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Do not blindly pursue high specific energy density. Safety is the first breakthrough in battery technology revolution.

On September 20-23, the “2nd GFM2018 Global Future Travel Conference” was held in Hangzhou. The conference integrates the world's top think tanks, industrial resources, core enterprises and government management departments to travel related industry system resources, and explore automotive and in-depth discussions. The future development trend and innovative practice of the travel industry will promote the upgrading and development of the automobile industry from the aspects of industrial policy, strategic path, technology and product innovation, and regulatory standards.

The conference will host more than 20 theme forums. The forum structure covers important aspects of the travel industry and is more open, diverse and close to industry practice. Among them, at the "Global New Energy Vehicle Innovation Conference" held on the afternoon of September 20th, Ouyang Minggao, an academician of the Chinese Academy of Sciences and executive vice president of China Electric Vehicles 100, gave a speech entitled "Safety of Power Battery for New Energy Vehicles". . The following is a record of the speech:

Dear guests, good afternoon! Just now Mayor Luo has said that the recent electric car accident is very interesting, so I will focus on the safety of electric vehicles today. I want to introduce to you in four aspects. The first is the statistics of electric vehicle accidents. This is a summary of the causes of spontaneous combustion of electric vehicles in foreign countries in recent years, mainly due to fire after collision. In fact, the fuel car will also catch fire after the collision, which is the situation of domestic fires. Domestic fires mainly have such characteristics: First, they are mainly ternary batteries, and lithium iron phosphate is also available, mainly ternary batteries, more than half. Second, the cylindrical battery is the main type. This is one of the more important types. Because it is a steel shell, the volume is tight, so once the thermal runaway occurs, it will explode and then ignite other batteries. Third, the proportion of accidents involving charging fires is relatively large. Generally speaking, if the battery does not get out of control after being discharged to a certain depth, the thermal runaway is generally in a fully charged state, so it is easy to cause when charging, because when charging, the battery is connected with the charging system, and it is thermally out of control. The easiest time, as well as the short circuit of high-voltage electrical appliances, etc., can easily lead to accidents. Also, from the perspective of the model, both the old and the new models have the battery system energy is not very high, because the accident is mainly the car installed in the past few years, the overall system is not very high energy, not We think of a very high specific energy battery.

The thermal runaway of the battery should be said to be the main cause of these accidents. What is the thermal runaway of the battery? When the battery temperature reaches a certain time, the battery will have a negative reaction of the chain, and the reaction will be exothermic, so the temperature will rise rapidly and the highest speed can reach The clock temperature rises close to 1 kilowatt, so its speed is very fast.

What is the cause of thermal runaway? First, the battery is overheated. Just now, the battery is hot and out of control. There are various reasons for overheating. It may be that the battery pack itself has uneven temperature and local temperature. High, overcharge and over discharge, external short circuit, internal short circuit, etc. These causes of heat, as well as mechanical reasons, such as water, seals, collisions and so on.

Let us look at the main reasons for these recent accidents, which we consider to be product quality issues. The product quality problem means that the product does not strictly comply with relevant technical standards and specifications during the design, manufacture, verification and use. There are three main types. First, battery product testing is insufficient. Second, reliability changes during vehicle use. Third, charging safety management technology has problems. Let us analyze these aspects.

First of all, battery product testing is insufficient. Since the policy cycle of subsidy declining is once a year, it does not match the product development cycle in general. For example, the improvement of our chemical material system generally takes more than one year, but because the company follows the subsidized baton, blind The pursuit of high specific energy shortens the time for test verification. Sometimes in order to shorten the development cycle, physical improvement methods are often preferred, such as thickening the battery active material and thinning the diaphragm, so that the battery will increase in energy, but the safety performance will decrease.

The second is that the means of electric battery test verification is imperfect and cannot reflect the conditions of use of real vehicles. A large number of enterprises have not established internal battery safety test standards, and some enterprises do not even have the ability to test battery safety. That is, it is jagged.

The third reason is that the reliability of the vehicle is reduced during the aging process. For example, the waterproof effect of the whole life cycle is not good. Generally, the sealing of our battery is to pass the IP67 standard. However, when the vehicle is used for a long time, the sealing will be worse, causing the vehicle to enter the water, which will easily cause a short circuit. For example, the laser-welded joint of the battery is prone to voids inside the solder joint, and these voids increase the impedance, which in turn causes high temperature points and causes thermal runaway. There is also the aging of the battery system and the high-voltage electrical appliance of the Charger. For example, the contactor when we charge is often broken, sometimes it will pull the arc, causing the high temperature and the burning or sticking of the surface of the contactor, which will short-circuit and heat up. These are the reasons for the loss of control.

The fourth reason is charging. The data communication during the charging process is not standardized. The manufacturers of BMS and the manufacturers of Chargers do not strictly implement the newly issued national standards. The function of charging is safe. It is reasonable to say that our battery management system has a good power-off function for charging. When it is charged, it is controlled by the battery management system. We do not strictly enforce the functional safety specification, which is ISO26262. This specification does not fully implement this specification at present, which is why we did not comply with the norm. The relevant standards for charging safety are not strictly enforced. For example, the adhesion of our charging relays should have diagnostic functions, but some do not have to save costs. The battery management system and the charging pile are not equipped with qualified insulation detection devices. The charging circuit formed by the vehicle and the charging pile does not meet the insulation voltage, creepage distance, overload, IP rating, insertion force, lock, temperature rise, lightning strike that meet the standard requirements. As required by various indicators, BMS does not strictly adhere to the specifications of charging guidance. Why is it a quality problem? We are in the design, manufacturing, use, and verification of various links, there is no strict compliance with standards and norms, of course, we also lack some, such as our security annual inspection, which is lacking, but this is not a business This is what the government has to do.

High-ratio energy batteries face more serious security technical challenges, so let me talk about this in the following.

According to the trend of energy development of China's new energy vehicles, we will soon move toward 300 watt-hours/kg of high-ratio energy batteries. Soon these products will enter the market, so-called high-nickel ternary 811 batteries are very It will soon enter the market, and these high-energy batteries will have higher requirements than the safety technologies faced by these relatively low-energy batteries. In this regard, Tsinghua University has built a battery safety laboratory to carry out related basic research and technology development. Here, we will briefly introduce the research and development results for your reference.

At present, the Battery Safety Laboratory of Tsinghua University has carried out extensive cooperation with domestic and foreign enterprises and research institutions, including large companies such as BMW, Mercedes-Benz and Nissan.

The research focuses on three aspects of thermal runaway, one is the cause of thermal runaway, including thermal, electrical, and mechanical reasons. The second is what is the mechanism of thermal runaway, so as to protect it at the material design level. The third is heat spread. Once the single battery can't prevent the heat from getting out of control, there must be a secondary protection means, that is, the spread of thermal runaway should be cut off at the system level, and the accident can be prevented as long as the spread is cut off. Our thermal runaway control of high specific energy batteries depends not only on the materials themselves, but also on the system level.

The first is about the mechanism and inhibition of thermal runaway. We carried out two experimental methods, one is a differential scanning calorimeter for thermal stability study of materials, and the other is an accelerated calorimeter for thermal runaway measurement of battery cells.

Several characteristic temperatures for thermal runaway of high specific energy cells. Generally speaking, when the battery temperature rises to a certain level, the battery will produce heat. We call this temperature T1. The heat generation can not be suppressed to a certain extent. The thermal runaway trigger is called T2. Finally, the temperature rises to the highest point. Call T3. The mechanism of thermal runaway is unclear mainly in the T2 to T3 phase. It is generally believed to be caused by an internal short circuit, which is true for conventional batteries, but we found in the study that the high specific energy is not exactly the same. We found that there was no internal short circuit and there was still thermal runaway. This is because the high-temperature energy battery's new high-temperature resistant membrane does not change above 200 degrees, and the electrolyte is almost completely evaporated. However, at 230-250 degrees, the oxygen released from the phase transition of the positive electrode material reacts with the negative electrode to generate an exothermic peak.

In addition, we look at the differences in ternary lithium-ion batteries of various nickel contents. Compared with the commonly used 622 or 532, the 811 battery has significantly higher exothermic peaks than the others, indicating that the thermal stability of the 811 is poor. After analysis, we have obtained the preliminary conclusion that the high nickel positive electrode has a great impact on the safety of the whole battery. The silicon carbon negative electrode has little effect on the safety in the initial stage, but the effect is relatively large after the cycle decay.

There are also a number of ways to improve this thermal stability difference, such as coating of materials. We have also discovered a new method, which uses single crystal particles instead of polycrystalline cathode materials. The thermal stability of the battery is very high. Good improvement, the corresponding security has also been improved.

The second is heat spread. The real accident is caused by heat spread. After a battery unit is out of control, all the battery packs are spread and a fire accident occurs.

According to our test of heat propagation and the heat transfer analysis of the simulation, a heat insulation method is designed, which is to add heat insulation material to the path of leading heat transfer. The experiment has found that the effect of the thermal runaway of the partition is indeed achieved. This kind of firewall technology has been adopted in the regulations of China's international electric vehicle thermal runaway.

The third aspect is the cause of thermal runaway and battery management. The first cause is the internal short circuit. The analysis of the battery and the accident battery shows that the uniform pole piece of the battery is broken after being used for a period of time, which is prone to localized lithium deposition, which leads to thermal runaway. In addition, the impurities in the manufacturing process will also cause internal short circuit. We call this cancer of the battery, because we don't know when it induces thermal runaway, and sometimes it will take a long time to generate an internal short circuit. To this end, we have invented an alternative experimental method for internal short circuits in batteries to achieve the desired internal short circuit by implanting a memory alloy in a specific battery. After our research, the internal short circuit was divided into four categories, in which the aluminum current collector and the negative electrode were the most dangerous internal short circuits. It is also necessary to give early warning. For this reason, we have done a series of studies and obtained a three-stage evolution of the internal short circuit. In the first stage, only the voltage is lowered, there is no temperature rise; in the second stage, there is a rise in temperature, and in the third stage, a sharp temperature rise occurs, that is, thermal runaway. According to this evolution process, we strive to identify the internal short circuit in the first two stages, and we can warn the internal short circuit that may cause thermal runaway 15 minutes in advance. This technology has already cooperated with the Ningde era.

The second aspect is charging. We have found out the mechanism of overheating out of control through test analysis. On this basis, the thermoelectric coupling model is used to predict the performance of battery overheating out of control. Overcharge accidents are generally micro-overcharges, such as battery inconsistencies, because of inconsistencies, some places in the charging process are already full, and some places are not full, it will lead to some full battery micro-overcharge, then Lithium is deposited on the negative electrode material to produce lithium dendrites, which are so-called lithium deposition, resulting in poor safety and short circuit.

In order to solve this problem, we have developed a lithium-free fast charge technology based on a reference electrode, which controls the potential of the negative electrode to be above zero (the lithium is precipitated below zero), which requires the addition of one electrode, that is, the three electrodes. On the basis of the three electrodes, feedback and observation can be based on the model. This is our lithium-free fast charging technology. After this technology is applied, no lithium is generated and the charging speed is accelerated.

The third reason is aging. The inconsistency of the battery after aging will increase, which is why the inconsistency in the increase in the number of battery cycles will become larger and larger, and as the capacity consistency becomes worse, the accuracy of battery management is poor. In addition, aging in a low temperature environment will seriously affect the thermal stability of the battery, and the self-generated heat temperature that causes thermal runaway will decrease, which is more likely to cause thermal runaway.

Through the analysis of these problems, we found that the core of the safety of the battery system is the development of advanced battery management systems. At present, in the battery management system, the domestic products are insufficient in function and accuracy, especially the safety functions are incomplete, so it is necessary to increase the research and development of the battery management system. Tsinghua has a rich battery management system and has obtained 65 patents. These patents have been applied in cooperation with well-known companies at home and abroad, and some of them have been licensed to Mercedes-Benz.

So how do we completely solve the problem of battery safety? In the near future, some technologies can be used to ensure safety. However, in the long run, forward-looking scientific research is needed to ensure the absolute safety of batteries. The high specific energy of lithium-ion power batteries is the development direction and trend of the whole world. We cannot develop high-energy energy batteries because of safety problems. The key is to grasp the balance between high specific energy and safety. For example, the intrinsic safety problem of high-nickel ternary lithium-ion battery, the mechanism is that the positive electrode will release oxygen, we can delay the oxygen release of the positive electrode and improve the stability through the modification of the interface; the other is to develop the next generation of solid electrolyte, from Fundamentally solve the problem of electrolyte burning.

Based on the comparison of power battery technology routes in various countries, lithium-ion batteries with short-term liquid electrolytes will develop in the next step toward solid-state batteries. Considering the cost of the battery and the development direction of the power battery, we suggest that China should follow a similar path, that is, short-term liquid electrolyte, develop high-nickel ternary positive electrode and silicon-carbon negative electrode, and prevent safety through battery management system and heat spread suppression. In the event of an accident, this type of battery can meet the requirements of the electric vehicle's 500km driving range. In the medium and long term, the transition from liquid electrolyte to all-solid-state batteries is expected to be industrialized in 2030.

In short, we must strive to solve the intrinsic safety of power batteries and ensure the healthy development of the new energy vehicle industry. A summary of my report can be summarized as:

We must correctly look at the recent incidents of new energy vehicles, mainly due to product quality problems, failure to comply with technical specifications and technical standards, and short technical verification cycles.

Suggestions on policy include:

First, the original industrialization target (350 watt-hours/kg in 2020, 260 watt-hours/kg in system and 2000 life-cycles) is relatively high. From a safety perspective, I think it is not appropriate to force it.

Second, the subsidy policy should conform to the law of technological development. The improvement of energy density should not be too fast and should not be changed too frequently. This is my suggestion to the Ministry of Finance.

Third, the annual safety inspection specification for electric vehicles will be introduced as soon as possible. At the same time, in order to better handle and analyze electric vehicle accidents, it is best to have electric car black box, and the battery pack should have a fire safety interface. The current battery pack is very dead, which makes the fire extinguishing difficult. These are the right The Ministry of Public Security's recommendations.

Finally, I think battery safety is the first key to the revolutionary breakthrough of battery technology, and the first key to the performance improvement of pure electric vehicles. The battery safety will become a bottleneck technology in the later stage of battery industry development, such as charging more than 300 kilometers in ten minutes. The electric fast charging technology will bring challenges to battery safety. The voltage is increased from 300V to 600V or even 800V. These are all related to safety and are the main battlefield for pure electric vehicles in the future. It can be said that safety is the lifeline for the sustainable development of electric vehicles. The national technology research and development of power batteries must take safety as the core, comprehensively upgrade the existing lithium-ion power battery system safety technology, and fully break through the new solid-state battery technology.

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