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Introduction: slurry coating is the next process after the preparation of slurry, the main purpose of this process is to evenly coat the slurry with good stability, good viscosity and good fluidity on the positive and negative current collectors. Electrode coating is of great significance to the capacity, consistency and safety of lithium battery batteries. According to incomplete statistics, the battery failure caused by the electrode piece coating process accounts for more than 10% of the lithium battery failure caused by all reasons, which is also one of the hot topics in the material craftsman group. Therefore, I summarized the coating process, told about the wrong places, and asked the bull to give me more advice.
Effect of coating process on the performance of lithium batteries
Electrode piece coating generally refers to a process in which the evenly stirred slurry is evenly coated on the current collector and the organic solvent in the slurry is dried. The effect of coating has an important impact on the battery capacity, internal resistance, cycle life and safety, and ensures that the pole piece is evenly coated. The selection of coating methods and control parameters have an important impact on the performance of lithium-ion batteries, which are mainly manifested in:
1) Coating drying temperature control: if the drying temperature is too low during coating, it cannot guarantee that the pole piece is completely dry, if the temperature is too high, it may be because the organic solvent inside the pole piece evaporates too quickly, and the surface coating of the pole piece cracks and falls off;
2) Coating surface density: if the coating surface density is too small, the battery capacity may not reach the nominal capacity, if the coating surface density is too large, it is easy to cause batching waste, and if the positive electrode capacity is excessive in serious cases, lithium dendrites will be formed due to the precipitation of lithium to puncture the battery separator and cause a short circuit, causing potential safety hazards;
3) Coating size: The coating size is too small or too large, which may cause the positive electrode inside the battery to not be completely wrapped 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 wrapped by the negative electrode, and the actual capacity of the positive electrode can not be played efficiently, and in serious cases, lithium dendrites will be formed inside the battery, which is easy to puncture the separator and cause the internal circuit of the battery;
4) Coating thickness: If the coating thickness is too thin or too thick, it will have an impact on the subsequent electrode rolling process, and the performance consistency of the battery electrode piece cannot be guaranteed.
In addition, electrode coating is of great significance to the safety of the battery. Before coating, it is necessary to do a good job of 5S to ensure that there are no particles, sundries, dust, etc. mixed into the pole piece during the coating process, if mixed with sundries, it will cause a micro-short circuit inside the battery, and in serious cases, the battery will catch fire and explode.
Selection of coating equipment and coating process
The coating process in a broad sense includes: uncoiling→ splicing→ tension control → pulling tabs→ coating → drying→ guiding → tension control→ guiding → winding and other processes. The coating process is complex, and there are 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 the dynamic tension in the coating process, the size of the air volume in the drying process and the temperature control curve will affect the coating effect, so it is extremely important to choose the appropriate coating process.
Generally, the selection of coating method needs to be considered from the following aspects, including: the number of layers to be coated, the thickness of the wet coating, the rheological properties of the coating solution, the required coating accuracy, the coating support or substrate, the coating speed, etc.
In addition to the above factors, it is also necessary to combine the specific conditions and characteristics of the pole piece coating. The characteristics of lithium-ion battery pole piece coating are: (1) double-sided single-layer coating; (2) The slurry wet coating is thicker (100~300μm); (3) The slurry is a non-Newtonian high-viscosity fluid; (4) The coating accuracy of the pole piece is high, which is similar to that of the film coating; (5) The coating support body is aluminum foil and copper foil with a thickness of 10~20μm; (6) Compared with the film coating speed, the electrode coating speed is not high. To sum up, general laboratory equipment often uses scraper type, consumer lithium-ion batteries mostly use roller transfer type, and power batteries mostly use slit extrusion method.
Fig.1 The main components of the coating machine equipment
Scraper coating: The working principle is shown in Figure 1, the foil substrate passes through the coating roller and is in direct contact with the slurry trough, the excess slurry is coated on 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, and at the same time, the excess slurry is scraped off and reflowed, and thus a uniform coating is formed on the surface of the substrate. The type of scraper is mainly a comma scraper. Comma scraper is one of the key components in the coating head, generally on the surface of the round roller along the bus bar processing into a comma-like cutting edge, this scraper has high strength and hardness, easy to control the coating amount and coating accuracy, suitable for high solid content and high viscosity slurry.
Fig.2. Comma scraper coater
Roller transfer type: the coating roller rotates to drive the slurry, adjusts the amount of slurry transfer through the comma scraper gap, and uses the rotation of the back roller and the coating roller to transfer the slurry to the substrate, the process is shown in Figure 2. Roller transfer coating consists of two basic processes: (1) the coating roller rotates to drive the slurry through the gap of the metering roller to form a slurry layer of a certain thickness; (2) The slurry layer of a certain thickness is rotated by the coating roller and the back roller in opposite directions, and the slurry is transferred to the foil to form a coating.
Fig.3. Schematic diagram of the roll coating scraper transfer coating process
Slit extrusion coating: As a precision wet coating technology, as shown in Figure 3, the working principle is that the coating liquid is extruded and sprayed along the gap of the coating mold under a certain pressure and flow rate and transferred to the substrate. Compared with other coating methods, it has many advantages, such as fast coating speed, high precision, and uniform wet thickness; The coating system is closed, which can prevent contaminants from entering during the coating process, and the slurry utilization rate is high, and the slurry properties can be kept stable, and multi-layer coating can be carried out at the same time. It can also adapt to different slurry viscosity and solid content ranges, and has stronger adaptability compared with the transfer coating process.
Fig.4. Slit extruder coater
Coating defects and influencing factors
Reducing coating defects, improving coating quality and yield, and reducing costs in the coating process are important contents that need to be studied in the coating process. Defects such as thick head and thin tail, thick edges, punctate dark spots, rough surface, and exposed foil often occur in the coating process. The thickness of the head and tail can be adjusted by the opening and closing time of the coating valve or the intermittent valve, the thick edge problem can be improved from the slurry properties, coating gap adjustment, slurry flow rate, etc., and the surface roughness and unevenness with stripes can be improved by stabilizing the foil, reducing the speed, adjusting the angle of the air knife, etc.
Substrate - slurry
The relationship between the basic physical properties of the slurry and the coating: in the actual process, the viscosity of the slurry has a certain impact on the coating effect, the electrode raw material, the slurry ratio ratio, and the viscosity of the slurry prepared when the binder type is selected is also different. When the viscosity of the slurry is too high, the coating is often not continuous and stable, and the coating effect is also affected
The uniformity, stability, edge and surface effects of the coating solution are influenced by the rheological properties of the coating solution, which directly determine the quality of the coating. The coating window can be studied by theoretical analysis, coating experimental technology, fluid mechanics finite element technology and other research methods, and the coating window is the process operation range that can be stably coated and uniform coating obtained.
Substrates - copper foil and aluminum foil
Surface tension: The surface tension of copper and aluminum foil must be higher than the surface tension of the coated solution, otherwise the solution will be difficult to spread flat on the substrate and lead to poor coating quality. A principle to be adhered to 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 the substrate can be adjusted by adjusting the formulation or by the surface treatment of the substrate. The measurement of the surface tension of both should also be a quality control test item.
Uniform thickness: In a process similar to squeegee coating, the thickness of the substrate banner is uneven, resulting in uneven coating thickness. This is because in the coating process, the coating thickness is controlled by the gap between the scraper and the substrate. If there is a place in the transverse direction of the substrate where the thickness of the substrate is lower, then there will be more solution passing through that place, and the coating thickness will be thicker, and vice versa. If you see the thickness fluctuations of the following substrate from the thickness gauge, the final coating thickness fluctuations will show the same deviation. In addition, deviations in the transverse thickness can lead to winding defects. So in order to avoid this defect, the thickness control of the raw material is very important
Static electricity: On the coating line, a lot of static electricity is generated on the surface of the substrate when it is unwound and passes through the roller. The generated static electricity can easily adsorb air and the ash layer on the roller, resulting in coating defects. In the process of discharging, static electricity will also cause electrostatic appearance defects on the coating surface, and even cause fire in more serious cases. In the winter when the humidity is low, the static electricity problem on the coating line is even more serious. The most effective way to reduce such defects is to keep the ambient humidity as high as possible, ground the coating wire, and install some antistatic devices.
Cleanliness: Impurities on the surface of the substrate can lead to some physical defects, such as protrusions, dirt, etc. Therefore, in the production process of the substrate, it is necessary to control the cleanliness of the raw materials. The in-line membrane cleaning roller is a more effective way to remove impurities from the substrate. Although it cannot remove all the impurities on the membrane, it can effectively improve the quality of raw materials and reduce losses.
Defect map of lithium battery pole piece
[1] Bubble defect in the anode coating of lithium-ion batteries
The negative electrode piece with air bubbles on the left and the 200x magnification of the scanning electron microscope on the right. In the process of mixing, transporting and coating, foreign substances such as dust or wool with large length are mixed into the coating solution or fall on the surface of the wet coating, where the surface tension of the coating changes due to the influence of external forces, and the slurry undergoes a slight transfer, forming circular traces after drying, and the middle is thin.
[2] Pinholes
One is the generation of bubbles (stirring process, conveying process, coating process); Pinhole defects caused by air bubbles are easy to understand, and air bubbles in the wet film migrate from the inner layer to the surface of the film, and rupture on the surface of the film to form pinhole defects. The bubbles mainly come from the poor fluidity, poor leveling, and poor release of bubbles in the coating during the stirring, coating liquid transportation and coating process
[3] Scratches:
Possible causes: foreign matter or large particles stuck in the slit gap or coating gap, poor quality of the substrate, resulting in foreign matter blocking the coating gap between the coating roller and the back roller, and mold lip damage
【4】Thick edge:
The reason for the thick edge is that the surface tension of the slurry is driven, so that the slurry migrates to the uncoated edge of the pole piece, and a thick edge is formed after drying
[5] Aggregate particles on the surface of the negative electrode
配方：球形石墨+SUPER C65+CMC+蒸馏水
Macroscopic morphology of the pole pieces from two different mixing processes: smooth surface (left) and large number of small particles on the surface (right)
配方：球形石墨+SUPER C65+CMC/SBR+蒸馏水
Amplified morphology of small particles on the surface of the electrode piece (a and b): agglomerates of conductive agents, not completely dispersed
Enlarged morphology of the smooth surface of the electrode: the conductive agent is fully dispersed and evenly distributed
[6] Agglomerate particles on the surface of the cathode
配方：NCA+乙炔黑+PVDF+NMP
During the stirring process, the ambient humidity is too high, resulting in the slurry being frozen, the conductive agent is not completely dispersed, and there are a large number of particles on the surface of the pole piece after rolling.
[7] Cracks in the electrode pieces of the water system
配方：NMC532/carbon black/binder= 90/5/5 wt%， 水/异丙醇（IPA）溶剂
极片表面裂纹光学照片，涂布面密度分别为 (a) 15 mg/cm2，(b)17.5 mg/cm2， (c) 20 mg/cm2和(d) 25 mg/cm2，厚极片更容易出现裂纹。
[8] Shrinkage porosity on the surface of the pole piece
配方：片状石墨+SP+CMC/SBR+蒸馏水
There are contaminant particles on the surface of the foil, and there is a low surface tension area on the wet film on the surface of the particles, and the liquid film migrates to the surrounding particles, forming a crater defect.
[9] Scratches on the surface of the pole piece
配方：NMC532+SP+PVdF+NMP
The slit is squeezed and coated, and the presence of large particles on the cutting edge causes foil leakage scratches on the surface of the pole piece
[10] Coated vertical strip road
配方：NCA+SP+PVdF+NMP
In the later stage of transfer coating, the water absorption viscosity of the slurry increases, and the coating is close to the upper limit of the coating window, and the leveling of the slurry is poor, forming a vertical channel.
[11] Roll pressure cracks in the area where the pole piece is not dry
配方：片状石墨+SP+CMC/SBR+蒸馏水
When coating, the middle area of the electrode piece is not completely dry, and the coating migrates when rolling, forming strip cracks.
[12] The edge of the pole piece is rolled into folds
Thick edges are formed by coating, rolled, and wrinkles are formed at the edge of the coating
[13] The negative electrode slitting coating is detached from the foil
Formula: natural graphite + acetylene black + CMC/SBR + distilled water, the proportion of active substances is 96%,
When the pole piece disc is slit, the coating is detached from the foil.
[14] Pole piece slitting burr
When the positive electrode piece disc is slitting, the foil burr is formed due to the secondary cutting due to the unstable tension control
[15] The pole piece is cut to the wavy edge
When the negative electrode piece disc is slitting, due to the inappropriate amount of knife overlap and pressure, the wavy edge and the incision coating fall off
[16] Other common coating defects are: air infiltration, transverse waves, sag flow, Rivulet, expansion, water pooling, etc
Defects can occur in any part of the process: the preparation of the paint, the production of the substrate, the handling of the substrate, the coating area, the drying area, the cutting, slitting, the rolling process, etc. What is the general logical way to solve defects?
1. In the process from pilot to production, it is necessary to optimize the formulation, coating and drying process of the product, and find a better or wide process window.
2. Control the quality of the product through some quality control means, statistical tools (SPC). Stable coating thickness is controlled by in-line monitoring, or the coating surface is inspected for defects by means of a visual system.
3. Adjust the process in time when there is a product defect to avoid the recurrence of defects.
Uniformity of the application
Coating uniformity refers to the consistency of the coating thickness or the amount of glue applied in the coating area. The better the consistency of the coating thickness or the amount of glue applied, the better the uniformity of the application, and vice versa. There is no uniform measurement of coating uniformity, which can be measured by the deviation or percentage of the coating thickness or gluing amount at each point in a certain area from the average coating thickness or gluing amount in the area, or by the difference between the maximum and minimum coating thickness or gluing amount in a certain area. The coating thickness is usually expressed in μm.
Coating uniformity is used to evaluate the overall gluing condition of an area. But in actual production, we are usually more concerned with the uniformity of the substrate in both the transverse and longitudinal directions. The so-called transverse uniformity is the uniformity in the direction of the coating width (or the transverse direction of the machine). The so-called longitudinal uniformity is the uniformity in the direction of the coating length (or the direction of travel of the substrate). The magnitude, influencing factors and control methods of transverse and longitudinal gluing errors are very different. In general, the larger the width of the substrate (or gluing), the more difficult it is to control the transverse uniformity. According to the practical experience of coating line for many years, when the width of the substrate is less than 800mm, the transverse uniformity is usually easy to guarantee; When the width of the substrate is 1300~1800mm, the transverse uniformity can often be controlled well, but there is a certain degree of difficulty, which requires a very professional level; When the width of the substrate is more than 2000mm, it is very difficult to control the transverse uniformity, and only a few manufacturers can handle it. When the production batch size (i.e. coating length) increases, longitudinal uniformity can become a greater difficulty or challenge than transverse uniformity.
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Article source: Mufeng Machinery
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