Non-aqueous carbon nanotube slurry and dispersion for positive electrode

Abstract

To provide non-aqueous carbon nanotube slurry having good handleability; and a composition which gives a positive electrode having low electric resistance.SOLUTION: Carbon nanotube slurry includes a carbon nanotube, a non-aqueous dispersant, and a non-aqueous polar solvent and has an alkaline-earth metal ion content of 5 ppm to 2000 ppm relative to the carbon nanotube. There is also provided a dispersion for positive electrodes formed by adding a positive-electrode active material, carbon black, graphite, and a binder to the carbon nanotube slurry.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a positive electrode dispersion used in producing a positive electrode of a lithium ion secondary battery or the like, and to a carbon nanotube slurry serving as an intermediate material therefor. [Background technology]

[0002] In recent years, the lithium-ion battery market has been attracting attention due to the widespread use of electronic devices and environmentally friendly mobility. Lithium-ion batteries are equipped with a negative electrode and a positive electrode containing active materials that allow lithium ions to reversibly enter and exit the battery, and a non-aqueous electrolyte in which they are immersed. The positive electrode is manufactured by coating a current collector plate such as aluminum foil with an electrode slurry consisting of the active material, a conductive material, and a binder.

[0003] Conventionally, the use of high surface area carbon materials, particularly graphite (graphite particles), in non-aqueous secondary batteries including lithium ion batteries has been studied. In particular, it is known that an intercalation compound is formed in graphite and the intercalation compound is used as an electrode material for secondary batteries, and many techniques have been disclosed.

[0004] Patent Document 1 discloses a slurry for an electrode film, which comprises: a carbon nanotube dispersion containing carbon nanotubes; a first polymer containing at least one selected from the group consisting of a hydroxyl group-containing structural unit and a heterocycle-containing structural unit and not containing a nitrile group-containing structural unit; a second polymer containing a nitrile group-containing structural unit; and a solvent, wherein the carbon nanotubes contain at least two types of carbon nanotubes that differ in fiber diameter distribution within a range of 100 nm or less in a fiber diameter distribution measured with a scanning electron microscope, and the two types of carbon nanotubes contain a component that exhibits a first fiber diameter distribution and a component that exhibits a second fiber diameter distribution; and an electrode active material.

[0005] Patent Document 2 discloses a method for manufacturing an electrode, which includes a carbon nanotube dispersion electrode active material containing bundled carbon nanotubes, a dispersion medium, and partially hydrogenated nitrile rubber having a residual double bond (RDB) value of 0.5 to 40 mass%, wherein the dispersed particle size of the carbon nanotubes has a particle size distribution D50 of 3 to 10 μm, and a binder resin to manufacture an electrode slurry, and a step of forming an electrode using the electrode slurry.

[0006] Patent Document 3 discloses an electrode for a non-aqueous electrolyte secondary battery, which contains an active material, a binder, carbon nanotubes, and a non-fibrous conductive carbon material, and is characterized in that a polyvinylpyrrolidone-based polymer is contained in an amount of 5 to 25 parts by mass per 100 parts by mass of the carbon nanotubes.

[0007] Patent Document 4 discloses an electrode slurry containing a carbon nanotube dispersion liquid including bundled carbon nanotubes, a dispersion medium, and a polyvinyl butyral resin having a mass average molecular weight of more than 50,000, and characterized in that the dispersed particle size of the bundled carbon nanotubes has a particle size distribution D50 of 3 to 10 μm, an electrode active material, and a binder resin.

[0008] Generally, non-aqueous slurries containing carbon nanofibers tend to produce slurries with low dispersibility and high viscosity. For this reason, dispersants are added, but the improvement effect is not always sufficient. On the other hand, Patent Documents 1 to 4 do not mention the metal content in carbon nanotube dispersions. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] JP 2022-99288 A (claims, examples, etc.) [Patent Document 2] JP 2020-19705 A (claims, examples, etc.) [Patent Document 3] International Publication No. 2012 / 114590 (Claims, Examples, etc.) [Patent Document 4] JP 2018-535284 A (claims, examples, etc.) Summary of the Invention [Problem to be solved by the invention]

[0010] Further improvements are needed for slurry compositions containing carbon nanofibers when preparing battery electrodes. Highly viscous slurries have poor coating properties on substrates when preparing electrodes, necessitating the use of increased amounts of solvent, which results in longer drying times after coating. Furthermore, electrodes prepared using such slurry compositions often have high electrical resistance, making them unsuitable for use in lithium-ion secondary batteries and other applications.

[0011] The problem to be solved by the present invention is to provide a carbon nanotube slurry having low viscosity, and to provide a positive electrode composition for producing a positive electrode having low electrical resistance and excellent charge / discharge characteristics. [Means for solving the problem]

[0012] The present inventors have discovered that the concentration of alkaline earth metal contained in a carbon nanotube slurry plays an important role in the behavior of carbon nanotubes in the slurry, and have completed the present invention.

[0013] The present invention relates to a carbon nanotube slurry comprising carbon nanotubes, a non-aqueous dispersant, and a non-aqueous polar solvent, wherein the content of alkaline earth metal relative to the content of carbon nanotubes is 5 ppm to 2000 ppm. Another aspect of the present invention relates to a positive electrode dispersion containing the carbon nanotube slurry, a positive electrode active material, carbon black, graphite, and a binder, and a positive electrode for a secondary battery produced from the positive electrode dispersion. [Effects of the Invention]

[0014] The carbon nanotube slurry of the present invention has high fluidity and is therefore easy to produce and handle. In addition, the positive electrode dispersion made using the carbon nanotube slurry of the present invention has low viscosity, so that it can be easily coated and dried to prepare a positive electrode. Furthermore, the positive electrode prepared from the positive electrode dispersion of the present invention has low electrical resistance and high capacity retention rate after charge / discharge and repeated cycle retention rate, so it is particularly useful as a positive electrode for a lithium ion secondary battery. DETAILED DESCRIPTION OF THE INVENTION

[0015] <Carbon nanotube slurry> The carbon nanotube slurry of the present invention comprises carbon nanotubes, a non-aqueous dispersant, and a non-aqueous polar solvent, and is characterized in that the content of alkaline earth metal relative to the content of carbon nanotubes is 5 ppm to 2000 ppm.

[0016] As the carbon nanotube, a carbon nanotube (CNT) having a shape essentially formed by rolling one surface of graphite into a cylindrical shape is preferred, and either a single-walled carbon nanotube in which one surface of graphite is rolled into one layer, or a multi-walled carbon nanotube in which one surface of graphite is rolled into two or more layers can be used.

[0017] Examples of the form of carbon nanotubes include, but are not limited to, graphite whiskers, filamentous carbon, graphite fibers, ultrafine carbon tubes, carbon tubes, carbon fibrils, carbon microtubes, and carbon nanofibers, and these may be used alone or in combination of two or more types (hereinafter simply referred to as "at least one type").

[0018] From the viewpoints of viscosity, conductivity, and stability of the slurry, the average outer diameter of the carbon nanotubes is preferably 1 nm or more and 90 nm or less, more preferably 3 nm or more and 30 nm or less, and even more preferably 3 nm or more and 15 nm or less. Here, the average outer diameter of the carbon nanotubes refers to the arithmetic mean value of the outer diameters of a sufficient number n of carbon nanotubes measured using an image of a transmission electron microscope at a magnification of 100,000 times or more.

[0019] The purity of the carbon nanotubes used in the present invention is preferably 90 to 100% by mass, particularly preferably 95 to 100% by mass. The purity of the carbon nanotubes is calculated based on the amount of impurities, with ash content measured in accordance with JIS K 1469 and JIS K 6218 being considered as impurities.

[0020] Specific examples of carbon nanotubes used in the present invention include NC7000 (average outer diameter 10 nm) manufactured by Nanocyl Corporation, Baytubes C150P (average outer diameter 11 nm) manufactured by Bayer, FloTube 9000 (average outer diameter 19 nm), FloTube 7320 (average outer diameter 9 nm), FloTube 7010 (average outer diameter 9 nm), FloTube 6810 (average outer diameter 8 nm), FloTube 6120 (average outer diameter 8 nm), FloTube 6100 (average outer diameter 8 nm), FloTube 2020 (average outer diameter 4 nm) manufactured by Cnano, and MEIJO eDIPS manufactured by Meijo Nano Carbon Co., Ltd. At least one selected from EC2.0 (average outer diameter 2.0 nm), KORBON-A7 (average outer diameter 1.2 nm) manufactured by KORBON Co., Ltd., NFT-7 (average outer diameter 30 nm) manufactured by Koatsu Gas Kogyo Co., Ltd., and NFT-15 (average outer diameter 30 nm) manufactured by Koatsu Gas Kogyo Co., Ltd. can be used.

[0021] In the carbon nanotube slurry of the present invention, the content of carbon nanotubes is preferably 0.05% by mass to 8.00% by mass, more preferably 0.10% by mass to 5.00% by mass, based on the total amount of the carbon nanotube slurry.

[0022] The carbon nanotube slurry of the present invention is blended with a non-aqueous dispersant. Non-aqueous dispersants include polymeric, low molecular weight, and inorganic types, but polymeric dispersants are preferred. More preferred is one or more polymeric dispersants selected from the group consisting of hydrogenated nitrile rubber, polyvinylpyrrolidone, and polyvinyl butyral.

[0023] The content of the non-aqueous dispersant in the carbon nanotube slurry of the present invention is preferably 0.01 mass % or more, more preferably 0.03 mass % or more, and even more preferably 0.05 mass % or more, relative to the total amount of the carbon nanotube slurry, and is preferably 5.00 mass % or less, more preferably 3.00 mass % or less, and even more preferably 2.00 mass % or less. The non-aqueous dispersant improves the fluidity of the carbon nanotube slurry to some extent.

[0024] The hydrogenated nitrile rubber used as the non-aqueous dispersant of the present invention is a hydrogenated copolymer of a conjugated diene compound, an α,β-unsaturated nitrile compound, and other copolymerizable comonomers. The hydrogenated nitrile rubber includes not only a completely hydrogenated copolymer but also a partially hydrogenated copolymer.

[0025] Examples of conjugated dienes that are raw materials for the copolymers that constitute the hydrogenated nitrile rubber include conjugated diene compounds having 4 to 6 carbon atoms, such as 1,3-butadiene, isoprene, and 2,3-methylbutadiene. These may be used alone or in combination of two or more.

[0026] Examples of the α,β-unsaturated nitrile that is a raw material for the copolymer that constitutes the hydrogenated nitrile rubber include acrylonitrile and methacrylonitrile, and one of these may be used alone, or a mixture of two or more of them may be used. The content of structural units derived from an α,β-unsaturated nitrile in the hydrogenated nitrile rubber is preferably 20 to 50% by mass, more preferably 25 to 45% by mass, based on the total mass of the hydrogenated nitrile rubber.

[0027] Furthermore, other copolymerizable comonomers that serve as raw materials for the copolymers that constitute hydrogenated nitrile rubber include aromatic vinyl compounds (e.g., styrene, α-methylstyrene, vinylpyridine, fluoroethyl vinyl ether), α,β-unsaturated carboxylic acids (e.g., acrylic acid, methacrylic acid, maleic acid, fumaric acid), esters or amides of α,β-unsaturated carboxylic acids (e.g., methyl (meth)acrylate, ethyl (meth)acrylate, n-dodecyl (meth)acrylate, methoxymethyl (meth)acrylate, hydroxyethyl (meth)acrylate, polyethylene glycol (meth)acrylate, (meth)acrylamide), and anhydrides of α,β-unsaturated dicarboxylic acids (e.g., maleic anhydride, itaconic anhydride, citraconic anhydride). However, the examples are not limited to these.

[0028] The molecular weight of the hydrogenated nitrile rubber is preferably 10,000 to 700,000, more preferably 50,000 to 600,000, and even more preferably 100,000 to 350,000. Here, the mass average molecular weight is a polystyrene-equivalent molecular weight measured by gel permeation chromatography (GPC). When the hydrogenated nitrile rubber has a mass average molecular weight within the above range, carbon nanotubes can be effectively dispersed in a solvent.

[0029] As the polyvinylpyrrolidone used as the non-aqueous dispersant of the present invention, a polymer of N-vinyl-2-pyrrolidone can be used. The viscosity average molecular weight of polyvinylpyrrolidone is preferably 5,000 or more, more preferably 7,000 or more, and even more preferably 10,000 or more, and is preferably 300,000 or less, more preferably 200,000 or less, and even more preferably 100,000 or less.

[0030] The viscosity average molecular weight of the polyvinylpyrrolidone can be measured by the molecular weight measurement method described in Kobunshi Ronbunshu, vol. 38, No. 7, pp. 457-463 (July 1981). Specifically, it can be measured by the following procedure: (1) The sample is purified by precipitation by adding it to an excess of acetone. This procedure is repeated twice, and then the sample is dried under reduced pressure until the acetone odor disappears. (2) Prepare an aqueous solution of the purified sample, and use an Ubbelohde viscometer (water, 120 seconds) to calculate the viscosity average molecular weight using the following formula.

[0031] [Formula 1] [η]=0.393M 0.59 The above equation is the relationship between intrinsic viscosity [η] and viscosity-average molecular weight, obtained by substituting the constants K and a in the Mark-Kuhn-Houwink equation (Mark-Houwink-Sakurada equation) below with reference to the above-mentioned collection of papers. Note that other methods may be used to measure viscosity-average molecular weight as long as they can produce equivalent results.

[0032] The polyvinyl butyral used as the non-aqueous dispersant of the present invention may be a resin obtained by reacting polyvinyl alcohol with butyral. This resin contains a cyclic group in which butyral is added to two hydroxyl groups, and may also contain residual hydroxyl groups derived from the polyvinyl alcohol. The weight average molecular weight of the polyvinyl butyral is preferably 50,000 or more, more preferably 100,000 to 500,0000. Two or more types of polyvinyl butyral having different weight average molecular weights may be combined.

[0033] The carbon nanotube slurry of the present invention contains a non-aqueous polar solvent, which is an organic solvent having polarity. The non-aqueous polar solvent used in the present invention is substantially free of water. The water content is, for example, preferably 2000 ppm or less, more preferably 1500 ppm or less, and even more preferably 1000 ppm or less. If the non-aqueous polar solvent contains a large amount of water, it is undesirable because it will cause decomposition of the coexisting positive electrode active material in the positive electrode dispersion prepared using the non-aqueous polar solvent.

[0034] Examples of the non-aqueous polar solvent to be used 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;

[0035] Examples include glycerols such as glycerol, diglycerol, and triglycerol; 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, dimethylformamide, and dimethylacetamide; ketones such as acetone; and esters such as ethylene carbonate, ethyl methyl carbonate, and dimethyl methyl carbonate.

[0036] The non-aqueous polar solvent used in the present invention is preferably an aprotic polar solvent, more preferably an amide or an ester, and even more preferably N-methyl-2-pyrrolidone.

[0037] The carbon nanotube slurry of the present invention contains carbon nanotubes, a non-aqueous dispersant, and a non-aqueous polar solvent, but other materials may be added as long as they do not change the spirit of the present invention.

[0038] The carbon nanotube slurry of the present invention has an alkaline earth metal content of 5 ppm to 2000 ppm, preferably 10 ppm to 1500 ppm, more preferably 15 ppm to 1000 ppm, and most preferably 20 ppm to 600 ppm relative to the carbon nanotube content. In the present invention, the ratio of the total amount of elements belonging to alkaline earth metals to the carbon nanotubes in the carbon nanotube slurry is referred to as the alkaline earth metal content relative to the carbon nanotubes.

[0039] Here, the alkaline earth metal content (ppm) is the value obtained by dividing the total amount (μg) of alkaline earth metal atoms contained in a given amount of carbon nanotube slurry by the mass (g) of carbon nanotubes contained in the carbon nanotube slurry.

[0040] The alkaline earth metal content refers to the total amount of alkaline earth metal atoms contained in the carbon nanotube slurry, regardless of the chemical form of the element. Alkaline earth metals include calcium, strontium, barium, radium, beryllium, and magnesium, and it is necessary to control the total amount of these metal atoms. The alkaline earth metal contained in the carbon nanotube slurry can be quantitatively determined by measuring it using an ICP (inductively coupled plasma emission spectrometer).

[0041] The alkaline earth metal content of the carbon nanotube slurry of the present invention can be adjusted by controlling the alkaline earth metal content of the raw carbon nanotubes, non-aqueous dispersant, and non-aqueous polar solvent. In particular, many commercially available non-aqueous dispersants contain alkaline earth metals, and the alkaline earth metal content of the carbon nanotube slurry can be adjusted by selecting a non-aqueous dispersant product containing an appropriate amount of alkaline earth metal.

[0042] The carbon nanotube slurry of the present invention has a low viscosity. Although the mechanism by which the viscosity of the carbon nanotube slurry is reduced is not clear, it is presumed that the amount of alkaline earth metal contained in the slurry reduces the Coulomb force due to the surface charge between the carbon nanotubes, thereby increasing the affinity between the carbon nanotube chains and the non-aqueous polar solvent.

[0043] The carbon nanotube slurry of the present invention can be prepared by putting its constituent components, that is, at least the carbon nanotubes, the non-aqueous dispersant, and the non-aqueous polar solvent, into a disperser and mixing them.

[0044] Examples of dispersing machines that can be used include homomixers, high-pressure homogenizers, ultra-high-pressure homogenizers, ultrasonic dispersing machines, planetary mixers, combimixes, kneaders, planetary mixers, combimixers, kneaders, planetary mixers, kneaders, Henschel mixers, ball mills, bead mills, thin film rotary high-speed agitators, screw mixers, paddle mixers, disperser mixers, turbine mixers, propeller mixers, blenders, ultrasonic homogenizers, colloid mills, pebble mills, beaters, disc refiners, conical refiners, double disc refiners, grinders, etc. It is also preferable to use two or more types of dispersing machines in combination.

[0045] <Dispersion for positive electrode> By mixing at least a positive electrode active material, carbon black, graphite, and a binder with the carbon nanotube slurry of the present invention, a positive electrode dispersion for producing a positive electrode of a secondary battery can be prepared.

[0046] Examples of the positive electrode active material include composite oxides of lithium and transition metals such as 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, and lithium-nickel-cobalt-manganese-aluminum composite oxide; transition metal sulfides such as TiS2, FeS, and MoS2; MnO, VO5, and VO 13 , transition metal oxides such as TiO2, and olivine-type lithium phosphate.

[0047] The olivine-type lithium phosphate oxide contains, for example, at least one element selected from the group consisting of Mn, Cr, Co, Cu, Ni, V, Mo, Ti, Zn, Al, Ga, Mg, B, Nb, and Fe, as well as lithium, phosphorus, and oxygen. These compounds may have some elements partially substituted with other elements to improve their properties.

[0048] A preferred positive electrode active material is a lithium-nickel composite oxide, and more preferably, the lithium-nickel composite oxide is represented by the formula: LiNiXM1YM2ZO2 (M1 and M2 are at least one metal element selected from the group consisting of 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 alone or in combination of two or more.

[0049] In the positive electrode dispersion of the present invention, the content of the positive electrode active material is preferably 40 to 70 mass %, more preferably 45 to 65 mass %, based on the total amount of the positive electrode dispersion, thereby ensuring the capacity of the battery.

[0050] The positive electrode dispersion of the present invention contains non-fibrous conductive carbon materials other than carbon nanotubes, namely, carbon black and graphite, which are materials that exhibit the effect of assisting conductivity in a positive electrode for a secondary battery produced from the positive electrode dispersion.

[0051] Carbon black that can be used is produced by incomplete combustion of hydrocarbons such as heavy aromatic oils and gases, and preferably acetylene black produced by thermal decomposition of acetylene.

[0052] The amount of carbon black in the positive electrode dispersion of the present invention is preferably 10% by mass or more and 2000% by mass or less, more preferably 50% by mass or more and 1500% by mass or less, and even more preferably 100% by mass or more and 1000% by mass or less, relative to the amount of carbon nanotubes.

[0053] As the graphite, layered graphite having a hexagonal plate-like structure can be used. The amount of graphite in the positive electrode dispersion of the present invention is preferably 10% by mass or more and 2000% by mass or less, more preferably 50% by mass or more and 1000% by mass or less, and even more preferably 100% by mass or more and 750% by mass or less, relative to the amount of carbon nanotubes.

[0054] The positive electrode dispersion of the present invention contains a binder. The binder acts to enhance adhesion between the coated positive electrode dispersion and the positive electrode substrate when preparing the positive electrode, and for example, polyvinylidene fluoride (PVDF) is used. The amount of binder in the positive electrode dispersion of the present invention is preferably 0.1% by mass or more and 20% by mass or less, more preferably 0.5% by mass or more and 10% by mass or less, and even more preferably 0.8% by mass or more and 5% by mass or less, based on the total amount of the positive electrode dispersion.

[0055] The positive electrode dispersion of the present invention may further contain other components within the scope of the present invention, such as a sulfide solid electrolyte, an oxide solid electrolyte, a dry polymer electrolyte, a gel polymer electrolyte, or a quasi-solid electrolyte.

[0056] It has been believed that the lower the content of metal ions other than lithium ions that make up the battery positive electrode, the better the battery performance. However, the positive electrode dispersion containing carbon nanotubes of the present invention contains alkaline earth metals in a predetermined amount range, and the positive electrode for a secondary battery made from such a positive electrode dispersion unexpectedly exhibits good battery performance.

[0057] The positive electrode dispersion of the present invention can be prepared by putting the carbon nanotube slurry, the positive electrode active material, carbon black, graphite, and a binder into a disperser and mixing them.

[0058] Examples of dispersing machines that can be used include homomixers, high-pressure homogenizers, ultra-high-pressure homogenizers, ultrasonic dispersing machines, planetary mixers, combimixes, kneaders, planetary mixers, combimixers, kneaders, planetary mixers, kneaders, Henschel mixers, ball mills, bead mills, thin film rotary high-speed agitators, screw mixers, paddle mixers, disperser mixers, turbine mixers, propeller mixers, blenders, ultrasonic homogenizers, colloid mills, pebble mills, beaters, disc refiners, conical refiners, double disc refiners, grinders, etc. It is also preferable to use two or more types of dispersing machines in combination.

[0059] <Positive electrodes for lithium-ion secondary batteries> The positive electrode dispersion of the present invention can be applied to an electrode substrate and dried to obtain a positive electrode for a secondary battery. The positive electrode dispersion of the present invention has a low viscosity, so that the operation of applying the dispersion to an electrode substrate can be carried out smoothly. The resulting positive electrode for secondary batteries has low electrical resistance, can withstand high C-rate charge / discharge, and has excellent cycle characteristics, and can be effectively used in lithium-ion secondary batteries. [Example]

[0060] Example 1 <<Preparation of carbon nanotube slurry>> 100 parts by mass of carbon nanotube slurry was prepared by performing 10 dispersion operations using a high-pressure homogenizer at a dispersion pressure of 100 MPa using 4 parts by mass of multi-walled carbon nanotubes (FT7010, manufactured by Cnano), 1 part by mass of dispersant A (hydrogenated nitrile rubber 1) as a dispersant, and 95 parts by mass of N-methyl-2-pyrrolidone as a non-aqueous polar solvent.

[0061] <Alkaline earth metal content> The carbon nanotube slurry thus prepared was precisely weighed, burned, and the resulting ash was dissolved in a predetermined amount of aqueous nitric acid to obtain a sample solution. The concentrations of various alkaline earth metals were measured using a sequential ICP atomic emission spectrometer, and the alkaline earth metal content in the entire sample solution was calculated and summed to obtain the alkaline earth metal amount (μg). This alkaline earth metal amount (μg) was divided by the carbon nanotube amount (g) calculated from the carbon nanotube concentration in the carbon nanotube slurry to obtain the alkaline earth metal content (ppm) relative to the carbon nanotubes. The results are shown in Table 1.

[0062] <viscosity> The prepared carbon nanotube slurry was subjected to shear rate of 0.1 s at a sample temperature of 25°C using a rheometer (Anton Paar, Modular Compact Rheometer MCR102, 50 mm diameter, 2° cone). -1 or 1000s -1 The viscosity was measured at each temperature, and the results are shown in Table 1.

[0063] <<Preparation of Positive Electrode Dispersion>> 20 parts by mass of the carbon nanotube dispersion liquid prepared above, 100 parts by mass of LiNi as a positive electrode active material 0.5 Co 0.2 Mn 0.3 O2, 1.5 parts by mass of acetylene black, 1.0 part by mass of graphite, 2 parts by mass of polyvinylidene fluoride (Solef5130, manufactured by Solvay) as a binder, and 30 parts by mass of N-methyl-2-pyrrolidone as a solvent were mixed and mechanically dispersed using a planetary mixer to prepare a dispersion for the positive electrode.

[0064] <viscosity> The prepared positive electrode dispersion was subjected to the same procedure as above at a sample temperature of 25°C and a shear rate of 0.1 s -1 or 1000s -1 The viscosity was measured at each temperature, and the results are shown in Table 2.

[0065] <Coating stability> The resulting positive electrode dispersion was applied to one side of aluminum foil at a wet film thickness of 100 μm using a comma coater with a coating width of 200 mm. Streaks that appeared in the resulting coating film and were 10 mm or longer were considered defects and evaluated according to the following criteria. When multiple streaks appeared, the total length of each was used as the standard. The results are shown in Table 2. When the viscosity of the positive electrode dispersion was high, streaks tended to appear as coating defects. Evaluation criteria: ◎: No streaks longer than 10 mm were present. ○: The total length of the muscle was 10 mm or more and less than 50 mm. △: The total length of the muscle was 50 mm or more and less than 100 mm. ×: The total length of the muscle was 100 mm or more.

[0066] <<Preparation of a positive electrode for a secondary battery>> The obtained dispersion for the positive electrode was applied to one side of a PET film (Toray Industries, Inc., Lumira #100-T60) using an applicator so that the liquid film was 50 μm thick, and then dried at room temperature for 30 minutes, and then further dried at 80°C for 5 minutes to prepare a positive electrode for a secondary battery.

[0067] <Electrode sheet resistance> The electrode sheet resistance (kΩ / □) of the fabricated positive electrode for secondary battery was measured using a device consisting of a four-point probe with a probe spacing of 10 mm and a resistance meter (Hioki E.E. Corporation, Milliohm HiTester 3227). The results are shown in Table 2.

[0068] <Capacity retention rate after 10C charge / discharge> The resulting positive electrode dispersion was coated on one side of aluminum foil using a comma coater with a coating width of 200 mm to a wet film thickness of 100 μm, and then dried for 5 minutes in a drying oven at 100°C to obtain an electrode. Five coin-type secondary batteries were fabricated using the resulting electrode as the positive electrode, a carbonate-based solvent as the electrolyte, a polypropylene film as the separator, and metallic lithium as the negative electrode. The prepared coin secondary batteries were charged and discharged once at a charge / discharge rate of 1C (a) and then once at a charge / discharge rate of 10C (b) to measure the discharge capacity, and the capacity retention rate after 10C charge / discharge was calculated using Equation 2. The average capacity retention rate for five coin secondary batteries was calculated and used as the capacity retention rate after 10C charge / discharge (%). The results are shown in Table 2.

[0069] [Formula 2] Capacity retention rate after 10C charge / discharge=b / a×100(%)

[0070] <100 cycles maintenance rate> Using the prepared coin secondary batteries, the initial discharge capacity (A) at 0.2 C and the discharge capacity (B) when charged and discharged at 0.2 C after 100 cycles of charging and discharging at 10 C were measured, and the 100 cycle retention rate was calculated using Equation 3. The average value for five coin secondary batteries was calculated, and the average value was applied to the following evaluation criteria to evaluate the 100 cycle retention rate. The results are shown in Table 2.

[0071] [Formula 3] 100 cycles retention rate = B / A x 100 (%)

[0072] Evaluation criteria: ◎: 98% or more ○: 95% or more to less than 98% △: 85% to less than 95% ×: Less than 85%

[0073] Example 2 A carbon nanotube slurry was prepared in the same manner as in Example 1, except that 1 part by mass of single-walled carbon nanotubes (TUBALL, manufactured by OCSIAL) was used instead of 4 parts by mass of multi-walled carbon nanotubes, the amount of dispersant A (hydrogenated nitrile rubber 1) was changed to 0.25 parts by mass, and the amount of solvent was increased. The measurement results of the alkaline earth metal content (ppm) and viscosity of the carbon nanotube slurry are shown in Table 1. Next, a positive electrode dispersion was prepared using the prepared carbon nanotube slurry in the same manner as in Example 1, and a positive electrode for a secondary battery was fabricated from the positive electrode dispersion. Table 2 shows the measurement results of the electrode sheet resistance, capacity retention rate after 10 C charge / discharge, and 100 cycle retention rate of the fabricated positive electrode for a secondary battery.

[0074] Examples 3 to 5 A carbon nanotube slurry was prepared in the same manner as in Example 1, except that 1 part by mass of a dispersant shown in Table 1 was used instead of 1 part by mass of dispersant A (hydrogenated nitrile rubber 1). The alkaline earth metal content (ppm) and viscosity of the carbon nanotube slurry were measured and shown in Table 1. Next, a positive electrode dispersion was prepared using the prepared carbon nanotube slurry in the same manner as in Example 1, and a positive electrode for a secondary battery was fabricated from the positive electrode dispersion. Table 2 shows the measurement results of the electrode sheet resistance, capacity retention rate after 10 C charge / discharge, and 100 cycle retention rate of the fabricated positive electrode for a secondary battery.

[0075] Comparative Examples 1 to 4 A carbon nanotube slurry was prepared in the same manner as in Example 1, except that 1 part by mass of a dispersant shown in Table 1 was used instead of 1 part by mass of dispersant A (hydrogenated nitrile rubber 1). The alkaline earth metal content (ppm) and viscosity measurement results of the carbon nanotube slurry are shown in Table 1. The alkaline earth metal content (ppm) relative to the carbon nanotubes in the carbon nanotube slurry did not fall within the range specified in the present invention. Next, a positive electrode dispersion was prepared using the prepared carbon nanotube slurry in the same manner as in Example 1, and a positive electrode for a secondary battery was fabricated from the positive electrode dispersion. Table 2 shows the measurement results of the electrode sheet resistance, capacity retention rate after 10 C charge / discharge, and 100 cycle retention rate of the fabricated positive electrode for a secondary battery.

[0076] [Table 1]

[0077] [Table 2]

[0078] As is clear from the results in Table 1, the carbon nanotube slurries of Examples 1 to 5, in which the content (ppm) of alkaline earth metal relative to the carbon nanotubes in the carbon nanotube slurry was within the range specified in the present invention, had lower viscosity than the carbon nanotube slurries of Comparative Examples 1 to 4, in which the content (ppm) was outside the range specified in the present invention. Therefore, the carbon nanotube slurries of the present invention had high fluidity and were easy to produce and handle.

[0079] As is clear from the results in Table 2, the secondary battery positive electrodes of Examples 1 to 5, which were made from positive electrode dispersions prepared using carbon nanotube slurries in which the alkaline earth metal content (ppm) relative to the carbon nanotubes in the carbon nanotube slurry was within the range specified in the present invention, had lower electrode sheet resistance and higher capacity retention after charge / discharge and repeated cycle retention than the secondary battery positive electrodes of Comparative Examples 1 to 4, which were made from positive electrode dispersions prepared using carbon nanotube slurries in which the alkaline earth metal content (ppm) relative to the carbon nanotubes in the carbon nanotube slurry was outside the range specified in the present invention. Therefore, the electrodes made from the positive electrode dispersions containing the carbon nanotube slurry of the present invention were useful as secondary battery positive electrodes. [Industrial Applicability]

[0080] The present invention can be used to manufacture positive electrodes for lithium ion secondary batteries.

Claims

1. A carbon nanotube slurry comprising carbon nanotubes, a non-aqueous dispersant, and a non-aqueous polar solvent, the carbon nanotube slurry having an alkaline earth metal content of 5 ppm to 2000 ppm relative to the carbon nanotube content.

2. 2. The carbon nanotube slurry according to claim 1, wherein the non-aqueous dispersant is at least one selected from the group consisting of hydrogenated nitrile rubber, polyvinylpyrrolidone, and polyvinyl butyral.

3. 2. The carbon nanotube slurry according to claim 1, wherein the carbon nanotube content is 0.05% by mass or more and 8.00% by mass or less, and the non-aqueous dispersant content is 0.01% by mass or more and 5.00% by mass or less, based on the total amount of the carbon nanotube slurry.

4. A positive electrode dispersion comprising the carbon nanotube slurry according to any one of claims 1 to 3, a positive electrode active material, carbon black, graphite, and a binder.

5. A positive electrode for a secondary battery, produced from the positive electrode dispersion according to claim 4.

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