Failure Investigation of LiFePO4 Cells under Overcharge Conditions

The failure mechanism of commercial A123 18650 LiFePO4 battery in the overcharge failure process at 1C and 110% SOC has been studied. It was found that, during the normal cycle, the anode and cathode voltages were from -0.25 V to 0.25 V and 2.60 V to 4.50 V respectively, while in the 10% overcharge process, the anode voltage was between -0.4V to 0.22V (vs. Li/Li+) and the cathode voltage changed from 2.20 V to 4.86 V (vs. Li/Li+). The overcharge failure was studied using SEM and EDX. It was found that coding spots appeared in the anode side. Fe, Ni and Ge were found only in the coding spots by performing EDX, indicating the micro shorting by metal dendrite is the main reason of the failure of battery during the 10% overcharge process. It could be concluded from the results of High Energy Synchrotron X-ray Diffraction (HESXRD) that Fe was not from the decomposition of LiFePO4. The potential of Fe/Fe2+ oxidation/reduction pair in the LiPF6/EC-EMC electrolyte system was found to be 2.2 V/2.7 V. It was concluded that Fe cations could not only be reduced on the surface of graphite, but also at the cathode side during the overcharge process. Compared with normal cycle, under the overcharge process, the impurities containing iron in the raw material was more conducted to dendrite formation leading to micro-shorting because of the wider reduction voltage range in the overcharge process.