An Experimental and Computational Study of Mechanically and Dynamically High Loaded Separators for Lithium-Ion Batteries

Abstract

Abstract In recent years, the automotive market has been pushed to significantly reduce CO2 emissions and increase the development and production of electrically powered powertrains. Currently, the best industrial storage method for electric energy is the lithium-ion battery, which is already used in the automotive industry and in consumer electronics (e.g., cell phones and laptops). The Li-ion cell is considered the most promising energy storage source currently available on the market. Motor vehicles are exposed to various crash and abuse scenarios under different load conditions. With the increasing use of these batteries in vehicles, safety has become a concern due to unintended failures and accidents. For this purpose, a detailed study of the separator in terms of mechanical stiffness as a function of fabrication direction and loading rate was carried out. Based on the results a new material model was developed in which the orthotropic-visco-elastic and orthotropic-visco-plastic mechanical behavior of a polymeric material can be considered. In addition, based on uniaxial tensile tests with local strain measurement, a novel failure criterion for finite element analysis was developed to predict the effects of high mechanical loads in terms of triaxiality, large plastic strains, orthotropy and viscosity. Finally, a simulation model for a PE separator was developed combining the new failure with a G’Sell and Swift-Voce strengthening rule. This model was able to predict the anisotropic behavior of the PE separator during deformation and failure. The modeling method presented in this work allowed accurate prediction of the strength of the tested PE separator. The proposed new failure criterion predicted the failure strain for all three directions within a range of 3% and agreed very well with the tests performed. Some small deviations occurred due to the simplified boundary conditions as well as the influence of the element length. However, it should be noted that the real tests showed scatter in a much larger range. The quantitative comparison was made based on the elongation response at fracture of the three differently oriented specimens (MD, DD, TD). The present failure criterion and detailed calculation method should be useful in the development of batteries and be used as an important new computational tool for the for evaluating the safety of lithium-ion batteries under high mechanical load and crashworthiness. The proposed failure criterion can also be combined with other constitutive material laws. The results of these investigations are implemented in the failure model based on Elham Sahraei’s model (/FAIL/SAHRAEI) in the explicit crash solver OpenRadioss™.