Lithium-ion batteries are currently the most common chemical energy storage power source. From mobile phones to laptops to wearable mobile devices, all rely on lithium-ion batteries to provide energy. While enjoying the convenience brought by lithium-ion batteries, several Samsung mobile phone lithium-ion battery fires and explosions have caused us to pay attention to the safety of lithium-ion batteries. There are many factors that cause the safety risk of lithium-ion batteries. Generally speaking, they are divided into two parts: "internal factors" and "external factors". "External factors" are mainly the battery is exposed to external forces, causing deformation and other risks, causing internal positive A short circuit occurred between the negative electrodes, resulting in fire and explosion. "Internal factors" are mainly internal defects caused by factors such as design and processing, such as excess internal electrodes, negative electrode analysis, and other factors, which cause internal battery short circuits and cause battery safety risks.
Among them, negative electrode lithium precipitation is an important factor that causes frequent accidents in lithium-ion batteries. There are many factors that cause negative lithium evolution in lithium ion batteries. For example, the design of the positive and negative electrode redundancy is insufficient, the battery is charged at low temperature, and the charging current is too large. It may cause lithium precipitation in the negative electrode, which will not only lead to less lithium resources available for lithium-ion batteries, decrease in capacity, but also form lithium dendrites in the negative electrode. Lithium dendrites continue to grow with the cycle of lithium-ion batteries. Eventually it will penetrate the diaphragm and cause a short circuit between the positive and negative electrodes. Therefore, how to avoid negative electrode lithium evolution is a key issue in the design process of lithium-ion batteries. Today, I will bring you friends to discuss the conditions and mechanism of lithium precipitation of lithium ion battery negative electrode.
Low temperature is an important factor that induces lithium precipitation in lithium-ion batteries. Under low temperature conditions, the lithium insertion kinetics of the negative electrode becomes worse, the specific capacity of the negative electrode decreases, and it is easy to form a lithium plating layer or even lithium branches on the surface of the negative electrode under a large charging current. Therefore, it is necessary to make a detailed study on the characteristics and mechanism of lithium precipitation of lithium ion batteries at low temperatures. Christianvon Luders and others from the Technical University of Munich, Germany, studied the characteristics and mechanism of lithium evolution of commercial 18650 lithium-ion batteries at -2 ° C by means of static voltage and neutron diffraction. Studies have shown that the charge rate exceeds C / 2. Under the circumstances, the amount of lithium precipitation will be significantly increased. For example, in the case of C / 2, the lithium plating on the surface of the negative electrode 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 in the graphite structure depends on the number of lithium coatings, and revealed that the standing voltage is closely related to the amount of lithium evolution.
In the experiment ChristianvonLuders used an 18650 battery, with a positive electrode of NCM111 and a negative electrode of graphite. At a C / 20 rate of -2 ° C, the battery is limited by the electrolyte diffusion conditions and the dynamic conditions of the positive and negative electrode active materials. It can only display about 87% of the capacity at 25 ° C, which is about 1682.21mAh. The following table is the battery charge capacity at -2 ℃ at different rates. From the data we can notice that with the increase of the charging current, the temperature of the battery during the charging process gradually increases, which has a certain impact on the accuracy of the low temperature performance measurement of the battery, but is limited by the thermal conductivity of the 18650 battery. The phenomenon is unavoidable.
The neutron diffraction data clearly reveals the process of Li + embedded in the graphite structure of the anode. At a C / 20 charge rate, Li + first reacts with graphite to form LiC12. When the battery charge capacity reaches 1009mAh (about 50% SoC), it begins to appear. The diffraction peak of LiC6, when the battery is charged to 1687mAh, the intensity of the diffraction peak of LiC6 greatly increases, exceeding the intensity of the diffraction peak of LiC12. In contrast, after charging at 1C rate, the intensity of the diffraction peak of LiC6 is lower than that of LiC12, which indicates that Li + is not 100% converted in the graphite structure, and only a part of lithium is embedded in the crystal structure of graphite. Another part of lithium was precipitated as metallic lithium, but no electric lithium diffraction peak was seen on the diffraction curve, which indicates that the amount of lithium analyzed in this part is relatively small and cannot be detected by means of neutron diffraction.
After the charging is completed, the battery needs to stand for 4 hours. The neutron diffraction test is performed on the battery after standing. The specific results are shown in the following figure. As can be seen from the curve, the diffraction of LiC6 after 4 hours of standing The peak intensity is significantly enhanced, and the intensity of the diffraction peak of LiC12 is significantly reduced, especially for the battery charged at 1C rate. This change is more significant, which is mainly due to the "rebalance" of lithium concentration between the internal parts of the negative electrode. However, compared with the C / 20 rate battery, the peak of LiC6 in the 1C rate battery is significantly lower, which indicates that part of the lithium deposited on the negative electrode surface is irreversible.
In addition to neutron diffraction, ChristianvonLuders also tested the battery voltage curve during the battery standstill, as shown in the figure below. As can be seen from the figure, a battery with a charge rate of more than C / 2 appeared in the voltage standstill process. For voltage platform, the length of this voltage platform is 2h for C / 2 charged batteries, and for battery charged at 1C, the length of this voltage platform is 3h. According to the neutron diffraction data, it can be known that the voltage platform mainly corresponds to the process in which the precipitated lithium is re-embedded into the graphite crystal structure.
The amount of lithium evolution caused by different rates is shown in the figure below. It can be seen from the figure that the amount of lithium evolution of the battery gradually increases with the increase of the charging rate, especially after the rate exceeds C / 2, the lithium evolution of the battery The amount of lithium has increased significantly, but it should be noted that even at a small rate of C / 20, about 3% of the amount of lithium evolution still occurs.
The work of ChristianvonLuders reveals the chemical reaction history of Li + embedded in the negative electrode during the charging process of lithium ion batteries, and the reaction characteristics of negative electrode lithium evolution at low temperature, which provides important clues for studying the decay mechanism of lithium ion batteries at low temperature. It provides a useful reference for the management strategy of lithium-ion battery packs at low temperatures.