At present, new energy industries such as electric vehicles and energy storage batteries are rapidly developing around the world. As a recognized ideal energy storage component, power lithium batteries are also receiving high attention. The coating machine is a key process equipment for producing power lithium battery pole pieces. At present, the process for coating lithium battery pole pieces is mainly scraper type, roller type transfer type and slit type extrusion type. In the process of work, all three types of coating methods have come into contact with each other. The general laboratory equipment adopts scraper type. 3C battery adopts roller coating transfer type, while power battery uses slit extrusion type.
Scraper coating
The working principle is shown in Fig. 1. The foil substrate passes through the coating roller and directly contacts with the slurry tank. The excess slurry is coated on the foil substrate. When the substrate passes between the coating roller and the scraper, the scraper and the substrate are The gap between the materials determines the thickness of the coating and at the same time scrapes the excess slurry back and creates a uniform coating on the substrate surface. The squeegee type is mainly a comma blade. The comma blade is in the coating head. One of the key components is generally formed on the surface of a round roll along the generatrix to form a comma-like cutting edge. This blade has a high strength and hardness, and it is easy to control the coating amount and coating accuracy. It is suitable for high solid content and high viscosity pastes. material.
Figure 1 comma blade coating diagram
Roller transfer
The rotation of the coating roller drives the slurry. The squeegee blade gap is used to adjust the slurry transfer amount, and the slurry is transferred to the substrate by the rotation of the back roller and the coating roller. The process is shown in Figure 2. The roller coating transfer coating contains Two basic processes: (1) The coating roller rotates and drives the slurry through the gap of the metering roller to form a certain thickness of the slurry layer; (2) A certain thickness of the slurry layer transfers the slurry through the opposite coating roller and the back roller. Form a coating on the foil.
Fig. 2 Schematic diagram of roller coating process
Slot extrusion coating
As a kind of precise wet coating technology, as shown in Fig. 3, the working principle is that the coating liquid is pressed and sprayed along the gap of the coating die at a certain pressure and is transferred to the substrate. The cloth method has many advantages, such as high coating speed, high precision, and uniform wet and thick coating; the coating system is closed, and pollutants can be prevented from entering during the coating process. The slurry utilization rate is high and the slurry can maintain stable properties. At the same time for multi-layer coating. And can adapt to different slurry viscosity and solid content range, compared with the transfer coating process has a stronger adaptability.
Figure 3 Slot extrusion coating schematic
To form a stable and uniform coating, it is necessary to satisfy these conditions at the same time in the coating process:
(1) Slurry properties are stable, settlement does not occur, viscosity, solid content, etc. do not change.
(2) The slurry feeding is stable, and a uniform and stable flow is formed inside the die.
(3) Coating process within the coating window, forming a stable flow field between the die head and the coating roll.
(4) The foil is stable, no belt slippage, severe jitter and wrinkles.
The coating operation window is an important process parameter for slit coating. In the actual production, when the process parameters exceed the range of the operation window, coating defects will occur. The coating film has many types of defects, and the reasons are different. This paper mainly focuses on the slit extrusion coating of lithium-ion batteries, analyzes several common defects, and presents corresponding solutions. Common defects are defects, edge effects, sawtooth defects, etc.
1 point defect
1.1 Blowholes: First, bubbles are generated (stirring process, transport process, coating process); Pinhole defects caused by bubbles are easier to understand, bubbles in the wet film migrate from the inner layer to the film surface, and cracks form on the film surface to form pinholes. Defects. Bubbles mainly come from stirring, coating fluid transport and coating process.
1.2 Foreign material shrinkage hole: Various particles (dust, oil, metal particles, etc.) are generated. The presence of foreign particles causes the surface of the wet film at the surface of the particles to have a low surface tension region, and the liquid film radiates outwardly around the particles to form a shrinkage point. Defects, as shown in Figure 4. The main preventive measures are: fluid filtration and iron removal, environmental dust control, and substrate surface cleaning.
Fig. 4 The surface tension at the foreign particles is low and the coating fluid migrates to the surrounding area
1.3 Agglomerate particle bulge: If the slurry is stirred unevenly, the conductive agent does not disperse, and such defects occur when agglomerates are formed. As shown in FIG. 5, large surface bulges appear on the surface of the pole pieces, and these areas are enlarged. Observed, found that this is agglomerates of conductive agents. This defect is mainly to improve the slurry mixing process to eliminate.
Fig.5 Aggregate particle SEM morphology
2 linear defects
2.1 Scratches: Line thin areas or leaky foil lines parallel to the coating direction, as shown in Figure 6.
Fig. 6 Scratch defect of the coated pole piece
• Possible Causes
– foreign matter or large particles stuck in the slit gap or coating gap
- The quality of the substrate is poor, causing foreign matter to stop on the coating gap between the coating roller and the back roller.
– mold lip damage
• Countermeasures
– Remove particles from lip or coating gap, check die lip
2.2 Vertical bar: The corrugation parallel to the coating direction, as shown in Figure 7.
Fig. 7 Vertical stripe defect of coated pole piece
• Possible Causes
– Usually occurs at the upper speed limit near the coating window, the thin coating is more pronounced.
• Countermeasures
- Adjust the viscosity of the paste
- Reduce coating speed
– Reduce the coating gap between the coating roller and the back roller
2.3 Transverse pattern: The corrugation or line produced by a fixed interval perpendicular to the coating direction.
• Possible Causes
– Mechanical shock
– Travel speed fluctuations
– Periodic fluctuations in the flow of slurry feed
• Countermeasures
– Check if the mechanical disturbance frequency is the same as the frequency of the horizontal stripes
3 edge effect
3.1 Thick Edges: During the coating process, edges are often thick and thin in the middle, ie, thick edges. The reason for thick edges is the migration of substances driven by the surface tension. As shown in Figure 8, the edges of the wet film are at the beginning. Thinner, solvent volatilization rate is faster than the middle, resulting in rapid increase of edge solid content, the surface tension of the edge is much greater than the surface tension of the middle wet film, the larger surface tension at the edge and faster solvent evaporation drive the inner liquid to the edge , forming a thick edge after drying.
Figure 8 Process of thick edge phenomenon during drying
The application of thick edge phenomenon is an unfavorable defect. Measures to prevent and alleviate the thick edge phenomenon include:
(1) When the slurry flow is constant, decreasing the slit size can increase the exit speed of the slurry at the die, thereby reducing the drag ratio of the slurry, thereby reducing the thickness of the thick edge coating, but the slit size becomes smaller The pressure inside the die is greater, and it is more likely to cause the die outlet shape to expand, resulting in uneven lateral thickness of the coating, which requires a more precise coating equipment.
(2) The decrease of the coating gap can reduce the thickness and width of the thick edge coating.
(3) reduce the surface tension of the slurry, such as adding surfactants, etc., to suppress the slurry to the edge of the drying process of casting.
(4) Optimize the outlet shape of the slit gasket, change the direction and size of the slurry flow velocity, reduce the stress state of the edge slurry, and weaken the edge expansion effect of the slurry.
