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How Viscosity at Different Shear Rates Shapes
Coating Quality in Electrode Manufacturing
Controlling slurry viscosity is critical for achieving consistent and high‑quality electrode coatings. Because viscosity behaves differently under varying shear conditions, understanding both high‑shear and low‑shear regions is essential for optimizing the slot‑die coating process.
High‑Shear Viscosity: Influencing How the Slot‑Die Head Coats
During coating, the slurry experiences high shear as it passes through the slot‑die head. Most slot‑die coaters operate at roughly 100 s⁻¹ shear rate, meaning that any formulation changes that affect viscosity in this region will directly influence how the slurry forms a wet film on the current collector.
A slurry that is too viscous at high shear may resist flow through the die, leading to:
- Inconsistent wet thickness
- Streaking or ribbing defects
- Higher pump pressure requirements
Conversely, reducing high‑shear viscosity through improved dispersion can create a more uniform, stable coating profile. Although the exact shear rate depends on equipment geometry and flow conditions, simple industry equations allow manufacturers to estimate it for their specific slot‑die system. Understanding this value helps guide formulation adjustments and prevents coating variability.
Low‑Shear Viscosity: Determining the Final Wet Film Before Drying
Once the slurry leaves the slot‑die head and settles on the current collector, it enters a low‑shear, resting phase. This period - before heating and water removal - is where low‑shear viscosity becomes especially important.
Low‑shear viscosity influences:
- Wet film stability
- Edge definition and leveling
- The risk of slumping or runoff
- The uniformity of the final dried coating
If viscosity in this region drops too low, the slurry may spread excessively, leading to thin regions or uneven active‑material distribution. If it is too high, the coating may resist leveling, trapping variations that become defects after drying.
Balancing Both Regions for Optimal Performance
Because high‑shear and low‑shear viscosity affect different stages of coating, ideal slurry performance requires a careful balance. Adjustments made to improve flow through the slot‑die head must not compromise the slurry’s stability once coated. This makes dispersant selection and dosage an important lever in achieving a formulation that coats smoothly and dries uniformly.
By understanding how viscosity behaves across shear conditions, manufacturers can fine‑tune their electrode slurries for reliable coating quality, reduced defect rates, and improved downstream battery performance.
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Read our Publication: "Debundling of SWCNTs using a Non-Toxic, Low Carbon Footprint Dispersant"
Abstract
A fully aqueous, N-methyl-2-pyrrolidone–free strategy for debundling single-walled carbon nanotubes (SWCNTs) is reported using the renewable dispersant Vanisperse® LI. Dispersions at 2 mg mL−1 were subjected to probe ultrasonication at 0.3 W mL−1 and evaluated using oscillatory rheology. Complex viscosity (η*) exhibited a transient maximum (~75 min) consistent with the formation of a percolated fibrous network, followed by a decline as debundling progressed. An optimum dispersant coverage of ~1.5 mg m−2 minimized η*, while overdosing likely induced multilayer adsorption and bridging seen by a rapid increase in η*. A two-stage centrifugation at 10,000× g yielded storage-stable suspensions of debundled SWCNTs without ultracentrifugation. SEM confirmed substantial debundling into thin fiber-like bundles. By formulating a dispersion with a dispersant that has a significantly lower cradle-to-gate carbon footprint than both fossil-based and bio-based alternatives such as CMC, this work presents a more sustainable approach to producing debundled SWCNT dispersions for advanced material applications.
Technical Bulletin: Dispersion of Carbon Nanotubes with Vanisperse LI
Borregaard’s bio-based battery additives are designed for use in water-based electrode slurries and to stabilize aqueous carbon nanotube (CNT) dispersions. Obtained from sustainably sourced wood, our products are non-toxic, environmentally friendly, and highly effective.
Vanisperse LI provides a uniquely sustainable alternative and affords a lower CO2 footprint than petroleum-derived additives. Efficient dispersion of CNTs is critical for achieving uniform electrode composition and optimal battery performance. Sustainable water-soluble dispersants are essential to de-agglomerate CNTs in water, preventing the hydrophobic carbons from reforming agglomerates.