Lithium ion battery pole piece after slurry coating, drying and rolling, the formation of the current collector and the coating on both sides of the three-layer composite structure and then according to the battery design structure and specifications, we need to re-cut the pole piece. , Winding the battery, the pole piece according to the design width of the stripe; laminated batteries, pole pieces corresponding cut into pieces, as shown in Figure 1. Currently, Li-ion battery pole piece cutting process mainly uses the following three: 1) disc shears, (2) die punching, (3) laser cutting.
Figure 1 lithium-ion battery positive and negative pole piece schematic
During the process of pole piece cutting, the quality of the cutting edge of the pole piece has an important influence on the performance and quality of the battery, including: (1) burrs and impurities cause a short circuit in the battery and cause self-discharge or even thermal runaway; (2) Dimensional accuracy is poor, can not guarantee the negative electrode completely wrapped positive, or the separator is completely isolated positive and negative pole piece, causing battery safety problems; (3) material thermal damage, coating off, resulting in the loss of activity of the material, can not play a role; The irregularity of trimming edge causes the nonuniformity of charging and discharging of the pole piece, so the pole piece cutting technique needs to avoid these problems and improve the craft quality.
1, disc cut and cut
Disc cutting mainly the upper and lower disc knife, mounted on the cutter shaft, the use of rolling principle to sub-cut the thickness of 0.01 ~ 0.1 mm roll positive and negative pole piece. Disc on the cut The basic principle of technology, the quality of the pole piece cutting quality, process defects and cutter failure mode, has been sorted out, click the link to read:
Lithium battery pole piece cutting process basis
2, Die punching
Lithium-ion battery pole piece die-cutting process is divided into two types: (1) woodcutter die cutting, sharp blade mounted on the board, under certain pressure will be cutting edge pole piece.This process is simple mold, the cost Low, but not easy to control the quality of die-cutting, and now gradually be eliminated. (2) die cutting hardware, the use of punch and under the knife mode a very small gap on the film cutting, shown in Figure 2. Coating particles through The adhesive is connected together, during the punching process, under the action of stress peeling particles between the coating, the deformation of the metal foil deformation occurs after breaking strength to produce cracks, crack expansion separation, the metal foil fracture separation process Shown in Figure 3. Section of metal blanking section is divided into four parts: the collapse, shear zone, fault zone and burr.The section of the shear zone is wider, the smaller the height of the collapse and burr, blanking The higher the quality of the cross-section.
Figure 2 punching principle diagram
Figure 3 metal foil punching fracture process
Punching cutting process between punch and die punching gap, you can use the following formula (1) said:
(1)
Among them, CL is punching gap, D and d are the size of the upper and lower die, t is the thickness of the plate, shown in Figure 2. Considering the die wear, the effective blanking clearance Cle defined as equation (2):
(2)
Among them, the mold wear process simplified as shown in Figure 4a, the amount of mold wear a and b said that when the wear occurs, as a, b value changes, when the mold wear, the effective punch clearance Cle will be a corresponding increase, As shown in Fig.4 (b), 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 punching process.With the die wear, the punching clearance increases and the fillet of the die edge increases , The quality of the section of blanking parts will also change.
Figure 4 die wear and effective punching clearance
(a) die wear schematic, (b) limited die-cut gap increases with wear amount
3, laser cutting
Disc cutting and die cutting tool wear problems exist, which easily lead to process instability, resulting in poor quality of pole piece cutting, causing the battery performance degradation.Laser cutting has the advantages of high production efficiency, good process stability, has been in the industry Is applied to the cutting of the lithium ion battery pole piece. The basic principle is to irradiate the severed battery pole piece with a high-power density laser beam, so that the pole piece is quickly heated to a high temperature and rapidly melts, evaporates and ablates Or to achieve the ignition point and the formation of holes, with the beam moving in the pole piece, the hole to form a narrow width of the continuous slit, to complete the pole pieces of the cut.
Among them, the laser energy and cutting speed are the two main process parameters, which have a great influence on the cutting quality.Figure 5 shows the trimming morphology of single-coated negative plate under different laser cutting conditions, When the laser power is too low or the moving speed is too fast, the pole piece can not be completely cut, but when the power is too high or the moving speed is too low, the laser The role of the material area becomes larger, kerf size larger.
Figure 5 under different laser cutting conditions single-coated negative pole piece trimming morphology
Figure 6 under different laser cutting conditions single-sided coating positive pole piece trimming morphology
As the lithium ion battery pole piece is a double-sided coating + intermediate current collector metal layer structure, and the difference between the properties of the coating and the metal foil, the role of the laser response is not the same laser role in the negative graphite layer or positive Living layers, due to their high laser absorption and low thermal conductivity, require relatively low levels of melting and vaporization of the laser energy, whereas metal current collectors have a reflective effect on the laser and have fast thermal conduction, So the metal layer melting and vaporization of laser energy increases.Figure 7 is a single-sided coating of the negative electrode under the action of laser in the thickness direction of the copper component and temperature distribution, when the laser in the graphite layer, due to the characteristics of the material, graphite The main occurrence of vaporization, when the laser invades the metal copper foil, the copper foil begins to melt, the formation of molten pool.If the process parameters are inappropriate, there may be problems: (1) trimming coating shedding, exposing the metal foil, as shown in Figure 8 (2) There is a lot of chip foreign material around the trimming edge, which will cause the battery performance degradation and safety quality problems, as shown on the right of Figure 8. Therefore, when using laser cutting, According to the characteristics of living material and metal foil, optimize the appropriate process parameters in order to both complete cutting pole piece, but also form a good trimming quality, does not produce metal chip impurities.
Fig.7 Copper composition and temperature distribution in the thickness direction of the anode under the action of laser
Figure 8 Trimming Problem: Dew Foil and Chip Foreign Objects
