Comparative study on thermal runaway of commercial 14500, 18650 and 26650 LiFePO4 batteries used in electric vehicles

LiFePO4 batteries are widely regarded as safe. However, electric bus fires caused by overheating of LiFePO4 batteries have been reported. In this study, a pseudoadiabatic instrument was used to verify the thermal runaways of IFR 14500, A123 18650, A123 26650, and SONY 26650 cylindrical LiFePO4 lithium-ion batteries charged to 3.8 or 4.2V. At runaway maximum temperature lower than the auto-ignition temperature of electrolyte, both 14500 and 18650 LiFePO4 batteries were determined to be safer than 26650 batteries. The crucial temperatures for 26650 batteries are in the range of 240 - 280°C with a self-heat rate exceeding 100°C min−1. These high self-heat rates were not observed in 18650 and 14500 batteries. The effects of diameter on thermal runaway were visualized through the comparison of the runaway curves, crucial temperatures, maximum temperatures, maximum self-heat rates and total runaway energies of the batteries. The diameter effects on runaway hazards were investigated. The hazards of 26650 LiFePO4 batteries were caused by excessive heat because of internal short circuits above the crucial temperature, which were largely attributed to the collapse of the integrity of the separator. Therefore, the hazard of 26650 batteries is not caused by chemical activity.