| Lithium iron phosphate（LFP）batteries have been widely used in the field of energy storage,such as new energy vehicles,large-scale energy storage power stations,5G base stations,etc.,due to their advantages of good thermal stability,long cycle life,large capacity,and light weight.However,thermal runaway（TR）has always been the biggest safety issue in the application of LFP batteries.Once TR of a cell occurs due to short-circuit or other problems,it may cause the spread of TR and cause catastrophic fire and explosion accidents.Therefore,it is urgent to look for protection against heat.The out-of-control high-efficiency blocking technology minimizes the damage to the battery pack itself and the casualties caused by the monomer TR.In this thesis,starting from the TR hazard characteristics of a cell and battery packs,the TR propagation characteristics of LFP battery packs are analyzed,and the anti-extinguishing effect of liquid nitrogen on LFP batteries is explored.In the study,the TR characteristic test of the 32650 LFP batteries was carried out,and it was found that the TR process was divided into:passive heating stage,initial explosion stage,self-accelerating reaction stage,explosion stage,and smoldering stage.The SOC determines the severity of the TR,that is,the higher the SOC of the cell,the lower the explosion response temperature.The temperature rise rate and peak temperature during explosion are higher,and more energy is released.The higher the SOC,the more O2 will be consumed by the combustible gas during combustion,so the higher the consumption of O2 content and the greater the output of CO and CO2.The more the number of cells in the battery pack,the higher the temperature rise rate and peak temperature above the battery pack during the TR process are.The mass loss and oxygen consumption of the battery pack is not a simple multiple increase compared to the cells.Respectively,with the increase of the number of cells,the trends are of exponential and power functions increase.The TR propagation characteristics of the 32650 LFP battery pack are analyzed,and the explosion of a dangerous-level cell A can cause the adjacent cell B to experience TR behavior and explode.Moreover,after the initial explosion,the explosion time of cell B is earlier than that of the A dangerous cell,and the risk of TR of the battery pack increases.In view of the TR propagation characteristics of the battery pack,countermeasures and suggestions are put forward for its protection,and a method to block the TR propagation of the battery pack using liquid nitrogen injection is proposed.The low temperature and oxygen-deficient atmosphere caused by liquid nitrogen injection can quickly extinguish the flame and reduce the peak temperature reached.The influence of different liquid nitrogen injection durations on the TR blocking effect is analyzed,and it is found that the short-term liquid nitrogen injection has insufficient cooling depth and cannot block the TR propagation in the battery pack.The 30 s liquid nitrogen injection can prevent the occurrence of explosion and block the TR propagation in the battery pack.A large-capacity LFP battery fire extinguishing test is carried out to verify the fire extinguishing efficiency of liquid nitrogen.By comparing and analyzing the peak temperature,smoke generation time and quality loss of the battery after the flame is extinguished under different fire extinguishing conditions,it is found that the earlier the liquid nitrogen injection time,the better the cooling and extinguishing effect.In addition,the increase of the injection time can increase the cooling depth of the liquid nitrogen to the battery.The shortening of the distance between the injection port and the battery can reduce the loss of liquid nitrogen.The thesis has 63 pictures,19 tables,and 77 references... |
