I. Shrinkage
1.1 About crater
The round or nearly round depression on the surface of the electrode sheet, especially the negative electrode sheet, is called shrinkage hole or crater. Shrinkage, a coating defect, is common in the coating process of paint application, and is not unique to lithium-ion battery electrode coating. The figure below illustrates this.
1.2 Formation of shrinkage holes
There are various defects that can occur during the coating process, such as bubbles, fat edges, craters, polygonal depressions, orange peel, etc. Shrinkage is one of the most common problems. This is fundamentally due to the surface tension gradient generated during film formation, a phenomenon known as the Maragoni effect.
The mismatch of surface tension between materials is the main cause of shrinkage. Viscosity, fluidity, as well as drying air speed and oven temperature may change the surface tension and its course, thus inducing shrinkage.
The presence of low surface tension particles (such as powder, oil droplets, etc.) in the flowable film surface before curing, resulting in the central surface tension is low, the fluid to the pollutant as the center of migration in all directions, and ultimately the formation of the edge is higher than the center of the circular sag (shrinkage hole).
1.3 Prevention and control of shrinkage holes
Selection of compatible dispersants or dispersing media, reduce the presence of large particles with low surface tension (including large droplets) in the coating body; add hydrophobic surface-active dispersants and solvents, etc. can also control the degree of shrinkage.
Second, lithium battery wafer coating shrinkage solution strategy
1）Further modification of material surface structure
Through further modification of the graphite material surface, improve the polar groups, improve its hydrophilicity;Email:amy@etelux.com
(2) Adjust the molecular structure of the active ingredient of the adhesive
By adjusting the molecular structure of the active ingredient of the adhesive, reduce the content of polar groups, effectively reduce the surface tension of LA-type water-based adhesive products, and improve the degree of wettability of graphite materials.
3）Adding dispersant with surface activity
Add PVP, CMC and other surface-active dispersants in the slurry to ensure emulsification and dispersion of low surface tension particles, improve the dispersing ability of graphite materials in water, i.e., improve the degree of wetting of graphite materials by water.
4）Adding solvent
Add solvents with low surface tension (such as ethanol, etc.) in the slurry to improve the wetting degree of water to graphite materials.
5）Improve the stirring speed and extend the stirring time.
6) Increase the viscosity of the slurry.
7) Shorten the drying time appropriately.
