Analysis and modeling of cycle aging of a commercial LiFePO4/graphite cell

This paper presents cycle aging results of a commercial lithium-ion cell from a comprehensive, 29 month aging study and follows up on calendar aging results published previously [1]. We use a widely used commercial LiFePO4/graphite cell from Sony/Murata, which promises long calendar and cycle lifetime, that would make it suitable for stationary battery applications. The evolution of the cells' capacity and impedance are shown in a static cycle aging study for 19 test points with different combinations of temperature, C-rate, depth of discharge and state of charge. Based on the measurement data shown herein and the calendar aging model presented in the previous paper, a semi-empirical combined aging model is presented for the capacity loss and resistance increase. Two dynamic load profiles are used to experimentally validate the combined aging model. Absolute model errors below 1% for the capacity loss and below 2% for the resistance increase in both dynamic load profiles demonstrate that the combined aging model can predict the lifetime of LiFePO4/graphite battery cells for different applications and varying operation conditions adequately. In the cycle experiments, an unexpectedly strong, but partly reversible capacity loss is observed with shallow cycles at medium states of charge.