Sparse Cyclic Excitations Explain the Low Ionic Conductivity of Stoichiometric Li$_{7}$La$_{3}$Zr$_{2}$O$_{12}$

TL;DR

This study uses molecular dynamics simulations to analyze lithium ion transport mechanisms in different phases of LLZO, revealing that sparse, concerted excitations in tetragonal phase explain its lower ionic conductivity compared to cubic phase.

Contribution

It introduces a detailed mechanistic understanding of lithium transport differences in LLZO phases based on dynamical excitations and collective motion analysis.

Findings

Frequent excitations in cubic LLZO mirror fragile glass dynamics.

Sparse, concerted excitations in tetragonal LLZO involve closed-loop motion.

Lithium ordering in t-LLZO explains its lower ionic conductivity.

Abstract

We have performed long time-scale molecular dynamics simulations of the cubic and tetragonal phases of the solid lithium-ion--electrolyte Li$_{7}$La$_{3}$Zr$_{2}$O$_{12}$ (LLZO), using a first-principles parameterised interatomic potential. Collective lithium transport was analysed by identifying dynamical excitations; persistent ion displacements over distances comparable to the separation between lithium sites, and string-like clusters of ions that undergo cooperative motion. We find that dynamical excitations in c-LLZO are frequent, with participating lithium numbers following an exponential distribution, mirroring the dynamics of fragile glasses. In contrast, excitations in t-LLZO are both temporally and spatially sparse, consisting preferentially of highly concerted lithium motion around closed loops. This qualitative difference is explained as a consequence of lithium ordering in t-LLZO, and provides a mechanistic basis for the much lower ionic conductivity of t-LLZO compared to c-LLZO.