Over the past three decades, the energy density of lithium-ion batteries (LIB) has steadily increased to meet the growing demand for portable energy storage. This increase in energy density has made LIBs a potential safety hazard. In the worst-case scenario, uncontrolled heating in a LIB, known as thermal runaway, can result in serious damage or the destruction of the entire battery pack and its surroundings.

It is our goal to develop a virtual model of a LIB cell, that is capable of reproducing the LIB cell’s mechanical and thermal response under impact and crushing loads. In order to more accurately model and predict how LIB cells fail when subjected to mechanical stresses and loads, a detailed, heterogeneous finite element (FE) model is to be developed that captures the individual components of a single LIB cell with high resolution. Such a model would improve the prediction of safety and performance of LIB modules in a cost-effective manner.

The required level of detail in the FE model is achieved by characterizing the material behavior of the jellyroll components, e.g. anode, cathode, and separator. Both the anode and cathode foils are three-layer structures, with a metallic layer sandwiched between two active material layers, while the separator, typically made of a single- or multilayer polymer, prevents short circuits between the anode and cathode. The metallic layer in the anode foil is typically made of copper, while the active material is usually a graphite-based compound. In the cathode foil, the metallic layer is usually made of aluminum, while the active material is generally made of metal oxides.

A series of experimental tests were performed to determine the mechanical properties of the individual foils within the jellyroll. These tests, including macro- and nanoindentation as well as tensile tests, were performed on cleaned foils extracted from commercially available LIB cells. The focus was on indentation tests to evaluate the compressive behavior of the active material layers in the anode and cathode foils, as this was considered to be a critical factor influencing the force response of the battery to impact and crushing. A flattop macroscopic indenter with a surface area of 5 mm² was used to indent individual layers of the anode of a commercial battery cell, achieving high repeatability. This enabled detailed measurement of stress-strain curves for each layer up to 400 MPa. At approximately 350 MPa, a sudden drop in stiffness was observed. The cause of this behavior remains unclear, though one hypothesis is that, at this high stress level, pores collapse and individual grains are crushed.

Using the experimental results, material models for individual components will be developed through simulation tools. To reduce the computational demands of these models, surrogate models will be created and trained, enabling efficient simulations of complete LIB cells.