Numerical modelling of breathing mechanisms of Li-Ion batteries

Electrification in the automotive industry is gaining a main role in reducing greenhouse gas emissions and countering climate change. The most common batteries used in electric vehicles are Li-ion batteries thanks to their advantages such as high density, long life and high efficiency. Li-ion batteries change their thickness reversibly during charging and discharging as a result of Li intercalation and de-intercalation, respectively. As battery cells are commonly constrained within a module and so their expansion is limited, this so-called Breathing effect may provoke an increase in pretension force. This can result in asymmetric pressure distribution over the battery cell body, promoting ageing and reducing the safety of the cells themselves. The development of a finite element model able to describe the mechanical behaviour of Li-ions cells may help gain a deeper understanding of its effects. By modelling this behaviour in advance, it is possible to take it into account in the design of the module and thus improve the performance of the whole system. For this purpose, mechanical compression tests were performed involving the evaluation of the mechanical characteristics such as stiffness, hysteresis and energy dissipation of anode, cathode, separator and pouch of a commercial Li-ion battery. Subsequently, mechanical compression tests of the whole battery cell were used to validate the previously obtained results and to evaluate the dependency of stack pressure and SOC on the mechanical battery cell behaviour. The collected data was used to develop a finite element model able to illustrate the breathing behaviour of the battery cell under different boundary conditions, taking into consideration the dependence pretension and State of Charge (SOC).