In the battery formation process of lithium-ion batteries, in the initial stage of the formation of the SEI film on the surface of the negative electrode graphite, there exists the inorganic lithium salts Li2O, LiX (X=F, Cl, etc.) The high battery temperature performance is more stable, while the outer layer of the SEI film mainly generates organic lithium salts (ROCO2Li, ROLi, etc.), which have a loose structure and unstable performance.

Influence of battery formation charging cut-off voltage on battery performance

The influence of battery formation cut-off voltage of lithium cobalt oxide battery

Lithium cobaltate (LiCoO2) is used as the positive electrode active material and graphite is used as the negative electrode active material to form a battery system. By adjusting battery formation charging cut-off voltage, the influence of battery formation process on battery performance is investigated. The comparison experiment process is as follows:

  1. Assemble the 683064 soft-pack lithium-ion battery through the steps of slurry mixing, coating, drying, assembly, liquid injection, standing, battery formation, and volume separation.
  2. The batteries are divided into two groups, A/B. 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 battery capacity, initial efficiency, energy density, internal resistance, rate, impedance, high temperature storage and cycle performance.
Influence of battery formation charging cut-off voltage on battery performance


Effect of battery internal resistance

With the deepening of battery formation depth, the internal resistance of each group of batteries shows a decreasing trend after battery formation. The electrodes of the battery with a shallow battery formation depth are not fully activated, and the reaction between the electrolyte and the electrode material is not sufficient. A complete and dense SEI film has not been formed on the surface of the electrode, and the SEI film has a relatively high resistance.

The impact of high temperature cycle life of the battery

When the depth of battery formation reaches more than 60% of the rated capacity, continuing to increase the depth of battery formation has little effect on the high-temperature cycle performance of the battery.

The impact of high temperature cycle life of the battery


The effect of pressure on the formation process

The pressurized battery formation mentioned here refers to the soft-packed batteries, and other types of batteries will not be discussed. Since the soft-packed battery adopts an aluminum-plastic packaging structure, its appearance structure determines that the pole pieces cannot be closely arranged, and the pole pieces are easily separated.

Voids are generated, and the gas generated during battery formation process is also likely to remain between the pole pieces. The gas cannot be completely discharged in the subsequent Degas sealing, which will affect the performance of the battery. Therefore, it is considered to adopt the rolling process between the two charges of battery formation. 

The comparison system is a lithium iron phosphate soft-packed battery cell, the battery separator is a 25 μm polypropylene porous single-layer separator, and the electrolyte is 1mol/L LiPF6/(EC+EMC+DMC) (volume ratio 1:1:1), divided into Group A/B, in which the cells of group A did not perform pressurization and exhaust during battery formation process, the cells of group B increased the process of pressurization and exhaust during the two charging processes of battery formation, and the pressurization of group B was 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 battery formation.

The main reason is that the lithium-ion battery will generate a large amount of gas during battery formation process, and due to the structure of the pouch cell itself, the gas generated during battery formation process will increase the distance between the positive electrode, separator, and negative electrode, and hinder lithium ions from the positive electrode. In addition, the presence of gas will hinder the contact between the electrolyte and the positive and negative electrodes, making the local wetting performance of the negative electrode worse, and eventually resulting in a large number of unreacted dead zones on the negative electrode.

The effect of temperature on the battery formation results of lithium ion battery


The effect of temperature on the battery formation results of lithium ion battery

During the battery formation process, applying high temperature can reduce the viscosity of the electrolyte, accelerate the diffusion of ions, and ensure the rapid combination of electrons and ions under high current. Battery formation at high temperature can not only make the SEI film on the electrode surface react more fully, but also It can enhance the liquid absorption of the separator, which is beneficial to reduce the gas swelling of the battery.

However, high-temperature formation will reduce the stability of the SEI film and cause poor cycle performance of the battery, because high temperature will intensify the dissolution of the SEI film and the co-intercalation of solvent molecules, while the SEI film tends to be stable at low temperature. Studies have shown that the battery formation at low temperature is mainly based on solvent reduction, the reduction rate of lithium salt is slower, and the battery formation rate of SEI film is slower.

Therefore, the deposition of solvent products is more orderly and dense, which is more conducive to prolonging the service life of the battery. In this article, the flexible packaging lithium-ion battery of lithium cobalt oxide-graphite system is taken as the research object, and the influence of temperature on the formation effect in the high-temperature pressure formation process is studied. The battery formation process is divided into four situations, the temperature is 50°C, 60°C, 80°C and variable temperature formation.

The variable temperature formation means that the temperature of the two charging processes of the battery formation is different. The first charging stage of the battery formation is 40°C, and the second is The charging stage temperature is 70°C. The performance of the first 350 cycles is similar, and the initial capacity can reach 3000 mA h. After 400 cycles, the capacity retention rate begins to differ.

After 450 cycles, the capacity retention of schemes 1, 2, 3 and 4 The ratios are 90.57%, 90.63%, 93.44%, and 92.02%, respectively, indicating that the cycle performance of the high-temperature formation battery at 80°C is better than that of other schemes, which may be because the battery formation at 80°C has greater battery activity and thicker SEI films. It is beneficial to the cycle stability of the battery.

For the comparison of high-temperature battery formation or low-temperature battery formation, there is no definite result and it can be said that high-temperature formation must be better or low-temperature formation must be better. The choice of temperature in the battery formation process needs to be done according to different battery systems and different manufacturing processes. Experimental verification, select the battery formation process parameters suitable for the battery itself.

Optimization of formation process of NMC811 high-nickel lithium ion battery


Optimization of formation process of NMC811 high-nickel lithium ion battery

All the batteries in the experiment were first charged to 1.5V (to avoid corrosion of the negative electrode copper foil), and then left at 30 or 40°C for 6 hours to fully infiltrate the electrolyte, and then formed according to the system shown in the above table (where, 30h into the first charge and discharge of C/2, the second charge and discharge of C/10, the mark in the table is wrong).

From the average coulombic efficiency, the batteries formed at 30h (C/2 charge and discharge 2 times) and 26h (C/10 charge and discharge 1 time) have the best average coulombic efficiency, indicating that the two batteries have the best average coulombic efficiency in the first charge and discharge and the side effects are less.

However, the average coulombic efficiency of the 86h battery (C/10 charge and discharge 4 times) with the longest battery formation time is not the highest, which is mainly because at this small rate, the negative electrode is at a low potential for a long time, causing More side reactions and growth of SEI, thereby reducing the overall Coulombic efficiency of the battery.

Immediately after battery formation, the battery was dissected to analyze changes in the negative electrode. It can be seen that the negative electrode of the battery formed by C/10 low current (86h and 26h) does not have the phenomenon of metal Li precipitation, but the battery formed by C/2 larger rate (30h, 10h and 10h at 40℃) In the battery, obvious metal Li precipitation can be observed on the surface of the negative electrode, and the battery formed in 30h is lighter than the two batteries in the 10h cycle due to the use of C/10 cycle in the second cycle, which shows that Part of the metal Li formed during the first charging process is still active and can participate in the electrochemical reaction.

This experiment shows that during the first charging process of the battery formation, because the negative electrode has not yet formed a stable SEI film, the kinetic conditions are poor, so a small current must be used for the first charging to avoid the precipitation of metal Li on the surface of the negative electrode. Therefore, this also shows that soaking at 30°C for 6 hours can fully ensure the full soaking of the electrodes. Undoubtedly, for batteries with more electrode layers or winding structures, the soaking time should be extended appropriately to ensure full soaking of the electrodes and separator by the electrolyte.

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