Research On Laws Of Thermal Runaway And Thermal Propagation Of Li-ion Batteries

Because of its high energy density and long cycle life,lithium-ion batteries are widely used in electric vehicles and energy storage power stations.However,the frequent thermal runaway and thermal propagation accidents have become the problems that the large-scale application of lithium-ion batteries has to face.At present,there has been some research work on thermal runaway and thermal propagation.However,due to the high safety risk and complex reaction of thermal runaway and thermal propagation process,the existing research methods are still immature,especially for the key technology of thermal runaway and thermal propagation complex process research based on practical application conditions.Therefore,this paper focuses on solving the key technical problems in the study of thermal runaway and thermal propagation of lithium ion batteries,systematically studies the characteristics of heat generation and gas generation after thermal runaway of lithium ion batteries and the influence of electrothermal effect of parallel battery modules on thermal propagation,and proposes the boundary condition research scheme of thermal propagation blocking technology for battery modules to effectively block the thermal propagation and provide strong technical support to product development.In order to solve the problems that the triggering method of thermal runaway and the judging condition of thermal runaway of single cell in thermal runaway and thermal propagation experiments are not clear,9 kinds of lithium-ion batteries with significant differences in material system,shape structure,energy density and other aspects are selected as the experimental research object.The success rate,reaction mechanism and operability of thermal runaway by heating,penetration and overcharging methods are compared,it is found that heating is more suitable for triggering the thermal runaway of lithium ion batteries.Further analysis of the temperature and voltage data of 9 types of batteries in the process of heating thermal runaway found that the temperature rise rate≥1℃/s and the voltage drop rate>25%can be used as the judgment condition for the occurrence of thermal runaway of lithium-ion batteries.The result was quoted by the national standard GB38031-2020 Electric Vehicles Traction battery Safety Requirements.Based on the demand of thermal runaway warning for lithium ion batteries,the research on the expansion force change of ternary prismatic lithium-ion battery under normal use conditions and thermal runaway conditions the results show that the expansion force increases significantly before the thermal runaway occurs,reaching more than 4times of the normal use state of the battery.The expansion force signal has earlier identification than the temperature signal,and has the potential to be used as a warning condition for thermal runaway of lithium ion batteries.In order to reduce the influence of heating conditions on the thermal runaway test results,the thermal runaway results of lithium-ion batteries under different heating power of 25 square centimeter heating plates were studied.For 37 Ah prismatic ternary lithium-ion batteries and 54 Ah pouch ternary lithium-ion batteries,600 W heating was the most suitable.A calculation method of the optimal heating power based on the weight and rated energy of the battery was proposed for the ternary cylindrical lithium-ion batteries.After establishing the key conditions of the thermal runaway experiment,the gas generation characteristics of lithium ion battery after thermal runaway and its influence on thermal propagation were further studied.The thermal runaway heat generation characteristics of lithium-ion batteries were tested based on the traditional accelerated adiabatic calorimeter.A gas generation characteristic test device after thermal runaway was developed.The thermal runaway gas generation process of 40 Ah prismatic lithium-ion batteries was about 7s and the gas production was about 33 L.Amesim and Star CCM+ are used to simulate the thermal runaway and thermal propagation heat and gas generation process of the battery module,which simplifies the simulation model creation process compared with a single software.It is found that the gas ejected after the thermal runaway of the cell and the energy it carries make the temperature of the area far away from the thermal runaway cell also reach a higher state at the early stage of thermal runaway.As a result,the thermal propagation of the battery module broke the original order of thermal propagation,which proved that incorporating the actual gas production process during the thermal runaway into the simulation model could provide more accurate research results.In order to study the laws of thermal runaway and thermal propagation of parallel battery modules,a method for directly measuring the physical parameters such as the internal short circuit resistance and the electrothermal effect power of the thermal runaway battery during the thermal runaway process of the parallel circuit battery is proposed.The experimental study on the thermal runaway of the 4 parallel 25 Ah prismatic lithium iron phosphate battery module shows that the thermal runaway process in parallel circuit will go through the establishment of internal short circuit,violent reaction of thermal runaway and continuous electrothermal reaction after thermal runaway.The minimum short circuit resistance in the process is about 0.5 mΩ,The discharge peak current of the parallel battery is more than 1100 A for the thermal runaway cell,and the electrothermal effect power at the stable discharge stage is about 600 W to 900 W.It shows that the parallel circuit has an important influence on thermal propagation of the module.Through establishing the thermal runaway simulation research model of parallel battery module and conducting a series of studies,it is found that without considering the heat exchange between the system and the outside,the maximum temperature and heat release of the thermal runaway battery will be increased when the thermal runaway of the first triggered cell occurs in the module.In the subsequent thermal propagation process,if the change rate of electric energy stored by the cell with SOC is greater than the change rate of its thermal runaway release energy with SOC,the parallel circuit will accelerate the thermal propagation process.Otherwise,the parallel circuit will delay the thermal propagation process.In order to efficiently develop safer lithium-ion battery modules and systems,a simulation model was proposed to quickly obtain the boundary conditions that can effectively block the thermal propagation.This method was applied in the thermal safety design and development of a certain type of module,and the thermal conductivity boundary of thermal insulation materials that can effectively block thermal propagation under different space conditions and the flow boundary that can effectively block thermal propagation under different coolant temperature conditions were obtained.