Analysis of Common Defects in Lithium Battery Slot-Die Coating

Defects in lithium-ion battery coating originate from three main parts: the coating slurry, the coating window, and the coating drying process.

• In the slurry preparation process, incomplete dispersion introduces agglomerated particles; insufficient iron removal filtration introduces metal debris; and incomplete vacuum deaeration leaves behind numerous bubbles. Corresponding coating defects include agglomerates, metal particle contamination, pinholes, orange peel, etc.
• Coating process parameters (slurry volumetric flow rate, coating speed, slot gap, etc.) exceeding the coating window can lead to air entrainment, sagging, periodic horizontal stripes, and vertical stripes. Additionally, periodic vibrations from feed pumps, coating rolls, etc., can also produce periodic horizontal stripes.
• During the coating drying process, excessively high drying temperatures can cause defects such as binder migration, thick edges, and cracking.

To form a stable and uniform coating, the following conditions must be simultaneously met during the coating process:
① Stable slurry properties, no sedimentation, no changes in viscosity, solid content, etc.
② Stable slurry feeding supply, forming a uniform and stable flow state inside the die.
③ The coating process is within the coating window, forming a stable flow field between the die and the coating roll.
④ Stable foil movement, no web slippage, severe vibration, or wrinkles.

There are numerous types of coating film defects with varying causes. This article focuses on slot-die coating for lithium-ion batteries, analyzing several common defects and providing corresponding solutions. Common defects include point defects, edge effects, and jagged defects.

I. Point Defects

1. Pinholes
Definition: A defect where air escaping from within the material's pores forms holes during the coating drying process. Bubbles in the wet film migrate from the inner layer to the film surface and rupture, forming pinhole defects. Bubbles mainly come from stirring, coating liquid transport, and the coating process.
Improvement Measures: Apply vacuum, select CMC with good dispersibility and stable performance, adjust solid content, CMC and SBR ratio, etc., adjust stirring speed and time.

2. Crater (Shrinkage Hole)
Definition: A defect caused by low surface tension spots on the substrate surface.
Caused by various particles (dust, oil stains, metal particles, etc.). The presence of foreign particles creates low surface tension areas in the wet film at the particle surface. The liquid film migrates radially around the particles, forming crater-like point defects, as shown below.
Improvement Measures: Filter the coating liquid for iron removal, control environmental dust, clean the substrate surface.

3. Anode Coating Bubbles

Agglomerate Particle Protrusion: If the slurry is not stirred uniformly, and the conductive agent is not dispersed, forming agglomerates, this type of defect occurs. As shown below, large areas of protrusions appear on the electrode surface. Observing these areas under magnification reveals that these are agglomerates of the conductive agent.
Improvement Measures: Improve the slurry stirring process to eliminate them.

4. Orange Peel
Definition: A surface appearance defect where the coating resembles the texture of an orange peel.
Causes: During coating, due to solvent evaporation, temperature differences arise in different areas, creating a concentration difference between the upper and lower layers of the slurry. This forms a surface tension gradient and natural convection phenomenon, causing the coating solution to migrate, ultimately resulting in an uneven coating surface and orange peel defects. Excessively fast drying rates in the oven or high hot air velocity can cause the solution to solidify prematurely before leveling, also forming orange peel defects.
Improvement Measures:

1. Reduce the drying rate, allowing the solution sufficient time to level.
2. Add low-volatility solvents, surfactants, etc., to the solution to reduce temperature and concentration differences.

II. Line Defects

1. Scratches
Definition: The coating surface is scratched into linear grooves of varying lengths and irregular sides due to contact with sharp objects, as shown below.
Possible Causes: Large particle agglomerates, foreign objects, or large particles stuck in the slot gap or coating gap; poor substrate quality causing foreign objects to block the coating gap between the coating roll and the backing roll; damage to the die lip. Additionally, during equipment operation, uneven tension or inappropriate oven air volume can cause the electrode to scrape during travel, also causing scratch line defects.
Improvement Measures: Remove particles from the lip or coating gap, replace filters, inspect the die lip. For problems caused by scraping, stop the line to troubleshoot, find the scraping location, remove the obstacle, or appropriately adjust the oven air volume to solve the scratch line defect while ensuring electrode dryness.
Large particles in the slurry: Adjust CMC, increase stirring time. Using uniform CMC with good dissolution, minimal fiber residue, and gelation, good hydration effect, can well disperse material particles; increase stirring time, clean residual soft particles from screens, screen the slurry.

2. Vertical Streaks
Definition: Striped defects appearing in the coating, parallel to the running direction of the substrate, continuously extending in the length direction, as shown below.
Possible Causes: Usually occurs near the upper speed limit of the coating window, more obvious with thin coatings. Uneven distribution of slurry on the foil surface.
Improvement Measures: Adjust slurry viscosity (excessive dispersant added for uniform dispersion can make viscosity too high; non-Newtonian fluid behavior causes poor leveling after the slurry is transferred from the coater to the foil surface, failing to spread evenly.); reduce coating speed (reducing speed allows the slurry to lose "elasticity" and level promptly); reduce the coating gap between the coating roll and the backing roll; use well-dispersing dispersants with appropriate dosage.

3. Horizontal Streaks
Definition: Striped defects appearing in the coating, perpendicular to the running direction of the substrate, traversing the web width.
Possible Causes: Mechanical vibration; web speed fluctuation; periodic fluctuations in the flow of slurry supply.
Improvement Measures: Confirm if the mechanical disturbance frequency matches the horizontal streak occurrence frequency.

4. Wavy Edge
Definition: A defect characterized by wavy stripes appearing at the edge of the coating.
Two main reasons cause wavy edge line defects: one is related to the foil, e.g., unacceptable foil surface roughness, foil oxidation, secondary contamination during transportation or unwinding can all cause wavy edge problems. For such issues, replace with new foil for coating. Another reason is mismatch between slurry viscosity and shim extension. During coating, the supply tank's prolonged stirring may cause viscosity decrease, or poor wettability of the conductive agent during stirring severely affects slurry viscosity. Viscosity changes (decrease) mean the current shim extension is no longer suitable; replace with a larger extension shim to solve wavy edge caused by viscosity reduction.

5. Cracking
Definition: A defect where interlaced cracks appear on the coating surface.
Causes: Excessive or overly rapid drying. Due to uneven heating of different parts of the electrode during stoppage, some edges dry too quickly, others too slowly during drying. If the internal stress generated within the coating is not fully released, it will crack.
Improvement Measures: Adjust drying time and speed; adjust solid content (low solid content can cause the coating to dry too quickly and excessively, leading to curling and cracking); due to coating material tension reasons, control cracking by adjusting oven air intake.

III. Edge Effects

1. Thick Edge (Edge Buildup)
Definition: A defect where the coating thickness at both edges of the coated width is higher than the average thickness. Often, the edges are thick and the center is thin, i.e., thick edge.
Cause: Mass migration driven by surface tension. As shown below, initially, the edge of the wet film is thinner, solvent evaporates faster than in the center, causing the solid content at the edge to rise rapidly. The surface tension at the edge becomes much greater than that of the central wet film. The larger surface tension at the edge and faster solvent evaporation drive liquid from the inner part towards the edge, forming a thick edge after drying.

The thick edge phenomenon is an unfavorable defect. Measures to prevent and mitigate it include:
① With constant slurry flow rate, reducing the slot size increases the slurry exit velocity at the die, thereby reducing the drag force ratio of the slurry, thus reducing the thickness of the thick edge. However, a smaller slot size creates higher internal pressure in the die, more easily causing die lip swell, leading to uneven coating transverse thickness uniformity, requiring higher precision coating equipment.
② Reducing the coating gap can limitedly reduce the thickness and width of the thick edge.
③ Reduce the surface tension of the slurry, e.g., by adding surfactants, to inhibit slurry flow towards the edge during drying.
④ Optimize the exit shape of the slot shim to change the flow velocity direction and magnitude of the slurry, reduce the stress state of the edge slurry, and weaken the edge swell effect of the slurry.

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2. Edge Depression
Definition: Similar to edge depression type line defects.
Cause: A key factor causing edge depression line defects is related to coating pressure. When the coating pressure inside the die cavity is relatively low, edge depression occurs at the edge positions of the electrode coating.
Improvement Measures: One method is to add Teflon on both sides of the die cavity to increase edge coating pressure, improve edge coating amount, and solve edge depression. The other is to reduce shim thickness, also to increase cavity coating pressure and avoid edge depression.

3. Wrinkling
Definition: A defect where wrinkles appear on the coating during the drying process.

Main factors include: unwinding/rewinding tension, roll ratios, copper foil specifications (more common in wide, thin types), rewind shaft diameter, PET film winding density, etc.
Improvement Countermeasures: Optimize tension curves, change roll ratios, adjust regulating rolls, and increase the winding density of the PET film over rolls.

4. Weak Edge / Fuzzy Edge
Definition: The color difference between the coated material and the edge material is small, with no clear boundary; inaccurate edge detection due to a weak edge can lead to inaccurate distance measurement, subsequently affecting follow-up deviation correction linkage.
Causes: Material swaying left and right; different materials/slurries have different grayscale; interference from appearance defects at the coating edge, etc.
Improvement Measures: Optimize the imaging system and edge detection algorithm.

Other common coating defects include: Air entrainment, transverse waves, sagging, Rivulet, expansion, ponding, etc.

The Complexity of Coating Defects

Coating defects, from their causes to elimination methods and even control methods, are quite complex.

First, the factors causing defects are complex. Every unit operation or process step in the coating process can affect coating quality, leading to quality defects in the final product. Besides the direct effects of defect-causing factors, interactions between different factors may also cause defects.

Secondly, pinpointing the source of defects is complex. Defects may appear in the same process step or only after several operations. Sometimes coating streaks are found at the coating station, but mixed or contamination defects only become visible after drying ends. Even substrate defects might not be detected until the final product is rewound or after slitting.

Thirdly, the occurrence of defects is random. Defects can happen at any stage of the product development cycle, from lab-scale slurry preparation, to pilot coating, to scaled-up production. Each stage may have different defects, and elimination in the initial stage does not guarantee a defect-free next stage. More complexly, similar appearance defects can have many causes, and the cause may differ each time, such as with bubbles, spots, or ripples.

Finally, defects can also be determined by the initial design of the coater and the performance of each hardware unit on the coating production line. A coater might be reasonable for the product it was initially customized for, but product changes or cost variations can make the coater unable to meet requirements.

The analysis above highlights the intricate challenges in lithium-ion battery electrode coating. Achieving consistent, high-quality results requires not only deep technical knowledge but also precision equipment, stable materials, and a seamlessly integrated production line.

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