CR1625 battery.Basics of lithium battery pole disc slitting process

　　The production of pole pieces is the basic process for manufacturing lithium-ion power batteries, which has very high requirements on the accuracy, intelligence level, and reliability of production performance of the equipment [1]. The pole piece slitting machine is an equipment that slits the rolled battery pole pieces according to the battery specifications. The main technical requirement is that the pole pieces after slitting should not have wrinkles or powder removal. It requires high dimensional accuracy of the strips and at the same time extremely high precision. The burrs on the edge of the film should be small, otherwise dendrites will be generated on the burrs to pierce the separator, causing a short circuit inside the battery [2]. The disc slitting knives mainly include upper and lower disc knives, which are installed on the knife shaft of the slitting machine. They use the rolling shear principle to slit rolls of aluminum foil, copper foil, and positive and negative electrode sheets with a thickness of 0.01~0.1mm. wait. Here we organize, share and learn with you the basic knowledge of lithium battery pole disc slitting process.

　　1. Principle of pole piece disk cutting

　　Referring to the metal plate disc slitting process, we first introduce the basic principles of pole piece disc slitting. As shown in Figure 1, this is a schematic diagram of a pair of ordinary disc cutters slitting the plate. First, when the plate comes into contact with the AB points of the upper and lower blades, the plate will be elastically deformed by the pressure of the upper and lower blades, and due to the existence of moments, the plate will bend, and a shear stress will occur inside the material near the gap. main stress. As the blade points A and B rotate to positions C and D, plastic deformation occurs when the internal stress state meets the plastic conditions. As the shearing process continues, the shear force on the plate becomes larger and larger, and it enters the shear yield state. The shear deformation zone begins to produce macroscopic slip deformation, and the upper and lower circular knife shear edges begin to cut into the material. At this time, the material at the cutting edge undergoes plastic deformation (observed in the feeding direction in Figure 1), shear plastic slip is formed, and the cross section is bright. As the cutterhead continues to rotate, the plastic deformation of the material intensifies, the material will undergo work hardening, and its stress state will also change, thus causing microscopic cracks to appear inside the material. As the deformation continues, these microcracks merge into the main The crack transforms into crack expansion and separation, and the cross section forms a tear zone.

　　Figure 1 Schematic diagram of the disk slitting process

　　Compared with metal sheet slitting, the cutting method of lithium battery pole disc shears has completely different characteristics:

　　(1) When cutting pole pieces, the upper and lower disc knives have back angles, similar to scissor blades, 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 slitting the sheet, there are basically rubber rollers on the upper and lower sides to balance the shear force and shearing moment generated by the upper and lower knives during shearing and avoid significant deformation of the sheet. 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 occur in the coating and expand and separate.

　　2. Factors affecting pole piece cutting quality

　　There are many factors that affect the quality of the burr, such as the size of the burr, the cross-sectional morphological characteristics, and the dimensional accuracy of the pole piece. According to the existing theory, it can be summarized as follows: the physical and mechanical properties of the pole piece, the thickness of the pole piece, and the lateral pressure of the upper and lower paired tools. (Parameter c in Figure 1), the overlap amount of the upper and lower paired tools (Parameter δ in Figure 1), edge wear status, bite angle (Parameter α in Figure 1), disc cutter accuracy, etc.

　　(1) Influence of physical and mechanical properties of materials. Generally speaking, if the material has good plasticity, cracks will appear later during shearing. The material is sheared to a greater depth, and the proportion of bright bands in the resulting section will be larger; while for materials with poor plasticity, under the same parameter conditions , it is easy to break, the proportion of the tear zone in the cross section will be larger, and the bright zone will naturally be smaller.

　　(2) The influence of the lateral pressure of the upper and lower paired tools (the 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 on the fracture surface overlap and the stress and strain state of the shearing force are closely related to the magnitude of the lateral pressure. If the lateral pressure is too small, defects such as uneven cutting surfaces and material loss may occur during pole piece slitting. If the lateral pressure is too high, the tool will be more likely to wear and have a shorter service 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 pole piece. A reasonable overlap amount is conducive to the bite of the tool. Its impact includes issues such as the quality of shearing, the size of the burrs, and the speed of tool edge wear.

　　(4) The influence of bite angle (parameter α in Figure 1). In disc slitting, the bite angle refers to the angle between the shear segment and the center line of the sheet being sheared. As the bite angle increases, the horizontal component of the shear force will also increase. If the horizontal component force is greater than the feeding tension of the pole piece, the plate will either slip or arch in front of the circular knife and cannot be sheared. As 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 be determined with reference to actual working conditions.

　　In the pole piece slitting process, the lateral pressure and overlap amount of the tool are the main adjustment parameters of the disc cutter part, which need to be adjusted in detail according to the nature and thickness of the pole piece. In the past equipment manufacturing and process, tool adjustment often lacked precise numerical parameters, but relied on experience to make corresponding adjustments based on the pole piece batch. With the advancement of equipment technology, tool adjustment technology has also continued to advance and become numerical. At present, there is an automatic adjustment device for the lateral pressure cylinder of the pole piece slitting machine. When pole piece slitting, the cylinder pressure is set to adjust the lateral pressure of the cutter to control the slitting quality.

　　3. Main defects of pole piece slitting

　　Figure 2 shows the typical morphology of the pole piece’s cut section. The main particles of the coating on the fracture surface are peeled off and fractured, while the current collector is plastically cut and torn. When the compaction density of the pole piece coating increases and the bonding force between particles increases, some particles of the pole piece coating also appear to be cut off. The main defects in pole piece slitting include the following:

　　Figure 2 Typical morphology of pole piece cut section

　　(1) Burr

　　Burrs, especially metal burrs, are extremely harmful to lithium batteries. Larger metal burrs directly pierce the separator, causing a short circuit between the positive and negative electrodes. The pole piece slitting process is the main process that produces burrs in the lithium-ion battery manufacturing process. Figure 3 shows the typical morphology of metal burrs produced by pole piece slitting. Current collector burrs are formed during pole piece slitting, and the size reaches more than 100 μm. Table 1 shows the experimental results of LiFePO4 pole piece slitting burrs. The number and size of burrs can be controlled by adjusting the cutter chamfering, tool lateral pressure and rewinding and unwinding tension.

　　Figure 3 Metal burrs for pole piece slitting

　　Table 1 LiFePO4 pole piece slitting burr experimental results

　　In order to avoid this situation, it is most critical to find the most suitable lateral pressure and tool overlap based on the properties and thickness of the pole piece when adjusting the tool. In addition, the edge quality of the pole piece can be improved by cutting and chamfering, and retracting and unwinding tension.

　　(2) Wavy edge

　　Figure 4 shows the material dropout and wavy edge defects that exist when the pole piece is slit. The wavy edge. When wavy edges appear, edge correction jitter will occur during pole piece slitting and winding, which will cause process accuracy. In addition, it will also have a negative impact on the final thickness and shape of the battery.

　　Figure 4: Material loss during pole piece cutting and wavy edge defects

　　(3) Powder loss

　　As shown in Figure 4, the powder falling off from the pole piece will affect the battery performance. When the positive electrode powder falls off, the battery capacity is reduced. When the negative electrode powder falls off, the negative electrode cannot wrap the positive electrode, which can easily cause lithium precipitation.

　　The above quality problems are mainly solved by finding suitable tool adjustment parameters.

　　(4) The size does not meet the requirements

　　The pole piece slitting machine slits the rolled battery pole pieces according to the battery specifications, which requires high dimensional accuracy of the slit pole pieces. When designing a wound battery, the separator should wrap the negative electrode to prevent direct contact between the positive and negative electrodes to form a short circuit. The negative electrode should wrap the positive electrode to prevent the lithium ions in the positive electrode from being absorbed by the negative active material during charging. Generally, the negative electrode and The size difference between the separator, negative electrode and positive electrode is 2-3mm, and as the specific energy requirements increase, this size difference continues to decrease. Therefore, the dimensional accuracy of the pole pieces is required to be higher and higher, otherwise the battery will have serious quality problems.

　　4. Main failure modes of disc cutters

　　During the pole piece slitting process, the quality of the slitting disc blade directly affects the pole piece slitting performance. 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, etc. are all important. The disc cutter has a certain lifespan during continuous operation. In the slitting process, the life of the cutter needs to be managed. Generally, the main failure mode of the pole piece disc slitting knife is tool wear, as shown in Figure 5. Specifically include:

　　Figure 5 Tool wear morphology

　　(1) Fatigue failure

　　There is lateral pressure on the pole piece slitting disk knife, and the upper and lower knives overlap each other on the side of the knife edge. During the continuous slitting process, the local stress on the cutting edge is large and concentrated. Due to frequent extrusion, shearing, friction and cyclic interactions, Under the action of changing mechanical loads, fatigue micro-cracks are prone to appear in the cutting edge area, and then under the action of cyclic stress, they will expand along the disc cutter side toward the center. When the cracks grow to a certain extent, the stress reaches the strength limit of the tool material. This will cause local micro damage to the cutting edge of the tool.

　　(2) Adhesive wear

　　Adhesives with extrusion marks can also be found in the tool wear area. These adhesions are formed by the adhesion of pole chips to the tool. This is because the side of the disc cutter is in contact with the cutting surface of the pole piece, and there is a large contact pressure, so that the slit material part is cold welded to the side of the disc cutter. These bonding layers continuously fall off and then re-form during the cyclic slitting process. The shedding process may cause the hard phase of the tool to fall off and aggravate the wear process. This situation is most likely to occur when slitting aluminum foil. Due to the low melting point of aluminum, the cutting chips adhere to the side of the tool, causing tool wear and shortening its life.

　　(3) Abrasive wear

　　Although the hardness of chips and pole pieces is lower than that of the tool, they often contain some extremely high hardness tiny hard points that can carve grooves on the surface of the tool. This is abrasive wear.

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