What are the factors that affect the formation of 24V50AH lithi

  • Introduction: 

    During the formation of the 24V50AH lithium battery, the SEI film on the graphite surface of the negative electrode is in the initial stage of formation, and the inorganic lithium salts {Li2O, LiX (X=F, Cl, etc.)} near the graphite surface are mainly composed of inorganic lithium salts. The electrolyte and high-temperature performance are more stable, while the outer layer of the 24V50AH lithium battery SEI film is mainly composed of organic lithium salts (ROCO2Li, Roli, etc.), which is loose in structure and unstable in performance. Next, Chinese 24V50AH lithium battery manufacturers will introduce the three major factors affecting the formation process of 24V50AH lithium batteries: charging cut-off voltage, pressure, and temperature.

    1. Analysis of the effect of the formation charging cut-off voltage on the performance of the 24V50AH lithium battery

    1.1 The influence of cobalt acid 24V50AH lithium battery into a cut-off voltage on the performance of 24V50AH lithium battery

    For lithium cobalt oxide (LiCoO2) as the positive active material and graphite as the negative active material, a 24V50AH lithium battery system was formed. By adjusting the formation charge cut-off voltage, the influence of the formation process on the performance of the 24V50AH lithium battery was investigated. The comparison experiment process is as follows:

    1. Assemble the 683064 soft pack 24V50AH lithium battery through the steps of mixing, coating, drying, assembling, liquid injection, standing, forming, and dividing.

    2. The 24V50AH lithium battery is divided into two groups, A. The cut-off voltage of group A is 3.8V, and the cut-off voltage of group B is 3.7V.

    3. Comparative analysis of the capacity, first efficiency, energy density, internal resistance, rate, impedance, high-temperature storage, and cycle performance of the 24V50AH lithium battery.

    Basic performance comparison

    The performance comparison results are shown in the following table. The capacity, specific capacity, first efficiency, energy density, and gas production deformation rate of the 24V50AH lithium battery formed by the low cut-off voltage are all better than those of the high cut-off voltage because the two schemes A/B The specific capacity of the positive electrode is the same. The difference between the full 24V50AH lithium battery mainly comes from the change of the negative electrode. The active lithium ions consumed by the B scheme are less than that of the A scheme, which makes the first efficiency and capacity higher, and the overall thickness is smaller.

    Performance comparison of two 24V50AH lithium batteries with cut-off voltage

    EIS comparison

    The difference in the internal resistance properties of the cells mainly comes from the membrane impedance and the ion transfer impedance. Because the thickness of the SEI film under the B process condition is relatively thin, the membrane impedance and ion transfer impedance are both lower than those of the A process condition. 

    24V50AH lithium battery

    1.2 The influence of the formation depth of iron phosphate 24V50AH lithium battery on the performance of 24V50AH lithium battery

    The olivine-type LiFePO4 is used as the positive electrode material, graphite is used as the negative electrode material, and the ternary electrolyte is ethylene carbonate (EC), diethyl carbonate (DEC), and methyl ethyl carbonate (EMC) in a volume ratio of 1:1:1 The ratio, the LiPF6 supporting electrolyte is 1mol/L, and the 1865140 square power 24V50AH lithium battery with a rated capacity of 10Ah is assembled. The formation depth was controlled by adjusting the formation time, and the electrical properties of the 24V50AH lithium battery were tracked and tested to analyze the influence of different formation depths on the electrical performance of the 24V50AH lithium battery.

    By adjusting the formation and charging time, the formation depth of the 24V50AH lithium battery of Group I~IV is controlled to be 40%, 60%, 80%, and 100% of the rated capacity, respectively.

    24V50AH lithium battery internal resistance influence

    The internal resistance of the 24V50AH lithium battery changes with the deepening of the formation depth. After the formation of the 24V50AH lithium battery in each group, the internal resistance tends to decrease. The 24V50AH lithium battery with a shallow formation depth is not fully activated, and the electrolyte and electrodes of the 24V50AH lithium battery are not fully activated. The material reaction is not sufficient, a complete and dense SEI film has not been formed on the surface of the 24V50AH lithium battery electrode, and the SEI film has a large impedance.

    1 C charge-discharge cycle life curve of each group of 24V50AH lithium batteries in a constant temperature environment of 45 °C: The cycle life of the 24V50AH lithium battery in groups II to IV is basically the same as the rate of change in the number of charges and discharges. After 100 cycles, the cycle life of the 24V50AH lithium battery in group IV The capacity retention rate is the highest at 99.22%. The capacity decay rate of group I 24V50AH lithium battery is relatively fast, especially after the 30th cycle, the capacity of group I 24V50AH lithium battery has dropped significantly. The rate is only 97.51%. It shows that when the formation depth reaches more than 60% of the rated capacity, continuing to deepen the formation depth has little effect on the high-temperature cycle performance of the 24V50AH lithium battery.

    2. The effect of pressure on the formation process of 24V50AH lithium battery

    The pressurization mentioned here refers to soft-packed cells, and other types of cells are not discussed. Since the soft-packed 24V50AH lithium battery adopts an aluminum-plastic packaging structure, its external structure determines that the pole pieces cannot be closely arranged. It is easy to create gaps between the 24V50AH lithium battery and the gas generated during the formation of the 24V50AH lithium battery is also easy to remain between the pole pieces. The gas cannot be completely discharged in the subsequent Degas seal, thus affecting the performance of the 24V50AH lithium battery. Therefore, consider charging twice during the formation. A rolling process is used to remove the gas between the pole pieces.

    The comparison system is a lithium iron phosphate soft-packed battery, the diaphragm of a 24V50AH lithium battery is a 25μm polypropylene porous single-layer diaphragm, and the electrolyte is 1mol/L LiPF6/(EC+EMC+DMC) (volume ratio 1:1:1), divided into two groups A/B, in which the cells of group A are not pressurized and exhausted during the formation process, and the cells of group B are added to the process of pressurization and exhaust during the two charging processes of formation, and the pressure of group B is respectively B1 (pressure value is 4-6N), B2 (pressure value is 20-30 N), B3 (pressure value is 50-60 N), compare the performance of A/B two groups of cells after formation.

    The capacity of the cells after formation is compared, and the comparison results are shown in the following table. The capacity of group A (without the pressurization process) is smaller than that of group B (with the pressurization process), and the capacity of group B cells increases with the increase of pressure. Increase.

    Capacity comparison results of pressurized and unpressurized cells

    24V50AH lithium battery

    The main reason for this is that the 24V50AH lithium battery will generate a large amount of gas during the formation process, and due to the structure of the soft-pack battery itself, the gas generated during the formation process will increase the distance between the positive electrode, the diaphragm and the negative electrode, hindering lithium The transmission of ions from the positive electrode sheet to the negative electrode sheet, in addition, the presence of gas will also hinder the contact of the electrolyte with the positive electrode and the negative electrode, which makes the local wetting performance of the negative electrode worse, and finally leads to a large number of unreacted dead zones on the negative electrode sheet. The appearance of death will increase the possibility of lithium precipitation of the 24V50AH lithium battery, and will seriously affect the performance of the 24V50AH lithium battery itself.

    Dismantling diagram of the battery cell when the formation process is not pressurized or the pressure is less than the process requirements

    3. The effect of temperature on the formation of 24V50AH lithium battery

    In the process of formation, applying high temperature can reduce the viscosity of the electrolyte, accelerate the diffusion of ions, and ensure that electrons and ions are rapidly combined under high current; formation at a high temperature can not only make the SEI film on the surface of the electrode react more fully but also It can enhance the liquid absorption of the diaphragm, which is beneficial to reduce the inflation of the 24V50AH lithium battery.

    However, the high-temperature formation will reduce the stability of the SEI film, resulting in poor cycle performance of the 24V50AH lithium battery. This is because high temperatures will aggravate the dissolution of the SEI film and the co-insertion of solvent molecules, while the SEI film tends to be stable at low temperatures. Studies have shown that the formation at low temperatures is mainly based on solvent reduction, the reduction rate of lithium salts is slow, and the formation rate of SEI film is slow, so the deposition of solvent products is more orderly and dense, which is more conducive to prolonging the service life of 24V50AH lithium battery. In this paper, the flexible packaging 24V50AH lithium battery of lithium cobalt oxide-graphite system is taken as the research object, and the effect of temperature on the formation effect in the high-temperature pressure formation process is studied. The chemical composition process is divided into 4 cases, the temperature is 50℃, 60℃, 80℃ and the temperature change, the temperature change means that the temperature of the two charging processes is different, the first charge stage is 40℃, the second charge The temperature during the charging phase was 70°C.

    The performance of the first 350 cycles is similar, and the initial capacity can reach 3000 mAh, but after 400 cycles, the capacity retention rate begins to differ. The rates are 90.57%, 90.63%, 93.44%, and 92.02%, respectively, indicating that the cycle performance of the 24V50AH lithium battery turned into a 24V50AH lithium battery at a high temperature of 80°C is better than the 24V50AH lithium battery of other solutions. The thicker SEI film is beneficial to the cycle stability of the 24V50AH lithium battery.

    For the comparison of high-temperature formation and low-temperature formation, there is no exact result and it can be said that high-temperature formation is definitely good, or low-temperature formation is certain. The choice of temperature in the formation process should be made according to different 24V50AH lithium battery systems and different manufacturing processes. The specific experimental verification, select the suitable formation process parameters for the 24V50AH lithium battery itself.

    Well, the above is the whole content of the 24V50AH lithium battery brought to you by Microcharge today. I hope this article is helpful to you. For more information on lithium batteries, please refer to the following:

    Lithium battery overcharge mechanism and overcharge prevention measures

    The effect of fast charging on the positive electrode of lithium battery