Battery manufacturing can be divided into three stages. The first stage is electrode production (including slurry mixing, electrode coating, drying, rolling, slitting and making electrode), the second stage is battery cell assembly (including winding/stacking, shelling, liquid injection and sealing), and the third stage is battery cell activation (including formation, capacity division, detection and sorting). As one of the key components of the battery, the battery electrode design, material selection and preparation process directly affect the comprehensive performance of the battery.
In the battery manufacturing process, the coating process plays a key role. The quality of pole piece coating, such as coating thickness uniformity, surface density distribution and defects, has a great impact on the consistency, cycle life, energy density, safety performance and other aspects of the battery.
In order to improve the process quality of pole piece coating and improve coating efficiency, we must first understand the development of coating and select a suitable coating method. Secondly, we must reduce the cost of experimental trial and error through process simulation, explore the factors affecting coating quality, and achieve the purpose of guiding production by optimizing and improving various parameters. Finally, based on online detection technology, the quality of coating is monitored online to avoid production defects caused by uncontrollable factors such as human and environmental factors.
This article will discuss the research status of coating process in battery manufacturing from three aspects: coating method, coating process simulation and coating detection, so as to promote the improvement of electrode coating process quality, coating efficiency and production quality control.
1.Electrode Coating Method
Coating technology was first used in the paper industry and plastic packaging, and has now become an indispensable part of the new energy battery manufacturing field. However, there are currently dozens of coating methods in the coating industry, which can apply slurry to different foils. For different foils and slurries, the coating structure is also different. Choosing the right coating method is crucial and directly affects the production efficiency and quality of the battery.
Common lithium-ion battery coating methods are shown in the figure above. With the increasing demand in the battery manufacturing market, coating methods are also constantly developing. Common coating methods include manual coating, scraper coating, transfer coating, and the current mainstream slit extrusion coating.
The development of the battery pole coating industry aims to improve coating quality and efficiency to meet the needs of the new energy industry for battery manufacturing.
1.1 Manual coating
Manual coating is a coating method based on manual operation. Its working principle is shown in the figure below. The foil to be coated is placed on an adsorbent platform, and the stirred slurry is evenly poured on one end of the platform. The slurry is evenly coated on the surface of the foil by slowly moving the scraper roller to achieve manual coating.
This coating method is easy to operate and is mainly used in Laboratory battery research. It is highly flexible and can adapt to the needs of different slurries and foils. However, it has great limitations. First, the coating efficiency is low. It can only coat foils of fixed size each time. It is completely dependent on manual labor and cannot be mass-produced. Second, the coating quality is greatly affected by manual operation, and it is difficult to ensure the uniformity of coating thickness and width. Although manual coating has certain value in experimental research, it cannot meet the quality and efficiency requirements of the battery manufacturing industry, and thus a coating method that replaces manual labor with machines has emerged.
1.2 Scraper coating
As shown in the figure, the scraper coating consists of a scraper and a coating roller. The slurry is stored in the storage tank. The slurry is evenly coated on the copper foil or aluminum foil by the rotation of the coating roller. The coating thickness can be controlled by adjusting the gap between the scraper and the coating roller. At the same time, the excess slurry is scraped off and returned to the storage tank to ensure the accuracy and efficiency of the coating process.
The scraper coating is simple to operate and is often used in experimental research and suitable for small-batch production. The coating thickness depends on the gap between the foil and the scraper. During the coating process, it is necessary to ensure that the scraper is parallel to the foil, otherwise it will lead to uneven lateral thickness of the coating. At the same time, this method has limited coating speed. If the coating speed is too high, it will cause the foil to vibrate, resulting in uneven coating thickness and slurry splashing. The scraper blade is prone to agglomeration and particles may form on the scraper.
1.3 Transfer coating
There are two common types of transfer coating: blade transfer coating [(a)] and double roller transfer coating [(b)]. The blade transfer coating machine has an additional back roller compared to the blade coater.
The coating principle is similar, but the difference is that the slurry is transferred to the copper foil or aluminum foil through the counter-rotation between the coating roller and the back roller. The double roller transfer coating replaces the traditional blade with a transfer roller, adjusts the transfer amount of the slurry through the gap between the two rollers, and then achieves the coating purpose through the counter-rotation between the back roller and the coating roller.
Transfer coating can effectively control the amount of slurry transferred and the uniformity of coating thickness by adjusting the amount of slurry transferred twice. Moreover, by changing the temperature of the transfer roller, the viscosity characteristics of the slurry during the coating process can be stably controlled. However, it is difficult to meet the coating requirements in terms of accuracy, quality and efficiency. First, the accuracy is limited by the manufacturing accuracy of the scraper or roller. Secondly, the slurry is exposed to the air, and it is difficult to ensure that the slurry is not affected by the environment. Finally, the coating gap needs to be debugged many times and relies on empirical methods. This method is time-consuming and reduces coating efficiency.
1.4 Slot extrusion coating
The working principle of slot extrusion coating is shown in the figure below. Under the action of pressure, the pre-mixed slurry enters the die head through the feed port at a certain flow rate. Under the action of pressure, the slurry is evenly coated on the surface of the foil.
Although the slit extrusion coating has a complex structure, it can accurately control the flow state of the slurry. The die head is a key component of coating, mainly composed of an upper die, a gasket, and a lower die. By replacing the gasket, various types of coating can be achieved. At the same time, by optimizing the structural shape of the internal cavity of the die head, the flow state and pressure distribution of the slurry inside the die head can be controlled. The structural shape of the gasket can not only achieve different types of coating (multi-striped coating, intermittent coating), but also improve the uniformity of the slurry at the outlet. Controlling the angle of the die head can increase the coating speed and improve the coating quality. High-precision coating is achieved by controlling various factors that affect the coating quality.
Although scraper and transfer coating can achieve standard coating, it is difficult to achieve high-precision, high-efficiency, and high-quality coating requirements. First of all, the accuracy is limited by the manufacturing accuracy of the die head or roller. The slurry is exposed to the air, and it is difficult to ensure that the slurry is not affected by the environment. The coating gap needs to be debugged many times and relies on empirical methods. This method is time-consuming and reduces coating efficiency. The two coating methods have limited ranges for slurry properties (solid content, viscosity), coating speed, and coating thickness, so the traditional coating method is no longer suitable for current market demand. Slit extrusion coating is widely used in the battery manufacturing industry due to its high efficiency and high quality.
Compared with the first three coating methods, slot extrusion has the characteristics of high coating accuracy, wide application range, high coating efficiency and automated production. First of all, extrusion coating is an advanced pre-metered coating technology. According to the given feeding speed, coating width, coating gap and foil speed, the coating amount can be estimated more accurately. Since it is coated in a closed environment, it has little effect on the viscosity characteristics of the slurry, and can achieve coating with a wide range of slurry viscosity. It can also achieve coating of different widths and thicknesses by changing the width and thickness of the gasket. At the same time, changing the gasket structure can achieve different types of coating, such as double-striped coating, intermittent coating, etc. It also has great advantages in coating efficiency. It can achieve double-sided coating, and both sides of the foil are coated and dried at the same time. Compared with single-sided coating, double-sided coating has high production efficiency and high production space utilization, which can ensure the consistency and uniformity of electrode coating. Finally, automated production can be achieved. The detection signal (surface density, thickness) fed back to the closed-loop die head by the detection equipment can be used to automatically adjust the parameters (coating speed, coating gap), accurately control the coating process, and achieve uninterrupted 24-hour coating. It greatly improves production efficiency and reduces production costs.
Under the current trend of rapid development of the new energy battery industry, slit extrusion coating can well meet the needs of battery manufacturers. First of all, in terms of coating efficiency, most manufacturers use slit extrusion for double-sided coating at the same time, with a maximum coating speed of up to 120m/min, and can also achieve coating of high solid content and high viscosity slurry. Slit extrusion coating is not only fast, but also has a wide range of applications, which meets the needs of the current development of the new energy battery manufacturing industry.
Under the development trend of intelligence, precision and efficiency in the battery manufacturing industry, the continuous improvement and application of slit extrusion coating technology provides reliable support for the development of the industry. How to optimize the structural parameters and process parameters of slit extrusion coating is an urgent problem for enterprises to solve. At present, with the help of process simulation, numerical simulation is used to optimize the structure and process parameters, and the influence of various factors on coating efficiency and quality is explored to reduce the production cost caused by trial and error and achieve the purpose of guiding production.
