1. The influence of coating process on the performance of lithium batteries
Polar coating generally refers to a process of uniformly coating a stirred slurry onto a current collector and drying the organic solvents in the slurry. The coating effect has a significant impact on battery capacity, internal resistance, cycle life, and safety, ensuring uniform coating of electrode sheets. The selection of coating methods and control parameters have a significant impact on the performance of lithium-ion batteries, mainly manifested in:
1) Coating drying temperature control: If the drying temperature during coating is too low, it cannot guarantee complete drying of the electrode. If the temperature is too high, it may cause cracking, peeling, and other phenomena on the surface coating of the electrode due to the rapid evaporation of organic solvents inside the electrode;
2) Coating surface density: If the coating surface density is too low, the battery capacity may not reach the nominal capacity. If the coating surface density is too high, it is easy to cause waste of ingredients. In severe cases, if there is an excess of positive electrode capacity, lithium dendrites may form due to lithium precipitation and pierce the battery separator, causing a short circuit and posing a safety hazard;
3) Coating size: If the coating size is too small or too large, it may cause the positive electrode inside the battery to not be completely covered by the negative electrode. During the charging process, lithium ions are embedded from the positive electrode and move to the electrolyte that is not completely covered by the negative electrode. The actual capacity of the positive electrode cannot be efficiently utilized, and in severe cases, lithium dendrites may form inside the battery, which can easily puncture the separator and cause damage to the internal circuit of the battery;
4) Coating thickness: If the coating thickness is too thin or too thick, it will affect the subsequent electrode rolling process and cannot guarantee the consistency of battery electrode performance.
In addition, electrode coating is of great significance for the safety of batteries. Before coating, 5S work should be done to ensure that no particles, debris, dust, etc. are mixed into the electrode during the coating process. If debris is mixed in, it can cause micro short circuits inside the battery, and in severe cases, it can lead to battery fire and explosion.
2. Selection of Coating Equipment and Coating Process
The general coating process includes: unwinding → splicing → pulling → tension control → coating → drying → correction → tension control → correction → winding and other processes. The coating process is complex, and there are also many factors that affect the coating effect, such as the manufacturing accuracy of the coating equipment, the smoothness of the equipment operation, the control of dynamic tension during the coating process, the size of the air flow during the drying process, and the temperature control curve. Therefore, choosing the appropriate coating process is extremely important.
Generally, when choosing a coating method, several factors need to be considered, 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 support or substrate, and the coating speed.
In addition to the above factors, it is also necessary to consider the specific situation and characteristics of electrode coating. The characteristics of lithium-ion battery electrode coating are: ① double-sided single-layer coating; ② The wet coating of the slurry is relatively thick (100-300 μ m); ③ The slurry is a non Newtonian high viscosity fluid; ④ The precision requirement for polar film coating is high, similar to that of film coating; ⑤ The coating support consists of aluminum foil and copper foil with a thickness of 10-20 μ m; ⑥ Compared to the coating speed of film, the coating speed of polarizer is not high. Taking into account the above factors, generally laboratory equipment adopts a scraper type, consumer lithium-ion batteries mostly use a roll coating transfer type, and power batteries mostly use a slit extrusion method.
Scraper coating: The foil substrate passes through the coating roller and directly contacts the slurry tank. Excess slurry is applied to the foil substrate. When the substrate passes between the coating roller and the scraper, the gap between the scraper and the substrate determines the coating thickness. At the same time, excess slurry is scraped off and refluxed, forming a uniform coating on the surface of the substrate. The main types of scrapers are comma scrapers. The comma scraper is one of the key components in the coating head. It is generally machined along the generatrix on the surface of the circular roller to form a comma like edge. This scraper has high strength and hardness, and is easy to control the coating amount and accuracy. It is suitable for high solid content and high viscosity slurries.
Roller coating transfer type: The coating roller rotates to drive the slurry, and the transfer amount of the slurry is adjusted through the gap between the comma scraper. The slurry is transferred to the substrate by the rotation of the back roller and the coating roller. The process is shown in Figure 2. Roller coating transfer coating involves two basic processes: (1) the rotation of the coating roller drives the slurry to pass through the gap between the measuring rollers, forming a certain thickness of slurry layer; (2) A certain thickness of slurry layer is transferred to the foil material by rotating the coating roller and back roller in opposite directions to form a coating.
Slit extrusion coating: As a precise wet coating technology, as shown in Figure 3, the working principle is that the coating liquid is extruded and sprayed along the gaps of the coating mold under a certain pressure and flow rate, and transferred to the substrate. Compared to other coating methods, it has many advantages, such as fast coating speed, high accuracy, and uniform wet thickness; The coating system is closed, which can prevent pollutants from entering during the coating process. The utilization rate of the slurry is high, and the properties of the slurry can be maintained stable. Multiple layers of coating can be carried out simultaneously. And it can adapt to different viscosity and solid content ranges of slurry, and has stronger adaptability compared to transfer coating process.
3. Coating defects and influencing factors
Reducing coating defects, improving coating quality and yield, and reducing costs during the coating process are important aspects that need to be studied in coating technology. The common problems in the coating process include thick head and thin tail, thick edges on both sides, point like dark spots, rough surface, and exposed foil. The thickness of the head and tail can be adjusted by the opening and closing time of the coating valve or intermittent valve. The problem of thick edges can be improved from the aspects of slurry properties, coating gap adjustment, slurry flow rate, etc. The surface roughness, unevenness, and stripes can be improved by stabilizing the foil material, reducing the speed, and adjusting the angle of the air knife.
Substrate - Slurry
The relationship between the basic physical properties of the slurry and coating: In the actual process, the viscosity of the slurry has a certain impact on the coating effect. The viscosity of the slurry prepared varies depending on the electrode raw materials, slurry ratio, and the type of binder selected. When the viscosity of the slurry is too high, coating often cannot be carried out continuously and stably, and the coating effect is also affected.
The uniformity, stability, edge and surface effects of the coating liquid are directly determined by the rheological properties of the coating liquid, thereby directly determining the quality of the coating. Theoretical analysis, coating experimental techniques, fluid dynamics finite element techniques, and other research methods can be used to study the coating window, which refers to the process operation range that can achieve stable coating and obtain uniform coating.
Substrate - Copper and Aluminum Foil
Surface tension: The surface tension of copper aluminum foil must be higher than that of the coated solution, otherwise the solution will be difficult to spread smoothly on the substrate, resulting in poor coating quality. One principle to follow is that the surface tension of the solution to be coated should be 5 dynes/cm lower than that of the substrate, although this is only rough. The surface tension of the solution and substrate can be adjusted by adjusting the formula or surface treatment of the substrate. The measurement of surface tension for both should also be included as a quality control testing item.
Uniform thickness: In processes similar to scraper coating, uneven thickness across the substrate can lead to uneven coating thickness. Because in the coating process, the coating thickness is controlled by the gap between the scraper and the substrate. If there is a lower thickness of the substrate in the transverse direction, more solution will pass through that area and the coating thickness will be thicker, and vice versa. If the thickness fluctuation of the substrate is observed from the thickness gauge, the final film thickness fluctuation will also show the same deviation. In addition, lateral thickness deviation can also lead to defects in winding. So in order to avoid such defects, it is important to control the thickness of the raw materials
Static electricity: On the coating line, a lot of static electricity is generated on the surface of the substrate during unwinding and passing through the roller. The generated static electricity can easily adsorb air and ash layers on the roller, causing coating defects. During the discharge process, static electricity can also cause electrostatic appearance defects on the coating surface, and more seriously, it can even cause fires. If the humidity is low in winter, the static electricity problem on the coating line will be more prominent and severe. The most effective way to reduce such defects is to keep the environmental humidity as high as possible, ground the coating line, and install some anti-static devices.
Cleanliness: Impurities on the surface of the substrate can cause physical defects such as bumps, dirt, etc. So in the production process of the substrate, it is necessary to control the cleanliness of the raw materials well. Online film cleaning rollers are a relatively effective method for removing substrate impurities. Although not all impurities on the membrane can be removed, it can effectively improve the quality of raw materials and reduce losses.
4. Defect map of lithium battery electrodes
【1】 Bubble defects in the negative electrode coating of lithium-ion batteries
【2】 Pinhole
【3】 Scratches
【4】 Thick edge
【5】 Aggregated particles on the negative electrode surface
【6】 Aggregated particles on the surface of the positive electrode
【7】 Water system polar crack
【8】 Surface shrinkage of polarizer
【9】 Scratches on the surface of the polarizer
【10】 Apply vertical stripes
【11】 Rolling cracks in the partially dried area of the polarizer
【12】 Wrinkles on the edge of the polarizer roller
【13】 Negative electrode slitting coating and foil detachment
【14】 Polar slice cutting burrs
【15】 Polar slice cutting wave edge
5. Uniformity of coating
The so-called coating uniformity refers to the consistency of coating thickness or adhesive distribution within the coating area. The better the consistency of coating thickness or adhesive amount, the better the uniformity of coating, and vice versa. There is no unified measurement index for coating uniformity, which can be measured by the deviation or percentage deviation of the coating thickness or adhesive amount at each point in a certain area relative to the average coating thickness or adhesive amount in that area, or by the difference between the maximum and minimum coating thickness or adhesive amount in a certain area. The coating thickness is usually expressed in µ m.
Coating uniformity is used to evaluate the overall coating condition of an area. But in actual production, we usually care more about the uniformity in both the horizontal and vertical directions of the substrate. The so-called lateral uniformity refers to the uniformity in the coating width direction (or machine lateral direction). The so-called longitudinal uniformity refers to the uniformity in the coating length direction (or substrate travel direction).
There are significant differences in the magnitude, influencing factors, and control methods of horizontal and vertical glue application errors. In general, the larger the width of the substrate (or coating), the more difficult it is to control the lateral uniformity. Based on years of practical experience in coating, when the substrate width is below 800mm, lateral uniformity is usually easily guaranteed; When the width of the substrate is between 1300-1800mm, the lateral uniformity can often be controlled well, but it is difficult and requires a considerable level of professional expertise; When the width of the substrate is above 2000mm, it is very difficult to control the lateral uniformity, and only a few manufacturers can handle it well. As the production batch (i.e. coating length) increases, longitudinal uniformity may become a greater challenge or difficulty than transverse uniformity.