lithium 18650 li ion battery.Wang Qingsong, University of Science and Technology of my country: Research on thermal runaway propagation and barrier mechanism of lithium-ion batteries

Wang Qingsong, University of Science and Technology of my country: Research on thermal runaway propagation and barrier mechanism of lithium-ion batteries

Wang Qingsong, University of Science and Technology of China: Research on the propagation and blocking mechanism of thermal runaway in lithium-ion batteries. From the perspective of the entire system, many accidents are caused by thermal runaway of a single battery cell, through convection or heat conduction, and direct heating by flames. , thermal runaway spreads to the entire battery pack, eventually causing an uncontrollable fire and explosion accident. We start from the thermal runaway mechanism of the battery and conduct experiments and simulations

From the perspective of the entire system, many accidents are caused by thermal runaway of a single battery cell. Through convection or heat conduction, as well as direct flame heating, thermal runaway spreads to the entire battery pack, eventually causing an uncontrollable fire and explosion. ACCIDENT. We start from the thermal runaway mechanism of the battery and study the thermal runaway propagation process and blocking methods through experiments and simulations.

——Wang Qingsong, University of Science and Technology of my country

From August 26 to 28, 2020, under the guidance of the my country Energy Research Association, Zhongguancun Administrative Committee, and Zhongguancun Science City Administrative Committee, the Energy Storage Special Committee of the my country Energy Research Association, Zhongguancun Energy Storage Industry Technology Alliance, and the Engineering Heat of the Chinese Academy of Sciences The "9th Energy Storage International Summit and Exhibition" co-sponsored by the Institute of Physics was held in Beijing. The theme of the summit focused on "gathering the momentum of energy storage in the past ten years and creating new opportunities for the industry during the 14th Five-Year Plan". A forum to commemorate the tenth anniversary of the Energy Storage Alliance was held at the same time. Polaris Energy Storage Network and Polaris Power APP will broadcast the entire summit live.

At the "Energy Storage Safety and Standards" sub-forum held on August 28, Wang Qingsong, State Key Laboratory of Fire Science, University of Science and Technology of my country, gave a keynote report on "Research on Thermal Runaway Propagation and Barrier Mechanism of Li-ion Batteries".

Wang Qingsong, University of Science and Technology of my country

Wang Qingsong: I am very grateful to the Zhongguancun Energy Storage Alliance for giving me such a good opportunity to share with you my experience and some work progress in lithium-ion battery energy storage and safety. What I am sharing with you today is about thermal runaway of lithium-ion batteries. The research on transmission and isolation mechanisms was jointly completed by Dr. Li Huang, a recently graduated doctoral student in my research group, so it is also a task that our team completed together. First, let’s give a brief overview from the research background. We know that lithium-ion batteries are not only used in energy storage, but also in other fields, such as portable devices, mobile phones, laptops, special aerospace, ships, etc. It is becoming more and more widespread. Of course, energy storage is our most widely used aspect. However, fire accidents of lithium-ion batteries often occur. In the field of energy storage, we know of more than 20 fire accidents in Korean energy storage power stations, which greatly hinders its entire development. development of the industry.

The main components of lithium-ion batteries include positive electrode materials, negative electrode materials, electrolytes, and separators. Among them, the electrolyte is more likely to catch fire. Because the electrolyte is made of organic carbonate materials, there are many safety hazards. However, from the perspective of the entire system, the fire of a single battery cell is the cause. Gradually expanding and spreading, after one cell catches fire, thermal runaway will be transmitted to the entire battery pack through convection or heat conduction and direct flame heating, eventually causing an uncontrollable fire and explosion accident. Like the picture on the right, there are many photos after the accident. It can also be seen that some places were severely burned, and some places were only slightly burned but not completely burned, indicating that there is a process of thermal runaway propagation inside. The reason for the spread of thermal runaway is that many batteries are put together. The energy and heat caused by a battery running out of control are transmitted to the surroundings in a variety of ways. If it is not effectively blocked, it will cause the entire battery module to lose control. In the meantime A lot of toxic and harmful gases will appear, causing uncontrollable losses. We have done a lot of experiments, and this is also the process of module-level uncontrolled transmission, which ultimately led to the expansion of the accident.

As far as the current research status at home and abroad is concerned, some early research has been done abroad, but most of them were focused on small batteries like 18650. Recently, our research group has done a lot of research on the safety of large-capacity lithium-ion batteries, with battery capacities ranging from 30AH to 50AH to 300AH. From the goal of our research, we must first understand the stability of battery materials, so that we can understand the heat generation rules of internal materials. The second is to look at the characteristics of thermal runaway of a single battery cell. The evolution process of thermal runaway and the mechanism of thermal runaway propagation are obtained. Finally, the process of thermal runaway propagation is simulated through simulation.

The research route starts from the heat generation rules of battery materials, including the positive electrode, negative electrode, electrolyte, separator, etc. This can provide some key parameters for the next experiment and modeling, such as reaction kinetic parameters and the loss of control of the entire battery cell. Characteristic behavior, then to the characteristics of energy migration, temperature changes, and propagation behavior of the entire module, and finally taking corresponding blocking measures to see if we can effectively block the thermal runaway of the battery. This is the overall route we take.

In terms of the thermal stability of materials in the first part, we used calorimetric analysis methods to study the decomposition reactions of the battery separator, electrolyte, negative electrode material and positive electrode material. Through these, we can obtain the trigger temperature and total temperature of each thermal reaction process. The amount of heat released provides some key parameters for later modeling of thermal runaway. It can be seen that the temperature triggered by the thermal reaction is that the negative electrode material is smaller than the electrolyte and positive electrode materials, and the heat release occurs most in the electrolyte. In addition, we also studied the total heat production characteristics of the positive and negative electrode systems with added electrolyte load, and when a single positive electrode and negative electrode were separated, to obtain the process of each reaction peak, and further obtain the dominant reaction processes at different stages. Here we analyze the two materials of lithium iron phosphate and NCM. Other material systems can also be analyzed. In this way, the relationship between some important chemical reaction processes and heat production can be obtained, which also provides a basis for subsequent work. Analyze the corresponding basis for supply.

Next is the study of thermal runaway of a single battery cell. We use an adiabatic acceleration calorimeter to keep the battery in a relatively adiabatic process, that is, there is no exchange of energy and heat with the outside world. Therefore, the heat that causes thermal runaway of the battery comes entirely from the battery. The heat generated by the self-heating reaction can be quantified more clearly. In this way, we tested the runaway process of the 38AH square ternary battery and the 2AH 18650 ternary battery. It can be seen that when it is heated in the early stage, when the battery temperature gradually increases to a certain level, its internal thermal reaction is triggered, and the battery begins the self-heating stage. It can be seen that there is a small fluctuation during the temperature rise, and the temperature drops slightly. This is because the safety valve of the battery opens and takes away a large amount of heat. Based on these characteristics and behaviors, we can divide the four stages of the battery runaway process. The first stage starts from room temperature to 96oC. Under EV-ARC heating, the battery gradually heats up. During this process, the internal thermal reaction of the battery is not triggered. . The second stage starts from 96oC to 134oC. During this process, due to the melting of the separator, the battery begins to undergo some micro short circuits, causing the battery temperature to increase, and the battery self-heating rate begins to increase. In the third stage, the battery temperature is higher and the internal heat generation rate is faster. When it reaches about 160oC, the battery is triggered to thermal runaway. The final stage is the process of thermal runaway.

Based on these research results, a simple analysis can be done on the cause of the internal runaway of the battery, because there is an attached SEI film on the surface of the battery negative electrode. At about 90oC, the SEI film begins to decompose. After decomposition, the battery negative electrode loses protection. As a result, the negative electrode of the battery reacts with the electrolyte, emitting heat and causing the temperature to rise, gradually melting the internal diaphragm, causing an internal short circuit of the battery, which in turn releases a large amount of heat and a certain amount of gas, causing the battery safety valve to rupture. The heat generated by these reactions will cause the internal temperature of the battery to rise, and the increase in temperature will further accelerate the progress of these reactions, forming a positive feedback process of heat, which will eventually cause the battery to run out of control at a certain critical point. We can compare some dynamic parameters related to the internal short-circuit temperature and thermal runaway trigger temperature of different SOC batteries to evaluate the safety level of different battery systems. For the same battery system, it can be seen that the internal short-circuit temperature gradually decreases with the increase of SOC.

Only with the study of individual cells can we better analyze the process of thermal runaway propagation of battery modules. We made 1×1, 1×3, 1×5 and 3×3 structures. The 1×1 structure is two side by side. The 1×3 structure is triggered by a thermal runaway battery that causes the rear battery to run away. There is also the 3×3 The method is that one thermal runaway battery triggers thermal runaway of surrounding batteries. At the same time, we consider the impact of factors such as SOC, battery model, battery spacing, and heating power on the spread of thermal runaway. Regarding the battery pack with a 1×1 structure, in the early stage, after the battery is heated, its internal self-reaction is gradually triggered. In the second stage, the battery safety valve begins to open and releases a certain amount of smoke, which will later produce gas. gradually increases, and will enter a completely out-of-control state when it reaches a certain level in the later stage. Finally, the out-of-control temperature of the battery can reach 700~800oC, accompanied by a relatively strong jet fire phenomenon. In addition, we can see that as As SOC increases, the opening temperature of the safety valve shows a certain downward trend, but it is not particularly obvious. In addition, as the SOC increases, the energy released and the peak temperature of the battery during thermal runaway become higher and higher, and the severity of thermal runaway becomes more and more serious.

Regarding the battery system with a 1×1 structure, we can analyze how much air convection and thermal radiation transfer heat to the next battery. As can be seen from the figure above, in the early stage, heat transfer is mainly carried out by air convection. As the temperature increases, the battery The temperature difference gradually becomes obvious, and the radiation heat transfer gradually increases in the later stage. After increasing the distance between cells, this ratio changes slightly. It can be seen that after the distance is increased, the most important thing is radiation, which is the proportion of blue below. For the 3×3 structure, a heating tube with the same size as the battery is used to replace the thermal runaway trigger battery, which is battery 0. It heats the surrounding batteries 1 and 2 and triggers their runaway. These two batteries The loss of control will release more heat, which will later cause batteries 3 to 5 to gradually show a step-like process of out-of-control propagation, and finally batteries 6 to 8 will lose control. For the battery module, basically under this experimental working condition It will be found that the spread of thermal runaway will eventually cause thermal runaway of the entire module. In the later period, a 4×4 module was also made, and it spread faster as it went further.

We also conducted experiments and modeling studies on the propagation of thermal runaway in square batteries. We arranged thermocouples on the front wall, rear wall, upper wall and side wall of the battery to test the temperature change rules. On the left, use a 300W heating plate to heat battery No. 1. We will see later whether batteries No. 1 to No. 5 will spread out of control. It can be seen that after the thermal runaway was triggered, battery No. 1 expanded and released gas, then entered a stage of violent jet fire, and then entered a stage of relatively stable combustion. After that, thermal runaway gradually spread to the following batteries. Without blocking measures, No. 1, No. 2, No. 3, No. 4, and No. 5 all lost control. It can be seen that the temperature changes on the front and rear walls of the battery are It is very regular and shows a step-by-step transfer process. The section where the temperature is slightly flat is the temperature transfer process inside the battery. In addition, we can see that there is a time difference between the sudden rise in temperature of the front and rear walls of the battery. This is the process of thermal runaway spreading inside the battery core. That is, the local high-temperature area of the battery core near the front wall of the battery is triggered and gradually undergoes thermal runaway. The process of spreading to the entire battery, this time process generally has a statistical law, which is the time of heat diffusion inside the battery. For a 100% SOC battery, this time is basically 10 seconds. For a 50% SOC battery, the delivery time is longer, 39 seconds.

Taking the thermal runaway propagation process between batteries No. 3 and No. 4 as an example, we conduct a theoretical analysis of the mechanism of thermal runaway propagation. First, battery No. 3 loses control, and its temperature quickly reaches a peak, and battery No. 4 is heated violently. , causing the surface temperature of AA battery to rise rapidly. When the internal cell temperature of the No. 4 battery gradually rises to the thermal runaway triggering temperature, the No. 4 battery first undergoes local thermal runaway and quickly expands to the whole. Regarding the battery module with 100% SOC, the average time from the thermal runaway of the previous battery to the thermal runaway of the next battery is about 87 seconds. The battery pack with 50% SOC will take longer, 307 seconds.

This is the thermal runaway propagation mechanism. The thermal runaway propagation dynamic model is briefly modeled and analyzed later. The heat that appears inside the battery comes from the test results of the corresponding thermal analysis we did in the first part, and the thermal parameters obtained It can be input into the battery thermal runaway model. We did a modeling study on the previous 38AH lithium-ion battery thermal runaway. The more critical parameters here are also obtained through experiments, such as: SEI film decomposition, the reaction temperature triggered by the negative electrolyte, the key temperature for converting electrical energy into thermal energy, the reaction temperature of the positive electrode and the electrolyte, and The temperature at which the positive electrode and the electrolyte react again, as well as the decomposition process of the electrolyte, etc., are all based on the previous experimental results. At the same time, we use the key parameter data of the experiment to verify the model. You can see that the red line is simulated, and the blue line is the experimental temperature rise curve. From this comparison, the early temperature rise process basically maintains It is completely consistent. There is a slight difference when the temperature rises to the front in the later period. This may be related to the battery expansion and the thermal runaway process being too rapid. It does not affect the entire thermal runaway triggering process. We think the simulation results are quite reasonable. of.

Later, a simple modeling study was conducted on the propagation of thermal runaway, and the influence of insulation materials, released energy, ambient temperature and battery spacing on the propagation of thermal runaway was also analyzed. Some of the control equations used here include the temperature rise rate and battery thermal conservation equations, as well as the exchange of heat between the battery environment. The battery's own heat release includes many, such as SEI film, positive and negative electrodes and electrolyte decomposition, etc. , The heat transfer methods between batteries include convection, radiation and heat conduction. If the two batteries are not in contact, there will be no heat conduction process, as well as the heat dissipation process between the battery and the environment. After passing some important thermal conservation equations, the simulation results are compared with the experimental results. This is the experimental result without a heat insulation layer between the batteries. It can be seen that the experimental value and the model runaway trigger temperature are relatively close. The experiment was at 287oC, and the simulation It comes out at 293oC, which is a difference of more than ten degrees Celsius. After adding the heat insulation layer, the difference between the experimental results and the model results is also more than ten degrees Celsius. We believe that within this range, although there is a certain temperature difference in this simulation, Still acceptable. Later, the diffusion time and temperature distribution inside different batteries can be simulated. The results from No. 1 to No. 5 are relatively close. For example, in No. 1 battery, the heat transfer time inside the battery in the experiment was 10 seconds, while the simulation The result is 9 seconds, which is still very close and generally acceptable.

What follows is the impact of battery and ambient temperature on the propagation of thermal runaway, as well as the changing trends and patterns. A unified analysis can be conducted based on these models. That is to say, when the distance between batteries increases,