Theory of layered-oxide cathode degradation in Li-ion batteries by oxidation-induced cation disorder

TL;DR

This paper presents a free energy model for layered-oxide cathode degradation in Li-ion batteries, linking defect formation and electrostatic forces to capacity loss, and suggests strategies to improve battery longevity.

Contribution

It introduces a comprehensive free energy model for cation disorder in layered-oxide cathodes, integrating electrostatic, defect, and entropy effects, to predict degradation mechanisms.

Findings

Lower-voltage cycling reduces cathode degradation.

Model aligns with experimental degradation observations.

Guides development of more durable cathode materials.

Abstract

Disorder-driven degradation phenomena, such as structural phase transformations and surface reconstructions, can significantly reduce the lifetime of Li-ion batteries, especially those with nickel-rich layered-oxide cathodes. We develop a general free energy model for layered-oxide ion-intercalation materials as a function of the degree of disorder, which represents the density of defects in the host crystal. The model accounts for defect core energies, long-range dipolar electrostatic forces, and configurational entropy of the solid solution. In the case of nickel-rich oxides, we hypothesize that nickel with a high concentration of defects is driven into the bulk by electrostatic forces as oxidation reactions at the solid-electrolyte interface reduce nickel and either evolve oxygen gas or oxidize the organic electrolyte at high potentials (>4.4V vs. Li/Li+). The model is used in battery cycling simulations to describe the extent of cathode degradation when using different voltage cutoffs, in agreement with experimental observations that lower-voltage cycling can substantially reduce cathode degradation. The theory provides a framework to guide the development of cathode compositions, coatings and electrolytes to enhance rate capability and enhance battery lifetime. The general theory of cation-disorder formation may also find applications in electrochemical water treatment and ion separations, such as lithium extraction from brines, based on competitive ion intercalation in battery materials.