BALLISTIC SIMULATION METHOD FOR LITHIUM-ION BATTERIES USING THICK SHELL COMPOSITES IN LS-DYNA

Lithium-ion (Li-ion) batteries have become an important energy storage solution for a wide range of applications from consumer electronics to automobiles. In particular, the automotive industry’s push for improved fuel efficiency has led to the development of electric and hybrid-electric vehicles, many of which use Li-ion batteries. In addition to these fuel-saving motivations for Li-ion batteries, the US Army has its own unique mission requirements for onboard energy storage and available power, which could potentially be addressed at least in part, by Li-ion batteries. However, military ground vehicles are also subject to harsh operating conditions and abuse conditions that can cause failures of onboard equipment. Due to complex nature of the batteries, it is numerically challenging to capture the behavior of these batteries under abuse conditions such as a high-energy impact event. Each battery cell is made up of several layers and sub-layers of different materials. If a finite element model is created by meshing each sub-layer with sufficient detail to perform a ballistic simulation, even a single battery cell will have millions of degrees of freedom (DOF). In a hybrid-electric vehicle battery there are typically tens of cells in a module and hundreds of cells in a full pack. Therefore, the computational challenge associated with using finite element analysis (FEA) for ballistic simulations becomes daunting. In this work, a new method is proposed that employs a Thick Shell Composite (TSC) representation of Li-ion batteries using the commercially available FEA software, LS-DYNA. This approach shows promise for modeling the battery at the module or full-pack level with significantly reduced computational cost compared to a more traditional modeling approach. Individual layers are embedded into this TSC in order to reduce the number of finite elements in each cell significantly. Several of these cells are then assembled either in series or parallel to represent the module and the full battery pack. The TSC numerical model is validated by running a simulation of a bullet impact test and comparing the results to the equivalent physical test. The TSC model predictions show good agreement with the experimental results. In addition, three different impact angle scenarios (oblique, vertical, and horizontal) are simulated for one module of a generic Li-ion battery using the new approach. The extent of the predicted battery damage due to these different impact loading conditions are compared both qualitatively and quantitatively. INTRODUCTION In July 2011, the Advanced Vehicle Power and Technology alliance (AVPTA) was announced as a partnership between U.S. Department of Energy (DOE) and the U.S. Army to develop new technologies for both commercial and military ground vehicles that will help address top priorities such as decreasing petroleum dependence, increasing ground vehicle fuel efficiency, and enhancing nation’s energy security infrastructure. The US Army Tank Automotive Research, Development and Engineering Center (TARDEC) and the DOE Vehicle Technology Office (VTO) are collaborating as part of this partnership. One area of collaboration is the development of computational modeling and simulation (M&S) and design tools in the Computer Aided Engineering of Batteries (CAEBAT) program. An important overall goal of CAEBAT is to advance the state of the art physics-based simulation tools for predicting the battery response in order to accelerate fielding safe and robust lithium-ion (Li-ion) batteries [1] that are suitable for use in both commercial and military vehicles with high UNCLASSIFIED: Distribution Statement A. Approved for public release; distribution is unlimited Proceedings of the 2016 Ground Vehicle Systems Engineering and Technology Symposium (GVSETS) UNCLASSIFIED: Distribution Statement A. Approved for public release; distribution is unlimited Ballistic Simulation Method for Lithium-Ion Batteries Using Thick Shell Composite Modeling. – Babu, Castanier, and Ding Page 2 of 7 onboard electrical power demands. One of the specific objectives is to accelerate the comparative analysis of alternative Li-ion battery designs as drop-in replacements to lead acid 6T batteries. Another objective is to modify and extend the CAEBAT battery simulation tools to predict the response of Li-ion battery modules under abuse conditions experienced in military applications, including high-energy impact events such as bullet strikes. The focus of this study is to develop finite element models and associated simulation methods that are suitable for predicting the response of Li-ion batteries to high-energy impact, crash, and blast events at a reasonable computational cost. In particular, an efficient method is developed for ballistic M&S of Li-ion batteries by using a Thick Shell Composite (TSC) representation of Li-ion batteries using the commercially available FEA software, LS-DYNA. This approach shows promise for modeling the battery at the module or full-pack level with significantly reduced computational cost compared to a more traditional modeling approach. Individual layers are embedded into this TSC in order to reduce the number of finite elements in each cell significantly. In order to test and validate the M&S approach, a model of a generic but representative battery pack is built and integrated with a steel casing model. This is a notional system that does not represent a specific battery mounted in a particular ground vehicle. However, from a numerical perspective, it does entail the same high level of computational demands that would be required to simulate an actual vehicle-mounted battery system. This generic battery model has therefore been used for research purposes to run initial ballistic simulations for an integrated batteryvehicle structure, in order to examine the computational efficiency of various approaches, as well as to establish standard procedures and best practices for future M&S efforts. In particular, the M&S analysis method presented in this paper focuses on the system-level mechanical failure of batteries due to high-speed bullet impact, which is very challenging to capture from a computational perspective. PROBLEM STATEMENT A battery is made up of many cells that are combined in series and/or parallel into modules. These modules are then combined in a full battery pack. There are different form factors for battery cells. In this paper, pouch cells are considered, which have a plate-like form factor. A single pouch cell is typically comprised of 16 to 24 layers, with the precise number depending upon capacity and size requirements. The notional battery system used in this study has 24 layers in each pouch cell. Figure 1 shows the sub-layer thicknesses of a single layer in the Li-ion battery cell. The average thickness of the full layer is 265 microns (0.265 mm). One cell with 24 layers plus the pouch material is approximately 7.2 mm thick. Figure 1: Single layer of a Li-ion battery cell. Figure 2: Dimensions of a pouch cell (24 layers). Figure 3: One module consisting of 12 pouch cells in series. Figure 2 shows the dimensions of a single pouch cell. Figure 3 shows one module of the generic battery model with 12 cells in series. This configuration requires over one million finite elements to represent one layer of the pouch cell and over 24 million elements for one pouch cell. Computationally, it is very challenging to run such a large UNCLASSIFIED: Distribution Statement A. Approved for public release; distribution is unlimited Proceedings of the 2016 Ground Vehicle Systems Engineering and Technology Symposium (GVSETS) UNCLASSIFIED: Distribution Statement A. Approved for public release; distribution is unlimited Ballistic Simulation Method for Lithium-Ion Batteries Using Thick Shell Composite Modeling. – Babu, Castanier, and Ding Page 3 of 7 model even at the single-cell level. With this approach, the full pack would have over 100 million elements, which makes it extremely challenging or even impossible to carry out simulations with the available resources. Therefore, meshing the cells at the micromechanical scale may not be the best approach. However modifying the original CAEBAT [2, 3] model may work for a single cell, or for a few cells, but it becomes infeasible for a module or a full pack. This is especially true for models to be used for system-level bullet penetration simulations, given the very large number of elements associated with these models. As an alternative approach, one could represent the complex Liion cell materials as an equivalent homogeneous material and model as a single thickness layer. However, this requires an equivalent or homogeneous material property for the single-cell configuration, which may or may not provide the true or best behavior of the materials at high-strain, highspeed impact loading. In this paper, a new approach is proposed, which is called the System Level Ballistics Simulation Method for Li-ion Batteries. The key idea is to represent a single cell layer as a thick shell composite (TSC) in LS-DYNA [4] with one integration point for each material. By adopting this strategy, all of the different materials are represented independently at the microscale at each layer. This approach seeks a balance between fidelity and cost: it eliminates the limitations due to simplifying assumptions associated with a homogeneous representation of complex materials in Li-ion batteries, yet it also provides a way to represent a module with much fewer than the approximately 20 million elements required for a more detailed finite element representation.