| With the popularization and application of lithium-ion batteries in electric vehicles,the demand for lightweight and high-endurance vehicles has driven the increase in energy density and format of power batteries.Large-format high-specific-energy pouch lithium-ion batteries are favored by the market due to their high energy density.However,the safety issues such as thermal runaway are still the biggest obstacle to development,especially for such large-format high-nickel batteries.At present,domestic and foreign researches mostly focus on small-capacity cylindrical or prismatic batteries,but the lumped parameter method used is no longer suitable for large-format batteries.And the research on high nickel batteries(LiNi0.8Co0.1Mn0.1O2 cathode)is mostly based on the material level,and there is little research on the battery level or even the battery pack level.Therefore,the purpose of this thesis was to fundamentally understand the thermal runaway behavior and characteristics of large-format highspecific-energy pouch lithium-ion batteries.The study attempted to investigate the thermal runaway behavior and heat and gas production characteristics of single cells,combustion behavior and thermal runaway propagation mechanism of single cells,and thermal runaway propagation behavior and characteristics of multiple cells,through experiments,combined with theoretical analysis and numerical calculation.The major work was summarized as follows:(1)Thermal runaway behavior and heat and gas production characteristics of the single cell.To explore the thermal runaway behavior of large-format highspecific-energy pouch lithium-ion batteries,EV+ARC and full-component gas analysis were used to investigate characteristics of heat and gas production.It was found that the surface temperature of the large-format battery was non-uniform during the thermal runaway process.The commonly used lumped parameter method is not suitable for characterizing the heat production characteristics of this large-format high-specificenergy pouch battery after severe temperature rise of thermal runaway.Therefore,a method for characterizing the surface temperature of large-format pouch batteries based on the distributed parameter method is established,so that the reaction heat of the cell can be calculated more reasonably.The violent heat production of the thermal runaway process accounts for about 88%of the total heat production.The venting characteristics of cells with 0%,50%,and 100%state of charge(SOC)were obtained,and it was found that with the increase of SOC,the gas production composition of the battery became more complex,and the typical gas concentration was higher.The gas and heat production of the cells also increased accordingly.The gas production mechanism of the thermal runaway cell is revealed,and it is believed that the increase in the amount of lithium intercalation in the negative electrode enables more gas production reactions to occur more thoroughly.Three kinds of venting rate prediction methods based on mass loss,temperature and pressure change,and gas concentration are given,and the corresponding applicable conditions are proposed.(2)Combustion behavior and thermal runaway propagation mechanism of the single cell.Based on the oxygen consumption method and the distributed parameter method,combustion behavior and the surface temperature distribution of the single cell after thermal runaway were studied.The heat release rate(HRR)of the battery fire measured by the oxygen consumption method was modified by measuring the oxygen production of the fully charged cathode,and it was found that the heat release corresponding to the self-released oxygen of the battery accounted for about 5%of the total heat release.The effects of different SOC and radiant heat flux on the combustion behavior of large-format high-specific-energy pouch lithium-ion cells were investigated,and it was found that 100%SOC and 0%SOC cells showed two different combustion behaviors.The ignition time of the cell decreased with the increase of the radiant heat flux,but the radiant heat flux had little effect on the total burning time,HRR,and total heat release(THR)of the 100%SOC cell.The relationship model between the cell ignition time and the radiant heat flux and the description model for the centerline temperature distribution of the stable combustion flame are established.The phenomenon of thermal runaway propagation in the width direction of the cell was found,and the thermal runaway propagation mechanism in the large-format pouch cell is revealed,that is,the hot flammable gas generated by thermal runaway,the jet fire of the thermal runaway cell interacts with the component materials that reach the activation temperature,providing a comprehensive thermo-mechanical-chemical effect for the thermal runaway propagation inside the cell.(3)Thermal runaway propagation behavior and characteristics of multiple cells.The distribution parameter method was used to investigate the temperature variation of the multilayer cell interfaces during the thermal runaway propagation process under the heating trigger mode.And the thermal runaway propagation process between the cells was analyzed.The calculation results showed that the main control factor triggering the thermal runaway of adjacent cells was the conduction heat between the cells,and the radiant heat and convective heat of the jet flame were both an order of magnitude lower than the conduction heat.The influence and mechanism of thermal insulation materials with different thermophysical properties on thermal runaway propagation between multiple cells were clarified.It is considered that the thermal diffusion coefficient is the key to influencing the thermal runaway propagation behavior of thermal insulation materials,and should be used as a key parameter for the selection of thermal insulation materials. |
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