Research On Fracture Mode And Mechanical Properties Of Separator For Lithium-ion Power Battery

With the rapid development of electronic products and the promotion of new energy vehicles,people have more and more demands on batteries.The many advantages of lithium-ion batteries ensure great potential for future development and application.As an important part of lithium-ion battery,the separators prevent contact between the positive and negative electrodes,allowing lithium ions to pass through and blocking the passage of electrons.In lithium-ion batteries,the performance of the separator directly affects the internal resistance,capacitance and cycle life of lithium-ion batteries.The separators act as the physical barriers to prevent contact between the positive and negative electrodes,and their structural integrity is critical to battery safety.Therefore,it must be ensured that the separator has sufficient mechanical properties to avoid contact between the positive and negative electrodes.In order to understand the fracture mode and mechanical properties of the separator under different loads in lithium-ion battery,this paper uses Instron 5544 to conduct different tensile,tear and creep experiments on different types of separator materials.Electron microscopy,high-speed cameras,anddigital image correlation are used to research the mechanical properties and fracture modes of separator materials.It not only has important significance for understanding the mechanical properties of separator materials,but also provides parameters basis for numerical simulation of lithium ion power battery separators.It also reduces the amount of measurement work for the manufacturer and helps separator manufacturers optimize separator performance.(1)In the unnotched samples tensile test,since the separator is a typical anisotropic material,uniaxial tensile tests were conducted in multiple directions for different separator materials.The tensile strength,failure strain and fracture mode of the separator material were studied with different loading angles.The influence of coating on the mechanical properties of the separator material was analyzed.The effect of tensile load on the porosity and pore shape of the separator was studied at microscale.The results show that the trigonometric function equation can well fit the variation of tensile strength with loading angle;except for the 0degree specimens along the transverse direction fracture,the other loading angle specimens are broken along the machine direction.The ability of the coating to withstand loads is much less than that of the base film.The coated separator will not break immediately after reaching the tensile strength,showing a certain angle between the stress-strain curve and the vertical direction;the tensile loading reduces the porosity of the separator by causing deformation or even closure of the pores.(2)All cases in which the sample may be broken or may be torn when the samples have a notch are considered.In the tensile test,the notched specimens and the unnotched specimens in different directions were compared and analyzed to obtain the influence of the existence of the notch and the depth and position of the notch on the mechanical properties and fracture mode of the separator material.The fracture toughness and tear properties of the separator material were measured by the essential work of fracture(EWF)and tear propagation resistance(TPR).The results show that when there is a notch in the separator material,it will show the same notch effect as the metal material;the notch will increase the elastic modulus of the separator material and reduce the plasticity;the larger the notch depth,the larger the elastic modulus of the separator;The failure strain decreases as the depth of the notch increases.For quasi-static experiments of notched specimens,the location of the notch has no effect on the mechanical properties and fracture mode of the separator material.The fracture toughness of the separator material in different directions also has anisotropy,and the fracture mechanism is independent of the length of the ligament.In the tearing experiment,tear propagation resistance(TPR)and the fracture mode of various separator materials in different directions were obtained.The results show that there is obvious anisotropy in different directions,and the TPR value of the 90 degree sample is the largest.The fracture mode was also different from that in the tensile test,and all the samples were torn along the MD.(3)Through creep experiments on the four separator materials in the 0degree direction,the relationship between creep value and creep resistance of the separator material under different tensile strengths as a function of material thickness and the coating was analyzed.The Kelvin-Voigt model was used to fit the average creep compliance of its linear viscoelastic phase.When the separator is coated or thick,there is a large creep compliance value;when the fixed tensile strength value in the separator creep test is greater than or less than the yield strength of the material itself,the creep rate increases at different speeds;increasing the number of Kelvin-Voigt element models can improve the accuracy of the fit;for the four dry uniaxially stretched separators used in this paper,two Kelvin-Voigt element models can accurately fit the average creep compliance curve of the linear viscoelastic stage.