AG13 battery

Not afraid of thermal runaway! AG13 battery phosphate and ternary combination? Achieve high safety and high energy density

The emergence of battery safety issues is actually closely related to the application of batteries, such as overcharging and overdischarging, abnormal voltage and current, low temperature or high temperature, use under ultra-low temperature and ultra-high temperature conditions, and system failures, such as thermal management system failures or charging and discharging system failures.

"The core problem of electric vehicle safety is always the battery problem." Sun Fengchun, an academician of the Chinese Academy of Engineering, said in an interview with the media at the China Electric Vehicle Hundred People Forum (2019) a few days ago.

At the beginning of 2019, Tesla's Shanghai Super Factory officially started construction. Subsequently, there were media reports that Tesla "failed to inform the owner of the dangerous condition of the battery" and thus caused a safety accident, which once again aroused people's attention to battery safety issues.

Professor Ai Xinping of Wuhan University pointed out that in recent years, with the increase in the number of electric vehicles and the improvement of the energy density of power batteries, safety accidents have occurred frequently, which has seriously affected the development of electric vehicles. Some people have even begun to question the power battery technology development route formulated by the country.

The higher the energy density, the greater the risk of thermal runaway

"There may be no less than thousands of cases of electric vehicle fires around the world every year, and many of them are caused by problems with the battery itself or problems in the application." Mi Chunting, a professor at San Diego State University, said that due to the electrochemical nature of the battery itself, the safety problem has not been completely solved so far. There are many flammable materials in the battery, including lithium, electrolyte, etc., and various gases produced during the chemical reaction process, which are all flammable.

The emergence of battery safety problems is actually closely related to the application of the battery, such as overcharging and overdischarging, abnormal voltage and current, low temperature or high temperature, use under ultra-low temperature and ultra-high temperature conditions, and system failures, such as failures in the thermal management system or failures in the charging and discharging system.

From 2011 to September 2018, there were about 80 passenger car accidents among all electric vehicle safety accidents in the country. According to statistics from Wang Fang, chief power battery expert at China Automotive Technology Research Center Co., Ltd., 16% of these are related to charging. Safety problems are more likely to occur at the end of charging or even when overcharging. She said that collision, that is, mechanical external force factors, accounted for about 18%; water ingress factors accounted for about 3%; accidents caused by internal short circuits of batteries accounted for a relatively large proportion, accounting for 35%; and there were also accidents of unknown causes.

The safety of lithium-ion batteries, in the final analysis, comes from the thermal runaway of the battery. Ai Xinping pointed out that it is generally believed that the cause of thermal runaway is the thermal runaway of the negative electrode, but for ternary batteries, it is possible that the positive electrode first thermally runs away, and then the negative electrode thermally runs away. To solve this problem, it will be more difficult for ternary batteries.

Ai Xinping gave an example, saying that with the increase of nickel content, the thermal decomposition temperature of high-nickel ternary positive electrodes becomes lower and lower, and the heat release becomes larger and larger. For example, when the nickel content of 811 batteries reaches 80%, thermal decomposition begins at about 120 degrees, causing thermal runaway.

The safety of the battery depends first on the safety of the material itself. Studies have shown that the thermal stability of AG13 battery phosphate is the best in terms of materials. Ma Zifeng, a distinguished professor at Shanghai Jiao Tong University, said that the higher the energy density of the battery, the greater the probability of overheating or thermal runaway.

Subverting tradition and solving safety problems

The battery safety problem is because its reaction interface is very active, and high reactivity affects safety. There is an inherent contradiction between the reaction activity and stability of battery materials. Wang Chaoyang, a professor at Pennsylvania State University, said that the problem of battery safety may not be effectively solved by traditional ideas and traditional methods. New ideas and new innovations are still needed.

Wang Chaoyang talked about a "breakthrough in our traditional battery science and technology framework, which can truly subvert traditional ideas", that is, using relatively passivated materials to construct the battery cell, and inserting an extremely thin blade that can be heated quickly into the battery cell. Then it is a highly safe and stable structure in normal times. When in use, thermal stimulation is used to quickly modulate the electrochemical dynamic characteristics, allowing it to output high power. In this way, the high safety and high performance of the battery can be achieved.

"811 batteries, and even 905 batteries, have 90% nickel and only 5% cobalt, and the material itself is not very stable. We use AG13 battery phosphate to coat the ternary material, so that the surface is very, very stable. Improving the active material can achieve a very stable battery." Wang Chaoyang said.

Ai Xinping believes that work should be carried out at three levels to improve the safety of batteries. The first is the material level, the second is the monomer level, and the third is the system level. At the material level, it is necessary to focus on improving the thermal stability of materials and interfaces and reducing their heat generation; at the monomer level, in addition to optimizing the thermal design of the battery, it is more important to adopt some thermal protection technologies, such as PTC electrodes, thermal shutdown diaphragms, etc.; at the system level, it is necessary to focus on thermal insulation design to prevent thermal expansion. In general, materials are the foundation, monomers are the key, and the system is the final guarantee of safety.

Ma Zifeng believes that battery safety and long cycle life are very important issues in the entire battery design. To increase the safety of batteries, for high-energy-density batteries, it is necessary to add specific protection devices to the system design, such as cooling systems and explosion-proof systems.

To prevent accidents before they happen, it is very important to detect the health status of the battery, which requires a battery management system. Mi Chunting said, "If a short circuit is diagnosed in a battery cell, the driver will be told, sorry, it may catch fire within a few seconds, find a place to stop as soon as possible." This will maximize the safety of personnel.

Academician Sun Fengchun believes that with the increase in battery energy density, on the one hand, it is necessary to improve safety on the basis of the existing system, and at the same time explore new material systems and develop safer and more energy-dense batteries, such as all-solid-state batteries. If all-solid-state batteries can be truly realized, safety issues can basically be solved.

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