Failure and constitutive behavior of a Li-ion pouch cell under mechanical loading

The constitutive mechanical behavior of the individual components in Lithium-ion cells has a fundamental influence on the development of internal electrical short-circuits in crash-relevant load scenarios. These short circuits can result in explosive, thermally unstable states (so called thermal runaways). The experimental characterization of mechanical properties of single components but also of entire cells is therefore a central aspect in the safety-related assessment of battery systems. This paper presents and compares experimental results of the mechanical characterization of individual cell components as well as whole pouch-cells under different loading patterns. Especially, the different mechanical behavior of the active materials NMC and graphite was investigated in dry and wet conditions. In compression tests, the presence of the electrolyte reduced the stress levels by about 100 % for the graphite layered anode (Cu) and by about 20 % for the NMC layered cathode (Al) compared to dry conditions. The separator displayed an anisotropy with tensile strengths differing by a factor of three between the longitudinal and transversal orientations. For investigating the failure of a whole pouch-cell, interrupted flat-punch and hemispherical-punch indentation tests were performed. Post-mortem CT analysis revealed that crack development is rather gradual than abrupt. The initiation and propagation of the failing cell structure were examined and related to the characteristics of the individual cell components. It could be concluded that for a physical based modeling of the deformation and fracture processes within the cell, understanding the mechanical behavior on component and on cell level is crucial.