Patent Description:
Carbon nanostructures (CNSs) refer collectively to nanosized carbon structures having various shapes, such as nanotubes, nanohairs, fullerenes, nanocones, nanohorns, and nanorods. Carbon nanostructures can be widely utilized in a variety of technological applications because they possess excellent characteristics.

Carbon nanotubes (CNTs) are tubular materials consisting of carbon atoms arranged in a hexagonal pattern and have a diameter of approximately <NUM> to <NUM>. Carbon nanotubes exhibit insulating, conducting or semi-conducting properties depending on their inherent chirality. Carbon nanotubes have a structure in which carbon atoms are strongly covalently bonded to each other. Due to this structure, carbon nanotubes have a tensile strength approximately <NUM> times greater than that of steel, they are highly flexible and elastic, and are chemically stable.

Carbon nanotubes are divided into three types: single-walled carbon nanotubes (SWCNTs) consisting of a single sheet and having a diameter of about <NUM>; double-walled carbon nanotubes (DWCNTs) consisting of two sheets and having a diameter of about <NUM> to about <NUM>; and multi-walled carbon nanotubes (MWCNTs) consisting of three or more sheets and having a diameter of about <NUM> to about <NUM>.

Depending on their diameter, length, and chirality, carbon nanotubes may exhibit unique optical, electrical, thermal, and mechanical properties.

However, the advantageous properties of carbon nanotubes are often unrealized on a macroscopic level for several reasons. The first reason is a tendency of carbon nanotubes to crystallize in rope-like structures which become entangled into networks. Strong Van-der-Waals interactions between two carbon nanotubes lead to their alignment and to their consequent packing into ropes. This aggregation of carbon nanotubes often adversely affects both their electrical and mechanical behavior. The second reason is the insolubility or poor dispersion of carbon nanotubes in common organic solvents and polymer matrixes.

In order to address these problems, researchers have developed many methods for preparing uniform and stable carbon nanotube dispersions.

<CIT>, relates to a dispersion liquid containing a dispersion liquid medium, a polymer dispersion aid, and carbon nanotubes dispersed in the dispersion liquid medium. The dispersion liquid is selected from the group of water, acetone, nitrile, alcohol, dimethylformamide, N-methylpyrrolidone, pyrrolidone derivative, butyl acetate, methoxypropyl acetate, alkylbenzene, cyclohexane derivative and mixtures thereof; poly (vinylpyrrolidone) is disclosed as the dispersion aid among a wide variety of different polymers.

<CIT>, relates to a composite conductive paste comprising between <NUM> and <NUM> parts of carbon nanotubes, between <NUM> and. <NUM> parts of reduced expanded graphite, between <NUM> and <NUM> parts of polyvinylpyrrolidone, between <NUM> and <NUM> parts of N-methyl pyrrolidone and between <NUM> and <NUM> part of a complexing agent, wherein the complexing agent is at least one of ethylenediaminetetraacetic acid, sulfosalicylic acid, and acetylacetone.

<CIT>, relates to a carbon nanotube dispersion, comprising a carbon nanotube, a solvent, such as an amide-based organic solvent such as N-methyl-<NUM>-pyrrolidone and N-ethyl-<NUM>-pyrrolidone , and a dispersant such as polyvinylpyrrolidone.

<CIT>, relates to a mixture of an amide-based organic solvent and a polyvinylpyrrolidone for dispersing a carbon nanotube.

<CIT>, relates to a dispersible single-walled and multi-walled carbon nanotubes prepared by dissolving surfactants in a solvent to form a solution; adding carbon nanotubes to the solution to form a mixture, wherein the surfactant comprises polyvinylpyrrolidone, and wherein the solvent may be N-methylpyrrolidone.

<CIT>, discloses a carbon nanotube dispersion solution consisting of carbon nanotubes, an amide-based polar organic solvent, and polyvinylpyrrolidone, wherein the amide-based polar organic solvent is N-methylpyrrolidone (NMP).

<CIT> discloses a carbon nanofiber dispersion comprising an amide-based organic solvent, an amine-compound and a polymer which is polyvinylpyrrolidone, a copolymer of vinylpyrrolidone and vinylacetate, a copolymer of vinylpyrrolidone and vinylimidazole, or a copolymer of vinylpyrrolidone and vinylcabrolactam, or a combination thereof. The amide-based organic solvent, is selected from dimethylformamide, diethylformamide, dimethylacetamide, N-methylpyrrolidone and the like, while the amine-based compound is selected from <NUM>-methylaminoethanol, <NUM>-aminoethanol, isopropanolamine, n-propylamine, <NUM>-ethoxyethylamine, ethanolamine and the like.

<CIT>) discloses a carbon fiber dispersion liquid containing a polymer-based dispersant, the polymer being selected from a cellulose derivative, polyvinylpyrrolidone, polyvinyl alcohol, polyvinyl acetal, and polyamic acid salt, or a mixture of two or more thereof. The dispersion liquid further may comprise N-methyl-<NUM>-pyrrolidone and an amine compound such as trimethylamine.

<CIT>) discloses a carbon nanotube dispersion having N-methyl-<NUM>-pyrrolidone as a solvent and having excellent viscosity, dispersibility, and storage stability despite having a high carbon nanotube concentration, and providing a cell electrode composite layer having uniform and satisfactory coating film properties and low electrode plate resistance. The carbon nanotube dispersion is characterized by containing carbon nanotubes, polyvinylpyrrolidone, N-methyl-<NUM>-pyrrolidone, and an amine compound as a dispersing auxiliary. The carbon nanotube dispersion contains from <NUM> parts by weight to less than <NUM> parts by weight of polyvinylpyrrolidone and from <NUM> parts by weight to <NUM> parts by weight of an amine-based compound with respect to <NUM> parts by weight of the carbon nanotubes. The amine-based compounds are selected from the group consisting of an aliphatic primary amine, an aliphatic secondary amine, an aliphatic tertiary amine, an amino acid, an alkanolamine, a polyoxyalkylene alkylamine, a polyamine, and a nitrogen-containing alicyclic heterocyclic compound. The alkanolamines, as disclosed in the description, are dimethylaminoethanol, monoethanolamine, diethanolamine, methyldiethanolamine, and triethanolamine. The influence of iron-free catalytic remnants on the dispersion process of MWCNT's, containing said remnants, and on the final viscosity of the MWCNT dispersion is not dealt with.

Carbon nanotubes have attracted attention as potential electrode backbone or buffer materials in lithium batteries.

Typical lithium-ion batteries utilize carbon anodes (negative electrode) and lithiated transition metal oxide cathodes (positive electrode) situated on opposite sides of a microporous polymer separator.

A lithium-ion cell begins life with all of the lithium in the cathode and upon charging, a percentage of this lithium is moved over to the anode and intercalated within the carbon anode.

A failure in lithium-ion batteries is the result of a formation of dendrites within the battery. Dendrites are microscopic metal deposits that can form within the cell. Dendrite formation generally begins in the anode and creates an internal shortcut when it extends through the separator to the cathode.

When iron impurities from any electrode dissolve in the electrolyte, there is a significant risk that these impurities migrate on the anode side and initiate dendrite growth by deposition. Because of this, iron-free materials are required as electrode material.

When using MWCNT's as electrode material, the risk of battery failure caused by those dendrites arises.

Consequently, MWCNT's comprising interstitial iron-components obtained by a process using a catalytic system comprising an iron-based graphitization catalyst should be avoided.

Therefore the present inventors have successfully produced iron-free supported catalyst and MWCNT's using said iron-free supported catalysts, yet they came to the conclusion that the dispersion mediums, disclosed in the prior art, are not sufficient to efficiently disperse the MWCNT's prepared from said iron-free catalysts, said MWCNT's comprising iron-free catalyst remnants.

Due to a longer length of the MWCNT's prepared from iron-free catalysts, and the resulting nano- and microstructures formed, the dispersion in N-methylpyrrolidone comprising polyvinylpyrrolidone creates gel-like highly viscous dispersions, which are very difficult to process, i.e. good homogeneity is hardly obtained and application on the electrode surface for lithium battery applications is barely practicable.

The aim of the present invention is to disclose a dispersion medium for smoothly dispersing MWCNT's prepared in a process involving iron-free catalysts for a method for the preparation of the MWCNT dispersion, as well as its use in batteries.

The present invention discloses a carbon nanotube dispersion comprising:.

Preferred embodiments of the present invention disclose one or more of the following features:.

The present invention further discloses a method for the production of the MWCNT dispersion comprising the steps of:.

A preferred embodiment of the method relates to the dispersing device comprising a bead mill with spherical beads having a diameter comprised between <NUM> and <NUM>, preferably between <NUM> and <NUM>.

The present invention further discloses an electrode comprising the carbon nanotube dispersion and an electrode active material.

The present invention further discloses a battery electrode mixture layer comprising the carbon nanotube dispersion formed into a layer, characterized by a surface resistivity of less than <NUM>,<NUM> Ohm/square, as measured by a Keithley <NUM> Multimeter in a <NUM> film, obtained from coating the dispersion on a polyethylene terephthalate film and evaporating the solvent.

The present invention discloses a multi-walled carbon nanotube dispersion having excellent viscosity, dispersibility, and storage stability and providing a cell electrode composite layer having uniform and satisfactory coating film properties and low electrode plate resistance; said multi-walled carbon nanotube dispersion comprising:.

The multi-walled carbon nanotubes (MWCNT's) of the present invention are obtained from a process using an iron-free supported catalysts and, as a result, still comprise catalyst remnants (also called catalyst residues).

The MWCNT's comprise between <NUM> and <NUM>% by weight, preferably less than <NUM> % by weight of catalyst remnants, said remnants comprising metal compounds of at least two metals selected from the IUPAC group <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM> and <NUM>.

More specifically the multi-walled carbon nanotubes comprise catalyst remnants, said remnants comprising metal compounds of at least three metals selected from the group consisting of aluminum, cobalt, molybdenum and vanadium.

The MWCNT's of the present invention are substantially free of iron. By substantially free, the present invention means less than <NUM> ppm, preferably less than <NUM> ppm, more preferably less than <NUM> ppm, most preferably less than <NUM> ppm.

The inventors have observed that MWCNT's prepared from iron-free supported catalysts are longer and present a different aspect ratio and particular nano- and microstructures when compared to MWCNT's prepared from iron-comprising supported catalysts.

These differences result in a more difficult dispersion process, and in dispersions with higher viscosity when said dispersions are prepared from current dispersion mediums.

The dispersion medium of the present invention comprises a polar amide based solvent selected from the group consisting of dimethylformamide, diethylformamide, dimethylacetamide, N-methyl-<NUM>-pyrrolidone and mixtures thereof.

Preferably the polar amide based solvent of the dispersion medium of the present invention is N-methyl-<NUM>-pyrrolidone.

Polyvinylpyrrolidone for being used in the dispersion medium of the present invention is characterized by a K-value comprised between K-<NUM> and K-<NUM>, more preferably between K-<NUM> and K-<NUM>.

The amine-based compound for being used in the dispersion medium of the present invention is selected from the group consisting of an aliphatic primary amine, an aliphatic secondary amine, an aliphatic tertiary amine, an amino acid, an alkanolamine, and mixtures thereof.

Examples of aliphatic primary amines for being used in the dispersion medium of the present invention are ethylamine, octylamine, laurylamine, myristylamine, stearylamine, and oleylamine.

Examples of aliphatic secondary amines for being used in the dispersion medium of the present invention are diethylamine, dibutylamine, and distearylamine.

Examples of aliphatic tertiary amines for being used in the dispersion medium of the present invention are triethylamine, dimethyloctylamine, dimethyldecylamine, dimethyllaurylamine, dimethylmyristylamine, dimethylpalmitylamine, dimethylstearylamine, dimethylbehenylamine, dilaurylmonomethylamine, and trioctylamine,.

Examples amino acids for being used in the dispersion medium of the present invention are alanine, methionine, proline, serine, asparagine, glutamine, lysine, arginine, histidine, aspartic acid, glutamic acid, and cysteine.

Preferably the dispersion medium of the present invention comprises an alkanolamine of the formula:.

NH<NUM> - CR1,R2 - CH,R2 - (CH<NUM>)n - OH.

More preferably, the dispersion medium of the present invention comprises an alkanolamine selected from the group consisting of <NUM>-amino-<NUM>-methyl-<NUM>-propanol, <NUM>-amino-<NUM>-methyl-<NUM>-butanol, <NUM>-amino-<NUM>-methyl-<NUM>-butanol, <NUM>-amino-<NUM>-methyl-<NUM>-butanol, <NUM>-amino-<NUM>-methyl -<NUM>-pentanol and mixtures thereof.

Most preferably, the dispersion medium of the present invention comprises <NUM>-amino-<NUM>-methyl-<NUM>-propanol.

Preferably, the dispersion medium of the present invention comprises N-methyl-<NUM>-pyrrolidone, polyvinylpyrrolidone K30 and <NUM>-amino-<NUM>-methyl-<NUM>-propanol.

Preferably, the dispersion medium of the present invention comprises:.

The MWCNT's of the present invention are prepared using iron-free catalysts.

Preferably, the iron-free supported catalyst is an iron-free two-component catalyst or a three-component supported catalyst, the support comprising aluminum oxide (Al<NUM>O<NUM>) and/or aluminum hydroxide (Al(OH)<NUM>) and aluminum oxide hydroxide (AlO(OH)) as confirmed by X-Ray diffraction.

Preferably, the iron-free two-component catalyst comprises a first cobalt-based catalytic component and a second vanadium-based catalytic component, both preferably in the form of oxide.

Preferably, the iron-free three-component graphitization catalyst comprises a first cobalt-based catalytic component, a second vanadium-based catalytic component and a third molybdenum-based catalytic component, all preferably in the form of oxide.

The aluminum-based support of both, the two-component and the three-component supported catalysts, comprises at least <NUM>% by weight of aluminum oxide hydroxide.

The two-component and the three-component supported catalysts have a purity of at least <NUM>%.

For the preparation of MWCNT, the supported iron-free catalyst is brought into contact with a carbon source in the gas phase.

The use of the supported catalyst allows for growth of the carbon nanotubes by chemical vapor synthesis through decomposition of the carbon source, leading to the production of the carbon nanotube aggregate.

Any carbon source that can supply carbon and can exist in the gas phase at a temperature of <NUM> or more may be used without particular limitation for the chemical vapor synthesis. The gas-phase carbonaceous material is preferably a compound consisting of up to <NUM> carbon atoms, more preferably a compound consisting of up to <NUM> carbon atoms.

The iron-free catalyst according to the present invention allows for the production of MWCNT at a carbon yield comprised between <NUM> and <NUM>% by weight, preferably between <NUM> and <NUM>% by weight, more preferably between <NUM> and <NUM>% by weight.

The carbon yield, in % by weight, is defined as: <MAT> wherein mtot is the total weight of product after reaction and mcat is the weight of the catalyst used for the reaction.

The MWCNT's obtained by the above method comprise between <NUM> and <NUM> % by weight, preferably <NUM> and <NUM> % by weight of the catalyst residues.

The weight percentage of catalyst residues is determined by thermogravimetric analysis or by calcination in air.

The MWCNT dispersion is prepared in a dispersion device generally used for dispersing pigments and the like.

Examples of a dispersion device include for instance homomixers, planetary mixers, colloid mills, cone mills, ball mills, sand mills, pearl mills, wet jet mills, roll mills and bead mills.

Preferably, the dispersion device comprises a bead mill with spherical beads having a diameter comprised between <NUM> and <NUM>, preferably between <NUM> and <NUM>.

In the method for preparing the MWCNT dispersion, the quantities (or weight percentages) of MWCNT's are quantities (or weight percentages) of MWCNT's comprising between <NUM> and <NUM>% by weight of iron-free catalyst residues, further referred to as iron-free catalyst residues comprising MWCNT. The catalyst residues are intrinsically present in the MWCNT's as a result of their preparation. Thus, for a dispersion of <NUM>% by weight of iron-free catalyst residues comprising MWCNT, comprising between <NUM> and <NUM>% by weight of iron-free catalyst residues, said dispersion will comprise between <NUM> and <NUM>% by weight of pure MWCNT's.

First, a dispersion medium is prepared by mixing for at least <NUM> minutes, preferably for at least <NUM> minutes, the amide-based solvent, the amine-based compound and polyvinylpyrrolidone in a dispersion device, said mixture comprising:.

the total of amide-based solvent, polyvinylpyrrolidone and amine-based component being <NUM>% by weight.

Then, <NUM>% by weight of iron-free catalyst residues comprising MWCNT's are slowly added while mixing for at least <NUM> minutes, preferably for at least <NUM> minutes.

Subsequently, the mixing is switched off to allow the trapped air to be removed.

A further amount of iron-free catalyst residues comprising MWCNT's, up to <NUM>% by weight, is slowly added while continuing the mixing for at least <NUM> minutes, preferably for at least <NUM> minutes, until a shiny homogeneous dispersion free of visible aggregates is obtained.

The amounts of amide-based solvent, polyvinylpyrrolidone, amine-based compound and iron-free catalyst residues comprising MWCNT's are selected in such a way that the dispersion of iron-free catalyst residues comprising MWCNT's is characterized in that the weight ratio of:.

In the present invention, by mixing is meant dispersing and homogenizing.

The dispersion of iron-free catalyst residues comprising MWCNT's is considered as being completed when the dispersed iron-free catalyst residues comprising MWCNT aggregates are characterized by a D90 equal to or less than <NUM>, as measured by a Malvern Mastersizer M3000 wherein D90 represents the equivalent spherical diameter where <NUM>% by volume of the aggregates lies below.

The particle size distribution of dispersions is measured via laser diffraction technique using a Malvern Mastersizer M3000. The samples are diluted in a water-based fluid carrier after automatized cleaning and verification of the background from the particle analyzer software. Once the obscuration is within the limits, <NUM> measurements are recorded and averaged. Measurements are done at ambient temperature.

The dispersion of iron-free catalyst residues comprising MWCNT's is characterized by a complex viscosity at <NUM>% deformation, as obtained from amplitude oscillatory sweep tests with from <NUM> to <NUM>% deformation at a fixed frequency of <NUM>, of <NUM> Pa. Preferably, the complex viscosity at <NUM>% deformation is less than <NUM>,<NUM> Pa. s, more preferably less than <NUM>,<NUM> Pa. s, most preferably less than <NUM> Pa. s, as measured on a control stress rheometer Anton Paar MCR-<NUM> at <NUM>.

In order to assess the performance of the iron-free catalyst residues comprising MWCNT dispersion of the present invention in either cathode or anode of batteries, in particular lithium-ion batteries, the resistivity is measured as surface resistivity in a film of <NUM> microns. A dispersion of iron-free catalyst residues comprising MWCNT's is coated over the PET (polyethylene terephthalate) and solvent is evaporated at <NUM> in an oven before measurements. Afterwards, a Keithley <NUM> Multimeter is employed to measure <NUM> points on the surface of the film and make the average.

The following illustrative examples are merely meant to exemplify the present invention but they are not intended to limit or otherwise define the scope of the present invention.

<NUM> parts by weight of water, at <NUM>, comprising <NUM> parts by weight of citric acid and <NUM> parts by weight of malic acid, were added to <NUM> parts by weight of ammonium metavanadate and <NUM> parts by weight of ammonium heptamolybdate tetrahydrate, and mixed during <NUM> minutes using a paddle mixer, resulting in a first aqueous solution.

Similarly, <NUM> parts by weight of water, at <NUM>, were added to <NUM> parts by weight of cobalt(II) acetate tetrahydrate and mixed during <NUM> minutes using a paddle mixer, resulting in a second aqueous solution.

The second aqueous solution was added to the first aqueous solution and mixed during <NUM> minutes using a paddle mixer.

To the mixture of the first and the second aqueous solution, <NUM> parts by weight of aluminum hydroxide (Apyral® <NUM> SM - Nabaltec), with specific surface area (BET) of <NUM><NUM>/g, was added and mixed during <NUM> minutes using a paddle mixer.

The paste thus obtained was then transferred to ceramic crucibles with a large opening and subjected to a heating process, wherein the paste was heated to <NUM> with a heating gradient of <NUM>/min and an air flow of <NUM><NUM>/h and maintained at <NUM> for <NUM> hours.

After <NUM> hours at <NUM>, the paste was further heated to a temperature of <NUM> with a heating gradient of <NUM>/min. and maintained at <NUM> for <NUM> hours while maintaining an air flow of <NUM><NUM>/h.

The solid material thus obtained was cooled down to room temperature and ground, by means of a conical grinder, to a powder characterized by a volume median particle diameter (D<NUM>) of <NUM>.

The iron-free supported catalyst is characterized by.

<NUM> of the iron-free supported graphitization catalyst of example <NUM> was spread in a quartz vessel which subsequently was brought in the center of a quartz tube-type reactor with an inlet and an outlet.

The center of the quartz tube reactor where the vessel comprising the catalyst is located was heated to a temperature of <NUM>.

Subsequently, ethylene gas, nitrogen and hydrogen were allowed to flow through the quartz tube reactor at a flow rate of <NUM> I/min (C<NUM>H<NUM>); <NUM> I/min (N<NUM>) and <NUM> I/min (H<NUM>) during <NUM> minutes.

The MWCNT is produced at a carbon yield of <NUM> % by weight and comprises <NUM>% by weight of catalyst remnants.

MWCNT dispersions, in N-methyl-<NUM>-pyrrolidone, comprising <NUM>% by weight of iron-free catalyst residues comprising MWCNT's of example <NUM> were prepared according to the formulations as in table <NUM>,.

In table <NUM>, AMP stands for <NUM>-amino-<NUM>-methyl-<NUM>-propanol.

In table <NUM>, the MWCNT dispersions were prepared in a pilot production line operating in circulation mode using a bead mill with beads of <NUM> diameter and a premixing equipment consisting of a tank with a mixer.

The iron-free catalyst residues comprising MWCNT dispersions of examples <NUM>, <NUM> and <NUM> were characterized by a D90 of <NUM> (amine-based compound: <NUM>-amino-<NUM>-methyl-<NUM>-propanol), <NUM> (amine-based compound: n-octylamine) and <NUM> (amine-based compound ethanolamine) respectively, as measured by a Malvern Mastersizer M3000. By way of comparison, the MWCNT dispersion, prepared under the same dispersion conditions with a comparable energy input, but not comprising the amine-based compound, is characterized by a D90 of <NUM>.

The MWCNT dispersions of examples <NUM>, <NUM> and <NUM> were tested for surface resistivity. Surface resistivity values of <NUM>Ω/sq (amine-based compound: n-octylamine), of <NUM>Ω/sq (amine-based compound: <NUM>-amino-<NUM>-methyl-<NUM>-propanol) and of <NUM>Ω/sq (amine-based compound: ethanolamine) were respectively measured. By way of comparison, the surface resistivity for the MWCNT dispersion, not comprising the amine-based compound, equals <NUM>Ω/sq. According to the inventors, the surface resistivity values for the n-octylamine comprising dispersion medium, for the <NUM>-amino-<NUM>-methyl-<NUM>-propanol comprising dispersion medium , for the ethanolamine comprising dispersion medium and for an identical dispersion medium but not comprising an amine-based compound, are comparable given the experimental error on surface resistivity measurements.

Increasing the polyvinylpyrrolidone (PVP-content) and the <NUM>-amino-<NUM>-methyl-<NUM>-propanol (AMP-content) has a huge effect on the complex viscosity, and on D90, while surface resisitivity remains substantially unchanged within experimental error, as clearly appears from example <NUM>.

Claim 1:
A carbon nanotube dispersion comprising:
- multi-walled carbon nanotubes, comprising between <NUM> and <NUM>% by weight of iron-free catalytic remnants, said remnants comprising one or more iron-free metal oxide compound(s) of at least three metals selected from the group consisting of aluminum, vanadium, cobalt and molybdenum;
- an amide-based solvent selected from the group consisting of dimethylformamide, diethylformamide, dimethylacetamide, N-methyl-<NUM>-pyrrolidone and mixtures thereof;
- polyvinylpyrrolidone; and
- an amine-based compound selected from the group consisting of an aliphatic amine, an amino acid, an alkanolamine, and mixtures thereof.