1. Preface

Lithium-ion battery R&D production process found that the conductivity of the cathode and anode active material particles can not meet the requirements of the electron migration rate, so the battery manufacturing process needs to add a conductive agent, the main role is to enhance the electronic conductivity. Conductive agent between the active substance particles, active substance particles and collector play a role in conducting electrons, collect micro-current, thereby reducing the contact resistance of the electrode, effectively reducing the polarization phenomenon of the battery. Commonly used conductive agents in 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 hybrid conductive slurry, etc.) , such as Figure 1 for the lithium-ion battery electrode sheet in the distribution of the distribution of conductive agents schematic diagram.

Figure 1. Schematic representation of conductive agent distribution 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 representation of possible binding mechanisms of PVDF in Li-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 electrode end. With the development of the industry, the compaction density of cathode and anode 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 electrodes 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 electrodes. Compared with single powders, lithium-ion battery electrodes 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 by combining the PRCD series equipment to further clarify the differences in compaction and compression performance before and after powder mixing.

2. Test Method

2.1 Test equipment: PRCD3100 (IEST) series equipment is used to evaluate the compaction and compression performance of powder materials.

Figure 3. Appearance & Structure Schematic of PRCD Series

2.2 Sample preparation and testing

Different ratios of mixed powders were prepared by mixing the powders according to the ratios of NCM:PVDF=19:1 and NCM:PVDF:SP=18:1:1, respectively, and the compaction density, unloading rebound, and steady-state stress-strain properties of the powders were tested in the range of 10-350 MPa.

3. Test Results

In this paper, the dry powder mixing experiment was used to simulate the slurry making proportioning process of the electrode wafer process, and then the powder compression and compaction performance tests were conducted for SP, NCM and mixed powders NCM+PVDF and NCM+SP+PVDF, respectively. The thickness monitoring of different powders was carried out under the pressurized and unpressurized mode as in Figure 4(a), and the absolute value of the unpressurized thickness minus the pressurized thickness was defined as the thickness rebound of the material, and Figure 4(b) shows the comparison of the rebound of different materials under the pressurized and unpressurized conditions. From the test results, SP powder has the largest amount of rebound, followed by NCM+SP+PVDF hybrid powder, while NCM and NCM+PVDF hybrid powder have very small amount of rebound. Comparing the single NCM powder and the hybrid powder, the thickness rebound of the hybrid powder after adding PVDF under the same test condition slightly increased, while the thickness rebound of the hybrid powder after adding PVDF and SP at the same time had a more substantial increase, which is mainly considered to be the change caused by the addition of SP with a larger thickness rebound. In addition, with the increase of pressurization pressure, the calculated thickness rebound amount after unloading of SP powder showed a decreasing trend, while both NCM and NCM-based hybrid powders showed an increase in thickness rebound after unloading with the increase of pressurization pressure, and the results were consistent with the parallel sample testing of each powder, respectively.

Powder compression and compaction process and powder flow and rearrangement, elastic and plastic deformation, crushing and other phenomena associated directly by the powder particle size and its distribution, particle shape, surface roughness, particle toughness, additives, and many other factors, unpressurized experiments in the process of the differences between the different powder test results are also associated with it. Conductive carbon black SP is a kind of amorphous carbon, it is made of primary particles with a diameter of about 40nm (primary structure) agglomerated into primary aggregates with a diameter of 150-200nm (secondary structure), and then through the soft agglomeration and artificial compression and other subsequent processing and become, the whole is a grapeshot chain structure, a single carbon black particles have a very large specific surface area.SP is dispersed into active substances with primary aggregates with a diameter of 150-200nm in the lithium-ion battery. Native aggregates are dispersed around the active material to form a multi-branched chain-like conductive network, thus reducing the physical internal resistance of the battery and improving electronic conductivity. Due to this morphological structural feature, SP nanoparticles have stronger interactions with each other and accumulate relatively large elastic strains during compression, with a large thickness rebound after decompression. On the other hand, the active NCM is micrometer particles and has a relatively high modulus of elasticity, which results in a small elastic strain during compression and a small rebound thickness.

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

To further explore the possible correlations, the stress-strain and compaction density properties of different powder materials are further tested in this paper in conjunction with the steady-state experimental model. As shown in Figure 5(a), pressurization and depressurization were carried out under steady state pressure to monitor the thickness of different powders. Taking the thickness under initial pressure of 10 MPa as the base thickness, the thickness deformation of different powders under pressurized or unpressurized conditions was 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 for different materials are summarized in Table 1. From the stress-strain curves of different powders, it is obvious that there is a significant difference between the powders, and 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. The difference in the stress-strain curves further clarifies that the premixing of SP and PVDF powders with NCM can directly cause changes in the stress-strain properties of the materials, and such changes are consistent with the results of the unpressurization measurements. This indicates that the addition of PVDF powder to NCM powder increases the compressive strain of the mixed powder due to the elasticity of PVDF granular powder, and the irreversible strain is also slightly increased. The SP nanoparticles with hyperbranched chain-like structure have the highest compressive strain and the highest rebound. When they were added to the NCM powders, the stress-strain curve of the hybrid powders changed relatively significantly, and both reversible and irreversible strains increased substantially. This indicates that the conductive agent SP has a relatively large effect on the compaction density of the mixed powders, or electrodes.

Figure 5. Steady state test: (a) Steady state mode pressure variation; (b) Stress-strain curves for different powders

Table 1. Comparison of deformation data for different powders

According to the synthesis of the compression process of mixed powders can be seen, the actual powder compression filling process and powder material particle size distribution, morphology and other factors are closely related to the production process of the electrode of compression is actually embodied in the flow of the powder rearrangement, elasticity and plastic deformation process, in addition to the physical properties of the powder with the main material indicators have a direct correlation with the additives in the process ratios and moisture is also a key influence indicators. One of the common additives that affect the compression and compaction properties of the powder mainly includes flow aids, binder and conductive agent, binder for the soluble polymer materials with a bonding effect, the actual electrode process, it is wrapped in the surface of the active material, filling in the particles between the gaps; the actual electrodein the binder will increase the flow resistance, reduce the flow performance; in the presence of binder, different conductive agent on the compaction density also have different effects.

The experimental design of this paper is based on the NCM base powder, which is premixed with binder PVDF and conductive agent SP respectively, also to correlate the physical property indexes of the wafer level from the powder level. From the compression performance test results, it is clear that there is a significant change in the compression performance of the powder end after the addition of binder and conductive agent, and from the compaction density results, SP < NCM + PVDF + SP < NCM + PVDF < NCM, this result can be directly related to the amount of unloading rebound and changes in the steady state morphology. From a comprehensive point of view, the mixed powder after adding PVDF and SP needs more pressure to reach the same compaction density as the original powder of NCM, i.e., the two substances introduced in the experimental setup reduced the compaction density of the basic powder from the viewpoint of the powder layer; thus, it seems that the correlation between the pure powder mixture and the compression and compaction of the electrode needs to be further explored, and the next step can be the systematic exploration of the compression and compaction of the powder and the electrode after the slurry drying and dispersion, to explore the compression and compaction of the powder and the electrode. The next step is to systematically investigate the compression and compaction of the powder with the electrode after drying and dispersing the slurry, and to explore a new method to predict the performance of the electrode layer by the powder layer in the process of process development.

Figure 6. Compacted density measurement results of different powders

4. Summary

In this paper, the premixing of NCM+PVDF and NCM+PVDF+SP was carried out with NCM as the main material and reference to the premixing part of the dry mixing process, and the compaction density and compression performance of different mixed powders were evaluated by combining with the PRCD series of instruments, which further clarified the difference of compression and compaction performance before and after the mixing of powders, and clarified that the compression and compaction performance of NCM material had obvious changes after the addition of PVDF and SP. It is clear that the compression and compaction properties of NCM materials have obvious changes after the addition of PVDF and SP, and the correlation between the compression and compaction properties of the powder layer and the electrode layer can be evaluated in the process of process development by combining the current test methods with more reasonable experiments.

5. References

[1] mikoWoo @ Ideal Life. Theory and Process Basis of Lithium-ion Battery Electrodes.

[2] Zhong X , Han J , Chen L, et al. Binding mechanisms of PVDF in lithium ion batteries [J]. Applied Surface Science, 2021, 553(4):149564.

[3] BRUCE P G，SCROSATI B，TARASCON J M. Nanomaterials for Rechargeable lithium batteries[J]. Angew Chem Int Ed Engl，2008，47(16)：2930-2946.

[4] Kazuaki Kisu, Shintaro Aoyagi, et al. Internal resistance mapping preparation to optimize electrode thickness and density using symmetric cell for high-performance lithium-ion batteries and capacitors[J]. Journal of Power Sources, 2018, 396:207-212.

[5] Yang Shaobin, Liang Zheng. Principles and applications of lithium-ion battery manufacturing process.