A Shrinking-Core Model for the Degradation of High-Nickel Cathodes (NMC811) in Li-Ion Batteries: Passivation Layer Growth and Oxygen Evolution

Abir Ghosh,Monica Marinescu,Gregory Offer,Jamie M Foster

doi:10.1149/1945-7111/abdc71

Abstract

A degradation model for high-nickel positive electrode materials that undergo a structural reorganisation involving oxygen loss and the formation of a disordered (spinel or rock-salt structure) passivation layer is presented for the first time. The model is a thermally coupled continuum model based on the single-particle model and is based upon a LiNi0.8Mn0.1Co0.1O2 (NMC811) layered oxide in this instance. The theoretical framework assumes a shrinking core mechanism, where lattice oxygen, [O], release occurs at the interface between the bulk and the passivation layer, and the rate of reaction is controlled by either [O]-diffusion through the passivation layer or the reaction kinetics at the interface. As the passivation layer grows, the core of active positive electrode material shrinks giving rise to both loss in active material (LAM) and loss in lithium inventory (LLI) through trapping lithium in the passivation layer, giving rise to capacity fade. The slower diffusion of lithium through the passivation layer also gives rise to power fade. The model predicts two limiting cases, “diffusion dominated” if [O]-diffusion is slow, and “reaction dominated” if [O]-diffusion is fast, relative to the reaction rate of [O]-release and also the thickness of the passivation layer.

Highlights

High-nickel NMC structures are thermally less stable at high temperatures ⩾170 °C rather than at room temperature.[15,16,17] Li-ion-depleted layered oxide NMC811 structures are thermodynamically unstable even at battery operating temperature (20 °C–80 °C) when cycled at higher potentials (∼4.3 V vs Li/Li+)
A single-particle models (SPMs) is common in Li-ion battery modelling and the model assumes that even though each electrode is comprised of many different electrode particles, the behaviour of each of these particles is sufficiently similar that no significant errors are engendered by solving in just one representative particle.[59,60]
The SPM is chosen for this study because it is the simplest model within which to demonstrate a new degradation model and is suitable to treat the operational conditions considered in this study

Summary

Introduction

High-nickel NMC structures are thermally less stable at high temperatures ⩾170 °C rather than at room temperature.[15,16,17] Li-ion-depleted layered oxide NMC811 structures are thermodynamically unstable even at battery operating temperature (20 °C–80 °C) when cycled at higher potentials (∼4.3 V vs Li/Li+). Considering the above, we have focused on exploring the influences of the [O]-diffusivity (Do) through the shell, reaction rate constants (K1 and K2), Li-ion-diffusivity (Ds) through the shell, amount of Liions lost (f) in shell formation, and initial shell thickness (Rp − si) on the evolution of the cell behaviour to uncover the physics of this degradation mechanism.

Results

Conclusion