Today, we'll be discussing the topic of "Safety Issues of High-Energy-Density Power Batteries." Safety is a topic of great concern. In recent years, with the increase in the number of electric vehicles and the improvement in the energy density of our power batteries, safety incidents have occurred. These incidents have seriously affected the development of electric vehicles, and some have even begun to question the development roadmap for power battery technology set by our country. Why do lithium-ion batteries pose safety risks? What factors can cause lithium-ion battery safety incidents? Are there ways to improve battery safety? I would like to take this opportunity to have a brief discussion with you.
The safety of lithium-ion batteries ultimately boils down to thermal runaway. Besides the familiar normal charge and discharge reactions, lithium batteries actually contain potential negative reactions. These negative reactions don't occur within the battery's normal temperature and voltage range. However, when the battery temperature or charging voltage is too high, these negative reactions can be triggered. If this heat is not dissipated in time, it will cause a rapid increase in battery temperature and pressure, ultimately leading to thermal runaway. The positive electrode has the largest mass proportion in the battery and typically releases the most heat; therefore, the positive electrode has a significant impact on battery safety. This explains why the safety of lithium-ion batteries varies depending on the type of positive electrode used.
In the event of a safety incident, the earliest reaction leading to thermal runaway is now generally considered to be the decomposition of the SEI film on the negative electrode surface. We know that the potential of a fully charged negative electrode is essentially the same as that of lithium metal, which is highly reactive. This means that after a full charge, the negative electrode's potential is extremely reactive. When the electrolyte is depleted, a strong reduction occurs on the electrolyte surface, and the reduction products deposit on the negative electrode surface, forming the SEI film. It is this film that allows lithium-ion batteries to function normally. However, this SEI film is unstable. When the temperature reaches 120, 130, or 140 degrees Celsius, depending on the specific assembly and additives, it may undergo thermal decomposition. After decomposition, the negative electrode is exposed, and the electrolyte comes into direct contact with it. The electrolyte then undergoes a violent reduction decomposition on the negative electrode surface, releasing a large amount of flammable gas and heat. This heat will cause the battery temperature to rise further. When the battery temperature rises to the positive electrode decomposition temperature, which is about 180-200 degrees Celsius, the positive electrode decomposes, generating a large amount of heat. More seriously, the positive electrode decomposition process releases atomic oxygen, which is oxygen that cannot be loaded in time. This atomic oxygen is highly reactive and will cause the electrolyte to be directly oxidized and decomposed. The heat generated by the electrolyte decomposition is very large, causing a large amount of heat to accumulate inside the battery in a short period of time.
Our understanding of thermal runaway is generally as follows: when abuse conditions such as short circuits and overcharging cause the battery temperature to rise by more than 100 degrees Celsius, a series of potential negative reactions will be triggered. If the heat dissipation is lower than the heat generation level, the temperature will rise further. We know that the reaction rate of chemical reactions increases exponentially with increasing temperature. Therefore, the higher the temperature, the faster these negative reactions will increase, eventually causing the battery to enter an uncontrollable self-heating state, which we call thermal runaway, leading to explosion and combustion. Why do electric vehicles generally burn rather than explode? Because electric vehicles are typically designed with safety valves. When the pressure reaches six or eight atmospheres, pressure is limited. During this pressure-limiting process, the electrolyte's dispersion point is very low, only between ten and thirty degrees Celsius. When these vapors are ejected, the friction with the safety valve is enough to ignite them, causing combustion. This is why our power batteries often burn.
This graph shows that different cathode materials have different thermal stability. For example, lithium iron phosphate is relatively safe because it basically does not decompose or generate heat in the 200-400°C range, making it relatively safe. Why "relatively"? Because while the heat generation at the cathode is gone, the heat generation at the anode and the oxidative decomposition of the electrolyte still exist, hence the term "relatively safe." However, for ternary materials, even within the 300°C range (and varying depending on the specific materials used), there is significant exothermic decomposition, generating a large amount of heat.
As shown in the diagram on the right (PPT), with increasing nickel content, the thermal decomposition temperature of high-nickel ternary cathodes decreases while the heat release increases. For example, when the nickel content reaches 0.8%, as you can see in 811, thermal decomposition begins at around 120 degrees Celsius. We generally believe that the first reaction to thermal runaway should be thermal runaway at the negative electrode, but for ternary cathodes, it's possible that thermal runaway occurs first at the positive electrode, followed by thermal runaway at the negative electrode. Solving this problem will be much more difficult for ternary cathodes.
We say that short circuits and overcharging are the causes of battery temperature rise, ultimately leading to negative reactions. What factors lead to short circuits and overcharging? You might think, "Just prevent short circuits and overcharging, right?" But in reality, we can't do that. Here, I'd like to introduce three aspects: process factors, material factors, and application processes. For example, we all know that in batteries, there's a layer of conductive dust on the separator and electrode surfaces. Misalignment between the positive and negative electrodes, even with electrode burrs, uneven electrolyte distribution leading to localized lithium plating, etc., can all cause overcharging. Additionally, if the positive electrode material contains certain metallic impurities, during charging, these impurities will oxidize and dissolve the positive electrode. The metal ions dissolved in the electrolyte will immediately be reduced at the negative electrode, depositing on its surface and causing a short circuit. You might say that during assembly, we screen batteries and eliminate short-circuited batteries, so this problem is impossible. Yes, that's for batteries with obvious short-circuit characteristics, which we can control. However, if these conditions cause a short circuit but don't manifest, it's a potential short-circuit hazard without exhibiting any short-circuit characteristics. We know that batteries expand and contract during charging and discharging. For example, when charging, the negative electrode changes volume, increasing in size, and the battery expands. When discharging, the negative electrode decreases in size, and the battery contracts. During this repeated expansion and contraction, previously non-short-circuited sites may become short-circuited, leading to these phenomena. Sometimes, during analysis, we find that a car, even after being fully charged, catches fire several hours later; this could be due to this very reason.
Furthermore, overcharging, especially excessive charging, can lead to localized overcharging because the surface current distribution of our electrodes is uneven. The higher the current, the greater the unevenness, potentially causing localized overcharging. Even with coatings, uneven electrolyte distribution and uneven electrode spacing can cause uneven current distribution and localized overcharging. Additionally, if the positive electrode's performance degrades too quickly during cycling, coupled with reduced capacity, it can also lead to overcharging. From a management perspective, BMS malfunctions, battery management system crashes, or charging relay failures can all cause overcharging. Therefore, it's difficult to avoid these abuse conditions during application, and battery safety hazards are inevitable. The safety of a single cell is relative, not absolute. Where should we focus our efforts? Besides improving battery safety, we should focus on the work currently being done by Professor Ouyang and his team—conducting more work at the system level. How to prevent catastrophic performance-related accidents caused by thermal runaway in a single cell is the work we need to do at the system level.
Here I outline the development process of a safety incident. From a system perspective, thermal runaway in a battery is not a rapid process; it's a gradual one. First, a single cell experiences thermal runaway due to a short circuit, leading to thermal runaway of the entire module. This module runaway then triggers thermal runaway in critical modules, and so on, eventually causing thermal runaway of the entire system. It's a continuous process. Therefore, to improve battery safety, we should work on three levels: materials, individual cells, and the system. At the materials level, we need to focus on improving the thermal stability of materials and interfaces to reduce heat generation. At the individual cell level, besides optimizing battery thermal design, it's crucial to develop thermal protection technologies such as PTC electrodes and thermally shut-off separators. At the system level, Professor Ouyang and his team are focusing on thermal insulation design to prevent thermal expansion. Overall, I believe materials are fundamental, individual cells are key, and the system is the final guarantee of safety.
I will now elaborate on how to improve battery safety. First, regarding improving the thermal stability of materials and interfaces, we have several solutions. The first is surface coating. We know that the thermal decomposition of the positive electrode and the resulting oxygen evolution and interfacial reactions significantly affect battery safety. We can coat the positive electrode active surface with a thermally stable protective layer. The idea that Professor Wang Chaoyang just mentioned is essentially this. By coating an active or high-nickel positive electrode surface with a relatively low-energy, but also low-activity, active material, we can reduce the direct contact between the highly active material and the electrolyte, even without affecting the material's thermal stability, thus reducing negative reactions. This is very effective. These coatings include phosphates, oxides, fluorides, and polymers. Here are two examples similar to what Professor Wang Chaoyang just mentioned. For instance, after coating the surface with a phosphate film or lithium phosphate, the decomposition temperature and heat generation of the ternary material are reduced.
Another aspect is the material we need to construct with a concentration gradient. We know that high-nickel cathodes are unsafe, not only because of their poor thermal stability, but more importantly because nickel has a very strong catalytic effect on the oxidative decomposition of the electrolyte. Therefore, the higher the nickel content, the more oxidative decomposition and interfacial reactions occur in the electrolyte. As we know from safety studies, the heat release of the material itself isn't that large, but with the addition of the electrolyte, its heat generation temperature and heat output increase dramatically. Why? Often, the interfacial reactions of the electrolyte account for a significant portion. If we use high-nickel as the core and some low-nickel materials as the shell, creating a concentration gradient between the inside and outside, this helps reduce the reactivity of the material interface and improves battery safety.
Third, the stability of the SEI film is crucial to battery safety. If we can employ methods to increase the decomposition temperature and thermal stability of the SEI film, it will play a vital role in battery safety. Current research shows that some organic lipids, some organophosphates, and even some fluorinated lithium salts can effectively improve the thermal stability and decomposition temperature of the negative electrode SEI film; examples will not be detailed here.
At the individual cell level, how do we proceed? Besides the usual thermal safety design and optimization, more importantly, we need to establish self-excited thermal protection for each cell. What does this mean? It means enabling the cell to adjust its current or power output based on its sensed temperature. The technical principle is to use temperature-sensitive materials to cut off electron or ion transport on the electrodes at dangerous temperatures, even shutting down the battery reaction. When a battery reaction stops, its heat generation naturally ceases. For example, we know of a material called PTC (Potentially Transmitted Tolerant). This material is characterized by changing from a highly conductive state to an insulating state as the temperature rises to a certain level. If we use materials with PTC characteristics as a base layer for the electrode fluid, a conductive agent for the electrode, or a surface modification layer for the active material, then when a short circuit causes the battery temperature to rise, the PTC material will rapidly increase in temperature. This will significantly reduce the temperature of the electron materials on the electrodes, even cutting off electron transport and preventing the battery temperature from rising further.
Additionally, we can achieve ion transport. For example, we can modify the surface of the electrode with an oligosaccharide or monomer. Under normal conditions, these electrodes are porous, allowing electrolyte ions to pass through. At high temperatures, the oligosaccharide can polymerize to form a polymer film, which cuts off the contact between the active material and the electrolyte, shutting down the battery reaction. I'll give a few examples below. For instance, with our PTC electrode based on a conductive polymer-carbon black composite conductive agent, we can see that at 120 degrees Celsius, the battery essentially cannot discharge. However, this conductive agent significantly improves the safety of the ternary material under overcharge and short-circuit conditions.
Another approach we developed involves a microsphere-modified separator. We modify the surface of a conventional separator with a layer of fusible microspheres. At room temperature, these microspheres allow ions to pass through. However, as the temperature rises, the microspheres melt, sealing the pores in the separator and cutting off ion transport between positive and negative charges, thus shutting down the battery reaction. Our research shows that when used with ternary lithium batteries, it exhibits significant advantages over a blank battery in short-circuit tests, crush tests, and overcharge tests.
Furthermore, we can also use the modified separator in the high-nickel 811 system. We compared it with a conventional separator and found that its normal cycle performance was unaffected. However, during overcharge, the battery with the modified separator reached twice the voltage, and its surface temperature decreased by 25 degrees Celsius. During a short circuit, the modified separator battery performed significantly worse than the reference battery. During nail penetration, the reference battery experienced thermal runaway, while the battery with the modified separator did not.
What do these examples illustrate? They show that we do have solutions; with enough effort, we can address battery safety issues. We acknowledge that batteries do have safety concerns, but we are not without options. Theoretically and technically, we have many avenues to address these issues. However, these methods currently face some challenges in practical application. For instance, when modifying the separator, from a safety perspective, we aim for a higher melting point, ideally 70 or 80 degrees Celsius, even though normal batteries don't reach that temperature. However, batteries have very strict requirements for water content control; our battery cells require a drying temperature of 110 degrees Celsius. This creates a technological contradiction. How do we resolve these contradictions? This is a problem that needs to be addressed in the development of this technology.
In conclusion, I believe that by exploring some new approaches, it is possible to solve the battery safety issue.
Finally, I would like to summarize and look ahead at security issues based on my understanding.
First, safety issues have become increasingly serious as battery energy density increases, but this should not negate the technological route and development trend of power batteries.
Second, battery safety is a serious technical challenge, but it is controllable and preventable. Correctly addressing and actively exploring new safety technologies will help promote the progress of battery technology.
Third, improving the thermal stability of materials/interfaces, developing cell self-excited thermal protection technology, and system thermal expansion prevention technology can effectively improve the safety of battery systems, and further research is needed!
