Effects of Conductive Agents and Binders on Compression and Compactability of NCM Powders

Effects of Conductive Agents and Binders on Compression and Compactability of NCM Powders

In the field of energy development, lithium-ion batteries have gradually become an important component of power sources (medical equipment, entertainment equipment, computers, communication equipment, electric vehicles, spacecraft, etc.) due to their advantages of low cost, environmental friendliness, high specific energy, light weight, and no memory effect. Lithium-ion battery positive electrode active materials often use transition metal oxides, such as layered lithium cobalt oxide, lithium nickel oxide, lithium nickel cobalt oxide, or lithium iron phosphate, and negative electrodes often use graphite, silicon-based materials, etc. as active materials.

During the development and production process of lithium-ion batteries, it was found that the conductivity of the positive and negative active material particles cannot meet the requirements of the electron migration rate. Therefore, conductive agents need to be added during the battery manufacturing process. The main function is to improve the electronic conductivity. The conductive agent conducts electrons and collects microcurrents between the active material particles and between the active material particles and the current collector, thereby reducing the contact resistance of the electrode and effectively reducing the polarization of the battery. Commonly used conductive agents for lithium batteries can be divided into traditional conductive agents (such as carbon black, conductive graphite, carbon fiber, etc.) and new conductive agents (such as carbon nanotubes, graphene and its mixed conductive slurry, etc.). Figure 1 is a schematic diagram of the distribution of conductive agents in lithium-ion battery pole pieces.

Figure 1. Schematic diagram of the distribution of conductive agents in lithium-ion battery electrodes ^[1]

The main function of lithium-ion battery binders is to bind the active material powders. Binders can tightly attach the active material and the conductive agent to the current collector to form a complete electrode, prevent the active material from falling off and peeling off during the charge and discharge process, and can evenly disperse the active material and the conductive agent, thereby forming a good electron and ion transmission network and achieving efficient transmission of electrons and lithium ions. Commonly used binders include polyvinylidene fluoride (PVDF), carboxymethyl cellulose (CMC), styrene-butadiene rubber (SBR), polyvinyl pyrrolidone (PVP), polymethyl methacrylate (PMMA), polyacrylonitrile (PAN), polyacrylic acid (PAA), etc. The mechanism of action of binders in lithium-ion battery research has always been the focus of attention. Zhong et al. ^[3] analyzed the binding effect between active particles and binders through density functional theory (DFT) simulation calculations and explored the binding mechanism. The results of process simulation and theoretical calculation show that in the LFP system, the binding effect between LFP and PVDF is much greater than that between PVDF and Al, while in the NCM system, the binding effect between NCM and PVDF is weaker than that between PVDF and Al; scanning electron microscopy and Auger electron spectroscopy (AES) analysis also show that PVDF has good bonding properties in NCM batteries. Figure 2 shows the possible binding mechanism of PVDF in different battery systems. 

Figure 2. Schematic diagram of possible incorporation mechanisms of PVDF in lithium-ion batteries ^[2]

In the research of lithium-ion battery powders, the compaction density is closely related to the energy density of the battery. In the design process of lithium-ion batteries, the initial focus was on the compaction density of the pole piece end. With the development of the industry, the compaction density of positive and negative electrode powders has gradually become a key reference indicator for process modification and sample batch stability monitoring. At present, the compaction density evaluation of single powders is relatively mature, but the correlation between the compaction density of powders and the compaction density of pole pieces is still the focus of attention of industry researchers. Since the research process is greatly affected by the process ratio, there is no clear conclusion on the current research results of the correlation between powders and pole pieces. Compared with single powders, lithium-ion battery pole pieces are added with auxiliary materials such as conductive agents, adhesives and other additives, and their influence on the comprehensive compaction density cannot be ignored. This article focuses on NCM materials, and refers to the premixing part in the dry mixing process. The powder premixing of NCM+PVDF and NCM+PVDF+SP is carried out respectively, and the compaction density and compression performance of different mixed powders are evaluated in combination with the PRCD series equipment, so as to further clarify the differences in compaction and compression performance before and after powder mixing.

1. Test Method

1.1 Test equipment: PRCD3100 (IEST-Yuan Neng Technology) series equipment is used to evaluate the compaction and compression performance of powder materials.

Figure 3. PRCD series appearance & structure diagram

1.2 Sample preparation and testing:

Mixed powders with different ratios were prepared by fully mixing NCM:PVDF=19:1 and NCM:PVDF:SP=18:1:1, and the powder compaction density, pressure relief rebound and steady-state stress-strain performance tests were carried out in the range of 10-350MPa.

2. Test results

In this paper, a dry powder mixing experiment is used to simulate the slurry preparation and proportioning process of the electrode process, and then the powder compression and compaction performance tests are carried out on SP, NCM and mixed powders NCM+PVDF and NCM+SP+PVDF respectively. The thickness of different powders is monitored under the pressure-unloading mode as shown in Figure 4 (a), and the absolute value of the pressure-unloading thickness minus the pressure-applied thickness is defined as the thickness rebound of the material. Figure 4 (b) is a comparison of the rebound of different materials under the pressure-unloading conditions. From the test results, the rebound of SP powder is the largest, followed by NCM+SP+PVDF mixed powder, while the rebound of NCM and NCM+PVDF mixed powder is very small. Compared with single NCM powder and mixed powder, the thickness rebound of the mixed powder after adding PVDF under the same test conditions increases slightly, while the thickness rebound of the mixed powder after adding PVDF and SP at the same time increases significantly, mainly considering the changes caused by the addition of SP with a large thickness rebound. In addition, with the increase of pressurization pressure, the rebound thickness of SP powder after pressure release shows a downward trend, while the rebound thickness of NCM and NCM-based mixed powders after pressure release first increases and then tends to be stable with the increase of pressurization pressure. Parallel sample tests were carried out on each powder, and the results were consistent.

The compression and compaction process of powders is related to the flow and rearrangement of powders, elastic and plastic deformation, crushing and other phenomena, and is directly affected by many factors such as powder size and its distribution, particle shape, surface roughness, particle toughness, additives, etc. The difference in the test results of different powders during the pressure relief experiment is also related to it. Conductive carbon black SP is an amorphous carbon, which is agglomerated from primary particles (primary structure) with a diameter of about 40nm into primary aggregates (secondary structure) of 150-200nm, and then processed by soft agglomeration and artificial compression. The overall structure is grape chain, and a single carbon black particle has a very large specific surface area. In lithium-ion batteries, SP is dispersed around the active material in the form of 150~200nm primary aggregates to form a multi-branched conductive network, thereby reducing the physical internal resistance of the battery and improving the electronic conductivity. Due to this morphological and structural feature, the interaction between SP nanoparticles is relatively strong, and relatively large elastic strain will accumulate during the compression process, and a large thickness rebound will occur after pressure relief. Active NCM is a micron particle with a relatively high elastic modulus. During compression, the elastic strain is small and the rebound thickness is also small.

Figure 4. Pressure relief test: (a) Pressure change in pressure relief mode; (b) Thickness rebound curve

To further explore the possible correlation, this paper further tests the stress-strain and compaction density properties of different powder materials in combination with the steady-state experimental mode. As shown in Figure 5 (a), the thickness of different powders is monitored by applying pressure and unloading pressure under steady-state pressure. Taking the thickness under the initial pressure of 10 MPa as the basic thickness, the thickness deformation of different powders under pressurization or unloading conditions is calculated to obtain the stress-strain curves of different powder materials shown in Figure 5 (b). The maximum deformation, reversible deformation and irreversible deformation results of different materials are summarized in Table 1. It can be clearly seen from the stress-strain curves of different powders that there are significant differences between the powders. After the materials are pressed to the same pressure, the maximum deformation is: SP>NCM+PVDF+SP＞NCM+PVDF＞NCM, and the irreversible deformation and reversible deformation have the same trend. It can be further clarified from the difference in stress-strain curves that the premixing of SP and PVDF powders with NCM can directly cause changes in the stress-strain properties of the material, and this change is consistent with the results of the unloading test. This shows that when PVDF powder is added to NCM powder, the compressive strain of the mixed powder will increase, and the irreversible strain will also increase slightly, because PVDF granular powder has a certain elasticity. SP nanoparticles with a hyperbranched structure have the largest compressive strain and the largest rebound. When they are added to NCM powder, the stress-strain curve of the mixed powder changes significantly, and both the reversible strain and the irreversible strain increase significantly. This shows that the conductive agent SP will have a relatively large impact on the compaction density of the mixed powder or electrode.

Figure 5. Steady-state test: (a) steady-state mode pressure change; (b) stress-strain curves of different powders

Table 1. Comparison of deformation data of different powders

According to the above-mentioned compression process of mixed powders, the actual pressure filling process of powders is closely related to factors such as the particle size distribution and morphology of powder materials. The compression of the pole piece during production is actually reflected in the flow rearrangement, elastic and plastic deformation process of the powder. In addition to being directly related to the physical properties of the main material powder, additives and moisture in the process ratio are also key influencing indicators. Among them, the common additives that affect the compression and compaction performance of powders mainly include flow aids, binders and conductive agents. Binders are soluble polymer materials with bonding effects. In the actual pole piece process, they are wrapped on the surface of the active material and filled between the particle gaps. In the actual pole piece, the binder will increase the flow resistance and reduce the flow performance. In the presence of binders, different conductive agents also have different effects on the compaction density.

The experimental design of this paper is based on NCM basic powder, and the binder PVDF and the conductive agent SP are added and premixed respectively, which is also to de-correlate the physical properties of the electrode layer from the powder level. From the compression performance test results, it can be clearly seen that the compression performance of the powder end has changed significantly after adding the binder and the conductive agent, and from the compaction density results, SP＜NCM+PVDF+SP＜NCM+PVDF＜NCM, and this result can also be directly related to the change of pressure relief rebound and steady-state deformation. On the whole, the mixed powder after adding PVDF and SP requires a greater pressure to reach the same compaction density as the original NCM powder, that is, from the powder level, the two substances introduced in the experimental setting reduce the compaction density of the basic powder; it seems that the correlation between the compression and compaction of the simple powder mixing and the electrode needs to be further explored. In the next step, the compression and compaction of the powder and the electrode after the slurry is dried and dispersed can be systematically explored to explore new methods for predicting the performance of the electrode layer at the powder level during process development.

Figure 6. Results of compaction density measurement of different powders

3. Conclusion

This article focuses on NCM materials, and refers to the premixing part in the dry mixing process. It premixes NCM+PVDF and NCM+PVDF+SP powders respectively, and uses PRCD series equipment to evaluate the compaction density and compression performance of different mixed powders. It further clarifies the differences in compression and compaction performance before and after powder mixing, and clarifies that the compression and compaction performance of NCM materials have obvious changes after the addition of PVDF and SP. The process development process can combine the current test methods to design more reasonable experiments to evaluate the correlation between the compression and compaction performance of the powder level and the electrode level.

4. References

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