New energy vehicles mainly refer to vehicles that use unconventional vehicle fuels as their power source. At present, the most common ones on the market are pure electric vehicles and hybrid electric vehicles. Just as ordinary cars need internal combustion engines, electric vehicles need power lithium-ion batteries to supply forward energy. At present, lithium-ion batteries are the most important type of power lithium-ion batteries.

Following the rules of science and not blindly developing safe and reliable technology first, people from academia and industry have called for the safety of lithium-ion battery development at the China Yichun 2018 Global Lithium Industry Chain Summit Forum held in Beijing from October 17 to 18.

Li-ion battery (Li-ion battery) is developed from Li-ion battery. Lithium-ion battery has a long history in people's life, such as button battery belongs to the lithium ion battery. The cathode material of a lithium-ion battery is manganese dioxide or thionyl chloride, and the cathode is lithium metal. After the battery is assembled, the battery will have voltage and will not be charged. Lithium-ion batteries are generally prohibited from charging, because lithium dendrites tend to form in the process of charging and discharging, resulting in internal short circuit of the battery.

In 1992, Sony invented a battery with carbon materials as the negative electrode and lithium compounds as the positive electrode. In the process of charging and discharging, there are only lithium ions, and no lithium metal exists, which is now the lithium ion battery. Since then, Japan's Sony Energy Development Company and Canada's Moli Energy Company respectively successfully developed a new type of lithium ion battery (hereinafter referred to as lithium ion battery). Lithium-ion batteries are already widely used in a variety of hand-held electronic products and electric cars.

For lithium-ion batteries, the performance is good or bad in two indicators: one is the charge-discharge ratio, which represents the charging speed of the battery; One is energy density, which determines how many kilometers a car can go. However, the blind pursuit of these two indicators, to a large extent at the expense of safety factors.

Fast-charging technology is not yet a solution

At least 60 percent of these fires occurred during or just after charging, indicating a major problem with charging. National 863 electric vehicle major special power lithium battery pool test center director Wang Zidong said.

In the charging process of lithium-ion battery, lithium ions are inserted and disembedded between the two electrodes, and oxidation reaction does not occur in the positive and negative electrodes. However, Zi Dong Wang proposed that the current charging method and the use of the process of charging are REDOX reaction, not lithium-ion battery to follow their own laws should be some charging method. Previous experiments by Prince's team have shown that current charging methods can reduce battery life by around 30 per cent. Therefore, in such a case, Wang Zi Dong thinks that charging with a large current should not be considered.

The charge-discharge ratio of lithium-ion battery refers to the charge-discharge current per unit rated capacity. For example, the charge-discharge ratio is 0.2C when the battery with rated capacity of 100Ah is charged and discharged at 20A. General lithium ion battery charging current set between 0.2C and 1C, the greater the current, the faster charging, but at the same time the battery heating phenomenon is also more serious. 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 can result in insufficient capacity, because the electrochemical reactions within the battery take time. Pour beer and the same, pour too fast will appear foam, but discontent.

According to Qi Lu, director of the Laboratory of New Energy Materials and Technology at Peking University, today's multiple positive metal composite oxide batteries need to charge in 8 minutes, which in theory requires about 10C multiplier to do. This energy is unimaginable.

Lu is one of China's pioneers in the field of lithium-ion battery research. He was the chief scientist of the lithium-ion battery project for clean energy electric vehicles in the 2008 Olympic Games. At the time, three-component batteries could be recharged in less than five minutes. In the experiment, the ternary lithium ion battery in the process of rapid charging, heat can not be released quickly, making the possibility of explosion greatly increased. For safety reasons, Chiru says the technology cannot be used in pure electric cars, but only in battery-powered hybrids.

In addition, the rapid charging of power lithium-ion batteries also faces a very real problem: the urban power infrastructure is unable to meet the demand. Suppose a bus uses a 150kWh battery, which takes 5 minutes to charge, and a bus needs 100kW power supply capacity. If hundreds of buses are charged at the same time, there will be a great impact on the power grid. 

Today's urban grid simply cannot do that. Chiru said.

At present, Wang's team is studying how to adjust the charging method according to the characteristics of the battery during the charging process. After changing the different charging methods, the battery life of the standard charging method is 500 times, but under the new method, the battery can be charged 1,000 times, effectively slowing down the attenuation of the battery. So Wang said that even if there are many bottlenecks, lithium-ion batteries must have a charging method that is particularly suitable for them.

Its Lu believes that the most appropriate at this stage is to use the parking space as a unit of wiring charging, two or three hours, five or six hours, or even a night, which is the charging technology can be done. To promote the steady, safe and healthy development of electric vehicles by first developing safe and reliable charging methods.

Energy density is a contradiction to safety

In February 2018, the Ministry of Finance, the Ministry of Industry and Information Technology and other four ministries and agencies jointly issued the Notice on Adjusting and Improving the Financial Subsidy Policy for the Promotion and Application of New Energy Vehicles, which cancelled the subsidy for pure electric vehicles with a driving range of less than 150 kilometers, and increased the subsidy for pure electric vehicles with a driving range of more than 300 kilometers to 34,000 yuan. The subsidy is increased to 50,000 yuan for vehicles over 400 kilometers.

Wang Yunshi, director of China's Transportation Energy Center at the University of California, Davis, analyzed that this means that after pure electric vehicles reach a driving range similar to that of gasoline vehicles, the longer the driving range, the better. The new policy may hope to promote the development of power lithium battery through the requirement of energy density of power lithium battery system.

The energy density of lithium-ion batteries (Wh/kg) refers to the amount of energy that can be stored per unit weight of the battery, which is determined by the material properties of the battery. According to the energy density of ordinary lead-acid batteries is about 40Wh/kg, if the use of lead-acid batteries to drive a family car for more than 200 kilometers, it will take nearly 1 ton of batteries. Therefore, the greater the energy density of the battery, the longer the range of the car will be if the battery weight is controlled within a certain range.

The higher the energy density is supposed to be, the better, but batteries are small devices with a high concentration of energy, and when more energy is packed into a smaller volume, if it is not used properly, such as a rise in temperature or a sudden and violent collision, the consequences can be comparable to those of a bomb.

According to the latest data released by the New Energy Research Institute, about 95 percent of battery packs with 120Wh/kg have been installed since June 2018, up from just 7.3 percent a year ago in June. In other words, the energy density of domestic battery systems has improved at an alarming rate that far exceeds that of overseas batteries.

Ratio of installed battery packs with different energy densities in the output of electric vehicle passenger vehicles in China from 2017 to 2018.8 Source: ZhenLi Research

Despite the huge increase in the number of high-energy density battery packs installed, the question of how to balance energy density with safety remains to be solved.

Today, energy density is definitely inversely proportional to battery safety, and we haven't figured it out yet. Chiru said.

At present, China's domestic mass production of lithium ion batteries, the theoretical cell energy density of 300 to 400Wh. If there is no way to break through this upper limit, the material space can be enlarged by reducing the friction diaphragm, so as to increase the energy density. The purpose of the diaphragm is to separate the positive and negative poles of the battery, to prevent the two poles from short-circuiting and to allow the electrolyte ions to pass through.

This is the simplest, but also the most dangerous. "Prince Dong said.

Zi-dong wang, after his team understand samsung Note7 fire events, check found that samsung Note7 battery diaphragm at about 45 micron to 46 microns thick, in the case of homogeneous materials used, some power lithium battery manufacturers even considering with 10 microns, 8 microns thick diaphragm, in his view the idea is very bold.

Due to the phenomenon of particles can not be prevented in the process of battery manufacturing, the actual diaphragm will bring some subtle industrial injuries. Under the premise of no breakthrough in the material, the ultra-thin diaphragm, combustible electrolyte, and the undercurrent surging dendrite, if in this link, it will be buried in the risk of explosion.

Before mastering the ignition law of lithium-ion batteries, it is a problem that we cannot ignore to control the balance between energy density, safety and life. "Said Prince Winter.

In fact, the problem of energy density and safety has plagued the development of lithium-ion battery technology not only in China, but also in South Korea and Japan, which are leading in the industry. Seun Ghoonhan, an analyst at Deutsche Securities Korea, says that no company can say that its technology path is entirely certain, but that every company thinks its batteries are the safest. Given that many other industries have solved many safety problems through standardization and standardization, these safety problems faced by the development of lithium ion battery industry today may also be solved by standardization. However, these problems in the development bottleneck period do not affect the fast charging technology and improving energy density are still the future direction of technology development.

Only after the standardization and standardization of safety indicators will it be easier for technology development to determine what is safer and what is unsafe. Han said.

On the other hand, high energy density means high density of materials, and high density of materials will determine the amount of energy stored. When a material reaches a safe limit of thickness but falls short of expectations, many people turn to new materials. According to Wang Zidong, if there is no breakthrough in materials, the problem of high energy density will be stalled for a long time in the bottleneck period, maybe even 10 or 50 years.

And about recently very hot graphene, nano materials, etc., its Lu is not optimistic. He said, including the previous use of lithium ferrous sulfate, these materials are actually low density materials, and the ternary material polymaterial isopensity is much higher, the future can even be made higher density.

Graphene conducts electricity well from a material point of view, but is the concept the same from material to battery to electric car? Chilu says nanomaterials will have few concrete applications in this field.

Regardless of whether it is fast charging or high energy density, Lu believes that we must guard against conceit and rashness, especially for solid-state lithium ion batteries, which can guarantee energy density and safe to use. Before the emergence of electrolyte materials with good electrical conductivity, we should not have too many expectations for the industrialization of such batteries.