A Simplified Electrochemical and Thermal Aging Model of LiFePO4-Graphite Li-ion Batteries: Power and Capacity Fade Simulations

In this paper, an isothermal physics-based aging model from the literature is modified and extended to simulate both capacity and power fade of a commercial LiFePO 4 -graphite Li-ion battery. Compared to the isothermal reference, the mechanism of porosity modification due to the Solid Electrolyte Interphase (SEI) film growth at the negative electrode is integrated in the present electrochemical and thermal model to establish theoretical correlations between capacity and power fade of the system. The aging model includes different contributions of the cell impedance increase such as the SEI film resistance and the electrolyte mass transport resistance due to the mitigation of the negative electrode porosity. Experimental databases from literature and specific experiments coupling endurance tests and Electrochemical Impedance Spectroscopy results, are used to calibrate and validate the correlated power and capacity loss simulations for both calendar and classic galvanostatic cycling operating conditions. The analysis of the experimental data points out that an additional possible aging mechanism such as cracking and fracture of the SEI layer could play an important role for cycling operating conditions and accelerate the electrochemical mechanisms. The impact of physical and design parameters on the power and capacity theoretical correlations are discussed. The limits of applicability of the present model are also discussed in this paper.