Electric car battery thermal runaway causes and preventive measures
April 21, a Tesla Model S at the Shanghai underground garage spontaneous combustion, spontaneous combustion of video spread rapidly on the network. Many media stressed: \”The smoke from the fire to only four seconds, no time to escape.\” This makes many people feel very unsafe electric vehicles. And so many batteries, once afault, the whole car and the people inside it are at stake. This paper tries for everyone to do a reading. What is What is said first thermal runaway, according to standard \”Electric Vehicle Safety requirements for lithium-ion secondary battery\” in the definition of, thermal runaway of the battery cell refers to a chain reaction caused by heat from dramatic changes in the rate of temperature rise of overheating, fire, explosion phenomenon. New energy vehicles battery system generally consists essentially of the battery module, the BMS battery management system, thermal management system as well as some other electrical and mechanical systems. Manufacturers OEMs, battery or power systems, battery systems in the design and production, to consider multiple factors. For cheap, they want low-cost battery systems; car to run away, mileage long, they want high energy density; for safety, they want a battery system safe. These are important factors of new energy vehicles, whether large-scale application. But, unfortunately, often between these factors are bound to each other, you can not pursue anything extreme. We can look at the chart. In order to pursue a longer life mileage, we have to use higher and higher energy density batteries: from lead-acid batteries, nickel cadmium batteries, lithium iron phosphate, three batteries, but also more and more high energy density battery triples . However, the higher the energy density of the battery, even lower thermal stability. Therefore, in order to better use these high energy density batteries, security has been further attention. Most important, it is to prevent thermal runaway. In the lithium-ion battery over-discharge, needling, the event of collision, are likely to cause an exothermic chain reaction, resulting in thermal runaway, form smoke, fire and even explosion. In addition to these extreme cases, the battery will conventionally generates heat, heat dissipation is not good design, it may lead to thermal runaway. Individual cells (i.e., batteries) due to its own a certain resistance, charge and discharge at the same time, some heat is generated, so that their temperature increases. When the temperature exceeds itself between its normal operating temperature range, the battery performance and life willaffected. During operation of system conditions in different applications, it will generate a lot of heat, gathered in a small battery housing. If the heat can not be dissipated quickly in a timely manner will affect the battery life in the system, and even thermal runaway, leading to electrical and other core caught fire and exploded. Tesla models, characterized by a long mileage, but also continue to improve the electrical energy density of the core implementation. Currently three yuan NCA batteries Tesla used, due to the high energy density, while acupuncture test may burn fiercely, and other than batteries, naturally not good. From the point of view but also the material properties of the ternary is lower than the decomposition temperature of the battery, lithium iron phosphate, in the same high temperature environment, the probability of occurrence of thermal runaway than lithium iron phosphate. Thus, thermal runaway of the battery Tesla reason, from the cell itself and the external environment two look. The former and the latter will have some impact. causes thermal runaway Tesla This is not the first case of spontaneous combustion from electric cars on fire, it will not be the last. From years electric vehicle accidents occur view, the reason is formed mainly outside the needling impact extrusion or immersion of seal failure, and excessive heat concentration at high temperature charge-discharge process, batteries itself such as leakage, with electricity the core internal short circuit caused by thermal runaway. In the case study of electric car spontaneous combustion process, you can see the hot charging and discharging causes uncontrolled minority. Lithium-ion battery during thermal runaway occurs, as the temperature rising successively through the following process will not instantly a complete process: high temperature capacity fade the SEI film exploded → negative → – → electrolyte membrane reactor the decomposition reaction of the positive electrode melting process → → → decomposition reaction of the electrolyte solution, the negative electrode and the electrolytic solution → combustion reaction of the adhesive and the like. from the components of the lithium ion battery cell of view, possible causes thermal runaway occurred mainly in the electrolyte, positive and negative electrodes thermally stable material. Such as a lithium ion battery during charging, the surface of metal lithium can be deposited lithium dendrite-like twigs, accumulated to a certain extent it will pierce the cell membrane, resulting in direct contact with positive and negative short circuit lead to thermal runaway. As another example, when the overcharge, heat is generated lithium ions from the positive electrode reacts with the electrolyte solution overflow. Heating was continued heat in turn trigger cell with a solvent metal lithium, lithium carbon mixed with a solvent of the reaction, the gas can not generate heat immediately discharged in a very small space, so the battery to explode. forTo prevent thermal runaway, To batteries, modules, systems do prevention and control of insulation, but the cell and module level of insulation prevention and control, had not been too much concern to the industry. Therefore, the overall design of the battery system, it is necessary to influence the environmental temperature and cell position monomers consideration where battery modules. In the design of the battery module arrangement, if the alignment between the cell or module is compact, and there is no heat and insulation measures, during charging and discharging the battery temperature rises rapidly serious security risks. In order to improve the safety of the battery, the need to avoid or suppress the occurrence of thermal runaway. Overall, the lithium ion battery industry to solve the problem of thermal runaway, a major improvement from two batteries and a system protection. Thermal runaway working solution In order to solve thermal runaway, the monomer level dielectric material of the core can be improved. For example, positive and negative electrode material coating treatment, a flame retardant electrolyte solution, or the like to the direction of development of solid-state battery. Key narrative about some domestic and foreign companies, in terms of battery thermal runaway do the job. First look at the thermal management of batteries. domestic battery boss Ningde era, aerogels are placed between the batteries. Airgel both high performance and thin insulation thickness, it may be extremely limited in the insulation space and a certain thickness can be better delaying runaway heat. Including SAIC age we have the same program. The GM program, similar to the combination of Tesla and Ningde era. They are placed between the cooling plate and the foam cell, so that the heat insulating effect of the foam and cooling the sheet while achieving. However, within the existing scope of the study, foam is mainly used to improve the cycle life of batteries, thermal runaway and not delay. Look at system-level protection. Further, the first battery industry encapsulants commonly used, may be seen as an immersion solution. This program, there is no advantage in terms of energy density, and therefore in the context of subsidies to encourage energy density, out of the market. After the exit of subsidies, potting program that can win back market remains to be seen. BYD old battery, liquid cooling plate scheme, can ensure that the battery in the normal working temperature range. The cooling plate may be by soldering, welding friction stir several ways, like borehole. But in the long-term external shock and vibration environment, how to ensure that no leakage is to be considered. For a more into aStep save space and improve efficiency, liquid cooling box have begun to have a certain market. However, once the liquid cooling box collision, deformation and foreign invasion, might have actually hurt the batteries. Thus the liquid-cooled tank scheme for large commercial vehicles, commercial vehicles because of the battery pack to protect the battery compartment generally, rather than directly placed on the chassis as passenger cars, direct contact with the outside. Remove the three common scenario, a new heat pipe solutions began to emerge. It can meet the high-temperature heat of the battery pack and the low temperature conditions required preheating duplex, fast response, good temperature uniformity. Several more, can be said to be liquid-cooled solutions. When the liquid cooling scheme, the housing needs to be taken into account insulation, meaning that: a, maintaining the temperature inside the system, improve the low temperature charging and discharging, extend the service life; two, reduce the influence of high temperature thermal radiation of the internal temperature of the road system; Third when fire occurs or external temperature, holding the temperature of the battery pack normal, slow battery thermal runaway and improve safety. Overall, alone or a combination of the foregoing embodiment and a plurality of thermal runaway can not completely avoid the occurrence of thermal runaway, such as box sealing failure constantly changing environment, resulting in leakage, but also may cause thermal runaway, A poor thermally conductive material selected risky. These in turn is the subject of another in-depth study of the needs. standards for thermal runaway aspects thermal runaway is not only attach importance to the industry, national authorities, industry standards set forth requirements. At present, the Chinese out of control standards in battery safety involving heat, including the \”electric vehicle safety requirements,\” \”electric bus safety requirements\” and \”electric vehicles using lithium-ion battery safety requirements\” (draft stage, will be released soon), etc. . \”Electric Vehicle safety requirements for lithium-ion batteries\” stressed in the introductory part in determining the electric vehicle lithium ion battery cells, the battery pack design or system which uses, subject to the following priorities: First, if possible, preference safe materials, try to avoid using prone to insulation failure, thermal runaway or materials fire burning; secondly, if you can not implement these principles, then the need to develop protection measures to reduce or eliminate the likelihood of the risk occurring. Standard was also clear that the lithium-ion battery cells and battery systems in accordance with the requirements of the test method over-discharge, short circuit, heating, and extrusion temperature cycling test, should be no fire, no explosion. standard lithium ion battery pack or system manufacturer may choose toIn one of two ways a battery pack or a lithium ion diffusion system thermal analysis or verification. Thermal diffusion is completed in accordance with Appendix occupant protection analysis and validation or completion of the thermal diffusion tests with reference to the Appendix. In the thermal diffusion occupant protection analysis and verification report this specification was specified: a lithium ion battery pack or system before Since a single battery thermal runaway caused by heat diffusion, leading to the danger of the passenger compartment occurs five minutes, should provide an advance warning signal (serving the whole hot car accident alarm) to alert the crew to evacuate. If the heat diffusion dangerous situation does not occur the vehicle occupant, it is considered that requirements are met. But the thermal diffusion occupant protection analysis and thermal diffusion test and verification report these two aspects, but as an appendix at the end, only requires manufacturers to provide the final structure and description data, and there is no uniform standard techniques. I think there should be a follow-up uniform standards to certified or not, the test records and disclosure with regard to thermal runaway necessity to remind the manufacturers to do related work. In addition, \”Electric Bus Safety Requirements\” rechargeable energy storage system (or pods mounted) between the cabin and the insulation material should be fire isolation, combustion characteristics of the flame retardant heat insulating material should meet a predetermined A in GB 8624-2012 level requirements, and less than a thermal conductivity equal to 0.04 W at 300 ℃ / (m? K). In summary, many domestic and foreign enterprises and national standards, in solving thermal runaway done a lot of work. There is no need to mention the electric car scared to believe in the safety of electric vehicles continues to increase. Also hope to have more professional people to something more universal, so that thermal runaway better prevention and control.