Why is it not suitable for electric buses to install ternary lithium batteries?
Източник:
Преди 8 часа
【Резюме】:
Zhang Xiangmu, Director of the Equipment Industry Department of the Ministry of Industry and Information Technology, said at the 2016 China Electric Vehicles Conference Forum that passenger cars will be suspended from being included in the “New Energy Vehicle Promotion and Application Recommended Model Catalogue”, which expresses the country’s safety concerns. “In fact, no passenger car was included in the new car list of the Ministry of Industry and Information Technology on January 14 this year.

Zhang Xiangmu’s reasons are that high ternary specific energy and good cycle performance are important development directions for lithium batteries in the future. However, my country started late in this field, and there are not enough product development and verification for passenger car safety. . The Ministry of Industry and Information Technology is considering further raising the technical threshold of new energy vehicle product safety, and organizes risk assessments of vehicles such as ternary lithium battery buses under the current safety standard system. He made it clear that before the evaluation is completed, the ternary lithium battery bus will be suspended from being included in the list of recommended models for the promotion and application of new energy vehicles.

■Ternary materials are flammable at high temperatures

Ternary materials have relatively large safety hazards. Huang Xuejie, a researcher at the Institute of Physics of the Chinese Academy of Sciences, said that based on past accident experience, the nickel-cobalt-aluminum ternary polymer 18650 battery will spontaneously ignite at a temperature above 180°C, and it is difficult to control after a fire, while lithium iron phosphate materials will be affected at a temperature above 250°C. It will heat up and is relatively safe.

From the material properties alone, both materials will decompose when they reach a certain temperature. The ternary lithium material will decompose at about 200℃, and the lithium iron phosphate will decompose at 700~800℃. In addition, the chemical reaction of the ternary lithium material is fierce, which will release oxygen molecules, and the electrolyte will burn rapidly at high temperatures, causing a chain reaction. Lithium iron phosphate will not release oxygen molecules when it is decomposed, and the combustion will not be so violent. Therefore, judging from the characteristics of the cathode material of lithium batteries, ternary lithium materials are more likely to catch fire or cause accidents than lithium iron phosphate materials.

Large battery capacity leads to poor heat dissipation

In addition to the safety risks of ternary materials, the large capacity of passenger car batteries also increases the degree of danger of ternary batteries. Huang Xuejie said that the safety of a battery is inversely proportional to the energy stored in the battery. The greater the energy that can be released, the greater the safety risk that exists. The battery capacity of a pure electric vehicle is usually tens of KWH. The capacity is several hundred KWH. As the battery capacity increases, the battery is also added, and its heat dissipation performance is poor, and the possibility of safety accidents increases.

Liu Yanlong, secretary-general of the China Chemical and Physical Power Supply Industry Association, believes that the battery pack’s formation technology, charging method, usage method, maintenance method, protection system reliability, and communication protocol effectiveness will all affect the safety of the battery pack.

lack of a judging system

Due to the lack of a relevant evaluation system in my country, it is difficult to evaluate the final safety status of the ternary battery system. The design of the battery pack will be carried out according to the conditions of use of the vehicle. By strengthening the monitoring of the entire production process, the safety of the ternary battery system can be effectively improved. However, the final safety should be judged by the corresponding evaluation system. Liu Yanlong told reporters that my country still lacks a corresponding evaluation system.

Wang Binggang, a special expert from the Ministry of Science and Technology, agrees with Liu Yanlong’s point of view. Wang Binggang believes that the choice of battery materials cannot fundamentally solve the battery safety problem, and strict battery product quality requirements and supervision methods should be formulated. In this regard, Wang Binggang put forward two suggestions. One is to formulate clear safety assessment indicators for battery components and battery systems; the other is to implement strict monitoring of new energy buses and clarify the monitoring content and responsibilities.

AVIC Lithium Battery (Luoyang) Co., Ltd. Marketing Department Director Ruan Yiliang believes that the operating route and environment of buses are relatively fixed, and the number of batteries is not the key point. We should stick to the lithium iron phosphate route with stronger thermal stability and safety. Ruan Yiliang, director of the Marketing Department of AVIC Lithium Battery (Luoyang) Co., Ltd., believes that the bus operating route and environment are relatively fixed, and the number of batteries is not the focus of the bus. Therefore, we should stick to the lithium iron phosphate battery route with stronger thermal stability and higher safety. . Data shows that among the pure electric buses currently produced in my country, buses powered by lithium iron phosphate batteries account for 60% of the entire new energy bus market, and power lithium batteries of three materials account for 30% of the market. . Lithium iron phosphate battery is still the mainstream direction in the field of electric buses, and it is also the best choice. “If the ternary lithium battery has an internal short circuit or the anode material meets water, an open flame will appear.” Therefore, 18650 batteries are generally protected by a steel case. Tesla’s battery pack is composed of approximately 7,000 18650 batteries. Therefore, although Tesla has carried out all-round protection of the battery pack, there is still a hidden danger of fire in extreme collision accidents.

The lithium-ion iron phosphate battery is much more stable. Even if the panel is punctured or short-circuited, it will not explode and burn, and will not catch fire when exposed to a high temperature of 350°C (three lithium batteries cannot hold it at 180-250°C). Therefore, in terms of safety performance, lithium-ion iron phosphate batteries are better.

Lithium batteries are energy-intensive and have higher voltages, so under the same weight, a car can go far and faster with a larger battery pack. The Tesla ModelS has a cruising range of more than 400 kilometers when fully charged.

However, since the ternary lithium battery needs to be combined with a large number of single cells, the battery pack is very large. Compared with the relatively compact lithium iron phosphate battery, although it is superior in power output, it also has to bear the impact of side use on the internal space.