Laboratory lithium-ion coin cells are used by researchers for rapid electrochemical performance testing of newly synthesized cathode materials, anode materials, or novel electrolytes/additives. Their characteristics of speed, low cost, and standardization have greatly promoted the advancement of lithium-ion battery technology. However, during the coin cell slurry preparation stage, the difficulty in dispersing conductive carbon black (such as Super P, acetylene black, etc.) due to its large specific surface area, high surface energy, and tendency to agglomerate is a common and challenging problem.
This leads to uneven slurry, poor conductive network formation. It ultimately affects the electrode’s conductivity, mechanical strength, and the battery’s electrochemical performance. The main reasons for difficult carbon black dispersion and the corresponding solutions are as follows:
1 Physical and Chemical Characteristics of Carbon Black
Carbon black particles are extremely fine, have a huge specific surface area, and very high surface energy. Strong van der Waals forces exist between particles, making them highly prone to agglomeration into hard aggregates that are difficult to break apart. The surface of most carbon black is hydrophobic, resulting in poor compatibility with commonly used polar solvents (like NMP) and difficulty in being wetted by the solvent, which causes particle aggregation. Carbon black particles typically form branched or grape-like chain aggregates (primary structure), which can further agglomerate with each other (secondary structure). Breaking this structure requires sufficient energy.
Therefore, during slurry preparation, select conductive carbon black types with relatively better dispersibility (e.g., those with surface treatment). If necessary, vacuum dry the carbon black before use (e.g., 80-120°C for several hours) to remove adsorbed moisture and gases.
2 Solvent Issues
Excessive solvent moisture content: This is one of the most common reasons. NMP is highly hygroscopic. Moisture can:
Cause hydrolysis of NMP, producing organic amines that alter the system’s pH and viscosity.
Form a “water film” on the hydrophobic surface of carbon black, hindering its wetting by the solvent.
Cause side reactions with the binder PVDF, affecting its solubility and dispersion stability.
Promote capillary forces between carbon black particles, exacerbating agglomeration.
Additionally, impurities in the solvent may interfere with the dispersion process or adsorb onto the carbon black surface.
Strictly control solvent moisture: Use high-purity NMP and, if necessary, perform strict dehydration treatment before use (e.g., molecular sieve dehydration, distillation, inert gas purging).
Control the ambient humidity in the slurry preparation room (typically required <30% RH, lower is better) and use sealed containers for operations.
3 Binder Issues
PVDF not fully dissolved in NMP, forming gels or microgels, can encapsulate carbon black particles, making them harder to disperse and leading to so-called “fisheyes” or granules.
Excessively high PVDF concentration or molecular weight: Leads to excessively high slurry viscosity, reducing the efficiency of shear force transmission and making it difficult to effectively disperse carbon black agglomerates.
PVDF molecular chains may also adsorb onto the carbon black surface; if the adsorption is too strong or improper, it can also affect dispersion.
Use PVDF with a suitable molecular weight and good solubility.Ensure PVDF is completely dissolved in NMP before adding other materials, forming a uniform, transparent colloid solution. Appropriate heating (e.g., 50-60°C) and thorough stirring can be applied during dissolution.
4 Slurry Preparation Process Issues
(1) Improper addition sequence
Incorrect addition order is a critical factor leading to dispersion failure.
Adding the active material (e.g., LFP, NCM) too early: Active material particles are relatively large. If added first or simultaneously with the conductive agent, these larger particles can “shield” the conductive agent, preventing it from being fully exposed to shear forces, and the conductive agent can become encapsulated by the active material, forming agglomeration centers.
Improper method of adding the conductive agent: Dumping the conductive agent in all at once causes localized high concentration, instantly forming hard lumps that are difficult to break up.
Solutions:
Optimizing the addition sequence is extremely critical:
Solvent (NMP) + Binder (PVDF): First, mix most of the NMP (about 70-80% of the total) with the PVDF. Stir thoroughly at an appropriate temperature until fully dissolved, forming a uniform, transparent PVDF colloid solution.
Conductive Agent (Carbon Black + small portion of remaining NMP): Pre-mix the conductive agent (carbon black) with a small, reserved portion of NMP (about 10-20%) to form a low-solid-content conductive agent slurry/paste. Then, under high-speed stirring (high shear rate), slowly and in batches, add this conductive agent slurry to the PVDF colloid solution from step 1. This step is key for dispersing carbon black! Maintain high-speed stirring for a sufficient time (e.g., 30-60 minutes) to ensure the carbon black is fully dispersed and its agglomerates are broken down.
Active Material (LFP/NCM, etc.): After confirming the carbon black is well dispersed, reduce the stirring speed (to prevent re-agglomeration) and add the cathode active material slowly and in batches. After addition, adjust the stirring speed as needed (medium-high speed) for homogenization, avoiding excessive shear that could damage the active material particles.
Viscosity Adjustment (remaining NMP): Add the reserved remaining NMP (about 10%) as needed to adjust to the target viscosity.
Deaeration and Aging: Use low-speed stirring for deaeration, or vacuum deaeration. Allow appropriate aging time for the slurry state to stabilize.
(2) Insufficient Stirring Speed and Shear Force:
Dispersing carbon black requires a sufficiently high shear rate (high rotational speed) to overcome the cohesive forces of agglomerates. Inefficient stirrer blade design or excessively low rotational speed fails to provide effective shear. Insufficient dispersion time prevents the adequate breakdown of agglomerates.
Ensure carbon black dispersion occurs under conditions of low viscosity (only solvent + binder + solvent for a small amount of conductive agent) and high shear force. Absolutely avoid adding dry powder carbon black directly into a high-viscosity slurry or adding it simultaneously with a large quantity of active material.
Optimize stirring speed and time: The rotational speed during the dispersion phase must be sufficiently high (specific value depends on the equipment, but significantly higher than the mixing phase) and guarantee sufficient dispersion time for the shear force to work. Insufficient time is a common mistake.
(3) Unreasonable Stirring Program:
Adopt a “step-wise dispersion” strategy: Clearly distinguish between the “wetting/mixing” phase (low speed) and the “dispersion” phase (high speed). The high speed/high shear rate must be used during the carbon black dispersion phase.
Control slurry temperature: The dispersion process may generate heat. Excessively high temperature can cause solvent evaporation or side reactions. Use a cooling jacket if necessary to control temperature (e.g., <40°C). Note: Heating may be required when dissolving PVDF.
Control final solid content/viscosity: Excessively high overall slurry viscosity severely weakens the transmission efficiency of shear force, making dispersion difficult. While ensuring coating performance, appropriately reducing the solid content during the initial dispersion phase favors carbon black dispersion. The final viscosity is adjusted using the reserved solvent.
5 Equipment Issues
Improper mixer type and blade design: Using mixers unsuitable for high viscosity or high shear demands (e.g., simple paddle stirrers), or blades that cannot generate sufficient shear flow and circulatory flow, leading to dead zones.
Dead zones in the container or blades: Cause localized slurry to not participate in effective mixing and dispersion.
Select high-shear dispersion equipment: Such as planetary mixers, dual planetary mixers, high-speed dispersers, or in-line high-shear dispersion equipment. Avoid using simple stirring equipment with insufficient shear force.
Optimize blade design: Select blade combinations that generate strong shear flow and good circulatory flow (e.g., saw-tooth dispersion disc + anchor paddle).
Ensure equipment is clean and free of residue: Clean thoroughly before and after each use to prevent dried slurry residue from becoming nucleation sites for agglomeration.
6 Environmental Factors
High ambient humidity accelerates solvent (NMP) moisture absorption, worsening the moisture problem. Control the ambient humidity in the slurry preparation room stringently.
Strengthening Monitoring and Quality Control of the Slurry Preparation Process:
Online Monitoring: Monitor stirring power/torque, temperature, and vacuum level (if applicable) in real-time.
Slurry Testing:
Fineness Gauge Test: A quick, intuitive method to assess the maximum particle size in the slurry and judge the degree of dispersion. A qualified slurry should meet the target fineness (e.g., ≤20µm).
Viscosity and Rheological Properties: Measure viscosity and its change with shear rate (rheological curve). A well-dispersed slurry typically exhibits more stable rheological behavior.
Resistivity/Conductivity: Measure the slurry’s resistivity. A well-dispersed slurry has a more complete conductive network, resulting in lower and more stable resistivity.
Stability Test: Observe the slurry for sedimentation or flocculation under static conditions or low-speed stirring.
Microscopic Morphology Observation (SEM/TEM): Observe the distribution state of carbon black on the surface of active materials using dried slurry powder or coated electrodes. This is the most direct method for evaluating dispersion effectiveness.
Περίληψη
Solving carbon black dispersion problems requires systematic thinking and precise operation. Controlling solvent moisture, optimizing the addition sequence (ensuring carbon black is dispersed under low viscosity and high shear), and guaranteeing sufficient shear force and dispersion time are the three most critical elements. Simultaneously, selecting appropriate equipment, controlling the environment, and strictly monitoring raw material and process quality are also crucial. Before resorting to dispersing agents, it is essential to exhaust all process optimization measures. Through the comprehensive measures outlined above, the issue of poor carbon black dispersion in cathode slurry can be effectively resolved, enabling the preparation of high-performance lithium-ion battery electrodes.
The production of conductive carbon black for lithium batteries almost exclusively utilizes a jet milling process. This is because jet milling effectively disperses carbon black aggregates to a conductive size in a low-pollution manner. It can also preserve their inherent, crucial chain structure to the greatest extent possible. This ensures the final product possesses excellent conductive properties. Lithium battery materials have a very low tolerance for metallic impurities (such as Fe, Cu, and Zn). This can severely impact battery life and safety. Φρέζα με πίδακα operates on the principle of particle-to-particle collisions. This relatively gentle action effectively breaks up larger aggregates without excessively damaging their valuable internal chain structure.
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