Model-based fast charging of lithium-ion batteries: Impact of thermal gradients on the degradation of parallel-connected cells

Fast charging is key to increase the convenience and acceptance of battery electric vehicles. However, there are challenges at the battery system level that are not yet sufficiently understood. Due to performance limitations of the vehicle’s thermal management system, thermal gradients occur between the individual battery cells. Since the current distribution between parallel-connected cells cannot be actively controlled, avoiding overload and accelerated degradation is challenging, especially at high currents. In this study, a thermally homogeneous module consisting of two parallel-connected cells and a second module with a 10 °C temperature gradient are tested for 1200 fast charging cycles applying a model-based fast charging protocol. A thermal battery test bench is used to heat and cool the cells before, during, and after the fast charging event according to state-of-the-art thermal management strategies. Cycle life results reveal that the warmer cell in the module with gradient experiences a higher current load at the beginning of life (BoL), with convergent behavior over lifetime. The warmer cell exhibits a higher capacity fade and resistance increase than the other cells. Electrochemical impedance spectroscopy (EIS) shows an increase of the solid electrolyte interface (SEI) and charge transfer (CT) resistance, with the first dominating. Differential voltage analysis (DVA) reveals accelerated cathode degradation for the cell at elevated temperatures. Therefore, reducing thermal gradients and paying closer attention to the cathode when developing future fast-charging protocols is crucial.