Some common deficiencies in the extrusion coating of lithium - ion batteries

At present, new energy industry including electric cars and energy storage battery, is developing rapidly worldwide. Recognized as the ideal energy storage component, power lithium battery has been paid close attention to. Coating machine is the key power lithium battery pole piece production process equipment. At present, lithium battery pole piece coating technology mainly includes knife, roller transfer type and slit extrusion type, etc. I work in process, the three coating method are approached. General laboratory equipment adopts scraper type, 3 c battery uses the transfer roller type, and power battery use the slit extrusion type more.

Blade coating

Working principle is shown in figure 1, foil substrate through the applicator roll and direct contact with the slurry chute, excess slurry coating on the foil substrate, between the base material through the paint roller and scraper, scraper gap between substrate and determines the coating thickness, scrape off excess slurry return at the same time, and thus in the substrate surface to form a uniform layer of coating. Main comma blade scraper type. Comma blade is one of the key components of coating head, generally in the round roller surface along the busbar processing into shaped like a comma blade, the blade with high strength and hardness, easy to control the coating weight and coating precision, suitable for high solid content and high viscosity of the slurry.

Figure 4 comma blade coating

Transfer roller type

Coating roller rotation driven pulp, stock transfer by comma blade clearance to adjust, and use the back roller, coating roller and the rotation of the moving paste to the substrate, the technological process is shown in figure 2.Roller transfer coating consists of two basic processes: (1) the applicator roll rotation driven pulp through the measurement of roll gap, forming a certain thickness of pulp layer;(2) through a certain thickness of pulp layer direction relative to the paint roller and back roller rotating transfer slurry coating is formed on the foil.

Figure 2 roller scraper transfer coating process diagram

Slit extrusion coating

As a sophisticated wet coating technology, as shown in figure 3, the working principle of coating liquid under certain pressure must flow along the coating mold gap squeeze out and transfer to the substrate. Compared to other coating method, has many advantages, such as the coating speed, high precision, wet thickness; Coating system is closed, in can prevent pollutants from entering the coating process, high utilization rate of slurry and able to maintain stability of the slurry properties, can be simultaneously multilayer coating. And can adapt to different slurry viscosity and solid content range, compared with the transfer type coating process has stronger adaptability.

Figure 3 slit extrusion type coating

To form a stable and uniform coating, coating process needs to be at the same time satisfy the several conditions:

(1) pulp properties is stable, not settling, such as viscosity, solid content don't change.

(2) slurry on the material supply is stable, homogeneous steady flow state is formed within the die head.

(3) coating process in the range of the coating window between the die and the roller coating to form stable flow field.

(4) go foil is stable, don't walk with sliding, severe vibration and fold.

Coating operation window is slit coating is an important process parameters, in the actual production, when the process parameters are beyond the scope of operation window, coating defect will be produced. Many species of coated film defects, for different reasons, this article is aimed at lithium ion batteries slit extrusion coating, several common defects is analyzed, and corresponding solutions are given. Common defect is a little defect, edge effect, saw tooth defects, etc.

1 point defects

1.1 stomata: one is air bubbles (mixing process, transport process, coating process);Air bubble produced by pinhole defect is easier to understand, in the wet film bubble from the inner to the membrane surface migration, the pinhole defects on the membrane surface rupture. Bubbles, mainly from mixing of fluid transport and coating process.

1.2 foreign material shrinkage cavity: all kinds of particles, dust, grease, metal particles, etc.).The presence of foreign particles in particle surface area of the wet film with low surface tension, the liquid film to FaSheZhuang around particles migration, formation of shrinkage cavity point defects, as shown in figure 4.Preventive measures mainly include: coating liquid filter in addition to iron, environmental dust control, base material surface clean.

Figure 4 foreign particles in surface tension is low, with fluid migration

1.3 aggregate particles bump: if the slurry mixing unevenly, conductive agent did not spread out, the formation of aggregate can produce this kind of defects, as shown in figure 5, the sheet surface the bulge of large area, enlarge the local observation, found that this is the aggregate of conductive agent. This kind of defect is mainly to improve pulp mixing process to eliminate.

Figure 5 aggregate particles bump SEM morphology

2 linear defects

Nick: 2.1 with thin coating direction parallel linear area or leakage foil line, as shown in figure 6.

Figure 6 coated sheet was scratch defects

Possible reasons for

Card - foreign or large particles inside a slit gap or coated on clearance

- quality of base material, resulting in a foreign body block on the coating of paint roller and back roller clearance

- mold die lip lesions

countermeasures

- remove lip or coated particles, check the die lip clearance

2.2 bar: parallel to the direction of coating of corrugated, as shown in figure 7.

Figure 7 coated sheet bar defects

Possible reasons for

- usually occurs near the speed of the coating window cap, thin coating is more obvious.

countermeasures

- adjust the viscosity of slurry

- slows the coating

- reduce the paint roller and back roller coating gap between

2.3 horizontal lines: in vertical direction, the coating produced by fixed interval corrugated or lines.

Possible reasons for

- mechanical shock

- walk speed variation

- the flow of the slurry feed cyclical fluctuations

countermeasures

- confirm that mechanical disturbance frequency and frequency horizontal stripes are the same

3 edge effect

3.1 thick edge: coating process, often appear edge thick, thin phenomenon among them, the thick edge. Causes of webbing is driven by surface tension of the material transfer. As shown in figure 8, in the beginning, the edge of the wet film is thinner, solvent evaporation speed faster than in the middle, lead to edge higher solid content quickly, on the edge of the surface tension is greater than the middle of the wet film surface tension, the surface tension of the edge is bigger and faster solvent volatilization move to the edge of the inside liquid, form a thick edge after drying.

Figure 8 thick edge phenomenon in the process of drying process

Coated webbing phenomenon is a kind of bad defects, prevent and relieve the thick edge phenomenon are:

(1) the slurry flow must be, to decrease the size of slot increases the exit velocity of pulp in the die head, thereby reducing the drag force ratio of the thick liquid material, thus reducing thickness and the thickness of the coating, but the slit size smaller internal die head pressure is greater, the expansion of the exports are more likely to cause the die shape, resulting in coating thickness transverse inhomogeneity, it need higher precision coating equipment.

(2) coating clearance limited to decrease thickness and coating thickness and width.

(3) reduce the surface tension of the thick liquid material, such as adding surfactants, such as inhibition of slurry to the edge of the stretch film during drying.

(4) to optimize the slit gasket export shape, change the direction of slurry flow speed and size, the stress state of the lower edge of the pulp decrease pulp edge expansion effect.

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