Cycling Degradation of Lithium-Ion Batteries Module from Electric Vehicle

A commercial 26650-type lithium-ion module (15P4S) of LiFePO4/graphite from an electric vehicle was cycled at the charge-discharge rate of 2 C at room temperature (20 +/- 2 degrees C) for 400 times until its state-of-health (SOH) declined below 60%. Its degradation was investigated by capacity fading characterization, electrochemical impedance spectroscopy (EIS), field emission scanning electron microscopy (FE-SEM), energy dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). The results show that the battery module's capacity degradation is mainly related to the increase of internal resistance. The patchy image on the aged graphite surface indicates that part of the graphite is detached from the current collector. The rise of P-F and Li-F content connotes the thickening of the SEI film. The (002) peak position in the XRD patterns of the ageing graphite electrode shifts to a low angle direction, suggesting an increase in the graphite layer distance. The appearance of the FePO4 phase in the XRD patterns indicates a reduction in active lithium. Therefore, the capacity loss of the battery module is mainly caused by the performance deterioration of graphite and LiFePO4 materials together.