CR1620 battery

Pole sheet production is the basic process for manufacturing CR1620 battery

Pole sheet production is the basic process for manufacturing CR1620 battery, and it has very high requirements for the accuracy, intelligence level, and reliability of production performance of the equipment. The pole sheet slitting machine is an equipment that slits the rolled battery pole sheets according to the battery specifications. The main technical requirements are that the pole sheets after slitting cannot have wrinkles or powder removal, and the slitting size accuracy is high. At the same time, the burrs on the edge of the pole sheet are small, otherwise dendrites will be generated on the burrs and pierce the diaphragm, causing a short circuit inside the battery. The disc slitting knives mainly include upper and lower disc knives, which are installed on the knife shaft of the slitting machine and use the rolling shear principle to slit aluminum foil, copper foil, positive and negative pole sheets with a thickness of 0.01~0.1mm. Here, the basic knowledge of the lithium battery pole sheet disc slitting process is sorted out, shared and learned with everyone. 1. The principle of pole sheet disc slitting refers to the disc slitting process of metal plates. The basic principle of pole sheet disc slitting is first introduced. As shown in Figure 1, this is a schematic diagram of a pair of ordinary disc cutters slitting a plate. First, when the plate contacts the AB points of the upper and lower blades, the plate will be subjected to the pressure of the upper and lower blade surfaces and produce elastic deformation, and due to the existence of torque, the plate will bend, and stress dominated by shear stress will be generated inside the material near the gap. As the blade points A and B rotate to positions C and D, plastic deformation occurs when the internal stress state meets the plastic condition. As the shearing process continues, the shear force on the plate becomes larger and larger, entering the shear yield state, and the shear deformation zone begins to produce macroscopic slip deformation. The upper and lower circular knife shear blades begin to cut into the material. At this time, the material attached to the blade edge produces plastic deformation (Figure 1 Observation of the feed direction), shear plastic slip is formed, and the cross section is bright. As the cutter disc continues to rotate, the degree of plastic deformation of the material intensifies, the material will undergo work hardening, and its stress state will also change, resulting in micro cracks inside the material. As the deformation continues, these micro cracks merge into main cracks, transform into crack expansion and separation, and a tearing zone is formed on the cross section.

Figure 1 Schematic diagram of the disc slitting process Compared with the metal sheet slitting process, the cutting method of the lithium battery pole disc shear has completely different characteristics: (1) When cutting the pole piece, the upper and lower disc knives have a back angle, similar to the blade of scissors, and the blade width is particularly small. There is no horizontal gap between the upper and lower disc knives (the parameter c shown in Figure 1 is equivalent to a negative value), but the upper and lower knives are in contact with each other and there is lateral pressure. (2) When cutting the sheet, there are basically rubber rollers on the upper and lower sides to balance the shear force and shear torque generated by the upper and lower knives during shearing, avoiding large deformation of the sheet. However, there are no upper and lower rollers for pole piece slitting. (3) The pole piece coating is a composite material composed of particles and has almost no plastic deformation ability. When the internal stress generated by the upper and lower disc knives is greater than the bonding force between the coating particles, cracks will appear in the coating and expand and separate. 2. Factors affecting the quality of pole piece slitting There are many factors that affect the quality of burr size, cross-sectional morphology and pole piece size accuracy. According to existing theories, they can be summarized as: physical and mechanical properties of pole piece, pole piece thickness, lateral pressure of upper and lower paired tools (parameter c in Figure 1), overlap of upper and lower paired tools (parameter δ in Figure 1), edge wear state, bite angle (parameter α in Figure 1), disc cutter accuracy, etc. (1) Influence of physical and mechanical properties of materials. Generally speaking, the better the plasticity of the material, the later the crack will appear during shearing, the deeper the material is sheared, and the larger the proportion of the bright band in the resulting cross section; while the material with poor plasticity is prone to fracture under the same parameter conditions, the proportion of the tear band in the cross section will be larger, and the bright band will naturally be smaller. (2) Influence of lateral pressure of upper and lower paired tools (parameter c in Figure 1 is equivalent to a negative value). In the slitting of pole pieces, the lateral pressure of the tool is one of the key factors affecting the slitting quality. During shearing, whether the upper and lower cracks of the fracture surface overlap and the stress-strain state of the shear force are closely related to the size of the lateral pressure. If the lateral pressure is too small, the electrode slitting may have defects such as uneven slitting sections and material dropout. If the pressure is too large, the tool is more likely to wear and have a shorter life. (3) The influence of the overlap amount of the upper and lower paired tools (parameter δ in Figure 1). The setting of the overlap amount is mainly related to the thickness of the electrode. A reasonable overlap amount is conducive to the bite of the tool. Its influence includes the quality of shearing, the size of burrs, and the speed of tool edge wear. (4) The influence of the bite angle (parameter α in Figure 1). In disc slitting, the bite angle refers to the angle between the shear section and the center line of the cut sheet. As the bite angle increases, the horizontal component of the shear force will also increase. If the horizontal component is greater than the feed tension of the electrode, the sheet will either slip or arch up in front of the circular knife and cannot be sheared. If the bite angle decreases, the diameter of the blade will increase, and the size of the slitting machine will also increase accordingly. Therefore, how to balance the bite angle, blade diameter, sheet thickness and overlap must refer to the actual working conditions. In the pole piece slitting process, the lateral pressure and overlap of the tool are the main adjustment parameters of the disc cutter part, which need to be adjusted in detail according to the properties and thickness of the pole piece. In the previous equipment manufacturing and process, the tool adjustment often lacks precise numerical parameters, but is adjusted accordingly according to the pole piece batch based on experience. With the advancement of equipment technology, the tool adjustment technology has also been continuously improved and digitized. At present, there is an automatic adjustment device for the lateral pressure cylinder of the pole piece slitting machine tool. When the pole piece is slitting, the lateral pressure of the tool is adjusted by setting the cylinder pressure to control the slitting quality. 3. The main defects of pole piece slitting Figure 2 is a typical morphology of the pole piece slitting section. The main particles of the coating on the fracture surface are peeled off and broken, and the current collector undergoes plastic cutting and tearing. When the compaction density of the pole piece coating increases and the bonding force between the particles increases, some particles of the pole piece coating are also cut off. The main defects in the electrode slitting include the following:

(1) Burrs Burrs, especially metal burrs, are extremely harmful to lithium batteries. Large metal burrs directly pierce the diaphragm, causing a short circuit between the positive and negative electrodes. The electrode slitting process is the main process for burr generation in the lithium-ion battery manufacturing process. Figure 3 shows the typical morphology of metal burrs generated by electrode slitting. During the slitting, the electrode forms a collector burr with a size of more than 100μm. Table 1 is the experimental results of LiFePO4 electrode slitting burrs. The number and size of burrs are controlled by adjusting the cutter chamfer, the tool lateral pressure, and the winding tension.

Figure 3 Pole slitting metal burrs

Table 1 LiFePO4 electrode slitting burr experimental results In order to avoid this situation, it is most critical to find the most suitable lateral pressure and tool overlap according to the nature and thickness of the electrode when adjusting the knife. In addition, the edge quality of the electrode can also be improved by adjusting the cutter chamfer and winding tension. (2) Wavy edge Figure 4 shows the defects of material drop and wavy edge when the electrode is cut. Wavy edge. When wavy edge appears, the edge correction will jitter when the electrode is cut and wound, which will cause process accuracy. In addition, it will have an adverse effect on the final thickness and morphology of the battery.

Figure 4 Pole sheet cutting material drop and wavy edge defects (3) Powder drop As shown in Figure 4, the powder drop of the electrode will affect the battery performance. When the positive electrode drops powder, the battery capacity is reduced, and when the negative electrode drops powder, the negative electrode cannot wrap the positive electrode, which is easy to cause lithium precipitation. The above quality problems are mainly solved by finding suitable knife adjustment parameters. (4) The size does not meet the requirements. The electrode slitting machine cuts the battery electrode after rolling according to the battery specifications, and requires high dimensional accuracy of the cut electrode. When designing a wound battery, the diaphragm should wrap the negative electrode to avoid direct contact between the positive and negative pole pieces to form a short circuit, and the negative electrode should wrap the positive electrode to avoid lithium deposition when the lithium ions in the positive electrode are not accepted by the negative electrode active material during charging. Generally, the size difference between the negative electrode and the diaphragm, and the negative electrode and the positive electrode is 2-3mm, and as the specific energy requirement increases, this size difference continues to decrease. Therefore, the requirements for the accuracy of the pole piece size are getting higher and higher, otherwise the battery will have serious quality problems. 4. The main failure mode of the disc cutter is during the pole piece slitting process. The quality of the slitting disc blade directly affects the performance of the pole piece slitting. The slitting machine needs to run for a long time, so the slitting blade needs to have good mechanical properties, such as hardness, manufacturing process and wear resistance. The disc cutter has a certain life span during continuous operation. The life span of the cutter needs to be managed during the slitting process. Generally, the main failure mode of the pole disc cutter is tool wear, as shown in Figure 5, including:

(1) Fatigue failure The pole disc cutter has lateral pressure, and the upper and lower cutters overlap each other on the side of the blade. During the continuous slitting process, the local stress on the blade is large and concentrated. Due to frequent extrusion, shearing, friction and cyclic alternating mechanical loads, fatigue microcracks are prone to appear in the blade area. Then, under the action of cyclic stress, they will expand toward the center along the side of the disc cutter. When the crack grows to a certain extent and the stress reaches the strength limit of the tool material, it will cause local micro damage to the tool edge. (2) Adhesive wear Adhesives with extrusion marks can also be found in the tool wear area. These adhesives are formed by the pole chip debris adhering to the tool. This is because the side of the disc cutter is in contact with the pole piece slitting surface and there is a large contact pressure, which makes the slitting material partly cold welded to the side of the disc cutter. These adhesive layers are constantly falling off and then re-formed during the cyclic slitting process. The shedding process may cause the hard phase of the tool to fall off, aggravating the wear process. This situation is most likely to occur when slitting aluminum foil. Due to the low melting point of aluminum, the slitting chips and the side of the tool are bonded together, causing tool wear and shortening the life. (3) Abrasive wear Although the hardness of the slitting chips and pole pieces is lower than that of the tool, they often contain some tiny hard points with extremely high hardness, which can scratch grooves on the surface of the tool. This is abrasive wear. Note: Figures 2 to 5 in the article are all derived from production practice or process experiment results.

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