1. Preface

The actual electrode process design, the active material, especially the cathode electrode material conductivity properties is relatively poor, the electronic transmission path is mainly realized through the conductive agent path, such as Figure 1 for the ideal state of the lithium electrode microstructure schematic diagram. The conductivity properties of the electrode sheet affects the basic performance of the battery, which is not only limited to the power performance of the cell, but also affects the reliability of the cell and the safety performance. The resistance of the electrode can better evaluate the performance of the electronic conductive network or the uniformity of the microstructure of the electrode during the electrode manufacturing process, and help to study and improve the formulation of the electrode as well as the control parameters of the mixing, coating and rolling processes. Lithium current research more and more researchers in the focus on the conductivity properties of the electrode level at the same time, but also pay attention to the conductivity properties of lithium powder materials, and try to find out the correlation between the two, in order to directly predict the conductivity properties of the electrode level from the powder material level.

Cathode and anode powder materials, diaphragm, electrolyte, conductive agent, binder, collector, etc. are the main raw materials for lithium-ion battery manufacturing; lithium-ion battery production is to optimize the process conditions of these raw materials into the battery process. The parameters of these raw materials need to be targeted to change the process conditions for optimization and adjustment, in order to obtain the optimal electrical properties of lithium-ion batteries. Lithium-ion battery positive and negative electrode parameter design is the key to the development of lithium-ion battery process, including the active substance load, porosity, thickness, and the ratio between the active substance, conductive agent and binder. Among them, the category, content and performance of the conductive agent are the key factors affecting the electron transfer in the charging and discharging process of lithium-ion batteries, while the conductivity properties directly determine the advantages and disadvantages of electrochemical performance.

Figure 1. Schematic diagram of 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 conductivity properties of the mixed powder. At the same time, the slurry is prepared and coated on the powders with the same ratio, and the conductivity properties 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.

2. Test Methods

2.1 Test equipment: PRCD3100 (IEST ) series of powder resistivity tester is used to determine and evaluate the conductivity properties of powder materials; BER2500 (IEST) series of electrode resistivity tester is used to evaluate the conductivity properties of the electrode.

Figure 2. (a) Schematic diagram of the appearance & structure of the PRCD series; (b) Appearance & structure of BER series.

2.2 Sample preparation and testing

2.2.1 Mixed powders with different NCM ratios were prepared according to the ratios of NCM:PVDF=19:1 and NCM:SP:PVDF=18:1:1, respectively, and the resistance of the powders was tested in the range of 10-200 MPa;
2.2.2 Slurry preparation according to the proportioning parameters in Table 1, manual coating with a 200 μm squeegee, and electrode resistance testing of the produced electrodes.

Table 1. Electrode preparation slurry ratio

3. Test results

Powder resistance tests were carried out on SP, NCM and hybrid powders NCM+PVDF and NCM+SP+PVDF, respectively, and it can be seen from the test results in Table 2 and Figure 3 that the conductivity properties of the NCM active powders was obviously deteriorated by the addition of the binder PVDF, both at low and high pressure strengths, and the electronic conductivity properties of the hybrid powder after the addition of the SP, which has a better electrical conductivity, was Obviously improved. Lithium-ion battery cathode active powder conductivity properties assessment process, electronic conduction is mostly particle-to-particle contact conduction, after adding poor conductivity properties PVDF powder, PVDF powder will reduce the original active particles between the contact ratio, resulting in a change in the overall conduction path, which leads to a decline in the overall conductivity properties. Conductive agent in the lithium-ion battery electrode is to provide the role of electronic transmission channels, the appropriate amount of conductive agent content can obtain relatively high discharge capacity and better cycling performance, conductive agent content is too high or too low will have an impact on the electrical properties, conductive agent content is too high will reduce the relative content of the active substance, so that the battery’s specific capacity is reduced, and the content of the electronic conduction channels is too low, for high-current charging and discharging The utilization rate of the active substance in the electrode is also relatively low. From the test results of powder resistance, the conductive agent SP relative to the active powder NCM has very superior conductivity properties, in the NCM + PVDF mixed powder after adding SP, the full pressure range of powder conductivity properties has also been significantly improved, mainly because of the addition of good conductivity properties SP, the actual powder measurement process of the electronic conduction path changes, more along the good conductivity properties of the SP path transmission. The better SP path transmission also further clarifies the main role of the conductive agent in the electrode of lithium-ion batteries.

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 properties of the materials, for different powders in accordance with the ratios in Table 1 for slurry preparation, and combined with a scraper for the same conditions of manual coating, made of electrode after the electrode resistance test. As shown in Table 3 & Figure 4 for the resistance test results of the electrode, the conductivity properties of the electrode under different ratios varies greatly, compared with Table 2 & Figure 3 powder mixing resistance results, in the electrode level also presents the phenomenon of rapid improvement of the conductivity after the addition of conductive agent SP, and further clarifies that no matter in the pre-mixed powder level or the electrode level of the conductive agent can be effective in enhancing the conductivity properties of the pathway. The optimization of the ratio of the conductive agent in the development stage of the actual electrode process is also crucial, and the dosage of the 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, the more the conductive agent dosage, the conductive network should be used as a reference to determine the optimal ratio of the conductive network overdiffusion theoretical model of systematic experiments.

Table 3. Comparison of electrode resistance test results

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

4. Summary

In this paper, we adopt PRCD series powder resistance testing equipment and BER series electrode resistance testing equipment to systematically evaluate the resistance performance from the two levels of mixed powder and electrode, clarify the role and influence of conductive agent, and make clear the correlation of trend between different levels, based on which we can further optimize the system formulation, and make a preliminary estimation of electrode performance from powder performance to provide a new way of thinking for the research and development of lithium-ion battery process. Process development for lithium-ion batteries to provide a new way of thinking.

5. References

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