For lithium-ion batteries, thermal runaway is the most serious safety incident. Thermal runaway of lithium-ion batteries is caused by the heat generation rate being much higher than the heat dissipation rate. A large amount of heat accumulates inside the lithium-ion battery, causing the temperature of the lithium-ion battery to rise rapidly, causing the separator to shrink and melt, and the positive and negative active materials to decompose spontaneously. The exothermic reaction causes lithium-ion batteries to catch fire and explode. Avoiding thermal runaway is the ultimate goal pursued by countless lithium-ion battery designers. However, to achieve this goal, we first need to have a clear and comprehensive understanding of the reaction process of the thermal runaway process of lithium-ion batteries. However, the sealing of lithium-ion batteries The structure is the first hurdle that hinders us. The fully sealed structural design makes it very difficult to observe the internal reactions of lithium-ion batteries. Secondly, the high temperature in thermal runaway is the second hurdle that hinders us. The high temperature of thousands of degrees will burn out any remaining evidence. Finally, the high speed of thermal runaway is the third hurdle that hinders us. The time for lithium ions to explode in thermal runaway is often less than 0.01s, which also makes it particularly difficult to track the reaction process. The first level: sealing First let’s look at the first level. Lithium-ion batteries generally adopt square or cylindrical hard-shell sealing structures or soft-pack sealing structures. The common feature is that it is difficult for external detection equipment to enter the interior of the lithium-ion battery. Therefore, in order to track the internal reactions of lithium-ion batteries during thermal runaway, this problem must first be solved. There are two ideas to solve this problem: 1. Internal implantation. We can place a thermocouple inside the lithium-ion battery to track the internal temperature changes of the lithium-ion battery in real-time during thermal runaway. We can also place an FBG fiber detector inside the lithium-ion battery to monitor the temperature of the lithium-ion battery during thermal runaway. Tracking and analyzing changes in temperature and pressure, which is also a widely used method at present.

　　2. Perspective technology. Although the sealed structure of lithium-ion batteries can block visible light, it cannot block high-energy ray technology. Therefore, using high-energy rays and particles to track the behavior of lithium-ion batteries in thermal runaway is also a very effective method. For example, we have previously Donal P. Finegan and others from City College London used high-speed X-ray photography to comprehensively track the internal reaction process of lithium-ion batteries during thermal runaway, revealing the explosion-proof valve of the 18650 battery during thermal runaway. working principle. Neutrons are uncharged, so their penetrating ability is very strong. In recent years, they have been widely used to study the internal reaction mechanisms of lithium-ion batteries. For example, engineers from the German Bosch Company used neutron diffraction technology to analyze the electrolyte in lithium-ion batteries. The internal wetting process has been tracked and studied. Through neutron diffraction technology, we can "directly see" the wetting process of electrolyte in lithium-ion battery cells. Therefore, neutron diffraction technology also has great potential to be applied to lithium-ion batteries. in the study of thermal runaway processes.

　　The second level: When the high-temperature lithium-ion battery is thermally out of control, a large amount of chemical energy stored inside the lithium-ion battery is released in a short period of time. The heat generation rate is much higher than the heat dissipation rate of the lithium-ion battery, which causes the lithium-ion battery to The temperature rises rapidly in a short period of time. Research shows that the temperature of lithium-ion batteries can reach more than 1000°C during thermal runaway, which will even melt the copper foil inside the lithium-ion battery (the melting point of Cu is 1085°C). The extreme high temperature will destroy all All possible remaining evidence was burned, so it is difficult to infer the cause of thermal runaway from the remains of lithium-ion batteries after thermal runaway. Extremely rapid cooling can effectively solve this problem. For example, we have previously reported that Professor Ouyang Minggao of Tsinghua University put the battery in thermal runaway into liquid nitrogen to quickly cool down the lithium-ion battery, thereby achieving the "first The evidence of "Crime Scene" was fixed, which also helped Professor Ouyang discover that even if the separator does not melt and shrink, the lithium-ion battery may still cause thermal runaway of the lithium-ion battery through the "shuttle" of O2 between the positive and negative electrodes. , which also opens a new door for the study of thermal runaway.

　　The third level: High-speed lithium-ion batteries react very quickly in thermal runaway. Especially when an explosion occurs due to thermal runaway, the reaction time is often less than 0.01s. This also results in most methods often causing inaccuracies in observation due to insufficient time accuracy. Accurately, in order to solve this problem, X-ray high-speed photography technology has made a grand appearance. In order to study the internal reaction process of lithium-ion batteries in acupuncture experiments, Donal P. Finegan and others used X-ray high-speed photography technology with a frame rate of 2000fps. (resolution 10um) and 5130fps (resolution 20um). At such a high frame rate, we can basically clearly see the entire reaction process of thermal runaway, but even at such a high frame rate, it is still difficult to capture the lithium-ion battery. The reaction process at the moment of explosion (the time is often less than 0.01s) requires the introduction of synchrotron radiation technology. The intensity of the synchrotron radiation light source is much higher than that of ordinary X-rays, so it can achieve shorter exposure times. The European Synchrotron Radiation Center uses the synchrotron radiation light source to achieve an astonishing frame rate of millions of frames per second. It provides advanced observations of processes such as glass breaking and arcing. With the help of synchrotron radiation technology, Donal P. Finegan successfully observed the moment when the 18650 battery exploded. The X-ray shooting speed reached 20272 frames/second. He successfully observed how the explosion-proof valve of the 18650 battery acted during the explosion. work.

　　Thermal runaway of lithium-ion batteries seriously threatens the life and property safety of users. Therefore, the study of the mechanism of thermal runaway is particularly important. In the past, due to limitations of experimental conditions, we could only indirectly measure the changes in shell temperature and battery voltage. to infer some reactions inside lithium-ion batteries. The advancement of technology not only allows us to detect changes in the internal temperature and pressure of lithium-ion batteries in real time, but also the blessing of high-speed photography technology allows us to intuitively observe the high-speed reaction process inside the lithium-ion battery during thermal runaway, which is very important for Our understanding of the reaction mechanism of lithium-ion batteries is of great significance and provides guidance for the design of safer lithium-ion batteries.

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