Simplified modeling of pouch cells under different loadings
Due to increasing requirement on the reduction of CO2-Emissions, the meaning of E-Mobility becomes more and more important. The related development of efficient Li-ions with high charge densities has also a direct impact on the automotive industry. This applies in particular to the crash safety of Li-ion-battery-powered vehicles. The structure of Li-ion batteries is in principle a repetitive layered system. One cell unit consists of two very thin metal foils (typically Al and Cu), which coated with active materials (lithium-metal-oxide and graphite). These coated layers represent the anode and the cathode of one unit-cell which are separated by thin polymeric films (separator-foils). A typical pouch cell consists of hundred and more layers which are embedded in an electrolyte within a bag (pouch, coffee bag). Local deformations resulting from uncontrolled crash loads can lead to critical damage in the separator, which causes an intrinsic short circuit and subsequently an unstable state (thermal runaway) that can result in battery explosions. Therefore, the simulations of intrinsic mechanical deformation states are crucial to predict electrical short circuits. However, in a full crash simulation of an electrical vehicle it is impossible to model the intrinsic layered system of each pouch cell, because there are hundreds of cells installed and each cell consists of more than hundred layers. Due to these facts, it is necessary to develop and apply simplified models, which use a homogenization approach based on the strong repetitive intrinsic layered cell structure. For this modelling approach, anisotropic compressible plasticity models should be well suited, because it is to be expected, that the deformation behavior of the cells is different in the layered thickness direction in relation to the in-plane loadings of the parallel layers. In the presented work, different anisotropic and isotropic plasticity models in LS-Dyna were used to describe the mechanical behavior of pouch cells in a simplified manner and were compared to each other. The calculated results are compared to experimental investigations of representative crash loading scenarios, e.g. bending, indentation, intrusion and compression in different cell directions. Furthermore, the evaluated intrinsic mechanical quantities (volumetric strain) are presented to investigate the possibility of formulating a mechanical based electrical short circuit criterion for all experimental tested loading cases. The result suggests the conclusion for a development of a user defined material model to simulate the whole testing regime consistent with the experimental observations for the mechanical and electrical behavior of the pouch cell.
Ministerium für Wirtschaft, Arbeit und Tourismus Baden-Württemberg WM BW