External heating-induced thermal runaway and gas venting characteristics of polymer lithium-ion cells with LiNixCoyMnzO2 cathode

Thermal runaway is an important issue in the process of storage and application of lithium-ion cells. Herein, to better grasp gas venting characteristics and assess its risk, several sets of thermal runaway experiments and hazard analysis were conducted on polymer lithium-ion cells under external heating. Results demonstrate that 50–100% SOC cells exhibit fierce venting accompanied by higher surface temperature, temperature rate, voltage dropping, and mass loss compared to 25% SOC cells. The gas centerline temperature distribution covered with venting oscillation conforms to piecewise exponential expressions of height and heat release rate. The venting rate and internal pressure were deduced by temperature and mass loss, which increase monotonously with SOC increasing. The maximum heat release rate increases from 1.45 kW to 5.54 kW for 50–100% SOC cells, while it rapidly rises to 73.3 kW for 100% SOC cells under a secondary ignition. The heat release rate predicted by mass loss or flame surface area coincides well with experimental results. Besides, the asphyxiant harm of the gas emission was also evaluated by the fractional effective dose method. The findings give guidance for process safety evaluation, fire prevention and rescue in a real practical engineering utilization of cells.