Carbon nanotube slurry and slurry for electrode

Abstract

Provided are a carbon nanotube dispersion liquid which can be easily produced, and a carbon nanotube dispersion liquid which is suitable for manufacturing an electrode for a high-performance lithium-ion secondary battery or the like. More specifically, provided is a carbon nanotube slurry containing at least carbon nanotubes, a dispersant, and an aqueous solvent. The carbon nanotubes have a BET specific surface area of 70 m2 / g to 180 m2 / g, and the peak intensity ratio G / D in Raman spectroscopy is 1.0 to 2.5. The peak intensity ratio G / D represents a ratio where G is the maximum intensity of a G-band scattered light peak in the range of 1570 to 1620 cm-1, and D is the maximum intensity of a D-band scattered light peak in the range of 1320 to 1370 cm-1 in the Raman spectrum.

Description

Carbon nanotube slurry and electrode slurry

[0001] The present invention relates to an electrode slurry suitable for producing electrodes for secondary batteries such as lithium ion secondary batteries, and a carbon nanotube slurry used in the production thereof.

[0002] With the widespread use of electric vehicles and the trend toward smaller, lighter, and more powerful portable devices such as mobile phones and laptop computers, secondary batteries with high energy density, as well as higher output and longer life, are being demanded. Against this background, non-aqueous secondary batteries, including lithium-ion batteries using non-aqueous electrolytes, have come to be used in many devices due to their high energy density and high voltage characteristics, and their development is being actively pursued.

[0003] Patent Document 1 discloses a slurry for forming an electrode of a lithium ion battery, which contains an electrode active material, a conductive additive, a binder, and a polar solvent, and is characterized in that the conductive additive has an average particle size of 500 nm or less when dispersed.

[0004] Patent Document 2 describes a carbon nanotube dispersion liquid containing carbon nanotubes (A), a solvent (B), and a dispersant (C), in which the carbon nanotubes (A) have a peak at a diffraction angle 2θ=25°±2° in powder X-ray diffraction analysis, with a half-width of the peak being 2° to 6°, and a peak width of 1560 to 1600 cm in Raman spectrum. －１ The maximum peak intensity in the range of 1310 to 1350 cm －１ The carbon nanotube dispersion liquid is characterized in that, when the maximum peak intensity within the range is defined as D, the G / D ratio is 0.5 to 5.0, and the dispersant (C) is a vinyl alcohol skeleton-containing resin.

[0005] JP 2006-309958 A (claims, examples, etc.) JP 2020-11873 A (claims, examples, etc.)

[0006] When carbon nanotube slurry is used to form electrodes for lithium-ion secondary batteries, it is generally applied to a current collector such as aluminum foil and then dried. In this case, it is desirable to reduce the amount of solvent contained in the slurry. However, reducing the amount of solvent usually increases the viscosity of the carbon nanotube slurry, making it difficult to apply it uniformly.

[0007] Furthermore, when dispersing carbon nanotubes, a common procedure is to mix powdered carbon nanotubes into a solvent, premix them to make them uniform (hereinafter referred to as "premixing"), and then disperse the premixed mixture using a disperser. In this case, depending on the carbon nanotubes used, the carbon nanotube slurry may have a very high viscosity, making premixing difficult. Furthermore, it may be difficult to feed the solution to the disperser, which may place too much strain on the disperser in the early stages of dispersion, making it difficult to operate the disperser.

[0008] The present invention aims to provide a carbon nanotube slurry that uniformly disperses carbon nanotube material and has low viscosity. The carbon nanotube slurry is used as an electrode slurry, and the electrodes made from this electrode slurry are required to have high electrical conductivity.

[0009] As a result of extensive research into the above-mentioned conventional problems, the present inventors have found that a slurry containing carbon nanotubes (CNTs) having specific properties is suitable, and have completed the present invention based on this finding.

[0010] That is, the present invention provides a method for producing a dispersion medium containing at least carbon nanotubes, a dispersant, and an aqueous solvent, wherein the carbon nanotubes are 70 mm or less in length. ２ / g~180m ２ / g and a peak intensity ratio G / D in Raman spectroscopy of 1.0 to 2.5. (The peak intensity ratio G / D is the ratio of the peak intensity G / D to the peak intensity D in Raman spectroscopy of 1.0 to 2.5.) －１ ~1620cm －１The maximum intensity of the G-band scattered light peak in the range of 1320 cm －１ ~1370cm －１ The maximum intensity of the D-band scattered light peak in the range is defined as D, and this represents the ratio.)

[0011] Furthermore, the present invention preferably comprises a step of subjecting a slurry containing 0.4 mass % of carbon nanotubes to a temperature of 25° C. and 38.3 seconds. －１ The carbon nanotube slurry has a viscosity at 2000 kJ / min (mPa·s) of 5.0 mPa·s / μm, and a particle size distribution d90 of the carbon nanotubes contained in the carbon nanotube slurry is B (μm).

[0012] The carbon nanotube slurry of the present invention is a slurry in which the carbon nanotube material is uniformly dispersed and has a relatively low viscosity. Therefore, premixing is easy, and the liquid can be fed to a disperser and the dispersion operation in the disperser can be performed without imposing a load. As a result, the carbon nanotube slurry of the present invention is easy to prepare.

[0013] In particular, the electrode slurry containing carbon nanotubes used in the present invention can be used to prepare electrodes with low electrical resistance and excellent conductivity, making the electrode slurry of the present invention suitable as an electrode material for lithium ion secondary batteries and the like.

[0014] Carbon nanotube materials used in the present invention are cylindrical substances composed only of carbon and having a diameter on the order of nanometers. They include single-walled carbon nanotubes as well as multi-walled carbon nanotubes in which two or more carbon nanotubes with different diameters are stacked in layers.

[0015] The BET specific surface area of ​​the carbon nanotubes used in the present invention is 70 m ２ / g or more 180m ２ / g, preferably less than 100m ２ / g and over 170m ２ / g, more preferably less than 110m ２ / g and over 160m ２The BET specific surface area of ​​the carbon nanotubes can be measured using a specific surface area measuring device.

[0016] When carbon nanotubes having a BET specific surface area within the above range are used, the amount of solvent held by the carbon nanotubes during premixing is small due to the relatively small BET specific surface area, and swelling of the carbon nanotubes is suppressed. This makes it easy to add a solvent to the carbon nanotubes and premix them. Furthermore, the slurry after premixing is less susceptible to the fiber length of the carbon nanotubes and has a low viscosity, which makes it easy to send the solution to a disperser, reduces the load on the disperser in the early stages of dispersion, and prevents difficulty in operating the disperser.

[0017] On the other hand, when carbon nanotubes have a BET specific surface area exceeding the above range, the solvent is held by the carbon nanotubes during premixing, resulting in a carbon nanotube slurry with a very high viscosity, which may make premixing difficult.Furthermore, it may become difficult to feed the liquid to the disperser, which may place too much strain on the disperser in the early stages of dispersion, making it difficult to operate the disperser.

[0018] One of the characteristics of the carbon nanotubes used in the present invention is that their BET specific surface area is within a relatively small range. The mechanism by which the carbon nanotubes used in the present invention exhibit the effects of the present invention is presumed to be as follows. That is, carbon nanotubes generally exist in a state of bundles of tubes in a powder state. If there are gaps in the bundles or if the bundles are in a non-uniform and sparse state, the BET specific surface area becomes large. Carbon nanotubes used in the present invention that have a BET specific surface area within a small range have very few gaps in the bundles and are in a uniform and dense state, which suppresses swelling due to the infiltration of solvents and suppresses an increase in the viscosity of the slurry.

[0019] The carbon nanotubes used in the present invention are carbon nanotubes having a peak intensity ratio G / D of 1.0 to 2.5 in Raman spectroscopy. －１~1620cm －１ The maximum intensity of the G-band scattered light peak within the range of 1320 cm －１ ~1370cm －１ The maximum intensity of the D-band scattered light peak within the range of 1 / 2 to 1 / 3 is defined as D, and the ratio is expressed as the ratio. The Raman spectrum can be measured by detecting the Raman scattered light using a Raman spectrometer.

[0020] The peak intensity ratio G / D is related to the crystallinity of the carbonaceous material. If the crystallinity of the carbonaceous material is too high, the carbon edges decrease due to the development of the graphite structure, reducing the number of electrolyte coordination sites, resulting in problems such as reduced low-temperature properties and increased resistance. Furthermore, if the crystallinity of the carbonaceous material is too low, the amount of amorphous material increases, increasing electrical resistance and reducing the utilization efficiency of the electric double layer at the interface between the electrolyte and the electrode material. The use of carbon nanotubes with a G / D value within the above range has the advantage that, when an electrode is fabricated, it has few defects, is highly resistant to high-voltage operation, and maintains high conductivity over a long period of time.

[0021] The carbon nanotubes used in the present invention preferably have a fiber length of 30 μm to 1000 μm and a fiber width of 3 nm to 20 nm. The fiber length and fiber width of the carbon nanotubes can be measured using an electron microscope. When carbon nanotubes having a fiber length and fiber width within the above ranges are used, the carbon nanotubes are in a thin, long fibrous state, which has the advantage of improving entanglement with the active material and resulting in a low-resistance electrode.

[0022] On the other hand, when carbon nanotubes with a shorter fiber length are used, the conductive path is easily broken when an electrode is fabricated. When carbon nanotubes with a fiber length exceeding the above range are used, the fibers may become entangled during premixing of the carbon nanotube slurry, resulting in a very high viscosity, which may make premixing difficult. Furthermore, it may become difficult to feed the solution to the disperser, which may place too much strain on the disperser in the initial stage of dispersion, making it difficult to operate the disperser. Alternatively, the viscosity may become so high that similar problems may occur.

[0023] Furthermore, the carbon nanotubes used in the present invention preferably exhibit a peak at a diffraction angle 2θ=25°±2° in powder X-ray diffraction analysis, with the peak half-width being 1.3° to 2.9°. Powder X-ray diffraction can be performed using an X-ray diffractometer. The half-width of the 2θ peak detected by powder X-ray diffraction analysis is an indicator of the layer structure of the carbon nanotubes. It is believed that the smaller the half-width of the peak, the more layers the multi-walled carbon nanotubes have, and conversely, the larger the half-width of the peak, the fewer layers the carbon nanotubes have. Using carbon nanotubes with a peak half-width within the above range has the advantage of achieving both electrical conductivity and dispersibility.

[0024] The carbon nanotubes used in the present invention are produced, for example, by a fluidization method in which a raw material gas is introduced into a high-temperature chamber containing a powdered catalyst and grown on the catalyst surface in a fluidized state. Alternatively, they are produced by a substrate method in which a raw material gas is introduced into a high-temperature chamber containing a stationary substrate with a catalyst attached and grown chemically on the substrate. Carbon nanotubes produced by the above method or any other method can be used as long as they have the properties specified in the present invention.

[0025] The carbon nanotubes used in the present invention are preferably those that have been subjected to one or a combination of the following treatments: pulverization, classification, and demetallization. These treatments allow various properties of the carbon nanotubes to be adjusted.

[0026] The pulverization operation is an operation for pulverizing carbon nanotubes to an appropriate size. Applicable pulverization operations include dry pulverization using a pin mill, pulverizer, hammer mill, jet mill, ball mill, Henschel mixer, or attritor, and wet pulverization using an ultrasonic disperser, disperser, homomixer, planetary mixer, high-pressure homogenizer, paint conditioner, colloid mills, bead mill, cone mill, wet jet mill, or thin-film rotary high-speed mixer.

[0027] The classification operation is an operation for adjusting the size of carbon nanotubes. Applicable classification operations can be carried out using a device that utilizes gravity, inertial force, or centrifugal force, or a device that utilizes a filter, in a dry method or a wet method.

[0028] Carbon nanotubes may contain heavy metals of Group VIII, Group VIIA, and Group VIA elements, such as Fe, Co, and Ni, derived from the manufacturing process. The demetallization process is a process for removing the amount of heavy metals contained in the carbon nanotubes and adjusting their content. Applicable demetallization processes include acid treatment, base treatment, and calcination in an inert atmosphere, and can be carried out singly or in combination of two or more of these. The amount of heavy metals contained in the carbon nanotubes can be measured by calcining the carbon nanotubes, extracting them with an acidic aqueous solution, and then using an ICP optical emission spectrometer.

[0029] The total content of heavy metals contained in the carbon nanotubes after the demetallization operation is preferably less than 5000 ppm, more preferably less than 3000 ppm, and even more preferably less than 1000 ppm. If the total content of heavy metals contained in the carbon nanotubes is 5000 ppm or more, the heavy metals will dissolve from the electrodes into the electrolyte in the produced secondary battery, degrading the charge / discharge cycle characteristics and making it impossible to achieve a long battery life, which is undesirable.

[0030] The carbon nanotube content in the carbon nanotube slurry is preferably 0.1 to 3.0% by mass, more preferably 0.2 to 1.5% by mass, and even more preferably 0.4 to 1.0% by mass, based on the total mass of the carbon nanotube slurry. When the carbon nanotube content is within this range, the slurry can be uniformly applied to a current collector, and the performance of a secondary battery electrode made from the slurry can be ensured.

[0031] The carbon nanotube slurry of the present invention is prepared by mixing and homogenizing at least carbon nanotubes having the above-described properties, a dispersant, and an aqueous solvent. The amount of the aqueous solvent blended into the carbon nanotube slurry is 88.0 to 99.9 mass %, preferably 95.0 to 99.8 mass %, of the total amount of the slurry.

[0032] The carbon nanotube slurry of the present invention is a slurry in which carbon nanotubes are uniformly dispersed in an aqueous solvent and has a low viscosity. The viscosity of the carbon nanotube slurry of the present invention is 25° C. and a shear rate of 38.3 s －１ In this case, the viscosity is preferably 5 to 500,000 mPa·s, more preferably 10 to 10,000 mPa·s, even more preferably 20 to 1,000 mPa·s, and still more preferably 30 to 700 mPa·s.

[0033] In the present invention, it is preferable that the carbon nanotube slurry contains 0.4 mass % of the carbon nanotubes and is subjected to a temperature test at 25° C. for 38.3 seconds. －１ When the viscosity at the time of the carbon nanotube slurry is A (mPa·s) and the particle size distribution d90 of the carbon nanotubes contained in the carbon nanotube slurry is B (μm), the value of A / B is 5.0 (mPa·s / μm) or more.

[0034] Here, the slurry viscosity (A) was measured using an E-type viscometer at a shear rate of 38.3 s -1 This refers to the viscosity value of an aqueous carbon nanotube slurry with a concentration of 0.4 mass% measured at a sample temperature of 25°C. When the carbon nanotube concentration of the carbon nanotubes in the carbon nanotube slurry in which carbon nanotubes are dispersed is higher than 0.4 mass%, the viscosity is measured after diluting the slurry with the aqueous solvent used in preparation to make the carbon nanotube concentration 0.4 mass%. When the carbon nanotube concentration in the carbon nanotube slurry is lower than 0.4 mass%, the viscosity is measured after volatilizing the solvent from the slurry to make the concentration 0.4 mass%. In the present invention, this measured value is called the LD viscosity (mPa s) of the carbon nanotubes.

[0035] Furthermore, the particle size distribution d90(B) of the carbon nanotubes is the particle size value (μm) corresponding to the 90% cumulative distribution percentage of the particle size of the carbon nanotubes measured using a laser diffraction / scattering particle size distribution measuring device.

[0036] The present inventors have found that when the ratio A / B, which is the ratio of the slurry viscosity (A) to the particle size distribution d90 (B) of the carbon nanotube slurry, is 5.0 or more, the surface resistance of an electrode fabricated using an electrode slurry containing this carbon nanotube slurry is low. This finding is useful for fabricating electrodes with low surface resistance. Although the mechanism by which this phenomenon occurs is unclear, it is presumed that the degree of entanglement between the carbon nanotubes and the active material is increased, resulting in high conductivity and low surface resistivity.

[0037] The aqueous solvent used in the carbon nanotube slurry of the present invention may be water (for example, ion-exchanged water, distilled water, or tap water) or an aqueous mixed solvent consisting of water and a water-soluble solvent.

[0038] Examples of water-soluble solvents that can be used in the aqueous mixed solvent include alkylene glycols such as ethylene glycol, triethylene glycol, tetraethylene glycol, propylene glycol, dipropylene glycol, 1,2-propanediol, 1,3-propanediol, 1,2-butanediol, 2,3-butanediol, 1,3-butanediol, 1,4-butanediol, 1,2-pentanediol, 1,5-pentanediol, 2,5-hexanediol, 3-methyl-1,3-butanediol, 2-methylpentane-2,4-diol, 3-methylpentane-1,3,5-triol, and 1,2,3-hexanetriol; polyalkylene glycols such as polyethylene glycol and polypropylene glycol; glycerols such as glycerol, diglycerol, and triglycerol;

[0039] Examples include lower alkyl ethers of glycols such as ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, diethylene glycol monomethyl ether, and diethylene glycol mono-n-butyl ether; amides such as N-methyl-2-pyrrolidone, 1,3-dimethyl-2-imidalidinone, N,N-dimethylacetamide, and N,N-dimethylformamide; and ketones such as acetone.

[0040] The content of the water-soluble solvent in the aqueous mixed solvent is preferably 0.1 to 30 mass % of the total amount of the mixed solvent, and more preferably 1 to 20 mass %. Within this range, good flowability of the carbon nanotube slurry can be obtained.

[0041] The dispersant is a polymer that disperses carbon nanotubes well in a solvent to form a stable slurry. Preferred dispersants for the carbon nanotube slurry of the present invention include polyvinylpyrrolidone, carboxymethylcellulose or its salts (alkali metal salts such as Li, Na, and K), cellulose nanofibers, modified polyvinyl butyral, and acrylate polymers.

[0042] The content of the dispersant in the carbon nanotube slurry of the present invention is preferably 3 to 300 parts by mass, more preferably 5 to 200 parts by mass, even more preferably 8 to 150 parts by mass, and particularly preferably 10 to 100 parts by mass, relative to 100 parts by mass of the carbon nanotube material. When the amount of the dispersant is within this range, the dispersibility of the carbon nanotubes in the slurry can be exhibited, while the performance of the secondary battery electrode produced from the slurry can be ensured.

[0043] In addition to the carbon nanotubes, dispersant, and aqueous solvent, the carbon nanotube slurry of the present invention can further contain a conductive material other than carbon nanotubes. By adding the conductive material, the conductivity of the secondary battery electrode produced from the slurry can be increased.

[0044] The content of the conductive material to be mixed is preferably 0.5 to 10 mass %, more preferably 0.5 to 7 mass %, and particularly preferably 0.5 to 5 mass %, based on the total amount of the carbon nanotube slurry.

[0045] Usable conductive materials include conductive carbon particles and conductive carbon fibers made of graphite-type carbonaceous materials. Usable conductive carbon particles include carbon black particles such as acetylene black and ketjen black. Conductive carbon fibers include carbon nanofibers other than carbon nanotubes.

[0046] The conductive carbon particles have a specific gravity difference of ±0.2 g / cm from the carbon nanotube material. ３ A particle diameter within this range is preferred because separation due to differences in specific gravity is unlikely to occur when the carbon nanotube slurry or electrode slurry is stored. Furthermore, from the viewpoints of conductivity and stability, the primary particle diameter of the conductive material particles is more preferably 1 to 70 nm, preferably 1 to 50 nm, and even more preferably 1 to 40 nm. The primary particle diameter of the conductive carbon particles refers to the arithmetic mean value of the outer diameters of a sufficient number n measured using an electron microscope image.

[0047] From the viewpoints of conductivity and stability, the fiber width of the conductive carbon fiber is preferably 1 to 500 nm, more preferably 1 to 400 nm, and even more preferably 1 to 200 nm. It is particularly preferably 1 to 90 nm, 3 to 30 nm, or even more preferably 3 to 15 nm. The fiber width of the conductive carbon fiber refers to the arithmetic average value of a sufficient number (n) of fiber widths measured using an image under an electron microscope.

[0048] The carbon nanotube slurry of the present invention can be produced by a dispersion step in which at least the carbon nanotubes, the dispersant, and the aqueous solvent are charged into a mixer and stirred and mixed. Other components such as a conductive material may be added during the dispersion step to prepare the slurry, or may be added to the slurry separately from the dispersion step.

[0049] The dispersion step in producing a carbon nanotube slurry can be carried out by dispersion treatment using, for example, an ultrasonic disperser, mixers such as a Disper, a homomixer, a planetary mixer, a paint conditioner, colloid mills, a media-type disperser such as a bead mill, a ball mill, a sand mill, an attritor, a pearl mill, or a Coball mill, a (high-pressure) homogenizer, a wet jet mill, a wet cavitation mill, a thin film rotary high-speed mixer, or a media-less disperser such as a cone mill, or other dispersing device such as a roll mill. From the viewpoints of dispersion stability and dispersion efficiency, preferred dispersing devices are a (high-pressure) homogenizer, a wet cavitation mill, and a bead mill.

[0050] The carbon nanotube slurry of the present invention has excellent fluidity. Specifically, the slurry has a shear rate of 38.3 s －１ The viscosity at 25°C can be set to preferably 5 to 500,000 mPa·s, more preferably 10 to 10,000 mPa·s, even more preferably 20 to 1,000 mPa·s, and even more preferably 30 to 700 mPa·s.

[0051] When preparing the carbon nanotube slurry of the present invention, the viscosity of the slurry can be adjusted to fall within the above range by appropriately selecting the dispersion conditions (dispersion time, dispersion strength) or the type or concentration of the dispersant. The viscosity value of the carbon nanotube slurry can be measured using an E-type rotational viscometer (TV-22 model, manufactured by Toki Sangyo Co., Ltd.).

[0052] <Slurry for Electrode> The carbon nanotube slurry of the present invention can be suitably used as an electrode slurry for producing an electrode for a secondary battery such as a lithium ion battery by adding an active material to the carbon nanotube slurry. That is, one aspect of the present invention is an electrode slurry containing the carbon nanotubes, a dispersant, an aqueous solvent, and an active material.

[0053] The electrode slurry of the present invention can be used for producing both a negative electrode and a positive electrode, but is particularly suitable for producing a negative electrode. That is, one aspect of the present invention is a negative electrode slurry containing the carbon nanotubes, a dispersant, an aqueous solvent, and a negative electrode active material.

[0054] The negative electrode slurry of the present invention includes the carbon nanotube slurry having the above-described configuration and at least a negative electrode active material. The negative electrode active material can be metal oxide-based active material particles, silicon-based active material particles, or spherical graphite, and metal oxide-based negative electrode active material particles are particularly suitable.

[0055] As the metal oxide-based negative electrode active material particles, for example, titanium oxide can be used. The titanium oxide is not particularly limited as long as it can absorb and release lithium. Examples of titanium oxide include spinel-type lithium titanate, ramsdellite-type lithium titanate, titanium-containing metal composite oxide, and titanium dioxide (TiO) having a monoclinic crystal structure. ２ (B)), and anatase type titanium dioxide, etc. can be used.

[0056] The spinel-type lithium titanate includes Li ４＋ｘ Ti ５ O １２ (x varies in the range of -1≦x≦3 depending on the charge / discharge reaction). Ramsdellite-type lithium titanate includes Li ２＋ｙ Ti ３ O ７ (y varies in the range of -1≦y≦3 depending on the charge / discharge reaction). ２ (B) and anatase type titanium dioxide include Li １＋ｚ TiO ２ (z varies in the range of −1≦z≦0 due to the charge / discharge reaction).

[0057] Examples of titanium-containing metal composite oxides include metal composite oxides containing Ti and at least one element selected from the group consisting of P, V, Sn, Cu, Ni, and Fe. Examples of metal composite oxides containing Ti and at least one element selected from the group consisting of P, V, Sn, Cu, Ni, and Fe include TiO ２ -P ２ O ５ , TiO ２ -V ２ O ５ , TiO ２ -P ２ O ５ -SnO ２ , TiO ２ -P ２ O ５ -MeO (Me is at least one element selected from the group consisting of Cu, Ni and Fe).

[0058] Such a metal composite oxide preferably has a microstructure with low crystallinity, in which a crystalline phase and an amorphous phase coexist, or in which an amorphous phase exists alone, which can further improve cycle performance.

[0059] In the negative electrode slurry of the present invention, the content of the negative electrode active material is preferably 30 to 60 mass %, more preferably 35 to 55 mass %, based on the total amount of the negative electrode slurry. When the content of the negative electrode active material in the negative electrode slurry is within this range, the fluidity of the slurry can be maintained while ensuring the performance of the produced electrode.

[0060] The content of the carbon nanotube material in the negative electrode slurry is preferably 0.05 to 5 parts by mass, more preferably 0.05 to 3 parts by mass, and even more preferably 0.05 to 1 part by mass, relative to 100 parts by mass of the negative electrode active material. When the content of the negative electrode active material in the negative electrode slurry is within this range, the fluidity of the slurry can be maintained while ensuring the performance of the produced electrode.

[0061] The negative electrode slurry of the present invention contains the carbon nanotube slurry and the negative electrode active material having the above-described configuration, and may also contain a solid electrolyte such as a sulfide solid electrolyte, an oxide solid electrolyte, a dry polymer electrolyte, a gel polymer electrolyte, or a quasi-solid electrolyte, as necessary.

[0062] <Binder> The electrode slurry preferably further contains a binder. For example, various emulsion-type polymers can be used as the binder. Specifically, polyimide-based resins; fluorine-based resins such as polyvinylidene fluoride (PVdF), polytetrafluoroethylene, tetrafluoroethylene-hexafluoropropylene-vinylidene fluoride copolymers, hexafluoropropylene-vinylidene fluoride copolymers, and tetrafluoroethylene-perfluorovinyl ether copolymers; elastomer-based resins such as ethylene-propylene-diene copolymers (EPDM), nitrile-butadiene rubber (NBR), and styrene-butadiene rubber (SBR); and acrylic-based resins can be used as emulsion-type binders. Furthermore, solutions of acrylic acid-based resins and cellulose-based resins can also be used as soluble polymers. Two or more binders can be used in combination.

[0063] The amount of binder used is preferably 0.05 to 5 parts by mass, more preferably 0.1 to 4.5 parts by mass, and even more preferably 0.5 to 4.5 parts by mass, per 100 parts by mass of the active material in each electrode slurry for secondary batteries. When the amount of binder added is within this range, an electrode with high adhesion to the current collector is obtained without adversely affecting the battery capacity or charge / discharge characteristics.

[0064] The amount of solvent contained in the electrode slurry is preferably 0.5 to 80 mass %, more preferably 1 to 70 mass %, based on the total amount of the electrode slurry, since an appropriate viscosity is required when the electrode slurry is applied to a current collector.

[0065] In addition to the above components, the electrode slurry may further contain a leveling agent, a solid electrolyte, an antiseptic, etc. as appropriate.

[0066] The electrode slurry can be prepared by mixing the carbon nanotube slurry, the active material of the secondary battery, and optionally a binder, an aqueous solvent, and other components. For example, a twin-screw mixer can be used for the mixing operation.

[0067] The electrode slurry of the present invention is applied to a current collector, which is a conductive member of a secondary battery such as a lithium-ion secondary battery, and then dried to produce an electrode. The obtained electrode achieves high output and battery performance that can withstand repeated charge and discharge over a long period of time as an electrode for a secondary battery.

[0068] In other words, by using the electrode slurry of the present invention, a secondary battery electrode having battery performance such as high output and the ability to withstand repeated charge and discharge for a long period of time can be obtained. The carbon nanotube slurry of the present invention uniformly disperses carbon nanotubes and the like and has a low viscosity, so that carbon nanotubes and the like can be uniformly coated on a current collector at a high concentration.

[0069] An electrode can be produced from the electrode slurry of the present invention as follows. First, the electrode slurry is applied to a current collector. The current collector is a conductive member that serves as an electrode substrate for a secondary battery such as a lithium-ion secondary battery. The material and shape of the current collector used as the electrode substrate are not particularly limited, and can be appropriately selected to suit the secondary battery to be used. Examples of materials for the current collector include metals and alloys such as aluminum, copper, nickel, titanium, and stainless steel. Furthermore, as the shape of the current collector, a flat metal foil is generally used, but a foil with a roughened surface, a perforated foil, or a mesh-like foil can also be used.

[0070] Examples of methods for applying the electrode slurry to a current collector include die coating, dip coating, roll coating, doctor coating, knife coating, spray coating, gravure coating, screen printing, and electrostatic coating. After coating, the surface may be smoothed using a lithographic press, a calendar roll, or the like.

[0071] Next, the current collector coated with the electrode slurry is dried. This produces an electrode in which an electrode film is formed on the current collector. Methods for drying the coated electrode slurry include natural drying, a blower dryer, a hot air dryer, an infrared heater, and a far-infrared heater. The thickness of the produced electrode, including the thickness of the current collector, is generally 1 μm or more and 500 μm or less, and preferably 10 μm or more and 300 μm or less.

[0072] An electrode produced from an electrode slurry using the carbon nanotube slurry of the present invention has a low electrical resistance and is suitable for use as an electrode for a lithium ion secondary battery or the like.

[0073] <Lithium-ion secondary battery> A secondary battery is typically composed of a positive electrode, a negative electrode, an electrolyte, a non-aqueous solvent, and a separator, if necessary, and can be formed into various shapes depending on the intended use, such as a cylindrical shape, a prismatic shape, a gum shape, a coin shape, a button shape, a pin shape, or a paper shape. An electrode prepared by coating the electrode slurry described above can be used as the positive electrode or negative electrode of the lithium-ion secondary battery. Hereinafter, a lithium-ion secondary battery constructed using an electrode prepared from the electrode slurry of the present invention will be described.

[0074] The electrolyte can be a Li salt in which ions are mobile, such as LiBF ４ , LiClO ４ , LiPF ６ , LiAsF ６ , LiSbF ６ , LiCF ３ SO ３ , Li(CF ３ SO ２ ) ２ N, LiC ４ F ９ SO ３ , Li(CF ３ SO ２ ) ３ C, LiI, LiBr, LiCl, LiAlCl, LiHF ２ , LiSCN, and LiBPh ４ (wherein Ph represents a phenyl group).

[0075] Examples of non-aqueous solvents include aprotic polar solvents such as carbonates such as ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate; lactones such as γ-butyrolactone, γ-valerolactone, and γ-octanoic lactone; ethers such as tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolane, 4-methyl-1,3-dioxolane, 1,2-methoxyethane, 1,2-ethoxyethane, and 1,2-dibutoxyethane; esters such as methyl formate, methyl acetate, and methyl propionate; sulfoxides such as dimethyl sulfoxide and sulfolane; and nitriles such as acetonitrile. These solvents may be used alone or in combination.

[0076] Examples of the separator include polyethylene nonwoven fabric, polypropylene nonwoven fabric, polyamide nonwoven fabric, and these fabrics that have been subjected to a hydrophilic treatment.

[0077] Examples of embodiments of the present invention will be described below. The properties of the carbon nanotubes (CNTs) used in preparing the carbon nanotube slurries in the following examples and comparative examples were measured by the following methods. The measurement results of the carbon nanotube properties are shown in Table 1.

[0078] (Properties of Carbon Nanotubes) [BET Specific Surface Area] Carbon nanotubes were collected, precisely weighed using an electronic balance, and dried at 110°C for 30 minutes while degassing. Thereafter, the BET specific surface area was measured by the single-point BET method using a fully automatic specific surface area measuring device (Macsorb model HM-1208, manufactured by Mountec Co., Ltd.).

[0079] [Raman Spectrum] Carbon nanotubes were collected, dispersed in water, and then applied to a test piece with an applicator. After drying at 80°C, the sample was mapped in a 100 μm square section using a Raman spectrometer (DXR2xi, manufactured by Thermo Fisher Scientific) and the Raman spectrum was measured. －１The absorbance G at the peak top position within the range of 1310 to 1350 cm －１ The absorbance D at the peak top position within this range was measured, and the G / D ratio was calculated.

[0080] [Fiber Length and Fiber Width] Carbon nanotubes were collected and observed using a scanning electron microscope (Hitachi High-Tech Corporation, S-3400N; SEM), and the arithmetic mean value of the fiber lengths of 10 carbon nanotubes measured using images at a magnification of 1000 times was calculated. In addition, carbon nanotubes were observed using a transmission electron microscope (Hitachi High-Tech Corporation, H-7650; TEM), and the arithmetic mean value of the fiber widths of 10 carbon nanotubes measured using images at a magnification of 50,000 times was calculated.

[0081] [Peak Half Width] Carbon nanotubes were sampled and observed using a powder X-ray diffraction analyzer (Miniflex 600, manufactured by Rigaku Corporation), and the average value of the 2θ peak half width of 10 carbon nanotubes was determined.

[0082]

[0083] Example 1 Preparation of Carbon Nanotube Slurry A glass bottle (M-140, manufactured by Kakuyo Glass Co., Ltd.) was charged with 0.4 parts by mass of commercially available carbon nanotubes A as carbon nanotubes, 0.2 parts by mass of sodium carboxymethyl cellulose (CMC, 1190, manufactured by Daicel Millize Co., Ltd.) as a dispersant, and 99.4 parts by mass of water as a solvent, and the mixture was stirred (premixed) for 1 hour using a disper at a stirring speed (300 rpm) that did not introduce bubbles, to obtain a premixed slurry. The fluidity of the premixed liquid was visually evaluated as follows: A: The premixed liquid flows well. B: The premixed liquid does not flow easily. C: The premixed liquid does not flow.

[0084] ​Next, 300 parts by mass of zirconia beads (bead diameter Φ0.5 mm) were added, the glass bottle was capped, and the premixed liquid was vigorously shaken by hand to allow the beads to blend. Subsequently, the premixed liquid in the above state was subjected to mechanical dispersion treatment for 2 hours using a paint shaker (manufactured by Seiwa Giken Co., Ltd.) and then removed. The movement of the beads was confirmed, and the above mechanical dispersion treatment was carried out again to separate the beads, thereby obtaining a carbon nanotube slurry (CNT dispersion liquid) after mechanical dispersion treatment. The fluidity of the carbon nanotube slurry after mechanical dispersion treatment was visually evaluated as follows, using the ease of movement of the beads in the slurry as an index: A: The beads in the slurry move well. B: The beads in the slurry do not move easily. C: The beads in the slurry do not move very easily. D: The beads in the slurry do not move.

[0085] [LD Viscosity A of Carbon Nanotube Slurry] A carbon nanotube slurry having a carbon nanotube concentration of 0.4% by mass was measured using an E-type viscometer (TV-22 model, manufactured by Toki Sangyo Co., Ltd.) with a 1°34′ cone at a shear rate of 38.3 s －１ The mixture was rotated at 100°C, and the viscosity was measured at a sample temperature of 25°C. The obtained viscosity value was expressed as LD viscosity (A). When the concentration of carbon nanotubes dispersed in the carbon nanotube slurry was higher than 0.4% by mass, the slurry was diluted with water to 0.4% by mass, and the viscosity was measured after adjusting the concentration to 0.4% by mass. When the concentration of carbon nanotubes in the carbon nanotube slurry was lower than 0.4% by mass, the solvent was volatilized from the slurry to a concentration of 0.4% by mass.

[0086] [Particle size distribution d90 of carbon nanotubes] The carbon nanotube slurry obtained by mechanical dispersion treatment was sampled, and the particle size distribution of the carbon nanotubes was measured using a laser diffraction / scattering particle size distribution analyzer LA-960S. The particle size value (μm) corresponding to the 90% cumulative distribution percentage in the volume-based particle size distribution was determined and used as the particle size distribution d90(B). The ratio A / B (mPa s / μm) of the LD viscosity (A) to the particle size distribution d90(B) was calculated.

[0087] [Temporal stability of carbon nanotube slurry] The carbon nanotube slurry obtained by mechanical dispersion treatment was collected and placed in a glass bottle with a cap (M-140, manufactured by Kakuyo Glass Co., Ltd.) in an amount of 100 parts by mass, and then left to stand in a thermostatic chamber at 50°C for 3 months. The state of the slurry in the glass bottle after storage was confirmed visually. The state of the carbon nanotube slurry was visually evaluated according to the following criteria: A: The carbon nanotube slurry was uniform and no separation was observed B: The carbon nanotube slurry was slightly non-uniform and some separation of the carbon nanotubes was observed C: The carbon nanotube slurry was non-uniform and significant separation of the carbon nanotubes was observed

[0088] <Preparation of Electrode Slurry> Carboxymethyl cellulose (CMC; 1190, manufactured by Daicel Miraize Co., Ltd.) as a binder and a negative electrode active material (LTO-2S, manufactured by BEAM Advanced Energy Materials Co., Ltd.) were blended with the carbon nanotube slurry prepared by the mechanical dispersion treatment as described above in amounts such that the CMC was 0.2 parts by mass and the negative electrode active material was 100 parts by mass relative to 0.4 parts by mass of the carbon nanotubes contained in the slurry, and the mixture was kneaded until uniform using a mixer (THINKY MIXER ARE-310, manufactured by THINKY CORPORATION) to prepare an electrode slurry.

[0089] The prepared electrode slurry was applied to a soda glass piece using an applicator to a film thickness of 50 μm, and the coated glass piece was then dried on a hot plate at 90° C. for 10 minutes to prepare a model electrode for a lithium battery negative electrode.

[0090] [Surface Resistivity of Electrode] The surface resistivity (Ω / □) of the obtained electrode was measured using a resistivity meter (Mitsubishi Chemical Analytech Co., Ltd., Loresta GP, MCP-T610, four-point probe, ASP pin spacing 5 mm).

[0091] Examples 2 to 6, Comparative Examples 1 to 6 Carbon nanotube slurries were prepared in the same manner as in Example 1, except that carbon nanotubes A were either used as they were or replaced with other varieties of carbon nanotubes listed in Table 1, the dispersant was changed, or the number of mechanical dispersion treatments was increased or decreased as needed. The fluidity and other properties of the obtained carbon nanotube slurries were evaluated in the same manner as in Example 1. Furthermore, electrode slurries were prepared using each of the prepared carbon nanotube slurries, and model negative electrodes were fabricated in the same manner as in Example 1. The surface resistivity (Ω / □) of each of the obtained model electrodes was measured in the same manner as in Example 1. The carbon nanotube slurry compositions and evaluation results for Examples 1 to 6 and Comparative Examples 1 to 6 are shown in Table 2.

[0092]

[0093] The carbon nanotube slurries of Examples 1 to 6, which are within the scope of the present invention, exhibited relatively low viscosity and could be easily prepared, as is clear from Table 2. In particular, when the carbon nanotubes of Examples 1 to 6, which are within the scope of the present invention, were used, the fluidity of the slurry was good during and after premixing, and the movement of the beads was also good before and after the mechanical dispersion treatment using a paint shaker, confirming that a carbon nanotube slurry was obtained as a dispersion with good operability.

[0094] The coating operation was easy when producing electrodes from the electrode slurries prepared using the carbon nanotube slurries of Examples 1 to 6 within the scope of the present invention. As is clear from Table 2, the electrodes of Examples 1 to 6 produced from the electrode slurries using the carbon nanotube slurries within the scope of the present invention exhibited low electrical resistivity.

[0095] ​In contrast, the carbon nanotube slurries obtained in Comparative Examples 1 to 3 and Comparative Examples 5 to 6, in which the ratio A / B of the LD viscosity A (mPa·s) of the slurry to the d90 particle size distribution B (μm) of the carbon nanotubes was less than 5.0 (mPa·s / μm), had poor stability over time and were difficult to handle. Furthermore, the electrodes produced from the carbon nanotube slurries in Comparative Examples 1 to 3 and Comparative Examples 5 to 6 had high surface resistivities. Note that in Comparative Example 4, the viscosity of the carbon nanotube slurry was too high to disperse the carbon nanotubes, making it impossible to prepare an electrode slurry or produce an electrode.

[0096] The carbon nanotube slurry of the present invention can be suitably used as an electrode slurry, from which electrodes for lithium secondary batteries can be easily produced.

Claims

1. A method for producing a dispersion medium comprising at least carbon nanotubes, a dispersant, and an aqueous solvent, the carbon nanotubes being 70 mm or longer. ２ / g ~ 180m ２ / g and a peak intensity ratio G / D in Raman spectroscopy of 1.0 to 2.

5. －１ ~1620cm －１ The maximum intensity of the G band scattered light peak in the range of 1320 cm －１ ~1370cm －１ The maximum intensity of the D band scattered light peak in the range is defined as D, and the ratio is expressed as D.

2. The carbon nanotube slurry according to claim 1, wherein the carbon nanotubes have a fiber length of 30 μm to 1000 μm and a fiber width of 3 nm to 20 nm.

3. The carbon nanotube slurry according to claim 1, wherein the carbon nanotube content is 0.1% by mass to 3.0% by mass.

4. In the carbon nanotube slurry, a slurry containing 0.4 mass % of the carbon nanotubes was subjected to 25° C. and 38.3 s －１ 2. The carbon nanotube slurry according to claim 1, wherein the viscosity at d90 is A (mPa·s) and the particle size distribution d90 of the carbon nanotubes contained in the carbon nanotube slurry is B (μm), and the value of A / B is 5.0 (mPa·s / μm) or more.

5. A slurry for an electrode comprising at least the carbon nanotube slurry according to any one of claims 1 to 4 and an active material.