Carbon nanotubes for solvent-based dispersions, solvent-based dispersion, electrode slurry, and electrode film

Abstract

In the present invention, the BET specific surface area is 500 m2 / g to 1300 m2 / g, the peak intensity ratio G / D in Raman spectroscopy is 30 to 150, and the average diameter is 1 nm to 5 nm. The present invention excludes single-walled carbon nanotubes. (The intensity ratio G / D represents the ratio of G and D in a Raman spectrum obtained by Raman spectroscopy, where G is the maximum intensity of a G-band scattered light peak in the range of 1570 cm-1 to 1620 cm-1 and D is the maximum intensity of a D-band scattered light peak in the range of 1320 cm-1 to 1370 cm-1.)

Description

Carbon nanotubes for solvent-based dispersions, solvent-based dispersions, electrode slurries, and electrode films

[0001] The present invention relates to carbon nanotubes for solvent-based dispersions, solvent-based dispersions using the carbon nanotubes for solvent-based dispersions, electrode slurries using the solvent-based dispersions, and electrode films using the electrode slurries.

[0002] In recent years, with the spread of electronic devices and mobility considering environmental aspects, lithium-ion secondary batteries have attracted attention. Conductive aids are used in lithium-ion secondary batteries to reduce the resistance of electrodes.

[0003] Carbon nanotubes (CNTs), which can reduce resistance with a small amount of use compared to carbon materials used as conventional conductive aids, have attracted attention, and the development of their dispersions (dispersion liquids) has been underway.

[0004] Carbon nanotubes include single-walled carbon nanotubes in which one plane of graphite is wound into one layer, and multi-walled carbon nanotubes wound into two or three or more layers. In Patent Document 1, a dispersion using single-walled carbon nanotubes is proposed. Single-walled carbon nanotubes are described as being superior in flexibility compared to multi-walled carbon nanotubes and having excellent battery performance (cycle characteristics) when used as a conductive agent for electrodes.

[0005] Further, Patent Document 2 describes that it is preferable to use single-walled carbon nanotubes from the viewpoints of viscosity, conductivity, and cost. Although only one example of an embodiment using multi-walled carbon nanotubes is disclosed, this embodiment is a combination with single-walled carbon nanotubes.

[0006] Japanese Patent No. 7194860 International Publication No. 2023 / 286793

[0007] As described above, in Cited Documents 1 and 2, it is described that single-walled carbon nanotubes are more suitable for dispersions than multi-walled carbon nanotubes from viewpoints such as viscosity (dispersion stability) and conductivity.

[0008] The object of the present invention is to provide carbon nanotubes other than single-walled carbon nanotubes, which have excellent dispersion stability when used as a dispersion and conductivity when used as an electrode film, a solvent-based dispersion using this carbon nanotube for solvent-based dispersions, an electrode slurry using this solvent-based dispersion, and an electrode film using this electrode slurry.

[0009] According to our investigations, we have found that the above objective can be achieved by using carbon nanotubes other than single-walled carbon nanotubes that have properties equivalent to single-walled carbon nanotubes, that is, by using carbon nanotubes other than single-walled carbon nanotubes that have predetermined properties, and thus we have completed the present invention.

[0010] In other words, according to the present invention, (1) the BET specific surface area is 500 m ２ / g or more 1300m ２ Carbon nanotubes for solvent-based dispersions, excluding single-walled carbon nanotubes, having a peak intensity ratio G / D of 30 or more and 150 or less in Raman spectroscopy, and an average diameter of 1 nm or more and 5 nm or less. (However, the above intensity ratio G / D is defined as the Raman spectrum obtained by the above Raman spectroscopy, at 1570 cm⁻¹.) －１ ~1620cm －１ The maximum intensity of the G-band scattered light peak in the range is G, 1320 cm. －１ ~1370cm －１When the maximum intensity of the D-band scattered light peak within the range is defined as D, the ratio is represented. (2) The carbon nanotube for an aqueous dispersion according to claim 1, which satisfies the following (i) to (iii): (i) In differential thermal analysis when the temperature is raised from 200 °C to 1000 °C at a rate of 10 °C / min, it has an exothermic peak at 500 °C or higher and 800 °C or lower. (ii) The total content of cobalt, iron, copper, zinc, nickel, chromium, manganese, and molybdenum is 7000 ppm or less. (iii) The surface oxygen content is 2.5 atm% or less. (3) A solvent-based dispersion containing at least an organic solvent as a solvent and the carbon nanotube for a solvent-based dispersion according to (1) or (2). (4) The solvent-based dispersion according to (3), wherein the organic solvent is at least one selected from lactam-based, ester-based, and alcohol-based solvents. (5) The solvent-based dispersion according to (3), containing at least one selected from polyvinylpyrrolidone, polyacrylonitrile-modified rubber, and polyvinyl butyral resin as a dispersant. (6) An electrode slurry using the solvent-based dispersion according to (3). (7) An electrode film using the electrode slurry according to (6) is provided.

[0011] According to the present invention, there are provided carbon nanotubes excluding single-walled carbon nanotubes, which are excellent in dispersion stability when made into a dispersion and conductivity when made into an electrode film, a solvent-based dispersion using the carbon nanotubes for a solvent-based dispersion, an electrode slurry using this solvent-based dispersion, and an electrode film using this electrode slurry.

[0012] (Carbon Nanotubes) Hereinafter, the carbon nanotubes for a solvent-based dispersion of the present invention will be described. The carbon nanotubes for a solvent-based dispersion of the present invention have a BET specific surface area of 500 m ２ / g or more and 1300 m ２ / g or less, a peak intensity ratio G / D in Raman spectroscopy of 30 or more and 150 or less, an average diameter of 1 nm or more and 5 nm or less, and exclude single-walled carbon nanotubes.

[0013] The BET specific surface area of the carbon nanotubes used in the present invention is 500 m ２ / g or more, preferably 550 m２ / g or more, more preferably 600m ２ It is 1300m or more per gram. ２ / g or less, preferably 1250m ２ / g or less, more preferably 1200m ２ The BET specific surface area is less than or equal to / g. The BET specific surface area of ​​carbon nanotubes can be measured using a specific surface area measuring device. More specifically, carbon nanotubes can be collected, accurately weighed using an electronic balance, dried at 110°C for 30 minutes while degassing, and then the BET specific surface area can be measured using a fully automatic specific surface area measuring device (Macsorb model HM-1208, manufactured by Mountec Co., Ltd.) by the BET single-point method.

[0014] Furthermore, the peak intensity ratio G / D of the carbon nanotubes used in the present invention in Raman spectroscopy is 30 or more, preferably 35 or more, more preferably 40 or more, and 150 or less, preferably 145 or less, more preferably 140 or less.

[0015] Here, the intensity ratio G / D is 1570 cm⁻¹ in the Raman spectrum obtained by the Raman spectroscopy method. －１ ~1620cm －１ The maximum intensity of the G-band scattered light peak in the range is G, 1320 cm. －１ ~1370cm －１ This expression represents the ratio of the maximum intensity of the D-band scattered light peak in the specified range, where D is the maximum intensity of the D-band scattered light peak. Here, the Raman spectrum is obtained, for example, by placing a carbon nanotube in a Raman microscope (ThermoScientific DXR2xi) and performing measurements using a laser wavelength of 532 nm. The measurement conditions are: objective lens magnification 20x, aperture 50 μm confocal pinhole, exposure time 0.1 s, laser output 2 mW, number of scans 10, and measurement wavelength 100–3400 cm. -1 That's what I decided.

[0016] Furthermore, the average diameter of the carbon nanotubes used in the present invention is 1 nm or more, preferably 1.5 nm or more, and 5 nm or less, preferably 4.5 nm or less.

[0017] The average diameter of carbon nanotubes was measured using electron microscope images, and the arithmetic mean of the average diameters of a sufficient number of samples (e.g., 10 to 20 nanotubes) was taken. Specifically, carbon nanotubes were observed using a transmission electron microscope (Hitachi High-Tech Corporation, H-7650; TEM), and the arithmetic mean of the diameters of 10 carbon nanotubes measured using images at 50,000x magnification was used.

[0018] Furthermore, the carbon nanotubes of the present invention preferably have an exothermic peak between 500°C and 800°C in differential thermal analysis (DTA) when heated from 200°C to 1000°C at a rate of 10°C / min. The exothermic peak was measured according to JIS K 0129. Specifically, the temperature of the carbon nanotubes and a reference material was changed under atmospheric conditions and a heating rate of 10°C / min, and a curve (DTA curve) with the temperature difference on the vertical axis and temperature on the horizontal axis was obtained, with the largest peak being defined as the exothermic peak. When the exothermic peak is within the above range, the amount of impurities contained in the carbon nanotubes is reduced. As a result, the nanotubes interact with each other and stick together easily, and the effect of suppressing warping and cracking when formed into a coating film is obtained.

[0019] Furthermore, the total content of cobalt, iron, copper, zinc, nickel, chromium, manganese, and molybdenum in the carbon nanotubes of the present invention is preferably 7,000 ppm or less, more preferably 6,000 ppm or less, and even more preferably 5,000 ppm or less. The total content of cobalt, iron, copper, zinc, nickel, chromium, manganese, and molybdenum contained in the carbon nanotubes can be quantified by ICP (inductively coupled plasma atomic emission spectroscopy). Specifically, it can be calculated by extracting the metals contained in the carbon nanotubes by acid decomposition of the carbon nanotubes and analyzing the extract by ICP. Here, in carbon nanotubes, cobalt, iron, copper, zinc, nickel, chromium, manganese, and molybdenum can each exist as elemental metals, metal oxides, and composite oxides thereof, but the total content in the present invention is the content converted to elemental metals. When the total content of cobalt, iron, copper, zinc, nickel, chromium, manganese, and molybdenum is within the above range, that is, when the metal content is low, the safety of secondary batteries can be improved. Furthermore, the nanotubes interact with each other and tend to stick together, which helps to suppress warping and cracking when applied as a coating.

[0020] Furthermore, the surface oxygen content of the carbon nanotube of the present invention is preferably 2.5 atm% or less, more preferably 1.9 atm% or less, and even more preferably 1.2 atm% or less. The surface oxygen content can be measured by X-ray photoelectron spectroscopy (XPS) and is expressed as the ratio of oxygen atoms to carbon atoms on the carbon nanotube surface (atm%). When the surface oxygen content is within the above range, conductivity is improved. In addition, good battery performance can be obtained as an electrode film.

[0021] Carbon nanotubes essentially have a cylindrical shape formed by winding a single sheet of graphite. Examples include single-walled carbon nanotubes, which are made by winding a single sheet of graphite in one layer, and multi-walled carbon nanotubes, which are made by winding two or three or more layers of graphite. However, the carbon nanotubes used in this invention are those other than single-walled carbon nanotubes. Other carbon nanotubes that can be used include multi-walled carbon nanotubes, double-walled carbon nanotubes, and few-walled carbon nanotubes.

[0022] Furthermore, examples of carbon nanotube forms used in the present invention include graphite whiskers, filamentous carbon, graphite fibers, ultrafine carbon tubes, carbon tubes, carbon fibrils, carbon microtubes, and carbon nanofibers, but are not limited to these. These may be used individually or in combination of two or more types.

[0023] Furthermore, the purity of the carbon nanotubes used in this invention is preferably 90 to 100% by mass, and particularly preferably 95 to 100% by mass. The purity of the carbon nanotubes is calculated based on the amount of impurities, with the ash content measured according to JIS K 1469 or JIS K 6218 being treated as an impurity.

[0024] (Solvent-based dispersion) The carbon nanotubes for solvent-based dispersions of the present invention can be used in solvent-based dispersions that include at least an organic solvent as a solvent and the carbon nanotubes for solvent-based dispersions. The solvent-based dispersion may also optionally contain at least one dispersant selected from polyvinylpyrrolidone, polyacrylonitrile-modified rubber, and polyvinyl butyral resin.

[0025] (Solvent) The solvent-based dispersion of the present invention contains an organic solvent as a solvent. There are no particular restrictions on the organic solvent, but it is preferable to use at least one selected from lactam-based organic solvents, ester-based organic solvents, and alcohol-based organic solvents. Examples of lactam-based organic solvents include N-methyl-2-pyrrolidone (hereinafter sometimes referred to as "NMP"). Examples of ester-based organic solvents include butyrate esters such as methyl butyrate, ethyl butyrate, propyl butyrate, butyl butyrate, amyl butyrate, hexyl butyrate, heptyl butyrate, and octyl butyrate, as well as acetate esters such as ethyl acetate, propyl acetate, butyl acetate, amyl acetate, hexyl acetate, heptyl acetate, and octyl acetate. Examples of alcohol-based organic solvents include ethanol, 1-hexanol, 2-hexanol, 3-hexanol, n-butanol, and tert-butanol. Among these, NMP, butyl butyrate, and ethanol are preferred. Other solvents may be mixed and used within a range that does not hinder the effects of the present invention.

[0026] (Dispersant) As the dispersant used in the solvent-based dispersion of the present invention, it is preferable to use at least one selected from polyvinylpyrrolidone, polyacrylonitrile-modified rubber, and polyvinyl butyral resin. The dispersant functions as a polymer that disperses carbon nanotubes well in the solvent and provides a stable slurry.

[0027] Polyvinylpyrrolidone (hereinafter sometimes referred to as "PVP") is not particularly limited, but for example, a weight-average molecular weight of preferably 70,000 or less, more preferably less than 70,000, even more preferably 68,000 or less, particularly preferably 65,000 or less, preferably 10,000 or more, more preferably 12,000 or more, and even more preferably 15,000 or more can be used.

[0028] The polyvinyl butyral resin is not particularly limited, but for example, one with a weight-average molecular weight of preferably 170,000 or less, more preferably 16.5 or less, even more preferably 160,000 or less, preferably 10,000 or more, more preferably 15,000 or more, and even more preferably 20,000 or more can be used.

[0029] The weight-average molecular weight of the dispersant can be measured, for example, by gel permeation chromatography (GPC); for example, under the following measurement conditions: instrument: HLC-8320GPC (Tosoh Corporation), columns: 2 SuperAWM-H, detector: HLC-8320GPC built-in RI detector, sample concentration: 0.2-0.6 wt%, flow rate: 0.6 mL / min, injection volume: 10 μL, column temperature: 40°C, eluent: DMF (containing 10 mM-LiBr).

[0030] The amount of dispersant contained in the solvent-based dispersion of the present invention is preferably 3 parts by mass or more, more preferably 5 parts by mass or more, even more preferably 8 parts by mass or more, particularly preferably 10 parts by mass or more, preferably 300 parts by mass or less, more preferably 280 parts by mass or less, even more preferably 260 parts by mass or less, and particularly preferably 250 parts by mass or less, per 100 parts by mass of carbon nanotubes in the solvent-based dispersion. When the amount of dispersant is within this range, it is possible to ensure the performance of the electrode film made from the electrode slurry while exhibiting the dispersibility of carbon nanotubes in the solvent-based dispersion and the electrode slurry described later.

[0031] (Conductive Material) In addition to the carbon nanotubes, dispersant, and solvent mentioned above, the solvent-based dispersion of the present invention may further contain conductive materials other than carbon nanotubes. By incorporating conductive materials, the conductivity of electrodes for secondary batteries made from the slurry can be improved.

[0032] The content of the conductive material to be blended is preferably 0.1 to 10% by mass, more preferably 0.1 to 7% by mass, and particularly preferably 0.1 to 5% by mass, based on the total amount of the solvent-based dispersion. Examples of conductive materials include carbon black particles such as acetylene black and Ketjen black, and carbon nanofibers.

[0033] (Other Components) The solvent-based dispersion of the present invention may contain other components in addition to those described above, depending on its intended use. Examples of other components include pH adjusters, anti-settling agents, wetting agents, emulsifiers, anti-sagging agents, defoaming agents, leveling agents, and plasticizers.

[0034] For example, pH adjusters can be used from the viewpoint of preventing corrosion of current collectors and ensuring compositional stability. At least one of the following can be used: ammonia, urea, monoethanolamine, diethanolamine, triethanolamine, aminomethylpropanol, sodium tripophosphate, alkali metal salts of carbonic acid or phosphoric acid such as sodium carbonate, or alkali metal hydroxides such as sodium hydroxide.

[0035] (Carbon nanotube content in solvent-based dispersion) The carbon nanotube content in the solvent-based dispersion of the present invention is preferably 0.1% by mass or more, more preferably 0.2% by mass or more, even more preferably 0.3% by mass or more, preferably 3.0% by mass or less, more preferably 2.5% by mass or less, and even more preferably 2.0% by mass or less, based on the total mass of the solvent-based dispersion. When the carbon nanotube content is within this range, the electrode slurry described later can be uniformly coated onto the current collector, and the performance of the electrode film made from the electrode slurry can be ensured.

[0036] (Method for producing solvent-based dispersions) The solvent-based dispersion of the present invention can be obtained, for example, by adding a carbon material containing carbon nanotubes, a solvent, a dispersant or conductive material used as needed, and other components, stirring and mixing, and then going through a dispersion process.

[0037] The above dispersion can be carried out by using dispersion devices such as ultrasonic dispersers, mixers such as dispersers, homomixers, rotational mixers, Henschel mixers, and planetary mixers, paint conditioners, colloid mills, bead mills, ball mills, sand mills, attritors, pearl mills, and coball mills, media-less dispersers such as (high-pressure) homogenizers, wet jet mills, wet cavitation mills, thin-film swirling high-speed mixers, and cone mills, and other roll mills. From the viewpoint of the stability of the dispersion process and the dispersion efficiency, preferred dispersion devices are (high-pressure) homogenizers, wet cavitation mills, and bead mills.

[0038] Distributed processing may be performed using the same distribution device or multiple times using multiple distribution devices. For example, premixing may be performed using mixers, and then distributed processing may be performed using a media-type distributer.

[0039] Furthermore, the viscosity after dispersion treatment (e.g., product viscosity) is preferably 5 mPa·s or more, more preferably 10 mPa·s or more, even more preferably 20 mPa·s or more, preferably 2000 mPa·s or less, more preferably 1900 mPa·s or less, and even more preferably 1800 mPa·s or less. Here, the above viscosity was measured using an E-type rotational viscometer (TV-22 model, manufactured by Toki Sangyo Co., Ltd.) at a shear rate of 38.3 s. －１ These values ​​were measured under conditions of 25°C. Furthermore, if the viscosity after dispersion treatment (e.g., product viscosity) is within the above range, the fluidity of the electrode slurry, as described later, will increase, making it possible to uniformly coat the electrode slurry onto the current collector at a high concentration.

[0040] (Applications of Solvent-Based Dispersions) The solvent-based dispersion of the present invention can be mixed with an electrode active material, a binder as needed, and other components to form an electrode slurry (positive electrode slurry or negative electrode slurry). Furthermore, an electrode film can be formed using the electrode slurry and used as a positive or negative electrode.

[0041] (Positive electrode slurry) The positive electrode slurry of the present invention comprises a solvent-based dispersion and a positive electrode active material having the above configuration. The positive electrode active material can be a substance that helps lithium ions to reversibly enter and exit the positive electrode of a lithium-ion secondary battery.

[0042] Examples of positive electrode active materials include lithium-nickel composite oxide, lithium-cobalt composite oxide, lithium-manganese composite oxide, lithium-nickel-manganese composite oxide, lithium-nickel-cobalt composite oxide, lithium-nickel-aluminum composite oxide, lithium-nickel-cobalt-aluminum composite oxide, lithium-nickel-manganese-cobalt composite oxide, lithium-nickel-manganese-aluminum composite oxide, lithium-nickel-cobalt-manganese-aluminum composite oxide, and other composite oxides of lithium and transition metals, as well as TiS ２ FeS, MoS ２ Transition metal sulfides such as MnO, V ２ O ５ , V ６ O １３ , TiO ２ Examples include transition metal oxides and olivine-type lithium phosphate oxides.

[0043] Olivine-type lithium phosphate oxide is a compound containing at least one element from the group consisting of Mn, Cr, Co, Cu, Ni, V, Mo, Ti, Zn, Al, Ga, Mg, B, Nb, and Fe, lithium, phosphorus, and oxygen. Olivine-type lithium phosphate oxide may also be a compound in which some of the aforementioned elements are substituted with other elements to improve its properties.

[0044] A preferred cathode active material is a lithium-nickel composite oxide, and more preferably, a material of the formula: LiNi Ｘ M1 Ｙ M2 Ｚ O ２ The active materials for the positive electrode are lithium-nickel composite oxides or lithium phosphate, represented as follows: (M1 and M2 are at least one metallic element from among Al, B, alkali metals, alkaline earth metals, and transition metals; 0.8 ≤ X ≤ 1.0, 0 ≤ Y ≤ 0.2, 0 ≤ Z ≤ 0.2). These positive electrode active materials may be used individually or in combination of two or more.

[0045] In the cathode slurry of the present invention, the content of the positive electrode active material is preferably 50 to 70% by mass, and more preferably 50 to 63% by mass, relative to the total amount of the positive electrode slurry. When the content of the positive electrode active material in the positive electrode slurry is within this range, the fluidity of the positive electrode slurry can be maintained while ensuring the performance of the electrode produced.

[0046] Furthermore, the carbon nanotube material content in the cathode 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, per 100 parts by mass of the cathode active material.

[0047] (Negative electrode slurry) The negative electrode slurry of the present invention, as an electrode slurry, comprises a solvent-based dispersion and a negative electrode active material having the above configuration. The negative electrode active material can be metal oxide-based active material particles, silicon-based active material particles, or spheroidal graphite, and metal oxide-based negative electrode active material particles can be used in particular.

[0048] As 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 is capable of intercalating and deintercalating lithium, but examples include spinel-type lithium titanate, ramsdellite-type lithium titanate, titanium-containing metal composite oxides, and titanium dioxide (TiO2) having a monoclinic crystal structure. ２ (B)), as well as anatase-type titanium dioxide, etc., can be used.

[0049] As for spinel-type lithium titanate, Li ４＋ｘ Ti ５ O １２ Examples include (where x changes in the range of -1 ≤ x ≤ 3 due to the charge-discharge reaction). As for ramsdelite-type lithium titanate, Li ２＋ｙ Ti ３ O ７ (Y changes in the range of -1 ≤ y ≤ 3 due to the charge-discharge reaction), etc. ２ (B) and as anatase-type titanium dioxide, Li １＋ｚ TiO ２ (z changes in the range of -1 ≤ z ≤ 0 due to the charge-discharge reaction.)

[0050] 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. For example, a metal composite oxide containing Ti and at least one element selected from the group consisting of P, V, Sn, Cu, Ni, and Fe is 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 Examples include -MeO (where Me is at least one element selected from the group consisting of Cu, Ni, and Fe).

[0051] Such metal composite oxides preferably have low crystallinity and a microstructure in which crystalline and amorphous phases coexist, or in which the amorphous phase exists alone. This microstructure can further improve the cycling performance.

[0052] In the anode slurry of the present invention, the content of the anode active material is preferably 30 to 60% by mass, and more preferably 35 to 55% by mass, relative to the total amount of the anode slurry. When the content of the anode active material in the anode slurry is within this range, the fluidity of the anode slurry can be maintained while ensuring the performance of the electrode produced.

[0053] Furthermore, the carbon nanotube material content 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, per 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 negative electrode slurry can be maintained while ensuring the performance of the manufactured electrode.

[0054] (Binder) The above-mentioned positive electrode slurry and negative electrode slurry preferably each further contain a binder. Examples of binders include polyimide resins, polyvinylidene fluoride (PVdF), polytetrafluoroethylene, tetrafluoroethylene / hexafluoropropylene / vinylidene fluoride copolymers, hexafluoropropylene / vinylidene fluoride copolymers, tetrafluoroethylene / perfluorovinyl ether copolymers, and other fluororesins, polyolefin resins such as polyethylene and polypropylene, polyvinylpyrrolidone, polyvinyl alcohol, styrene-butadiene rubber (SBR), acrylic resins, carboxymethylcellulose or its metal salts. These binders may be used individually or in combination of two or more types.

[0055] The amount of binder added is preferably 0.05 parts by mass or more, more preferably 0.1 parts by mass or more, even more preferably 0.5 parts by mass or more, preferably 5 parts by mass or less, and more preferably 4.5 parts by mass or less, per 100 parts by mass of active material in the positive electrode slurry or negative electrode slurry. When the amount of binder added is within this range, electrodes with high adhesion to the current collector can be obtained without adversely affecting the battery capacity or charge / discharge characteristics.

[0056] The amount of solvent contained in the electrode slurry is preferably 0.5% by mass or more, more preferably 1% by mass or more, preferably 80% by mass or less, and more preferably 70% by mass or less, relative to the total amount of the electrode slurry, since an appropriate viscosity is required when coating the electrode slurry onto the current collector. Note that additional solvent may be added to adjust the amount of solvent contained in the electrode slurry. As the solvent, any solvent usable in the above-mentioned solvent-based dispersion may be used, or other solvents may be used within a range that does not hinder the effects of the present invention. Furthermore, one solvent may be used alone, or two or more solvents may be used in mixture form.

[0057] (Conductive particles) Conductive particles can also be added to the electrode slurry. By adding conductive particles, the conductivity of the secondary battery electrodes made using the solvent-based dispersion of the present invention can be increased.

[0058] In addition to the above components, the electrode slurry may also contain, as needed, leveling agents, solid electrolytes (sulfide solid electrolytes, oxide solid electrolytes, dry polymer electrolytes, gel polymer electrolytes, pseudo-solid electrolytes, etc.), preservatives, and other additives.

[0059] Electrode slurry can be prepared by mixing the solvent-based dispersion of the present invention with a positive electrode active material or a negative electrode active material, and optionally adding a binder, solvent, and other components as needed. For the mixing operation, for example, a twin-screw kneader can be used.

[0060] An electrode film can be formed by applying the electrode slurry of the present invention onto a current collector and drying it, thereby producing an electrode (positive or negative electrode). The solvent-based dispersion of the present invention uniformly disperses carbon nanotubes and the like, and is a low-viscosity slurry. Therefore, the electrode slurry of the present invention using this slurry can uniformly coat carbon nanotubes and the like onto a current collector at a high concentration. An electrode can be produced from the electrode slurry of the present invention (positive electrode slurry or negative electrode slurry) as follows.

[0061] First, an electrode slurry is applied to the current collector. The current collector is a conductive material that serves as the 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 one can be appropriately selected to suit the secondary battery to which it is applied. Examples of current collector materials include metals and alloys such as aluminum, copper, nickel, titanium, or stainless steel. In addition, while a flat metal foil is generally used as the shape of the current collector, foils with roughened surfaces, perforated foils, and mesh-shaped foils can also be used.

[0062] Methods for coating the electrode slurry onto the current collector include die coating, dip coating, roll coating, doctor coating, knife coating, spray coating, gravure coating, screen printing, or electrostatic coating. Surface smoothing treatment using a flatbed press or calender roll may also be performed after coating. Next, the current collector coated with the electrode slurry is dried. This produces an electrode with an electrode film formed on the current collector. Methods for drying the electrode slurry after coating include natural drying, forced-air drying, hot-air drying, infrared heating, and far-infrared heating. The thickness of the produced electrode, including the thickness of the current collector, is generally between 1 μm and 500 μm, preferably between 10 μm and 300 μm.

[0063] Electrodes prepared from an electrode slurry (positive electrode slurry or negative electrode slurry) using a solvent-based dispersion containing carbon nanotubes for solvent-based dispersions of the present invention have high conductivity and can therefore be suitably used as electrodes for lithium-ion secondary batteries and the like, which have excellent battery characteristics. Furthermore, the carbon nanotubes for solvent-based dispersions of the present invention have better dispersibility compared to single-walled carbon nanotubes, resulting in superior productivity for solvent-based dispersions, electrode slurries, and electrode films.

[0064] (Lithium-ion secondary battery) A lithium-ion secondary battery typically consists of a positive electrode, a negative electrode, an electrolyte, a non-aqueous solvent for the electrolyte, and a separator provided as needed. It can be in various shapes depending on the intended use, such as cylindrical, prismatic, gum-shaped, coin-shaped, button-shaped, pin-shaped, or paper-shaped. It is preferable to use an electrode made by coating at least one of the positive or negative electrode of the lithium-ion secondary battery with the electrode slurry of the present invention (positive electrode slurry or negative electrode slurry). The following describes a lithium-ion secondary battery configured using an electrode made from the positive electrode slurry or negative electrode slurry of the present invention. For the positive electrode, an electrode made by coating and drying the above-mentioned positive electrode slurry containing positive electrode active material onto a current collector can be used. For the negative electrode, an electrode made by coating and drying the above-mentioned negative electrode slurry containing negative electrode active material onto a current collector can be used.

[0065] For the electrolyte, a lithium salt that allows ion movement can be used. For example, 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 Li SCN and LiBPh ４ (However, Ph represents a phenyl group.)

[0066] Examples of non-aqueous solvents for electrolytes include 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 aprotic polar solvents such as nitriles such as acetonitrile. These solvents may be used individually or in mixtures of two or more. Examples of separators include polyethylene nonwoven fabric, polypropylene nonwoven fabric, polyamide nonwoven fabric, and those made by applying a hydrophilic treatment to these materials.

[0067] The solvent-based dispersion containing carbon nanotubes of the present invention has low viscosity and excellent handling properties, and exhibits good conductivity when used as an electrode film with an electrode slurry. In other words, it is suitable for the manufacture of electrodes for high-efficiency lithium-ion secondary batteries and the like.

[0068] The present invention will be described in more detail below with reference to examples and comparative examples, but the present invention is not limited to these examples. In the following, "parts" refers to mass unless otherwise specified.

[0069] (Example 1) <Preparation of solvent-based dispersion> Carbon nanotube (CNT) A (BET specific surface area: 862.1 m²) ２0.2 parts of (peak intensity ratio G / D by Raman spectroscopy: 60.53, average diameter: 1.7-4.8 nm) and 0.2 parts of PVP (Disp. A; PVP-K30, weight-average molecular weight (Mw) 40,000) as a dispersant were mixed with the remainder (99.6 parts) and stirred (premixed) for 1 hour using a disperser at a stirring speed that did not entrain bubbles to obtain a premixed liquid. A shear rate of 38.3 s was measured using an E-type rotational viscometer (TV-22, manufactured by Toki Sangyo Co., Ltd.) on this premixed liquid. －１ The viscosity value was measured at 25°C.

[0070] The above premixing liquid was introduced into a horizontal bead mill disperser, followed by the addition of 970 parts by mass of zirconia beads (bead diameter Φ0.5 mm). Dispersion was then performed at a peripheral speed of 14 m / s. After that, the beads were separated to obtain a solvent-based dispersion in which carbon nanotubes were uniformly dispersed. The obtained solvent-based dispersion was subjected to a shear rate of 38.3 s using an E-type rotational viscometer (Toki Sangyo Co., Ltd., TV-22 model). －１ The viscosity value was measured at 25°C.

[0071] (Examples 2-10 and Comparative Examples 1-6) Solvent-based dispersions were prepared in the same manner as in Example 1, except that the types and amounts (in parts) of carbon nanotubes (CNTs), dispersants, and solvents were changed to those listed in Table 1.

[0072] In the examples and comparative examples, the average diameter of the carbon nanotubes was measured using electron microscope images, and the arithmetic mean of the average diameters of a sufficient number of samples (e.g., 10 to 20 nanotubes) was taken. Specifically, carbon nanotubes were collected, observed using a transmission electron microscope (Hitachi High-Tech Corporation, H-7650; TEM), and the arithmetic mean of the diameters of 10 carbon nanotubes measured using images at 50,000x magnification was calculated and used as the average diameter.

[0073] Furthermore, the peak intensity ratio G / D of carbon nanotubes in Raman spectroscopy is 1570 cm⁻¹ in the Raman spectrum obtained by Raman spectroscopy. －１ ~1620cm －１The maximum intensity of the G-band scattered light peak in the range is G, 1320 cm. －１ ~1370cm －１ This represents the ratio of the maximum intensity of the D-band scattered light peak within the specified range, where D is denoted as the maximum intensity. Here, the Raman spectrum was obtained by placing a carbon nanotube in a Raman microscope (ThermoScientific DXR2xi) and measuring it using a laser wavelength of 532 nm. The measurement conditions were: objective lens magnification 20x, aperture 50 μm confocal pinhole, exposure time 0.1 s, laser output 2 mW, number of scans 10, and measurement wavelength 100–3400 cm. -1 That's what I decided.

[0074] Furthermore, the BET specific surface area of ​​carbon nanotubes was measured using the BET single-point method with a Macsorb model HM-1208 manufactured by Mountec Co., Ltd., under the conditions of pretreatment at 110°C for 30 minutes.

[0075] Furthermore, the exothermic peak in the thermal analysis was measured in accordance with JIS K 0129. Specifically, the temperature of carbon nanotubes and a reference material was varied from 200°C to 1000°C under atmospheric conditions and a heating rate of 10°C / min. A curve (DTA curve) with the temperature difference on the vertical axis and temperature on the horizontal axis was obtained, and the largest peak was identified as the exothermic peak.

[0076] Furthermore, the total content of cobalt, iron, copper, zinc, nickel, chromium, manganese, and molybdenum in the carbon nanotubes was quantified by ICP (inductively coupled plasma atomic emission spectroscopy). Specifically, the metals contained in the carbon nanotubes were extracted by acid decomposition of the carbon nanotubes, and the extract was analyzed by ICP to calculate the total content. Here, in carbon nanotubes, cobalt, iron, copper, zinc, nickel, chromium, manganese, and molybdenum can exist as elemental metals, metal oxides, and composite oxides thereof, but the total content in this invention is the content converted to elemental metals.

[0077] Furthermore, the surface oxygen content of carbon nanotubes was measured using X-ray photoelectron spectroscopy (VersaProbeII, ULVAC-PHI) and expressed as the ratio of oxygen atoms to carbon atoms (atm%) on the carbon nanotube surface. The sample, ground in a mortar, was placed on carbon tape, placed in the XPS measurement device, and then vacuumed. The ratio of oxygen atoms to carbon atoms on the carbon nanotube surface was calculated using the cumulative measurement values ​​from 64 trials.

[0078] <Evaluation Method> The evaluation of the examples and comparative examples (dispersibility (dispersion time), dispersion viscosity, and surface resistance) was performed as follows.

[0079] (Pre-mixing fluidity) The fluidity of the slurry after pre-mixing was visually evaluated as follows, using the ease with which the beads move in the slurry as an indicator: A: Beads move easily B: Beads do not move easily C: Beads do not move

[0080] (Dispersion Viscosity) The viscosity values ​​of the solvent-based dispersions obtained in the examples and comparative examples were evaluated according to the following criteria. The results are shown in Table 1. A: Viscosity is less than 400 mPa·s B: Viscosity is 400 mPa·s or more and less than 1000 mPa·s C: Viscosity is 1000 mPa·s or more

[0081] (Surface resistance (conductivity)) To 100 parts of titanium oxide as the negative electrode active material, the solvent-based dispersion obtained in the examples and comparative examples was added so that the amount of carbon nanotubes was 0.3 parts and the amount of binder was 2.5 parts, and the negative electrode active material content was adjusted to 50% by mass of the total amount of negative electrode slurry. This mixture was then mixed using a twin-screw kneader to obtain an electrode slurry.

[0082] An electrode slurry was applied to a current collector by hand using a film applicator (manufactured by BEVS Industrial Co., Ltd.), dried in an oven at 100°C for 20 minutes to form an electrode film of 0.5 to 30 μm thickness, and electrodes were obtained. The surface resistivity (Ω / □) of the fabricated electrodes was measured using a resistivity meter (Mitsubishi Chemical Analytec Co., Ltd., Rolester GP, MCP-T610, four-probe, ASP pin spacing 5 mm). Lower surface resistivity indicates better conductivity. Surface resistivity was evaluated based on the following criteria. The results are shown in Table 1. A: Surface resistivity is less than 5000 Ω / □ B: Surface resistivity is 5000 Ω / □ or more

[0083]

[0084] As shown in Table 1, the BET specific surface area is 500 m². ２ / g or more 1300m ２ The solvent-based dispersion containing carbon nanotubes for solvent-based dispersions, excluding single-walled carbon nanotubes, exhibited excellent dispersibility and low viscosity, and the electrode film formed from the electrode slurry containing this solvent-based dispersion exhibited excellent conductivity.

[0085] The carbon nanotubes of the present invention can be mixed with an organic solvent to form a solvent-based dispersion with excellent dispersibility. This solvent-based dispersion can be mixed with an electrode active material to form an electrode slurry, which can be suitably used in the production of electrodes such as those for lithium secondary batteries.

Claims

1. BET specific surface area is 500 m² ２ / g or more 1300m ２ Carbon nanotubes for solvent-based dispersions, excluding single-walled carbon nanotubes, having a peak intensity ratio G / D of 30 or more and 150 or less in Raman spectroscopy, and an average diameter of 1 nm or more and 5 nm or less. (However, the above intensity ratio G / D is defined as the Raman spectrum obtained by the above Raman spectroscopy, at 1570 cm⁻¹.) －１ ~1620cm －１ The maximum intensity of the G-band scattered light peak in the range is G, 1320 cm. －１ ~1370cm －１ (When D is the maximum intensity of the D-band scattered light peak in the specified range, this represents the ratio.) 2. Carbon nanotubes for aqueous dispersions according to claim 1, satisfying the following (1) to (3): (1) Having an exothermic peak between 500°C and 800°C in differential thermal analysis when heated from 200°C to 1000°C at a rate of 10°C / min. (2) The total content of cobalt, iron, copper, zinc, nickel, chromium, manganese, and molybdenum is 7000 ppm or less. (3) The surface oxygen content is 2.5 atm% or less.

3. A solvent-based dispersion comprising at least an organic solvent as a solvent and carbon nanotubes for solvent-based dispersions according to claim 1 or 2.

4. The solvent-based dispersion according to claim 3, wherein the organic solvent is at least one selected from lactam-based, ester-based, and alcohol-based solvents.

5. The solvent-based dispersion according to claim 3, comprising at least one selected from polyvinylpyrrolidone, polyacrylonitrile-modified rubber, and polyvinyl butyral resin as a dispersant.

6. Electrode slurry using the solvent-based dispersion according to claim 3.

7. An electrode film using the electrode slurry described in claim 6.

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