Numerical Simulation Of Fracture Behavior For Active Particle In Electrode During The Charge/Discharge Process

The fracture of active particles in Li-ion batteries is one of the most important reasons for capacity fading.The numerical simulation method can be used to figure out the fracture mechanism of active particles,which can help us to optimize the structure of Li-ion batteries and charge/discharge strategies,thus improving the performance of the Li-ion batteries.In this paper,the numerical method is applied to simulate the evolution of internal cracks of active particles under charge or discharge conditions and the fracture behavior of active particles.The main contents are as follows:A spherical particle model with center crack is established by combining coupled diffusion-stress problem and extended finite element?XFEM?method.The effects of particle size,initial crack length,charge/discharge rate?C-rate?and crack surface diffusion?CSD?on the Li-ion concentration distribution,stress distribution and fracture behavior of active particles were studied by numerical method.The fracturing process occurs according to the following stages:no crack growth,stable crack growth,and unstable crack growth.CSD can reduce the difference of Li-ion concentration and stress in active particles,so it can also prevent the breakage of active particles,delay the time of entering the next fracture stage.The longer the initial crack length,the more obvious the effect of delay.A model for the interaction of active particles and binder was established.The modified J-integral formula is used to study the relationship between crack length and J-integral to derive the worst crack length.Sequential coupled and fully coupled diffusion-stress results are compared to study the effect on Li-ion concentration,stress and J-integral.The effects of adhesive attachment area and binder stiffness on J-integral are studied.The maximum J-integral of both surface crack and center crack appear at r=0.6R.Adhesive attachment area also affects the electrochemical performance of the battery,resulting in inhomogeneous distribution of the Li-ion concentration of the active particles,making complete charging and discharging more difficult.The elastic modulus of the binder greatly affects the driving force of crack propagation.The softer binder makes the maximum stress of the active particles smaller and the cracks are less likely to propagate.Based on experimental observations of LiNi_xCo_yMn_1-x-yO₂?NMC?particle morphology,a model of a secondary particle composed of randomly distributed primary particles is established using a fractal algorithm.The finite element method with cohesive crack modeling is employed to simulate the intraparticle fracture within the secondary particle.The different fracture energies are analyzed during the lithiation process and the crack behavior during the lithiation/de-lithiation cycling is studied.The fracture energy has a significant influence on crack density and crack branching.Model I cracks appear at the center of the secondary particle during the lithiation process along with the crack branching phenomenon.A model?crack appears on the surface of the secondary particle during the lithiation process.The damage during the electrochemical?lithiation/de-lithiation?cycling results in cracks within the secondary particle,which can penetrate the entire particle after certain lithiation/de-lithiation cycles.