Date: 2025-12-19 hits: 105
I. Hazards Caused by Cracking
Any form of crack appearing on the coating surface after drying, whether it's an almost imperceptible "mud crack" or a macroscopic crack that penetrates the entire coating, is a fatal threat to battery performance:
Damage to the conductive network: Cracks will sever the carefully constructed conductive agent network within the coating, interrupting the electron transport path. This prevents the active material in the cracked area from participating in charge and discharge reactions, resulting in a direct loss of capacity.
Increased internal resistance and heat generation: Electrons need to travel longer paths, significantly increasing the battery's internal resistance. During high-current operation, localized heating becomes severe, accelerating battery aging.
Structural collapse: During subsequent rolling and slitting processes, cracks become stress concentration points, leading to the coating peeling and detaching from the current collector, resulting in electrode sheet scrap.
Safety risks: The detached active material may form micro-short circuits inside the battery or promote the growth of lithium dendrites during cycling, puncturing the separator and causing serious safety hazards.
II. Analysis of the Causes of Cracking
The essence of cracking is that the internal stress generated in the coating during the drying process exceeds its own bonding strength. Our analysis focuses on this core contradiction.
1. Slurry Formulation
The slurry is the foundation of the coating, and its properties determine the quality of the coating.
Low solid content: The proportion of solvent (usually NMP or water) in the slurry is too high, and the proportion of solid substances (active material, conductive agent, binder) is too low. During drying, the evaporation of a large amount of solvent leads to severe shrinkage of the coating, generating huge shrinkage stress, which can easily cause the coating to crack.
Insufficient binder: Insufficient binder content (such as PVDF, SBR, CMC), uneven dispersion, or weak binding force prevents the binder from effectively "holding" all the particles together to form a whole. When stress occurs, cracking first occurs at the weakest bonding points.
Poor slurry rheology: The flow and deformation capabilities of the slurry are unsuitable. For example, if the slurry is too thixotropic, the viscosity recovers rapidly after the shear stops during coating, which is not conducive to surface leveling and may lead to cracking.
Conductive agent "bridging" problem: If high aspect ratio carbon nanotubes (CNTs) are not well dispersed, they can easily form a rigid network. This network itself is brittle and more likely to break under shrinkage stress, rather than deforming with the overall structure.
2. Drying Process
Drying is the direct process that generates stress, and improper processes are a major cause of cracks.
Excessive drying rate (most common reason): This is a typical case of "haste makes waste." If the drying temperature is set too high, especially in the first stage (preheating zone), it will lead to rapid evaporation of the solvent on the coating surface. When the internal solvent continues to vaporize due to heating, a huge vapor pressure is formed, leading to cracks.
Unreasonable drying gradient: The temperature, airflow, and wind speed settings in each temperature zone are mismatched, failing to create a smooth solvent gradient, resulting in the coating experiencing uneven stress at different drying stages.
3. "Interfacial Mismatch" between Current Collector and Coating
The bonding strength between the coating and the current collector (aluminum foil/copper foil) is crucial.
Current collector surface contamination: Oil stains, oxide layers, and dust on the foil surface will severely weaken the adhesion between the coating and the substrate. Stress will first be released from these weak bonding points, causing the coating to "peel off" and crack from the current collector.
Poor foil surface morphology: An overly smooth foil surface provides insufficient anchoring effect, preventing the binder from effectively interlocking, resulting in naturally weaker bonding strength.
4. "Mismatched Equipment and Parameters"
Excessive coating thickness: Applying an excessively thick coating in a single pass makes it difficult for the internal solvent to escape effectively, leading to uneven drying and inconsistent shrinkage between the inner and outer layers, resulting in more severe stress accumulation and a high risk of cracking. For thick electrodes, a "multi-layer coating" strategy is usually required.
Improper gap setting: For doctor blade coating, a mismatch between the gap setting and the slurry properties can also affect the uniformity and density of the initial coating, indirectly leading to cracking.
III. Solutions for Coating Cracking
To solve the cracking problem, a systematic approach is necessary, focusing on the "slurry-equipment-process" three dimensions.
Optimizing the Slurry for a Strong Foundation
Adjusting Solid Content: While ensuring coating leveling, appropriately increase the solid content of the slurry. This is the most effective way to reduce overall shrinkage and minimize shrinkage stress at the source. Strengthening the Binding System:
Ensure sufficient binder content, especially for thick electrodes or active materials with high specific surface area. Optimize binder dispersion to ensure uniform distribution in the slurry, forming a continuous and stable three-dimensional network.
For water-based systems, consider adding a small amount of toughening latex (such as acrylics) to improve coating flexibility and resist stress.
Improving Rheology: By adjusting dispersants and controlling slurry viscosity, ensure good leveling and moderate thixotropy, allowing the coating to fully relax before drying.
Precise Drying Control
Setting a Stepwise Drying Curve: This is the core of process control. A stepwise heating mode of "low temperature-medium temperature-high temperature" must be used. Preheating zone: Use a lower temperature and larger airflow to allow the surface solvent to evaporate slowly and evenly, keeping the surface "open" and providing an escape channel for internal solvents. Main drying zone: Gradually increase the temperature to drive out the "skeleton solvent" inside the coating. Cooling and final drying zone: Ensure that the solvent is completely removed.
The core principle is: "The evaporation rate of surface solvent must always be lower than the migration rate of internal solvent to the surface." Control wind speed and airflow: Excessive wind speed will accelerate the evaporation of surface solvent; a uniform and gentle airflow should be maintained.
3. Quality Assurance Interface
Strictly control the quality of the current collector materials: monitor their surface tension, roughness, and cleanliness.
Pre-treatment: If necessary, the foil material can be subjected to plasma cleaning or coated with an underlayer to significantly improve its surface energy and adhesion.
IV. Summary
Coating cracking is a typical "stress-strength" problem in lithium-ion battery manufacturing. It requires us to consider all aspects, from the slurry to the coating process parameters, to solve this issue.
