Is the battery thermal failure is an important reason for the fire lithium battery?

　　Author ：Iflowpower – Portable Power Station Supplier

On September 20, 2018, the world's future travel high-level forum opened in Hangzhou International Expo Center. This forum was hosted by my country Electric Vehicle Hundreds and my country's Informatization Hundred People Club, focusing on the integration of electricization and intelligence, webinization, sharing, and is divided into two parts: high-level forums and exhibitions. As a deep support medium for the world's future travel high-level forum and international exhibition (GFM2017), the EV century will bring users a comprehensive and rich forum report.

In the speech link of the main forum, Academician of the Chinese Academy of Sciences, Ouyang Ming, an executive vice chairman of my country Electric Motors, delivered a speech on electric vehicle safety and spontaneous combustion. There is such a shortcoming in the domestic fire: first, it is the three-yuan battery, and lithium iron phosphate is also, but it is important to be a ternary battery, more than half of the proportion. Second, the cylindrical battery is mainly, this is one of the more important types, because it is a steel shell, the structure is compact, so once the thermal out of control occurs, it will explode, which will ignite other batteries.

Third, the accident of the incident of charging fire is relatively large. Generally speaking, if the battery does not be hot after discharging to a certain depth, the thermal out of control is generally in full, so it is easy to cause when charging, because the battery and charging system are connected together, and it is thermal out-of control When it is easiest, there is a short circuit of high voltage electrical appliances, etc., it will be easy to cause an accident.

Also, from the perspective of the model, the new and old models have spontaneous combustion, the battery system is not necessarily very high, because the importance of the accident is that the car in the first few years is not what we think is very high. Compared to energy batteries. Battery thermostat should be said to be the main cause of these accidents, what is the thermal out of control of batteries? The battery temperature reaches a pressing battery will have a luggage negative reaction, the heat of the heat, so the temperature rises rapidly, the maximum speed can reach the temperature rose to 1000 degrees per second, so its speed is very fast.

What is the cause of thermal out of control? The first is that the battery is overheating. Just said that the battery is hot and it will be hot. There are various reasons for overheating.

It may be that the battery pack itself is uneven, there is a local area temperature, overcharge, outside The reason for this electricity, internal short circuit, etc. will exotherm, as well as mechanical reasons, more water, not good, collision, etc. Let's take a look at the main reason for these accidents, we think it is product quality problem.

Product quality issues refers to the product in design, manufacturing, verifying, without strict compliance with relevant technical standards and norms. There are three types of three categories, first, battery product test verification; second, reliability variation during vehicle use; third, charging safety management technology has problems. Let's analyze these aspects.

First, battery product test is insufficient. Since the policy cycle of subsidizes is once a year, it is not very matching with the development cycle of the product. For example, the improvement of our chemical materials system is generally more than one year, but because the company follows the warning of the subsidy, blindly Pursue high-than-specific energy, shorten the time of test verification.

Sometimes in order to shorten the development cycle, the physical improvement method is often preferred, such as thickening the battery active material, thin diaphragm, so that the battery is increased, but the safety performance is lowered. The second is that the means of electric battery test is not perfect, and the conditions of use of the real car cannot be reflected. Big large part of the company does not establish the company's internal battery safety test standard, some companies have no capacity of battery safety testing, production Quality is uneven.

The third reason is just now, the reliability decreases during the use of aging. For example, the waterproofing effect is poor in the full life cycle. Generally, the seal of our battery is to pass the IP67 standard, but after the vehicle is used, the seal will be deteriorated, resulting in water in water, will be easily short-circuited.

Also, such as laser welding of the battery, the inside of the welding point is prone to voids, which will cause new impedance, which in turn leads to high temperature points, causing thermal out of control. There is also the aging of the battery system and charger high-voltage electrical appliances. For example, the contactor we charge frequently open, sometimes it will arc, resulting in this burning or adhesion of the high temperature and contactor surface, will be short-circuited, will fever, These are the reasons for heat loss.

The fourth reason is to charge, data communication is not standardized during charging, and the manufacturer of BMS manufacturers and charging has no strict implementation of newly promulgated national standards. The functional safety of charging, according to our battery management system, the charging is very good power-on function, and when it is controlled by the battery management system, we currently do not have strict implementation of functional safety norms, is ISO26262 This specification, there is currently no complete implementation of this norm, which is caused by the reasons why we have not complied with the norm. The relevant standards for charging safety are not strictly enforced.

For example, our charging relay should have diagnostic functions, but some in order to save costs. Battery management system and charging pile There is no equipment qualified insulation detection device, and the charging circuit formed by the vehicle and charging pile does not meet the insulation voltage of standard requirements, climbing distance, overload, IP level, insertion force, lock, temperature rise, lightning strike All indicators require that BMS has not strictly abide by charging guidance. Why is it a quality problem? That is, we are in design, manufacturing, use, and verify all aspects, no strict compliance between standards and norms.

Of course, we lack some, such as our safety annual inspection, this is missing, but this is not a company. This is the government. Things to do.

High-than-energy battery faces a more severe security technology challenge, so I will talk about this problem below. According to the trend of my country's new energy vehicle power lithium battery than energy development, we will soon move forward to 300 Watt / kg, soon these products will enter the market, which is the so-called high nickel ternary 811 battery. Soon will enter the market, these high-specific energy batteries will be higher than the safety technology faced by these relatively low than energy batteries.

In this regard, We Tsinghua University specializes in the basic research and technology development of battery safety laboratories. Here, give you a brief introduction to R & D results for your reference. At present, Tsinghua University's battery safety laboratory has carried out extensive cooperation with domestic and foreign companies and research institutions, including BMW, Mercedes, Nissan.

The research focus is in three aspects of thermal out-of control, one is the cause of heat, including heat, electricity, and machinery. Second, what is the mechanism of thermal out-of control, which is protective in material design level. The third is the heat spread, once the cell battery does not stop the heat loss, there is a secondary protection means, that is, the spread of thermal out-of-control in the system level, as long as the spread can prevent accidents.

We have a high-than-energy battery thermal out-of control, not only by material itself, but also from the system level. The first is the mechanism and suppression of thermal out-of control. We carried out from two experimental means, one is a differential scanning calorimeter for material thermal stability research, one is an acceleration thermometer for battery monomeric heat loss measurement.

Several characteristics of high-proportioned energy battery thermal out of control. In general, when the battery temperature rises to a certain extent, the battery will be self-produced. We call this temperature T1, and the heat generation occurs to a certain extent, which cannot suppress, thermal out-of-control trigger, called T2, the last temperature rises to the highest point TH.

Thermostat mechanism is unclear is an important thing that happens in T2 to T3. It is generally considered to be due to short circuits, which is true for conventional batteries, but we found that it is not entirely in the study. We found that there is no internal short circuit, which is hot out of control.

This is because the high temperature-resistant high-temperature novel diaphragm of the high-specific energy battery has not changed, and the electrolyte is basically completely evaporated, but at 230-250 degrees, the oxygen and the negative electrode reactive in the positive electrode material phase change appear. In addition, let's take a look at the differences in the three-dimensional lithium-ion battery of different nickel content. The 811 battery is currently more than 622 or 532, and the exothermic peaks of 811 are significantly higher than that, indicating that the thermal stability of the 811 is poor.

After analysis, the preliminary conclusion we got is that the high nickel positive electrode has a large influence on all battery safety, and the negative electrode of the silicon charcoal is not large, but the influence is relatively large after the cycle attenuation. There is also a series of improvement paths, such as the coating of the material, and we have found a new method, which is to replace the positive material of the polycrystalline with single crystal particles. The thermal stability of the battery is very Good improvement, corresponding security has good improvement.

The second is that the heat spread, the real accident is caused by thermal spread, that is, after a battery monomer is completely out of control, all battery packs are all spread, and the fire will happen. According to our test and simulation of thermal out-of-control spread, a method of insulating is designed to add heat-insulating materials on the path of leading heat transfer. The experimental discovery has indeed achieved the effect of separation heat loss spread.

This kind of firewall technology has been adopted in the regulations that have spread in my country's international electric vehicles. In the third aspect, it is the cause of heat loss and battery management. The first incentive is inner short circuit, and the analysis of the battery and accident battery has been found that the uniform pole when the battery is manufactured, and the rupture of the folded area will occur after a period of time, which is easy to occur, which is prone to the lithium control, resulting in heat loss.

In addition, impurities in the manufacturing process also cause internal short circuits, we put this called battery's cancer, because I don't know when it is induced, and sometimes it is often short-circuited after a long time. To this end, we invented an alternative experimental method of short-circuit in the battery, and achieves expected inside short circuits by implantation of memory alloys in a particular battery. After we study, the internal short circuit is divided into four categories, of which the aluminum concentration fluid and negative electrode are the most dangerous internal short circuit.

It is also necessary to war well in advance, and we have made a series of research, and obtained the three-stage evolution process of internal short circuits. In the first stage, only the voltage is lowered, there is no temperature rise; the second stage has the rise in temperature, and the third stage has a sharp temperature rise, which is thermal out of control. According to this evolution process, we strive to discriminate the inner short circuit in the first two phases, and it will be possible to trigger the internal short-circuit warning of the thermal out-of-control in advance.

This technology has cooperated with Ningde Times. The second aspect is charging, we have clearly premise the transfection and out-of control mechanism through test analysis. On this basis, through the thermoelectric coupling model to predict the performance of the battery overhang.

The recharge accident is generally micro-charge, such as the inconsistency of the battery, because the inconsistency, there is already a place in the charging process, and some places are not full, it will lead to some full-filled batteries, then Lithium lithium in the negative electrode material, a lithium lactary crystal is the so-called lithium, resulting in a short circuit, resulting in short circuit. In order to solve this problem, we have developed the value-based lithium fast charge technology based on the reference electrode, control the potential of the negative electrode in zero (lithium under zero), which is added to add an electrode, ie three electrodes. Based on the three-electrode, feedback and observation can be performed based on the model.

This is our inexperimental lithium fast charge technology. After this technology application, there is no lithium to occur, and the charging speed is accelerated. The third reason is aging.

The inconsistency after the aging of the battery will be expanded, which is the cause of inconsprection of the number of battery cycles to become increasingly, and as the capacity consistency is poor, the accuracy of battery management is very poor. In addition, the aging in the low temperature environment severely affects the thermal stability of the battery, and the self-generating temperature of the thermal out-of control will decrease, which is more likely to cause thermal out of control. Through the analysis of these problems, we found that the core of ensuring the safety of battery system is the development of advanced battery management system.

At present, in terms of battery management systems, domestic products are insufficient, and the accuracy is insufficient, especially security functions, so it is necessary to increase the research and development of battery management systems. Tsinghua's accumulation of the battery management system is relatively abundant, and has obtained 65 patents, these patents have been applied in the cooperation of famous domestic and foreign companies, some of which are also authorized to give Mercedes-Benz Motors. 21110032FEC24409A60490.

JPG So how do we completely solve battery safety problems? Recently, you can guarantee safety through some technologies, but in the long run, it is necessary to protect the absolute safety of the battery. Lithium-ion power lithium battery high ratio can be world-wide development direction and trends, we can't develop high-specific energy batteries because of security issues, the key is to grasp the balance between high specific energy and security. For example, the intrinsic security problem of the high nickel ternary lithium ion power lithium battery is that the mechanism is that the positive electrode will release oxygen.

We can delay the positive release of oxygen through the modification of the interface; improve the stability; then, one is to develop the next generation of solid electrolytes, Fundamentally solve the problem of electrolyte combustion. Based on the comparison of the power lithium battery technology route, a short time is a lithium-ion battery of liquid electrolyte, and the next step will develop in the direction of the solid state battery. Comprehensively consider the development direction of battery cost and power lithium battery, we recommend that my country should also take a similar path, which is a short time is liquid electrolyte, developing high nickel ternary positive and silicon-negative electrode, and suppresses the battery management system and thermal spread.

Prevent safety accidents, such batteries can meet the requirements of 500 kilometers of electric vehicles. The medium and long-term, gradually transition from the liquid electrolyte to the full solid state battery, estimated in the 2030 full solid state battery will receive industrial application. In short, we must strive to solve the problem of dynamic lithium battery intrinsic security, guarantee the healthy development of new energy automotive industries.

The summary of my report can be summarized as: We must correctly look at the recent new energy cars to fire, and its important cause is product quality problems, no compliance with technical specifications and technical standards, technical verification cycles short, etc. In policy recommendations include: First, the original industrialization goals (2020 units reached 350 watt-hour / kg, the system 260 watt / kg, the cycle life 2000 times) is high, from the safety perspective, I think it is not advisable to implement it. Second, subsidy policies should meet the law of technology development, and the improvement of energy density should not be too fast, should not change over frequency, this is my recommendation to the Ministry of Finance.

Third, launch an electric car safety annual inspection specification as soon as possible. At the same time, in order to better handle and analyze electric car accidents, it is best to have electric cars black box. At the same time, the battery pack should have fire safety interface.

At present, the battery pack is very dead, leading to the difficulty of fire fighting, these are right. Ministry of Public Security. Finally, I think that battery safety is the first key points of battery technology revolutionary breakthroughs.

It is also the first key to the performance improvement of pure electric vehicles. The battery safety will become a bottleneck technology, such as 10 minutes, more than 300 kilometers. Electric fast charge technology will bring challenges to battery safety.

The voltage increases from 300V to 600V or even 800V. These are all relevant to security, and the main battlefield competition in pure electric vehicles in the future. It can be said that security is the life line of electric vehicles sustainable development.