Experimental investigation of environmental pressure effects on thermal runaway properties of 21700 lithium-ion batteries with high energy density

As lithium-ion batteries (LIBs) with higher energy density are becoming more widely applied, especially in aviation field, understanding the potential thermal hazards of which at low pressure is beneficial to evaluate the safety during storage, transport and use. In this paper, thermal runaway (TR) behaviors of 21700 cylindrical LIBs with high energy density under various pressures were investigated, and characteristic parameters at low pressure such as surface temperature of LIBs and mass loss were analyzed. The results showed that combustion behaviors of LIBs could be weaker as the pressure decreased. Besides, Tmean was used to characterize the temperature variation of the TR. Several critical parameters such as Topen, Ttr, Tmax, and tinterval during the TR of LIBs were analyzed, respectively. The results indicated that Ttr followed an upward trend as the environmental pressure decreased from 101 to 30 kPa. The tinterval reduced from 48 s to 22 s when the pressure declined from 101 to 30 kPa. A heat transfer model under external heating could be used to explain the trend in TR influence at low pressure, and the temperature change rate (dTdt) was related to the internal and external pressure of the battery. In addition, the relationship between the mass loss rate (MLR) and the pressure was analyzed, which indicated that environmental pressure had a little significance on mass loss for 21700 LIBs. Moreover, for given test conditions, the phenomenon of flame lift-off was not observed at different pressures, and the jet flame height reached the maximum at 50 kPa, which was 0.28 m. To quantify the maximum jet flame height, an approximate power law dependence between flame height and pressure could be deduced. This study might be helpful to further understand the fire risk of the LIB jet fires under low pressure.