An in-depth explanation of the process route for coating electrode films

01 Selection of coating method

One of the key factors in successfully coating electrode films is choosing the appropriate coating method. There are approximately more than 20 different coating methods that can be used to apply liquid slurry to a substrate, and each technique has many specific configurations, offering a variety of options. During the research and development stage of lithium-ion battery laboratories, homemade simple coating experimental apparatus such as scraper, doctor blade, or extrusion were used for electrode film coating experiments, but only a small amount of samples were coated for experimental research, and the results were not ideal and encountered various issues.

Generally, the selection of coating method needs to consider several aspects including the number of layers to be coated, the thickness of the wet coating, the rheological properties of the coating liquid, the required coating accuracy, the coating substrate or base material, and the coating speed.

How to choose a suitable coating method for electrode slurry?

In addition to the above factors, it is also necessary to consider the specific conditions and characteristics of electrode film coating.

The characteristics of electrode film coating in lithium-ion batteries are as follows:

① Double-sided single-layer coating

② Thick wet coating (100-300μm)

③ Non-Newtonian high-viscosity fluid

④ High requirement for coating accuracy, similar to film coating accuracy

⑤ Coating substrate is aluminum foil and copper foil with a thickness of 10-20μm

⑥ Compared to film coating, the coating speed for electrode films is not high

We first consider the selection of coating technology based on the number of coating layers.

Electrode films need to be coated on both sides of the metal foil. Currently, there are techniques that allow simultaneous coating on both sides of the substrate. However, if the simultaneous double-sided coating method is chosen, the drying and transportation of the coated films will become extremely complex and difficult to operate. Therefore, the coating technology route determines the use of single-layer coating, with another side being coated after drying. Considering that electrode film coating involves thick coating layers, methods such as scraping, doctor blade, and air knife coating are only suitable for thin coating layers and not suitable for electrode slurry coating. Among the remaining coating methods, dip coating is the simplest, but its coating thickness is affected by the viscosity of the coating slurry and the coating speed, making it difficult to achieve high-precision coating. Taking into account the special requirements of electrode slurry coating, extrusion coating or roll coating can be chosen.

Extrusion coating

Extrusion coating technology is an advanced technique that can be used for coating high-viscosity fluids and achieve high-precision coatings. To obtain a uniform coating using slot die extrusion coating, it is necessary to design and operate the extrusion die within the appropriate range, which is referred to as the "coating window" in coating technology.

The design of the extrusion die has a significant impact on coating accuracy. Therefore, detailed data on the rheological properties of the coating slurry is required for the design. Once the extrusion die is designed based on the provided rheological data, changes in the rheological properties of the coating slurry can potentially affect the coating accuracy. Extrusion coating equipment is relatively complex, and specialized technical expertise is required for operation.

Roll coating

Roll coating is a mature coating technique that can achieve good coating uniformity with high-precision coating rolls and precision bearings. Roll coating can be applied to the coating of electrode slurry. There are multiple forms of roll coating, with the distinction between forward roll and reverse roll coating based on the rotation direction of the rolls. In addition, there are more than 10 types of roll coating configurations, including 3-roll and 4-roll configurations. Which roll coating configuration is better depends on the rheological properties of the various slurries. The selected roll coating configuration, structural dimensions, operating conditions, physical properties of the coating liquid, and other factors must be within a reasonable range, entering the coating window of operation conditions, in order to achieve coating without defects.

Key Technologies in Polar Plate Coating

Among all coating products, the coating accuracy required for polar plates is the highest, so many technologies in film coating are the basis for solving polar plate coating. However, the unique requirements of polar plate coating must be addressed with special techniques.

Coating of High Viscosity Polar Plate Slurry

The viscosity of polar plate slurry is extremely high, exceeding the viscosity of general coating liquids. Moreover, a large coating amount is required, making it difficult to achieve uniform coating using conventional coating methods. We compared and analyzed various coating methods, designed and verified various experimental plans based on their flow mechanisms, combined with the rheological characteristics and coating requirements of polar plate slurry, and found several suitable coating methods for polar plate slurry. These methods successfully solved the challenge of continuous and uniform coating of high viscosity polar plate slurry.

"Polar Plate Segmented Coating and Double-sided Lamination Technology"

Whether it is film coating or other coating products, the majority of them are coated continuously on a film width. However, lithium-ion battery polar plates require segmented coating, and the length of each segment of polar plate needed for the production of different types of lithium-ion batteries varies. If continuous coating is used and then cut into segments of fixed lengths to produce polar plates, the coating layer needs to be scraped off at one end of each segment during battery assembly to expose the metal foil. The process of continuous coating and fixed-length cutting has low efficiency and cannot meet the requirements for large-scale production. Therefore, we propose using fixed-length segmented coating to perform coating according to the required coating and blank lengths for battery specifications. It is difficult to achieve length segmentation coating for different battery specifications using simple mechanical devices. In the design of the coating head, we use computer technology to design the polar plate coating head as an integrated and intelligent control coating device combining light, machinery, and electricity. Operational parameters are input into the computer via a keyboard before coating, and the coating process is controlled automatically by the computer, achieving fixed-length segmented and double-sided coating. Therefore, the coating machine can arbitrarily set the coating and blank lengths for segmented coating to meet the coating requirements of various types of lithium-ion battery polar plates.

"High-Efficiency Drying Technology for Thick Coatings of Polar Plate Slurry"

Polar plate slurry coatings are relatively thick and require a large amount of coating, resulting in a high drying load. The efficiency of conventional hot air convection drying or cylinder contact drying is low. We applied efficient drying technology from film drying to polar plate drying, using optimized design of hot air impinging drying technology, which significantly improved the drying efficiency. This technology enables uniform and rapid drying, without issues such as external dryness and internal moisture or surface cracking on the dried coating.

"Transportation Technology for Polar Plate Substrates (Polar Plates) in Coating Production Lines"

In the polar plate coating production line, from unwinding to rewinding, there are many processes involved, such as coating and drying, which require multiple points of substrate (polar plate) propulsion. This is similar to film coating and drying production lines. We successfully applied film coating machine transmission technology to polar plate coating. Considering that the substrate is extremely thin aluminum foil or copper foil with poor rigidity, which is prone to tearing and wrinkling, we adopted special technical devices in the design to ensure that the polar plate remains flat in the coating area and strictly control the tension gradient of the entire substrate path to keep the entire path tension within the safe limit. In the transmission design of the coating production line, we used DC motor intelligent speed control technology to keep the speed of the coating point and substrate path stable, thereby ensuring the longitudinal uniformity of the coating. In the transmission path design of the coating machine, automatic alignment devices were installed at key locations such as coating and rewinding to ensure accurate coating of the slurry on the substrate, leaving uniform margins on both sides. When rewinding the polar plate, it can achieve neat edges, creating favorable conditions for the subsequent processes of polar plate production.

03 Polar Plate Coating Process Flow

The general process flow of polar plate coating is as follows: unwinding -> splicing -> tensioning -> tension control -> automatic alignment -> coating -> drying -> automatic alignment -> tension control -> automatic alignment -> rewinding

The substrate (metal foil) for coating is dispensed from the unwinding device and fed into the coating machine. The leading and trailing ends of the substrate are connected on the splicing table to form a continuous belt, which is then sent to the tension adjustment device and automatic alignment device for adjusting the tension and position of the substrate path before entering the coating device. The polar plate slurry is applied in segmented coating according to the predetermined coating amount and blank length. In double-sided coating, it automatically tracks the front-side coating and blank length. The wet polar plate after coating is sent to the drying section for drying, with the drying temperature set according to the coating speed and thickness. After drying, the polar plate is adjusted for tension and aligned automatically before rewinding, ready for further processing in the next step.

04 Equipment Installation, Commissioning, and Coating Situation

The developed equipment consists of mechanical equipment, electrical control, drying ventilation, and other systems. After installation, mechanical tests, electromechanical integration tests, and comprehensive operation tests were conducted sequentially, all meeting the design and operational requirements. The coating was carried out according to the technical requirements and design specifications for lithium-ion batteries. Coating conditions: aluminum foil with a thickness of 20μm as the substrate, a width of 350mm, and a coating speed of 5m/min. Under these conditions, single-sided fixed-length coating and double-sided lamination coating were performed using the slurry, while drying was conducted simultaneously. The entire production line operated stably, with uniform coating and drying.

05 Sample Test Results

Coating Uniformity: The relative deviation range of the coating amount was 2.22% to -1.85%, with an absolute error of 4.07%.

Spotted Particle Phenomena on Positive and Negative Pole Polar Plates during Slurry Coating

Causes: (Note: Oxidation of the foil and poor appearance are excluded as possibilities)

- If the slurry is left for too long, typically more than 12 hours, this problem may become more severe. Please pay attention to whether this is the case and control the storage time.

- The viscosity of the slurry is too low (viscosity decreases over time). It is recommended to control the viscosity above 2500 mPa·s.

- The slurry storage temperature is too high, and spots occur when the temperature exceeds 35 degrees. It is recommended to control the temperature around 25 degrees.

- Formulations without conductive carbon are prone to this problem. Unfortunately, there is no good way to solve it temporarily. A possible solution is to reduce the stirring speed, as long as the slurry is evenly mixed and well dispersed, and there are no clogging issues during coating. The speed of the transfer storage tank should also be lower.

- For slurry that already exhibits spotted particles, it is recommended to mix it with several batches of high viscosity fresh slurry. Alternatively, when coating the second side, lightly wet the substrate with lint-free paper. (Note: This method may easily result in tailing/size abnormalities