On October 8th, the 2019 International Battery Safety Workshop (IBSW) was held in Beijing. Professor Ouyang Minggao, a member of the Chinese Academy of Sciences and a professor at Tsinghua University, gave a speech on battery thermal runaway.
Here is the full text of the speech:
Ladies and gentlemen, good morning! I am from Tsinghua University. First, let me introduce our New Energy Power System Research Group at Tsinghua University. Since 2001, we have been the major R&D team for the national key program on new energy vehicles and the leading team for the Chinese side of the U.S.-China Clean Energy Research Center for Clean Vehicles. Our team mainly focuses on three aspects: power batteries, fuel cells, and hybrid power.
For power batteries, we mainly focus on safety; for fuel cells, we mainly focus on durability; for hybrid power, we mainly focus on emissions control of internal combustion engines. These are our three main research areas.
Today, I will mainly introduce our research results on safety. The Battery Safety Laboratory at Tsinghua University was founded in 2009, with a focus on battery safety, specifically, battery thermal runaway. Below, I will introduce our progress in research on thermal runaway.
As you know, safety is a major concern for electric vehicles due to various reasons leading to accidents. Once thermal runaway is triggered in a battery, it will spread throughout the entire battery system and eventually cause an accident.
These are some of our partners in battery safety research, including major global automakers and battery manufacturers, as well as major domestic automakers and battery manufacturers. We also license intellectual property patents and technologies to domestic and foreign companies.
This is our Battery Safety Laboratory. Many conference delegates visited our laboratory yesterday. You are welcome to visit and exchange ideas.
We have a series of testing methods in our Battery Safety Laboratory, among which a distinctive one is the adiabatic thermal runaway test using ARC. We are the first in the world to carry out ARC tests on large-capacity power batteries.
After a large number of experiments and studies, we have summarized three characteristic temperatures of battery thermal runaway: T1, the self-heating starting temperature; T2, the thermal runaway triggering temperature; and T3, the maximum temperature of thermal runaway. We have also conducted various types of power battery tests, and all of them comply with this rule.
Among them, T2 is the most critical. Everyone is quite clear about the reaction at T1, which usually starts with the SEI film. T3 depends on the entire reaction enthalpy, and T2 is still not very clear, but it is also the most critical. Why can a slow temperature rise suddenly trigger a sharp temperature rise, and the temperature rise rate can reach 1000 degrees/second or even higher? This is the key factor in triggering thermal runaway.So, through our research on T2, we discovered three main reasons. The first one, as everyone is aware of, is internal short circuit, which is ultimately related to the diaphragm, that is, the positive and negative poles short-circuiting through the diaphragm. Additionally, we found that the oxygen release of the positive electrode material and the lithium deposition of the negative electrode material, as well as the breakthrough of the diaphragm, are the three main reasons that lead to thermal runaway and ultimately the formation of T2.
Next, I will introduce our progress in both mechanism and thermal runaway control for these three reasons, as well as the suppression of heat propagation, which is our last resort to prevent safety accidents, including:
Firstly, internal short circuit and BMS. It is clear that there are mechanical reasons such as collisions and mechanical damage, which eventually result in the diaphragm tearing, or electrical reasons such as overcharging, lithium dendrite, and piercing the diaphragm, or overheating, which ultimately leads to diaphragm breakdown. All of these reasons are related to internal short circuits, but the extent and evolution of internal short circuits vary. They all lead to diaphragm breakdown and diaphragm melting. Therefore, we use a combination of DSC and calorimetry to analyze the mechanism based on material exothermicity and the thermal runaway experiment of a single cell after overheating, which is combined with material exothermicity characteristics analysis to explain the mechanism of thermal runaway after overheating. We can see that the melting of the diaphragm will lead to internal short circuits, and the temperature rise will directly lead to T2 formation, resulting in thermal runaway, which is a more common cause. We have also used many other auxiliary methods, including various material analysis methods, as well as the analysis of various substances using the coupling of thermogravimetry and mass spectrometry.
This is our basic analytical method, which can be applied to the analysis of various batteries and mechanisms.
This is the first and relatively well-known method of thermal runaway, and from a design perspective, we can do a lot of work to prevent it, such as ensuring that the diaphragm is not too thin and that its strength is sufficient. However, the common problem is internal short circuit, so we must prevent it and study it. The internal short circuit experiment is relatively complicated, and there are no mature standardized methods available. Therefore, we have invented a new method, which is to implant shape memory alloy into the battery and heat it to a certain temperature to trigger thermal runaway when the sharp point of the memory alloy pops up.From literature and our own research, there are four main types of internal short circuits. Some internal short circuits can immediately lead to thermal runaway, while others evolve slowly and may not be dangerous at first, but become dangerous later on. Some internal short circuits remain stable, while others transition from a slow evolution to a sudden change. There are various types of internal short circuits. We have conducted some simulation analyses, but I will not go into detail here.
Voltage drop is the first sign of this type of internal short circuit. Temperature rise only occurs in the second stage, and eventually leads to thermal runaway. Therefore, for slow evolving internal short circuits, we should detect and diagnose them during the first stage of voltage drop to prevent further deterioration. This is our internal short circuit detection algorithm for series-connected battery packs, which includes analyzing voltage consistency. If the voltage of a single battery drops, it may indicate the possibility of an internal short circuit. However, if this cannot be confirmed, we will add temperature sensing. If there is still uncertainty, we will add a sensor for combustible gas. This way, we have methods to detect both slow and sudden evolving internal short circuits.
For example, the identification of voltage consistency for series-connected battery packs employs specific algorithms. I will not go into detail here, and everyone can see clearly that a cell with a voltage drop is easily identified.
Of course, we need to use a series of engineering methods. Only a simple algorithm is not enough, we need to include a lot of engineering experience to make judgments. This requires a database, so we choose to cooperate with companies. In summary, we can provide warnings based on this aspect. For sudden-evolving internal short circuits, such as micro short circuits, due to fast charging, the battery will experience deformation and strain during the charge and discharge process, which can lead to sudden deterioration of a micro short circuit. This is similar to plaque in the human body’s blood vessels suddenly causing a blood clot after a certain period of time. It is difficult to detect with only voltage and temperature, because it is too slow. By using a combustible gas sensor, we can even provide a thermal runaway warning at least three minutes in advance. In conclusion, we have developed a new generation of battery management systems based on these algorithms, with safety as the core.
The second part, is about the second mechanism we just mentioned. Does only internal short circuit lead to thermal runaway? Is thermal runaway impossible without internal short circuit? Actually, no. Even without internal short circuits, thermal runaway can still occur. As the separator continues to strengthen, and at the same time, the nickel content of the positive ternary material increases, the oxygen release temperature of the positive electrode material continuously decreases. This means the thermal stability of the positive electrode material becomes worse and worse. However, as the separator becomes better, the weak link will gradually shift to the positive electrode material.This is the experiment we conducted. Even without an internal short circuit, there was still thermal runaway. When we removed the electrolyte, there was still thermal runaway. Moreover, we observed a peak in heat release in the center where the positive and negative electrodes were combined. When the positive and negative electrode powders were combined after charging, there was a significant peak in heat release, which was caused by the reaction between the two electrodes. The root cause of this reaction is the exchange of materials between the positive and negative electrodes, specifically the release of oxygen from the positive electrode to the negative electrode, resulting in a violent reaction and thermal runaway.
For thermal runaway without an internal short circuit, we can establish a model based on the aforementioned secondary reactions through DSC multi-rate scanning. By calculating the reaction constants using this method, and combining energy and mass conservation, we can accurately simulate the complete process of the thermal runaway and the results fit well with the experimental data. With this approach, we can move from empirical trial-and-error to model-based design. However, we need a lot of databases. These databases relate to the relationship between the heat of reaction and the heat release power of various materials.
Based on the database, there are two main areas for improvement in materials: the positive electrode material and the electrolyte. First, by moving from polycrystalline to single crystal materials, we can increase the release temperature of oxygen by 100 degrees, and the characteristics of the thermal runaway change accordingly. For example, using a high concentration of electrolyte is also a viable option. Of course, solid-state electrolytes are currently being explored, and the relationship between the heat of reaction and heat release power of highly concentrated electrolytes has shown promise. For example, the heat release power and the thermal degradation of a concentrated electrolyte both decrease. Furthermore, the positive electrode does not react with the electrolyte, because our new electrolyte, DMC, evaporates at 100 degrees. This indicates that the future development of electrolytes is not limited to only solid-state electrolytes, but also includes additives to the electrolyte, highly concentrated electrolytes, and new types of electrolytes.Part III, on Lithium Degradation and Charging Control. As we know, I discussed earlier about lithium battery degradation that occurs over time. What would happen to the safety of the battery throughout its entire lifecycle? We discovered that the most significant factor affecting the safety during the entire lifecycle is the lithium degradation. The safety of the battery will not decrease if there is no lithium degradation, and the only reason it worsens is due to the occurrence of lithium degradation. We have found a series of evidence, such as low-temperature fast charging, where the temperature of T2 gradually drops after low-temperature fast charging, and thermal runaway occurs earlier, resulting in the battery’s capacity degradation from 100% to 80%, which correspondingly implies lithium degradation formed by low-temperature charging from new batteries to old batteries. Another evidence is fast charging, in which the temperature of T2 drops after fast charging, from 200 degrees Celsius to 100 degrees Celsius when the battery is new to when it is old, and thermal runaway occurs earlier and faster. The reason for this is also lithium degradation. We can observe that the intensity of lithium degradation is different with different outcomes in terms of lithium production. Greater production of lithium generates more heat, which would result in lithium precipitating from electrolytes and inducing excessive heat, leading to thermal runaway. Therefore, just as we study internal short circuits, we must study lithium degradation. But how do we study it? Firstly we can observe the process of lithium degradation. This is the charging process, after which lithium precipitates and then retreats, which is a reversible process. The red line in the previous experiment reveals some reversible lithium, while some lithium cannot be embedded again. This gives us a clue – can we detect the intensity of lithium degradation by detecting the intensity of reversible lithium? For instance, if there is no phenomenon where lithium is returned during slow charging, this is the result of the standard polarization without any peaks. Thus, the peak is a good signal, and the endpoint of the peak indicates the end of the platform. This represents the intensity of lithium degradation related to the total lithium production, predicted by a mathematical formula.
We discovered from the experiment that this is a charging and resting process, and we can detect the degree of lithium degradation by observing the process. This allows us to see the result after the battery is charged entirely. However, can we avoid lithium degradation during the charging process itself? To prevent as much lithium degradation as possible during charging, we need the help of our model.This is our simplified P2D model, which shows the negative electrode potential. As mentioned earlier, the negative electrode potential is related to lithium deposition. By controlling the overpotential of the negative electrode, we can ensure the absence of lithium deposition. Using this model, we can derive the charging curve with no lithium deposition by keeping the negative electrode potential above zero. We can use a three-electrode system to calibrate this curve and develop our charging algorithm. In collaboration with businesses, we have shown that the algorithm can completely prevent lithium deposition. However, as battery performance deteriorates over time, we require feedback. Therefore, we have developed a feedback algorithm for lithium deposition control, which uses an observer to measure the negative electrode overpotential. This is similar to our State-of-Charge estimation algorithm. With a feedback of the terminal voltage, we can perform real-time control of the battery charging to avoid lithium deposition.
During this process, we were wondering if we could use a negative electrode overpotential sensor. Therefore, we further researched and developed the overpotential sensor. Traditional three-electrode systems have limited lifetimes, and hence cannot be used as sensors. In collaboration with the chemistry department, we have made a breakthrough in developing an overpotential sensor, with a tested lifetime of more than five months. Since we only use this sensor during fast charging, a lifetime of five months should suffice. Our next step is to develop a feedback control algorithm for charging based on the negative electrode overpotential sensor.
In the fourth part, we focus on thermal runaway and its suppression. If the previous three methods fail, it will lead to thermal runaway and potentially an explosion when the battery is punctured or crushed mechanically. This is the propagation process of thermal runaway, with a temperature field test conducted for a parallel battery system. When a single cell undergoes thermal runaway, it causes a short circuit, the voltage drops, and the current flows towards the short-circuited cell. As a result, the voltage across the other cells increases, and if the cell is not taken out, it will eventually cause thermal runaway. For a series cell system, the propagation is purely a heat transfer process.
In another scenario, orderly propagation leads to fierce propagation and eventually combustion and/or explosion due to combustion.This is the entire process of the system and the PACK propagation. The propagation is regular, starting from D2, then U2, and D1 almost at the same time, followed by others. However, the design of the battery pack is very important due to the insulation. Therefore, our goal is to design based on model simulation, because the process is very complex, and it is very difficult to rely on experience alone. As a result, we have conducted a detailed study on parameter calibration, which is a very skillful process. With the calibrated model, we can design the insulation and heat dissipation of the battery pack. There are also some necessary measures that must be taken together, such as our company’s firewall technology. We have conducted a large number of experiments on the entire battery pack in the field, comparing a traditional battery pack with the one equipped with the firewall. Through this, we can really prevent it from spreading. We have also participated in the formulation of a series of international regulations regarding this area of work. In addition, there is a process of spraying, which is very complicated, and we are still working on a more accurate simulation model. From experiments, we can see that there are solid, liquid, and gaseous states, and the latter is often flammable gas or fuel. The solid state consists of solid particles, which often form flames. Our work involves collecting particles, like a traditional car, and diluting flammable gas beyond its ignition range.There are three stages of thermal runaway: initiation, propagation, and escalation. There are various reasons for initiation, as I have mentioned several before. Of course, there is also the collision mechanical part of our vehicles, which I didn’t mention. What we have been discussing mainly are these factors, which currently do not have regulations to govern them, but we believe it will be needed in the future. The second stage is when thermal runaway occurs. We have mentioned three temperatures, of which the reasons for T2 are shown here. Internal battery eruption and ignition are mainly determined by the state and boiling point of the electrolyte, with three stages of eruption, followed by ignition. To prevent this, we need to eliminate all these factors with certain measures. Lastly, there is escalation, which can be either anticipated or sudden, like a fire eruption that can lead to intense burning. All the issues mentioned here have solutions.
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This article is a translation by ChatGPT of a Chinese report from 42HOW. If you have any questions about it, please email bd@42how.com.