CR2450 battery.Basic analysis of lithium battery pole piece rolling process

　　The general process flow for manufacturing lithium-ion battery pole pieces is as follows: active materials, binders and conductive agents are mixed to prepare a slurry, which is then coated on both sides of a copper or aluminum current collector. After drying, the solvent is removed to form pole pieces and pole piece particles. The coating is compacted and densified, then cut or slit into strips. Rolling is the most commonly used compaction process for lithium battery pole pieces. Compared with other processes, rolling changes the hole structure of the pole piece greatly, and it also affects the distribution state of the conductive agent, thereby affecting the electrochemical performance of the battery. In order to obtain the optimal hole structure, it is very important to fully understand and understand the roller compaction process.

　　Basic process of rolling process

　　In industrial production, lithium battery pole pieces are generally compacted by continuous rolling using a pair of roller machines, as shown in Figure 1. During this process, the pole pieces coated with particle coatings on both sides are fed into the gap between the two rollers. The coating is compacted under the action of linear load. After coming out of the roller gap, the pole piece will elastically rebound and increase in thickness. Therefore, the roll gap size and rolling load are two important parameters. Generally, the roll gap should be smaller than the required final thickness of the pole piece, or the load can cause the coating to be compacted. In addition, the rolling speed directly determines the holding time of the load acting on the pole piece, and also affects the rebound of the pole piece, ultimately affecting the coating density and porosity of the pole piece.

　　Evolution of pole piece microstructure during rolling process

　　In the actual rolling process, as the rolling pressure changes, the compaction density of the pole piece coating has a certain pattern. Figure 2 shows the relationship between the density of the pole piece coating and the rolling pressure.

　　The curve I area is the first stage. The pressure at this stage is relatively small, the particles in the coating are displaced, and the pores are filled. When the pressure increases slightly, the density of the pole pieces increases rapidly, and the relative density of the pole pieces changes regularly.

　　The curve II area is the second stage. At this stage, the pressure continues to increase, and the density of the pole piece has increased after being compressed. The pores are filled and the slurry particles create greater resistance to compaction. The pressure continues to increase, but the density of the pole pieces increases less. Therefore, the displacement between the slurry particles has been reduced, and the large-scale deformation of the particles has not yet begun.

　　The curve III area is the third stage. When the pressure exceeds a certain value, the density of the pole piece will continue to increase as the pressure increases, and then gradually level off. This is because when the pressure exceeds the critical pressure of the slurry particles, the particles begin to deform and break, and the pores inside the particles are also filled, causing the pole piece density to continue to increase. But when the pressure continues to increase, the change in pole piece density gradually becomes gentle.

　　The actual pole piece rolling process is very complex. In the first stage, although the densification of the powder is dominated by the displacement of the slurry particles, there is also a small amount of deformation. In the third stage, densification is dominated by deformation of slurry particles, with a small amount of displacement also present.

　　In addition, due to the differences in the properties of the positive and negative electrode materials themselves, the microstructure changes during the rolling process of the positive and negative electrode sheets are also different. The positive electrode particle material has high hardness and is not prone to deformation, while the graphite negative electrode has low hardness and will undergo plastic deformation during the compaction process, as shown in Figure 3. Moderate compaction will reduce the plastic deformation of graphite, resulting in less resistance to lithium ion insertion and extraction, and better battery cycle stability. Excessive load may cause the particles to break. Since the active material in the positive electrode has poor conductivity, compared with the negative electrode, changes in the distribution of the conductive agent caused by the rolling process have a more obvious impact on electron conduction.

　　Effect of compaction density on electrochemical performance

　　In the battery pole piece, electron conduction mainly passes through, while lithium ion conduction mainly occurs through the electrolyte phase in the porous structure. The electrolyte fills the pores of the porous electrode, and lithium ions conduct through the electrolyte in the pores. The conduction of lithium ions Properties are closely related to porosity. The greater the porosity, the higher the volume fraction of the electrolyte phase and the greater the effective conductivity of lithium ions. Electrons are conducted through solid phases such as living matter or carbon colloid phase, and the volume fraction and tortuosity of the solid phase directly determine the effective electronic conductivity. Porosity and solid phase volume fraction are contradictory. Large porosity will inevitably lead to a decrease in solid phase volume fraction. Therefore, the effective conduction characteristics of lithium ions and electrons are also contradictory.

　　On the one hand, compacting the pole pieces improves the contact between particles in the electrode, as well as the contact area between the electrode coating and the current collector, reducing irreversible capacity loss, contact internal resistance and AC impedance. On the other hand, if the compaction is too high, the porosity is lost, the tortuosity of the pores increases, the particles are oriented, or the binder on the surface of the active material particles is squeezed, limiting the diffusion and intercalation/deintercalation of lithium salts, and the diffusion of lithium ions. The resistance increases and the battery rate performance decreases.

　　Influence rules of rolling process parameters

　　As mentioned earlier, the rolling process directly determines the porous structure of the pole piece, but what impact do rolling process parameters such as linear load and speed have on the microstructure of the pole piece? Chris Meyer, a researcher at the Technical University of Braunschweig in Germany, and others have done relevant research.

　　Their research found that the compaction process of lithium-ion battery pole pieces also follows the exponential formula (4) in the field of powder metallurgy, which reveals the relationship between coating density or porosity and compaction load.

　　The researchers conducted rolling experiments on the NCM ternary positive electrode sheets and graphite negative electrode sheets shown in Table 1 to study the influence of rolling process parameters on the density and porosity of the electrode sheet coating. According to the calculation of the physical true density of the material, when the porosity is 0%, the density of the positive electrode coating should be 4.3g/cc and the density of the negative electrode coating should be 2.2g/cc. In fact, the parameters obtained by fitting the experimental data (see Table 2) show that the maximum density reached by the positive electrode coating is about 3.2g/cc, and the negative electrode is about 1.7g/cc.

　　Figure 4 shows the relationship between the rolling line load and the coating density of the positive and negative electrodes. Experimental data points were collected under different loads and rolling line speeds, and then the exponential equation (4) was used to fit the data, and the corresponding equations were obtained. The combined parameters are listed in Table 2. Expressed as the compaction resistance of the coating, lower values indicate that the coating density reaches its maximum value faster as the line load increases, while higher resistance values indicate that the coating density reaches its maximum value more slowly. It can be seen from Figure 4 and Table 2 that the rolling speed has a small effect on the coating density, and smaller speeds lead to a slight increase in coating density. In addition, the compaction process of the positive and negative electrode pieces is very different. The compaction resistance of the positive electrode piece is about twice that of the negative electrode. This is caused by the difference in material properties of the positive and negative electrodes. The positive electrode particles have a large hardness and a large compaction impedance. The negative electrode particles have low hardness and low compaction resistance, making them easier to roll and compact.

　　In addition, the influence of the rolling process is analyzed from the perspective of pore structure. There are two main types of pores in the coating of battery electrodes: pores inside the granular material, with sizes ranging from nanometers to sub-microns; and pores between particles, with sizes in the micron range. Figure 5 shows the pore size distribution in the positive and negative electrode pieces under different rolling conditions. First of all, it is obvious that compaction of the electrode pieces can reduce the pore size and pore content. As the compaction density increases, the pore size of the negative electrode decreases more significantly compared with the positive electrode, which is due to the fact that the negative electrode coating has low compaction resistance and is easier to be compacted by rollers. At the same time, the data shows that the rolling speed has a small effect on the pore structure.

　　From the perspective of the porosity of the coating, the relationship between the rolling line load and the porosity of the coating can also be obtained by fitting the exponential equation. Figure 6 shows the relationship between the line load and the porosity of the positive and negative electrode plate coatings. Different The positive and negative electrode sheets were rolled under linear load, and the porosity was calculated through physical true density. The porosity of the coating was also measured experimentally. The obtained data points were plotted and linearly fitted. The results are shown in Figure 6 Show.

　　The rolling process has a huge impact on the microstructure of the lithium battery pole pieces, especially the porous structure. Therefore, the rolling process strongly affects the battery performance. In short, in the research and development of lithium battery technology, we also need to pay special attention to the manufacturing process.

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