Abstract: The traditional commercial fire extinguishing agent is not effective in suppressing the thermal runaway fire and reignition of lithium ion battery. Liquid nitrogen is an efficient fire extinguishing agent, and its cooling effect on restraining thermal runaway of lithium-ion batteries has been verified by experiments of many research institutions. However, at present, there is a lack of theoretical analysis methods for the influence of liquid nitrogen fire extinguishing dosage and fire extinguishing efficiency. Based on the theoretical analysis of liquid nitrogen fire extinguishing mechanism, the theoretical analysis and calculation method of liquid nitrogen cooling and inerting are studied and determined. In the method, experimental studies on thermal runaway heat release, heat absorption of liquid nitrogen, and inert amount of liquid nitrogen were carried out, and the effectiveness of the method was verified by full-scale experiments. Taking the thermal runaway of 280 Ah lithium iron phosphate battery as an example, the minimum amount of liquid nitrogen for inerting explosion suppression and cooling is calculated according to the theoretical analysis method. After 83.39 kg of liquid nitrogen was applied to the explosion test chamber with an effective volume of 38.88 m3, the explosion occurred 48 times at a 24-hour interval. The effectiveness and rationality of the theoretical analysis method are effectively verified by practical cases, and the proposed theoretical analysis method provides evaluation means and methods for the design and effectiveness analysis of liquid nitrogen fire suppression and explosion suppression, so as to further effectively enhance the safety of lithium iron phosphate battery storage system.

Key words: liquid nitrogen, thermal runaway, cooling, inerting, lithium iron phosphate battery