What are the cutting methods and characteristics of lithium battery pole pieces?
After slurry coating, drying and rolling, the lithium battery pack pole pieces form a three-layer composite structure of current collector and coating on both sides. Then according to the battery design structure and specifications, we need to cut the pole piece again. Generally, for wound batteries, the pole pieces are divided into strips according to the design width; for laminated batteries, the pole pieces are cut into pieces accordingly. At present, the lithium battery pack pole piece cutting process mainly adopts the following three types: (1) disc shear slitting, (2) die punching, (3) laser cutting.
In the process of lithium battery 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 will cause the battery Internal short circuit causes self-discharge or even thermal runaway; (2) Poor dimensional accuracy, it is impossible to ensure that the negative electrode completely wraps the positive electrode, or the separator completely separates the positive and negative pole pieces, causing battery safety problems; (3) Material thermal damage, coating peeling, etc , Causing the material to lose activity and fail to function; (4) The unevenness of the cutting edge causes unevenness in the charging and discharging process of the pole piece. Therefore, the lithium battery pole piece cutting process needs to avoid these problems and improve the quality of the process.
1. Disc cutter slitting
The disc slitting mainly includes upper and lower disc cutters, which are installed on the cutter shaft of the slitting machine and use the principle of rolling shears to divide Cut the positive and negative pole pieces into rolls with a thickness of 0.01~0.1 mm.
The die-cutting process of lithium battery pole pieces is divided into two types: (1) the die-cutting of the wood board knife, the sharp blade is installed on the wood board, Under certain pressure, the blade is cut open the pole piece. This process mold is simple and low cost, but the punching quality is not easy to control, and it is gradually eliminated. (2) Metal mold punching, using the extremely small gap between the punch and the lower die to cut the pole piece. The coating particles are connected together by a binder. During the punching process, the coating particles peel off under the action of stress, and the metal foil will undergo plastic strain. After reaching the fracture strength, cracks will occur and the cracks will propagate and separate. The cross-section of the metal material punching piece is divided into 4 parts: collapse, shear zone, fracture zone and burr. The wider the shear band of the section, the smaller the collapse angle and the height of the burr, and the higher the quality of the section of the punching piece.
3. Laser cutting of lithium battery pole pieces
There are tool wear problems in both disc slitting and die cutting, which can easily cause process instability and lead to poor cutting quality of pole pieces , Causing battery performance degradation. Laser cutting has the characteristics of high production efficiency and good process stability. It has been applied to the cutting of lithium-ion battery pole pieces in industry. The basic principle is to irradiate the cut battery pole pieces with a high-power density laser beam to make the pole pieces It is quickly heated to a high temperature, quickly melts, vaporizes, ablates or reaches the ignition point to form a hole. As the beam moves on the pole piece, the hole continuously forms a narrow slit to complete the cutting of the pole piece.
Among them, laser energy and cutting movement speed are the two main process parameters, which have a huge impact on cutting quality. When the laser power is too low or the moving speed is too fast, the pole piece cannot be completely cut, and 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 is larger.
Since the lithium battery pole piece is a double-sided coating with a middle current collector metal layer structure, and the properties of the coating and the metal foil are different, the response to the laser action is also different. When the laser acts on the negative graphite layer or the positive active material layer, they have high laser absorption rate and low thermal conductivity. Therefore, the coating requires relatively low melting and vaporization laser energy, while the metal current collector is very effective for the laser. It has a reflective effect, and heat conduction is fast, so the melting and vaporization of the metal layer increases the laser energy.
When the lithium battery pole piece cutting process parameters are not suitable, problems may occur: (1) The trimming coating falls off and the metal foil is exposed; (2) A large number of chips and foreign objects appear around the trimming. These will lead to battery performance degradation, safety and quality problems. Therefore, when laser cutting is used, it is necessary to optimize the appropriate process parameters according to the characteristics of the living material and the metal foil to completely cut the pole piece and form a good trimming quality without leaving metal chips and impurities.
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