Lithium-ion battery pole disc slitting process -Lithium - Ion Battery Equipment

Analysis of basic knowledge of lithium-ion battery pole disc slitting process -Lithium - Ion Battery Equipment

1. Principle of pole piece disk cutting

Referring to the metal plate disc slitting process, we first analyze the basic principles of pole piece disc slitting. First, when the plate comes into contact with the Ab point 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 the moment, the plate will bend, and a shear stress will appear 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. Macroscopic slip deformation begins to occur in the shear deformation zone, and the upper and lower circular knife shear edges begin to cut into the material. At this time, the material at the edge of the cutting edge undergoes plastic deformation, shear plastic slip is formed, and the cross section is bright. As the cutterhead continues to rotate, the degree of plastic deformation of the material intensifies, the material will undergo production 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.(Lithium - Ion Battery Equipment)

Compared with metal sheet slitting production, the cutting method of lithium-ion 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, but the upper and lower disc 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 of the upper and lower knives during shearing, and to prevent 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 in 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 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 pairs of cutting tools. , the superposition amount of the upper and lower paired tools, the wear status of the cutting edge, the bite angle, the accuracy of the disc cutter, 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 upper and lower pairs on the lateral pressure of the tool. 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 upper and lower pairs of tools on the superposition amount. The setting of the overlay amount is mainly related to the thickness of the pole piece. A reasonable overlay amount is conducive to the bite of the tool. Its influence includes the quality of shearing, the size of the burrs and the speed of tool edge wear.

(4) In disc slitting, the bite angle refers to the angle between the shear section 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 stacking amount must be determined by referring to the actual working conditions.

In the pole piece slitting process, the lateral pressure and superposition amount of the tool are important adjustment parameters for the disc cutter part, and they must be adjusted according to the properties and thickness of the pole piece. In the past equipment manufacturing and technology, tool adjustment often lacked precise numerical parameters, but relied on relevant 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 cutting quality.

3. Main disadvantages of pole piece slitting

When the compaction density of the pole piece coating increases and the bonding force between particles strengthens, some particles of the pole piece coating also appear to be cut off. Major defects in pole piece slitting include the following:

(1) Burr

Burrs, especially metal burrs, are extremely harmful to lithium-ion batteries. Larger metal burrs penetrate straight through the separator, causing a short circuit between the positive and negative electrodes. The pole piece slitting process is an important process for the occurrence of burrs in the lithium-ion battery manufacturing process.

In order to prevent this situation from happening, it is most critical to find the most suitable lateral pressure and tool stacking amount 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) Wave 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.

(3) Powder loss

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. Important 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 it is important for the slitting blade to have good mechanical properties, such as hardness, manufacturing process and wear resistance. The disc cutter has a certain lifespan while it continues to operate. The life of the cutter must be managed during the slitting process. Generally speaking, the main failure mode of the pole piece disc slitting knife is tool wear.

(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 slitting process, the local stress on the cutting edge is large and accumulated. Due to frequent extrusion, shearing, friction and cyclic interactions, Due to changing mechanical loads, fatigue micro-cracks are prone to appear in the cutting edge area, and then under cyclic stress, they will expand along the side of the disc toward the center. When the cracks rise to a certain level, 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.



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