Atomistic origins of asymmetric charge-discharge kinetics in off-stoichiometric LiNiO$_2$

TL;DR

This study uses first-principles kinetic Monte Carlo simulations to uncover atomistic mechanisms behind asymmetric charge-discharge kinetics in off-stoichiometric LiNiO₂, explaining capacity loss and kinetic bottlenecks at the atomic level.

Contribution

It provides a detailed atomistic understanding of charge-discharge asymmetry and the effects of Ni antisite defects in LiNiO₂, which was not fully understood before.

Findings

Li transport is hindered at charge and discharge ends due to different rate-limiting steps.

Ni_Li antisite defects increase overpotential during discharge but not during charge.

The model reproduces experimental trends in capacity loss and overpotential with varying Ni_Li and temperature.

Abstract

LiNiO$_2$ shows poor Li transport kinetics at the ends of charge and discharge in the first cycle, which significantly reduces its available capacity in practice. The atomistic origins of these kinetic limits have not been fully understood. Here, we examine Li transport in LiNiO$_2$ by first-principles-based kinetic Monte Carlo simulations where both long time scale and large length scale are achieved, enabling direct comparison with experiments. Our results reveal the rate-limiting steps at both ends of the voltage scan and distinguish the differences between charge and discharge at the same Li content. The asymmetric effects of excess Ni in the Li layer (Ni$_\textrm{Li}$) are also captured in our unified modelling framework. In the low voltage region, the first cycle capacity loss due to high overpotential at the end of discharge is reproduced without empirical input. While the Li concentration gradient is found responsible for the low overpotential during charge at this state of charge. Ni$_\textrm{Li}$ increases the overpotential of discharge but not charge because it only impedes Li diffusion in a particular range of Li concentration and does not change the equilibrium voltage profile. The trends from varying the amount of Ni$_\textrm{Li}$ and temperature agree with experiments. In the high voltage region, charge becomes the slower process. The bottleneck becomes moving a Li from the Li-rich phase (H2) into the Li-poor phase (H3), while the Li hopping barriers in both phases are relatively low. The roles of preexisting nucleation sites and Ni$_\textrm{Li}$ are discussed. These results provide new atomistic insights of the kinetic hindrances, paving the road to unleash the full potential of high-Ni layered oxide cathodes.