Physics-based modelling of LiFePO4-graphite Li-ion batteries for power and capacity fade predictions: Application to calendar aging of PHEV and EV

In this paper, an isothermal physics-based aging model of Li-ion battery from the literature is modified and extended to predict both capacity and power fade of a commercial LiFePO4-graphite system. Compared to the isothermal reference mathematical framework, the present electrochemical and thermal model integrates the mechanism of porosity modification due to the SEI film growth into the negative electrode in order to propose theoretical correlations between capacity and power fade of the system. Based on the porous electrode theory, the aging model integrates different contributions of the cell impedance increase, such as the SEI film resistance and the electrolyte mass transport resistance due to the decrease of the negative electrode porosity. Experimental data coupling endurance tests and Electrochemical Impedance Spectroscopy (EIS) results, are used to validate the theoretical power and capacity fade correlations for calendar operating conditions. The model is then used to discuss the impact of calendar operating conditions on the aging and lifetime of PHEV and EV battery packs. Based on the simulation results, strategies are proposed to extend the battery life during the parking mode of the vehicle.