Impact of the Energy Landscape on the Ionic Transport of Disordered Rocksalt Cathodes

TL;DR

This study reveals that in disordered rock-salt cathodes, ion transport is heavily influenced by the energy landscape, with only certain stable sites facilitating diffusion, leading to significantly reduced ionic conductivity.

Contribution

The paper introduces a kinetic Monte Carlo simulation approach to account for site stability and energy landscape effects on ion transport in disordered cathodes, challenging traditional equal-probability models.

Findings

Li-site participation in diffusion is limited to a fraction connected by low-barrier channels.

Effective migration barriers are higher than the lowest barriers due to site stability effects.

Cation disorder can reduce ionic diffusion coefficients by over 100 times.

Abstract

Traditional approaches to identify ion-transport pathways often presume equal probability of occupying all hopping sites and focus entirely on finding the lowest migration barrier channels between them. Although this strategy has been applied successfully to solid-state Li battery materials, which historically have mostly been ordered frameworks, in the emerging class of disordered electrode materials some Li-sites can be significantly more stable than others due to a varied distribution of transition metal (TM) environments. Using kinetic Monte Carlo simulations, we show that in such cation-disordered compounds only a fraction of the Li-sites connected by the so-called low-barrier ``0-TM" channels actually participate in Li-diffusion. The Li-diffusion behavior through these sites, which are determined primarily by the voltage applied during Li-extraction, can be captured using an effective migration barrier larger than that of the 0-TM barrier itself. The suppressed percolation due to cation disorder can decrease the ionic diffusion coefficient at room temperature by over 2 orders of magnitude.