Unveiling the relationship between the energy and gas released during the thermal runaway of Li-ion cells and their stored electrical energy

The use of lithium-ion (Li-ion) batteries has rapidly increased due to improvements in performance, durability, and cost-effectiveness, driven by the growth of electric vehicles (EVs), renewable energy storage, and portable electronics. However, the safety of Lithium-ion batteries remains a major concern due to the risk of thermal runaway, which can result in fast heat release, the ejection of particles and gases, and possibly fire and explosion. Therefore, the reliable characterization of thermal runaway energy and gas volume release is essential. In this study, 29 thermal runaway tests of large-capacity prismatic and pouch NMC (Nickel Manganese Cobalt oxide cathode) cells were analysed. Thermal runaway energy, normalized by the electrical energy of the cell at the time thermal runaway occurs, was constant and equal to 1.25 during all the tests performed under vacuum and inert atmosphere. Similarly, the normalized number of moles generated during thermal runaway remained constant at 4.9 × 10 -3 mol·kJ -1 . When the tests were conducted in air, the large quantity of oxygen available led to additional combustion and energy release. The normalized thermal runaway energy increased to 5.0, while the normalized average number of moles decreased to 4.2 × 10 -3 mol·kJ -1 . Thermodynamic equilibrium calculations of vented gas combustion with air show that gas phase combustion alone does not explain this energy increase, the contribution of graphite is therefore demonstrated. The two conditions-under vacuum and in air- represent the two limiting cases of air access in a typical battery module, therefore the data presented in this work can be useful for evaluating thermal runaway propagation and casing integrity in a battery module during the design phase. • Thermal runaway energy is proportional to the embedded electrical energy • Number of gas moles releases is proportional to the embedded electrical energy • Tests in air released four times more energy compared to an inert atmosphere • Gas phase combustion alone does not account for the increase in energy release • Graphite combustion can significantly contribute to the total energy release