced activation energies.

LiNbO₃ amorphous phase also shows the site exchange migration mechanism.

Abstract

Li containing transition metal oxides are known as good ionic conductors. Performing classical molecular dynamics simulations, the diffusion behavior of Li ions is investigated in crystalline and amorphous phases with the stoichiometries Li$_2$ZrO$_3$ and LiNbO$_3$. We first demonstrate the stability of the crystal structures for the used interatomic potential model and then analyze the amorphous phases, which result from melt-and-quench simulations, in terms of radial distribution functions. Diffusivities of Li ions in those systems are obtained from a statistical Arrhenius analysis of mean square displacement curves at different temperatures. The crystalline phase of Li$_2$ZrO$_3$ exhibits two well-defined migration mechanisms: vacancy-mediated migration is dominant below and a site exchange of Li ions above a crossover region between about 1700 and 1800 K. The latter mechanism also prevails in the amorphous phases of Li$_2$ZrO$_3$ with a strongly reduced activation energy, which is due to a smaller equilibrium separation of Li ions as in the crystal structure. This migration mechanism is found in amorphous LiNbO$_3$, too.