Heat Transfer And Flow Mechanism Of Thermal Runaway And Its Propagation Of Lithium-ion Batteries

Lithium-ion batteries are widely used in electrochemical energy storage power stations,new energy vehicles,mobile electronic equipment,and other aspects because of their high energy density,low self-discharge,high voltage,and long life.However,heat is generated during the charging/discharging process of lithium-ion batteries,and the accumulation of heat will cause the temperature rising in battery.When the temperature is higher than the onset temperature of thermal runaway,a series of chain exothermic side reactions inside the lithium-ion batteries will be triggered,resulting in thermal runaway of the battery,and even lead to combustion,explosion and other accidents.Based on the lithium-ion electrochemical reaction principle,thermal runaway heat generation principle and the nonlinear computational fluid dynamics（CFD）theory of high-speed turbulent thermal fluid,this work constructs pseudo-two-dimensional（P2D）electrochemical-thermal model,thermo-electrical model,thermal runaway model and Spalart-Allmaras（S-A）turbulence model based on The Reynolds-Averaged Navier-Stokes（RANS）equation.The internal electrochemical heat generation,polar ohm heat generation,thermal runaway induced by local high temperature of the polar ear and its propagation,as well as the thermal runaway propagation mechanism induced by high temperature and high speed venting gas of the battery are investigated.The main conclusions are as follows:（1）By constructing a P2D/three-dimensional thermal model,the charging/discharging performance and heat generation characteristics of pouch-type lithium-ion batteries at different temperatures（-20℃,-10℃,5℃,0℃,5℃,10℃,20℃,and 25℃）and different rates（0.1C,0.5C,1C,2C and 3C）are studied.The heat generation mechanism of Li-ion batteries at different temperatures and different C-rates is also revealed.The results show that at low temperature（-20℃）,the pouch-type lithium-ion battery can only maintain the normal working state at very low charge/discharge ratio（0.1C）.Only when the temperature is higher than 0℃,the battery can work normally at high charge/discharge rate.Under different temperature conditions,the total heat generation of the battery increases with the decrease of temperature.In the charging process,when the temperature is-5℃and the charging rate is 3C,the total heat generation of the battery is the highest,about 40.85 k J.In the discharge process,when the temperature is 0℃and the discharge rate is 3C,the heat generation of the battery is the highest,about 33.74 k J.The total heat generation of the battery is mainly composed of polarized heat generation,ohmic heat generation and reversible heat generation.Under the condition of 0℃,ohmic heat generation is the main heat source constituting the total heat generation.When the temperature is higher than 5℃,polarization heat generation becomes the main heat source.Reversible heat production is relatively small and almost unchanged at different charge/discharge rates.（2）By combining experiment and numerical calculation,the heat generation and heat transfer of the tabs in the charging/discharging process of pouch-type lithium-ion battery are studied.Firstly,the battery performance and temperature rise at different temperatures and charge/discharge rates are tested experimentally,and the temperature distribution on the surface of the lithium-ion battery is studied by infrared camera.In addition,by constructing the thermal-electric model of lithium-ion battery,the heat generation and temperature rise of positive tab under high current density and the effective heat dissipation strategy under high temperature are studied.The results show that the temperature rise of the positive tab is much higher than that of the body and negative tab of the lithium-ion battery,and the temperature rise of the tab increases with the increase of temperature and charge/discharge rate.The temperature rise of the positive tab in the charging process is much higher than that in the discharge process,and the temperature difference is about 20.2℃,indicating that the charging process has more thermal risk than the discharge process.The positive tab has a fast temperature response during charging/discharging.Under the condition of 35℃and 2C,the positive tab can reach 70.4℃and 52.3℃,respectively,within 100 s after charging/discharging.In addition,the numerical results show that under the fast-charging condition（6C）,the maximum temperature of the positive lug can reach about 350℃(T_amb=35℃,2C),and it will cause significant local high temperature of the battery material near the positive tab,which seriously affects the thermal safety of the battery.When the external heat transfer coefficient is greater than 100 W·m^-2·K^-1,the temperature at the positive tab of the battery can be controlled below 100℃.Due to the special structure of the tabs of the pouch-type lithium-ion battery,forced convection heat dissipation can be used as an effective method to control the temperature.（3）By combining the thermal-electrical model of lithium-ion battery with the lumped thermal runaway model,the propagation of thermal runaway in battery modules caused by different parameters of battery modules（heat insulation layer,tab series and parallel connection,etc.）are studied.The thermoelectric model is used to calculate the heat generation at the tab and the battery body during the normal charging and discharging process,while the lumped heat runaway model is used to calculate the heat generation of thermal runaway within the battery module.The results show that when the applied current density is high（3C,4C,5C and 6C）,the tab overheating and induces thermal runaway of the battery.The trigger time decreases with the increase of the applied current density.The trigger time of thermal runaway at 3C rate is about 9 times that at 6C rate.There is a strong functional correlation between trigger time and current density at different rates.In the battery module without insulation,the trigger time of thermal runaway is significantly delayed,and the delay time decreases with the increase of current density.In addition,both series and parallel tab connection delay the trigger time of thermal runaway of Cell A in the battery module,because heat is transferred through the connection between the tabs before thermal runaway occurs.The inhibition effect of electrode parallel on thermal runaway of Cell A is more obvious,and the inhibition effect decreases with the increase of the rates.In addition,at low current density（less than 4C）,the tab connection can significantly slow thermal runaway propagation within the battery module,while at high current density,the tab connection can accelerate thermal runaway propagation.A series of electrodes would accelerate the spread of thermal runaway to Cell B,while a parallel connection would accelerate the spread of thermal runaway to Cell C.（4）Based on the time function of temperature and flow rate in the exhaust process of lithium-ion battery,S-A turbulence model based on RANS equation is constructed to study the basic parameters affecting the battery exhaust process,including key parameters such as the top spacing of battery modules,battery spacing,exhaust valve size,and exhaust valve position.And the dimensionless factor is used to evaluate the influence of all the key parameters on the thermal runaway propagation process.The results show that the top spacing mainly affects the high temperature and high velocity flow of the battery,but has little effect on the battery exhaust and thermal runaway propagation.The cell spacing can affect the flow state of the gas discharged from the battery in the battery module,thus affecting the overall air flow and heat transfer characteristics in the battery module.In addition,when the size of the exhaust valve is small（1mm）,the high-temperature gas cannot be discharged quickly,so the thermal runaway of the battery module is quickly triggered.However,when the size of the exhaust valve is large（5 mm and 10 mm）,the high-temperature gas can be quickly discharged from the battery module,so as to prevent the battery in the battery module from being triggered thermal runaway and prevent the spread of thermal runaway.The location of the exhaust valve also affects the flow and heat transfer status of high temperature and high-speed fluid in the battery module.When the exhaust valve is at the bottom,it is more conducive to the discharge of gas,thus preventing the spread of runaway heat.Through the temperature-based dimensionless factor analysis of all module structures,it can be seen that the exhaust valve size has the most significant influence on the exhaust process and thermal runaway spreading process of lithium-ion battery,and reasonable exhaust valve size design can effectively avoid the thermal runaway caused by battery exhaust problems.In summary,this dissertation has carried out relevant research on the heat generation,thermal runaway and its spreading process of lithium-ion batteries,as well as the influence mechanism of high temperature and high-speed venting gas of batteries on the thermal runaway propagation process.The research methods and results have certain reference value in revealing the thermal runaway and spreading mechanism and improving the thermal safety of lithium-ion batteries.This dissertation has 85 figures,8 tables and 226 references.