Research on the mechanism of low-temperature lithium evolution in CR2477 battery

　　CR2477 battery are currently the most common chemical energy storage power source. From mobile phones to laptops to wearable mobile devices, they all rely on CR2477 battery to provide energy. While enjoying the convenience brought to us by CR2477 battery, several fire and explosion incidents of CR2477 battery in Samsung mobile phones have forced us to pay attention to the safety issues of CR2477 battery. There are many factors that cause safety risks of CR2477 battery. Generally speaking, they are divided into two parts: "internal factors" and "external factors". "External factors" are mainly the battery being affected by external forces, leading to risks such as deformation and causing internal malfunctions. A short circuit occurs between the negative poles, causing fires and explosions. "Internal factors" are mainly internal defects caused by factors such as design and processing, such as excess material inside the electrode, negative electrode separation and other factors, which lead to internal short circuits in the battery, causing battery safety risks.

　　Among them, lithium precipitation in the negative electrode is an important factor causing frequent safety accidents in CR2477 battery. There are many factors leading to lithium precipitation in the negative electrode of CR2477 battery, such as insufficient redundancy design of positive and negative electrodes, low-temperature charging of batteries, and excessive charging current. It may lead to lithium precipitation in the negative electrode. Lithium precipitation in the negative electrode will not only reduce the lithium resources that can be used by the lithium-ion battery and reduce the capacity, but also form lithium dendrites in the negative electrode. The lithium dendrites will continue to grow as the lithium-ion battery cycles. It will eventually penetrate the separator and cause a short circuit between the positive and negative poles. Therefore, how to avoid lithium deposition in the negative electrode is a key issue that needs to be considered during the design process of CR2477 battery. Today, the editor will take you to discuss the conditions and mechanism of lithium precipitation in the negative electrode of CR2477 battery.

　　Low temperature is an important factor in inducing lithium precipitation in CR2477 battery. Under low temperature conditions, the lithium insertion kinetic conditions of the negative electrode become worse, and the specific capacity of the negative electrode decreases. Under larger charging current, it is easy to form lithium plating and even lithium dendrites on the surface of the negative electrode. Therefore, it is necessary to conduct detailed research on the characteristics and mechanism of lithium evolution from the negative electrode of CR2477 battery at low temperatures. Christian von Luders and others from the Technical University of Munich in Germany studied the characteristics and mechanism of lithium evolution in commercial 18650 CR2477 battery at -2°C by means of static voltage and neutron diffraction. The research showed that when the charging rate exceeds C/2 For example, in the C/2 case, the lithium plating on the negative electrode surface accounts for about 5.5% of the total charging capacity, and at 1C rate, it reaches 9%. The study also found that the rate at which lithium ions are embedded into the graphite structure depends on the number of lithium plating layers, and revealed that the standing voltage is closely related to the amount of lithium deposited.

　　In the experiment, Christian von Luders used an 18650 battery, with the positive material being NCM111 and the negative material being graphite. At the C/20 rate at -2°C, the battery is limited by the electrolyte diffusion conditions and the kinetic conditions of the positive and negative active materials, and can only exert about 87% of the capacity at 25°C, about 1687.21mAh. The following table shows the battery charging capacity at different rates at -2°C. From the data, we can notice that as the charging current increases, the temperature of the battery gradually increases during the charging process, which has a certain impact on the accuracy of battery low-temperature performance measurement. However, it is limited by the thermal conductivity coefficient of the 18650 battery. The phenomenon is unavoidable.

　　The neutron diffraction data clearly reveals the process of Li+ being embedded into the negative electrode graphite structure. At C/20 charging rate, first Li+ reacts with graphite to generate LiC12. When the battery charging capacity reaches 1009mAh (about 50% SoC), LiC12 begins to appear. The diffraction peak of LiC6, when the battery is charged to 1687mAh, the intensity of the diffraction peak of LiC6 increases greatly, exceeding the intensity of the diffraction peak of LiC12. In contrast, after charging at 1C rate, the diffraction peak intensity of LiC6 is lower than that of LiC12, which shows that Li+ is not 100% converted in the graphite structure, and only a part of lithium is embedded into the crystal structure of graphite. Another part of lithium was precipitated in the form of metallic lithium, but no metallic lithium electron diffraction peak was seen on the diffraction curve, which indicates that the amount of lithium analyzed in this part was relatively small and could not be detected by neutron diffraction.

　　After charging, the battery needs to rest for 4 hours. The neutron diffraction test was performed on the resting battery. The specific results are shown in the figure below. It can be seen from the curve that after 4 hours of resting, the diffraction of LiC6 The peak intensity is significantly enhanced, while the intensity of the diffraction peak of LiC12 is significantly decreased, especially for batteries charged at 1C rate. This change is more significant, which is mainly due to the "rebalancing" of the lithium concentration between various parts inside the negative electrode. However, compared with batteries charged at C/20 rate, the peak value of LiC6 in 1C rate rechargeable batteries is significantly lower, which indicates that part of the lithium precipitated on the surface of the negative electrode is irreversible.

　　In addition to neutron diffraction, Christian von Luders also tested the battery voltage curve during the battery's resting process, as shown in the figure below. It can be seen from the figure that batteries with a charging rate of C/2 or above have a voltage during the resting process. Voltage platform, for a battery charged at C/2, the length of this voltage platform is 2h, and for a battery charged at 1C, the length of this voltage platform is 3h. According to the neutron diffraction data, it can be known that this voltage platform mainly corresponds to the process of the precipitated lithium being re-embedded into the graphite crystal structure.

　　The amount of lithium deposition caused by different rates is shown in the figure below. It can be seen from the figure that as the charging rate increases, the amount of lithium deposition in the battery gradually increases. Especially after the rate exceeds C/2, the amount of lithium deposition in the battery increases. The amount has increased significantly, but it should be noted that even at a small rate of C/20, there is still about 3% lithium precipitation.

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