What Is Selection of Coating Methods?

What Is Selection of Coating Methods?

There are many coating methods. Dip coating methods include dip coating, roll coating, blade coating, and wire-wound rod coating, etc. Preset quantity coating methods include slide coating, slot-die coating, extrusion coating, and curtain coating, etc. We have already discussed several coating methods in detail above. Below we briefly introduce the other coating methods.

Dip coating is a coating method in which the web is continuously immersed in and pulled out of the slurry tank, and the excess slurry flows back into the tank, as shown in Figure 5-89. Dip coating is usually double-sided coating, so the pulling direction of the web is generally perpendicular to the liquid surface to ensure that the coating thickness on both sides of the web is consistent. Usually, the coating thickness depends on viscosity, density, line speed, and pull-out angle. The slurry coating thickness on the web surface becomes thicker as the slurry viscosity and coating speed increase. Of course, dip coating is also suitable for intermittent operation of complex workpieces, but when the solvent volatilization rate is fast, there will be flow.

Fig 5-89 Dip Coating Method

Blade coating is a coating method that uses a blade to reduce the slurry coating thickness on the coating roll or web to the specified thickness, as shown in Figure 5-90. Among them, the coating thickness and coating weight of air-knife coating depend on the air-knife pressure and the air jet velocity, and it is especially suitable for coating low-viscosity or medium-low-viscosity water-based slurries. When using organic solvent-based slurries, care should be taken to avoid the formation of explosive mixtures from large amounts of air and flammable solvent vapor.

Fig 5-90 Blade Coating

Curtain coating is a coating method in which the slurry is extruded from a slot and falls directly onto the running web in the form of a liquid curtain. It belongs to the preset quantity coating method, as shown in Figure 5-91. Because the distance between the extrusion die lip and the web is large and there is no air bubble on the curtain, it has outstanding advantages in eliminating defects such as stringing and scratches. Due to the greater kinetic energy of the curtain, it helps to stabilize the meniscus, so higher coating speed can be achieved. The line speed can reach more than 100 m/min and is increasingly adopted in high-precision battery coating solutions.

Figure 5-91 Falling curtain coating; Figure 5-92 Wire rod coating

Wire-wound rod coating is a coating method that uses a wire-wound metering rod to uniformly coat liquid on a soft web, as shown in Figure 5-92. The wire-wound rod is made by tightly winding polished stainless steel wire around a core rod. The wire-wound rod is also called a coating rod or doctor rod. When the winding wire is very thin, the coating thickness accuracy can reach within a few microns. Because low-viscosity liquid easily flows on the wound metal wire, it is suitable for coating low-viscosity liquids. Using special coating rods, the coating viscosity can be relatively high, and the coating thickness can reach 225 μm. The rod coating speed is generally limited to 304 m/min.

The selection of coating method is a systematic project with many factors to consider. The following are the main factors that should be considered when selecting a coating method:

Most coating methods are suitable for coating one layer at a time, and another layer is coated after the previous layer is dried. Some methods can coat multiple layers at the same time, such as slide coating, which can coat at least 9 layers simultaneously in color film coating. The coating head of curtain coating is the slide coating head, and the coating layer also flows down at the edge, so multilayer coating can also be carried out. Slot-die coating and extrusion coating usually perform single-layer coating, but they can also have two or three extrusion slots for multilayer coating widely required by lithium-ion battery manufacturers.

Wire-wound rod coating is suitable for thin-layer coating; extrusion coating, reverse roll coating, and curtain coating are suitable for thick coating, which can reach 400–750 μm. Generally speaking, the thinner the coating, the greater the coating difficulty. It should be noted that the thickness mentioned here is the wet coating thickness, and the difference between dry coating and wet coating is very large.

Viscosity and viscoelasticity are physical quantities that reflect rheological properties. Each coating method has a certain range of suitable viscosity and shear rate. The slurry viscosity is best selected according to the viscosity measured at the coating shear rate, because the viscosity changes with the shear rate. For lithium battery slurry formulation provided by leading lithium-ion battery material suppliers, precise viscosity control has become a key factor in process stability. However, in the preset quantity coating process, the shear rate to which the slurry is usually subjected is too high, so that it is difficult to achieve such high shear rate with current instruments, so the viscosity estimation is rough, and finally the experimental results shall prevail. Although viscoelasticity is very important, it is difficult to predict. Some viscoelasticity helps to improve the operation of certain coatings, but high viscoelasticity can cause delayed dragging defects.

Precision slot-die coating, slide coating, and curtain coating have higher coating precision, while the precision of other coating methods depends on fluid properties, roller geometry, rotation speed, and other factors. Every coating method has a wide coating range, which depends on the structure and operation mode of the coating device. Only by very finely optimizing the battery electrode coating equipment can good coating results be obtained.

The web can be non-permeable or permeable. For permeable web, the pores can be sealed before coating. It is also necessary to consider the roughness and surface tension on the web. The surface tension of the slurry must be lower than that of the web.

Coating speed is related to production efficiency. When possible, the faster the coating speed, the better. All coating methods have coating speed limits, but some methods are better at high-speed coating. Curtain coating requires a minimum flow rate to ensure that the curtain does not tear, so high-speed coating cannot be carried out when the layer is thin. In slide coating, when the coating layer is very thin, the coating layer will be unstable. Higher speed and thicker coating layer help to avoid coating instability. Similarly, smooth and non-permeable web can be coated at higher speed. The coating speed is also related to the length of the drying section. The longer the drying section, the more favorable it is to increase the drying rate.

The coating methods and their applicable ranges are shown in Table 5-6.

Category	Coating method	Shear rate /s⁻¹	Viscosity /Pa·s	Wet thickness /μm	Coating precision /%	Maximum speed /(m/min)	Influence of web roughness
Leveling	Leveling	0.01～0.10	-	-	-	-	-
	Air knife	-	0.005～0.5	2～40	5	500	High
	Wire-wound rod coating	-	0.02～1	5～50	10	250	High
Single layer	Blade coating	1000～10000	0.5～40	1～30	-	1500	High
		20～40000
	Reverse roll coating	100～10000	0.1～50	5～400	5	300	Slight
		1000～100000
	Slot-die coating	3000～100000	0.005～20	15～250	2	400	Slight
	Extrusion coating	-	50～5000	15～750	5	700	-
Multilayer	Slide coating	1000～10000	0.005～0.5	15～250	2	300	Slight
		3000～12000
	Curtain coating	1000～10000	0.005～0.5	2～500	2	300	Slight

The coating methods for lithium-ion battery electrode sheets are primarily roll coating and extrusion coating. Reverse roll coating requires a higher shear rate, is suitable for slurry with slightly higher viscosity, obtains thicker coating film and good coating quality, and is currently the most commonly used coating method for lithium-ion batteries. Extrusion coating represents a relatively advanced coating technology in the industry. A certain pressure is applied to the slurry during coating, which can be used for coating of higher viscosity slurry. The obtained electrode sheet has higher precision and fast coating speed. With the continuous development of lithium battery technology and increasing demand from lithium-ion battery manufacturers for higher energy density and consistency, extrusion coating, supported by state-of-the-art battery electrode coating equipment and high-precision battery coating solutions, will be more widely used in next-generation production lines.