During the coating process, various defects may occur on the film surface, such as bubbles, fat edges, craters, polygonal depressions, orange peels, etc. Shrinkage cavities are one of the most common problems.The surface depression of the coating film, that is, the shrinkage cavity, is fundamentally caused by the surface tension gradient generated during the film formation. This phenomenon is called the Maragoni effect.

There are three basic components of coating slurry: binder,lithium battery powder material, dispersion medium, and there may also be various functional auxiliary materials.

The mismatch of surface tension between materials is the main cause of shrinkage cavities, but the viscosity of the coating, the fluidity of the liquid film, the wind speed and lab oven temperature for drying and film formation, the conditions of the heat treatment stage and equipment conditions, etc. may change the surface tension and Its action process, thereby inducing the generation of shrinkage cavities.

Due to the existence of particles with low surface tension (such as powder, oil droplets, etc., called "contaminants") in the flowable film surface before curing, the central surface tension is low, and the fluid migrates around the contaminants as the center, and finally A circular depression is formed with the edge higher than the center - shrinkage cavity. That is to say, there is a substance with low surface tension in the center of the shrinkage cavity, which has a surface tension difference with the surrounding paint. This difference is the driving force for the formation of the shrinkage cavity, which promotes the surrounding liquid to migrate away from the pollutants in a 360-degree direction.

How to prevent shrinkage cavities during the coating process;

1. Further modification of the surface structure of the material

By further modifying the surface of the graphite material, the polar groups are increased and its hydrophilicity is improved;

2. Adjust the molecular structure of the active ingredients of the adhesive

By adjusting the molecular structure of the active ingredients of the adhesive and reducing the content of its polar groups, the surface tension of the LA-type water-based adhesive product is effectively reduced, and the degree of wetting of the graphite material is improved.

3. Add surface active dispersant

Add PVP, CMC and other surface-active dispersants to the slurry to ensure the emulsification and dispersion of particles with low surface tension, and improve the dispersibility of graphite materials in water, that is, to improve the degree of wetting of graphite materials by water.

4. Add solvent

A solvent with low surface tension (such as ethanol, etc.) is added to the slurry to improve the wettability of the battery graphite material by water.

5. Increase the stirring speed and prolong the stirring time.

6. Increase the viscosity of the slurry.

7. Properly shorten the drying time.