Among the causes of lithium battery failure, the electrode coating process accounts for 10%！

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 electrode current collectors. Electrode coating is of great significance to the capacity, consistency, and safety of lithium batteries. According to incomplete statistics: battery failure caused by the electrode coating process accounts for more than 10% of all lithium battery failures.

Effect of coating process on lithium battery performance

Electrode coating generally refers to a process in which a uniformly stirred slurry is evenly coated on a current collector and the organic solvent in the slurry is dried. The coating effect has an important impact on battery capacity, internal resistance, cycle life and safety, ensuring uniform coating of electrodes. The selection of coating methods and control parameters have an important impact on the performance of lithium-ion batteries, mainly as follows:

1) Coating drying temperature control: If the drying temperature is too low during coating, the electrode cannot be guaranteed to be completely dry. If the temperature is too high, the organic solvent inside the electrode may evaporate too quickly and the electrode surface coating may crack or fall off. and other phenomena.

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 high, it will easily cause a waste of ingredients. In severe cases, if there is excessive positive electrode capacity, due to lithium The precipitation forms lithium dendrites that pierce the battery separator and 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. In this case, the actual capacity of the positive electrode cannot be used efficiently. In severe cases, lithium dendrites will form inside the battery, which can easily pierce the separator and cause the internal circuit of the battery.

4) Coating thickness: Coating thickness that is too thin or too thick will have an impact on the subsequent electrode rolling process and cannot guarantee the performance consistency of battery electrodes.

In addition, electrode coating is of great significance to the safety of the battery. Before coating, 5S work must 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 will cause a micro short circuit inside the battery, and in severe cases, the battery may catch fire and explode.

Coating equipment and coating process selection

The generalized coating process includes: unwinding → splicing → pulling → tension control → coating → drying → deviation correction → tension control → deviation correction → 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 dynamic tension during the coating process, the amount of air volume during the drying process, and temperature control Curves will affect the coating effect, so it is extremely important to choose the appropriate coating process.

Generally, the following aspects need to be considered when selecting a coating method, including: the number of coating layers, the thickness of the wet coating, the rheological characteristics of the coating liquid, the required coating accuracy, the coating support or substrate, and cloth speed, etc.

In addition to the above factors, the specific conditions and characteristics of electrode coating must also be combined. The characteristics of lithium-ion battery electrode coating are:

â‘  Single-layer coating on both sides;

② The slurry wet coating is thick (100~300μm);

â‘¢ The slurry is a non-Newtonian high-viscosity fluid;

â‘£ The electrode coating requires high precision. The coating accuracy is similar to that of film;

⑤ The coating support is aluminum foil and copper foil with a thickness of 10 to 20 μm;

â‘¥Compared with the film coating speed, the electrode coating speed is not high. Considering the above factors, general laboratory equipment often uses the blade type, consumer lithium-ion batteries mostly use the roller coating transfer type, and power batteries mostly use the slit extrusion method.

Blade 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 blade , the gap between the blade and the substrate determines the thickness of the coating, and at the same time, the excess slurry is scraped off and reflowed, thereby forming a uniform coating on the surface of the substrate. The blade type is mainly comma blade. The comma blade is one of the key components in the coating head. It is generally processed into a comma-like cutting edge on the surface of the round roller along the bus line. This blade has high strength and hardness, and is easy to control the coating amount and coating accuracy. It is suitable for High solid content and high viscosity slurry.

Roller coating transfer type: The rotation of the coating roller drives the slurry, and the comma blade gap is used to adjust the slurry transfer amount, and the rotation of the back roller and coating roller is used to transfer the slurry to the substrate. The process is shown in Figure 2.

Figure 2 Comma blade coating machine

Roller transfer coating consists of two basic processes:

(1) The rotation of the coating roller drives the slurry through the metering roller gap to form a slurry layer of a certain thickness; (2) The slurry layer of a certain thickness transfers the slurry to the foil through the rotation of the coating roller and the back roller in opposite directions. coating.

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 slit of the coating die under a certain pressure and flow rate and transferred to the substrate. Material. 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 to prevent contaminants from entering during the coating process, and the slurry utilization rate is high and the slurry can be maintained It has stable properties and can be coated with multiple layers at the same time. It can adapt to different slurry viscosity and solid content ranges, and has stronger adaptability than the transfer coating process.

Figure 3 Schematic diagram of roller blade transfer coating process

Coating defects and influencing factors

Reducing coating defects during the coating process, improving coating quality and yield, and reducing costs are important aspects that need to be studied in the coating process. Problems that often occur in the coating process are defects such as thick head and thin tail, thick edges on both sides, dotted dark spots, rough surface, and exposed foil. The thickness of the head and tail can be adjusted by the switching time of the coating valve or intermittent valve. The problem of thick edges can be improved from aspects such as slurry properties, coating gap adjustment, slurry flow rate, etc. Rough, uneven and striped surfaces can be improved by stabilizing the foil. , reduce the speed, adjust the air knife angle and other improvements.

1.Substrate - slurry

The relationship between the basic physical properties of slurry and coating: In the actual process, the viscosity of the slurry has a certain impact on the coating effect. The electrode raw materials, slurry proportions, and the viscosity of the slurry prepared when different types of binders are selected also different. 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 fluid are affected by the rheological properties of the coating fluid, which directly determines the quality of the coating. Research methods such as theoretical analysis, coating experimental technology, and fluid mechanics finite element technology can be used to study the coating window. The coating window is the process operating range in which stable coating can be achieved and a uniform coating can be obtained.

2.Substrate-copper foil and aluminum foil

Surface tension: The surface tension of the copper aluminum foil must be higher than the surface tension of the coated solution, otherwise the solution will be difficult to spread smoothly on the substrate, resulting in poor coating quality. One 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 formulation or surface treatment of the substrate. Surface tension measurement of both should also be used as a quality control test item.

Uniform thickness: In a process similar to blade coating, the thickness of the substrate is uneven, which 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 where the substrate thickness is relatively low in the transverse direction of the substrate, more solution will pass through that place and the coating thickness will be thicker, and vice versa. If the following thickness fluctuations of the base material are seen from the thickness gauge, the final coating thickness fluctuations will also show the same deviation. In addition, transverse thickness deviations can also lead to winding defects. So in order to avoid this defect, the thickness control of raw materials is very important.

Static electricity: On the coating line, a lot of static electricity will be generated on the surface of the substrate when the coating is unwinding and passing through the roller. The static electricity generated can easily absorb the air and the ash layer on the roller, causing coating defects. During the discharge process of static electricity, it will also cause static-like appearance defects on the coating surface, and more serious cases may even cause fires. If the humidity is low in winter, 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 as high as possible, ground the coating lines, and install some anti-static devices.

Cleanliness: Impurities on the surface of the substrate will cause some physical defects, such as protrusions, dirt, etc. Therefore, it is necessary to better control the cleanliness of raw materials in the base material production process. Online film cleaning rollers are a more effective method of removing substrate impurities. Although it cannot remove all impurities on the membrane, it can effectively improve the quality of raw materials and reduce losses.

Lithium battery electrode defect map

(1) Bubble defects in the negative electrode coating of lithium-ion batteries

The negative electrode with bubbles in the picture on the left, and the 200x magnification of the scanning electron microscope in the picture on the right. During the slurry mixing, transfer and coating processes, foreign matter such as dust or long lint is mixed into the coating liquid or falls on the surface of the wet coating, where the surface tension of the coating is affected by external forces, resulting in intermolecular forces. Changes occur, the slurry is slightly transferred, and after drying, circular traces are formed with a thin middle.

（2）Pinhole

One is the generation of bubbles (stirring process, transportation process, coating process); 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 burst on the film surface to form pinhole defects. Bubbles mainly come from poor fluidity and leveling of the paint during stirring, coating liquid transportation, and coating processes, and poor bubble release from the paint.

(3) Scratches

Possible reasons: Foreign matter or large particles are stuck in the slit gap or coating gap, the quality of the base material is poor, causing foreign matter to block the coating gap between.

the coating roller and the back roller, and the mold lip is damaged.

(4) Thick edges

The reason for the thick edge is that the surface tension of the slurry causes the slurry to migrate to the uncoated area at the edge of the electrode, forming a thick edge after drying.

(5) Aggregate particles on the negative electrode surface

Formula: spherical graphite+SUPER C65+CMC+distilled water. Macroscopic morphology of electrodes with 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. Magnified morphology of small particles on the electrode surface (a and b): agglomerates of conductive agent, not completely dispersed. Magnified morphology of an electrode with a smooth surface: the conductive agent is fully dispersed and evenly distributed.

(6) Aggregate particles on the cathode surface

Formula: NCA+acetylene black+PVDF+NMP. During the stirring process, the ambient humidity was too high, causing the slurry to be in a jelly state, the conductive agent was not completely dispersed, and there were a large number of particles on the surface after the electrode was rolled.

(7) Cracks in water-based electrodes

Formula: NMC532/carbon black/binder= 90/5/5 wt%, water/isopropyl alcohol (IPA) solvent; optical photos of electrode surface cracks, coating surface density are (a) 15 mg/cm2, (b) 17.5 mg/cm2, (c) 20 mg/cm2 and (d) 25 mg/cm2, thick electrodes are more prone to cracks.

(8) Shrinkage cavities on electrode surface

Formula: flake graphite+SP+CMC/SBR+distilled water. There are contaminant 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, the presence of large particles on the cutting edge causes foil scratches on the electrode surface.

(10) Coating vertical strips

Formula: NCA+SP+PVdF+NMP. In the later stage of transfer coating, the water absorption viscosity of the slurry increases, and it is close to the upper limit of the coating window during coating. The leveling property of the slurry is poor and vertical stripes are formed.

(11) Rolling cracks in the area where the electrode is not dry

Formula: flake graphite+SP+CMC/SBR+distilled water. During coating, the middle area of the electrode was not completely dry, and the coating migrated during rolling, forming strip-like cracks.

(12) Electrode rolling edge wrinkles

Thick edges are formed during coating, and wrinkles occur at the edges of the coating due to roller pressing.

(13) The negative electrode slitting coating is separated from the foil

Formula: natural graphite + acetylene black + CMC/SBR + distilled water, active material ratio 96%. When the electrode disc is cut, the coating will separate from the foil.

(14) Electrode cutting burrs

When the positive electrode disc is slit, the unstable tension control causes foil burrs to be formed by secondary cutting.

(15) Electrode cutting wavy edge

When the negative electrode disc is cut, due to improper overlap and pressure of the cutters, wavy edges are formed and the coating of the cut is peeled off.

(16) Other common coating defects include: air infiltration, transverse waves, vertical flow, Rivulet, expansion, water ripples, etc.

Defects may occur in any processing section: coating preparation, substrate production, substrate operation 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 testing to production, it is necessary to optimize the product formula, coating and drying processes, and find a better or wider process window. 2) Use some quality control methods and statistical tools (SPC) to control product quality. Control the stable coating thickness through online monitoring, or check whether there are defects on the coating surface using a visual appearance inspection system. 3) When product defects occur, make timely adjustments to the process to avoid recurrence of defects.

Coating uniformity

The so-called coating uniformity refers to the consistency of coating thickness or glue amount distribution within the coating area. The better the consistency of 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. It can also be measured by Measured by the difference between the maximum and minimum coating thickness or amount of glue applied in a certain area. Coating thickness is usually expressed in μm.

Coating uniformity is used to evaluate the overall coating condition of an area. But in actual production, we are usually more concerned about the uniformity in both the transverse and longitudinal directions of the substrate. The so-called transverse uniformity is the uniformity in the coating width direction (or machine transverse direction). The so-called longitudinal uniformity is the uniformity in the coating length direction (or the direction of travel of the substrate). The size, influencing factors and control methods of horizontal and vertical gluing errors are very different. In general, the wider the substrate (or coating), the more difficult it is to control lateral uniformity. According to many years of practical experience in coating lines, when the width of the substrate is below 800mm, the lateral uniformity is usually easy to ensure; when the width of the substrate is 1300 to 1800mm, the lateral uniformity can often be controlled well but there is a certain degree of difficulty. It requires a very professional level; and 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. When the production batch (i.e. coating length) increases, longitudinal uniformity may become a greater difficulty or challenge than transverse uniformity.