Mechanical Failure Analysis of Graphite Anode Particles with PVDF Binders in Li-Ion Batteries

Abstract

The mechanical failure of electrodes is one of the main mechanisms of degradation in Li-ion batteries. Many studies have been carried out on the fracture inside a single particle; however, studies on particles with binders are limited. In this study, we investigated stress development under the anisotropic lithium flux in a graphite particle partially attached to an ionically non-conductive polyvinylidene difluoride (PVDF) binder. The study reveals that the location of the maximum stress depends on the particle sizes and C-rates due to the anisotropic lithium intercalation and the mechanical failure can initiate with high probability either at the particle center or at the binder interface according to different sizes and C-rates. The simulations showed that small particles under low charging rates tend to produce high stress at the edge of the particle/binder interface, while large particles under high charging rates tend to generate high stress inside the particle. The possible fracture locations are determined by competition of the interface fracture due to expansion and at the inner fracture due to the gradients of lithium concentration. We also studied the effect of binder geometries on the level of stress and found that the stress concentration near the binder edges increases the possibility of the binder debonding as the binder size increases and the angle between the binder and particle decreases. The study will expand to crack propagation in the cluster of particles, eventually linking to the capacity fade issue due to mechanical failures.