Research On The Fire Risk Of Lithium Ion Battery And The Critical Condition Of Thermal Runaway Behavior

Due to the merits of high voltage,good cycle performance,high energy density and friendly to the environment,lithium ion batteries have been the power sources of most electron devices and energy storage power for peak loading and frequency regulation in interconnected power system.With the developing of new electrode materials,the energy density and electric chemical performance of batteries get great development.However,it also brings the decline of safety performance,which easily induce the accident of fire and explosion.This thesis mainly studied the fire safety of lithium ion batteries from following aspects:Designing a testing platform for fire risk of large scale lithium ion batteries,studying the fire hazard of 50 Ah lithium phosphate cells and lithium titanate cells,and reveling the influence of state of charge(SOC)and electrode materials.The platform can measure the heat release rate,surface temperature,mass loss and combustion behavior of large scale lithium ion batteries.Results show the combustion behavior of cells in high SOC appears multi jet flow.And in second jet flow period,surface temperature of full charged cells increase rapidly.In addition,the increase of SOC can improve the heat release rate of cells and decrease the combustion or ignition time.Through comparing the construction of materials before and after tests and heat generation of components,it was found the second jet flow is mainly induced by the reactions of cathode materials,and this period should belong to battery thermal runaway behavior.Studying the fire spread behavior between lithium titanate batteries in rombus and parallel arrangement,and obtaining the criticality of fire spread by Semenov and Frank-Kamenetskii models.The fire spread behavior is mainly induced by heat transfer between cell surface and fire heating.But the fire heating could make the onset temperature of battery fire and ignition time become irregular.The heated cells could have "smoldering period" and the potential of explosion.The kinetic parameters of exothermic reactions inside cell were obtained by C80.Through calculating by Semenov and Frank-Kamenetskii models,the equivalent critical ambient temperature inducing fire spread between lithium titanate batteries was obtained between 123.1 and 139.2℃.Creating the cricality model of battery thermal runaway through revising the classic Semenov and Thomas models.Predicting the influence of heat convection coefficient,cell size and shape to critical condition of battery thermal runaway on base of the revised model.This study revised Semenov and Thomas models considering the heat contribution of four exothermic reactions to battery runaway behavior,the consumption of reactant before cell get thermal runaway and the meachanism function of reactions.The critical condition of lithium cobalt battery calculated by revised thermal models is verified by the results of oven model.Through calculating by revised models,the critical ambient temperature of lithium titanate battery is revised at 425.9 K(h=20 W m-2 K-1),442.4 K(h=200 W m-2 K-1).Results also show the critical ambient temperature increase 25℃ when the heat convection coefficient varies from 5 W m-2 K-1 to 40 W m-2 K-1.To the contrary,it decreases 14℃when the radius of cell increase from 5 mm to 20 mm.And in the same volume,square cells are safer than cylindrical cells.The hot box experiments obtained the temperature variation of 18650 NCM/graphite cells with five degrees of SOC from 0%to 100%under four kinds of environment temperature(145 ℃,165 ℃,185 ℃,205 ℃).According to the temperature variation,the ambient conditions can be devided into safe,critical and danger regions.Among these regions,the onset temperature of critical region decrease from 205 ℃ to 165 ℃ when SOC increase from 0%to 100%,and the interval of this region get thinner.Analyzing the element events of battery fire,and proposing a semiquantitative evaluation method to classify battery safety.According to the three elements of fire,15 element events and 36 ways to induce battery fire can be obtained through fault tree.Combining the hazard severity(S)and likelihood(L)of battery accident in different failure modes,the hazard risk number(HRN)of battery can be obtained.The safety performance of battery can be evaluated through this parameter.