Let's take a look at the basic principle of coating, which is simply to coat the slurry on the metal foil and then dry. Taking transfer coating as an example.Through this schematic diagram, we can see that the coating equipment includes these parts: coating roller, back roller and scraper. The slurry is transferred by the rotation of the coating roller, then the thickness of the slurry is controlled by the scraper, and finally the slurry is transferred to the foil through the extrusion of the coating roller and the back roller. This is a more common coating method in industrialization.
These are several common coating methods, such as the extrusion coating. At present, this coating method is widely used, with high coating accuracy and fast speed. However, the cost of the equipment is high, and it is relatively troublesome to replace the slurry. The third method in the figure is immersion coating, which is useful for some treatments on the surface of the separator, and the amount of coating is very small.
Drying mainly including these parts. First unwinding, then coating, followed by oven drying, and finally rewinding. For the cathode slurry, due to the NMP, it is necessary to make a recovery system to recycle the NMP, otherwise the environmental protection will not pass.
Here is a detailed description of the coating mechanism, which is the most critical part, we should pay attention to the control points: the liquid level sensor is based on the height of the liquid level to remind whether to feed, so that the liquid level can be basically controlled to achieve stability within a certain range; The temperature of the coating process should also be controlled, so that the viscosity of the slurry will not fluctuate greatly; The squeegee gap and coating speed directly affect the density of the coating surface; The last point is the speed ratio of the coating and the back roller, which may be easier to understand depending on the real thing, simply put, the rotation speed of the two rollers can be different, which also affects the areal density of the coating, that is, the coating weight.
Another important part is the drying system, which is actually the oven. The temperature change throughout the baking process is a process of first heating up, then constant temperature, and finally cooling. The heating process should not be too fast, and the constant temperature process temperature is also critical, because the volatilization at this time is the largest, if these twoparts are not well controlled, it is often easy to occur some of the problems mentioned on the right: cracking, rolling, undrying, and material dropping.
At present, the main coating is continuous and intermittent coating. The conventional foil thickness for aluminum foil is the thinnest 9um, and the anode electrode is 6um, because the energy density should be considered in industrial applications, that is, the thickness of the battery should be thin, and for some materials, such as lithium iron phosphate,will use carbon-coated aluminum foil. The maximum coating width is 650mm, and the fast coating speed is 70m/min.
Next, I will show you some photos of the coating process, mainly according to the previous mention, from unwinding, coating, baking, and rewinding. Next, let's take a brief look at the extrusion coating process.
Extrusion coating and transfer coating are similar, the main difference is that in the coating part, use extrusion head, measured by a metering pump, and then sprayed. There are also three things in the equipment: a transfer tank, an online thickness monitoring instrument (some are online weight testers), and a dust and dehumidification cover, which are commonly used in the industry today.
The last part is the items to be monitored during the coating process, which are generally the coating weight, size and appearance. For coating weight, there are currently online monitoring means, which can achieve real-time online monitoring without downtime. There is also a means of online monitoring of the coating size, of course, before that, it was measured with a soft ruler when the machine was stopped. Finally, the coating appearance needs to be confirmed whether the coating process is baked and dried, whether there is rolling, cracking, material dropping, particles, etc.
