Car 18650 battery supplier to share with you: Lithium-ion battery is 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.
Lithium Ion Battery
While enjoying the convenience brought by the high temperature resistance lithium ion battery, several incidents of fire and explosion of lithium ion batteries of Samsung mobile phones 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”. 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. 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. The 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 the precipitation of 18650 cylindrical battery. At low temperature, the lithium insertion kinetics of the negative electrode becomes worse, the specific capacity of the negative electrode decreases, and it is easy to form on the surface of the negative electrode under a large charging current. Lithium plating, and even lithium dendrites, it is necessary to make a detailed study on the characteristics and mechanism of lithium precipitation of lithium ion batteries at low temperatures. Studies have shown that the amount of lithium evolution will increase significantly when the charging rate exceeds C / 2. For example, at C / 2, the surface of the negative electrode is about 5.5% of the total charging capacity. At 1C, then Reached 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.
Battery charging capacity at different rates. With the increase of the charging current, the temperature of the battery gradually increases during the charging process, which has a certain impact on the accuracy of the low temperature performance measurement of the battery, but it is difficult to avoid the phenomenon due to the thermal conductivity of the 18650 battery.
After charging at a rate of 1C, 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. Only a part of lithium is embedded in the crystal structure of graphite, and another part is lithium. The form of lithium precipitated, which indicates that the amount of lithium analyzed is relatively small and cannot be detected by means of neutron diffraction.
After charging, the battery needs to stand for 4 hours. The neutron diffraction test is performed on the battery after standing. After 4 hours of standing, the intensity of the diffraction peak of LiC6 is significantly increased, while the intensity of the diffraction peak of LiC12 is significantly decreased This change is more significant especially for batteries charged at 1C rate, which is mainly due to the "rebalance" of lithium concentration between the various parts inside the anode. 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.
A voltage platform appeared during the voltage standstill. For a C / 2-charged battery, the length of the voltage platform is 2h, and for a 1C-charged battery, the length of the voltage platform is 3h. Neutron
Increasing the charging rate The amount of lithium evolution of the battery gradually increases, especially after the rate exceeds C / 2, the amount of lithium evolution of the battery has increased significantly, but it should be noted that even at a small rate of C / 2, it still appears The amount of lithium evolution is about 3%.
During the charging process of lithium-ion batteries, the chemical reaction history of Li + embedded in the negative electrode and the reaction characteristics of negative electrode lithium precipitation at low temperature provide important clues for studying the decay mechanism of lithium-ion batteries at low temperature. Management strategies at low temperatures provide useful lessons.