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After slurry coating, drying and rolling, the lithium ion battery electrode sheet forms a three-layer composite structure of fluid collector and two-layer coating. Then according to the battery design structure and specifications, we need to cut the pole piece again. Generally, for the winding battery, the pole pieces are divided according to the design width; For the laminated battery, the pole slices are cut into corresponding slices, as shown in Fig. 1. At present, lithium-ion battery electrode slice cutting technology mainly adopts the following three kinds :(1) disc cutting, (2) die cutting, (3) laser cutting.
In the process of electrode slice cutting, the quality of electrode slice cutting edge has an important influence on the performance and quality of the battery, including :(1) Burr and impurities, which will cause short circuit in the battery, self-discharge and even thermal runaway; (2) poor dimensional accuracy, can not ensure that the negative electrode is completely wrapped in the positive electrode, or the diaphragm is completely separated from the positive and negative electrode pieces, causing battery safety problems; (3) material thermal damage, coating peeling, etc., resulting in material loss of activity, unable to play a role; (4) the unevenness of the cutting edge causes the unevenness of the charging and discharging process of the pole plate. Therefore, the pole slice cutting process to prevent these problems, improve the quality of the process.
1. Disc shearing and cutting
Disc slitting is important for upper and lower disc knives, mounted on the cutter shaft of the slitting machine. The principle of rolling shear is used to slitting positive and negative electrode pieces with a thickness of 0.01~0.1mm into rolls. The basic principles of disc slitting technology, factors affecting pole slitting quality, process defects and cutter failure modes have been summarized.
Fundamental technology of electrode slice and disc cutting for lithium ion battery
2, die cutting
Lithium-ion battery pole cutting process is divided into two kinds :(1) wood knife die punching, sharp blade is installed on the board, a certain pressure will be used to cut the blade pole piece. This process mold is simple, low cost, but the cutting quality is not easy to control, is now gradually eliminated. (2) Metal mold punching, using the punch and the very small clearance of the lower die to cut the pole piece, as shown in Figure 2. The coating particles are connected together by the binder. During the punching and cutting process, the coating particles are peeled off under the use of stress. The metal foil has a plastic strain, and cracks appear after reaching the breaking strength. The section of metal material punching is divided into four parts: collapse Angle, shear zone, fracture zone and burr. The wider the shear band of the section, the smaller the collapse Angle and the burr height, the higher the section quality of the punching piece.
Where, Cl is punching clearance, D and D are sizes of upper and lower die heads, and T is thickness of plate, as shown in Fig. 2. When the wear of the die is taken into account, the effective punching clearance CLE is defined in Formula (2) :
Where, the die wear process is simplified as shown in Fig. 4A, and the die wear amount is represented by A and B. When wear occurs, with the change of values of A and B, when die wear occurs, the effective punching clearance CLE will increase accordingly, as shown in Fig. 4B, and the effective clearance satisfies the relation of Formula (2). The punching clearance and the wear of the die edge have an important influence on the cutting process. With the wear of the die, the punching clearance increases, the fillet of the die edge increases, and the quality of the cross section of the punching piece will also change.
3, laser cutting
Cutting tool wear is a problem in both disk cutting and die cutting, which is easy to cause instability of the process, resulting in poor cutting quality of pole pieces, resulting in a decline in battery performance. Process of laser cutting with high efficiency, good stability characteristics, has been in industry application in lithium ion battery pole piece cutting, its basic principle is to use the high power density laser beam was cut battery pole piece, make the pole piece was quickly heated to high temperature, rapid melting, evaporation and ablation or achieve ignition point and form holes, with the moving of the beam on the sheet, The hole continues to form a very narrow slit to complete the cutting of the pole piece.
Among them, laser energy and cutting moving speed are two important technological parameters, which have a great influence on cutting quality. Fig. 5 shows the trimming morphology of the single-side coated negative electrode piece under different laser cutting process conditions, and Fig. 6 shows the trimming morphology of the single-side 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 plate can not be completely cut, and when the power is too high or the moving speed is too low, the laser area of material use becomes larger, the cutting size is larger.
Because the lithium ion battery electrode is double-sided coating + intermediate collector metal layer structure, and the properties of the coating and metal foil are very different, the response to the laser use is not the same. Laser applications in the anode graphite layer or the anode material, because they have very high laser absorption rate, coefficient of thermal conductivity is low, therefore, coating to relatively low melting and vaporization of laser energy, fluid and metal sets of laser reflection USES, and heat conduction is fast, so the higher layer metal melting and vaporization of laser energy. Figure 7 shows the copper composition and temperature distribution of the negative electrode of the single-side coating in the direction of the thickness of the electrode sheet under the laser application. When the laser is used in the graphite layer, due to the characteristics of the material, the graphite vaporizes significantly. When the laser intrudes into the metal copper foil, the copper foil begins to melt, forming a molten pool. When the process parameters are not appropriate, problems may occur :(1) The trimming coating falls off, exposing the metal foil, as shown in Figure 8 on the left; (2) there are a lot of chip foreign bodies around the cutting edge. These will lead to battery performance degradation, safety and quality problems, as shown in the right figure of Figure 8. Therefore, when laser cutting is used, according to the characteristics of live material and metal foil, optimize the appropriate process parameters, in order to fully cut the pole sheet, and form a good edge quality, there is no residual metal chip impurities.