Efficient dispersion of single-walled carbon nanotubes (SWCNTs) using high-pressure homogenization technology

Carbon nanotubes (CNTs) are hollow nanocylinders composed of carbon atoms. They were first reported in 1991 by Sumio Iijima. This was followed by the introduction of single-walled carbon nanotubes (SWCNTs), which are single-layer graphene tubes with diameters ranging from 1 to 2 nm, described by NEC and IBM in 1993. The structure of CNTs consists of sp² hybridized carbon atoms arranged in a hexagonal lattice. CNTs are classified into two categories: single-walled CNTs (SWCNTs, which have one layer) and multi-walled CNTs (MWCNTs, which have four or more layers).

SWCNTs and MWCNTs have excellent but different properties. SWCNTs must be distinguished from MWCNTs, as the characteristics of SWCNTs are generally superior to those of MWCNTs. In contrast to SWCNTs, the properties of MWCNTs diminish with the presence of structural defects and vary with an undefined diameter that is influenced by the number of layers.

SWCNTs are hollow cylinders, which results in a lower density compared to light metals like aluminum. The structure of SWCNTs consists of a hexagonal lattice formed by sp²-hybridized carbon atoms. The covalent sp² bonds between adjacent carbon atoms provide significant mechanical strength, enabling SWCNTs to exhibit a tensile strength greater than that of steel. Additionally, SWCNTs possess an unusually long mean free path (~1 μm) at room temperature, facilitating ballistic conduction. This extended mean free path also contributes to low power consumption and allows for the observation of quantum mechanical phenomena at room temperature. However, commercially available SWCNTs exhibit varying electrical conductivity due to reductions caused by lattice vibrations between MWCNT layers and additional scattering from impurities generated during the synthesis of SWCNTs, such as amorphous carbon and aromatic hydrocarbons. Despite this, SWCNTs still have higher electrical conductivity than copper. Additionally, the current density of SWCNTs exceeds 10^9 A/cm², surpassing that of copper due to the absence of electromigration breakdown. The strong carbon-carbon (C-C) bonds and low atomic mass of SWCNTs result in very high phonon frequencies and acoustic velocities, leading to minimal Umklapp scattering in phase space. SWCNTs efficiently transfer heat through acoustic phonon modes, demonstrating superior thermal conductivity compared to copper. The thermal conductivity of real materials is constrained by the anharmonic transmission of phonons. The thermal stability of SWCNTs is attributed to the robust covalent bonds between carbon atoms, and their hexagonal lattice structure contributes to their thermal resilience. Consequently, SWCNTs can maintain thermal stability up to 400°C in an atmospheric environment and up to 1000°C in a vacuum. Furthermore, the thermal stability of SWCNTs increases with their diameter and length.

There are three primary methods for synthesizing carbon nanotubes (CNTs): arc discharge, laser ablation, and chemical vapor deposition (CVD). Among these, CVD is the most suitable method for commercialization due to its advantages, including low cost, mass production capabilities, high purity, high yield, and moderate synthesis temperature. Particularly for lithium-ion battery applications, single-walled carbon nanotubes (SWCNTs) exhibit the following characteristics: low cost, high output (> 5 tons/year), high carbon nanotube content (> 99%), and minimal metal impurities (such as Fe < 100 ppm).

During the synthesis process, carbon nanotubes (CNTs) often become contaminated with amorphous carbon, residual metal catalysts, and fullerenes. Consequently, purification methods are necessary to enhance the quality of CNTs for commercialization. These methods include chemical etching, which removes contaminants using acid, and physical separation, which employs filters to isolate the contaminants from the CNTs.

Carbon nanotubes (CNTs) tend to aggregate and agglomerate during the preparation process due to van der Waals forces. To enhance their performance and facilitate their application in commercial products, it is essential to achieve a uniform dispersion of highly aggregated CNTs in a liquid medium. Dispersion refers to the distribution of one material, such as CNTs, within the continuous phase of another material, such as a solvent or polymer. Optimal CNT dispersion can be attained through a combination of dispersive mixing and distributive mixing techniques. Dispersive mixing, the initial step in the dispersion process, employs mechanical energy—such as shear stress and impact—generated by various machines, including ball mills and homogenizers. This energy effectively breaks apart and separates individual CNT fibers from the formed aggregates. After that, distributive mixing ensures that the carbon nanotubes (CNTs) are uniformly distributed within the liquid. During this process, chemical dispersion occurs as the dispersant wraps around the surfaces of the individual CNTs. This wrapping enhances the solubility of the CNT surfaces in the liquid, thereby improving the quality of dispersion and preventing re-aggregation through mechanisms such as steric hindrance and electrostatic repulsion. The process of dispersant wrapping is referred to as "non-covalent functionalization". To commercialize single-walled carbon nanotubes (SWCNTs) in the battery industry, non-covalent methods utilizing dispersants can achieve low-cost, high-quality dispersion. Among the three critical steps of synthesis, purification, and dispersion, the dispersion process is the most vital for the commercialization of SWCNTs, as it allows for optimal dispersion in liquids tailored to specific applications, ultimately enhancing the performance of commercial products.

Due to its high specific surface area, single-walled carbon nanotubes (SWCNTs) exhibit strong van der Waals forces between individual nanotubes, which results in inherent challenges such as poor dispersibility and a tendency to agglomerate. Although dispersing SWCNTs into a liquid phase is essential for many applications, the development of effective dispersion processes for SWCNTs has lagged behind that of other nanoparticles. In numerous studies, dispersion is primarily achieved through surface functionalization. For instance, SWCNTs can be oxidized using oxygen plasma and acid treatments to enhance their solubility in polar solvents. However, the industrial application of these methods poses significant challenges due to low productivity and safety concerns.

SWCNT aqueous dispersions may contain surfactants such as sodium dodecylbenzenesulfonate (SDBS), cetyltrimethylammonium bromide (CTAB), and Triton X-100, as well as polymer dispersants like carboxymethylcellulose (CMC), polyvinylpyrrolidone (PVP), and polyacrylic acid (PAA). These dispersants can be utilized individually or in combination. In non-battery applications, dispersants with smaller molecular weights are typically preferred because they can be relatively easily removed after the coating process. However, for applications involving SWCNT dispersions as battery electrode components, polymer dispersants are favored due to their long-term stability, enhanced electrode adhesion, and improved thermal stability.

Dispersants must be selected based on their intended use, and determining the appropriate amount of dispersant is crucial. Excessive use of dispersant may compromise the results. Conversely, using less dispersant than necessary can lead to inadequate coverage of the surface of the single-walled carbon nanotubes (SWCNTs), exposing free areas that may serve as bridge points, thereby inducing SWCNT aggregation in solution. Since various physical properties of SWCNTs, such as specific surface area and aspect ratio, influence the determination of the required amount of dispersant, the precise quantity needed must be established through multiple experiments and optimization.

Among various dispersants, CMC is widely used in the battery industry due to its low cost and environmentally friendly properties as a biopolymer. CMC is adsorbed onto the surface of SWCNT by encapsulation or non-covalent functionalization, which helps prevent SWCNT agglomeration. The hydrophobic cellulose backbone of CMC interacts with the surface of SWCNTs via van der Waals forces, while the hydrophilic sodium carboxylate groups enhance its solubility in water. Additionally, CMC improves the dispersibility of the negative electrode active material within the electrode composition, preventing the formation of clusters and thereby reducing electrode resistance. However, CMC has limitations as a dispersant; specifically, its solution viscosity is relatively high, making it challenging to create high-concentration SWCNT dispersions. As battery manufacturers seek to utilize higher concentrations of SWCNT dispersions to lower costs, it is increasingly important to identify or develop dispersants that can reduce viscosity while maintaining dispersion performance comparable to that of CMC.

In addition, selecting the appropriate dispersion process is as crucial as choosing the right dispersant. A well-balanced combination of the physical dispersion method and the non-covalent modification process, aided by the dispersant, is essential for achieving a high-quality dispersion of single-walled carbon nanotubes (SWCNTs). The most prominent physical dispersion strategies include ball milling, ultrasonic cavitation, and high-pressure homogenization.

High-pressure homogenization causes minimal direct damage to particles and is more effective at dispersing single-walled carbon nanotubes (SWCNTs) while preserving their physical properties compared to other techniques. This method also demonstrates excellent reproducibility and is easy to scale up. One of its most significant advantages is that most mass production equipment can supply fluids continuously, allowing for unlimited maximum production capacity and very fast production speeds.

In the operation of high-pressure homogenization, parameters such as pressure, passes, fluid feed rate, and channel size must be carefully optimized. Additionally, because the fluid must pass through narrow gaps or microchannels, the viscosity of the fluid being treated needs to be taken into account.

In summary, both the appropriate dispersion method and dispersant are crucial and should be selected based on the physical properties of SWCNTs, solution viscosity, matrix type, and other relevant factors. Research has shown that highly dispersed SWCNTs have significant potential for enhancing the energy density and rate performance of lithium batteries, enabling long-distance travel (over 640 km) and rapid charging (< 15 min) of electric vehicles. SWCNT dispersions can be utilized not only to enhance anodes but also to improve the electrical and thermal conductivity of cathodes, separators, modules, and battery packs. Additionally, they can be applied in various other contexts, such as dry cathode coating materials and conductive additives for next-generation batteries, including all-solid-state batteries. Industries across the board should consider leveraging the exceptional properties of SWCNTs to enhance their performance. Regardless of the application, it is highly likely that SWCNTs will be employed in the form of dispersions. Therefore, the functionalization, dispersion methods, and dispersants of SWCNTs will remain critical areas of research both now and in the future.

Since its establishment, Antuos Nanotechnology (Suzhou) Co., Ltd., a subsidiary of Duoning, has been dedicated to independent research and development, as well as the integration of advanced pharmaceutical equipment and technologies into local industries. The company offers state-of-the-art pharmaceutical equipment solutions for scientific research institutions and pharmaceutical companies. It has attracted a global clientele and has become a preferred choice for many users.

Antuos specializes in nano-preparation technology, bioengineering technology, and nano-chemical technology. Its primary products are utilized in the research, development, and production of liposome drugs, microsphere drugs, biological vaccines, diagnostic reagents, and more. The company offers a comprehensive range of high-pressure homogenizer products and is recognized as a leader in high-pressure homogenization technology. Antuos's products have been extensively adopted by major research institutions and pharmaceutical companies, spanning various industries, including the biological industry (protein drugs, diagnostic reagents, enzyme engineering, human vaccines, veterinary vaccines, etc.), the preparation industry (fat emulsions, liposomes, nanoparticles, microspheres, etc.), the food industry (beverages, dairy products, food additives, etc.), and the chemical industry (new energy batteries, nanocellulose, coatings, papermaking, polymer materials, etc.).