Abstract: This study started from the thermal runaway hazard characteristics of lithium iron phosphate battery packs, experimentally study the thermal runaway characteristics of the 32650 LiFePO4 battery and its thermal runaway propagation process between battery packs. Explore the use of liquid nitrogen spraying to extinguish the thermal runaway propagation between battery packs, and analyze the fire prevention and extinguishing effectiveness of liquid nitrogen on lithium?ion batteries. The experimental results show that the thermal runaway stage of monomer lithium battery can be divided into five stages: passive heating stage, initial explosion stage, self?reaction stage, explosion stage and negative combustion stage. The temperature change curve of monomer battery is inverted "V" shape. Liquid nitrogen can effectively extinguish the thermal runaway propagation between battery packs and greatly reduce the peak temperature of the explosive battery. And the longer the spraying time, the more obvious to prevent the spread of thermal runaway battery group. With 30 s liquid nitrogen spraying, except for battery A1, no safety value pressure relief occured in other batteries.

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