Carbon nanotube, carbon nanotube dispersion liquid, slurry for electrode, and electrode film

Abstract

In the present invention, the elongation of a carbon nanotube thread is 3 mm or more when a test piece of carbon nanotubes is drawn at a speed of 10 mm / min.

Description

Carbon nanotubes, carbon nanotube dispersions, electrode slurries, and electrode films

[0001] The present invention relates to carbon nanotubes, a dispersion using the carbon nanotubes, an electrode slurry using the dispersion, and an electrode film using the electrode slurry.

[0002] In recent years, lithium-ion rechargeable batteries have attracted attention due to the spread of electronic devices and environmentally friendly mobility solutions. Lithium-ion rechargeable batteries utilize conductive additives to reduce electrode resistance.

[0003] Compared to conventional carbon materials used as conductive additives, carbon nanotubes (CNTs), which can reduce resistance with small amounts of use, have attracted attention, and the development of their dispersions is progressing (see, for example, Patent Document 1). When drying electrode slurries containing such carbon nanotubes, warping or cracking due to shrinkage sometimes occurs in the coating film. Warping or cracking leads to a deterioration in battery performance when used in lithium-ion secondary batteries.

[0004] Japanese Patent Publication No. 2023-24526

[0005] The object of the present invention is to provide carbon nanotubes that can suppress warping and cracking when formed into a coating film, a carbon nanotube dispersion using these carbon nanotubes, an electrode slurry using this dispersion, and an electrode film using this electrode slurry.

[0006] While carbon nanotubes with elongation properties are suitable for handling as spun yarn, their handling is inferior when used as a conductive additive, and therefore they have not been actively used in that role. Our research has shown that elongation carbon nanotubes have fewer defects and uniform fiber thickness and length, thus eliminating warping and cracking. Furthermore, it is believed that optimizing their usage could improve battery performance.

[0007] That is, according to the present invention, (1) a carbon nanotube having an elongation of 3 mm or more, preferably 7 mm or more, more preferably 10 mm or more of a carbon nanotube yarn when a test piece of the carbon nanotube is pulled at a speed of 10 mm / min, (2) the carbon nanotube according to (1) having a peak intensity ratio G / D in Raman spectroscopy of 0.5 or more and 3.0 or less (where the intensity ratio G / D is 1570 cm －１ to 1620 cm －１ in the range of the maximum intensity of the G-band scattered light peak in the Raman spectrum obtained by the Raman spectroscopy is G, and 1320 cm －１ to 1370 cm －１ in the range of the maximum intensity of the D-band scattered light peak is D, and the ratio is represented). (3) A carbon nanotube dispersion liquid containing at least a solvent and the carbon nanotube according to (1) or (2), (4) An electrode slurry using the carbon nanotube dispersion liquid according to (3), and (5) An electrode film using the electrode slurry according to (4) are provided.

[0008] According to the present invention, there are provided a carbon nanotube capable of suppressing warping and cracking when formed into a coating film, a carbon nanotube dispersion liquid using this carbon nanotube, an electrode slurry using this dispersion liquid, and an electrode film using this electrode slurry.

[0009] It is a photograph showing the measurement situation of the elongation (extensibility) of the carbon nanotube yarn.

[0010] (Carbon nanotube) Hereinafter, the carbon nanotube of the present invention will be described. The carbon nanotube of the present invention has an elongation of 3 mm or more, preferably 7 mm or more, more preferably 10 mm or more of a carbon nanotube yarn when a test piece of the carbon nanotube is pulled at a speed of 10 mm / min.

[0011] Here, in the present invention, the elongation of the carbon nanotube yarn (hereinafter also referred to as extensibility) is a value measured as follows.

[0012] Set one side of a sheet-like carbon nanotube test piece on the specimen stage of the automatic load testing machine MAX series (MAX-1KN: Nihon Keisoku System Co., Ltd.) and fix the other side with a vise. Move the testing machine in the direction where the carbon nanotube on the y-axis extends at a speed of 10 mm / min, measure the length when all the threads are broken, and take it as the elongation of the carbon nanotube thread. The test piece of the carbon nanotube should be at least 5 mm × 5 mm or more. Also, when fixing the carbon nanotube test piece, fix it with a force that does not break the test piece. When the spinnability is within the above range, warping and cracking of the coating film can be suppressed.

[0013] In addition, the peak intensity ratio G / D in the Raman spectroscopy of the carbon nanotubes used in the present invention is preferably 0.5 or more, more preferably 0.7 or more, still more preferably 0.9 or more, and preferably 3.0 or less, more preferably 2.7 or less, still more preferably 2.3 or less. When the intensity ratio G / D is within the above range, the effect of suppressing warping and cracking of the coating film can be obtained. When G / D exceeds the above range, the carbon nanotubes become hard, so the effect of suppressing warping and cracking of the coating film becomes low. On the other hand, when G / D is below the above range, the conductivity of the carbon nanotubes themselves tends to be low, and it becomes difficult to suppress warping and cracking of the coating film.

[0014] Here, the intensity ratio G / D refers to the ratio when, in the Raman spectrum obtained by the Raman spectroscopic method, the maximum intensity of the G-band scattered light peak in the range of 1560 cm －１ to 1600 cm －１ is G, and the maximum intensity of the D-band scattered light peak in the range of 1310 cm －１ to 1350 cm －１ is D. Here, the Raman spectrum can be obtained, for example, by placing carbon nanotubes on a Raman microscope (DXR2xi manufactured by Thermo Scientific) and performing measurement using a laser wavelength of 532 nm. The measurement conditions are: objective lens magnification 20 times, aperture 50 μm confocal pinhole, exposure time 0.1 s, laser output 2 mW, number of scan times 10 times, and measurement wavelength 100 to 3400 cm -1 .

[0015] Further, the carbon nanotubes of the present invention preferably have an exothermic peak at 600°C or higher and 800°C or lower in differential thermal analysis (DTA) when the temperature is raised from 200°C to 1000°C at a rate of 10°C / min. The exothermic peak is measured based on JIS K 0129. Specifically, in an air atmosphere and under the condition of a heating rate of 10°C / min, the temperatures of the carbon nanotubes and the reference substance are changed to obtain a curve (DTA curve) with the temperature difference on the vertical axis and the temperature on the horizontal axis, and the largest peak is taken as the exothermic peak. When the exothermic peak is within the above range, the impurities contained in the carbon nanotubes are reduced, so that the nanotubes interact with each other and are likely to stick together, and the effect of suppressing warping and cracking when forming a coating film can be obtained.

[0016] Further, the total content of cobalt, iron, copper, zinc, nickel, chromium, manganese and molybdenum in the carbon nanotubes of the present invention is preferably 5000 ppm or less, more preferably 3000 ppm or less, and still more preferably 1000 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 optical emission spectrometry). Specifically, it can be calculated by extracting the metals contained in the carbon nanotubes by acid-decomposing the carbon nanotubes and analyzing the extract by ICP. Here, in the carbon nanotubes, cobalt, iron, copper, zinc, nickel, chromium, manganese and molybdenum may exist as metal simple substances, metal oxides, and composite oxides thereof, etc., but the above total content in the present invention is the content converted to metal simple substances. When the total content of cobalt, iron, copper, zinc, nickel, chromium, manganese and molybdenum is within the above range, not only the safety of the secondary battery is improved, but also the nanotubes interact with each other and are likely to stick together, and the effect of suppressing warping and cracking when forming a coating film can be obtained.

[0017] 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 (atm%) on the carbon nanotube surface. When the surface oxygen content is within the above range, conductivity is increased, and good battery performance can be obtained as an electrode film.

[0018] The carbon nanotubes of the present invention are not particularly limited as long as they have a shape substantially formed by winding a single sheet of graphite into a tube, and may be single-walled carbon nanotubes in which a single sheet of graphite is wound in one layer, or multi-walled carbon nanotubes in which two or three or more layers are wound. Among these, multi-walled carbon nanotubes are preferred.

[0019] Furthermore, examples of carbon nanotube forms 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.

[0020] 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 in accordance with JIS K 1469 or JIS K 6218 being treated as an impurity.

[0021] (Carbon Nanotube Dispersion) The carbon nanotubes of the present invention can be used in a carbon nanotube dispersion containing at least a solvent and the carbon nanotubes of the present invention. The carbon nanotube dispersion may also contain a dispersant as needed.

[0022] (Solvent) The solvent used in the carbon nanotube dispersion is preferably at least one selected from water, 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.

[0023] (Dispersant) Examples of dispersants that can be used in the carbon nanotube dispersion of the present invention include carboxymethylcellulose (hereinafter sometimes referred to as "CMC") or its metal salt or ammonium salt, polyacrylic acid or its neutralized product, polyvinylpyrrolidone (hereinafter sometimes referred to as "PVP"), hydrogenated nitrile rubber (hereinafter sometimes referred to as "H-NBR"), polyvinyl butyral resin, and polyvinylidene fluoride (hereinafter sometimes referred to as "PVDF"). The dispersant functions as a polymer that disperses carbon nanotubes well in the solvent and provides a stable slurry.

[0024] The weight-average molecular weight of carboxymethylcellulose or its metal salt or ammonium salt is preferably 50,000 or more, more preferably 100,000 or more, even more preferably 200,000 or more, and preferably 600,000 or less. Examples of metal salts include alkali metal salts such as lithium, sodium, and potassium, and ammonium salts can also be used.

[0025] The weight-average molecular weight of polyacrylic acid or its neutralized product is preferably 10,000 or more, more preferably 200,000 or more, preferably 2,000,000 or less, and more preferably 1,000,000 or less. Examples of neutralized products include alkali metal salts such as lithium, sodium, and potassium, and ammonium salts can also be used.

[0026] Polyvinylpyrrolidone is not particularly limited, but for example, it can be used if its weight-average molecular weight is 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.

[0027] The hydrogenated nitrile rubber is not particularly limited, but for example, it can be used if its weight-average molecular weight is preferably 600,000 or less, more preferably less than 500,000, even more preferably 400,000 or less, particularly preferably 300,000 or less, preferably 50,000 or more, more preferably 70,000 or more, and even more preferably 100,000 or more. The content of nitrile group-containing monomer units can be preferably 50% by mass or less, more preferably 40% by mass or less. The amount of remaining double bonds in butadiene after hydrogenation can be preferably 10% or less, more preferably 5% or less, even more preferably 2% or less, preferably 0.1% or more, and more preferably 0.2% or more.

[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 165,000 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] While polyvinylidene fluoride is not particularly limited, for example, those with a weight-average molecular weight of preferably 2 million or less, more preferably 1.5 million or less, preferably 100,000 or more, more preferably 300,000 or more, and even more preferably 500,000 or more can be used.

[0030] 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).

[0031] The amount of dispersant contained in the carbon nanotube 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, based on 100 parts by mass of carbon nanotubes in the carbon nanotube 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 carbon nanotube dispersion and the electrode slurry described later.

[0032] (Conductive Materials) In addition to the carbon nanotubes, dispersant, and solvent mentioned above, the carbon nanotube 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.

[0033] 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 carbon nanotube dispersion. Examples of conductive materials include carbon black particles such as acetylene black and Ketjen black, and carbon nanofibers.

[0034] (Other Components) The carbon nanotube 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.

[0035] 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.

[0036] (Carbon nanotube content in carbon nanotube dispersion) The carbon nanotube content in the carbon nanotube 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 carbon nanotube 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.

[0037] (Method for producing carbon nanotube dispersion) The carbon nanotube 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.

[0038] 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.

[0039] 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.

[0040] 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.

[0041] Furthermore, if the viscosity after dispersion treatment (for example, the 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.

[0042] (Applications of Carbon Nanotube Dispersion) The carbon nanotube dispersion of the present invention can be mixed with an electrode active material and other components such as a binder as needed 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.

[0043] (Positive electrode slurry) The positive electrode slurry of the present invention comprises a carbon nanotube 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.

[0044] 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.

[0045] 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.

[0046] 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 ２The 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.

[0047] 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.

[0048] 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.

[0049] (Negative electrode slurry) The negative electrode slurry of the present invention comprises a carbon nanotube 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.

[0050] 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.

[0051] As for spinel-type lithium titanate, Li ４＋ｘ Ti ５ O １２Examples 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.)

[0052] 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).

[0053] Such metal composite oxides preferably have low crystallinity and a microstructure in which a crystalline phase and an amorphous phase coexist, or in which the amorphous phase exists alone. This microstructure can further improve the cycling performance.

[0054] 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.

[0055] 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.

[0056] (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.

[0057] 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.

[0058] 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. Further solvent may be added to adjust the amount of solvent contained in the electrode slurry. As the solvent, a solvent that can be used in the carbon nanotube dispersion described above may be used, or other solvents may be used as long as they do not hinder the effects of the present invention. In addition, one type of solvent may be used alone, or two or more types of solvents may be used in mixture form.

[0059] (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 carbon nanotube dispersion of the present invention can be increased.

[0060] In addition to the above components, the electrode slurry may optionally contain leveling agents, solid electrolytes (sulfide solid electrolytes, oxide solid electrolytes, dry polymer electrolytes, gel polymer electrolytes, pseudo-solid electrolytes, etc.), preservatives, and other additives.

[0061] The electrode slurry can be prepared by mixing the carbon nanotube dispersion of the present invention with the positive electrode active material or the 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.

[0062] 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 carbon nanotube 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 a current collector with carbon nanotubes and the like 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.

[0063] 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.

[0064] 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.

[0065] Electrodes fabricated from an electrode slurry (positive electrode slurry or negative electrode slurry) using a carbon nanotube dispersion containing the carbon nanotubes for carbon nanotube dispersion 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 carbon nanotube dispersion of the present invention have better dispersibility compared to single-walled carbon nanotubes, resulting in superior productivity for carbon nanotube dispersions, electrode slurries, and electrode films.

[0066] (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 made into 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.

[0067] 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.)

[0068] 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.

[0069] The carbon nanotube dispersion containing carbon nanotubes for use in 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.

[0070] 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.

[0071] (Example 1) <Preparation of carbon nanotube dispersion> 0.4 parts of carbon nanotube (CNT) 1 (differential thermal peak 715°C, peak intensity ratio G / D by Raman spectroscopy: 1.9, metal ppm 629 ppm, extension 25 mm), 0.4 parts of carboxymethylcellulose sodium salt (CMC-Na) Daicel Corporation "CMC1110" (weight-average molecular weight 590,000) as a dispersant, and the remainder (99.2 parts) as water were mixed and stirred (premixed) for 1 hour using a disperser at a stirring speed that did not entrain bubbles to obtain a premixed liquid.

[0072] 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 carbon nanotube dispersion liquid in which carbon nanotubes were uniformly dispersed.

[0073] (Examples 2-7 and Comparative Examples 1-8) Carbon nanotube 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.

[0074] In the examples and comparative examples, the exothermic peaks in thermal analysis were measured in accordance with JIS K 0129. Specifically, the temperatures of carbon nanotubes and reference materials were varied from 200°C to 1000°C under atmospheric conditions and a heating rate of 10°C / min. A curve (DTA curve) was obtained with the temperature difference on the vertical axis and temperature on the horizontal axis, and the largest peak was identified as the exothermic peak.

[0075] Furthermore, the peak intensity ratio G / D of carbon nanotubes in Raman spectroscopy is 1560 cm⁻¹ in the Raman spectrum obtained by Raman spectroscopy. －１ ~1600cm －１ The maximum intensity of the G-band scattered light peak in the range is G, 1310 cm. －１ ~1350cm －１ This 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 was obtained by placing a carbon nanotube in a Raman microscope (ThermoScientific DXR2xi) and measuring with 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.

[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 elongation was measured as follows: A sheet-shaped carbon nanotube test specimen was set by clamping one end of the specimen on the sample stage of the MAX series automatic load testing machine (MAX-1KN: Nippon Keisoku System Co., Ltd.), and the other end was fixed with a vise. The testing machine was moved at a speed of 10 mm / min in the direction in which the carbon nanotube stretches along the Z axis, and the length when all the threads broke was measured and defined as the elongation of the carbon nanotube thread. The carbon nanotube test specimen was at least 5 mm x 5 mm, and when fixing the carbon nanotube test specimen, a force was applied that was not so strong as to break the specimen. Figure 1 shows a photograph of the measurement situation. In Table 1, "X" in the elongation column indicates that the elongation of the carbon nanotube thread was less than 3 mm.

[0078] <Evaluation Method> The evaluation of the examples and comparative examples (coating sagging, coating film warping, coating film cracking, and surface resistance) was performed as follows.

[0079] (Coating Sagging) To 100 parts of titanium oxide as the negative electrode active material, the carbon nanotube 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 in a twin-screw kneader to obtain an electrode slurry. This slurry was dropped and evaluated visually according to the following criteria: A: The coating liquid flows continuously when dropped. B: Almost all of the coating liquid flows continuously when dropped. C: Almost all of the coating liquid flows intermittently when dropped.

[0080] (Warping of the coating film) To 100 parts of titanium oxide as the negative electrode active material, the carbon nanotube 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 in a twin-screw kneader to obtain an electrode slurry. The electrode slurry was applied to the current collector by hand using a film applicator (manufactured by BEVS Industrial Co., Ltd.), and dried in an oven at 100°C for 20 minutes to form a coating film of 0.5 to 30 μm. The coating film was evaluated visually according to the following criteria: A: Almost no warping of the coating film B: Warping of the coating film is observed in several places C: Warping of the coating film is observed throughout

[0081] (Cracking of the coating) The coating was formed using the same procedure as for evaluating "warping of the coating" described above, and evaluated visually according to the following criteria: A: Almost no cracking of the coating B: Several cracks are visible in the coating C: Cracks are visible throughout the coating

[0082] (Surface resistance) To 100 parts of titanium oxide as the negative electrode active material, the carbon nanotube 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.

[0083] 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-prong 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 3000 Ω / □ B: Surface resistivity is 3000 Ω / □ or more and less than 7000 Ω / □ C: Surface resistivity is 7000 Ω / □ or more

[0084]

[0085] As shown in Table 1, carbon nanotube dispersions using carbon nanotubes in which the elongation of the carbon nanotube thread was 3 mm or more when the carbon nanotube specimen was pulled at a speed of 10 mm / min showed suppressed sagging during coating, and the coating film formed with this carbon nanotube dispersion showed suppressed warping and cracking. Furthermore, the electrode film formed with this carbon nanotube dispersion exhibited excellent conductivity.

Claims

1. A carbon nanotube in which the elongation of the carbon nanotube thread is 3 mm or more when the carbon nanotube test piece is pulled at a speed of 10 mm / min.

2. The carbon nanotube according to claim 1, wherein the peak intensity ratio G / D in Raman spectroscopy is 0.5 or more and 3.0 or less. (However, the intensity ratio G / D is defined as the peak intensity ratio at 1570 cm⁻¹ in the Raman spectrum obtained by the Raman spectroscopy.) －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.) 3. A carbon nanotube dispersion comprising at least a solvent and the carbon nanotube described in claim 1 or 2.

4. Electrode slurry using the carbon nanotube dispersion according to claim 3.

5. An electrode film using the electrode slurry described in claim 4.

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