Method for producing a suspension of carbon nanotubes

Abstract

A method for preparing a carbon nanotube suspension wherein the method comprises successive steps of (1) freezing a carbon nanotube suspension, the carbon nanotube suspension having a first viscosity and containing a solvent, not less than 0.2 wt.% and not more than 2 wt.% of single-walled and / or double-walled carbon nanotubes, and not less than 0.2 wt.% and not more than 3 wt.% of a dispersant to obtain a frozen carbon nanotube suspension; and (2) thawing the frozen carbon nanotube suspension to obtain a carbon nanotube suspension having a second viscosity, wherein the second viscosity is lower than the first viscosity. The resulting suspensions are particularly useful in the preparation of electrode pastes and elecrodes of Li-ion secondary batteries.

Description

[0001] IPC: C01B32 / 159; C01B32 / 158; C08J 3 / 02; H01M10 / 0525.

[0002] Method for producing a suspension of carbon nanotubes.

[0003] Field of the invention.

[0004] The invention relates to suspensions of single-walled and / or double-walled carbon nanotubes and their agglomerates, methods for their preparation, the use of these suspensions for the preparation of electrode pastes, electrode pastes, electrodes of lithium-ion batteries, and methods for manufacturing electrodes of lithium-ion batteries.

[0005] Background art.

[0006] Suspensions of single-walled and double-walled carbon nanotubes can be used to incorporate carbon nanomaterials into various coatings and composite materials, including electrode pastes (slurries) for manufacturing electrodes of lithium-ion batteries. Suspensions of single-walled and double-walled carbon nanotubes are sometimes also referred to as dispersions to emphasize that carbon nanotubes were dispersed in a solvent.

[0007] The use of single-walled and / or double-walled carbon nanotubes in the cathodes and anodes of lithium-ion batteries allows increasing the capacity of electrodes and batteries as a whole, reduce their internal resistance, and enhances the charge-discharge cycle life of lithium-ion battery electrodes, especially silicon-containing anodes of lithium-ion batteries, as demonstrated, for example, by the invention [RU2749904 Cl, 18.06.2021, IPC: C01B32 / 158, H01M4 / 02, H01M4 / 04, H01M4 / 13],

[0008] Single-walled and double-walled carbon nanotubes tend to assemble into bundles and agglomerates of more complex geometries. Agglomeration of carbon nanotubes into longer and thicker bundles is desirable in a number of applications, such as ensuring high electrical conductivity of the electrode, but it often leads to an increase in the sedimentation rate of dispersed agglomerates of carbon nanotubes, z.e., to a decrease in the stability of dispersions and / or reduced performance of the electrode paste. Reduced dispersion stability decreases dispersion storage time, limits logistical and technological flexibility, increases the risk of inhomogeneity during subsequent mixing with the electrode paste material, and, as a result, causes defects in lithium-ion battery production. To an even greater extent, the risks are associated with a decrease in the stability of the electrode paste containing single-walled and / or double-walled carbon nanotubes and their agglomerates.

[0009] Methods are known in the art to enhance the stability of dispersions by preventing agglomeration of dispersed particles, e.g., carbon nanotubes, using dispersing agents and surfactants. For single-walled carbon nanotubes, it is known, for example, that chlorosulfonic acideffectively shifts the equilibrium of the agglomeration process towards individual nanotubes [A. N. G. Parra- Vasquez, N. Behabtu, M. J. Green, C. L. Pint, C. C. Young, J. Schmidt, E. Kesselman, A. Goyal, P.M. Ajayan, Y. Cohen, Y. Talmon, R. H. Hauge, M. Pasquali " Spontaneous Dissolution of Ultralong Single- and Multiwalled Carbon Nanotubes" ACS Nano 2010 4 (7), 3969-3978], This approach, however, has the disadvantage that it reduces or completely eliminates the presence of long carbon nanotube bundles, the presence of which in some applications has the advantage of lowering the threshold of electrical percolation and increasing the electrical conductivity of the coating or composite material obtained using such dispersion, such as the electrodes of a lithium-ion batteries. Long carbon nanotube bundles also reinforce the composite material, e.g. help keeping physical integrity of electrodes during charge-discharge cycles.

[0010] On the other hand, the dispersions of single-walled or double-walled carbon nanotubes are characterized by high viscosity, which increases with the concentration of nanotubes and their bundles, as well as their length-to-thickness ratio [A. N. G. Parra- Vasquez, J. G. Duque, M. J. Green, M. Pasquali, " Assessment of length and bundle distribution of dilute single-walled carbon nanotubes by viscosity measurements." AIChE J., 60 (2014) 1499-1508], Additional agglomeration of carbon nanotube bundles into complex-shape agglomerates significantly increases viscosity of the dispersion, which creates technological difficulties in the further use of the dispersion, for example, while coating or pumping through technological lines in the production process.

[0011] In an attempt to minimize such difficulties, some inventions describe dispersions of carbon nanotubes whose viscosity is limited. To obtain such dispersions, it is proposed to use low-molecular polymer dispersants, e.g., low-molecular carboxymethylcellulose. Numerous applications formalize this obvious solution in different ways. For example, an application for invention [EP4270514A1, Panasonic Int Prop Management] claims a dispersion with a viscosity at 100 s'1of greater than or equal to 50 mPa s and less than or equal to 200 mPa s, obtained by using carboxymethylcellulose as a dispersant, in which a 3% aqueous solution of said carboxymethylcellulose has a viscosity at 100 s'1of greater than or equal to 2 mPa s and less than or equal to 200 mPas. Another invention application [W02022070810A1, Nippon Paper Industries Co Ltd.] claims a dispersion liquid containing carbon nanotubes and carboxymethylcellulose and / or its salt and limits the viscosity of 1 wt.% of an aqueous solution of said carboxymethylcellulose and / or its salt to the range of 1 to 1000 mPa s. Application for invention [WO2023121093, Betterial Co. Ltd] has claimed dispersions containing single-walled carbon nanotubes and a cellulose-based dispersant wherein the ratio {Degree ofsubstitution(DS) / (Weight average molecular weight of cellulosic dispersant)*106} is of 7 or more and 13 or less.

[0012] The low viscosity of carbon nanotube dispersions with low-molecular-weight dispersants have a technological advantage compared to high-viscosity dispersions, for example, when pumping through pipes in an industrial setting. However, at the same time, high quality of dispersion associated with high viscosity reduces the mobility of carbon nanotubes and their bundles, as well as the sedimentation rate of agglomerates, and thus increases the stability of dispersion, and, conversely, in dispersions with low viscosity, agglomerates mobility and sedimentation rate are higher. Thus, in the choice of carbon nanotube dispersions, a compromise has to be made between the high dispersion stability and low viscosity.

[0013] There are inventions in which gels with very high viscosity containing carbon nanotubes in a dispersed state are obtained. In the application for invention [W02006112162A1; NISSHINBO IND INC, MASUD A GEN, KATO YASUHARU, 26.10.2006; IPC: C08L75 / 04, C08K3 / 04, C09K3 / 16, C01B32 / 159, B82Y30 / 00, H01B1 / 12], a gel composition is offered, which comprises carbon nanotubes and a neutralized salt-type ion liquid obtained by a neutralization reaction of an acid, e.g., of benzoic acid or derivatives thereof, and a base. Such gel composition containing carbon nanotubes provides the possibility of indefinitely long storage without nanotubes agglomeration and their agglomerates sedimentation. However, the use of an ionic liquid as a dispersion medium imposes significant limitations on the applications of such dispersions. In the overwhelming majority of applications, including the electrode pastes preparation, such gel cannot be used directly and before the use would require to obtain a dispersion based on an aqueous and / or organic solvent, mainly containing electroneutral molecules. The stability of such dispersion will no longer be ensured by the stability of the original gel, nor the stability of the electrode paste will be ensured. In addition, in many applications, including those used in the preparation of electrode pastes for lithium-ion battery electrodes, the initial components of the ionic liquid may remain undesirable even after the dilution and dispersion in a solvent.

[0014] There are strategies where the dispersion composition is optimized to achieve a viscosity and / or complex modulus within a narrow range. This strikes a balance between the high viscosity needed to maintain the quality of carbon nanotube dispersion and the low viscosity required for efficient pumping and processing into the final product. For example, the patent [EP 3333946; LG CHEM, LTD., 17.03.2021; IPC: H01M4 / 62, H01M4 / 13, H01M10 / 0525, H01M4 / 139, H01M4 / 04, H01M4 / 02] declares a conductive material dispersed liquid, comprising bundle-type carbonnanotubes in a dispersion medium and a dispersant including a hydrogenated nitrile-based rubber, wherein a complex modulus, |G*|@1Hz, is in the range of 20 to 500 Pa when measured by a rheometer at a frequency of 1 Hz. The complex modulus of elasticity at a frequency of 1 Hz is used by the authors of the cited patent as a certain general rheological characteristic of a liquid: at the low value of the complex modulus of elasticity, the viscosity of the liquid is too small, at the high value of the complex modulus of elasticity, the viscosity of the liquid is too high and the further production process (electrode formation) ceases to be technological. As described in the cited patent, the viscosity of the dispersion with the complex modulus of elasticity mentioned above ranges from 2 to 20 Pa s (from 2000 to 20000 mPa- s) at the shear rate of 1 / (6.3 s). The cited invention does not focus on ensuring high dispersion stability, but the viscosity limitation at the shear rate of 1 / (6.3 s) is below 20 Pa s and is insufficient to maintain long-term stability of the dispersion, which is a disadvantage that hinders the efficient organization of the production process, including the efficient storage and transport of the dispersion.

[0015] The patent application [JP 6860740; TOYO INKMFG CO, 21.04.2021; IPC: H01M4 / 13, H01M4 / 62, H01M10 / 05], proposes to limit the complex modulus |G*| = (G'2+ G"2)1 / 2and at the same time to limit the value of the phase angle, z.e., the arctangent of the ratio of the loss modulus, G", and the storage modulus, G', measured at a frequency of 1 Hz. The patent claims a carbon dispersion liquid containing carbon nanotubes, carboxymethyl cellulose or a salt thereof, and water, wherein the carboxymethyl cellulose or its salt has a weight average molecular weight of 10,000 to 100,000 and a degree of etherification of 0.5 to 0.9; the product (X × Y) of the complex modulus of elasticity X (Pa) and phase angle Y (°) of the carbon nanotube dispersion is greater than or equal to 100 and less than or equal to 1500. That is, the suspension of the cited invention should meet condition (1), in which G" is the loss modulus and G' is the accumulation modulus, as measured by applying an oscillating shear strain at a frequency of 1 Hz.

[0016] 100 Pa° < (G'2+ G"2)1 / 2arctg(G" / G) < 1500 Pa° (1)

[0017] A simple mathematical analysis of condition (1) shows that it imposes no constraints on the accumulation modulus G', which determines the elastic properties of the dispersion. The quantity constrained in the invention monotonically depends on the loss modulus, G". Condition (1) constrains the loss modulus, which determines the viscous properties of the dispersion: G" < 26.18 Pa for large values of G'

[0018]

[0019] co, and with limited values of the storage modulus, it should be even smaller. At the same time, condition (1) also limits the phase angle, which must be less than 3 - 7.5 degrees to meet this condition at moderate values of the complex modulus of elasticity ranging from 200 to 500 Pa. Such values of the phase angle are typical for rigid gels. Thus, theinvention [JP 6860740] proposes the use of low viscosity dispersions rigid gels. The disadvantages of this solution are evident: the destruction of such a gel leads to the formation of large agglomerates, several tens of micrometers in size, which cannot be destroyed further due to the low viscosity of the dispersion and, consequently, low shear stresses. This makes it difficult to achieve homogeneity when mixing the dispersion with the active component of the electrode paste, and its rheological properties are not optimal, for example, the viscosity of the paste may be too low to ensure its stability. Another embodiment of the invention described in [JP 6860740] presents a dispersion that, in addition to the above limitation, is characterized by a phase angle greater than 15 degrees, but a complex modulus of elasticity less than 50 Pa, which means a very low viscosity and, consequently, low stability of the dispersion.

[0020] The invention [RU 2777040] provides a dispersion comprising water, a gelling agent and single-walled and / or double-walled carbon nanotubes, wherein the content of single-walled and / or double-walled carbon nanotubes is between 0.3 and 2 wt.% and the weight ratio of single-walled and / or double-walled carbon nanotubes to the gelling agent is not less than 0.05 and not more than 10, wherein the dispersion contains gel particles, formed by agglomerates of gelling molecules, which are physically interconnected in a weak gel network by the single-walled and / or doublewalled carbon nanotubes. The dynamic viscosity of such dispersion at rest during storage is high and exceeds 20 Pa s at a shear rate of 1 / 6.3 s'1or less, which makes it possible to store and / or transport the dispersion for a long time without agglomeration and / or sedimentation of carbon nanotubes, and the dynamic viscosity of the dispersion during processing is much lower: under 2 Pa s, at a shear rate of 18.6 s'1of higher. Such low viscosity is suitable for technological processes in which the dispersion is used, including the production of electrode paste.

[0021] Despite the above approaches, which offer a trade-off between the high viscosity needed for suspension stability and the low viscosity required for processing applications, it is preferable to avoid this trade-off at all. Ideally, it would be a high-viscosity suspension that remains stable during long-term storage or transportation, with the viscosity being reduced before use. The present invention provides a method for lowering the viscosity of a high-viscosity suspension of carbon nanotubes prior to its use.

[0022] Description of the invention.

[0023] The present invention provides a method for preparing a carbon nanotube suspension wherein the method comprises successive steps of (1) freezing a carbon nanotube suspension, the carbon nanotube suspension having a first viscosity and containing a solvent, not less than 0.2 wt.% and not more than 2 wt.% of single-walled and / or double-walled carbon nanotubes, and notless than 0.2 wt.% and not more than 3 wt.% of a dispersant to obtain a frozen carbon nanotube suspension; and (2) thawing the frozen carbon nanotube suspension to obtain a carbon nanotube suspension having a second viscosity, wherein the second viscosity is lower than the first viscosity.

[0024] This sequence of freezing and thawing leads to a controlled agglomeration of carbon nanotube bundles into secondary agglomerates of carbon nanotube bundles and dispersant. The term "controlled" means that agglomeration does not lead to complete sedimentation of particles in the suspension, but to the formation of secondary agglomerates with dimensions that depend on the conditions of freezing, primarily on the freezing rate (mm / min), which is determined by the temperature, the vessel geometry and the density of the heat flux removed from the vessel with the suspension. Thus, by adjusting these parameters, it is possible to control the topology and size of the agglomerates in the slurry and, consequently, its dynamic viscosity.

[0025] The secondary agglomerates are formed by congesting nanotube bundles at the boundaries of the solvent crystals and thus also consist of nanotube bundles and dispersant, but do not necessarily have a fibrous morphology themselves, they can be characterized by much smaller aspect ratios, e.g., less than 100 or less than 10 or even less than 2. At the same time, the size of the secondary agglomerates can be very significant: some secondary agglomerates can exceed 50 pm in size, but typically are larger than 5 pm, while some are smaller, down to 500 nm. It is preferable that for a part of the secondary agglomerates the least dimension is more than 5 pm, for some applications it is preferable that the part of the secondary agglomerates has the least dimension greater than 50 pm. Secondary agglomerates with the least dimension of more than 500 nm can be observed by optical microscopy, secondary agglomerates with the least dimension of more than 50 pm are visible to the naked eye.

[0026] It is preferrable that the volume-weighted distribution of particle sizes in the suspension, determined by laser diffraction, has a median size of D50 greater than 10 pm, and a value of D10 greater than 1 pm. Hereinafter, the volume-weighted particle size distribution (p.s.d.) refers to the distribution calculated from the laser diffraction, z.e., from measuring the angular pattern of intensity of light scattered from a laser beam passing through a suspension sample. Large particles scatter light at smaller angles, while small particles scatter light at larger angles. The angular scattering intensity data is analyzed to restore the particles size distribution using the Mie theory of light scattering. The particles’ size is reported as a volume equivalent sphere diameter (VESD). Laser diffraction provides an accurate p.s.d. only for spherical particles, and its application for measuring p.s.d. of particles with a high aspect ratio (e.g., fibers or nanotubes) is less straight forward and is still debated in literature. It is known that laser diffraction represents data on thesize of particles’ projection onto the plane normal to the laser beam (although it is usually reported as the volume equivalent sphere diameter). Therefore, the calculated size of non-spherical particles depends on their orientation relative to the beam. In the state-of-the-art laser diffraction particles size analizers non-spherical particles are aligned by laminar flow in the measuring cell. Therefore, the measured p.s.d. may differ significantly from that obtained using other methods. Despite these drawbacks, the ease of use and wide measuring range make laser diffraction the most widely used method for characterizing the dispersions of nanotubes. That is why, in this patent we characterize p.s.d. for suspensions of single-walled and / or double-walled carbon nanotubes in water and in N-methyl-2-pyrrolidone by means of laser diffraction. We acknowledge that these data do not represent the exact distribution of volume-equivalent sphere diameters of particles in these suspensions. However, these data are well reproducible and representative of the particle size and shape distribution. To avoid misinterpretation, we clearly note here that particle size distributions are measured by means of laser diffraction. In description below we will refer to such data as “particle size distribution” or “p.s.d.” without further mentioning laser diffraction.

[0027] It is important to note that the proposed method does not change the thickness of the bundles of single-walled and / or double-walled carbon nanotubes, z.e., their primary agglomerates (or aggregates) in which the single-walled and / or double-walled carbon nanotubes are firmly bound together by van der Waals π-π stacking. These primary agglomerates - bundles of nanotubes - combine into larger, but weaker, agglomerates. As such, when mixed into an electrode material, secondary agglomerates are relatively easily disassembled into primary agglomerates, i.e., primary bundles of carbon nanotubes.

[0028] Freezing and thawing can reduce the viscosity of a viscous suspension because the process disrupts the internal structure that originally made it highly viscous. Highly viscous suspensions behave that way because particles form interconnected networks or gels, z.e., secondary agglomerates. When the suspensions freeze, mechanical stress breaks the networks binding together the secondary agglomerates. Growing crystals physically tear apart gels, agglomerates, or entangled chains. During thawing, when the solvent crystals melt, the broken secondary agglomerates’ network does not re-form the same way. Particles - primary agglomerates of singlewalled and / or double-walled carbon nanotubes and dispersant - may become more dispersed, less aggregated and can reorganize into a weaker structure. The result is much lower resistance to flow, respectively lower viscosity. It is worth noting that the above described process of freezing and thawing does not influence the structure of the primary agglomerates, i.e., the carbon nanotubes bundles which are firmly bound together by van der Waals π-π stacking forces.Thus, freezing the suspension and its subsequent thawing makes it possible to obtain a suspension of agglomerates consisting of a dispersant and carbon nanotube bundles, whose viscosity (i.e. “the second viscosity”) is significantly lower than that of the original suspension of carbon nanotube bundles (i.e. “the first viscosity”), but the properties of the electrode material, in the preparation of which such a suspension is used, will be at least no worse than if the original suspension had been used before freezing.

[0029] It is worth noting that between the steps of freezing and thawing, the resulting composition can be stored for a long time and can also be transported in this state, if feasible in terms of the efficiency of the production process and logistics. After thawing, the resulting composition retains a low viscosity for a long time and can be transported.

[0030] The invented method is efficient in producing suspensions with decreased viscosity based on various solvents. In particular, it is useful for producing suspensions of single-walled and / or double-walled carbon nanotubes in water or in N-methyl-2-pyrrolidone. As a dispersant in the suspension, carboxymethylcellulose and / or its salt, polyvinylpyrrolidone, polyvinylidene fluoride, hydrogenated nitrile butadiene rubber, polyacrylic acid, and / or a salt thereof or a mixture thereof, may be used, but is not limited to the examples provided. In aqueous suspensions of the present invention, it is most preferred to use as a dispersant one of the following: carboxymethylcellulose, or a carboxymethylcellulose salt, or polyvinylpyrrolidone, or polyacrylic acid, or a salt of polyacrylic acid, or a mixture thereof. In some cases, it is preferable for two or more different dispersants to be present in the suspensions, in addition to the single-walled and / or double-walled carbon nanotubes.

[0031] The average molecular weight of the dispersant can range from 1 to 1500 kDa (including the endpoints), e.g., from 2 to 1200 kDa, for some applications it is preferred to range from 1 to 10 kDa, for others applications it is most preferred to be range from 10 kDa to 500 kDa, for other applications from 10 to 200 kDa, for other applications from 100 to 500 kDa, for other applications it is preferred to range from 500 to 1200 kDa. Manufacturers of such polymer dispersants sometimes describe the degree of polymerization by the viscosity of the dispersant solution. This is particularly common for carboxymethylcellulose and its salts, including Na-carboxymethylcellulose. For some applications, it is preferable to use Na-carboxymethylcellulose as the dispersant, wherein 2 wt.% aqueous solution of this Na-carboxymethylcellulose has a viscosity in the range from 1 to 20000 mPa s (including the endpoints) at a temperature of 25 °C and a shear rate of 100 s'1. It is more preferable to use Na-carboxymethylcellulose as the dispersant, wherein 2 wt.% aqueous solution of this Na-carboxymethylcellulose has a viscosity in the rangefrom 2 to 10000 mPa s (including the endpoints) at a temperature of 25 °C and a shear rate of 100 s’1. For some specific applications, it is more preferable that the viscosity of a 2 wt.% aqueous solution of Na-carboxymethylcellulose ranges from 1 to 200 mPa s (including the endpoints) at a temperature of 25 °C and a shear rate of 100 s’1. For example, Na-carboxymethylcellulose Cellogen 5 A with a viscosity of less than 5 mPa s for 2 wt.% aqueous solution at a shear rate of 100 s’1or Na-carboxymethylcellulose Walocel C30A or Na-carboxymethylcellulose Sunrose APP-84 (5 - 7 mPa s) or Sunrose A02SH (30-50 mPa s) or Blanose 7L (25-50 mPa s) can be used. For some other applications, it is preferred that Na-carboxymethylcellulose is used, which has the viscosity of a 2 wt.% aqueous solution in the range from 100 to 2000 mPa- s at a temperature of 25 °C and a shear rate of 100 s’1(e.g., Sunrose F03HC (150-250 mPa s) or Sunrose A20SH (900-1500 mPa s) or Walocel C1000A (550-800 mPa s) or Blanose 7M (300-600 mPa s)). For some other applications, it is preferable that Na-carboxymethylcellulose is used, which has the viscosity is 2 wt.% aqueous solution ranging from 1000 to 20000 mPa s at a temperature of 25 °C and a shear rate of 100 s’1(e.g., Sunrose F120MC (8000-15000 mPa s) or Walocel C2000A (1900-2800 mPa s)). It should be noted that the viscosity of Na-carboxymethylcellulose solutions depends on the concentration and on the shear rate. As a “rule of thumb”, doubling the concentration of Na-carboxymethylcellulose increases the viscosity by approximately tenfold.

[0032] For some polymers, for example, for polyvinylpyrrolidone, the ratio of viscosity and an aqueous solution concentration is commonly expressed as K-value, which is unambiguously described in the international standard [ISO 1628-1:2024 Plastics - Determination of the viscosity of polymers in dilute solution using capillary viscometers - Part 1: General principles], Fikentscher's value of viscosity characteristics K-value represents a viscosity index relating to viscosity average of the molecular mass distribution and is expressed by equation (1)

[0033] K = 1000 [formula as shown in image equation (1)] (1) where ηris relative dynamic viscosity of polymer solution (i.e. related to dynamic viscosity of solvent) and c is concentration of polymer, g / cm3.

[0034] Preferably, the polyvinylpyrrolidone used as a dispersant has a K-value of at least 10 and not more than 100. It is most preferable that the polyvinylpyrrolidone used as a dispersant has a K-value of at least 10 and not more than 40. For example, polyvinylpyrrolidone PVP Kollidon-12 (BASF) with K-value = 12, with a molecular weight of 2-3 kDa, can be used. Or polyvinylpyrrolidone PVP K-30 Povidon (Ashland) with a K-value = 27-33, with an average molecular weight of about 66.8 kDa, can be used. But the method can also be used for suspensionscontaining higher molecular weight polyvinylpyrrolidone, such as PVP Shokubai K-90 with K-value = 88-96, which has an average molecular weight of about 360 kDa.

[0035] The proposed method makes it possible to prepare suspensions of carbon nanotubes with reduced viscosity of suspensions with any of the dispersants mentioned above. The dispersant content in the suspension, at which it is possible to achieve a technical result, ranges from 0.2 to 3 wt.% (including the endpoints). The preferred dispersant content is not less than 50 and not more than 300 wt. parts per 100 wt. parts of containing single-walled and / or double-walled carbon nanotubes. Accordingly, for some applications, it is preferable for the dispersant content to range from 0.2 to 1 wt.%. Yet, for some other applications it is more preferable for the dispersant content to range from 0.6 to 2.4 wt.%

[0036] The presence of single-walled and / or double-walled carbon nanotubes (rather than multiwalled nanotubes) in the suspension is essential for achieving the desired technical result. It is known that single-walled carbon nanotubes are characterized by a small diameter, which is less than 4 nm in stable single-walled carbon nanotubes, for example, 1.5 nm, and their considerable length, which can exceed 5 pm. Thus, single-walled carbon nanotubes have a very high length-to-diameter ratio, which can exceed 3000. It is also known that for double-walled carbon nanotubes, the outer diameter usually does not exceed 6 nm, for example, it can be 2.8 nm, and their length can also exceed 5 pm.

[0037] The content of single-walled and / or double-walled carbon nanotubes in a suspension, required to achieve a technical result, is not less than 0.2 and not more than 2 wt.%. The preferred content of single-walled and / or double-walled carbon nanotubes in suspension is determined by the technological equipment used and the specifics of logistics. Preferably, the content of singlewalled and / or double-walled carbon nanotubes ranges from and including 0.2 wt.% upto and including 1.6 wt.%. For some applications, it is preferred that the content of single-walled and / or double-walled carbon nanotubes ranges from 0.3 to 0.6 wt.%, most preferably for these applications, the content ranges from 0.35 to 0.45 wt.%. For some other applications, it is preferred that the content of single-walled and / or double-walled carbon nanotubes in the suspension ranges from 0.6 to 1.4 wt.%, most preferably for these applications the content ranges from 0.7 to 1 wt.%.

[0038] Single-walled and / or double-walled carbon nanotube bundles are agglomerates of carbon nanotubes that interact via van der Waals forces (π-π stacking) and form a hexagonal close-packed stack. The presence of defects in single-walled and / or double-walled carbon nanotubes leads to defects in the nanotube bundle. Therefore, it is desirable that single-walled and / or double-walled carbon nanotubes contain as few defects as possible. Preferably, the single-walled and / or double-walled carbon nanotubes in the suspension are of high quality containing as few as possible of defects and impurities of other allotropic carbon modifications, such as soot or amorphous carbon. A quantitative indicator characterizing the content of defects in the structure of single-walled and / or double-walled carbon nanotubes is the ratio of the intensity of the G and D lines in the Raman spectrum - the larger this ratio, the fewer defects carbon nanotubes contain. It is preferable that the ratio of the intensities of the G and D lines in the 532 nm Raman scattering spectrum is at least 10, more preferable is at least 40, and even more preferable is at least 60.

[0039] It is important to note that the presence of impurities of other allotropic carbon modifications, such as soot, amorphous carbon, graphene, multi-walled carbon nanotubes, graphite-like particles, is tolerated in many end applications, and also does not have a significant effect on the rheology of the suspension before freezing or after thawing and, therefore, the suspension may contain these or other allotropic modifications of carbon.

[0040] The surface of single-walled and / or double-walled carbon nanotubes in the additive can be modified with functional groups. The expediency of surface modification of the SWCNT and / or DWCNT is determined by the specific formulation of the final electrode material. For some end applications, it is preferable that the surface of the carbon nanotubes in the additive contains functional groups containing elements with a Pauling electronegativity higher than carbon, e.g., hydroxyl, carboxylic, chlorine-containing: -Cl, -OC1, fluorine-containing, but not limited to the above examples. The presence of these groups enhances the adhesion of carbon nanotubes to the particles of the active component in the electrode material. Functional groups can be introduced onto the surface of carbon nanotubes by various methods known in the art. For example, carboxyl functional groups can be introduced onto the surface of carbon nanotubes by heat treatment in a nitric acid solution, and chlorine-containing functional groups can be introduced by one of the methods described in the invention EP4023598, but are not limited to the examples provided. The methods for functionalization carbon nanotubes are not within the scope of the present invention.

[0041] The ends of the single-walled and / or double-walled carbon nanotubes can be closed, the inner channel of the tubes is inaccessible to the molecules, or the ends of the single-walled and / or double-walled carbon nanotubes can be decapped. In this case, the inner channel of the nanotubes is accessible to molecules, e.g., it is accessible to water molecules and dissolved ions. This can be quantified by the BET method from nitrogen adsorption isotherms. The specific surface area of open SWCNT / DWCNT is significantly greater than that of closed ones and can even exceed the theoretical maximum specific surface area of an ideal graphene sheet, which is 1315 m2 / g. For some applications, it is preferable that at least a portion of the SWCNT and / or DWCNT in theadditive have open internal channels and a specific surface area, as determined by the BET method, of at least 800 m2 / g. The most preferable option for these applications is that the specific surface area of carbon nanotubes, as determined by the BET method, exceeds 1200 m2 / g.

[0042] A suspension of single-walled and / or double-walled carbon nanotubes and / or their agglomerates may contain impurities of metals of groups 8-11 of the Periodic Table of Elements or metal carbides used as a catalysts in the production of carbon nanotubes, for example, iron or cobalt or other metals, bimetallic particles or their alloys, which are inherent to the method of production of these carbon nanotubes. For some applications, including the preparation of electrode pastes (slurries) and the subsequent manufacture of the electrodes, it is desirable that the content of metal impurities of groups 8-11 of the Periodic Table of Elements in single-walled and / or double-walled carbon nanotubes and / or their agglomerates be not exceeding 1 wt. %. For some applications, it is more preferable that the content of metal impurities of groups 8-11 of the Periodic Table of Elements in single walled and / or double walled carbon nanotubes and / or their agglomerates is less than 0.1 wt.%. For other applications, on the contrary, there is no need to strictly limit the content of metal impurities of groups 8-11 of the Periodic Table of Elements, and their content in single-walled and / or double-walled carbon nanotubes and / or their agglomerates can be, for example, up to 15 wt.%.

[0043] The proposed sequence of freezing steps of a suspension containing carbon nanotubes and dispersant, followed by thawing of the resulting composite, makes sense if the initial suspension has a high dynamic viscosity - for example, the proposed method of reducing viscosity is preferable for initial suspensions with a viscosity greater than 2000 mPa s at a shear rate of 1.86 s'1and 25 °C. This method is especially efficient for suspensions with a viscosity greater than 4000 mPa s at a shear rate of 1.86 s'1and 25 °C, for example, for suspensions with a viscosity greater than 7000 mPa s at a shear rate of 1.86 s’1,and it is even more efficient for suspensions with a viscosity greater than 10000 mPa s at a shear rate of 1.86 s'1and 25 °C.

[0044] The dynamic viscosity of carbon nanotube suspensions is highly dependent on the shear rate. Some laboratories have adopted techniques where the viscosity of suspensions is determined at shear rates less than 1.86 s'1(e.g., 1 / 6.3 s'1or 1 s'1) or greater than 1.86 s'1(e.g., 18.6 s'1or 100 s'x). The viscosities determined at such shear rates will differ from those measured at 1.86 s'1. In the present disclosure, we rely on values measured at a shear rate of 1.86 s'1(z.e., by Brookfield SC4-21 Small Sample Adapter Spindle at rotational speed 2 rpm). In an embodiment, the first viscosity and the second viscosity are measured in the present disclosure at a shear rate of 1.86 s'1and 25 °C. To facilitate comparisons with data from other publications where viscosity is measured at othershear rates, we also provide viscosities measured at the spindle speed of 20 rpm corresponding to a shear rate of 18.6 s'1.

[0045] Freezing of the suspension should be carried out at a temperature below the freezing point of the solvent, but preferably above the temperature of homogeneous solvent nucleation, e.g., when using water as a solvent, freezing is most preferably carried out in the temperature range between -5 °C and -40 °C. Unlike the well-known freeze-cast processes, which aim to obtain a well-ordered porous structure in rigid ceramic or polymer composites, there is no need to strictly control the uniformity of the temperature field and the parallelism of the temperature gradient during freezing. Therefore, the shape of the containers in which freezing is carried out, as well as the design of the freezer, can be arbitrary, depending on what is available in production. Sometimes it is preferable to freeze the suspension in a flat layer of a thickness not more than 50 cm, more preferably in a layer with a thickness of not more than 30 cm, most preferably not more than 10 cm. In other cases, it is preferable to freeze the suspension in cylindrical containers with a diameter of not more than 40 cm, it is more preferable not more than 25 cm, most preferably not more than 10 cm. The power of the freezing system is chosen based on the required capacity and the desired freezing speed. It is preferable to use systems with a heat flux density of at least 100 W / m2and not more than 300 kW / m2, most preferably with a heat flux density of at least 200 W / m2and not more than 20 kW / m2, most preferably a heat flux density between 300 W / m2and 5 kW / m2. It is preferable that the freezing rate, ie., the linear velocity of the solvent crystallization front in the suspension, is not less than 1 pm / s and not more than 1 mm / s. It is most preferable that the linear velocity of the water crystallization front is between 2 pm / s and 50 pm / s, and most preferably between 2 pm / s and 20 pm / s. At this rate of freezing, primary agglomerates (nanotube bundles) are displaced by the crystallization front to the intergranular boundaries, where they form secondary agglomerates of nanotube bundles and dispersant.

[0046] It is possible and even preferable for convenient implementation, to carry out the freezing step by placing the container with the suspension in a thermostat with a temperature below the melting (freezing) point of the solvent, e.g., in the case of aqueous suspensions, with a temperature below 0 °C. It is preferable that the temperature of the thermostat be 10 °C lower than the melting point of the solvent or lower. Also, it is preferable that the thermostat temperature be higher than the temperature, which is 50 °C below the homogeneous solvent nucleation temperature (e.g., above -90 °C when the solvent is water). However, for some applications, freezing can also be carried out at lower thermostat temperatures, e.g., using a liquid nitrogen bath (-196 °C) as a thermostat. With such a rapid freezing of the suspension, the effect of viscosity reduction afterthawing will not be as significant, and the viscosity of the resulting suspension (the second viscosity) will be higher than what could be achieved under optimal freezing conditions. However, the freezing process in this case is simple and technological, which in some processes may be more important if the achieved viscosity reduction is acceptable.

[0047] The thawing step can be carried out either by natural thawing or by any other convenient method, for example, by placing the container with the frozen composition in a thermostat with a temperature not higher than the boiling point of the solvent, or by irradiating it with-infrared radiation. The frozen composition is electrically conductive, so it can be heated and thawed by passing a direct or alternating electric current through it, or in a microwave oven at a specific power input selected based on the available equipment and the desired thawing rate, e.g., between 1 and 10 kW / kg of the suspension, or by any another known method not limited to the examples provided.

[0048] The present invention also provides a suspension, which was prepared from the suspension of single-walled and / or double-walled carbon nanotubes by the above described sequence of freezing and thawing. In other words, the present invention provides a carbon nanotube suspension having a second viscosity and containing a solvent, not less than 0.2 wt.% and not more than 2 wt.% of single-walled and / or double-walled carbon nanotubes, and not less than 0.2 %.wt. and not more than 3 wt.% of a dispersant;wherein it has been prepared by a process comprising the following steps in the given order: (a) freezing a carbon nanotube suspension, the carbon nanotube suspension having a first viscosity and containing a solvent, not less than 0.2 wt.% and not more than 2 wt.% of single-walled and / or double-walled carbon nanotubes, and not less than 0.2 wt.% and not more than 3 wt.% of a dispersant, to obtain a frozen carbon nanotube suspension; (b) optionally storing and / or transporting the frozen carbon nanotube suspension; (c) thawing the frozen carbon nanotube suspension to obtain a carbon nanotube suspension having a second viscosity, wherein the second viscosity is lower than the first viscosity; (d) optionally stirring the carbon nanotube suspension having a second viscosity after step.

[0049] The suspension of agglomerates of carbon nanotube bundles and dispersant obtained by the above-described method is characterized by a dynamic viscosity (the second viscosity) significantly lower than that of the original dispersion before freezing (i.e. than the first viscosity). The viscosity of the resulting suspension depends on the freezing and thawing conditions, including the freezing temperature and the speed of the crystallization front.

[0050] It is preferable that the suspension has a viscosity of (i.e. the second viscosity is) not less than 10 and not more than 1000 mPa s at a shear rate of 1.86 s'1and 25 °C.It is preferable that suspension was prepared by the method, wherein after the step (c) a step (d) is performed, which is stirring the suspension obtained at step (c). This ensures the homogeneity of the dispersion and also increases the dynamic viscosity of the suspension obtained as a result of thawing the composition. The stirring step can be carried out, for example, using an overhead stirrer with a disk sawtooth impeller at an angular velocity ranging from 100 to 6000 rpm until the desired second viscosity is reached.

[0051] This step can also be carried out using other mixers, dispersers or homogenizers, not limited to the disc sawtooth impeller agitator, which is provided here for illustration as an example of a method that provides sufficiently good mixing and is capable of dispersing the secondary agglomerates of carbon nanotube bundles and dispersant, while at the same time is not being capable of disrupting van der Waals π-π stacking forces between single-walled and / or doublewalled carbon nanotubes, and therefore does not lead to noticeable destruction of the SWCNT and / or DWCNT primary bundles. It is preferrable that the suspension comprises agglomerates of carbon nanotubes and dispersant with the least dimension of more than 50 pm.

[0052] Above described suspension with reduced viscosity can be used to produce an electrode paste. The present invention claims a method for preparing an electrode paste containing an active electrode material, solvent, binder, dispersant and not less than 0.001 wt.% of single-walled and / or double-walled carbon nanotubes, wherein it includes step (El) of mixing the active electrode material and the above described suspension according and step (E2) of stirring the resulting mixture to a homogeneous paste. In some applications, it is preferable to also add a solvent and / or one or more binders and / or electrically conductive additives to the mixture in step (El). In other applications, it is preferable to carry out one or more separate steps of adding the solvent and / or one or more binders and / or electrically conductive additives to the mixture of the active electrode material and the suspension prior to step (E2). The solvent used in the preparation of the electrode paste may be the same as that in the suspension or may be another solvent, such as water, or a water-soluble organic solvent or a water-insoluble organic solvent.

[0053] The present invention claims an electrode paste comprising an active electrode material, a solvent, a binder, a dispersant, and single-walled and / or double-walled carbon nanotubes, wherein it contains at least 0.001 wt.% of single-walled and / or double-walled carbon nanotubes and is prepared by the method as described above.

[0054] The above description of the present invention can be summarized as follows below. The present invention provides:Method 1. A method for preparing a carbon nanotube suspension wherein the method comprises successive steps of (1) freezing a carbon nanotube suspension, the carbon nanotube suspension having a first viscosity and containing a solvent, not less than 0.2 wt.% and not more than 2 wt.% of single-walled and / or double-walled carbon nanotubes, and not less than 0.2 wt.% and not more than 3 wt.% of a dispersant to obtain a frozen carbon nanotube suspension; and (2) thawing the frozen carbon nanotube suspension to obtain a carbon nanotube suspension having a second viscosity, wherein the second viscosity is lower than the first viscosity.

[0055] Method 2. The method 1, wherein the method after the thawing step (2) comprises a step (3) of stirring the carbon nanotube suspension obtained at step (2).

[0056] Method 3. The method 1 or 2, wherein the first viscosity is greater than 2000 mPa s at a shear rate of 1.86 s'1and 25°C.

[0057] Method 4. Any one of methods 1 to 3, wherein during freezing at step (1) the heat flux density is not less than 100 W / m2and not more than 300 kW / m2and / or the crystallization front velocity is not less than 1 pm / s and not more than 1 mm / s.

[0058] Method 5. Any one of methods 1 to 4, wherein the solvent is water.

[0059] Method 6. The method 5, wherein the dispersant is carboxymethylcellulose, or a carboxymethylcellulose salt, or polyvinylpyrrolidone, or polyacrylic acid, or a salt of polyacrylic acid, or a mixture thereof.

[0060] Method 7. Any one of methods 5 or 6, wherein the dispersant is a carboxymethylcellulose salt with a degree of substitution from 0.4 to 1.2, in which a 2 wt.% aqueous solution thereof has a viscosity at shear rate of 100 s ' and 25 °C of not less than 2 mPa- s and of not more than 10 Pa- s, or polyvinylpyrrolidone with a K-value of not less than 10 and not more than 100, or a mixture thereof.

[0061] Method 8. Any one of methods 1 to 4, wherein the solvent is N-methyl-2-pyrrolidone. Suspension 9. A carbon nanotube suspension having a second viscosity and containing a solvent, not less than 0.2 wt.% and not more than 2 wt.% of single-walled and / or double-walled carbon nanotubes, and not less than 0.2 %.wt. and not more than 3 wt.% of a dispersant, which carbon nanotube suspension is obtainable by any one of methods 1 to 8.

[0062] Suspension 10. The suspension 9, wherein it has a second viscosity of not less than 10 and not more than 1000 mPa s at a shear rate of 1.86 s'1and 25 °C.

[0063] Suspension 11. A suspension 9 or 10, wherein it comprises agglomerates of carbon nanotubes and dispersant with the least dimension of more than 50 pm.Method 12. A method for preparing an electrode paste containing

[0064] an active electrode material, solvent, binder, dispersant, and not less than 0.001 wt.% of singlewalled and / or double-walled carbon nanotubes;

[0065] wherein it comprises

[0066] step (El) of mixing the active electrode material and the suspension according to claim 9 and step (E2) of stirring the resulting mixture to a homogeneous paste.

[0067] Method 13. The method 12, wherein the method further comprises adding a solvent and / or one or more binders and / or electrically conductive additives to the mixture at mixing step (El) and / or to the mixture of the active electrode material and the suspension in one or more steps prior to step (E2).

[0068] Paste 14. An electrode paste comprising an active electrode material, a solvent, a binder, a dispersant, and single-walled and / or double-walled carbon nanotubes;

[0069] wherein it contains at least 0.001 wt.% of single-walled and / or double-walled carbon nanotubes and is prepared by the method 12 or the method 13.

[0070] Method 15. A method for preparing an electrode paste containing

[0071] an active electrode material, solvent, and not less than 0.001 wt.% of single-walled and / or doublewalled carbon nanotubes;

[0072] wherein the method comprises the following steps in the given order:

[0073] (i) freezing a carbon nanotube suspension, the carbon nanotube suspension having a first viscosity and containing a solvent, not less than 0.2 wt.% and not more than 2 wt.% of single-walled and / or double-walled carbon nanotubes, and not less than 0.2 wt.% and not more than 3 wt.% of a dispersant, to obtain a frozen carbon nanotube suspension;

[0074] (ii) optionally storing and / or transporting the frozen carbon nanotube suspension

[0075] (iii) thawing the frozen carbon nanotube suspension to obtain a carbon nanotube suspension having a second viscosity, wherein the second viscosity is lower than the first viscosity;

[0076] (iv) optionally storing and / or transporting the carbon nanotube suspension having a second viscosity;

[0077] (v) optionally stirring the carbon nanotube suspension having a second viscosity before and / or after step (iv);

[0078] (vi) mixing the active electrode material and the carbon nanotube suspension having a second viscosity to give a mixture; and

[0079] (vii) stirring the resulting mixture to give a homogeneous paste.It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.

[0080] Brief Description of the Attached Drawings

[0081] The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention.

[0082] Fig i shows the optical microscopic images of suspensions of Example 3 (middle column) and Example 6 (right column) and the initial suspension, containing 0.8 wt.% single-walled carbon nanotubes TUB ALL™ and 0.8 wt.% CMC Cellogen 5 A before freezing (left column). The frame horizontal size is 800 pm for the top row, and 195 pm for the bottom row.

[0083] Fig.2 shows the dependence of capacity related to the initial capacity of the batteries according to Example 29 on the number of charge-discharge cycles (charge current 1C, discharge current 1C). Black symbols - with suspension according to Example 2 after freezing and thawing, white symbols - with suspension before freezing.

[0084] Fig.3 shows the dependence of capacity related to the initial capacity of the batteries according to Example 30 on the number of charge-discharge cycles (charge current 1C, discharge current 1C). Black symbols - with suspension according to Example 5 after freezing and thawing, white symbols - with suspension before freezing.

[0085] Detailed Description of Embodiments of the Invention.

[0086] Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings. For convenience, the information on the provided examples is also provided in the Tables 1-5 below.

[0087] Table 1 summarizes the viscosities of suspensions before and after procedure to lower the viscosity according to Examples 1-6.

[0088] Table 2 summarizes the viscosities of suspensions before and after procedure to lower the viscosity according to Examples 7-9.

[0089] Table 3 summarizes the viscosities of suspensions before and after procedure to lower the viscosity according to Examples 10-12.

[0090] Table 4 summarizes the viscosities of suspensions before and after procedure to lower the viscosity according to Examples 11 and 13-15.

[0091] Table 5 summarizes the viscosities of suspensions before and after procedure to lower the viscosity according to Examples 14 and 16-17.Table 6 summarizes the viscosities of suspensions before and after procedure to lower the viscosity according to Examples 18-28.

[0092] Examples.

[0093] The presented Examples 1-28 illustrate that the suspensions obtained according to the present invention have a significantly lower viscosity compared to the initial suspension of carbon nanotube bundles. Examples 29-30 illustrate that the properties of the electrode paste, in the preparation of which such a suspension was used, are no worse than if the original suspension before freezing had been used. In the examples, the viscosity was measured using Brookfield DV2T viscometer with SC4-21 Small Sample Adapter Spindle at 2 rpm (1.86 s'1) or 20 rpm (18.6 s'1). The values marked with an asterisk (*) were estimated from the measurements using RV04 spindle at 12 rpm and extrapolated based on the correlation between SC4-21 and RV04 measurements.

[0094] Examples 1-6 (thawing after freezing in thermostate with different temperatures).

[0095] The single-walled carbon nanotubes suspension TUBALL Batt H₂O™ (SKU 19HO57 batch N2.3.2.1) contains 0.8 wt.% single-walled carbon nanotubes TUBALL™ and 0.8 wt.% dispersant - sodium salt of carboxymethylcellulose Cellogen 5A. The single-walled carbon nanotubes TUBALL™ (SKU 01RW03 batch Nl.1.422.3) used with a diameter range from 1.2 to 2.1 nm and an average diameter of 1.68 nm are characterized by high purity (impurity content of less than 1 wt.%, including 0.4 wt.% of iron), a significant part of single-walled carbon nanotubes are open-ended, the specific surface area determined by the BET method is 1380 m2 / g, the ratio of the intensity of G / D bands in the Raman spectrum at the wavelength 532 nm is 86. The sodium salt of carboxymethylcellulose Cellogen 5Ais characterized by an esterification degree 0.7 - 0.8, the viscosity of its 2 wt.% aqueous solution is less than 5 mPa s. The suspension has a dynamic viscosity of 23500 mPa s at 1.86 s'1and 25 °C. To reduce the viscosity, the suspension was frozen in 50 ml conical centrifuge tubes (almost cylindrical tubes with a diameter of 27-29 mm) and placed in a thermostat at the following temperatures: -11 °C (Example 1), -15 °C (Example 2), -24 °C (Example 3), -35 °C (Example 4), -50 °C (Example 5), and -196 °C (Example 6). The heat flux density ranged from approximately 550 W / m2(Example 1) to approximately 10 kW / m2(Example 6). The solidification front velocity correspondingly ranged from 1.6 pm / s (Example 1) to 30 pm / s (Example 6). After freezing the resulting composition was thawed by placing the containers in a thermostat at a temperature of 25 °C. The thawed suspension was mixed using a four-blade impeller with a diameter of 27 mm in a glass beaker with a diameter of 50 mm at anangular velocity of 500 rpm for 2 minutes. Data on the viscosities of the resulting suspensions at shear rates of 18.6 s'1and 1.86 s'1at 25 °C are presented in Table 1. Optical micrographs of the initial suspension before freezing and the suspensions obtained in Examples 3 and 6 are shown in Figure 1. It can be observed that after thawing the suspension contains numerous agglomerates visible under an optical microscope with the least dimension being greater than 1 pm. Moreover, for suspension obtained in Example 6, numerous agglomerates were also visible by naked eye having the least dimension greater than 50 pm.

[0096] Examples 7-9 (thawing after freezing in thermostate with different temperatures, sodium CMC with high average molecular weight).

[0097] The single-walled carbon nanotubes suspension TUBALL Batt H₂O™ containing 0.4 wt.% single-walled carbon nanotubes TUB ALL™ with an average diameter of 1.4 nm and 0.6 wt.% dispersant - sodium salt of carboxymethylcellulose Blanose 7mf (etherification degree 0.65-0.9, average molecular weght 250 kDa, viscosity of its 2 wt.% aqueous solution in the range of 400-600 mPas). The suspension has adynamic viscosity of 17500 mPa- s at 1.86 s'1and 25 °C. Freezing and thawing were carried out under conditions described above for Examples 2, 3 and 6. Viscosity data obtained from the resulting suspensions at shear rates of 18.6 s'1and 25 °C and 1.86 s'1at 25 °C are presented in Table 2.

[0098] Examples 10-12 (agitation step after thawing).

[0099] The suspensions of Examples 10-12 were obtained in the same manner as in Examples 2-4, but after the thawing step, the suspension was stirred using a four- blade impeller with a 27 mm diameter at an angular velocity of 1000 rpm for 5 minutes. Viscosity data obtained from suspensions at shear rate of 1.86 s'1and 25 °C are presented in Table 3.

[0100] Example 13-15 (effect of the agitation step conditions on viscosity of suspension).

[0101] In Examples 13-15, the suspensions were obtained in the same way as in Example 11, but freezing was carried out in a tray 20 mm deep in a thermostat with a temperature of -24 °C. After thawing, it was stirred with a four-blade impeller with a diameter of 27 mm with an angular velocity of 500 (Example 13), 1000 (Example 11, see above), 2000 (Example 14) or 3000 (Example 15) rpm for 5 minutes. Viscosity data obtained from suspensions at shear rates of 1.86 s'1and 25 °C are presented in Table 4.

[0102] Examples 14, 16-17 (variation of freezing process geometry).

[0103] The freezing and thawing process was carried out as in Example 14: the initial suspension ofsingle-walled carbon nanotubes TUBALL Batt H₂O ™ containing 0.8 wt.% single-walled carbon nanotubes TUB ALL™ (average diameter of 1.58 nm) and 0.8 wt.% dispersant (sodium salt carboxymethylcellulose Cellogen 5 A), had a dynamic viscosity of 23500 mPa s at 1.86 s'1and 25 °C. Freezing was carried out by placing the container with the suspension in a thermostat at -24 °C. After thawing, the suspension was stirred for 5 minutes with a four-blade impeller (27 mm diameter) at angular velocity of 2000 rpm. The examples differ by the geometry of the container used for freezing as reported in Table 5. Viscosity data of the resulting suspensions at shear rates of 18.6 s'1and 1.86 s'1at 25 °C are also presented in Table 5.

[0104] Examples 18-28 (variation of SWCNT and DWCNT content, dispersant and solvent type). The suspensions with varying contents of SWCNT and DWCNT, different dispersants and solvents were tested by freezing in conical 50 ml tubes at - 35 °C followed by thawing at ambient temperature. All suspensions were agitated by a 4-lobe 27 mm impeller at 500 rpm for 5 minutes.

[0105] In Examples 18-20, Tuball SWCNT as in Examples 1-6 and 10-17 were used.

[0106] In Examples 21-24, a mixture of double-walled (30 wt.%) and single-walled (70 wt.%) carbon nanotubes was used, where the single-walled carbon nanotubes had a characteristic diameter of 1.38 nm and the double-walled carbon nanotubes had a diameter of 2.8 nm, the specific surface area determined by the BET method was 560 m2 / g, and the ratio of the intensity of the G / D bands in the Raman spectrum at a wavelength of 532 nm was 54.

[0107] In Examples 25-26, single-walled carbon nanotubes with surfaces modified (z.e., functionalized) with chlorine-containing groups, as described in patent EP 4023598, were used. SWCNT had an average diameter of 1.65 nm, a G / D ratio of 66, and a specific surface area of 1130 m2 / g, according to energy-dispersive spectroscopy, the chlorine content in SWCNT was 0.7 wt.%, and based on Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES), the SWCNT contained 0.46 wt.% of Croup 8 metal impurity - iron.

[0108] In Examples 27-28, single-walled carbon nanotubes TUBALL™ with SKU 01RW02 were used, having an average diameter of 1.62 nm, G / D bands intensity ratio is 46, capped (close-ended) nanotubes, specific surface area determined from nitrogen adsorption isotherms is 580 m2 / g; according to thermogravimetry analysis in a flow of 5% oxygen in Ar, the ash residue after oxidation of material at 950 °C is approximately 20 wt.%. X-ray diffraction analysis indicates the ash residue mainly contains iron oxide Fe₂O₃, and the used SWCNT contains nanodispersed metallic iron phase; according to the data of energy dispersive spectroscopy, the Fe content in theSWCNT is 14.2 wt.%, which aligns with the data of the ash mass. In Examples 19 and 25 polyvinylpyrrolidone (PVP) Ashland K30 was used as the dispersant with a K-value of 27-33 and an average molecular weight of ca. 66.8 kDa. Example 21 used PVP Shokubai K12 with K-value of 12 and an average molecular weight of 2-3 kDa, while Example 23 used PVP BASF K90 with K-value of 88-96 and an average molecular weight of ca. 360 kDa. In Example 20, PVDF Solef 5130 (Solvay) was used with a weight average molecular weight of 1000-1100 kDa, and Example 24 used PVDF Kynar HSV 900 (Arkema). Example 28 used Therban AT 3404 SF (Arlanxeo) as the dispersant, which is HNBR elastomer with ACN content of 33.3 wt.%, residual double bond content 0.4 %, and Mooney viscosity ML(l+4) @100 °C = 34 MU.

[0109] The viscosity data for the resulting suspensions at shear rates of 18.6 s'1and 25°C are presented in Table 6. The values marked with an asterisk (*) were estimated from measurements taken with LV-4(#64) spindle at 12 rpm, while those marked with (**) - from measurements with RV04 spindle at 12 rpm, with correlation between SC4-21, RV04 and LV-4(#64) measurements used for extrapolation.

[0110] Example 29. Anode paste and anode of a Li-ion battery, prepared using the suspension from Example 2.

[0111] The suspension from Example 2 was used to prepare an anode paste containing 26.47 wt.% of graphite active material, 19.29 wt.% silicon oxide active material, 51.78 wt.% of water as a solvent, 0.96 wt.% of the dispersant sodium salt of carboxymethylcellulose, 0.96 wt.% styrene-butadiene latex binder, 0.481 wt.% of conductive carbon black SuperP and 0.048 wt.% of single-walled carbon nanotubes.

[0112] The preparation of the anode paste was carried out through the following steps:

[0113] - mixing 114 g of 2 wt.% aqueous solution of sodium salt CMC, 1.2 g conductive carbon black SuperP, 15 g of single-walled carbon nanotubes suspension according to Example 2, 48 g of silicon oxide powder, and 65.88 g of graphite powder for 4 hours (step El, during which additional solvent (water), binder (sodium salt of CMC) and conductive additive (SuperP) were added);

[0114] - adding 4.8 g of 50 wt.% aqueous suspension of styrene-butadiene latex (additional step of adding the solvent and / or a binder to the mixture of the active electrode material and the suspension prior to step (E2));

[0115] - stirring for 30 minutes until a homogeneous paste was obtained (step E2).A similar paste was prepared with a suspension that was not subjected to freezing and thawing. The anodes were produced by applying the paste to copper foil using a coater, followed by drying at 60°C and calendering to a density of 1.3 g / cm3. The anode loading was 4.8 g / cm2Li-ion batteries with such anodes were assembled with an NCM811 cathode, loaded at 15 mg / cm2The batteries were tested at charge and discharge rates of 1 C in the voltage range of 2.7-4.2 V. Fig. 2 shows the graph of the batterie capacities as a function of the cycle number.

[0116] Example 30. Anode paste and anode of a lithium-ion battery prepared using the suspension from Example 5.

[0117] A suspension according to Example 5 was used to prepare an anode paste, which contains 37.24 wt.% active graphite material, 9.93 wt.% active silicon oxide material, 50.34 wt.% solvent (water), 0.99 wt.% dispersant sodium salt of carboxymethylcellulose, 0.99 wt.% styrene-butadiene latex binder, 0.50 wt.% conductive carbon black (SuperP), and 0.01 wt.% single-walled carbon nanotubes.

[0118] The preparation of anode paste was carried out in the following sequence of steps:

[0119] - mixing 495 g of a 2 wt.% aqueous solution of Na-CMC, 5 g of conductive carbon black (SuperP), 12 g of single-walled carbon nanotubes suspension from Example 5, 100 g of silicon oxide powder, and 375 g of graphite powder for 4 hours (step El, during which additional amount of solvent (water), binder (Na-CMC) and conductive additive (SuperP) were added);

[0120] - adding 20 g of 50 wt.% aqueous suspension of styrene-butadiene latex (steps of adding the solvent and / or a binder to the mixture of the active electrode material and the suspension prior to step (E2);

[0121] - stirring for 30 minutes until a homogeneous paste was obtained (step E2).

[0122] A similar paste was prepared using a suspension that was not subjected to freezing and thawing. The anodes were obtained by coating the paste onto copper foil using a coater, followed by drying at 60°C and calendering to a density of 1.3 g / cm3. The anode loading density was 6.4 g / cm2. Lithium-ion batteries with such anodes were assembled with the NCM811 cathode with a loading density of 15 mg / cm2.

[0123] The batteries were tested at charge and discharge rates of 1 C within the voltage range of 2.7-4.2 V. The capacities (relative to the initial capacity) of batteries vs. cycle number are shown in Fig. 3.Table 1.

[0124] Viscosity Viscosity Initial

[0125] Solidification after after T of

[0126] viscosity, Heat flow,

[0127] Example thermostate at front velocity, thawing, thawing,

[0128] W / m2@1.86 s’1,

[0129] freezing, °C pm / s @1.86 s’1, @18.6 s’1,

[0130] mPa s

[0131] mPa s mPa s 1 23500 -11 550 1.6 1800 14500 2 23500 -15 750 2.2 75 280 3 23500 -24 1200 3.6 63 225 4 23500 -35 1750 5.2 67 240 5 23500 -50 2500 7.5 152 590 6 23500 -196 10000 30 1430 9100

[0132]

[0133] Table 2.

[0134] Viscosity Viscosity Initial

[0135] Solidification after after T of

[0136] viscosity, Heat flow,

[0137] thawing, thawing, Example thermostate at front velocity,

[0138] W / m2@1.86 s’1,

[0139] freezing, °C pm / s @18.6 s1, @1.86 s’1, mPa s

[0140] mPa s mPa s 7 17500 -13 650 1.9 1630 10100 8 17500 -24 1200 3.6 1650 10900 9 17500 -196 10000 30 1770 11600

[0141]

[0142] Table 3.

[0143] Viscosity after Viscosity after

[0144] thawing and Initial viscosity, T of thermostate at

[0145] thawing, @1.86 s’1, Example

[0146] freezing, °C agitation, @1.86 s’1, @1.86 s’1, mPa s

[0147] mPa s

[0148] mPa s 10 23500 -15 280 550 11 23500 -24 225 275 12 23500 -35 240 500

[0149]

[0150] Table 4.

[0151] Viscosity after thawing Initial viscosity, Viscosity after thawing, Rate of impeller,

[0152] Example and agitation, @ 1.86 s’1, mPa s @ 1.86 s’1, mPa s rpm

[0153] @ 1.86 s’1, mPa s 11 23500 225 1000 275 13 23500 215 500 240 14 23500 220 2000 3150 15 23500 215 3000 6250

[0154]

[0155] Table 5.

[0156] Initial viscosity, Viscosity after Viscosity after Tank size

[0157] Example @ 1.86 s’1, thawing, @18.6 thawing, @1.86 geometry mm

[0158] mPa s s’1, mPa s s’1, mPa s 14 tray depth 20 23500 527 3150

[0159] Almost a length 110,

[0160] 16 23500 565 3520

[0161] cylinder dia. 28-29

[0162] 100(w) x 50(d)

[0163] 17 Rectangular 23500 473 2750

[0164]

[0165] x 100(h)Table 6.

[0166] Viscosity Contents of

[0167] Initial after thawing CNT:

[0168] Example Solvent Dispersant viscosity, @ and agitation,

[0169] Dispersant,

[0170] 18.6 s’1, mPa s @ 18.6 s’1, wt.%

[0171] mPa s 18 water Na-CMC Cellogen 5A 1.6: 1.6 9000* 1490* 19 water PVP Ashland K30 0.4: 0.8 1860 180 20 NMP PVDF Solef 5130 0.4: 0.8 1980 310 Na-CMC Blanose 7UL + 0.8:

[0172] 21 water 2450 370 PVP Shokubai K12 (0.4+0.4)

[0173] 22 water Na-CMC Blanose 7UL 1.6: 2.8 3500** 630** 23 water PVP BASF- K90 0.2: 0.4 1600 280 24 NMP PVDF Kynar HSV 900 0.4: 2.0 1470 270 25 water PVP Ashland K30 0.2: 0.6 1130 190 26 water Na-CMC Cellogen 5A 0.4: 0.6 640 72 27 water Na-CMC Cellogen 5A 0.3: 0.45 480 49 28 NMP HNBR Therban AT 3404 SF 0.4: 0.6 1950 675

[0174]

Claims

ClaimsClaim 1. A method for preparing a carbon nanotube suspension wherein the method comprises successive steps of(1) freezing a carbon nanotube suspension, the carbon nanotube suspension having a first viscosity and containing a solvent, not less than 0.2 wt.% and not more than 2 wt.% of singlewalled and / or double-walled carbon nanotubes, and not less than 0.2 wt.% and not more than 3 wt.% of a dispersant to obtain a frozen carbon nanotube suspension; and(2) thawing the frozen carbon nanotube suspension to obtain a carbon nanotube suspension having a second viscosity, wherein the second viscosity is lower than the first viscosity.Claim 2. The method of claim 1, wherein the method after the thawing step (2) comprises a step (3) of stirring the carbon nanotube suspension obtained at step (2).Claim 3. The method of claim 1, wherein the first viscosity is greater than 2000 mPa s at a shear rate of 1.86 s'1and 25°C.Claim 4. The method according to claim 1, wherein during freezing at step (1) the heat flux density is not less than 100 W / m2and not more than 300 kW / m2and / or the crystallization front velocity is not less than 1 pm / s and not more than 1 mm / s.Claim 5. The method of claim 1, wherein the solvent is water.Claim 6. The method of claim 5, wherein the dispersant is carboxymethylcellulose, or a carboxymethylcellulose salt, or polyvinylpyrrolidone, or polyacrylic acid, or a salt of polyacrylic acid, or a mixture thereof.Claim 7. The method of claim 6, wherein the dispersant is a carboxymethylcellulose salt with a degree of substitution from 0.4 to 1.2, in which a 2 wt.% aqueous solution thereof has a viscosity at shear rate of 100 s ' and 25 °C of not less than 2 mPa s and of not more than 10 Pa s, or polyvinylpyrrolidone with a K-value of not less than 10 and not more than 100, or a mixture thereof.Claim 8. The method of claim 1, wherein the solvent is N-methyl-2-pyrrolidone.Claim 9. A carbon nanotube suspension having a second viscosity and containinga solvent, not less than 0.2 wt.% and not more than 2 wt.% of single-walled and / or doublewalled carbon nanotubes, and not less than 0.2 %.wt. and not more than 3 wt.% of a dispersant; wherein it has been prepared by a process comprising the following steps in the given order:(a) freezing a carbon nanotube suspension, the carbon nanotube suspension having a first viscosity and containing a solvent, not less than 0.2 wt.% and not more than 2 wt.% of singlewalled and / or double-walled carbon nanotubes, and not less than 0.2 wt.% and not more than 3 wt.% of a dispersant, to obtain a frozen carbon nanotube suspension;(b) optionally storing and / or transporting the frozen carbon nanotube suspension;(c) thawing the frozen carbon nanotube suspension to obtain a carbon nanotube suspension having a second viscosity, wherein the second viscosity is lower than the first viscosity;(d) optionally stirring the carbon nanotube suspension having a second viscosity after step. Claim 10. The suspension of claim 9, wherein it has a second viscosity of not less than 10 and not more than 1000 mPa s at a shear rate of 1.86 s'1and 25 °C.Claim 11. The suspension of claim 9, wherein it comprises agglomerates of carbon nanotubes and dispersant with the least dimension of more than 50 pm.Claim 12. A method for preparing an electrode paste containingan active electrode material, solvent, binder, dispersant, and not less than 0.001 wt.% of singlewalled and / or double-walled carbon nanotubes;wherein it comprisesstep (El) of mixing the active electrode material and the suspension according to claim 9 and step (E2) of stirring the resulting mixture to a homogeneous paste.Claim 13. The method of claim 12, wherein the method further comprises adding a solvent and / or one or more binders and / or electrically conductive additives to the mixture at mixing step (El) and / or to the mixture of the active electrode material and the suspension in one or more steps prior to step (E2).Claim 14. An electrode paste comprising an active electrode material, a solvent, a binder, a dispersant, and single-walled and / or double-walled carbon nanotubes;wherein it contains at least 0.001 wt.% of single-walled and / or double-walled carbon nanotubes and is prepared by the method of claim 12.Claim 15. A method for preparing an electrode paste containingan active electrode material, solvent, and not less than 0.001 wt.% of single-walled and / or double-walled carbon nanotubes;wherein the method comprises the following steps in the given order:(i) freezing a carbon nanotube suspension, the carbon nanotube suspension having a firstviscosity and containing a solvent, not less than 0.2 wt.% and not more than 2 wt.% of singlewalled and / or double-walled carbon nanotubes, and not less than 0.2 wt.% and not more than 3 wt.% of a dispersant, to obtain a frozen carbon nanotube suspension;(ii) optionally storing and / or transporting the frozen carbon nanotube suspension;(iii) thawing the frozen carbon nanotube suspension to obtain a carbon nanotube suspension having a second viscosity, wherein the second viscosity is lower than the first viscosity;(iv) optionally storing and / or transporting the carbon nanotube suspension having a second viscosity;(v) optionally stirring the carbon nanotube suspension having a second viscosity before and / or after step (iv);(vi) mixing the active electrode material and the carbon nanotube suspension having a second viscosity to give a mixture; and(vii) stirring the resulting mixture to give a homogeneous paste.

Images