Since April 2019, combustion incidents of electric vehicles have occurred frequently. Tesla vehicles have experienced spontaneous combustion incidents one after another in Shanghai, Hong Kong, San Francisco, Belgium and other places, and Weilai ES8 has also suffered three spontaneous combustion incidents in just two months. In addition, according to data from the State Administration for Market Regulation, there were at least 40 fire accidents involving electric vehicles in my country in 2018. For this reason, the safety issues of electric vehicles, especially power batteries, have attracted great attention from the industry, and the essence of power battery safety is battery thermal runaway.

　　So what are the main causes of thermal runaway of power batteries? And what about the situation of battery thermal runaway? In response to this, on June 23, at the "China (Qinghai) Lithium Industry and Power Battery International Summit Forum" hosted by the Qinghai Provincial People's Government, the Ministry of Industry and Information Technology, the Ministry of Science and Technology, and the China Electric Vehicles Association of 100, the Chinese Academy of Sciences Ouyang Minggao, academician and executive vice chairman of the China Electric Vehicles Association of 100, elaborated.

　　Ouyang Minggao, Academician of the Chinese Academy of Sciences and Executive Vice Chairman of the China Electric Vehicles Association of 100

　　It is understood that in order to solve the problem of power battery safety, Tsinghua University, where Ouyang Minggao works, established a battery safety laboratory earlier to carry out research on power battery safety prevention and control. During the continuous testing process, the laboratory extracted three characteristic temperatures of battery thermal runaway, the starting temperature of self-generated heat T1, the triggering temperature of thermal runaway T2, and the maximum temperature of thermal runaway T3. Based on these tests, three thermal runaway trigger mechanisms were fully revealed: the first is the release of active lithium from the negative electrode, the second is internal short circuit, and the third is the release of active oxygen from the positive electrode.

　　Low temperature lithium iron phosphate battery 3.2V 20A -20℃ charging, -40℃ 3C discharge capacity ≥70%

　　Charging temperature: -20~45℃ -Discharge temperature: -40~+55℃ -40℃ Support maximum discharge rate: 3C -40℃ 3C discharge capacity retention rate ≥70%

　　Battery Charging Lithium Evaporation and Fast Charging Control

　　Analysis of recent charging accidents shows that improper rapid charging or overcharging causes lithium precipitation in the battery, which causes the thermal runaway temperature to drop significantly, from 219°C to 107°C, and reacts violently with the electrolyte, causing the battery to freeze at 107°C. Thermal runaway occurs.

　　Through experimental characterization, it was found that the generation of lithium precipitation can be clearly seen during fast charging. By studying the mechanism of lithium precipitation, the complete process of lithium precipitation was discovered, including the precipitation and re-intercalation of lithium on the surface of the negative electrode during the battery charging process, and the precipitation process. It is formed after the negative electrode has zero potential. After the battery stops charging, the potential will return to above zero potential. At this time, it will be re-embedded, and then all the reversible lithium will be completely dissolved, and the negative electrode will no longer react.

　　We established a simulation model for this mechanism, adding the process of lithium evolution reaction to the conventional battery quasi-two-dimensional (P2D) model, and based on this, we conducted simulation and verification. Judging from the simulation results, the voltage platform during the resting process of the battery after charging and lithium precipitation can be successfully simulated. This platform is the process of re-embedding. By differentially processing the above voltage platform, the time of the entire lithium evolution process can be quantitatively obtained. Using this time as a variable, we can establish an empirical formula to calculate the amount of lithium evolution.

　　First, a quasi-two-dimensional electrochemical mechanism model was established to predict the negative electrode potential, and based on this, the analytical expression of the optimal charging curve was obtained. Then, using the positioning of the charged negative electrode as a benchmark and adding a redundancy, it can be derived The optimal charging current of the battery is obtained. Based on this, we can control optimal charging. Based on the model-based negative electrode potential observer, we can compare the observed potential of the negative electrode potential with the reference potential, and adjust the charging current to make the potential difference tend to Zero can achieve fast charging without lithium precipitation.

　　The above model will cause errors as the battery decays, and the simulation results may not be accurate. Therefore, we developed a new reference electrode based on this, which directly feeds back the negative electrode potential. The traditional reference electrode has an extremely short lifespan. We have developed a new reference electrode with a lifespan of more than 5 months, and are continuing to optimize it. We hope that the reference electrode will The service life of the electrode is extended as much as possible, so that it can truly be used as a sensor. Before the application of on-board sensors, we applied it to the calibration of the charging algorithm, which can save a lot of time, because the traditional charging algorithm calibration requires disassembly and observation every time. After applying the reference electrode, there is no need to disassemble, and the charging is optimized with high efficiency. algorithm. At present, the charging algorithms of domestic companies are too simple. We have communicated with Nissan and their charging algorithm is based on a large amount of data MAP, so we must also make a good MAP so that the charging algorithm can consider various influences. Factors, the workload and experimentation of this process are very large. In order to solve this problem, a long-life reference electrode is used as a basis to calibrate a charging curve that is as close as possible to the optimal charging current.

　　In-battery short circuits and battery management

　　Internal short circuit is a common link in battery thermal runaway. Different types of internal short circuit may occur for various reasons, including mechanical deformation, extrusion, tearing, diaphragm rupture, overcharge and overdischarge, and extreme overheating. A more dangerous type of internal short circuit is a self-induced internal short circuit, such as the Boeing 787 accident, which is caused by the accumulation and evolution of impurities and particles introduced during the manufacturing process after long-term operation.

　　Dendrite growth can be simulated, but internal short circuiting is a phenomenon that is more difficult to reproduce experimentally, and a variety of alternative experimental methods need to be developed. We invented a new alternative experimental method to simulate internal short circuit testing, which mainly involves implanting a special memory alloy internal short-circuit trigger element with a spiked structure into the interior of the battery. The temperature rises to cause the spiked structure to rise and pierce the diaphragm. Simulate the internal short circuit process. Through this experiment, it was found that the main types of internal short circuits include four circuits: aluminum-copper, positive electrode-copper, aluminum-negative electrode, and positive electrode-negative electrode. Some of them will cause thermal runaway immediately, such as the contact between aluminum and the negative electrode; while the contact between the positive electrode and the negative electrode generally does not cause thermal runaway; the risk of contact between aluminum and copper is also relatively high, but it does not necessarily cause an internal short circuit immediately.

　　We build a simulation model for the thermal runaway internal short circuit, of which the most important thing is the fusing at the internal short circuit position. This fusing may cause the entire internal short circuit to terminate, or it may also lead to a more severe internal short circuit. To this end, we conducted an analysis of the various parameters that influence this fusing. We conducted a comprehensive analysis and summary of the evolution process of the entire internal short circuit. On this basis, we proposed that in order to prevent thermal runaway, the internal short circuit must be detected at an early stage.

　　One of the methods introduced is the internal short circuit detection method of series battery packs, which is mainly diagnosed based on consistency differences. Specifically, equivalent models with and without internal short circuit can be established. Based on this equivalent model and the average difference model, online parameter estimation is performed. After the internal short circuit, the potential and equivalent impedance change. We compare these two models. After parameter identification, we can finally find out which cell has the problem. By verifying the test results, it is obvious that a certain battery has an internal short circuit. But the algorithm is only a foundation. On this basis, we also need to combine a large amount of engineering experimental data to finally develop a practical detection algorithm. Of course, internal short-circuit detection alone is not enough. Comprehensive management of overcharge, over-discharge, SOP, etc. is required to achieve early warning of internal short-circuit and thermal runaway. This is a new generation of battery management system that takes safety as its core. comprehensive state estimation and fault detection.

　　Single battery thermal runaway and thermal design

　　There have been many changes in separator materials. From PE, PP, PE+Ceramic to PET materials, the heat-resistant temperature of separators has been very high, which can reach 300°C. At the same time, the cathode material has changed from early LFP to NCM111, NCM523, From NCM622 to now NCM811, the oxygen release temperature of the cathode material is gradually decreasing.

　　As these two technologies change, so do the mechanisms of thermal runaway. Most of the early batteries suffered from thermal runaway due to the collapse of the separator, which caused a large-scale internal short circuit. However, the mechanism of thermal runaway of the 811 cathode power battery with a high-temperature resistant separator currently used has changed, and the release of oxygen from the cathode material has become the main cause of thermal runaway. Experimental results show that in the absence of internal short circuit, thermal runaway will still occur if the separator is completely removed and the electrolyte is drained. When the positive and negative electrode powders are mixed for testing, a violent exothermic peak will appear. Through further analysis, it was found that the charged positive electrode material began to undergo a phase change at about 250°C and released active oxygen. The generated oxygen reacted with the negative electrode, and the heat release increased sharply. Therefore, in the new battery system, the positive and negative electrode oxidation-reduction reactions Generating a large amount of heat is the direct cause of thermal runaway, not just the collapse of the separator in the traditional battery system, which causes internal short circuit and triggers thermal runaway.

　　Based on the above-mentioned mechanism analysis, parameters related to exothermic side reactions of various battery materials are measured, and then thermal analysis kinetics is used for analysis and parameter optimization. Finally, by integrating all side reactions, the entire thermal runaway process can be predicted. Thus, accurate battery thermal runaway prediction can be used to guide battery safety design. Based on the statistics of thermal stability parameters of various battery material systems, a series of methods can be proposed to improve the thermal runaway characteristics of batteries, including positive electrode modification, negative electrode modification, improving the stability of the electrolyte, and using high thermal stability materials. Diaphragms, etc., the key lies in how to combine them. Only one of the methods is shown here. To optimize the morphology of the cathode material, the traditional ternary polycrystalline cathode is optimized into a ternary cathode with a single crystal large particle structure. The oxygen production of the single crystal cathode is 100°C later than that of the polycrystalline cathode. , the maximum temperature of thermal runaway has also been reduced.

　　Thermal spread and thermal management of battery systems

　　If all the previous methods fail, the problem must be considered from the perspective of the entire system. For example, a violent collision or the chassis is pierced by a sharp substance will cause immediate thermal runaway. This happens from time to time. This kind of thermal runaway can only be solved at the system level.

　　First, the thermal runaway propagation process was tested. It was obvious that the battery cells were thermally out of control one after another, like setting off firecrackers.

　　Secondly, a parallel battery module thermal spread test was conducted, and it was found that the unique characteristics of thermal runaway spread of parallel modules, that is, multi-stage V-shaped voltage drops; when the actual vehicle-level battery module is not suppressed, thermal runaway spreads in the battery Acceleration effects can occur in the module, eventually causing the entire module to explode violently.

　　Thirdly, the characteristics of the thermal runaway nozzle were tested. In a closed and constant-volume incendiary bomb, a high-speed camera was used to record the entire process of the thermal runaway eruption. From the test, it was found that the jet flow showed the characteristics of the coexistence of gas-liquid-solid three phases. The gas injection speed is as high as 137m/s.

　　Next, a lumped parameter thermal resistance model of the thermal runaway spread of the battery module and a three-dimensional simulation model of the thermal runaway spread of the power battery system are established. The most difficult part of the above models is how to determine the thermal physical parameters before and after the entire thermal spread process. If these parameters cannot be determined , the simulation results can only be good-looking but not easy to use. Our research group has developed a parameter estimation method, and the experiment and simulation can be in good agreement.

　　On this basis, a heat spread suppression design was carried out, including heat insulation design and heat dissipation design. The heat insulation design uses different insulation materials to prevent the heat spread of the module. The heat dissipation design uses different liquid cooling flow rates to suppress heat spread. In general battery systems, heat insulation and heat dissipation alone can solve the heat spread process, but in new battery systems, heat insulation and heat dissipation need to be combined to suppress heat spread. This is the so-called firewall technology.

　　Nowadays, heat spread technology has been applied in the formulation of international standards. There is currently no unified heat spread standard in the world, and China will soon introduce heat spread standards. Thermal spread is the last line of defense that leads to safety accidents. We must take care of this last line of defense and strive to promote China's experience to the world and become a global regulation.

　　Finally, let’s make a summary: Thermal runaway includes three processes: inducement, occurrence and spread. There are two main inducements. One is lithium precipitation caused by overcharging, fast charging, aging batteries, low-temperature charging, etc.; the other is internal lithium deposition caused by various reasons. short circuit. From the perspective of the safety of the system itself and the material system, the thermal safety design of single cells is carried out to suppress the spread of thermal runaway when other methods are not feasible.

　　Looking forward to the future, the energy density of lithium-ion batteries will continue to increase. The energy density of 300Wh/kg has been reached. The increase in volume and energy is an irreversible trend. Under this circumstance, the technical requirements for safety prevention and control will become higher and higher. We must focus on solving the safety issues of lithium-ion batteries, develop safer lithium batteries, and ensure the smooth development of the electric vehicle industry. On this basis, the expert group of the National New Energy Vehicle Key Project has also formed the next step of the lithium-ion power battery technology roadmap. This was made 2 years ago. We can continue to use it to form a high safety ratio. Energy batteries, in terms of cathode materials, have developed from the current high-nickel ternary materials to lithium-rich manganese-based materials. There is still a lot of room for development of cathode materials for lithium-ion batteries. From the perspective of the negative electrode, the current focus is on the silicon-carbon negative electrode. The next step is to gradually increase the proportion of silicon. When the proportion of silicon increases to a certain level, the problem of fast charging will be solved. What is more important at present is the electrolyte and separator. The electrolyte needs to add additives to form an interface with the positive and negative electrodes to prevent oxygen loss from the positive electrode and lithium evolution from the negative electrode. The solid electrolyte still needs some time to develop.

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