Why do electric vehicles have frequent combustion accidents?

　　1. The use of ternary lithium batteries is a technical reason for frequent car burnouts.

　　Tesla uses ternary lithium-ion batteries. Before last year, more than ten vehicles had caught fire. In May this year, four more vehicles burned successively, causing three deaths and one injury. From January to June this year, six spontaneous combustion accidents occurred in foreign new energy vehicles. The ternary lithium-ion batteries used in these vehicles were all from "famous families", and the production process should not be a major problem. The main causes of accidents are fires after serious collisions and spontaneous combustion for no reason, which has to be found in the inherent unsafety of ternary lithium-ion batteries.

　　The battery pack, like the gasoline tank, is a component containing high-energy substances and is the material basis for the safety of electric vehicles. The electrolyte in the lithium-ion battery is made of flammable solvents. The positive oxidant and negative reducing agent are only separated by a micron-thick separator. An internal short circuit will generate heat. In order to pursue high specific energy, companies now use thinner diaphragms, which are more prone to internal short circuits. When charging and discharging, the internal resistance of the battery generates heat, and more heat is generated at high rates. When reaching a certain temperature, the oxidant on the positive electrode is prone to chemical reactions with the electrolyte. Especially ternary materials will also decompose nascent oxygen. This is A state that is much more reactive than oxygen and can easily react chemically with reducing substances. A large amount of chemical reaction heat causes thermal runaway, which will produce a large amount of gas, causing the air pressure to rise, and then battery rupture, combustion, explosion and other accidents may occur.

　　Therefore, the oxidant on the positive electrode is different, and the temperature at which thermal runaway occurs is also different. The lower the thermal runaway temperature of the cathode material, the worse the safety of the battery. The safety of ternary cathode batteries is lower than that of lithium iron phosphate batteries and other batteries.

　　Because the ternary lithium-ion battery contains the "three elements of combustion" and can spontaneously ignite even when isolated from air, the fire in the ternary lithium-ion battery is difficult to extinguish and spreads rapidly, making it difficult for drivers and passengers to escape and obtain rescue. At present, my country's ternary lithium-ion battery has not completely passed the acupuncture test, but it is allowed to be used in passenger cars and even commercial vehicles. According to incomplete statistics, there were 21 burning incidents of electric vehicles in my country from January to September this year.

　　2. Focusing on the development of pure electric vehicles is the reason for the development route that frequently burns cars.

　　In recent years, the "three vertical" development routes of electric vehicles led by the Ministry of Science and Technology have changed again and again, from pure electric vehicles, hybrid vehicles, and fuel cell electric vehicles to pure electric vehicles, plug-in electric vehicles, and fuel cell electric vehicles. Then to pure electric vehicles, extended-range electric vehicles, and fuel cell electric vehicles. Among them, hybrid vehicles were changed to plug-in type after about two years; after maintaining for about five years, they were changed to extended-range type this year; pure electric vehicles and fuel cell electric vehicles remain unchanged.

　　At present, fuel cell vehicles have many problems to solve and are still far from marketization. Pure electric vehicles are the focus, but long-range pure electric vehicles under high subsidies need to solve five major anxieties. The first is range anxiety: even if you carry multiple batteries, you still have to worry about power outages; the car body is heavy and does not save electricity; the power consumption of the air conditioner seriously shortens the mileage. The second is safety anxiety: there are many batteries and high specific energy, which are very dangerous and prone to combustion and explosion accidents. The third is charging anxiety: charging piles must be densely packed, and it is difficult to meet the requirements even though it costs money and space. The fourth is price anxiety: large battery usage, high price, and low competitiveness. The fifth is battery anxiety: the battery life is shorter than that of the entire vehicle, and the second and third sets of batteries require users to pay separately.

　　After subsidies cease, cars with longer mileage and higher original subsidies will become harder to sell. At present, it is only suitable to build micro electric vehicles that consume less power and have little conflict between safety and mileage.

　　Pure electric vehicles pursue long mileage, which leads to excessive installation of batteries and reduced safety. This technological development path is the reason for frequent car fevers.

　　3. Heavy subsidies for long-mileage pure electric vehicles are the policy reasons for frequent car fevers

　　Since electric vehicle mileage indicators and subsidies are linked to pure electric mileage, a nationwide "artificial ternary trend" has been forced out; subsidies are linked to battery specific energy, resulting in an increasing amount of nickel in ternary batteries, and the ratio of nickel, cobalt and manganese. From 333, 523, 622, we are gradually moving towards 811, and the higher the energy density, the greater the risk.

　　Unreasonable subsidy policies are the reason for frequent car fevers.

　　4. Mileage, as the main aspect of the contradiction, is the reason for frequent car fever.

　　The main contradiction between electric vehicles is the conflict between safety and mileage. Mileage is considered to be the main aspect of the contradiction, and safety is considered to be the secondary aspect of the contradiction. This is an error in thinking and the root cause of problems in development routes, policies, and technologies.

　　To sum up, my country's new energy vehicle development route and policies mislead the general use and urgent use of ternary lithium-ion batteries, intensifying the contradiction between safety and mileage. The consequence is that electric vehicle burn-in incidents occur frequently. Moreover, the vehicle body is heavy, consumes a lot of power, and has high actual emissions, which deviates from the original intention of electric vehicles to save energy and reduce emissions.

　　Use range extension technology to solve the conflict between safety and mileage

　　In the past ten years, the author’s proposition can be summed up in two sentences. Principle: Make good use of safe and mature batteries to develop energy-saving and emission-reducing electric vehicles; Technical route: Use micro and small pure electric vehicles as a breakthrough, and develop extended-range pure electric drives for large and medium-sized vehicles. Micro-sized pure electric vehicles can use lead-acid batteries for low-speed vehicles, or lithium-ion batteries for high-speed vehicles, which is determined by the market; extended-range electric vehicles can solve the five major anxieties of pure electric vehicles, and are most feasible for marketization.

　　What is gratifying is that the author's proposition is gradually being accepted. In January this year, the Ministry of Science and Technology included extended-range vehicles in the "New Three Verticals"; in the "Automotive Industry Investment Management Regulations (Draft for Comments)" issued by the National Development and Reform Commission in July, extended-range vehicles were included in the category of pure electric vehicles. Once subsidies for new energy vehicles cease and the industry enters the marketization stage, it is expected that micro-sized pure electric vehicles and extended-range electric vehicles will develop rapidly.

　　The author divides the development of extended-range vehicle technology into three generations.

　　The first generation of extended-range electric vehicles is to install a range extender on a pure electric vehicle simply to increase the driving range. When the electricity is used up, the range extender generates electricity. Take the BMW i3 as an example. If a range extender is installed, the price will increase by 15%. With a 0.7L displacement engine, the fuel consumption of the vehicle in range extender mode is 5.35L per 100 kilometers. The battery of this small car is heavy, the range extender has high power and consumes a lot of energy, and the power generated by the range extender and the battery are simply connected in series, so the fuel consumption is high.

　　The second generation of extended-range electric vehicles technically optimizes the power system, reduces the size of the engine, optimizes engine energy efficiency, has fewer batteries and reduces costs, and reduces vehicle weight and is more energy-saving. Its advantages are that the battery pack will not be overcharged or overdischarged, with extended life and higher safety; the lithium iron phosphate battery has a suitable specific energy and further improved safety; there are fewer batteries and is less affected by the reduction and cancellation of subsidies; extended range driving It saves more than 50% of fuel when compared to fuel vehicles, which saves a lot of money; it can be charged without external charging, does not need to build charging piles, and can travel long distances; if charging conditions are available, the fuel saving rate within 100 kilometers in the city can reach more than 80%; it can still be used The production and refueling facilities of fuel vehicles are convenient for development; there are no five worries about mileage, safety, charging, price and battery.

　　The second-generation range extender is a fusion of fuel vehicles and electric vehicles. It changes the limitations of the first generation by simply extending the driving range and achieves energy conservation and emission reduction. This technology has been used in many vehicles, such as the 12-meter extended-range bus of Hualong New Energy Vehicle Co., Ltd., which uses lithium iron phosphate batteries and has a fuel consumption of 12L per 100 kilometers in urban bus mode and a fuel consumption of 16.3L per 100 kilometers in highway mode; Canadian Plan B company The extended-range truck can reduce pollutant emissions by 70%, with a fuel consumption of 17L per 100 kilometers; Nissan NOTE's e-POWER power system is a series hybrid, using a three-cylinder 1.3L displacement engine, a 1.5kWh battery, and a fuel consumption of only 2.9L per 100 kilometers. L; Shandong Dezhou Fulu Group's range-extended low-speed vehicle uses lead-acid batteries and a single-cylinder 0.2L engine with a fuel-electricity efficiency of 299g/(kW·h) and a fuel consumption of only 1.8L per 100 kilometers.

　　However, the second-generation range-extended electric vehicles also have shortcomings because the range-extender generates electricity to charge the battery, the battery supplies power to the motor, and all current flows through the battery pack. First, at least 10% of the energy is lost during the charging and discharging process of the battery; second, although the battery consumption is less than that of pure electric vehicles, because the power must meet the maximum speed requirements, the battery consumption still needs about 40% of that of pure electric vehicles, and the vehicle price is obviously Higher than fuel vehicles; third, there are more batteries and the body is heavier, which has the potential to reduce weight and save energy; fourth, the battery always works at high load, and its lifespan is affected.

　　At present, my country's Jiangsu Duke New Energy Vehicle Company has proposed "engine-generated direct-drive electric vehicles", referred to as "generated direct-drive electric vehicles", which can be called the third-generation range-extended technology. The electricity generated by the generator on the car does not need to go through the battery but directly drives the motor, which overcomes the shortcomings of the second-generation range extender, has all the advantages of the second generation, and can reduce battery charging-discharging energy loss by 10%. Its advantages are: the battery is further reduced, the weight of the vehicle is reduced, and the power consumption is reduced; the battery has fewer high-current working opportunities and the life is extended; the battery is used less, and the cost is further reduced; coupled with the high fuel saving rate, the total cost of the vehicle's entire life cycle Far lower than the same level of fuel vehicles. Moreover, this technology is suitable for various vehicles and helps save energy and reduce emissions.

　　Some people may say that the extended range still requires burning oil, which is not the ultimate goal. In this regard, the author would like to point out two points. First, if my country's automobile fuel consumption is reduced to less than half of the current level, 200 million tons of crude oil will be saved every year. This will effectively improve the environment, improve energy security, and also allow our country to transform from a major automobile country to a The automobile power has taken a big step forward. Secondly, pure electric vehicles may not be the ultimate goal. They have many batteries, are heavy, and consume a lot of power. Tesla’s punishment in Singapore illustrates its shortcomings. For new energy vehicles, energy conservation and emission reduction throughout their life cycle should be assessed.

　　Moreover, future extended-range electric vehicles can burn ethanol instead of oil, which saves energy without increasing CO2 emissions. Therefore, the author believes that range extension is not a "transition to pure electric vehicles", but the main force of future vehicles.

　　All in all, the main contradiction between electric vehicles is the contradiction between safety and pure electric range. Electric vehicles should reduce battery usage, improve safety, and reduce vehicle prices, rather than pursuing long mileage, installing more batteries, or desperately increasing specific energy and increasing risk. The first solution is to miniaturize products, and the second is to develop extended-range electric vehicles (or miniaturization + extended-range). This is the best technical route to cope with the decline of subsidies and move toward marketization. It is worth mentioning that range-extended electric vehicles and lithium iron phosphate batteries are an excellent pairing, solving the main contradiction between safety and mileage, and are most suitable for marketization. (The author is an academician of the Chinese Academy of Engineering)

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