Effect of coating process on lithium battery performance
Pole sheet coating generally refers to a process of evenly coating the stirred slurry on the current collector and drying the organic solvent in the slurry. The coating effect has an important impact on the battery capacity, internal resistance, cycle life and safety, ensuring that the pole sheet is evenly coated. The selection of coating method and control parameters have an important impact on the performance of lithium-ion batteries, mainly manifested in:
1) Coating and drying temperature control: If the drying temperature is too low during coating, the electrode cannot be completely dried. If the temperature is too high, the organic solvent inside the electrode may evaporate too quickly, causing cracks and peeling on the surface coating of the electrode.
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 waste of ingredients. In severe cases, if the positive electrode capacity is excessive, lithium dendrites will form due to the precipitation of lithium and pierce the battery separator to cause a short circuit, causing safety hazards.
3) Coating size: If the coating size is too small or too large, the positive electrode inside the battery may 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. In severe cases, lithium dendrites will form inside the battery, which can easily pierce the diaphragm and cause internal circuit damage to 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 performance consistency of the battery electrode.
In addition, electrode coating is of great significance to the safety of the battery. Before coating, the 5S work should be done well to ensure that no particles, debris, dust, etc. are mixed into the electrode during the coating process. If debris is mixed into the electrode, it will cause a micro short circuit inside the battery, and in severe cases, it will cause the battery to catch fire and explode.
Coating equipment and coating process selection
The coating process in a broad sense includes: unwinding → splicing → pulling → tension control → coating → drying → deviation correction → tension control → deviation correction → winding, etc. 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 stability of the equipment operation, the control of dynamic tension during the coating process, the size of the air volume during the drying process, and the temperature control curve will all affect the coating effect, so choosing a suitable coating process is extremely important.
Generally, the selection of coating method needs to consider the following aspects, including: the number of coating layers, the thickness of the wet coating, the rheological properties of the coating liquid, 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 pole piece coating accuracy is high, which is similar to the film coating accuracy; ⑤ the coating support is aluminum foil and copper foil with a thickness of 10-20μm; ⑥ compared with the film coating speed, the pole piece coating speed is not high. Considering the above factors, general laboratory equipment often uses a scraper type, consumer lithium-ion batteries mostly use a roller coating transfer type, and power batteries mostly use a slit extrusion method.
Blade coating:
The foil substrate passes through the coating roller and directly contacts the slurry tank. 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. At the same time, the excess slurry is scraped off and refluxed, thereby forming a uniform coating on the surface of the substrate. The main type of scraper is the comma scraper. The comma scraper is one of the key components in the coating head. Generally, the comma-like edge is processed along the generatrix on the surface of the round roller. This scraper has high strength and hardness, and is easy to control the coating amount and coating accuracy. It is suitable for slurries with high solid content and high viscosity.
Roller coating transfer: The coating roller rotates to drive the slurry, the comma scraper gap is used to adjust the slurry transfer amount, and the rotation of the backing roller and the coating roller is used to transfer the slurry to the substrate. The process is shown in Figure 1.
Figure 1 Comma blade coater
Roller transfer coating involves two basic processes:
(1) The coating roller rotates to drive the slurry through the gap between the metering rollers to form a slurry layer of a certain thickness;
(2) A slurry layer of a certain thickness is transferred to the foil through the rotation of the coating roller and the backing roller in opposite directions to form a coating.
Slit extrusion coating: As a precise wet coating technology, as shown in Figure 2, the working principle is that the coating liquid is extruded and sprayed along the gap of the coating mold at 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 pollutants from entering during the coating process, the slurry utilization rate is high, the slurry properties can be kept stable, and multiple layers of coating can be performed simultaneously. It can also adapt to different slurry viscosities and solid content ranges, and has stronger adaptability than the transfer coating process.
Figure 2 Schematic diagram of roller coating and blade transfer coating process
Coating defects and influencing factors
Reducing coating defects, improving coating quality and yield, and reducing costs during coating are important contents that need to be studied in coating process. Common problems in coating process are thick head and thin tail, thick edges on both sides, dark spots, rough surface, exposed foil and other defects. The thickness of the head and tail can be adjusted by the switching time of the coating valve or intermittent valve. The thick edge problem can be improved from the aspects of slurry properties, coating gap adjustment, slurry flow rate, etc. The rough surface with stripes can be improved by stabilizing the foil, reducing the speed, adjusting the wind knife angle, etc.
1.Substrate-Slurry
Relationship between the basic physical properties of slurry and coating: In the actual process, the viscosity of the slurry has a certain influence on the coating effect. The viscosity of the slurry prepared by different electrode raw materials, slurry ratios, and types of binders is also different. When the slurry viscosity is too high, the 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 affected by the rheological properties of the coating liquid, which directly determine the quality of the coating. The coating window can be studied by using theoretical analysis, coating experimental technology, fluid mechanics finite element technology and other research methods. The coating window is the process operation range in which stable coating can be carried out to obtain a uniform coating.
2. Substrate - copper foil and aluminum foil
Surface tension: The surface tension of copper and aluminum foil must be higher than the surface tension of the solution to be coated, otherwise the solution will be difficult to spread evenly on the substrate, resulting in poor coating quality. A principle that needs to be followed is that the surface tension of the solution to be coated should be 5 dynes/cm lower than that of the substrate, of course this is only rough. The surface tension of the solution and the substrate can be adjusted by adjusting the formula or surface treatment of the substrate. The surface tension measurement of both should also be used as a quality control test item.
Uniform thickness: In processes similar to blade coating, uneven thickness of the substrate cross section will lead to uneven coating thickness. Because in the coating process, the coating thickness is controlled by the gap between the blade and the substrate. If there is a place with a lower thickness in the lateral direction of the substrate, more solution will pass through that place and the coating thickness will be thicker, and vice versa. If the thickness of the substrate shown below fluctuates from the thickness gauge, the thickness fluctuation of the final coated film will also show the same deviation. In addition, lateral thickness deviation will also lead to defects in winding. Therefore, in order to avoid this defect, it is important to control the thickness of the raw materials.
Static electricity: On the coating line, a lot of static electricity will be generated on the surface of the substrate when the coating is unrolled and passes through the roller. The generated static electricity can easily absorb the air and the dust layer on the roller, causing coating defects. In the process of static electricity discharge, it will also cause static electricity-like appearance defects on the coating surface, and more seriously, it may even cause fire. If it is in winter with low humidity, the static electricity problem on the coating line will be more serious. The most effective way to reduce such defects is to keep the ambient humidity at a relatively high state as much as possible, ground the coating line, and install some anti-static devices.
Cleanliness: Impurities on the surface of the substrate can cause some physical defects, such as protrusions, dirt, etc. Therefore, the cleanliness of the raw materials needs to be better controlled in the production process of the substrate. Online film cleaning rollers are a relatively effective way to remove impurities from the substrate. Although it cannot remove all impurities on the film, it can effectively improve the quality of the raw materials and reduce losses.
Lithium battery electrode defect map
(1)Bubble defects in negative electrode coating of lithium-ion batteries
The left picture shows a negative electrode with bubbles, and the right picture is a 200-fold magnification of the scanning electron microscope. During the slurry mixing, transportation and coating process, foreign matter such as dust or long fluff is mixed into the coating liquid or falls onto the surface of the wet coating. The surface tension of the coating is affected by external forces, causing the intermolecular forces to change, and the slurry is slightly transferred. After drying, a circular mark is formed, which is thin in the middle.
(2)Pinhole
The first is the generation of bubbles (stirring, transporting, and coating processes); the pinhole defects caused by bubbles are relatively easy to understand. The bubbles in the wet film migrate from the inner layer to the film surface and rupture on the film surface to form pinhole defects. The bubbles mainly come from the poor fluidity of the coating during stirring, coating liquid transport, and coating processes, poor leveling, and poor bubble release of the coating.
(3)Scratches
Possible reasons: foreign matter or large particles are stuck in the narrow gap or coating gap, the substrate quality is poor, causing foreign matter to block the coating gap between the coating roller and the backing roller, and the mold lip is damaged.
(4)Thick edge
The reason for the thick edge is that the surface tension of the slurry drives the slurry to migrate to the uncoated area at the edge of the electrode, forming a thick edge after drying.
(5)Agglomerate particles on the negative electrode surface
Formula: spherical graphite + SUPER C65 + CMC + distilled water.Macroscopic morphology of electrodes produced by two different stirring processes: smooth surface (left) and a large number of small particles on the surface (right).
Formula: spherical graphite + SUPER C65 + CMC/SBR + distilled water.Enlarged morphology of small particles on the electrode surface (a and b): Agglomerates of conductive agent, not completely dispersed.Enlarged morphology of the electrode with smooth surface: the conductive agent is fully dispersed and evenly distributed
(6) Agglomerate particles on the positive electrode surface
Formula: NCA+acetylene black+PVDF+NMP.During the stirring process, the ambient humidity was too high, causing the slurry to become a jelly state, the conductive agent was not completely dispersed, and a large number of particles existed on the surface of the electrode after rolling.(7) Water system electrode cracks
Formula: NMC532/carbon black/binder = 90/5/5 wt%, water/isopropyl alcohol (IPA) solvent; optical photos of cracks on the pole piece surface, the coating surface density is (a) 15 mg/cm2, (b) 17.5 mg/cm2, (c) 20 mg/cm2 and (d) 25 mg/cm2, respectively. Thick pole pieces are more prone to cracks.
(8) Shrinkage holes on the electrode surface
Formula: flake graphite + SP + CMC/SBR + distilled water.There are pollutant particles on the surface of the foil, and there is a low surface tension area in the wet film on the surface of the particles. The liquid film migrates radially around the particles, forming shrinkage point defects.
(9) Scratches on the electrode surface
Formula: NMC532+SP+PVdF+NMP.During slit extrusion coating, large particles on the cutting edge can cause foil scratches on the electrode surface.
(10) Coating vertical stripes
Formula: NCA+SP+PVdF+NMP.In the later stage of transfer coating, the viscosity of the slurry increases due to water absorption, and it approaches the upper limit of the coating window during coating. The slurry has poor leveling properties and forms vertical stripes.
(11) Roller cracks in the electrode area that is not completely dry
Formula: flake graphite + SP + CMC/SBR + distilled water.
During coating, the middle area of the pole piece was not completely dry, and the coating migrated during rolling, forming strip cracks.
(12)Pole piece rolling edge wrinkles
The coating forms a thick edge phenomenon, and the coating edge produces wrinkles when rolled.
(13)Negative electrode slitting coating and foil separation
Formula: natural graphite + acetylene black + CMC/SBR + distilled water, the active material ratio is 96%. When the pole disc is cut, the coating is separated from the foil.
(14)Pole cutting burrs
When the positive electrode disc is cut, the unstable tension control causes the secondary cutting to form foil burrs.
(15)Pole cutting wave edge
When the negative electrode disc is cut, wavy edges and coating peeling off are formed due to inappropriate cutter overlap and pressure.
(16)Other common coating defects include: air infiltration, lateral waves, vertical flow, rivulet, expansion, water pools, etc.
Defects may occur in any process stage: coating preparation, substrate preparation, substrate handling, coating area, drying area, cutting, slitting, rolling process, etc. So what is the general logical method to solve defects?
1) In the process from pilot to production, it is necessary to optimize the product formula, coating and drying processes to find a better or wider process window.
2)Use some quality control methods and statistical tools (SPC) to control product quality. Use online monitoring to control stable coating thickness, or use a visual appearance inspection system (Visual System) to check whether there are any defects on the coating surface.
3)When product defects occur, adjust the process in a timely manner to avoid the recurrence of defects.
Coating uniformity
The so-called coating uniformity refers to the consistency of the coating thickness or glue amount distribution in the coating area. The better the consistency of the coating thickness or glue amount, the better the coating uniformity, and vice versa. There is no unified measurement index for coating uniformity. It can be measured by the deviation or deviation percentage of the coating thickness or glue amount at each point in a certain area relative to the average coating thickness or glue amount in the area, or by the difference between the maximum and minimum coating thickness or glue amount in a certain area. Coating thickness is usually expressed in µm.
Coating uniformity is used to evaluate the overall coating condition of an area. However, in actual production, we are usually more concerned about the uniformity in the horizontal and vertical directions of the substrate. The so-called horizontal uniformity is the uniformity in the coating width direction (or machine horizontal direction). The so-called vertical uniformity is the uniformity in the coating length direction (or substrate travel direction). The size, influencing factors and control methods of horizontal and vertical coating errors are very different. Generally speaking, the larger the width of the substrate (or coating), the more difficult it is to control the horizontal uniformity. According to many years of practical experience in coating online, when the substrate width is below 800mm, the horizontal uniformity is usually easy to guarantee; when the substrate width is between 1300 and 1800mm, the horizontal uniformity can often be controlled well but with certain difficulty, requiring a very professional level; and when the substrate width is above 2000mm, the control of the horizontal uniformity is very difficult, and only very few manufacturers can handle it well. When the production batch (i.e. coating length) increases, the vertical uniformity may become a greater difficulty or challenge than the horizontal uniformity.
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