Influence of concentration-dependent material properties on the fracture and debonding of electrode particles with core-shell structure

Abstract

Core-shell electrode particle designs offer a route to improved lithium-ion battery performance. However, they are susceptible to mechanical damage such as fracture and debonding, which can significantly reduce their lifetime. Using a coupled finite element model, we explore the impacts of diffusion-induced stresses on the failure mechanisms of an exemplar system with an NMC811 core and an NMC111 shell. In particular, we systematically compare the implications of assuming constant material properties against using Li concentration-dependent diffusion coefficient and partial molar volume. With constant material properties, our results show that smaller cores with thinner shells avoid debonding and fracture regimes. When factoring in a concentration-dependent partial molar volume, the maximum values of tensile hoop stress in the shell are found to be significantly lower than those predicted with constant properties, reducing the likelihood of fracture. Furthermore, with a concentration-dependent diffusion coefficient, significant barriers to full electrode utilisation are observed due to reduced lithium mobility at high states of lithiation. This provides a possible explanation for the reduced accessible capacity observed in experiments. Shell thickness is found to be the dominant factor in precluding structural integrity once the concentration dependency is accounted for. These findings shed new light on the performance and effective design of core-shell electrode particles.

Figures (9)

• Figure 1: Core-shell design: (a) SEM cross-sectional image of $\mathrm{LiMn_{0.85}Fe_{0.15}PO_4}$ particle coated with $\mathrm{LiFePO_4}$ adapted from oh2012double; (b) Schema of an ideal spherical particle with core-shell structure; (c) High-resolution FIB image depicting a cross-section of an NMC core-shell particle after a single charge, revealing shell fracture and core-shell debonding brandt2020synchrotron.
• Figure 2: Two failure modes of the core-shell structure: (a) fracture of the shell caused by large hoop stress during lithiation; (b) debonding at the core-shell interface induced by large radial stress during delithiation.
• Figure 3: Concentration-dependent material properties: (a) diffusion coefficient $D_{c,1}$ (NMC811) and $D_{c,2}$ (NMC111), measured using GITT method Gaoy2020Wu2012. (b) molar volume $\Omega_{c,1}$ (NMC811) and $\Omega_{c,2}$ (NMC111) derived from in situ X-ray diffraction data Biasi2017. The constant effective values are shown using dotted lines.
• Figure 4: Delithiation results along the radial direction: dimensionless concentration for (a) $\bar{D}$, $\bar{\Omega}$, (b) $D_c$, $\bar{\Omega}$, (c) $\bar{D}$, $\Omega_c$; hoop stress for (d) $\bar{D}$, $\bar{\Omega}$, (e) $D_c$, $\bar{\Omega}$, (f) $\bar{D}$, $\Omega_c$; and radial stress for (g) $\bar{D}$, $\bar{\Omega}$, (h) $D_c$, $\bar{\Omega}$, (i) $\bar{D}$, $\Omega_c$. An overline denotes constant (averaged) properties while a $c$ subscript denotes concentration-dependent properties. The radial position $r$ is normalised by the outer radius $b$ of the core-shell system.
• Figure 5: Lithiation results along the radial direction: dimensionless concentration for (a) $\bar{D}$, $\bar{\Omega}$, (b) $D_c$, $\bar{\Omega}$, (c) $\bar{D}$, $\Omega_c$; hoop stress for (d) $\bar{D}$, $\bar{\Omega}$, (e) $D_c$, $\bar{\Omega}$, (f) $\bar{D}$, $\Omega_c$; and radial stress for (g) $\bar{D}$, $\bar{\Omega}$, (h) $D_c$, $\bar{\Omega}$, (i) $\bar{D}$, $\Omega_c$. An overline denotes constant (averaged) properties while the subscript $c$ denotes concentration-dependent properties.