Anode-Controlled Fast Charging Optimization of Lithium Ion Batteries

Electric vehicles are an important element for a sustainable environmentally and climate-friendly mobility. But charging of these takes considerably more time than refueling of a conventional one. This can be an obstacle for many consumers. For an increased acceptance of electric vehicles, the general objective is to charge the battery up to 80% SoC in less than 15 minutes. To reduce the common charging time, higher charging currents are needed. In order to avoid accelerated aging of the battery due to this higher currents, advanced charging profiles have to be applied. During charging, the anode potential should always be above 0 V vs. Li/Li+ to prevent lithium plating which is a major degradation phenomenon and safety concern in the case of fast charging. Not only the anode potential and charge cut-off voltage but also the increased heat generation and growth of solid electrolyte interface (SEI) need to be considered for an optimal charging profile. In order to investigate different charging profiles for cylindrical lithium ion batteries, a multicriteria optimization was conducted considering the charging time tch, maximum temperature Tmax and capacity loss due to SEI growth Closs,SEI. For this, an electrochemical model developed at DLR was used together with a thermal model and SEI growth model from literature [1,2]. Two different 21700 cells were investigated, one with a high energy density and one with a high power capability. A pareto frontier of possible multi-stage constant current (MSCC) profiles was calculated for each cell to evaluate the correlation between tch, Tmax and Closs,SEI. By applying a weighted objective function defined optimized charging profiles are obtained depending on the importance of the three variables. The optimized profiles were validated with the 21700 cells and laboratory test cells with reference electrode. With this, it could be ensured that the simulated temperature and voltage progression matches the experiment and the given limits are not exceeded. [1] R. Richardson, S. Zhao, D. Howey, J. Power Sources 326 (2016) 377-388 [2] M. Safari, M. Morcrette, A. Teyssot, C. Delacourt, J. Electrochem. Soc. 156 (3) (2009) A145-A15