Evaluation of Thermal Runaway Venting Characteristics for High-Energy Density and High-Power Density 21700-Format Li-ion NMC Cells

The increasing use of lithium-ion batteries in aviation to support carbon-neutrality objectives raises critical safety challenges, particularly regarding thermal runaway and the venting of hot, flammable, and toxic gases. While previous studies have investigated cumulative gas release, there is a lack of transient experimental data on key venting parameters such as average ejection rate, duration, and gas temperature during thermal runaway. To address this gap, a custom low-cost test bench was developed to experimentally characterize gas venting behavior under critical thermal runaway conditions, where material ejection is maximized, using conduction heating. The tests were conducted in an inert nitrogen atmosphere to isolate gas venting from secondary reactions such as combustion, allowing a focused characterization of the venting phenomenon. The test bench consists of a sealed 45.8-liter pressure vessel instrumented with thermocouples and a high-speed pressure sensor enabling the evaluation of the total ejected gas, venting duration, and ejected gas temperature, with data sampled at 1 kHz. Tests were conducted on two commercial 21700-format NMC cells: a high-power cell (Molicel® P42a) and a high-energy cell (Samsung® 50e). Each cell type was tested three times, and the reported results reflect the average of those tests. Both cells triggered thermal runaway at temperatures between 210 and 220 ℃ and released comparable amounts of gas when normalized by capacity, 0.059 mol/Ah for the high-power cell and 0.072 mol/Ah for the high-energy cell. The high- power cell completed its venting in approximately 376 ms, while the high-energy cell did so in 291 ms. Ejected gas temperatures ranged from 640 to 960 °C. When adjusted to these ejection temperatures and standard atmospheric pressure, average ejection flow rates ranged between 50 and 100 L/s, with the high-power cell tending toward the lower end and the high-energy cell exhibiting higher flow rates. These findings provide essential data for designing safe venting and containment systems in aerospace environments.