Effect of the heating rate on the degassing and combustion of cylindrical Li-Ion cells

The thermal runaway (TR) of 18650 Li-ion cells under constant heating rate is investigated in a cone calorimeter. The effect of the heating rate was observed on the outgassing and cell casing temperatures, as well as on the thermal power of the flames produced and resulting smoke characteristics. Time-resolved analysis of outgassing composition based on four main gases was carried out using mass spectrometry and the size distribution of the particulate emission was measured with aerosol spectrometers. The results revealed that the heating rate had an impact on the course of TR events, with high heating rates resulting in more violent TR. The initial temperature of the first venting also increased with higher heating rates. Mass spectrometry measurements revealed that the first venting mainly released vaporized electrolyte (dimethyl carbonate, DMC) with a small quantity of CO2. During the combustion associated with the major venting event, the gases released were mainly CO2 and H2O, with small quantities of H2 and DMC, and sporadic detection of C3H6 and C6H6. The high-resolution size distributions obtained for the particulate emission showed a bimodal distribution, characteristic of very rich diffusion flames, but no significant effect of the heating rate.