Juner Zhu

Degradation of battery separators under charge–discharge cycles

Researchers have reported on the electrochemical aging of lithium-ion batteries. The mechanisms of battery capacity loss, such as consumption of electrolytes and fading of electrodes, commonly seen as fracture of coatings, have been studied intensively. The widely used polymeric separators sandwiched between cathode and anode, which do not directly contribute to the electrochemical properties of the cell, are usually taken as chemically, thermally and structurally stable materials. In this paper, the degradation of a dry processed trilayer separator due to charge–discharge cycles is investigated. It has been found that the separators that underwent higher cycles failed at lower lateral punch force and smaller deformation. Live cell tests also indicate that the deformation and force intensity at the onset of short circuit decreased for a cell after 1200 cycles compared to those for a non-cycled cell, when under lateral indentation. Different characterization methods were used to understand this charge–discharge induced mechanical aging. SEM through-thickness views of the separators show no significant pore size change, but reaction products accumulated in pores of the separator middle layer. FTIR (Fourier Transform Infrared) examination of the surfaces of those separators shows there was no apparent chemical bond change on the surface of the separator during charging and discharging process.

Extension of Non-Associated Hill48 Model for Characterizing Dynamic Mechanical Behavior of a Typical High-Strength Steel Sheet

Sheet metals usually exhibit a certain degree of plastic anisotropy because of the rolling effect. To characterize the anisotropic behavior in simulations related to large deformation, strain-rate independent phenomenological models are frequently used in quasi-static conditions. Two functions are generally included in such a model, i.e. the yield function and the plastic potential. The former limits the stress state within the yield surface while the latter determines the direction of the plastic strain increment.Traditional plasticity models mostly assume associated flow rule, in which the two functions mentioned above are identical. With the enhanced demand of accuracy, the forms of the associated models become too complex with more and more parameters to achieve an easy calibration procedure. Alternatively, in the past decade the non-associated models were increasingly used for sheet metals. Separate functions for the two aspects of plasticity lead to efficient characterization and convenient calibration.In numerical study of dynamic loading cases, how to characterize strain-rate dependence of plasticity is an important issue. Some visco-plastic models were developed to take the rate effect into account, e.g. Johnson-Cook and Cowper-Symonds models, where the isotropic J2 flow theory was commonly used. However, when the material is severely anisotropic, this approach is very likely to be insufficient, and a model including both anisotropy and rate dependence would be needed. Extending a non-associated anisotropic model to be rate-dependent is a promising approach which has not been published in open literature to the best knowledge of the authors.Objective of the present study is to develop an applicable model for characterizing dynamic mechanical behavior of a typical high-strength steel sheet. Two steps are performed. The material is investigated under quasi-static loading firstly. Tensile test results show an obvious anisotropy which cannot be described by traditional associated models. So the non-associated Hill48 model is chosen and calibrated. Accuracy of the model is verified by a quasi-static punching test. Thereafter the dynamic material properties are obtained by conducting tensile tests at quite a few strain-rate levels covering 0.0004–1200s−1. To characterize the positive strain-rate effect in strength, the non-associated Hill48 model is extended to be visco-plastic after checking two rate-dependence formulations in existing isotropic models. With implementing the extended model into a user subroutine of ABAQUS/explicit, simulations of the dynamic tension tests are run and compared to the real experiments. A good agreement between the simulated and the experimental result is achieved using the VUMAT.Copyright

Adhesion strength of the cathode in lithium-ion batteries under combined tension/shear loadings

To understand the failure mechanism and establish reliable deformation tolerances for lithium-ion batteries under mechanical loading, accurate testing and modeling of individual components are indispensable. This paper is focused on one of the most common failure scenarios, which is the de-bonding between the coating material and the current collector. A new specimen is carefully designed to measure the failure strength of the coating-foil interface. The electrode is bonded to two acrylic substrates using liquid formula glue for one side and gel formula glue for the other. Compared with conventional peeling tests using double-sided tape, the major advantage of this new specimen is that it realizes conducting shear tests. Using this special specimen, the failure strength of the coating-foil interface is obtained under combined tension/shear loadings. The new method is less susceptible to the testing conditions such as loading rate. For the cathode studied in this paper, the shear strength of the coating-foil interface turns out to be almost twice its tensile strength, which emphasizes the necessity of carrying out combined tension/shear loading tests. Moreover, a combined adhesion and cohesion failure mode is observed at the failure interface, where with larger shear component, the adhesion failure becomes dominant.

Testing and Modeling the Effect of Strain-Rate on Plastic Anisotropy for a Traditional High Strength Steel

The effect of strain-rate on plastic anisotropy of an HSLA340 steel sheet is investigated in the present paper. Uniaxial tension tests in seven different directions are performed under three strain-rates (one in quasi-static range and two in intermediate range). Flow stress and Lankford r-value are obtained from the test data to describe the plastic anisotropy of the material. Results show that the anisotropy varies with the increase in strain-rate. To model this rate-dependent anisotropic behavior, three associated flow rule based yield functions, Hill48, Yld96, and Yld2000_2d, are employed first for each strain-rate. Though the result cannot match the test data perfectly, it still seems acceptable considering the complexity of the trend of anisotropy. Accordingly, the coefficients of these models are studied, and it turns out that all of them change with the stain-rate in a similar trend. Based on this result, a rate-effect term is introduced to the coefficients of the models to characterize the rate-effect on plastic anisotropy. Finally, two issues about the modeling work are discussed. One is calibrating the Yld96 and Yld200_2d model with the non-associated flow rule, which means the coefficients of the functions are determined by flow stress only or r-value only separately. It is found that the accuracy of the characterization is largely improved because of the increase in model coefficients. The other issue that is discussed is the modeling of strain-rate effect on the plastic anisotropy. A new strategy is considered, in which the strain-rate effects in different directions are characterized first, followed by the anisotropy description. The results of the two strategies are compared and good agreement is achieved.Copyright