Phase field modelling of cracking and capacity fade in core-shell cathode particles for lithium-ion batteries

TL;DR

This work tackles mechanical degradation in core-shell Li-ion cathode particles by introducing a fully coupled chemo-mechano-damage framework based on a phase-field fracture formulation. The model links diffusion-induced stresses, interfacial debonding, and material damage within a unified AT2 phase-field approach, incorporating a diffuse core-shell interface and diffusion degradation due to cracking. Numerical case studies on NMC811@NMC532 reveal how initial defect location, core size, shell thickness, and charging rate govern cracking patterns and resulting capacity fade, with weaker core-shell bonding accelerating debonding and reducing final lithiation. The framework enables predictive guidance for microstructural design and manufacturing strategies to mitigate mechanical degradation and extend battery lifetime.

Abstract

Core-shell electrode particles are a promising morphology control strategy for high-performance lithium-ion batteries. However, experimental observations reveal that these structures remain prone to mechanical failure, with shell fractures and core-shell debonding occurring after a single charge. In this work, we present a novel, comprehensive computational framework to predict and gain insight into the failure of core-shell morphologies and the associated degradation in battery performance. The fully coupled chemo-mechano-damage model presented captures the interplay between mechanical damage and electrochemical behaviours, enabling the quantification of particle cracking and capacity fade. Both bulk material fracture and interface debonding are captured by utilising the phase field method. We quantify the severity of particle cracking and capacity loss through case studies on a representative core-shell system (NMC811@NMC532). The results bring valuable insights into cracking patterns, underlying mechanisms, and their impact on capacity loss. Surface cracks are found to initiate when a significantly higher lithium concentration accumulates in the core compared to the shell. Interfacial debonding is shown to arise from localised hoop stresses near the core-shell interface, due to greater shell expansion. This debonding develops rapidly, impedes lithium-ion transport, and can lead to more than 10\% capacity loss after a single discharge. Furthermore, larger particles may experience crack branching driven by extensive tensile zones, potentially fragmenting the entire particle. The framework developed can not only bring new insight into the degradation mechanisms of core-shell particles but also be used to design electrode materials with improved performance and extended lifetime.

Figures (15)

• Figure 1: Schematic representation of the multiphysics framework, showing the coupling among lithium-ion diffusion, strain/stress, and cracking.
• Figure 2: Schematic representation of an electrode particle of volume $V_s$ containing (a) a discrete sharp crack, described by $\Gamma$ and (b) a phase field crack, characterised by the phase field order parameter $\phi$ and the regularised crack surface $\Gamma_\ell$.
• Figure 3: A diffuse representation of the core-shell interface: (a) Radial distribution of $G_c$ under 3 assumptions for $G_{c,I}$; (b) Visual representation of the variation of $G_c$ with the assumption $G_{c,I}=0.1G_{c,ave}$.
• Figure 4: Key definitions and boundary value problem. (a)-(c) Schematics of three types of initial cracks in spherical core-shell particles; and (d) contour plot showing that a diffuse initial crack is used instead of a geometric one. In all figures, fully cracked regions ($\phi > 0.95$) are removed for the sake of better crack visualisation.
• Figure 5: Evolution of the state of lithiation (SOL) for a cracked particle. When assuming that the mechanical damage impedes diffusion ($D=D_0g(\phi)$), the state of lithiation evolution shows a capacity fade compared to full lithiation without coupling between damage and diffusion ($D_0$).