Coupled Mechano-chemical And Fracture Behavior Of Electrode Particles

The diffusion-induced stress（DIS）of the electrode active particles in the process of charge and discharge will lead to the particle cracking,which will lead to the capacity fading of the lithium battery.Establishing a mechanical-electrochemical coupling model to study the concentration distribution,stress evolution and fracture problems inside the active particles.In this study,concentration-dependent material properties are introduced to model the distribution of the concentrations and evolution of DIS in anisotropic active particles.The concentration-dependent diffusion coefficient increases the concentration gradient and thus the DIS,and the concentration-dependent elastic modulus hardening increases the internal DIS and thus the stress-enhanced diffusion of Li ions.Diffusion in the direction of a large diffusion coefficient enhances the diffusion in the direction of a small diffusion coefficient,which leads to an anisotropic concentration,concentration gradient and DIS.The anisotropic diffusion decreases the DIS in the particles,the greater the anisotropic difference within the particles is,the more obvious the decrease in the radial stress and hoop stress.The results can be comparable with many published experimental results of graphite and indicate that the role of concentration-dependent material properties and anisotropy in the particles cannot be ignored.A model for simulating the effects of cracks and binders on the Li-ion distribution,DIS,and J-integral of active particles is proposed in this paper.Comparisons are made between the two-way coupled stress-diffusion model and the one-way coupled model.The relationship of crack length and the J-integral or the fracture properties of active particles under the influence of binders are studied by a modified J-integral equation.Li ions are attracted by the tensile DIS at the crack tip.Tensile-compressive DIS conversion maximizes the J-integral.Additionally,the J-integral is negatively correlated with the attachment area of the binders,and the J-integral increases with the increasing elastic modulus of the binders.The relationship between DIS and stress-enhanced diffusion leads to the trend of J-integral.The results indicate that it is necessary to consider the influences of binders on the electrochemo-mechanical properties of active particles.The concentration-dependent material parameters are introduced into the calculation of J-integral.The concentration-dependent modulus hardening increases the J-integral inside the particles,while the concentration-dependent diffusion coefficient decreases the J-integral inside the particles.The larger the charge/discharge rate（C-rate）,the larger the J-integral in the active particles,and the easier the crack propagation.The particle size is positively correlated with the J-integral,and the larger the particle size is,the easier the crack propagation is.Therefore,high C-rate are not favored while satisfying other conditions.When making the electrode,try to choose smaller active particles,which will help to suppress the crack growth in the electrode.The geometric model of Li Ni_xCo_yMn_1-x-yO₂（NCM）particles is established by voronoi algorithm,and the cohesive element is used to simulate the fracture problem at the interface of primary particles.The influence of fracture energy,the key factor of crack formation,is discussed.The larger the fracture energy,the more difficult the particles are to crack.The smaller the fracture energy,the easier the particles are to crack.The higher the C-rate,the easier the particles are to crack.When the C-rate reached 5C,the propagation path of the main crack will change,and the growth of the branch crack will be inhibited by the main crack.The bond strength between the primary particles or the fracture energy of the active material decrease with the cycle of charge and discharge,which may be the reason for the formation of island particles and holes in the secondary NCM particles.