Diffusion Mechanisms in Li$_{0.5}$CoO$_2$ -- A Computational Study

TL;DR

This study uses atomistic simulations and DFT calculations to investigate Li-ion diffusion in Li$_{0.5}$CoO$_2$, revealing how different configurations affect diffusion barriers and explaining the observed diffusivity dip.

Contribution

It identifies the zig-zag Li arrangement as the lowest energy phase and explains the diffusivity dip through differing diffusion barriers in various configurations.

Findings

Zig-zag Li configuration is energetically favored over linear.

Diffusion barriers differ significantly parallel and perpendicular to the zig-zag phase.

The diffusivity dip is explained by the variation in energy barriers.

Abstract

An atomistic study of the order-effect occurring in Li$_{x}$CoO$_{2}$ at $x=0.5$ is presented and an explanation for the computationally and experimentally observed dip in the Li diffusivity is proposed. Configurations where a single half-filled Li layer arranged in either a linear or a zig-zag pattern are simulated. It is found that the lowest energy phase is the zig-zag pattern rather than the linear arrangement that currently is considered to be of lowest energy. Atomic interactions are modeled at the DFT level of accuracy and energy barriers for Li-ion diffusion are determined from searches for first order saddle points on the resulting potential energy surface. The determined saddle points reveal that the barriers for diffusion parallel and perpendicular to the zig-zag phase differ significantly and explain the observed dip in diffusivity.