Failure Theory Analysis Of Lithium Battery Electrode Materials Based On Fracture Mechanics

The electrode material of Li-ion battery will produce large volume deformation during charging and discharging,which will induce large diffusion stress and make the material fracture or shatter,which will directly lead to the decay of Li-ion battery capacity and reduce the cycle performance of the electrode material.In order to further clarify the failure mechanism of lithium battery electrode materials and predict their fracture and pulverization failure,this thesis carries out the analysis under the state of complete elasticity theory and ideal elastic-plasticity theory respectively,simulates the lithiation process of electrode materials with the help of finite element numerical simulation software,analyzes the evolution law of stress and strain in the lithiation process of electrode materials,obtains the lithiation of electrode materials with initial crack The fracture mechanism of the surface is obtained,and the stress damage of lithiation with and without initial crack is compared.The main contents of this thesis include the following.(1)A thin film electrode structure model for lithiation diffusion is developed,dislocation theory and fracture theory are introduced,ideal elastic-plastic deformation of the material is considered,and the dimensions,initial conditions and boundary conditions of the geometric model are determined.In order to describe the dynamic diffusion process and the corresponding stress evolution,the two-way diffusion-stress coupling analysis in the model is realized by the two-way thermal-stress coupling analysis module,which equates the diffusion equation with the heat conduction equation.(2)Qualitative analysis of concentration,dislocation,horizontal stress,vertical stress,Mises yield stress and strain field.The trends of the ideal elastoplastic and fully elastic concentration diagrams are slightly different because the volume change allows plastic deformation to have an effect on the concentration change.The ideal elastoplastic model has relatively low confining stresses due to the presence of plasticity,relieving its deformation.For perfectly elastic materials,at the end of lithiation,the horizontal stress is compressive and the vertical stress is tensile;while for elasto-plastic materials,at the end of lithiation,both horizontal and vertical stresses are tensile,providing a good basis for the cracking of lithium-ion electrode materials.Under the ideal elasto-plastic state,the surface on the von Mises yield stress of the thin film material will be the first to yield.In both states,the lithiation-induced shear strain field follows the law of reciprocal shear stress.(3)The stress evolution and crack types with initial cracks are investigated and the lithiation damage results with and without initial cracks are compared.Based on fracture mechanics and using the extended finite element,i.e.,sequential coupling method,the shear stress evolution,the maximum principal stress evolution,and the crack types on the upper surface in the lithiation damage model are investigated and analyzed.The shear stresses show that the tensile plastic flow located in the surface layer during lithiation leads to the morphological instability of the thin film active material,which leads to the destruction of the material.The material cracks when the maximum allowable principal tensile stress in the material parameters is reached.By studying the normal positive and shear stresses on the upper surface of the thin film active material during lithiation damage,the crack type of the material is found to be type II cracking.By comparing the lithiation damage of the material with and without the initial crack,it was found that the cracks in the thin film active material always preferentially crack in the direction of the prefabricated crack.