A Material Model for the Orthotropic and Viscous Behavior of Separators in Lithium-Ion Batteries under High Mechanical Loads

Abstract

The present study is focused on the development of a material model where the orthotropic-visco-elastic and orthotropic-visco-plastic mechanical behavior of a polymeric material is considered. The increasing need to reduce the climate-damaging exhaust gases in the automotive industry leads to an increasing usage of electric powered drive systems using Lithium-ion (Li-ion) batteries. For the safety and crashworthiness investigations, a deeper understanding of the mechanical behavior under high and dynamic loads is needed. In order to prevent internal short circuits and thermal runaways within a Li-ion battery, the separator plays a crucial role. Based on results of material tests, a novel material model for finite element analysis (FEA) is developed using the explicit solver Altair Radioss. Based on this model, the visco-elastic-orthotropic, as well as the visco-plastic-orthotropic, behavior until failure can be modeled. Finally, a FE simulation model of the separator material is performed, using the results of different tensile tests conducted at three different velocities, 0.1 mm·s−1, 1.0 mm·s−1 and 10.0 mm·s−1 and different orientations of the specimen. The purpose is to predict the anisotropic, rate-dependent stiffness behavior of separator materials in order to improve FE simulations of the mechanical behavior of batteries and therefore reduce the development time of electrically powered vehicles and consumer goods. The present novel material model in combination with a well-suited failure criterion, which considers the different states of stress and anisotropic-visco-dependent failure limits, can be applied for crashworthiness FE analysis. The model succeeded in predicting anisotropic, visco-elastic orthotropic and visco-plastic orthotropic stiffness behavior up to failure.

Stress Distribution Inside a Lithium-Ion Battery Cell during Fast Charging and Its Effect on Degradation of Separator

“…Equation ( 22) only considers elastic deformation and does not consider any plastic deformation. This is a safe assumption since the amount of deformation in this study is very small and does not exceed the elastic limit [8,14,[33][34][35]. It should be also noted that the strain induced by intercalation, denoted by ε D ij is exclusively applicable to the positive and negative electrodes.…”

Mechanical Behavior of Lithium-Ion Battery Separators under Uniaxial and Biaxial Loading Conditions

“…Concerning the rate sensitivity of microporous separators, there is a consensus in the literature: a positive strain rate dependency was noted under both tensile and compressive loadings owing to the viscoelastic behavior of separator materials, c.f. [ 27 , 28 , 29 , 30 , 31 ]. A number of studies thereby proposed a material model to describe the mechanical and fracture behavior of microporous separators; these include, among others, a linear viscoelastic model based on the Kelvin–Voigt model [ 28 , 32 ], a viscoelastic poroelastic model for a range of strain rates [ 33 ], an anisotropic crushable foam model [ 34 ], and a fully 3D microstructural model using the stochastic reconstruction approach [ 35 ].…”

An Experimental and Numerical Study on Charged 21700 Lithium-Ion Battery Cells under Dynamic and High Mechanical Loads

“…An additional physical effect was considered, which is the high orthotropic behavior of this material, due to its manufacturing processes. Within further investigations of the separator material, a new material model was developed based on the findings of these tests, which considers the visco-elasticity and visco-elasto-plasticity and orthotropic behavior of the separator material [22], used in 18650 cylindrical cells. A comparison between the widely-used 18650 vs. 21700 cells was provided by Quinn and Waldmann.…”