Research on the diffusion of thermal runaway in LR03 battery

　　Thermal runaway is one of the most serious safety issues of lithium-ion batteries. Thermal runaway can cause lithium-ion batteries to catch fire and explode, seriously threatening the lives and property safety of users. If there is anything more terrifying than thermal runaway of lithium-ion batteries, it must be the spread of thermal runaway within the battery pack. Usually LR03 battery are composed of hundreds to thousands of single lithium-ion batteries. Although a single lithium The risk of thermal runaway in ion batteries is relatively small, but in the face of large numbers, the risk of thermal runaway will greatly increase. Once one of the batteries suffers from thermal runaway, there is a high probability that it will trigger the thermal runaway of adjacent batteries, leading to The spread of thermal runaway within the battery pack poses a huge threat to the safety of electric vehicles. Therefore, people who make batteries often say that it is not terrible if one battery suffers from thermal runaway, but what is terrible is that a group of batteries suffers from thermal runaway.

　　Recently, Ahmed O. Said (first author) and Stanislav I. Stoliarov (corresponding author) of the University of Maryland, USA, studied the thermal runaway diffusion of the LCO/graphite system 18650 battery pack in N2 and atmospheric atmospheres. The research showed that in Due to the combustion reaction of combustible materials in the atmospheric atmosphere, the thermal runaway diffusion rate of the battery pack will be greatly accelerated.

　　In the experiment, the author used LCO/graphite material 18650 batteries as the research object, with a rated capacity of 2600mAh. 12 or 18 18650 batteries were fixed together using the stainless steel structure shown in the picture below. The individual cells were closely connected, but there was no busbar. links. In order to trigger the thermal runaway of one of the batteries, the author used the heating device shown in the figure below to heat the battery in the middle of the first row, triggering its thermal runaway.

　　The layout of 12 or 18 18650 batteries used in the experiment is shown in the figure below. The layout of 18 batteries was only tested in N2 atmosphere for thermal runaway diffusion, while the layout of 12 batteries was tested in air and N2 atmosphere respectively. .

　　In order to facilitate the control of the environmental conditions and boundary conditions of the battery module, the author places the above-mentioned 12 or 18 battery modules in a small wind tunnel as shown in the figure below. This wind tunnel can control the atmospheric atmosphere (N2 or air) in which the battery is located. , as well as parameters such as air flow rate.

　　When thermal runaway and pressure relief occur in lithium-ion batteries, some of the active materials, foils and other components in the lithium-ion battery will usually be taken out, causing the weight of the lithium-ion battery to change. Here, the author analyzes the lithium-ion battery before and after each thermal runaway. The change in weight and the total time of thermal runaway of the lithium-ion battery are used to calculate the active material loss rate of the lithium-ion battery, as shown in the following formula.

　　During thermal runaway, a variety of gases will be produced due to the decomposition of the electrolyte on the surfaces of the positive and negative electrodes. We can calculate the amount of different types of gases produced according to the following formula. For the combustible gas components produced during the thermal runaway process, such as CH4 and H2, the author also calculated the minimum combustion concentration of these combustible gases. The calculation shows that the minimum combustion concentrations of alkanes (such as CH4), CO and H2 in the air are respectively are 5%, 12.5% and 4%.

　　In the above experiments, although all lithium-ion batteries experienced thermal runaway, only a small number of batteries experienced shell cracking. Some lithium-ion batteries still maintained their original shape, and the batteries of other batteries were deformed (as follows) as shown in the figure). Under N2 atmosphere, about 15.6% and 14.6% of the batteries in the 18-cell and 12-cell battery packs had shell rupture. If in the air atmosphere, 23% of the batteries had shell rupture (12 battery packs). This is mainly due to Because the combustion reaction of lithium-ion batteries in an air atmosphere releases a large amount of heat.

　　During the thermal runaway process of lithium-ion batteries, a large amount of gas will be ejected and components such as active materials, electrolytes, and current collectors in the battery will be taken out, resulting in a reduction in the weight of the battery. The following table lists two types of The mass loss of the battery pack and the battery during thermal runaway in the two atmospheres. We can see that the size of the battery pack has no significant impact on the mass loss of the battery in thermal runaway, but the mass loss of the battery in the air atmosphere will be slight. Increase, which also shows that a single battery will eject more battery components during thermal runaway in an air atmosphere, thereby aggravating the spread of thermal runaway in lithium-ion batteries to a certain extent.

　　A large amount of heat will be generated during the thermal runaway process of lithium-ion batteries, which can be divided into two categories according to the source of heat:

　　1) Electrochemical heat. This part of heat mainly comes from the ohmic heat released by the internal short circuit of lithium ions, and the chemical heat released by the redox reaction between the active material and the electrolyte;

　　2) Combustion heat. This part is mainly caused by the battery ejecting a large amount of flammable gases and substances that react with O2 in the air during thermal runaway. The combustion reaction releases heat.

　　In the N2 atmosphere, since combustible gas cannot burn, the measured heat is the heat released by the electrochemical reaction. The figure below shows the thermal power change curves of 12 and 18 battery packs in N2. It can be seen from the figure The exothermic peak at the beginning was the exothermic process of the first thermal runaway battery, and then a large number of exothermic peaks began to appear in both battery pack sizes, which was a sign that thermal runaway began to spread within the battery pack. Based on the test data, the author calculated the average heat release of the lithium-ion battery during thermal runaway. In this experiment, the energy released by the LCO/graphite system 18650 battery was 56.6±2.5kJ/battery, 1.3±0.06kJ/g, 21.8±1.0 kJ/Ah, the ratio of the heat released during thermal runaway of a lithium-ion battery to its stored electrical energy is approximately 1.57, indicating that a considerable proportion of the heat released during thermal runaway comes from the redox reaction between the active material and the electrolyte.

　　If a lithium-ion battery undergoes thermal runaway in the atmosphere, the high-temperature combustible gas ejected from the lithium-ion battery will react with O2 in the atmosphere and release a large amount of heat. The picture below shows 12 battery packs experiencing thermal runaway in the air. From the figure, we can see that the thermal power when thermal runaway occurs in air is 4-5 times that of lithium-ion batteries when thermal runaway occurs in N2.

　　Ahmed O. Said's research shows that the atmosphere in which the battery is located will have a significant impact on the diffusion rate of thermal runaway in the lithium-ion battery pack. In the N2 atmosphere, the diffusion rate of thermal runaway in the battery pack is relative to the temperature, but in the air atmosphere Under the condition, the existence of combustion reaction will greatly increase the heat generation rate in the thermal runaway of lithium-ion batteries, so the diffusion rate of thermal runaway in the battery pack will also be greatly increased, seriously affecting the safety of the lithium-ion battery pack.

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