Study On Multiscale Finite Element Method Of Lithium Ion Diffusion-mechanics Coupling

Lithium-ion batteries rely on the diffusion of lithium ions between the positive and negative electrodes to work,in which the coupling of diffusion and stress in the electrode materials is an important factor that threatens the performance and safety of the battery.The diffusion behavior usually leads to uneven distribution of concentration,which leads to uneven deformation.Uneven deformation causes diffusion-induced stress,which may leads to cracking of the electrode material and in turn causes the electrode capacity to decay and the battery performance to deteriorate.In order to study the ions diffusion and stress evolution in this process and promote the improvement of mechanical behavior during the lithiation of electrode materials,the lithium ion diffusion-mechanics coupling model in electrode materials was established based on multiscale cohesive finite element method.The main research contents and achievements in this paper are as follows:1.By combining chemical energy with strain energy,the diffusion potential is applied into the multiscale cohesion finite element method based on Cauchy-Born rule.The diffusion-mechanics coupling equation in lithium ion diffusion process is established and the discrete form of finite element is derived.2.An analysis program of lithium ion diffusion-mechanics coupling effect based on triangular three-node elements is developed.In this program,implementation of the Cauchy-Born rule based on LJ potential function is realized and and the multiscale stress based on two-dimensional hexagonal unit cell is calculated.3.The numerical calculations of grainboundaries-free and grainboundariescontaining,crack-free and crack-containing were all established by taking thin film silicon of anode material as an example.The model containing the grain boundary utilizes the Voronoi tessellations to generate randomly shaped polycrystalline grain and the separation phenomenon of the grain boundary is simulated by the multiscale cohesive model.The simulation of the crack is also realized by the cohesive elements.4.The diffusion-mechanics coupling behavior of the electrode material under the condition of no grain boundary is calculated and analyzed,and the variation of the coupling field in the process of lithium ion inserted electrode material is simulated.The coupling field in the case of crack-free and crack-containing conditions is simulated and relative calculation analysis is performed.Electrode materials containing linear crack exhibit significant stress concentration at the crack tip during the intercalation of lithium ions,which can easily lead to structural failure.5.The diffusion-mechanics coupling behavior of the electrode material with grain boundary is calculated and analyzed,and the presence or absence of crack is also discussed separately.In the case where the grain diffusion coefficient is constant,the larger the diffusion coefficient of the grain boundary is,the lower the stress level of the structure is,and the smaller the grain density,that is,the larger the grain size is,the higher the stress level of the structure is.The electrode material of the polycrystalline structure is more stable than the case of no grain boundary.The above research results show that the multiscale cohesive finite element diffusion-mechanics coupling model proposed in this paper can simultaneously consider the condition of grain boundary and crack in the material to simulate the process of lithiation in electrode material and obtain reasonable concentration variation and stress evolution results.The proposed model has positive driving significance for exploring the mechanical behavior of lithium ion battery electrode materials,reducing material mechanical damage and improving battery performance.