Academician of the Chinese Academy of Sciences, Professor Ouyang Minggao, Tsinghua University, my country. Battery safety has a very important application value in transportation and modern travel, especially in energy security, is also global focus..
The US Department of Energy (DOE) and the German Science Institute (BMBF) and related internationally renowned scholars have launched an international battery safety seminar (IBSW), and continued in 2015 in Munich University in Germany, 2017 in Sandia National Experiment in the United States. Room, successfully held the first and second international battery safety seminars (IBSW). On October 7, 2019, the 3rd International Battery Safety Seminar was held in Beijing.
The General Assembly hosted by Tsinghua University Battery Safety Laboratory, the theme of the meeting is “safer high-than-high-specific battery for electric vehicles”. At the meeting, academician of the Chinese Academy of Sciences, Professor Ouyang Minggao, Tsinghua University, published the keynote speech, introduced the “Safety Research of Tsinghua University Motor Lithium Battery”. The content is organized, as follows: ladies, gentlemen, everyone is good! I am from Tsinghua University.
First of all, we introduce our Tsinghua University’s new energy power system research group. Since 2001, we since 2001 is the key special research and development team of national new energy vehicles, and it is also the lead team in China and the United States..
Our team is important for several research, including power lithium batteries, fuel power batteries and hybrid power. In terms of power lithium battery, we are important to do safety; we are important to do durability in fuel power batteries; in terms of hybrid, we are important to do emission control of internal combustion engine. So this is our important three focus points.
. Today I have given you an important introduction to our research results in safety..
Tsinghua University Battery Safety Lab is found in 2009. The focus is to do battery safety. Specifically, the battery’s thermal out of control.
Here I introduce us to the research progress in thermal out-of control.. Everyone understands that security is the problem of focus on electric vehicles, and there are various reasons to cause safety accidents.
. Once the thermal out of control is induced in a battery, the entire battery system will spread, and finally the accident is formed..
This is some of our partners in battery safety, including internationally important automakers and important battery manufacturers, as well as important automotive manufacturers and important battery manufacturers in China, and we also license intellectual property patents, domestic and foreign companies, etc.. This is our battery safety laboratory.
Yesterday, many participants have visited our laboratory. Welcome everyone to visit and exchange..
There is a series of test methods in our battery safety laboratories, which is a more distinctive thermal out-of control experiment with ARC to heat-out of control. We are the world’s unit of ARC experiments on large-capacity power lithium batteries..
After a large number of experimental studies, we summed up three characteristics of battery thermal out-of control, self-hot start temperature T1, thermal out-of control trigger T2, thermal out-of-control maximum temperature T3, we have also done a lot of type of power lithium battery test, In line with this law. The T2 is the most critical, what reacts T1 is more clear, usually the SEI film begins, T3 depends on the entire reaction enthalpy, T2 is not very clear, but it is also the most critical, why is there a slow rise The heat will suddenly cause a sharp heater, and the rate of lifting can reach 1000 degrees per second or more, which is the key to the cause of heat..
Therefore, through the exploration of T2, there are three important reasons. The first one is more clearly, it is inner short circuit. It is ultimately related to the diaphragm, which is short-circuited.
. There is also a newly illicited positive material release oxygen, the lithium lithium, summarizes the positive limit of oxygen, the negative lithium, the diaphragm collapse, these three reasons are ultimately the main reason for the formation of T2. Below I have introduced the three mechanisms mentioned earlier to the mechanism and the progress of thermal out-of control control, including the first, internal short circuit and the short circuit of our control, is BMS.
. Second, the thermal out of control and the thermal design of the battery caused by the positive limit. Third, the thermostat caused by the vigorous reaction of lithium lithium and electrolyte and our charging control.
If the three technologies, the three technologies can solve the thermal out-of control problem. We have the last trick, which is to suppress heat spread, we must understand the law of thermal spread, while suppressing the thermal spread, and ultimately prevent safety accidents. Let me introduce you to these four aspects: First, short circuit and BMS.
It is more clear that mechanical reasons, such as collision, mechanical, and finally the tear of the diaphragm, or the reason for electricity, charging over charge, branch crystal lithium, dendritic puncture, or overheating, of course, will eventually Overheating, overheating can lead to the collapse of the diaphragm, all the reasons are related to the short circuit, but it is not the same, the process of evolving is different, but it will last to the diaphragm crash and diaphragm melting. So we use the heating calorimeter and DSC, one is to explain its mechanism from the exotherm of material, one is to heat out of the entire single battery from the heat transfer of the entire single battery, and put the thermal out of control experimental heel material The thermal character is analyzed, which is the mechanism of thermal out of control after we are routine..
We can see that the melting of the diaphragm can cause internal short circuits, starting the temperature, and the diaphragm crashes will form T2, directly leading the thermal out of control, this is a more common reason. We also use a lot of other auxiliary means, including various material analysis methods, and a method of thermal weight and mass spectrometry to analyze various substances..
This is our basic analysis method, you can analyze a variety of batteries, various mechanisms.. This is the first, and it is also a kind of thermal out-of-control method, no matter what, we can do a lot of work from the design angle, not too thin, but the strength is enough, but the middle There is a common problem to short circuit, so we must prevent internal short circuits, we have to study the short circuit, short-circuited experiments are relatively complex, no mature norms, so we invented a new approach It is to implant the battery with memory alloy, heating to a certain temperature, let memory alloy sharp sharply, trigger heat out of control.
From literature and our own research, there are four types of important internal short circuits. Some short-circuits can immediately lead to thermal out-of control, but some short-circuited is slowly evolved, and some short-circuit may not be dangerous, but some short-circuited It will be very dangerous, and some short-circuited is always slow, and there are some internal short circuits from slowing to mutations, there are various types..
To this end, we have also conducted some simulation analysis, I am not detailed here.. In short, we finally discovered that the evolution of short circuits in the evolution type was the voltage drop, the first process is important to drop the voltage.
. It will be temperature rise in the second part, and finally form heat out of control..
So about this slow, we should in its first process, that is, the voltage drop stage is to detect it to troubleshoot, pick it up, to prevent it from further deterioration, this is our internal short circuit detection Algorithm, this is an algorithm for the series battery pack, including the first is analyzed from the consistency of the voltage, and a battery voltage is dropped, indicating that this battery may have an internal short circuit.. But if you can’t confirm, let’s add temperature.
If you have changed after the evolution, we add the combustible gas sensor, so there is a way to slow and mutation.. For example, the consistency identification of the series battery pack voltage, I don’t introduce the specific algorithm.
You can clearly see that the battery that is down on the voltage can be obvious.. Of course, we have to conduct a series of engineering methods, and there is a simple algorithm that is not enough.
It is also necessary to join the relevant experience of many projects to judge, this is the database, so we choose to cooperate with the company.. In short, we can warw well from this area, such as a micro-short circuit, because of the fast charge, because the battery will have a deformation during the charge and discharge, it will have a strain, which will cause sudden deterioration of micro-short circuit, like human blood vessels The plaque inside, suddenly the thrombosis is a press, if we use the voltage and temperature, it is too slow, it can’t see it, it is already hot when you see it.
. How to do? We must use this gas sensor, which can do at least 3 minutes in advance to perform thermal out-of-control warning. In short, we develop a new generation battery management system based on these algorithms.
. The second part is the second mechanism we just said, is it only short-circuited? Is there any heat loss without internal short circuit? In fact, there is no internal short circuit to have a thermal out of control..
As the diaphragm is constantly increasing, the nickel content of the positive electrode three-membered material is constantly increasing, its release temperature is constantly decreasing, that is, the thermal stability of the positive electrode material is getting worse, but our diaphragm will become better and better, so weak The link will slowly become a positive material. This is the experiment we did, there is no short circuit, there is heat out of control, we remove the electrolyte, there is heat out of control, and you can see it from the middle, there is a heat-free spike, this is positive and negative in one piece, fully completed The positive and negative powder is placed in a piece, there is a dramatic release peak, this is the reason why he triggered. Specifically, where is the hot peak? Positive electrode material phase change, free oxygen.
Look at the peak of the holland, when the positive and negative is combined, the negative electrode is oxidized. If there is no peak, it is closed, proves that the heat generated from the positive heterogenesis and the negative electrode reaction..
So what is this mechanism? It is the material exchange of the positive and negative electrode, which is the positive end of the oxygen to the negative electrode to form a dramatic reaction, which caused the thermal out of control.. With regard to the thermal out-of-control of the internal short circuit, we can establish a model according to all the side effects just all the side effects.
Through the multi-rate scanning of DSC, the reaction constant of all the side reactions can be calculated in this method, of course, through a certain method, finally Combined with energy conservation, quality conservation can calculate the complete process of the thermal out-of-control, and can be well complied with the experiment. In this way, we can develop from related experience to develop the model-based design, of course, there are many databases, no database is not, this is the reaction of the reaction of various materials and the relationship of the heat. Based on the database, we must of course improve the materials, the key improvements I think two, one is the improvement of the positive material, one is the electrolyte.
First of all, we can increase the temperature of the oxygen from polysantial to single crystal, and it can be seen that the characteristics of thermal out-of-control have changed.. For example, we use high concentration electrolytes, it is also a way.
Of course, everyone can explore more solid electrolytes. The solid electrolytes are very complicated. We believe that the concentrate itself has a good feature.
. For example, its thermal weight has dropped, and the exothermic power has dropped. From this middle we can see it, and the positive is not reacted with the electrolyte, because our new electrolysis quality is DMC, DMC is 100 degrees It has been evaporated.
This is what we believe that the next step of electrolyte is more than just solid electrolytes, more is from the additive of the electrolyte, high concentration electrolyte, and new electrolytes can be. Part III, about lithium lithium and charging control. Everyone understands that I will tell the lithium-ion battery.
After a battery will be attenuated, what will the full life cycle safety? We have found that the most important factors in the middle of the full-life cycle security is to analyze lithium, if there is no status of lithium-decreasing battery safety does not deteriorate, the only reason for deteriorating it is to analyze lithium. We can find a series of evidence, such as low temperature fast charge, low temperature fast charge, the temperature of T2 gradually decreases, and the heat loss occurred earlier, this is battery capacity attenuation, from 100% to 80%. Obviously correspond, lithically from the low temperature charging from the new battery to the old battery.
The other is fast charge. After the fast charge, it can be seen that the temperature drop in T2 is dropped to 100 degrees. From the beginning of the new battery 200 to more than 100 degrees, the heat loss occurred earlier, faster.
What is this reason? It is also lithium lithium, we can see that there are many lithiums, and the lithium has little significantly.. Analysis of lithium has a large amount of exotherm, so it is still a lithium, precipitation lithium will directly react with the electrolyte, causing a lot of temperature rise, can directly induce heat loss.
Therefore, we must study lithium, just like the short circuit in our study, how to study lithium studies? First we can see the process of lithium lithium. This is charging, charging is over, it can be seen that the lithium is beginning to start, there is a large part of the back, this is the process of lithium. The experiment just now can be seen from the Red Line, this is the activated lithium, reversible lithium.
There is also a part of death, reversible lithium, can be re-embedded, and the negative electrode is over-potential, and the excessive stage over-electricity increases to 0, which can be reversible to lithium. Of course, the dead lithium cannot be retrieved..
This gives us a prompt. Can we pass the process of reversible lithium to detect the amount of lithium, for example, it is going back this process, this process corresponds to a platform on a voltage, we have simulated, and found this platform..
When we are very low, there is no phenomenon, it is normal voltage to polarize, no this platform.. So this platform is a good signal, the end of the platform we can determine by differentiation, this is the end of the platform, representing the lithium quantity, and there is a relationship with the total amount of lithium, can predict the formula.
. We also found out from experiments that this is a charging, standing process..
We also see that the lithium can be seen from the middle, this is the result of the experiment.. So in this way we can find it after charging it, but this is a result after the charge, can we do not let it lithium in the process of charging? Ability to deal with lithium as much as possible, of course, this requires us to help our model.
This is the simplified P2D model we did, you can see the potential of the negative electrode, just say that the negative electrode potential and lithium lithium, as long as we control the over-potential of the negative electrode, we can guarantee the lithium. Through this model, you can derive the curve of lithium charging, we let the negative electrode potential is not less than zero, you can get the best charging curve for lithium lithium. We can use the three-electrode to calibrate this curve, which is our charging algorithm.
We have cooperated with the company, which can be clearly seen that using this algorithm can fully realize lithium, but this is a calibration process, over time Extending the battery’s attenuation performance is changeable, what do we do, we have to feedback, so we have given feedback to the control algorithm for lithium lithium, that is, there is an observer to observe the over-electricity of the negative electrode, this is an negative observation Overotic, this is the observer, actually is a mathematical model. This is very similar to our SOC, we have an observer algorithm, we have a feedback on the voltage, so that we can carry out the real-time control of lithium charging, and we also cooperate with the company..
In this process, we still have some regrets, can you directly use the sensor for an negative power? Therefore, further research is to develop this over-potential sensor.. Everyone understands the traditional three electrodes mentioned earlier.
Its life is limited, no way to use it as a sensor, and we have recently been cooperated with chemical system.. The chemical department Zhang Qiang team, because they are a team that is very related experience, breakthrough in this area, our test life can be greater than 5 months, more than 5 months should be used, because we actually When the application is only in the fast charge, it is not always used, and it is enough for 5 months.
. Next, our work is based on the feedback charging control of the negative overtest power sensor..
The fourth part, thermal out-of control, if we don’t work in front, it is the spread of thermal out-of control and our suppression method. Everyone understands that this mechanical abuse directly pierces or extruded the battery immediately formed a combustion explosion, which is the process of spread, this is the spread of our spread..
The first is the test of the temperature field. This is the spread process of our parallel battery pack. The mechanism of spread the process is above.
Why is it a section of the section, because when the first battery is thermostable, it will be shorted, all electricity They will come here, so they cause the voltage to drop, but once it will be broken, it goes back, this is the characteristics of parallel heat loss. This is a series battery group, and the series battery group is purely a heat transfer process..
This is another situation, the beginning of the order, finally spread, of course, because there is a combustion in the middle, not only heat transfer, this immediately leads to explosive accidents, combustion accidents, etc.. This is the process of the entire system, the entire PACK propagation process, its communication is regular, from D2 first to U2, D1 is almost simultaneous, then other, this is basically no longer, because there is insulation, this prompts Our design is still very important for battery packs.
. Accordingly, our purpose is of course based on model simulation design, because this process is very complicated, if only related experience is very difficult, this is what we do. Everyone must know, how to take the parameters of the simulation, you can adjust the parameters, but the number of parameters is meaningless, so we do a detailed study in parameters, how to take parameters is a very skillful process, I I don’t detail here, a series of methods.
With this model calibration model, we can design, this is the design of heat insulation.. The battery is only insufficient, and there is a cool design.
. There are also some battery insulation, heat dissipation must be all possible, this is the firewall technology developed by our students, insulation, heat dissipation, blocking through insulation, heat dissipation, and heat the energy, these two Cooperation. This is a lot of experiments, this is the experiment of the entire battery pack in the wild, a traditional battery pack, a battery pack with firewall.
The battery pack with firewalls just started this, the smoke is quite big, slowly, no burning, no hot spread, traditional battery packs to finally form hot spread and combustion. We can pass this, realize it really. This is about this work, we also participate in a series of international regulations.
. Now that we have further done this process is eruption, more complicated, now we have not added to the simulation, the eruption model is of course, but it is not accurate. It can be seen from the experiment that there is solid state, liquid, gaseous tri-state, this intermediate gaseous is some combustible gases, which is fuel, solid state is some solid particles, often form flames.
How to do? One is to collect particulate matter, just like a traditional car, to capture the particulate matter through the filter. The other is diluted, let the combustible gas beyond its fire range, this is what we are doing now..
Finally, I will make a summary. There are three processes of thermal out-of control, in which they have occurred..
In the induction, there are various reasons in the induction, I have said a lot, of course, there is another part of our collision machine, I didn’t say, now we are in front of these things, these things are still still No regulations are regulated, we feel that later is. Second, thermal out of control. We mentioned three temperatures, of which three reasons are shown here.
. There is eruption and fire inside the battery. It is important to be determined by the state of the electrolyte, the boiling point of the electrolyte.
. Finally, it is spread, and we can spread, there is a sudden spread, such as a fire, which is erupting to the flexible fire, and finally leads to severe burning, all the problems we have shown here is to solve of. .