What is lithium-ion battery electrode slitting?
Electrode Slitting Methods
Slitting is called shearing in mechanical processing, and according to the form of the cutting tools, it can be divided into shearing methods such as oblique-blade shearing, flat-blade shearing, rotary shearing (or roll cutting), and disc shearing.
In oblique-blade shearing, there is a fixed angle between the upper and lower blades, and the inclination angle is generally 1° ~ 6°. Generally, the upper blade is inclined, as shown in Figure 7-2(a). Since the upper and lower blades are not parallel, there is a force along the cutting direction, which easily causes distortion of the cut edge. However, the shearing area is small, and the shearing force and energy consumption are smaller than that of flat-blade shearing, so it is used for shearing thick plates in large and medium-sized shearing machines, and is generally not used for electrode slitting.
Flat-blade shearing has the same structure as oblique-blade shearing, except that the upper and lower blade edges are parallel, as shown in Figure 7-2(b). It achieves cutting without distortion and has good cutting quality, but the shearing force is large, and it is mostly used in small shearing machines and for blanking thin plates and films[1-2], as well as for electrode cross-cutting.
Rotary shearing is also called arc-edge roll cutting, which uses tools with an arc-shaped edge. The tools complete the shearing process by rotating and rolling around two fixed axes, as shown in Figure 7-2(c). It is mainly used to realize fixed-length cross-cutting, head and tail cross-cutting, and edge slitting. Generally, when shearing medium and thick plates, it features high quality, low energy consumption, long life, and high output[3].
Disc shearing is completed by the continuous rotation of two disc-shaped cutter wheels, one above and one below, as shown in Figure 7-3. During shearing, the uncoiled electrode sheet enters the disc cutter opening and is divided into multiple strips after cutting[1]. Disc shearing is widely used for the longitudinal slitting of thin plates, thin films, and metal foils.
In the production of lithium-ion batteries, the longitudinal cutting (slitting) of electrode sheets usually uses disc shearing, while cross-cutting uses flat-blade shearing. Automated production lines usually perform slitting first, and then cross-cutting. Generally, the following requirements are placed on the slitting of lithium-ion battery electrode sheets:
① High dimensional accuracy of the electrode sheet;
② The electrode sheet edges are flat and free of burrs, with few defects, and the electrode coating layer is not damaged;
③ High yield and high production efficiency.
Electrode Sheet Shearing Process
For plastic materials, the shearing process can be divided into three stages[5-6]:
① The blade begins to press into the plate, and the plate material between the cuts undergoes plastic deformation and flow until the amount of plastic deformation reaches the limit of plastic strain. This is called the first stage. During this stage, the plate material is sheared, forming a smooth shear surface, see Figure 7-4(a).
② The blade continues to press into the plate, and cracks nucleate, propagate, and penetrate in the plate material between the cuts. The plate material between the cuts undergoes tearing under the action of tensile stress until the upper and lower blades are on the same horizontal plane. This is called the second stage. In this stage, the plate material is torn, forming a dull fracture surface, see Figures 7-4(b) and (c);
③ The blade continues to press down. Due to plastic flow, the plate material between the cuts is extruded into the small gap where the blades overlap. As the overlap of the blade cuts increases, the plate material in the gap is torn and pulled off, forming burrs at the edge, see Figure 7-4(d), until the tools separate. This is called the burr formation stage. During the tool separation process, the tools will have a smoothing or extruding effect on the plate material, causing the morphology of the dull fracture surface and burrs to change, but the change is small.
The shear cross-section after cutting is shown in Figure 7-5, which consists of three parts: the smooth shear surface, the dull fracture surface, and the edge burr[5-6]. The dull fracture surface is also called the brittle fracture surface. Its width is related to the material's plasticity. For materials with better plasticity, the dull fracture surface is smaller, the smooth shear surface is larger, and the metal flow generated is greater, making it more prone to burrs. Conversely, for brittle materials, there is no smooth shear surface.
