There are some problems in the production and application of high-nickel materials: as the nickel content increases, the capacity decays due to multiple phase transitions during the charge and discharge process; the total amount of LiOH and Li2CO3 on the surface increases, and the slurry during the homogenization process will appear. The material is easy to gel, and the battery is easy to generate gas when stored at high temperature; the thermal stability becomes poor. This involves the matching of materials with electrolytes, binders, conductive agents, etc.
The binder can enhance the contact between the active material and the conductive agent as well as the current collector, keeping the structure of the pole piece stable during the charging and discharging process. The performance of the binder, such as adhesion, flexibility, alkali resistance, hydrophilicity, etc., directly affects the performance of the battery. It is a simple and practical method to optimize the overall performance of the battery by improving the binder. Polyvinylidene fluoride (PVDF), which is currently widely used in commercial lithium-ion batteries, is an excellent cathode binder with high dielectric constant, good electrochemical stability, and superior bonding properties.
In this paper, two kinds of modified PVDF binders are used. Due to different preparation processes, there are differences in molecular weight, types and positions of groups on the polymer chain. Glue A is prepared by an emulsion polymerization process. The monomers are dispersed in water by means of an emulsifier and mechanical stirring to form an emulsion, and then an initiator is used to initiate the polymerization of the monomers. The resulting latex polymer is demulsified, washed and dried to obtain the product PVDF; glue B is prepared by a suspension polymerization process. The monomer dissolved in the initiator is suspended in water in the form of droplets for free radical polymerization to generate a polymer, and the finished product is obtained by centrifugation, washing and drying. Compared with the emulsion polymerization process, the suspension polymerization process is simple in post-treatment because it does not need to use surfactants, and the impurity content is lower, the product particle size is larger, and the relative molecular mass and distribution are relatively stable.
1. Button battery production and performance test
The button battery adopts CR2032 battery shell, with NCM622 pole piece as the positive pole, lithium metal piece as the negative pole, adding a separator, adding an appropriate amount of electrolyte, the additive is vinyl chloride, and finally stamping and sealing to make an NCM/Li half-cell. All assembly procedures were performed in an argon atmosphere glove box. In this experiment, five slurry formulations (see Table 1) were used to investigate the influence of the composition on the NCM622 system when the total binder content was 2%, including the properties of the slurry, the pole piece and the capacity performance of the NCM/Li half-cell.
It can be seen from Figure 2 that the BS of formula 3 and 5 at the same height does not change much, and the value is small, and sample 1 has a certain fluctuation between 14 and 21 mm. The results of the slurry instability index (TSI) measured by the instrument are shown in Table 2. The TSI values of the slurry are all small, and the daily TSI values remain the same or have little difference, indicating that the slurry is stable under the three ratios. All are good. Therefore, the PVDF mixed adhesive selected in the experiment can well adapt to the characteristics of other materials in the formula, and has good anti-agglomeration and anti-suspension properties, so the slurry stability is good.
2-pole piece adhesion and sheet resistance test
Adhesion is an important performance index of binders for lithium ion batteries, that is, the peel strength between active materials and conductive agents and current collectors is tested. The adhesion test of the pole piece is carried out, and the results are shown in Figure 3.
The pole piece is subjected to a pair of roll cold pressing treatment to simulate the state of the battery pole piece. After rolling, the active material, conductive agent and binder on the pole piece will be in closer contact, so the bonding force and the pressing will be larger than the film. By comparison, it can be seen that only adding glue A, the bonding force is weak. With the increase of glue B content, the bonding force of the pole piece increases rapidly. If pure glue B is used, the pole piece before and after rolling shows very good quality. Bonding properties.
The surface resistance test of the pole piece is carried out, and the results are shown in Figure 4. The surface resistance of the pole piece before and after rolling is not much different, and the surface resistance of the formula 1-5 is not much different, indicating that after the total content of NCM622, conductive agent and binder is determined, the addition ratio of the mixed glue has an effect on the electrical conductivity of the pole piece. The effect is not large, and they all show a relatively ideal effect.
In addition, the flexibility test of the pressed pole piece was carried out, and it was wound according to the coating direction and the vertical direction of the slurry. It shows that the flexibility of the pole piece is good.
3 NCM/Li half-cell capacity calibration
The NCM/Li half-cell capacity was calibrated using 0.05C constant current charge and discharge, the cut-off voltage range was 3~4.2 V, and the nominal specific capacity was set to 165mAh/g. The first charge-discharge curves of the five formulations are shown in Figure 5. The first charge-discharge specific capacity and coulombic efficiency are shown in Table 3. The specific capacity of formula 1 is not very ideal, which may be due to the residual emulsifier and other impurities in pure rubber A. loss of electrochemical performance.
With the increase of the proportion of glue B in the composition, the specific capacity of the first discharge increased, indicating that the use of glue B can effectively improve the capacity of the NCM622 system. The determination of the formula must fully consider the balance of material properties, adhesion, surface resistance and other indicators, and find the best ratio. Both formulas 4 and 5 show relatively superior performance. Considering the material cost, formula 4 can be in-depth. Research.
In this paper, for the high nickel ternary material NCM622, the effects of PVDF binders prepared by two different processes on the homogenization process, pole piece properties, material capacity, etc. were investigated. Slurry fluidity.
Using different proportions of PVDF binder to homogenize the slurry, the stability of the slurry is good, and the prepared pole piece has good flexibility. Adjusting the mixing ratio can significantly affect the bonding performance of the pole piece, but has little effect on the electrical conductivity of the pole piece. Increasing the amount of glue B can improve the electrochemical performance of NCM622 system.