“Slow” meditation on “fast” charging of lithium-ion batteries: Can today’s electric vehicles be fast charged?

“Follow scientific laws, don’t be blind”, “Develop safe and reliable technology first”,“Can today’s electric vehicles be fast charged?” at the “my country Yichun 2018 Global Lithium Battery Industry Chain Summit Forum” held in Beijing from October 17th to 18th, from the academic industry Many people have called for the safety of lithium-ion battery development.

Lithium-ion battery (Li-ionbattery) is developed from lithium-ion battery (Lithiumbattery). Lithium-ion batteries have a long history in people’s lives, such as button batteries are lithium-ion batteries. The positive electrode material of lithium ion battery is manganese dioxide or thionyl chloride, and the negative electrode is metallic lithium. After the battery is packaged, the battery has voltage and is no longer charged. Lithium-ion batteries are generally prohibited from charging, because lithium dendrites are easily formed during the charging and discharging process, resulting in an internal short circuit in the battery.

In 1992, Sony Corporation of Japan invented a battery with carbon material as the negative electrode and lithium-containing compound as the positive electrode. During the charging and discharging process, only lithium ions exist, and no metal lithium exists, which is the current lithium ion battery. Since then, Japan’s Sony Energy Development Corporation and Canada’s Moli Energy Corporation have successfully developed new lithium-ion batteries (hereinafter referred to as “lithium-ion batteries”). At present, lithium-ion batteries have been widely used in various handheld electronic products and electric vehicles.

For lithium-ion batteries, there are currently two indicators of performance: one is the charge-discharge rate, which represents the charging speed of the battery; the other is the energy density, which determines how many kilometers a car can last. However, the blind pursuit of these two indicators largely sacrifices the safety factor.

‘Fast-charging technology has no way out yet’

At least 60% of these fire incidents occurred during or just after charging, indicating a major problem with charging. ” said Wang Zidong, director of the National 863 Electric Vehicle Major Project Power Li-ion Battery Detection Center.

During the charging process of lithium-ion batteries, lithium ions are intercalated and deintercalated back and forth between the two electrodes, and the positive and negative electrodes do not undergo oxidation reaction. However, Wang Zidong pointed out that the current charging method and charging process are all redox reactions, not the charging method that lithium-ion batteries should follow their own laws. The previous test results of Wang Zidong’s team showed that using the current charging method can reduce the battery life by about 30%. Therefore, in such a case, Wang Zidong believes that charging with a large current should not be considered.

The charge-discharge rate of a lithium-ion battery refers to the charge-discharge current per unit of rated capacity. For example, when a battery with a rated capacity of 100Ah is charged and discharged with 20A, the charge-discharge rate is 0.2C. Generally, the charging current of lithium-ion batteries is set between 0.2C and 1C. The larger the current, the faster the charging, but at the same time, the more serious the battery heating phenomenon is. At present, the charging capacity of pure electric vehicles is slow charging, basically between 0.3C and 0.5C. On the other hand, charging with too much current will result in insufficient capacity, because the electrochemical reaction inside the battery takes time. The same as pouring beer, if pouring too quickly, foam will appear, but it will be dissatisfied.

Qi Lu, director of the New Energy Materials and Technology Laboratory of Peking University, analyzed that today’s multi-element cathode metal composite oxide battery needs to be charged in 8 minutes, which theoretically requires a rate of 10C. “This energy is unimaginable.”

These technical bottlenecks are not actually new problems. Qi Lu is one of the pioneers in the field of lithium-ion battery research in my country. He served as the chief scientist of the power lithium-ion battery project for clean energy electric vehicles in the 2008 Olympic Games. They have done various experiments on various problems. At that time, the ternary material battery could already be charged in 5 minutes. In the test, the heat of the ternary lithium-ion battery cannot be released quickly during the fast charging process, which greatly increases the possibility of explosion. Taking into account safety issues, Qi Lu said that this technology cannot be used in pure electric vehicles, but only in battery hybrid vehicles.

In addition, the fast charging of power lithium-ion batteries also faces a very practical problem – the city’s electricity infrastructure cannot meet this demand. Assuming that a bus uses a 150kWh battery, it takes 5 minutes to charge a bus, and a bus needs a power supply capacity of 100kW. If there are hundreds of thousands of buses charging at the same time, it will cause a great impact on the power grid. .

“Today’s urban grid simply can’t do this,” Qilu said.

At present, Wang Zidong’s team is studying how to adjust the charging method according to the characteristics of the battery during the charging process. After changing different charging methods, the general standard charging method can charge 500 times of battery life. Under the new method, it can be charged 1,000 times, effectively slowing down battery degradation. Therefore, Wang Zidong said that even if there are many bottlenecks, lithium-ion batteries must have a particularly suitable charging method.

Qilu believes that at this stage, the most suitable way is to connect the parking space for charging, two or three hours, five or six hours, or even one night. This is completely possible with charging technology. By first developing a safe and reliable charging method, we will promote the steady, safe and healthy development of electric vehicles.

Energy density and security are a pair of contradictions

In February 2018, four ministries including the Ministry of Finance and the Ministry of Industry and Information Technology jointly announced the “Notice on Adjusting and Improving the Financial Subsidy Policy for the Promotion and Use of New Energy Vehicles”, canceling the subsidy for pure electric vehicles with a driving range of less than 150 kilometers, and for those with a driving range exceeding 150 kilometers. The subsidy for pure electric vehicles with 300 kilometers is increased to 34,000 yuan, and the subsidy for models with more than 400 kilometers is increased to 50,000 yuan.

Wang Yunshi, director of the my country Transportation Energy Center at the University of California, Davis, decomposes, which means that after pure electric vehicles reach a driving range similar to gasoline vehicles, the longer the driving range, the better. The new policy may hope to promote the development of power lithium-ion batteries through the requirements for the energy density of power lithium-ion battery systems.

The energy density (Wh/kg) of a lithium-ion battery refers to the amount of energy that the battery can store per unit weight, which is mainly determined by the material properties of the battery. According to the calculation of the energy density of the general lead-acid battery is about 40Wh/kg, if the lead-acid battery is used to drive the family car for more than 200 kilometers, the battery will be nearly 1 ton. Therefore, under the premise of controlling the weight of the battery within a certain range, the greater the energy density of the battery, the longer the cruising range of the car.

The energy density value should be as high as possible, but the battery is a small device with high energy concentration. When more energy is concentrated in a smaller volume, if it is used improperly, such as temperature rise or sudden violent collision, the consequences will occur. It can even be compared to a bomb.

Although the installed capacity of high-energy-density battery packs has greatly increased, the issue of how to balance energy density and safety has not been addressed.

“Today, there is no doubt that energy density is inversely proportional to battery safety, and we have not dealt with this problem well,” Qi Lu said.

At present, my country’s domestic mass-produced lithium-ion batteries have a theoretical cell energy density of 300 to 400Wh. In the absence of a breakthrough method for this upper limit, the material space can be enlarged by means of a friction-reducing diaphragm, thereby increasing the energy density. The purpose of the separator is to separate the positive and negative electrodes of the battery, guard against short-circuiting due to contact between the two electrodes, and allow the passage of electrolyte ions.

“This is the easiest and most dangerous method,” Wang Zidong said.

Wang Zidong analyzed that his research team had previously learned about the fire incident of Samsung Note7, and found that the separator used in Samsung Note7 batteries was about 45 microns to 46 microns thick. When using homogeneous materials, some power lithium-ion batteries Manufacturers are even considering 10-micron, 8-micron-thick separators, a “bold” idea in his view.

Because the phenomenon of particles falling off during the battery manufacturing process cannot be avoided, the actual diaphragm will have some slender “work injuries”. On the premise that the material is not broken, the ultra-thin diaphragm, flammable electrolyte, and undercurrent surging dendrites, If you take the risk in this link, it will bury the hidden danger of explosion.

“Before we have mastered the ignition law of lithium-ion batteries, it is a problem that we cannot ignore to control the balance between energy density, safety and lifespan.” Wang Zidong said.

In fact, the “fish and bear’s paw” problem of energy density and safety has not only plagued the development of lithium-ion battery technology in China, but also the industry-leading South Korea and Japan. Seunghoon Han, a decomposer from Deutsche Securities Korea, said that no company can say that its technology route is completely certain, but each company believes that their batteries are the safest. Given that many other industries have led to deal with many safety issues through standardization and standardization, these safety issues faced by the development of the lithium-ion battery industry today may also be handled by standardization. And these problems in the bottleneck period of development do not affect the fast charging technology and improving the energy density is still the future direction of technological development.

“Only after the safety indicators are standardized and normalized, it will be easier for technological development to determine what is safer and what is not safe,” Han said.

On the other hand, high energy density means high density materials, and high density materials will determine the size of the stored electrical energy. When the thickness of a material reaches a safe limit but is still lower than people’s expectations, many people turn their attention to seeking new materials. Wang Zidong believes that if there is no breakthrough in materials, the problem of breaking through high energy density will be stagnant in the bottleneck period for a long time, possibly 10 or 50 years.

However, Qi Lu is not optimistic about the recently popular graphene and nanomaterials. He said that, including the previously used lithium ferrous sulfate, these materials are actually low-density materials, while the density of ternary material multi-component materials is much higher, and the density can even be made higher in the future.

“From a material point of view, graphene has good electrical conductivity, but can the concept be the same from the material to the battery and then to the electric vehicle?” Qi Lu said, “Nanomaterials will not be used in detail in this field.”

Whether it is fast charging or high energy density, Qi Lu believes that we must guard against arrogance and rashness, and maintain vigilance, especially for technologies such as solid-state lithium-ion batteries that can ensure energy density and are safe to use. Until the material appears, there should not be too many expectations for the industrialization of such batteries.

“We still have to go all out to develop our feasible technology, and I think the most important thing is our ‘technology of tomorrow’.” Qilu said.

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