Non-aqueous carbon nanotube slurry and dispersion for positive electrode

Abstract

Provided are: a non-aqueous carbon nanotube slurry having favorable handleability; and a composition for providing a positive electrode having low electric resistance.　Moreover, provided are: a carbon nanotube slurry characterized by comprising carbon nanotubes, a non-aqueous dispersant, and a non-aqueous polar solvent, wherein the content of alkaline earth metal ions is 5-2,000 ppm with respect to the carbon nanotubes; and a dispersion for a positive electrode, the dispersion being obtained by adding a positive electrode active material, carbon black, graphite, and a binding material to the carbon nanotube slurry.

Description

Non-aqueous carbon nanotube slurry and dispersion for positive electrode

[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.

[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 to form an intercalation compound in graphite and use the intercalation compound 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 producing 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 produce 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 relative to 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, wherein 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. Therefore, dispersants are added, but the improvement effect is not always sufficient. Meanwhile, Patent Documents 1 to 4 do not mention the metal content in carbon nanotube dispersions.

[0009] JP 2022-99288 A (claims, examples, etc.) JP 2020-19705 A (claims, examples, etc.) WO 2012 / 114590 A (claims, examples, etc.) JP 2018-535284 A (claims, examples, etc.)

[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.

[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, and characterized in that the alkaline earth metal content is 5 ppm to 2000 ppm relative to the carbon nanotube content. The present invention also relates to a positive electrode dispersion comprising 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.

[0014] The carbon nanotube slurry of the present invention has high fluidity and is therefore easy to produce and handle. Furthermore, the positive electrode dispersion made using the carbon nanotube slurry of the present invention has low viscosity, making it easy to coat and dry to produce a positive electrode. Furthermore, the positive electrode electrode made from the positive electrode dispersion of the present invention has low electrical resistance and high capacity retention and repeated cycle retention after charge and discharge, making it particularly useful as a positive electrode for a lithium ion secondary battery.

[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 alkaline earth metal content is 5 ppm to 2000 ppm relative to the carbon nanotube content.

[0016] As the carbon nanotube, a carbon nanotube (CNT) having a shape substantially formed by rolling one surface of graphite into a cylindrical shape is preferable, 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 the 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 or JIS K 6218 being considered as an impurity.

[0020] Specific examples of carbon nanotubes used in the present invention include NC7000 (average outer diameter 10 nm) manufactured by Nanocyl, 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), and FloTube 2020 (average outer diameter 4 nm) manufactured by Cnano, and MEIJOeDIPS manufactured by Meijo Nanocarbon 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 Corporation, 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 carbon nanotube content 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 dispersants, 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 the conjugated diene that is a raw material for the copolymer that constitutes the hydrogenated nitrile rubber include conjugated diene compounds having 4 to 6 carbon atoms, such as 1,3-butadiene, isoprene, and 2,3-methylbutadiene. One of these may be used alone, or two or more may be used as a mixture.

[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 these may be used alone or in combination. The content of structural units derived from the 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 copolymer that constitutes the hydrogenated nitrile rubber include, but are not limited to, 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).

[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] The polyvinylpyrrolidone used as the non-aqueous dispersant of the present invention may be a polymer of N-vinyl-2-pyrrolidone. The viscosity-average molecular weight of the 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 Polymer Review Vol. 38, No. 7, pp. 457-463 (July 1981). Specifically, it can be measured by the following procedure: (1) The sample is added to an excess of acetone to purify by precipitation. This procedure is repeated twice, and then the sample is dried under reduced pressure until the acetone odor disappears. (2) An aqueous solution of the purified sample is prepared, and the viscosity-average molecular weight is calculated using an Ubbelohde viscometer (water, 120 seconds) using the following formula:

[0031] [Formula 1] [η]=0.393M ０．５９ The above equation is a relational expression between the intrinsic viscosity [η] and the 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 the viscosity-average molecular weight may be measured using other methods as long as they can produce equivalent results.

[0032] The polyvinyl butyral used as the non-aqueous dispersant of the present invention can 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 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 with different weight-average molecular weights may be combined.

[0033] The carbon nanotube slurry of the present invention contains a non-aqueous polar solvent. A polar organic solvent is used as the non-aqueous polar solvent. 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 of the alkyl ether 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 predetermined 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 metals contained in the carbon nanotube slurry can be quantified by measuring using an inductively coupled plasma (ICP) optical 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] <Positive Electrode Dispersion> A positive electrode dispersion for producing a positive electrode of a secondary battery can be prepared by mixing at least a positive electrode active material, carbon black, graphite, and a binder with the carbon nanotube slurry of the present invention.

[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; TiS ２ , FeS, MoS ２ transition metal sulfides such as MnO, V ２ O ５ , V ６ O １３ , TiO ２ and transition metal oxides such as those mentioned above, and olivine-type lithium phosphate oxides.

[0047] The olivine-type lithium phosphate 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: LiNiXM １ Y.M. ２ ZO ２ (M １ and M ２ is at least one metal element selected from Al, B, alkali metals, alkaline earth metals, and transition metals, and a lithium-nickel composite oxide expressed as 0.8≦X≦1.0, 0≦Y≦0.2, 0≦Z≦0.2. These positive electrode active materials may be used singly 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] The graphite may be layered graphite having a tortoiseshell structure made of hexagonal plate-like crystals. 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, based on 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 producing a positive electrode, and polyvinylidene fluoride (PVDF) is used, for example. The amount of binder contained 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 electrode for lithium ion secondary battery> A positive electrode for a secondary battery can be obtained by applying the prepared positive electrode dispersion to an electrode substrate and drying it. The positive electrode dispersion of the present invention has a low viscosity, so the application to the electrode substrate can be easily performed. The positive electrode for a secondary battery obtained in this manner has low electrical resistance, can withstand high C-rate charge / discharge, and can be usefully used in lithium ion secondary batteries as a secondary battery positive electrode with excellent cycle characteristics.

[0060] Example 1 Preparation of Carbon Nanotube Slurry 4 parts by mass of multi-walled carbon nanotubes (FT7010, manufactured by Cnano Corporation), 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 were subjected to 10 dispersion operations using a high-pressure homogenizer at a dispersion pressure of 100 MPa, to prepare 100 parts by mass of carbon nanotube slurry.

[0061] <Alkaline Earth Metal Content> The prepared carbon nanotube slurry was precisely weighed, burned, and the resulting ash was dissolved in a predetermined amount of nitric acid water 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 measured using a rheometer (Anton Paar, Modular Compact Rheometer MCR102, diameter 50 mm, 2° cone) 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 of the following temperatures. The results are shown in Table 1.

[0063] <<Preparation of Positive Electrode Dispersion>> 20 parts by mass of the carbon nanotube dispersion prepared above, 100 parts by mass of LiNi as a positive electrode active material, ０．5 Co ０．2 Mn ０．3 O ２ 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 a positive electrode.

[0064] <Viscosity> The prepared dispersion for a positive electrode was measured in the same manner 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 of the following temperatures. The results are shown in Table 2.

[0065] <Coating Stability> The obtained positive electrode dispersion was applied to one side of an aluminum foil at a wet film thickness of 100 μm using a comma coater with a coating width of 200 mm to a thickness of 10 m. Streaks that appeared in the obtained 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 streak length was 10 mm or more but less than 50 mm. Δ: The total streak length was 50 mm or more but less than 100 mm. ×: The total streak length was 100 mm or more.

[0066] <<Preparation of Positive Electrode for Secondary Battery>> The obtained positive electrode dispersion was applied to one side of a PET film (Lumilar #100-T60, manufactured by Toray Industries, Inc.) 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 prepared 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 (Milliohm HiTester 3227, manufactured by Hioki E.E. Corporation). 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 in a drying oven at 100°C for 5 minutes 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 discharge capacity (a) of each of the fabricated coin secondary batteries was measured after one charge / discharge at a charge / discharge rate of 1C, followed by one charge / discharge at a charge / discharge rate of 10C (b). The capacity retention rate after 10C charge / discharge was calculated using Equation 2. The average capacity retention rate for the five coin secondary batteries was calculated and recorded 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 Cycle Retention Rate> Using the prepared coin secondary battery, the initial discharge capacity (A) at 0.2 C and the discharge capacity (B) when charged and discharged at 0.2 C after 100 repeated charge and discharge cycles 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 cycle retention rate = B / A × 100 (%)

[0072] Evaluation criteria: ◎: 98% or more ○: 95% or more to less than 98% △: 85% or more 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 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 secondary battery positive electrode was fabricated from the positive electrode dispersion. Table 2 shows the measurement results of the electrode sheet resistance, capacity retention after 10 C charge / discharge, and 100 cycle retention of the fabricated secondary battery positive electrode.

[0074] Examples 3 to 5 Carbon nanotube slurries were 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 slurries were measured and shown in Table 1. Next, a positive electrode dispersion was prepared using the prepared carbon nanotube slurries in the same manner as in Example 1, and a secondary battery positive electrode 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 secondary battery positive electrodes.

[0075] Comparative Examples 1 to 4 Carbon nanotube slurries were 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 slurries were measured and shown in Table 1. The alkaline earth metal content (ppm) relative to the carbon nanotubes in the carbon nanotube slurries was not within the range specified in the present invention. Next, a positive electrode dispersion was prepared using the prepared carbon nanotube slurries in the same manner as in Example 1, and a secondary battery positive electrode was fabricated from the positive electrode dispersion. The electrode sheet resistance, capacity retention after 10 C charge / discharge, and 100 cycle retention of the fabricated secondary battery positive electrodes are shown in Table 2.

[0076]

[0077]

[0078] As is clear from the results in Table 1, the carbon nanotube slurries of Examples 1 to 5, 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 viscosity than the carbon nanotube slurries of Comparative Examples 1 to 4, 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 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 rates and repeated cycle retention rates after charge and discharge, compared to 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 not within 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 positive electrodes for secondary batteries.

[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, wherein the alkaline earth metal content is 5 ppm to 2000 ppm relative to the carbon nanotube content.

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. A 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 dispersion for a positive electrode 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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