18650 lithium battery 3.7 v

There are several levels to consider when studying the safety of 18650 lithium battery 3.7 v

The China Electric Vehicle Hundred People Forum (2019) was held at the Diaoyutai State Guesthouse in Beijing. Wang Fang, chief expert of the China Automotive Technology Research Center, delivered a keynote speech. The content of the speech is as follows:

In fact, when we are testing and evaluating key 18650 lithium battery 3.7 v, the first thing we consider is the evaluation of the performance of the whole vehicle from the perspective of the use of the whole vehicle and the requirements for the battery performance derived from the power battery. In fact, several leaders mentioned in their reports yesterday afternoon that for electric vehicles, among all the performance of electric vehicles, safety is the most important first performance. In fact, we have discussed a lot about it in the scope of 18650 lithium battery 3.7 v today, and everyone is actually most concerned about the safety of 18650 lithium battery 3.7 v.

In fact, when we are studying the safety of 18650 lithium battery 3.7 v, there are several levels to consider:

First, the safety of normal use under different temperatures, conditions, and various harsh environments is the safety evaluation of its availability.

Second, what cannot be avoided is that the safety hazards of the battery mentioned by the experts just now may exist, and the controllable safety evaluation once the hidden danger occurs.

Third, the evaluation of its loss of control after an accident.

Due to time constraints, we will only discuss part of the available and controllable evaluations today.

As you know, in addition to testing, I also devote a large part of my energy to the formulation of domestic national standards and international regulations. The premise of our formulation of domestic standards and international regulations is to study the possible failures or out-of-control conditions that may exist in the actual use of electric vehicles, and what the possible consequences of such conditions are. This is my team's analysis of all electric vehicle accidents from 2011 to September 2018. There are about 80 passenger car accidents. We analyzed the specific causes of the accidents. Of course, all of these causes are because we know the exact causes of some of them. I personally participated in the detailed investigation process of several accidents. Most of the causes of accidents actually come from the analysis of some media that we can search. I have made a statistics for you to see. 16% of them are related to the charging process. Mr. Ai also analyzed that during the use of electric vehicles, he analyzed overcharging. In fact, we have seen some accidents at the end of charging or during the charging process. Some are in the state of overcharging. This is also clear to everyone. At the end of charging or even when overcharging, its activity is relatively easy to cause safety problems. The second category, collision, is the influence of some mechanical external forces, which accounts for about 18%. The third category, water ingress, accounts for about 3%. In addition, there is a relatively large proportion of 35%. We have conducted a careful analysis and believe that some accidents are caused by internal short circuits of batteries, which is the problem mentioned by many experts just now.

There are some other reasons. I think they are unknown. I can't explain clearly what causes this thermal runaway, but what is certain is that the first place for these accidents to start is the battery pack. When we are actually working on global regulations, thermal diffusion is now a key topic. Here we have listed all the reasons for thermal diffusion in detail, including mechanical, electrical, thermal, and environmental factors. After listing them, everyone thinks that basically these possible reasons, we have corresponding projects at the international regulatory level to limit it and protect its safety. For example, collision, everyone thinks that our mechanical vibration, mechanical anti-collision and dynamic simulation collision can be guaranteed. For example, overcharging, overcharging protection can guarantee it. After we conduct a systematic analysis, there may be a smaller proportion than this in the accident-free spontaneous combustion, that is, it is believed that the hidden danger of internal short circuit in the actual production process leads to thermal runaway. This is a focus we want to pay attention to, and it is also not protected at the current regulatory level. It is also the current research of our experts on thermal diffusion.

In fact, in the process of global regulation, my point of view, when I communicate with global experts, I also put forward my point of view. In addition to the heat diffusion in the manufacturing process, due to the development of high-energy-density batteries, many uncertain factors have caused instantaneous thermal runaway, and in this case, we may be more concerned about the internal short circuit, rather than what causes the internal short circuit, and the thermal runaway caused by the internal short circuit.

The last part is actually the same as traditional cars. Because of other factors, the fire of electric vehicles is naturally caused. When we talk about the safety of 18650 lithium battery 3.7 v, we cannot do without analyzing the development of battery technology itself. As I said just now, when we analyzed this thermal characteristic, we also saw the rapid development of energy density. We analyzed the data based on the vehicle's certificate of conformity. As of October 2018, the average energy density of the battery and the system has reached 139.5 Wh/kg, and its grouping rate has reached 74%. We can see that from 90 Wh/kg in 2015 to 140 Wh/kg in 2018, the increase of 50 Wh/kg took only more than 3 years. This time is for product updates and iterations and reliability verification cycles. In the state of increased energy density of this system, the first thing everyone considers is the increase in the energy density of the single cell. This figure According to the data from our laboratory tests as of the end of 2018, a leader's report yesterday afternoon also mentioned that the energy density of the single cell has reached 260 Wh/kg. I would like to say that the energy density of our battery cells was 150 Wh/kg in 2012, 180 Wh/kg in 2015, and 260 Wh/kg in 2018. It took 3 years to increase from 120 to 150 by 30 Wh/kg, and it only took 3 years to increase from 180 to 260 by about 80 Wh/kg. I think this growth rate may imply some issues other than energy density that we need to pay more attention to.

We can see that the changes in the monomer system he improved, from lithium iron phosphate in 2012 to 333 in 2015, and then to 622 and 532 in 2018, were rapid changes. To be precise, it should be the end of 2017. In fact, in the process of transformation in 2018 and 2019, I think the previous one was from steady advancement to rapid running to 333 and 532, and then from 532 to 622. Basically, the trend we saw from 2018 to 2019 was that everyone had quickly jumped from 532 to 622 and quickly ran to the stage of 811. In this process, you can see that many experts have shared similar data just now. We see that batteries with different energy densities of different systems have different self-heating temperatures, self-heating time, and instantaneous temperature rise rates of heat generation, and they are increasing rapidly, not to mention the difference in oxygen evolution that an expert mentioned just now.

Another thing is that in order to achieve this high ratio, when manufacturing batteries, because the batteries need to be made as large as possible to meet the requirements of long battery life, you can see such a change, that is, the amount of its diaphragm becomes smaller, and the positive and negative pole pieces become thicker, which also increases the challenge of its safety. Just now, Professor Ai also talked about the attempts to modify the material of the battery cell, including the technology of the diaphragm, the improvement of the positive and negative electrodes and the electrolyte.

In fact, with the increase in energy density, I think everyone's safety is not only dependent on the level of the monomer, but a systematic project. For our testing, we are also concerned about the transition from the monomer level to the multi-level system evaluation stage, including the safety at the monomer level. The safety at the monomer level has become a level and a range, rather than an absolute safety concept. It is definitely a relatively safe concept. In this case, it extends to the level of safety at the material level, and it is possible to make improvements and improvements.

In addition, relying on the strengthening of the monitoring and protection functions of the BMS, as well as the protection of the battery shell, we hope to get a relatively safe battery system in the end.

We have also received support from the Ministry of Science and Technology's key R&D projects, and have carried out relevant research at several levels. Here is a brief introduction:

The first is the evaluation of the safety structure-performance relationship from the material to the battery level. Universities and research institutes have done a lot of work on the test and evaluation of material properties, and they also have very advanced instruments and testing methods to characterize the properties of materials. In the past ten years, we have also done a lot of research on the characteristics of batteries, and various safety test items. In fact, when I was testing, we called it a black box test, that is, after the battery is made from materials, what happened in the entire reaction process and the process of use, in fact, there are many things we don't know, so it's a black box test. In fact, we are now more hoping to improve the safety of the battery. We are concerned about what its process reaction is like during this use process, and whether I can observe this process reaction non-destructively, so as to better understand what role the material plays in the performance of the battery's safety performance and what kind of improvement it can make. So in this process, including the changes in the SCI membrane mentioned by the expert just now, it may be the focus of everyone's concern, and there are other factors.

In fact, what we are trying to do now is to use in-situ CT or in-situ neutron diffraction, in-situ scanning and other means to make the black box gradually transparent, so that we can see some of its changes during the entire use process, and some of its actual qualitative parameters can be quantitatively or as quantitatively as possible to understand its safety structure-activity relationship.

What I want to say about the battery cell level is the safety of the battery. Today, Professor Wang Chaoyang said that he hopes to make unmanaged batteries in the future. I think it may still be an ideal at present. The actual battery is an electrochemical carrier that changes dynamically throughout its life cycle. Its safety and availability boundary itself is a dynamic change. So we hope to understand, first, what is the possible level of its own safety. The second thing is to understand how its safety level changes under different life cycle states during its entire life cycle. We need to see that it is actually available. This is an animation we drew. You can see that its availability boundary has been shrinking. How can we ensure that the boundary we control of electric vehicles can always be safe during the entire life cycle? This is what we should pay attention to.

The third level is the management system. The increase in energy density has raised more and higher requirements for the management system, or placed more expectations on it. Now, for the management system, its evaluation is becoming more and more important. There are two national standards being drafted for the management system. One is the comprehensive requirements of the management system, and the other is the functional safety of the management system. You can take a look at these two standards. In fact, we divide the evaluation of the battery management system into five categories. One is balance and the other is SOX. Professor Ma just introduced some of his methods in detail. The actual SOH method will be established in the group standard of the society next Tuesday, and its specific test method will be quantified. In addition, electromagnetic compatibility functional safety. Another more important thing is the evaluation of thermal management. In fact, we are also working on a group standard. The specific draft of this group standard will be discussed next Wednesday. In this, we will focus on whether its thermal management cooling, heating, insulation, temperature equalization, and energy consumption can meet the actual use requirements of our thermal management during the actual use process, and whether it can reach the target value of the original design. When it comes to us, we may have to care about how to design the thermal management system. In fact, in the design of the thermal management system, we rely on a lot of test evaluation. In fact, we have also established an evaluation system for this test evaluation. The most important thing is the acquisition of the thermal characteristics of the battery. First, under different SOC states, different powers, different multiples, and different external environments, its heat generation characteristics, heat generation, heat generation power, and the maximum temperature value of temperature rise are the basic values of the temperature rise range. Second, look at its basic boundary, because the battery must have a thermal runaway boundary, and what we need to find is the value of its boundary. Third, under this boundary value, the way it transmits to its neighboring batteries, the speed of transmission, the heat transfer, and the boundary point where it may run away are the most basic values we need to obtain to make a good thermal management system.

The fourth level is the safety of the system. This is the "Mandatory Standard for Safety Requirements of 18650 lithium battery 3.7 v for Electric Vehicles" that the Ministry of Industry and Information Technology just posted online for comments on the 10th. I will not introduce the requirements of the battery system in this standard in detail. There are 15 items. The battery standards currently being implemented are quite different. One is the vibration test. I made the vibration road spectrum requirements based on the actual road spectrum collection. In addition, the requirement for immersion is to require that the vibration be done after a certain use state before doing the immersion test. Another very important thing is the test of thermal diffusion. Assuming that thermal runaway of a battery is inevitable, the probability of occurrence always exists. Although we hope to reduce this probability as low as possible, it is actually inevitable. In this case, what we require is, as I said just now, we are more concerned about the occurrence of internal short circuit, so we must first simulate the internal short circuit in different ways, and then when this kind of thermal diffusion and thermal runaway occurs, when a battery has thermal runaway, we have defined the conditions for the occurrence of this thermal runaway, including its temperature rise rate, the highest temperature value and the voltage drop.

After this situation occurs, we have to see its subsequent reaction. First of all, if there is no alarm signal for this accident, that is, it is now required that electric vehicles must be equipped with this kind of thermal runaway accident alarm signal. If there is no such alarm signal, or if the battery does not catch fire within 5 minutes after the accident alarm signal occurs, we believe that it is invalid and this product does not meet the requirements. The other two forms, if this kind of thermal runaway occurs again after 5 minutes, that is, the entire battery pack is on fire, we also recognize that it is safe, or the entire battery pack is not on fire in the end, that is the best state, why? Our basis is to give passengers enough time to escape within 5 minutes, or a certain amount of time, because the most basic concept of electric vehicle safety is personal safety, and other factors are not considered. So as long as there is enough time for people to escape, we think it is safe. So whether there is a fire or not within 5 minutesAll are acceptable.

But in fact, this is the draft of our current GB standard. There will still be some problems when we write it into global regulations. For example, I have listed the problems in six major points:

The first point is the various experiences mentioned by everyone just now. For example, if solid-state batteries are developed in the future, or even batteries that really won't catch fire are developed, of course this is our OK state. So what should we set in the current state? What are the conditions for exemption? That is to say, what is the condition for not doing thermal diffusion? Is the energy density value lower than a certain value, or what type of battery can be exempted? This is something we need to do.

The second point is high repeatability and repeatability. Because the method we use now, such as heating or acupuncture, etc., to make the battery thermal runaway, you will add heat from the outside, and this will affect the reproducibility of the battery experiment results, so this is what we need to study for repeatability.

The third point is that we also need to choose the point that triggers thermal runaway.

The fourth point is the reproducibility of the entire thermal runaway system.

The fifth point is safe escape time. In fact, the 5 minutes we set before was based on our fire drills in the past two years. In fact, everyone will also point out that the escape time of passenger cars and commercial vehicles is different, which requires us to do more research and determination.

The sixth point is the different components and levels of safety. This is related to the out-of-control evaluation we just mentioned. We also have a special domestic team studying it, that is, its toxic gas analysis, toxicological hazard level analysis and fire level analysis. We are also the topic of the second phase, and we are also doing a small advertisement, because the second phase is being carried out from 2018 to the end of 2020. There are four topics, thermal diffusion, vibration, seawater immersion, and toxic gas analysis. These four are basically proposed by our Chinese experts. I am also entrusted as the leader of the Chinese technical team in the second phase. Now we have established special domestic research teams for the four proposals. If you have these experiences in this regard, I hope you can give more support to our industry's work. This is also a work for China to participate in global regulations.

Finally, there is a summary:

First, we have always said that the performance evaluation of batteries is a comprehensive indicator system, and in this indicator system, safety is the most important, not energy density, so this is the most important point.

Second, when it comes to safety, it is a systematic project. Whether it is materials, batteries, BMS and systems, they should meet the safety requirements, and safety should not be transitioned to rely on a certain level of safety protection, so each level should have safety requirements.

Third, because it is a systematic project, whether it is the production and manufacturing of batteries or the production and manufacturing of PACK, the quality control level in its manufacturing process is very important, because after 2020, it has entered the post-subsidy era or the non-subsidy era. In this case, whoever has core competitiveness must be the enterprise with better quality control level in its production. Only those enterprises with core competitiveness have such enterprises.

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