Lithium Battery Coating Process | Analysis of Appearance Defects

Coating related defects

Battery Coating is the core basic process of lithium battery manufacturing, which directly determines the uniformity of the distribution of electrode active materials and the stability of the interface, and fundamentally affects the energy density, cycle life and safety of the battery;

At the same time, coating quality defects (such as thickness fluctuations, foreign matter, pinholes, etc.) will be magnified step by step in subsequent processes, eventually leading to a surge in the risk of battery failure and even thermal runaway, which can be regarded as the first lifeline of battery performance and safety.

1.Missing coating/leaking foil (as shown below)

• Description: The substrate (copper foil/aluminum foil) is partially or strip-shapedly not covered by the slurry, revealing the metallic luster.
• Features: Clearly visible metallic stripes or spots, with a strong contrast to the surrounding coating color.
• Possible causes:
  • The slurry delivery system is blocked (nozzle, pipe).
  • The coating head (slot die, comma blade) is blocked or damaged.
  • Uneven substrate tension causes partial detachment from the coating head.
  • The slurry has poor fluidity or a sudden change in viscosity.
• Impact: Severely reduces battery capacity and easily causes internal short circuit (direct contact between positive and negative poles), posing a huge safety hazard.

2.Scratches/brushing (as shown below)

• Description: Narrow, linear grooves or raised stripes appear on the coating surface. They may penetrate the substrate (hard scratches) or only damage the coating surface (soft scratches).
• Features: Continuous or intermittent lines along the coating direction (MD) or cross direction (CD).
• Possible causes:
  • The coating head (scraper, die lip) is damaged, burred or adhered with hard particles.
  • The surface of the guide roller and back roller is damaged or has foreign matter.
  • Hard large particles are mixed in the slurry.
  • The tension of the winding and unwinding is too large or unstable, resulting in increased friction between the substrate and the roller surface.
• Impact: Destroys the coating structure, affects the thickness uniformity, may cause local lithium deposition, increase internal resistance, and even micro short circuit.

3.Bubbles/Pinholes (as shown below)

• Description:
  • Bubbles: Gas trapped in the coating or at the interface between the coating and the substrate, which forms round or oval holes after drying (the surface may be closed or broken).
  • Pinholes: Tiny holes that penetrate the coating directly to the substrate (usually smaller and deeper than bubble holes).
• Features: Dense or scattered small holes can be seen on the surface or cross section of the coating. Bubble holes may be convex or concave, and pinholes directly expose the substrate.
• Possible causes:
  • Incomplete degassing of the slurry (gas is introduced during stirring, conveying, and standing).
  • Air is drawn into the coating process (poor coating head design, uneven substrate, too fast speed).
  • The substrate surface has poor wettability and the slurry cannot be fully spread and covered.
  • The solvent evaporates too quickly in the early stage of the drying process, and the surface crust seals the internal solvent vapor to form bubbles and burst.
• Impact: Reduce the active material loading, increase the local current density, easily cause lithium precipitation, reduce the cycle life and safety (short circuit is easy to occur at the pinhole).

4.White spots/dark spots (as shown below)

• Description: Areas of abnormal color (whiter or darker than normal) on the surface of a coating, usually accompanied by local changes in the composition or density of the material.
• Characteristics: Irregularly shaped spots or cloud-like areas.
• Possible causes:
  • Uneven dispersion of slurry, agglomeration of conductive agent or binder.
  • Uneven drying rate (uneven hot air distribution, too high or too low wind speed), resulting in solvent migration and component segregation.
  • Slurry sedimentation, insufficient stirring before coating.
  • Surface contamination of substrate (oil, dust).
• Impact: Affects the conductive uniformity and bonding strength of the coating, resulting in local electrochemical performance differences, which may increase internal resistance and accelerate attenuation.

5.Uneven thickness (as shown below)

• Description: The coating thickness in the cross direction (CD) or the longitudinal direction (MD) exceeds the tolerance range allowed by the process. Including overall thick/thin, thick edge/thin middle (or vice versa), periodic thickness fluctuation (“horizontal streaks”).
• Features: Need to be tested by a thickness gauge (β-ray, X-ray, laser), in severe cases, the gloss or color difference (such as whitening at the edge) can be seen by the naked eye.
• Possible causes:
  • Improper setting or adjustment of the coating head (die gap, scraper pressure).
  • Fluctuation of slurry viscosity and solid content.
  • Unstable substrate tension control.
  • Vibration and deformation of the backing roller or coating roller.
  • Fluctuation of feed pump flow.
• Impact: Directly affects the consistency of battery capacity, internal resistance, and rate performance. Too thick parts are prone to cracking, too thin parts are prone to puncture or insufficient capacity.

6.Edge Effect/Dog Ears

• Description: The coating thickness on both sides of the coating film in the width direction is significantly higher than that in the middle area, forming a shape similar to “dog ears”.
• Features: The coating is thickened on both sides of the edge.
• Possible causes:
  • Surface tension effect of the slurry at the die slit outlet (the meniscus at the edge causes increased flow).
  • The tension at the edge of the substrate is inconsistent with that in the middle.
• Impact: It affects the subsequent slitting and winding. The edge may accumulate too thickly after winding, causing battery deformation or uneven internal pressure.

7.Foreign matter/contamination

• Description: Particles, fibers, hair, metal shavings, dust, etc. that are not part of the slurry are embedded on or inside the coating.
• Characteristics: Foreign matter of various shapes.
• Possible causes:
  • Environmental cleanliness is not up to standard (dust in the air).
  • Introduction by human operators (hair, fiber).
  • Metal shavings from equipment wear.
  • Foreign matter mixed in during slurry preparation or transportation.
  • Contaminants carried by the substrate itself.
• Impact: One of the most dangerous defects, especially metal foreign matter, can easily cause internal short circuits in the battery and trigger thermal runaway. Non-metallic foreign matter may also affect coating uniformity and adhesion.

8.Orange peel texture (as shown below)

• Description: The coating surface presents irregular concave and convex ripples similar to orange peel.
• Features: There are no uneven tiny undulations in a specific direction.
• Possible causes:
  • Poor leveling of the slurry (high viscosity, strong thixotropy).
  • The surface solvent evaporates too quickly in the initial drying stage, and the flow compensation of the lower slurry is insufficient (driven by surface tension).
  • The surface roughness or tension of the substrate is uneven.
• Impact: Mainly affects the appearance, and may slightly affect the coating uniformity and contact in severe cases.

Substrate related defects

1.Wrinkles

• Description: The substrate is partially wrinkled and arched during the coating process.
• Features: Obvious wavy or strip-shaped creases, and the coating is also deformed.
• Possible causes:
  • Improper unwinding/rewinding tension control (too large, too small or unstable).
  • The guide rollers are not parallel or deformed.
  • The substrate itself has stress or uneven thickness.
  • Failure of the correction system.
• Impact: The coating is damaged, the thickness is uneven, and subsequent processes cannot be carried out, usually resulting in the entire roll being scrapped.

2.Tearing/Broken Belt

• Description: The substrate was pulled apart during the coating process.
• Features: The substrate was broken.
• Possible causes:
  • Tension is too high in an instant (such as sudden stop, too fast acceleration).
  • The substrate itself has defects (edge ​​cracks, internal damage).
  • The guide roller is stuck or does not rotate smoothly.
• Impact: Production interruption and waste.