I. Reasons for the appearance of false edges in lithium battery coating
1. Difference in interfacial tension gradient of slurry
High surface tension of PVDF binders: Traditional PVDF binders increase the surface tension of ceramic slurries, leading to interfacial tension differences (Marangoni effect) between the slurry and the positive electrode active layer (such as LFP), causing slurry migration and forming uneven or virtual edges in the fusion zone.
The improving effect of PAA binder: Using polyacrylic acid (PAA) as a binder can significantly reduce the surface tension of the slurry and reduce the interfacial tension gradient, thereby suppressing the virtual edge (experimental data show that the virtual edge length of PAA-based binders can be controlled within 0.01-0.06 mm, while that of PVDF-based binders is as high as 55.1%).
2. Insufficient rheological and wettability of the slurry
When the rheological properties (such as viscosity and flowability) and wettability of ceramic slurry are poor, incomplete edge coverage is likely to occur during high-speed coating.
3. Improper coating process parameters
Mismatched parameters such as coating speed and drying rate may result in insufficient leveling of the slurry in the edge area, excessive solvent evaporation, and the formation of false edges.
4. Equipment and Die Head Design
An improperly designed chamfered gasket or unsuitable exit shape in a slot coating die may weaken the stress state of the slurry at the edge, leading to excessive or insufficient thinning.
II. Measures to Improve the Coating Edge Problem
1. Change the adhesive: Use a low surface tension adhesive such as PAA.
2. Optimize the process: Adjust the coating speed and drying curve to ensure uniform solvent evaporation.
3. Equipment improvement: Optimize gasket design or adopt high-precision coating equipment.
In summary, the virtual edges are mainly caused by the imbalance of interfacial tension in the slurry and insufficient process control, and need to be solved through the coordinated optimization of materials, processes and equipment.
