Slot extrusion coating technology is an advanced pre-measured coating technology, can get higher precision coating, at present, the lithium ion battery industry has been widely used slit extrusion coating technology to manufacture battery pole piece. Slit extrusion coating schematic diagram shown in Figure 1, a certain flow of slurry from the extrusion head into the die cavity inside the mouth, and the formation of a stable pressure, the slurry is finally discharged at the die slit outlet , Coated on the foil material.
Figure 1 slit squeeze coating schematic
Extrusion die is a key component of lithium battery coating, which directly determines the quality and uniformity of the coating. Therefore, the die cost is generally greater than 30% of the whole coating machine. The coating die structure mainly includes the upper die, Under the mold, gasket three parts. Under the film has a special cavity, the upper die is relatively simple, the gasket located between the upper and lower mold can be selected according to different coating forms, as shown in Figure 2. Affect the coating thickness uniformity The main factors of the extrusion die cavity cavity exit speed uniformity, the substrate flatness, slurry uniformity and surface tension, etc., which extrusion die outlet speed uniformity is one of the main factors. The geometrical structure of the die head cavity directly affects the flow field shape of the cavity, and the optimization of the structural parameters can effectively improve the uniformity of the outlet velocity distribution.At present, there are many domestic companies that independently design and develop coated dies, and the design optimization includes:
(1) Interior design of coating die, such as gradient, hanger, single chamber and double chamber, etc. The goal is to maintain the flow rate of the coating liquid in the mold without generating static area or settlement Problems, so as to ensure uniform die slit exit speed to ensure the uniformity of the coating.
(2) feed location optimization design, such as the lower part of the feed head, the feed side of the die, etc., to change the fluid flow state to ensure that the die slit exit speed uniform.
(3) gasket design optimization.
Figure 2 extrusion coating die
Among them, the coating die edge is again the key to the entire die, as shown in Figure 2, when the slurry coating flow through the edge, the slit from the discharge, the edge of the performance index will have a direct impact Cloth effect.According to the characteristics of lithium-ion battery coating, extrusion die cutting edge should meet the following requirements:
(1) high precision edge size Lithium-ion battery coating general slurry wet thickness 100-300 μm, accuracy requirements<1%, 因此, 对刃口的尺寸要求也很高, 比如刃口要求锋利, 达到微米级精度; 刃口直线度高 (小于2μm/m) .
(2) Nozzle surface smooth, small roughness (Rz0.2μm below). Slurry is a particle suspension of active material, flowing through the edge and then spit out from the slit, if the stagnation of solid accumulation somewhere, will result in coating Uneven thickness, the formation of vertical tracks and other defects.This requires spout surface smooth, small roughness, does not cause solid particles clogging.
(3) cutting edge material hardness, wear resistance, long life. Positive and negative electrode slurry is composed of solid particles suspended in the coating process under the pressure of pulp slurry continuously from the edge of the slot , Solid particles will be sprayed on the surface of the blade to form abrasive wear, which requires cutting edge material hardness, wear-resistant, long-term use.
(4) Corrosion-resistant Lithium-ion battery slurry often contains organic solvents, polymer binders and other components, the positive slurry tends to be weakly alkaline, therefore, requires cutting edge corrosion resistance.
(4) Sharp blade edge, requires a certain toughness, will not fracture, requiring high compressive strength and bending strength, bending deformation will not occur.
Taking into account the specific use of the environment and cost factors, we should choose the appropriate extrusion die material, as shown in Table 1 can consider these different material combinations.
Table 1 extrusion die material options
Carbide is a powder metallurgy mainly consisting of a hard phase and a binder phase. The hard phase is hard, mainly a variety of carbides, the main carbides are: tungsten carbide (WC), titanium carbide, carbonization Tantalum and niobium carbide, cobalt as the binder phase. Japan's TF15 is a tungsten steel carbide, the grain size of this material is small, less than 1μm, taking into account the hardness and toughness, easy to form a sharp edge, the tip strength And high, the basic performance parameters shown in Table 2. Figure 3 is the ultra-hard alloy edge and stainless steel edge precision contrast, the corresponding use of these two different blade material die coating, coating thickness uniformity is also different , As shown in Figure 4. The cutting edge with excellent wear resistance of cemented carbide, enabling long life, high quality.
Table 2 TF15 basic performance parameters of the material
Figure 3 Superhard alloy blade edge and stainless steel edge comparison
Figure 4 tungsten carbide and stainless steel blade edge die coating thickness uniformity comparison
To produce high-quality batteries require high-precision equipment, equipment development also needs continuous improvement. The above summary, please correct me wrong place.
