Introduction to 18650 battery 3500mah pole piece cutting technology

　　During the pole piece cutting process, the quality of the pole piece's cutting edges has an important impact on battery performance and quality, including: (1) Burrs and impurities can cause short circuits in the battery, causing self-discharge and even thermal runaway; (2) Poor dimensional accuracy cannot ensure that the negative electrode completely wraps the positive electrode, or that the separator completely isolates the positive and negative electrodes, causing battery safety issues; (3) Thermal damage to the material, peeling of the coating, etc., cause the material to lose activity and fail to function; (4) The unevenness of the trimmed edges causes uneven charging and discharging processes of the pole pieces. Therefore, the pole piece cutting process needs to avoid these problems and improve process quality.

　　1. Disc shear cutting

　　Disc slitting mainly includes upper and lower disc knives, which are installed on the knife shaft of the slitting machine and use the principle of rolling shear to slit the positive and negative electrode pieces in rolls with a thickness of 0.01~0.1 mm. Regarding the basic principles of disc slitting technology, the factors affecting the quality of pole piece slitting, process defects and cutter failure modes, they have been sorted out before. Click the link to read:

　　Basics of 18650 battery 3500mah pole disc slitting process

　　2. Mold punching

　　The die-cutting process of lithium-ion battery pole pieces is divided into two types: (1) Wood board die cutting, a sharp blade is installed on the wood board, and the blade cuts the pole pieces under a certain pressure. This process has simple molds and low cost, but the punching quality is difficult to control and is currently being phased out. (2) Hardware die cutting, use the extremely small gap between the punch and the lower die to cut the pole piece, as shown in Figure 2. The coating particles are connected together through a binder. During the punching process, the coating particles are peeled off under the action of stress, and the metal foil undergoes plastic strain. After reaching the breaking strength, cracks occur, the cracks propagate and separate, and the metal foil The material fracture separation process is shown in Figure 3. The cross section of metal material punching parts is divided into 4 parts: sag, shear zone, fracture zone and burr. The wider the shear band of the section, the smaller the sag angle and burr height, and the higher the section quality of the punched parts.

　　The punching gap between the upper punch and the lower die in the punching process can be expressed by the following formula (1):

　　(1)

　　Among them, CL is the punching gap, D and d are the dimensions of the upper and lower dies, and t is the thickness of the sheet, as shown in Figure 2. When considering the wear of the mold, the effective punching clearance Cle is defined as formula (2):

　　(2)

　　Among them, the die wear process is simplified as shown in Figure 4a. The die wear amount is represented by a and b. When wear occurs, as the values of a and b change, when the die wear occurs, the effective punching clearance Cle will also increase accordingly.

　　The effective gap satisfies the relationship of equation (2). The punching gap and the wear of the die edge have an important impact on the punching process. As the die wears, the punching gap increases, the die edge fillet increases, and the cross-sectional quality of the punched parts will also change.

　　(a) Schematic diagram of mold wear,

　　(b) Curve relationship of limited punching clearance as wear amount increases

　　3. Laser cutting

　　Both disc slitting and die-cutting have tool wear problems, which can easily cause process instability, lead to poor cutting quality of pole pieces, and degrade battery performance. Laser cutting has the characteristics of high production efficiency and good process stability. It has been used in industry to cut lithium-ion battery pole pieces. The basic principle is to use a high-power density laser beam to irradiate the cut battery pole pieces to make the pole pieces It is quickly heated to a very high temperature and quickly melts, vaporizes, ablate or reaches the ignition point to form holes. As the beam moves on the pole piece, the holes continuously form narrow slits to complete the cutting of the pole piece.

　　Among them, laser energy and cutting movement speed are two main process parameters, which have a huge impact on cutting quality. Figure 5 is the cut edge morphology of the single-sided coated negative electrode piece under different laser cutting process conditions. Figure 6 is the cut edge morphology of the single-sided coated positive electrode piece under different laser cutting process conditions. When the laser power is too low or the moving speed is too fast, the pole piece cannot completely cut. When the power is too high or the moving speed is too low, the laser's action area on the material becomes larger and the slit size becomes larger.

　　Figure 5: Cut edge morphology of single-sided coated negative electrode piece under different laser cutting process conditions

　　Figure 6 Trimming morphology of single-sided coated cathode plates under different laser cutting process conditions

　　Since the lithium-ion battery pole piece has a structure of double-sided coating + intermediate current collector metal layer, and the properties of the coating and the metal foil are very different, the response to laser action is also different. When the laser acts on the negative electrode graphite layer or the positive electrode active material layer, because they have high laser absorption rate and low thermal conductivity, the coating requires relatively low melting and vaporization laser energy, and the metal current collector is very sensitive to the laser. It is reflective and conducts heat quickly, so the laser energy for melting and vaporizing the metal layer increases. Figure 7 shows the copper composition and temperature distribution in the thickness direction of the single-sided coated negative electrode under the action of laser. When the laser acts on the graphite layer, due to the characteristics of the material, the graphite mainly vaporizes. When the laser penetrates into the metal copper foil , the copper foil begins to melt and forms a molten pool. When process parameters are inappropriate, problems may arise:

　　(1) The coating on the trimmed edge peels off, exposing the metal foil, as shown in the left picture of Figure 8;

　　(2) A large number of chips and foreign matter appear around the cutting edge. These will lead to battery performance degradation and safety and quality problems, as shown on the right side of Figure 8. Therefore, when using laser cutting, it is necessary to optimize the appropriate process parameters based on the characteristics of the active material and the metal foil, so that the pole piece can be completely cut, and good edge quality can be achieved without leaving metal chips and impurities remaining.

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