Effect of conductive agent on the electronic conductivity of mixed powder & electrode

Positive and negative electrode powder materials, separators, electrolytes, conductive agents, binders, current collectors, etc. are the main raw materials for the manufacture of lithium-ion batteries; the production of lithium-ion batteries is the process of processing these raw materials into batteries under the optimal process conditions. Changes in the parameters of these raw materials require targeted optimization and adjustment of the process conditions to obtain lithium-ion batteries with optimal electrical performance. The design of the parameters of the positive and negative electrode sheets of lithium-ion batteries is the key to the development of lithium battery processes, including active material loading, porosity, thickness, and the ratio between active materials, conductive agents, and binders. Among them, the type , content, and performance of the conductive agent are key factors affecting the electron transmission during the charging and discharging process of lithium-ion batteries, and the electron conduction characteristics directly determine the quality of the electrochemical performance.

the actual pole piece process design, the electronic conductivity of active materials, especially positive electrode materials, is relatively poor, and the electron transmission path is mainly achieved through the conductive agent path. Figure 1 is a schematic diagram of the microstructure of a lithium electrode sheet under ideal conditions. 

The electronic conductivity of the pole piece affects the basic performance of the battery, not only limited to the power performance of the battery cell, but also affects the reliability and safety performance of the battery cell. The pole piece resistance can better evaluate the performance of the electronic conductive network or the uniformity of the electrode microstructure during the electrode manufacturing process, and help study and improve the electrode formulation and the control parameters of the mixing, coating and rolling processes. In the current research on lithium batteries, more and more researchers are paying attention to the electronic conductivity of lithium battery powder materials while paying attention to the electronic conductivity of the pole piece level, and try to find the correlation between the two, so as to directly estimate the electronic conductivity of the pole piece level from the powder material level.

                                                    Figure 1. Schematic diagram of ideal electrode microstructure

This article mainly combines the NCM523 series lithium-ion battery powder materials, combines the binder PVDF and the conductive agent SP for powder layer premixing, and evaluates the electronic conductivity of the mixed powder. At the same time, the slurry is prepared and coated on the powders with the same ratio, and the electronic conductivity of the finished electrode is evaluated. The influence of the conductive agent on the conductive performance of each layer is clarified, and its correlation is preliminarily explored.

1. Test Method

1.1 Test equipment: PRCD3100 (IEST-Yuan Neng Technology) series powder resistance meter is used to measure and evaluate the conductive properties of powder materials; BER2500 (IEST-Yuan Neng Technology) series electrode resistance meter is used to evaluate the conductive properties of the electrode.

Figure 2. (a) PRCD series appearance and structural diagram ;

(b) BER series appearance and structure diagram

1.2 Sample preparation and testing

1.2.1 Prepare mixed powders with different NCM ratios by mixing NCM: PVDF = 19: 1 and NCM: SP: PVDF = 18: 1: 1, and conduct powder resistance tests within the range of 10-200 MPa; 

1.2.2 Prepare the slurry according to the parameters in Table 1, apply it manually with a 200μm scraper, and test the electrode resistance of the prepared electrode.

Table 1. Pole preparation slurry ratio

2. Test results

The powder resistance tests were conducted on SP, NCM, and mixed powders NCM+PVDF and NCM+SP+PVDF. From the test results in Table 2 and Figure 3, it can be seen that, whether under low pressure or high pressure, after adding the binder PVDF to the NCM active powder, the electronic conductivity is significantly deteriorated, while after adding SP with good conductivity, the electronic conductivity of the mixed powder is significantly improved. In the process of evaluating the electronic conductivity of the positive active powder of lithium-ion batteries, the electronic conduction is mostly the contact conduction between particles. After adding PVDF powder with poor electronic conductivity, the PVDF powder will reduce the contact ratio between the original active particles, resulting in a change in the overall conduction path, which in turn leads to a decrease in the overall electronic conductivity. The role of the conductive agent in the lithium-ion battery electrode is to provide a channel for electron transmission. The right amount of conductive agent content can obtain a relatively high discharge capacity and good cycle performance. Too high or too low a conductive agent content will affect the electrical performance. When the conductive agent content is too high, the relative content of the active material will be reduced, which will reduce the specific capacity of the battery. When the content is too low, there are fewer electronic conduction channels, which is not conducive to large current charging and discharging, and the utilization rate of the active material in the electrode is relatively low. From the test results of the powder resistance, the conductive agent SP has very superior electronic conductivity compared to the active powder NCM. After adding SP to the NCM+PVDF mixed powder, the electronic conductivity of the powder in the full pressure range has also been significantly improved. This is mainly because after adding the SP with good conductivity, the change in the electronic conduction path of the actual powder measurement process is more along the SP path with good conductivity. It also further clarifies the main role of the conductive agent in lithium-ion battery electrodes.

Table 2. Powder resistance test results comparison table 

                           Figure 3. Powder resistance test results under 10MPa & 200MPa pressure conditions

In order to further clarify the correlation between the electronic conductivity of materials, slurry was prepared according to the ratio in Table 1 for different powders, and manual coating was performed under the same conditions with a scraper. After the electrode was made, the electrode resistance test was performed. Table 3 & Figure 4 show the test results of the electrode resistance. The electronic conductivity of the electrode under different ratios is quite different. Compared with the mixed powder resistance results in Table 2 & Figure 3, the electrode level also shows that the conductivity is rapidly improved after adding the conductive agent SP. It is further clarified that the addition of conductive agents at both the premixed powder level and the electrode level can effectively improve the electronic conductive path. The optimization of the conductive agent ratio is also crucial in the actual electrode process development stage. The amount of conductive agent is closely related to the particle size of the conductive agent material, the specific surface area of ​​the active material and other conditions. The larger the specific surface area of ​​the active material, the larger the particle size of the conductive agent, and the more conductive agent is used. The optimal ratio should be determined by systematic experiments based on the percolation theory model of the conductive network.

Table 3. Comparison of electrode resistance test results

Figure 4. Comparison of the average values ​​of electrode resistance test results under different formulations

3. Summary

This paper uses PRCD series powder resistance detection equipment and BER series electrode resistance detection equipment to conduct a systematic evaluation of resistance performance at two levels: mixed powder and electrode. It clarifies the role and influence of the conductive agent and the trend correlation between different levels. Based on this, the system formula can be further optimized, and the performance of the electrode level can be preliminarily estimated from the powder performance, providing a new idea for the research and development of lithium-ion battery technology.

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