CR2032 button cell batteries

Rolling process of lithium CR2032 button cell batteries pole piece

The general process flow of lithium-ion CR2032 button cell batteries pole piece manufacturing is: active material, binder and conductive agent are mixed to prepare slurry, then coated on both sides of copper or aluminum current collector, and then dried to remove solvent to form pole piece. The pole piece particle coating is compacted and densified, and then cut or stripped. Rolling is the most commonly used compaction process for lithium CR2032 button cell batteries pole pieces. Compared with other processes, rolling has a huge change in the pore structure of the pole piece, and it will also affect the distribution state of the conductive agent, thereby affecting the electrochemical performance of the CR2032 button cell batteries. In order to obtain the most optimized pore structure, it is very important to fully understand and understand the rolling compaction process. Basic process of rolling process In industrial production, lithium CR2032 button cell batteries pole pieces are generally rolled and compacted by a double roller machine, as shown in Figure 1. In this process, the pole piece with particle coating on both sides is fed into the gap between the two rollers. The coating is compacted under the action of the roller line load. After coming out of the roller gap, the pole piece will rebound elastically, resulting in an 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 compact the coating. In addition, the rolling speed directly determines the retention time of the load on the pole piece, and will also affect the rebound of the pole piece, and ultimately affect the coating density and porosity of the pole piece.

Figure 1 Schematic diagram of the pole piece rolling process At the rolling speed Vcal, when the pole piece passes through the roll gap, the line load can be calculated by formula (1): qL=FN/WC Where qL is the line load acting on the pole piece, FN is the rolling force acting on the pole piece, and Wc is the width of the pole piece coating. Evolution of the pole piece microstructure during rolling process The pole piece is compacted through the roll gap, and the coating density changes from the initial value ρc,0 to ρc. The compaction density ρc can be calculated by formula (2): Basic analysis of lithium CR2032 button cell batteries pole piece rolling process (2) Wherein, mE is the weight of the electrode piece per unit area, mC is the weight of the current collector per unit area, hE is the thickness of the electrode piece, and hC is the thickness of the current collector. The compaction density is related to the porosity of the pole piece. The physical coating porosity εc,ph can be calculated by formula (3), which means the volume fraction of the pores inside the particles and the pores between the particles in the coating: Basic analysis of lithium CR2032 button cell batteries pole piece rolling process (3) Wherein, ρph is the average physical true density of each component material of the coating. In the actual rolling process, as the rolling pressure changes, the compaction density of the pole piece coating has a certain regularity. Figure 2 shows the relationship between the pole piece coating density and the rolling pressure.

Figure 2 Relationship between the pole piece coating density and the rolling pressure Curve I area is the first stage. In this stage, the pressure 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 piece increases rapidly, and the relative density of the pole piece changes regularly. Curve II area is the second stage. During this stage, the pressure continues to increase, and the density of the pole piece has increased after compression. The pores have been filled, and the slurry particles have generated greater compaction resistance. The pressure continues to increase, but the pole piece density increases less. Therefore, the displacement between the slurry particles has decreased at this time, 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 pole piece density will continue to increase as the pressure increases, and then gradually slow down. 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. However, when the pressure continues to increase, the change in pole piece density gradually slows down. The actual pole piece rolling process is very complicated. In the first stage, although the densification of the powder body is mainly based on the displacement of the slurry particles, there is also a small amount of deformation. In the third stage, the densification is mainly based on the deformation of the slurry particles, and there is also a small amount of displacement. In addition, due to the differences in the properties of the positive and negative electrode materials themselves, the microstructure changes in the positive and negative electrode sheet rolling process are also different. The positive electrode particle material has high hardness and is not easy to deform, 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, reduce the resistance to lithium ion insertion and extraction, and improve the CR2032 button cell batteries cycle stability. However, excessive load may cause particle breakage. Due to the poor conductivity of the active material in the positive electrode sheet, the change in the distribution of the conductive agent caused by the rolling process has a more obvious effect on electronic conduction than the negative electrode.

Figure 3 Schematic diagram of displacement and deformation of positive and negative electrode sheet rolling particles Effect of compaction density on electrochemical performance In the CR2032 button cell batteries electrode, electron conduction mainly passes through, while lithium ion conduction mainly passes through the electrolyte phase in the porous structure. The electrolyte is filled in the pores of the porous electrode, and lithium ions are conducted through the electrolyte in the pores. The conduction characteristics of lithium ions are closely related to the porosity. The larger 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 active materials or carbon gel phases, and the volume fraction and tortuosity of the solid phase directly determine the effective conductivity of electrons. Porosity and the volume fraction of the solid phase are contradictory. A large porosity will inevitably lead to a decrease in the volume fraction of the solid phase. Therefore, the effective conduction characteristics of lithium ions and electrons are also contradictory. On the one hand, compacting the pole piece improves the contact between particles in the electrode, as well as the contact area between the electrode coating and the current collector, reducing the 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 adhesive on the surface of the active material particles is squeezed, which limits the diffusion and embedding/de-embedding of lithium salts, increases the diffusion resistance of lithium ions, and reduces the CR2032 button cell batteries rate performance. The influence of rolling process parameters As mentioned earlier, the rolling process directly determines the porous structure of the pole piece, and what kind of influence 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 conducted relevant research. They found that the compaction process of lithium-ion CR2032 button cell batteries 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. Basic analysis of lithium CR2032 button cell batteries pole piece rolling process (4) Among them, ρc,max and γc can be obtained by fitting experimental data, which respectively represent the maximum compaction density and coating compaction impedance that the coating can achieve under certain process conditions.

Table 1 Experimental parameters of positive and negative pole pieces The researchers conducted rolling experiments on the NCM ternary positive pole pieces and graphite negative pole pieces shown in Table 1 to study the influence of rolling process parameters on the density and porosity of the pole piece coating. According to the physical true density of the material, when the porosity is 0%, the positive electrode coating density should be 4.3g/cc and the negative electrode coating density should be 2.2g/cc. In fact, the parameters obtained by fitting the experimental data (see Table 2) show that the maximum density of the positive electrode coating is about 3.2g/cc and that of 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 electrode sheets. Experimental data points were collected under different loads and rolling line speeds, and then the exponential equation (4) was used to fit the data to obtain the corresponding equation fitting parameters, which are listed in Table 2. It is expressed as the compaction impedance of the coating. A lower value indicates that the coating density can reach the maximum value faster as the line load increases, while a higher impedance value indicates that the coating density reaches the maximum value more slowly. It can be seen from Figure 4 and Table 2 that the rolling speed has little effect on the coating density, and a lower speed leads to a slight increase in the coating density. In addition, the compaction process of the positive and negative electrode sheets is very different. The compaction impedance of the positive electrode sheet is about more than one times that of the negative electrode. This is caused by the difference in the characteristics of the positive and negative electrode materials. The positive electrode particles have a large hardness and a large compaction impedance, while the negative electrode particles have a small hardness and a small compaction impedance, which makes it easier to roll and compact.

Figure 4 Relationship between line load and compaction density of the positive and negative electrode sheet coatings

Table 2 Parameter values obtained by fitting under different rolling process conditions In addition, the influence of the rolling process is analyzed from the perspective of pore structure. The pores in the CR2032 button cell batteries pole piece coating mainly include two types: pores inside the granular material, which are nano-submicron in size; pores between particles, which are micron in size. Figure 5 shows the pore size distribution in the positive and negative pole pieces under different rolling conditions. First of all, it is obvious that the compaction of the pole piece can reduce the pore size and reduce the pore content. As the compaction density increases, the pore size of the negative electrode is more significantly reduced compared with the positive electrode. This is because the negative electrode coating has a low compaction impedance and is more easily compacted by rolling. At the same time, the data shows that the rolling speed has a small effect on the pore structure.

Figure 5 Pore size distribution under different rolling conditions From the perspective of the porosity of the coating, the rolling line load and the coating porosity can also be fitted by an exponential equation to obtain a law. Figure 6 shows the relationship between the line load and the porosity of the positive and negative pole piece coatings. The positive and negative pole pieces are rolled under different line loads, and the porosity is calculated by physical true density. At the same time, the porosity of the coating is also measured experimentally. The obtained data points are plotted and linearly fitted. The results are shown in Figure 6.

Figure 6 Relationship between line load and porosity of positive and negative electrode sheet coating The rolling process has a huge impact on the microstructure of lithium CR2032 button cell batteries sheet, especially the porous structure. Therefore, the rolling process strongly affects the CR2032 button cell batteries performance. In short, in the research and development of lithium CR2032 button cell batteries technology, we also need to pay special attention to the manufacturing process.

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