Comprehensive guide for battery coating machines

1、 coating machine overview

2、the purpose of coating

3、Preparation of coating materials

4、Coating process

5、Drying and baking

6、slit extrusion coating technology

7、Coating defect control

8、the structure and function of coating die head

9、Summary

In the production process of lithium batteries, the coating die as a key component plays a vital role. It includes coating material preparation, coating process, drying and baking, slit extrusion coating and coating defect control.

1、 coating machine overview

The technological advanced degree of coating machine equipment is mainly investigated in four aspects: coating technology, tension technology, correction technology, drying technology. Coating technology needs to meet the production requirements of different thickness, now the thickness of positive lithium Aluminum Foil has been thin to 6-8 microns, the thickness of negative lithium copper foil has been thin to 4.5-6 microns, the diaphragm coating is only a few microns, the graphene coating is even thinner, and different thickness also needs to develop different coating methods for customers. Ensure that the coating thickness accuracy of the slurry is controlled below 2 microns. Tension technology, due to the inevitable uneven tension state of the web along the coating direction, resulting in a lack of consistency in coating quality, so it is necessary to ensure that each section has a good tension control during the operation of the sheet. Correction technology, because the length of the coating equipment is more than tens of meters, there will be position deviation during the operation of the strip, in order to ensure that whether it is copper film aluminum film or very thin diaphragm can run smoothly and effectively on the strip, and achieve precision coating, it is necessary to choose different driving forms with responsive control systems to correct the deviation. Drying technology, coating production speed bottleneck is drying drying, the most direct means is to lengthen the bellows, but it will bring cost and occupation increase, after strengthening also need to enhance the correction and tension control, in order to further improve the drying efficiency, it is necessary to improve the control of the wind field, temperature field control, layout form, as far as possible to ensure the coating speed under the condition of reducing the bellows length.

2、the purpose of coating

The main purpose of coating is to coat the slurry with good stability, good viscosity and good fluidity evenly on the surface of the positive and negative electrodes. Its important significance for lithium batteries is mainly reflected in three aspects: consistency, cycle life and safety. During the coating process, if the thickness of the positive and negative slurry coating at the front, middle and back of the pole plate is inconsistent, or the parameters before and after the pole plate are inconsistent, it is easy to cause the battery capacity to be too low or too high, and lithium evolution may be formed during the battery cycle, affecting the battery life. The coating process should strictly ensure that no particles, debris, dust, etc. are mixed into the pole, if mixed with debris, it will cause micro short circuit inside the battery, and cause the battery to burst into flames in serious cases. Therefore, in order to make the middle of the winding process as uniform and tight as possible, the coating error of the positive and negative electrodes is required to be as small as possible, and the advanced degree of the coating machine will directly affect the chemical performance of the battery, as well as the yield of the final product (battery manufacturers usually require more than 99%).

Coater is the core equipment of the previous process. The coater has undergone the evolution of three structural types, namely, scraper type, transfer type and slit extrusion type. The scraper type is mainly used in laboratory conditions; Transfer coating is mainly used in the production of 3C batteries; Slit extrusion coating is mainly used in power batteries, and this type has increased rapidly in recent years due to the outbreak of power battery production demand. Extrusion coating technology, as the most advanced of the three technologies, can be used for coating higher viscosity fluids to obtain a higher precision coating.

3、Preparation of coating materials

Coating material preparation is the first step in the work of lithium battery coating die, which is directly related to the quality and stability of the subsequent coating process. In the preparation stage, it is necessary to carefully select positive and negative electrode materials, conductive liquids, adhesives, solvents and other materials. The selection of these materials needs to consider factors such as electrochemical performance, stability, reliability, and cost to ensure that the final lithium battery product has excellent performance and stability.

4、Coating process

The coating process is a key step in evenly coating the prepared electrode material on the conductive substrate. According to the different coating methods, it can be divided into scraping coating method, spraying method, dipping method and rolling coating method. Among them, the scraping coating method is widely used because of its simple operation and stable process. In the coating process, the cutting edge of the coating die head plays a key role, and the slurry continues to flow through the cutting edge and out of the slit to form a uniform coating.

5、Drying and baking

The coated electrodes need to be dried and baked. The purpose of drying is to remove volatile substances such as moisture and organic solvents in the coating, so that the electrode material is closely bonded to the conductive substrate. There are two drying methods: natural drying and oven drying, of which oven drying can better control the temperature and time, and improve the drying effect. Baking is an important step to further improve the density and chemical stability of the coating, by controlling the baking temperature and time to ensure that the coating quality meets the requirements.

6、slit extrusion coating technology

Slit extrusion coating, as a precise wet coating technology, has significant advantages in the work of lithium battery coating die. The technology uses the coating liquid to extrude along the gap of the coating die under a certain pressure and flow rate to achieve uniform distribution of the coating on the substrate. Compared with other coating methods, slit extrusion coating has the characteristics of fast coating speed, high precision and uniform wet thickness. At the same time, because the coating system is closed, it can effectively prevent pollutants from entering, improve the utilization rate of the slurry and maintain the stability of the slurry properties. In addition, the slit extrusion coating also has multi-layer coating ability, which has a strong adaptability to the slurry viscosity and solid content range.

7、Coating defect control

In the work of lithium battery coating die, the control of coating defects is very important. Common coating defects include thick head and thin tail, thick edges on both sides, spotty dark spots, rough surface and exposed foil. In order to control these defects effectively, we need to start from many aspects. First, by adjusting the opening and closing time of the coating valve or the intermittent valve, the problem of inconsistent head and tail thickness can be improved. Secondly, to solve the problem of thick edge, it can be optimized by adjusting slurry properties, coating gap and slurry flow rate. In addition, measures such as improving the stability of the foil, reducing the coating speed, and adjusting the Angle of the wind knife also help to improve the surface roughness and streaking.

8、the structure and function of coating die head

Coating die head is the core component in the coating process of lithium battery, its structure and function have a decisive influence on the coating effect. The coating die head is usually composed of a die core, a die sleeve and a die seat, which is designed and manufactured precisely to ensure that the slurry can be uniformly and stably sprayed through the slit. At the same time, the coating die also needs to have good wear resistance, corrosion resistance and thermal stability to meet various challenges in the coating process.

9、Summary

The working principle of lithium battery coating die head involves a number of complex links and precision technology, which requires comprehensive consideration of material preparation, coating process, drying and baking, slit extrusion coating and coating defect control. By continuously optimizing the coating process and improving the performance of the coating die, there are now many patented products, which can also further improve the quality and performance of lithium batteries to meet the growing market demand.