Lithium Grid News: The pole piece manufacturing process is mainly the coating and rolling process, in which the rolling process is critical. The pole piece rolling process mainly solves the following key problems:

(1) reducing the elongation and the broadness of the pole piece during the rolling process, and reducing the failure rate of the pore structure of the pole piece coating material;
(2) Improve the thickness uniformity of the pole piece coating to improve the cross-sectional shape of the pole piece.
(3) improving the compaction density consistency of the electrode material after the pole piece rolling;
(4) reducing the rebound of the surface electrode material after the pole piece rolling;

The compaction density of the surface coating material has a very important relationship with the electrochemical performance of the battery . Reasonable compaction density can effectively increase the electrochemical performance of the battery, reduce the contact resistance and AC impedance of the electrode, and increase the participation in the electrochemical reaction. The active material area, thereby significantly improving the electrochemical performance of the pole piece coating material. The manufacture of lithium-ion battery pole pieces belongs to the high-precision manufacturing category. The pole piece rolling is different from the strip and strip rolling. The strip and strip rolling is a process in which the metal material is longitudinally extended and laterally widened. The material density during the rolling process is not A change has occurred. The electrode material on the surface of the battery pole piece is a pore structure. During the rolling process, the electrode material on the positive and negative electrode sheets is compacted and the density changes. The pole piece rolling is a process in which the pore structure is filled and the coating particles are gradually compacted.

The purpose of the pole piece rolling is as follows:
1) Ensure that the surface of the pole piece is smooth and flat, and prevent the burr on the surface of the coating from piercing the diaphragm to cause a short circuit;
2) compacting the pole piece coating material to reduce the volume of the pole piece to increase the energy density of the battery ;　
3) bringing the active material and the conductive agent particles into contact more closely, and improving the electronic conductivity;　
4) Enhance the bonding strength between the coating material and the current collector, reduce the occurrence of powder falling out of the battery pole piece during the cycle, and improve the cycle life and safety performance of the battery.

Previously, the basic analysis of the roll process of lithium battery pole piece (click to read) article shared the basic knowledge of roll pressure, some people asked about the effect of roll pressure on the performance of battery pole and battery, this time share a piece of information, take some of the instructions lithium Effect battery pole piece roll temperature. The pole piece rolling is divided into two types: cold rolling and hot rolling. At present, hot rolling is widely used in foreign countries for pole piece rolling, and domestically, cold rolling is often used. Compared with cold rolling, hot rolling has the following main advantages:
1) can reduce the pole piece rebound by about 50%;
2) The thickness of the pole piece can be compressed to the thickness of the process demand by using a small rolling force, and the rolling force can be reduced by up to 62%;
3) Enhance the bonding force between the coating material and the current collector, reduce the occurrence of powder drop during the charge and discharge cycle of the battery, and improve the cycle life of the battery.
Liu Binbin et al. used LiFePO4 as the positive electrode material and lithium wafer as the negative electrode material to make the button-type lithium ion battery. The three parameters of surface density, compaction density and thickness consistency were used as indicators to investigate the rolling temperature of the positive electrode sheet to the battery pole. The effect of the electrochemical properties of the sheet and battery.

Figure 1 Pole thickness at different rolling temperatures

Figure 1 is a thickness curve of a pole piece coated with a thickness of 100 μm at different rolling temperatures, as shown in the figure, as the rolling temperature increases from 20 ° C to 90 ° C and then increases to 160 ° C, the pole piece The thickness deviation is reduced from ±1.9μm to ±1.3μm and then to ±0.8μm, and the thickness uniformity of the pole piece is gradually increased. This is because the deformation resistance of the pole piece coating decreases and the plasticity becomes better as the rolling temperature increases. Make the surface thickness of the pole piece more uniform.

Figure 2 SEM image of the surface of the pole piece coating material at different rolling temperatures

Figure 2 is the SEM image of the surface of the pole piece coating material at different rolling temperatures. As shown in the figure, when the rolling temperature is 20 °C, the surface of the pole piece coating has a relatively tight particle combination, and some areas are not tight enough. There is a small amount of micropores; when the rolling temperature is 90 °C, the surface of the pole piece coating is tightly bonded, the tight bonding area is increased, and the number of micropores is decreasing; when the rolling temperature is 160 ° C, the pole piece is coated. The degree of tight binding of the surface particles is further increased, the tightly bound region is further increased, and the number of micropores is further reduced. The difference in rolling temperature changes the deformation resistance of the coating, so that the surface of the pole piece coating material has different densities.

Figure 3 Coulombic efficiency of each sample cell

Fig. 3 shows the coulombic efficiency of each sample battery, and 9#, 8#, and 3# are batteries produced by rolling the pole pieces at 20 ° C, 90 ° C, and 160 ° C, respectively. As can be seen from Figure 3, the coulombic efficiency of the sample is also increased when the rolling temperature is increased from 20 ° C to 90 ° C and then increased to 160 ° C. The coulombic efficiency is the ratio of the specific discharge capacity to the charge specific capacity in the same charge and discharge cycle. When the thickness uniformity of the pole piece is increased, the resistance is lowered and the coulombic efficiency is correspondingly increased.

Figure 4 Cyclic volt-ampere performance of each sample cell

Fig. 4 is a cyclic voltammetry performance curve of each sample, and 9#, 8#, and 3# are batteries produced by rolling the pole pieces at 20 ° C, 90 ° C, and 160 ° C, respectively. It can be seen from the figure that when the rolling temperature is 160 °C, the upward oxidation peak and the downward reduction peak have better symmetry, the peak position difference is also the smallest, and the reversibility of charge and discharge is also the best, which proves the coulombic efficiency. It must be high.

source:　

[1] Liu Binbin. Theoretical and experimental research on precision manufacturing of power lithium-ion battery pole pieces [D]. Taiyuan University of Science and Technology, 2017.