Carbon nanotube dispersion

Abstract

To provide a new carbon nanotube dispersion having good dispersibility.SOLUTION: A carbon nanotube dispersion of the present invention comprises: at least a carbon nanotube, a solvent, and polyvinyl acetal. The ratio of an intensity of a peak at 2950 cm-1 to an intensity of a peak at 1650 cm-1 in FT-IR spectrum of the polyvinyl acetal obtained by ATR method is 50 or more. The mass average molecular weight of the polyvinyl acetal measured by gel permeation chromatography is 100,000 or less.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to a carbon nanotube dispersion, and in particular to a carbon nanotube dispersion for use as a positive electrode dispersion used in electrodes of lithium ion secondary batteries and the like. [Background technology]

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

[0003] For example, Patent Document 1 discloses an invention relating to a carbon nanotube dispersion with improved dispersibility, a method for producing the carbon nanotube dispersion, a method for producing an electrode slurry and an electrode using the carbon nanotube dispersion, and an electrode produced by the method and a battery including the electrode.

[0004] Furthermore, Patent Document 2 discloses a carbon nanotube dispersion and a carbon nanotube resin composition for obtaining a highly conductive electrode film by using specific carbon nanotubes.

[0005] Furthermore, Patent Document 3 discloses a carbon nanotube dispersion and a carbon nanotube resin composition for obtaining an electrode film with high adhesion and conductivity by using specific carbon nanotubes and a resin containing a vinyl alcohol skeleton. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Patent No. 6765685 [Patent Document 2] Patent No. 2020-01960 [Patent Document 3] Patent No. 2020-11873 Summary of the Invention [Problem to be solved by the invention]

[0007] However, all of the carbon nanotube dispersions and electrode dispersions still had high viscosities, which meant that the yields in the manufacturing processes for the carbon nanotube dispersions and electrode dispersions were insufficient, and the pressure loss during liquid transport was large, making it difficult to increase the solid content.

[0008] Furthermore, since the fluidity in the piping and coating section of the coating machine is still insufficient, problems occur when coating the electrode dispersion onto the current collector foil, causing defects in the electrode, and as a result, sufficient electrode yields are not obtained in some cases.

[0009] Furthermore, these conventional techniques still provide insufficient dispersion of carbon nanotubes, and therefore are unable to sufficiently reduce the resistance of electrodes using carbon nanotube dispersions, and do not have output characteristics or cycle characteristics that fully utilize the properties of carbon nanotubes.

[0010] Therefore, the present invention provides a novel carbon nanotube dispersion having good dispersibility. [Means for solving the problem]

[0011] As a result of intensive research, the present inventors have found that the above problems can be solved by the following means, and have completed the present invention. That is, the present invention is as follows: Aspect 1: A composition comprising at least carbon nanotubes, a solvent, and a polyvinyl acetal represented by the following formula (I): In the FT-IR spectrum of the polyvinyl acetal obtained by the ATR method, ―1 Intensity of the peak at 2950 cm-1 is 50 or more, and The polyvinyl acetal has a mass average molecular weight of 100,000 or less as measured by gel permeation chromatography. Carbon nanotube dispersion. [ka] (In the formula, l, m, and n are positive integers, and R is an alkyl group having 1 to 20 carbon atoms.) Aspect 2: The carbon nanotube dispersion according to aspect 1, wherein a 10% by mass solution of the polyvinyl acetal in NMP has an absorbance at 320 nm of 0.50 or more. <Aspect 3> The carbon nanotube dispersion according to aspect 1 or 2, wherein a 10 mass % solution of the polyvinyl acetal in NMP has a viscosity of 21.0 mPa·s or more and 31.0 mPa·s or less at a shear rate of 76.6 / s. <Aspect 4> The carbon nanotube dispersion according to any one of Aspects 1 to 3, wherein the polyvinyl acetal has a mass average molecular weight of 30,000 to 50,000 as measured by gel permeation chromatography. <Aspect 5> A carbon nanotube dispersion according to any one of aspects 1 to 4, and a positive electrode active material are included, The mass of the carbon nanotubes is 0.1 to 6.0 parts by mass relative to 100 parts by mass of the positive electrode active material. Dispersion for positive electrodes. Effect of the Invention

[0012] According to the present invention, it is possible to provide a novel carbon nanotube dispersion having good dispersibility. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] Carbon nanotube dispersion The carbon nanotube dispersion of the present invention is The present invention includes at least carbon nanotubes, a solvent, and a polyvinyl acetal represented by the following formula (I), In the FT-IR (Fourier transform infrared spectroscopy) spectrum of the polyvinyl acetal obtained by total attenuation measurement (ATR method), ―1 Intensity of the peak at 2950 cm -1 is 50 or more, and The polyvinyl acetal has a mass average molecular weight of 100,000 or less as measured by gel permeation chromatography. A carbon nanotube dispersion. [ka] (In the formula, l, m, and n are positive integers, and R is an alkyl group having 1 to 20 carbon atoms.)

[0014] According to the above-mentioned arrangement, a carbon nanotube dispersion having good dispersibility can be obtained.

[0015] In this specification, "2950 cm -1 The peak near the 2950 cm -1 200cm before and after -1 , front and rear 150cm -1 , front and rear 100cm -1 , front and back 50cm -1 , or 30cm before and after -1 This refers to the largest peak in the region of 3400 cm. -1 "Peaks near 1650cm" and " -1 The "peak in the vicinity" should be interpreted in a similar manner.

[0016] In the present invention, the ATR method uses diamond as the crystal, and is performed using a sample that has been dried in an environment with a dew point temperature of minus 30° C. for three days before measurement.

[0017] Each component of the present invention will now be described.

[0018] Carbon nanotubes As the carbon nanotube, a known carbon nanotube can be used.

[0019] The average particle size of the carbon nanotubes can be 100 nm or more, 200 nm or more, 300 nm or more, 500 nm or more, 700 nm or more, 1 μm or more, 2 μm or more, or 3 μm or more, and can be 20 μm or less, 15 μm or less, 10 μm or less, or 7 μm or less. Here, the average particle size adopted in this specification is appropriately selected depending on the size of the target carbon particles, and in the case of particles less than about 1 μm, it is the value of the histogram average particle size (D50) based on the scattering intensity distribution measured by the dynamic light scattering method, and in the case of particles of 1 μm or more, it is the value of the median size (D50) calculated on a volume basis in the laser diffraction method. Measurement by the dynamic light scattering method can be performed using, for example, DelsaMax CORE (Beckman Coulter, Inc.). Measurement by the laser diffraction method can be performed using, for example, a particle size distribution measuring device MT3300II (Microtrack Bell, Inc.). The average particle size of carbon nanotubes refers to secondary particles formed by the single fibers of carbon nanotubes forming bundles or balls, and can be measured by suspending the carbon nanotubes in any solution.

[0020] The carbon nanotube content in the nonaqueous slurry for a secondary battery positive electrode may be 0.1 mass% or more, 0.3 mass% or more, 0.5 mass% or more, or 0.6 mass% or more, and may be 10.0 mass% or less, 8.0 mass% or less, 5.0 mass% or less, 3.0 mass% or less, 2.5 mass% or less, 2.0 mass% or less, 1.5 mass% or less, 1.0 mass% or less, or 0.8 mass% or less, based on the total mass of the nonaqueous slurry for a secondary battery positive electrode.

[0021] <solvent> As the solvent, any organic solvent can be used, for example, a protic polar solvent or an aprotic polar solvent. These solvents can be used alone or in combination.

[0022] The protic polar liquid medium is generally a solvent containing acidic hydrogen. Examples of the protic polar liquid medium include a phenol-based liquid medium, a monohydric alcohol-based liquid medium, and a polyhydric alcohol-based liquid medium.

[0023] As the phenol-based liquid medium, for example, cresols and the like can be used.

[0024] As the monohydric alcohol liquid medium, ethanol, n-propanol, isopropanol, n-butanol, isobutanol, tert-butyl alcohol, 1-pentanol, isoamyl alcohol, sec-amyl alcohol, 3-pentanol, tert-amyl alcohol, n-hexanol, and the like can be used.

[0025] Examples of polyhydric alcohol liquid media that can be used include glycol liquid media such as propylene glycol, triethylene glycol, tetraethylene glycol, diglyme, and polyethylene glycols having a molecular weight of 600 or less, and glycerin.

[0026] The aprotic polar liquid medium is generally a solvent that does not contain acidic hydrogen. For example, N,N-dimethylformamide, N,N-dimethylacetamide, N-methyl-2-pyrrolidone, etc. can be used as the aprotic polar liquid medium. Among them, N-methyl-2-pyrrolidone is preferably used from the viewpoint of affinity with polyvinyl acetal.

[0027] <Polyvinyl acetal> The polyvinyl acetal in the carbon nanotube dispersion of the present invention is constituted by the following formula. [ka] (In the formula, l, m, and n are positive integers, and R is an alkyl group having 1 to 20 carbon atoms.)

[0028] The polyvinyl acetal of the present invention has a peak at 1650 cm as measured by FT-IR.―1 Intensity of the peak at 2950 cm -1 is greater than or equal to 50. This ratio may be 50 or greater, 55 or greater, 60 or greater, 65 or greater, 70 or greater, 75 or greater, 80 or greater, 85 or greater, 90 or greater, 95 or greater, or 100 or greater.

[0029] I don't want to be bound by theory, but this 1650cm ―1 The intensity of the peak at 2950 cm ―1 It is considered that the intensity of the peak at is large due to the ratio of the total of the acetal group and the acetyl group, i.e., the total value of l and m in the above formula (I). Therefore, it is considered that the ratio of the intensities of the above peaks indicates that the ratio of the acetal group is relatively large.

[0030] In addition, the polyvinyl acetal of the present invention has a peak at 3400 cm measured by FT-IR. ―1 Intensity of the peak at 2950 cm -1 The ratio of the intensities of the peaks at 3400 cm may be 2.0 or more, 2.5 or more, or 3.0 or more. ―1 It is believed that the intensity of the peak at is large due to the proportion of hydroxyl groups, i.e., the value of n in the above formula (I). For this reason, it is believed that a large intensity ratio indicates a relatively large proportion of acetal groups.

[0031] It is believed that the relatively large proportion of acetal groups leads to a stronger tendency for the polyvinyl acetal as a whole to be hydrophobic, which results in a high affinity for carbon nanotubes. As a result, the carbon nanotube dispersion is easily disintegrated at low shear rates, for example at 0.1 s -1 The viscosity at the time of the reaction can be reduced.

[0032] The carbon nanotube dispersion of the present invention having such a polyvinyl acetal is 0.1s-1 Viscosity at 1000s -1 can be 1000 or less, 900 or less, 800 or less, 700 or less, 600 or less, or 550 or less.

[0033] In the measurement of the viscosity ratio, the shear rate is set to 0.1 s -1 From 1000s -1 The shear rate was then changed to 1000 s with a logarithmic slope over 20 s. -1 From 0.1s -1 The viscosity was measured over a total of 40 seconds by changing the shear rate to 1000 s with a logarithmic gradient over 20 seconds. The viscosity at the 20 second point was -1 The viscosity was measured at 40 seconds and the shear rate was 0.1 s -1 This was taken as a measured value of viscosity.

[0034] The content of the above acetal groups in the polyvinyl acetal in the present invention can be 70 mass% or more, 75 mass% or more, 80 mass% or more, 81 mass% or more, 82 mass% or more, or 83 mass% or more, based on the mass of the polyvinyl acetal.

[0035] The number of carbon atoms in the alkyl group constituting R in the above formula (I) is not particularly limited, and may be 1 or more, 3 or more, 5 or more, 7 or more, or 9 or more, and may be 20 or less, 18 or less, 16 or less, 14 or less, or 12 or less. In particular, the polyvinyl acetal in the present invention may be polyvinyl butyral in which the number of carbon atoms in the alkyl group constituting R is 4.

[0036] The absorbance at 320 nm of a 10% by mass NMP solution of the polyvinyl acetal in the present invention may be 0.25 or more, 0.30 or more, 0.35 or more, 0.40 or more, 0.45 or more, 0.50 or more, 0.55 or more, 0.60 or more, or 0.65 or more, and among them, 0.50 or more is preferable from the viewpoint of improving dispersibility. This absorbance may be 0.90 or less, 0.85 or less, 0.80 or less, 0.75 or less, or 0.70 or less. In addition, in the measurement of this absorbance, the absorbance of NMP is used as a blank.

[0037] The viscosity of a 10% by mass NMP solution of the polyvinyl acetal in the present invention at a shear rate of 76.6 / s may be 10.0 mPa.s or more, 15.0 mPa.s or more, 20.0 mPa.s or more, or 21.0 mPa.s or more, or 70.0 mPa.s or less, 65.0 mPa.s or less, 60.0 mPa.s or less, 55.0 mPa.s or less, 50.0 mPa.s or less, 45.0 mPa.s or less, 40.0 mPa.s or less, 35.0 mPa.s or less, 33.0 mPa.s or less, 31.0 mPa.s or less, or 30.0 mPa.s or less. This viscosity can be measured using a cone-plate type viscometer under the condition of a measurement time of 60 s.

[0038] The weight average molecular weight of the polyvinyl acetal of the present invention measured by gel permeation chromatography is 100,000 or less. This weight average molecular weight may be 90,000 or less, 85,000 or less, 80,000 or less, 75,000 or less, 70,000 or less, 65,000 or less, 60,000 or less, 55,000 or less, or 50,000 or less. Such a weight average molecular weight reduces the viscosity of the polyvinyl acetal solution, and as a result, the aggregation of the obtained carbon nanotubes can be suppressed. This weight average molecular weight may be 10,000 or more, 15,000 or more, 20,000 or more, 25,000 or more, 30,000 or more, 35,000 or more, or 40,000 or more. This weight average molecular weight obtained by gel permeation chromatography is a converted value obtained using polystyrene as a standard polymer.

[0039] The polyvinyl acetal content may be 0.1 mass% or more, 0.3 mass% or more, 0.5 mass% or more, 0.7 mass% or more, or 0.9 mass% or more, and may be 10.0 mass% or less, 8.0 mass% or less, 5.0 mass% or less, 3.0 mass% or less, 2.5 mass% or less, 2.0 mass% or less, 1.5 mass% or less, 1.3 mass% or less, or 1.1 mass% or less, relative to the total mass of the carbon nanotube dispersion.

[0040] <Other polymers> As the polymer other than polyvinyl acetal, for example, a polymer that can be used as a binder can be used. For example, various emulsion type or solution type polymers can be used as such polymers, for example, fluorine-based polymers such as polyvinylidene fluoride (PVdF), tetrafluoroethylene-hexafluoropropylene polymer (FEP), tetrafluoroethylene-perfluoroalkylvinyl ether polymer (PFA), polytetrafluoroethylene (PTFE), etc., elastomer-based polymers such as ethylene-propylene-diene copolymer (EPDM), nitrile-butadiene rubber (NBR), styrene-butadiene rubber (SBR), etc., acrylic polymers, etc. can be used.

[0041] As the polymer, for example, natural polymers such as polysaccharides or synthetic polymers can be used. Such polymers are sometimes used as thickeners.

[0042] Examples of polysaccharides that can be used include gum arabic, tragacanth gum, guar gum, locust bean gum, alginic acid, carrageenan, gelatin, xanthan gum, welan gum, succinoglycan, diutan gum, dextran, methylcellulose, ethylcellulose, hydroxyethylcellulose, carboxymethylcellulose, starch glycolic acid, and salts thereof. Among these, carboxymethylcellulose is preferred from the viewpoint of dispersion stability.

[0043] Examples of synthetic polymers that can be used include water-soluble resins such as polyvinylpyrrolidone, polyvinyl methyl ether, polyacrylic acid and its salts, polyethylene oxide, vinyl acetate-polyvinylpyrrolidone copolymers, styrene-acrylic acid copolymers and their salts, and isobutylene-maleic anhydride copolymers and their salts.

[0044] As the polymer, for example, non-ionic dispersants such as polyalkylene oxides, polyvinyl ethers, chitins, chitosans, starch, etc. can be used. These polymers are sometimes used as dispersion assistants.

[0045] The polymer may be a dispersant. Specifically, the polymer may be a nonionic or anionic dispersant, or a polysaccharide. The nonionic dispersant may be the above-mentioned dispersing aid or polyvinylpyrrolidone, and the anionic dispersant may be an acrylic resin such as a styrene-acrylic resin, a urethane resin, a polyester resin, a polyvinyl chloride resin, or an epoxy resin.

[0046] The total content of other polymers may be 0.5 mass% or more, 1.0 mass% or more, 1.5 mass% or more, 2.0 mass% or more, or 2.2 mass% or more, based on the total mass of the carbon nanotube dispersion, and may be 15.0 mass% or less, 12.0 mass% or less, 10.0 mass% or less, 8.0 mass% or less, 6.0 mass% or less, 5.0 mass% or less, 4.5 mass% or less, 4.0 mass% or less, 3.5 mass% or less, 3.0 mass% or less, or 2.5 mass% or less.

[0047] <Proton concentration regulator> As the optional proton concentration regulator, at least one of the following can be used: ammonia, urea, monoethanolamine, diethanolamine, triethanolamine, aminomethylpropanol, sodium tripophosphate, sodium carbonate, and other alkali metal salts of carbonate or phosphoric acid, and alkali metal hydroxides, such as sodium hydroxide.

[0048] <Preservatives> Optional preservatives include at least one of phenol, sodium omadine, sodium pentachlorophenol, 1,2-benzisothiazolin-3-one, 2,3,5,6-tetrachloro-4(methylphenonyl)pyridine, alkali metal salts of benzoic acid, sorbic acid or dehydroacetic acid, benzimidazole compounds, alcohols such as phenoxyethanol, glycols such as 1,3-pentanediol, etc.

[0049] 《Dispersion for positive electrode》 The positive electrode dispersion of the present invention contains the above-mentioned carbon nanotube dispersion and a positive electrode active material.

[0050] The positive electrode dispersion of the present invention, which contains the carbon nanotube dispersion of the present invention having good dispersibility, can obtain good sheet resistance, capacity retention rate, and cycle retention rate.

[0051] In the positive electrode dispersion of the present invention, the mass of carbon nanotubes per 100 parts by mass of the positive electrode active material is 0.1 parts by mass or more, 0.3 parts by mass or more, 0.5 parts by mass or more, 0.7 parts by mass or more, or 0.9 parts by mass or more, and is 6.0 parts by mass or less, 5.5 parts by mass or less, 5.0 parts by mass or less, 4.5 parts by mass or less, 4.0 parts by mass or less, 3.5 parts by mass or less, 3.0 parts by mass or less, 2.5 parts by mass or less, 2.0 parts by mass or less, or 1.5 parts by mass or less.

[0052] The positive electrode dispersion may contain a polymer other than polyvinyl acetal. As such a polymer, those mentioned in relation to the carbon nanotube dispersion can be used.

[0053] The positive electrode dispersion may further contain a solvent. As the solvent, those listed for the carbon nanotube dispersion can be used.

[0054] The positive electrode dispersion may contain other conductive materials, such as carbon-based conductive materials other than carbon nanotubes.

[0055] The conductive material may be, for example, a carbon-based conductive material, which may be carbon fibers and / or carbon particles.

[0056] Carbon fibers include, but are not limited to, milled fibers, chopped fibers, etc. These may be used alone or in combination.

[0057] The average length of the carbon fibers can be 1 μm or more, 3 μm or more, 5 μm or more, 10 μm or more, or 15 μm or more, and can be 100 μm or less, 70 μm or less, 50 μm or less, or 30 μm or less.

[0058] Examples of carbon particles include graphene, graphite, and carbon black such as acetylene black and ketjen black, etc. These may be used alone or in combination.

[0059] The shape of the carbon particles is not particularly limited, and may be, for example, flat, array-like, spherical, or the like.

[0060] The average particle size of the carbon particles can be 100 nm or more, 200 nm or more, 300 nm or more, 500 nm or more, 700 nm or more, 1 μm or more, 2 μm or more, or 3 μm or more, and can be 20 μm or less, 15 μm or less, 10 μm or less, or 7 μm or less. Here, the average particle size adopted in this specification is appropriately selected depending on the size of the target carbon particles, and in the case of particles less than about 1 μm, it is the value of the histogram average particle size (D50) based on the scattering intensity distribution measured by the dynamic light scattering method, and in the case of particles of 1 μm or more, it is the value of the median size (D50) calculated on a volume basis in the laser diffraction method. Measurement by the dynamic light scattering method can be performed using, for example, DelsaMax CORE (Beckman Coulter, Inc.). Measurement by the laser diffraction method can be performed using, for example, a particle size distribution measuring device MT3300II (Microtrack Bell, Inc.).

[0061] The content of the carbonaceous conductive material in the electrode layer-forming non-aqueous dispersion may be 1.0 mass% or more, 1.5 mass% or more, 2.0 mass% or more, or 2.5 mass% or more, and may be 15.0 mass% or less, 12.0 mass% or less, 10.0 mass% or less, 8.0 mass% or less, 6.0 mass% or less, 5.0 mass%, 4.5 mass% or less, 4.0 mass% or less, 3.5 mass% or less, or 3.0 mass% or less, based on the total mass of the electrode layer-forming non-aqueous dispersion.

[0062] <Cathode active material> As the positive electrode active material, for example, metal compounds such as metal oxides and metal sulfides that can dope or intercalate lithium ions, and conductive polymers can be used. For example, oxides of transition metals such as Fe, Co, Ni, and Mn, composite oxides with lithium, and inorganic compounds such as transition metal sulfides can be used. Specifically, MnO, VO, VO 13 , transition metal oxide powders such as TiO2, layered lithium nickel oxide, lithium cobalt oxide, lithium manganate, spinel-structured lithium manganate, ternary lithium materials (NCM) such as LiNi 0.8 Co 0.1 Mn 0.1 Examples of the oxide include a composite oxide powder of lithium and a transition metal such as O2, a lithium iron phosphate material which is a phosphate compound having an olivine structure, and a transition metal sulfide powder such as TiS2 or FeS. Conductive polymers such as polyaniline, polyacetylene, polypyrrole, and polythiophene can also be used. The inorganic compounds and organic compounds described above may be mixed and used.

[0063] The content of the positive electrode active material may be 40 mass% or more, 45 mass% or more, 50 mass% or more, 55 mass% or more, 60 mass% or more, or 65 mass% or more, and may be 80 mass% or less, 75 mass% or less, or 70 mass% or less, relative to the mass of the positive electrode dispersion. EXAMPLES

[0064] The present invention will be specifically described with reference to examples and comparative examples, but the present invention is not limited to these.

[0065] <<Preparation of carbon nanotube dispersion>> Example 1 100 parts by mass of the carbon nanotube (CNT) dispersion of Example 1 was prepared by dispersing 4 parts by mass of carbon nanotubes (FT7010, Cnano), 1 part by mass of polyvinyl butyral A (PVB-A), and 95 parts by mass of N-methyl-2-pyrrolidone as a solvent using a high-pressure homogenizer (dispersion pressure 100 MPa, 10 passes).

[0066] Examples 2 to 3 and Comparative Examples 1 to 5 Carbon nanotube dispersions of Examples 2 to 3 and Comparative Examples 1 to 5 were prepared in the same manner as in Example 1, except that the same amount of a dispersant shown below and in Table 1 was used instead of PVB-A. PVB-B, PVB-C, PVB-D, PVB-E, PVB-F, PVB-G: Polyvinyl butyral PVP: Polyvinylpyrrolidone

[0067] The estimated ratios of functional groups in PVB-B and PVB-F are as follows: PVB-B: Polyvinyl butyral containing 83% butyral groups by mass, 3% acetyl groups by mass, and 14% vinyl alcohol groups by mass. PVB-F: Polyvinyl butyral containing 78% butyral groups by mass, 3% acetyl groups by mass, and 19% vinyl alcohol groups by mass.

[0068] <<Preparation of Positive Electrode Dispersion>> 25 parts by mass of the prepared carbon nanotube dispersion, 2 parts by mass of acetylene black as a conductive material, and 100 parts by mass of LiNi as a positive electrode active material. 0.8 Co 0.1 Mn 0.1O2, 2 parts by mass of polyvinylidene fluoride as another polymer, and 21 parts by mass of N-methyl-2-pyrrolidone as a solvent were dispersed by a planetary mixer to prepare 100 parts by mass of a dispersion for a positive electrode.

[0069] <Dispersant properties> <FT-IR Intensity> For each PVB, the FT-IR spectrum was measured and the -1 Near, 2950cm -1 Near and 1650cm -1 The peak intensity values ​​around the peak were obtained, and the specified intensity ratio was calculated based on these values. The ATR method was used for the measurements, and diamond was used as the crystal. Each PVB was dried for three days in an environment with a dew point temperature of minus 30 degrees Celsius before the measurements.

[0070] Absorbance and Viscosity A 10% by mass solution of each dispersant in N-methylpyrrolidone was prepared, and the absorbance of this solution was measured at 320 nm, with the absorbance of N-methylpyrrolidone used as a blank.

[0071] The viscosity of this solution was measured using a cone-plate viscometer at a shear rate of 76.6 / s for a measurement time of 60 seconds.

[0072] <Weight average molecular weight> The weight average molecular weight of each dispersant was measured by gel permeation chromatography. The weight average molecular weight was determined by a conversion value obtained using polystyrene as a standard polymer.

[0073] The physical properties of the dispersant are shown in Table 1.

[0074] [Table 1]

[0075] "evaluation" Viscosity of Dispersion Each of the prepared CNT dispersions and the positive electrode dispersions obtained using them were subjected to shear rate measurements at a sample temperature of 25°C and a shear rate of 0.1 s -1 and 1000s -1 The viscosity was measured at a shear rate of 0.1 s -1 From 1000s -1 The shear rate was then changed to 1000 s with a logarithmic slope over 20 s. -1 From 0.1s -1 The viscosity was measured over a total of 40 seconds by changing the shear rate to 1000 s with a logarithmic gradient over 20 seconds. -1 The viscosity was measured at 40 seconds and the shear rate was 0.1 s -1 This was taken as a measured value of viscosity.

[0076] <Coating stability> The obtained dispersion for positive electrode was applied to 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 on the obtained coating film and were 10 mm or longer were evaluated according to the following criteria. When multiple streaks appeared, the total length of each streak was used as the standard. Evaluation criteria: ◎: No streaks longer than 10 mm were present. ○: The total muscle length was 10 mm or more and less than 50 mm. △: The total length of the muscle was 50 mm or more and less than 1000 mm. ×: The total length of the muscles was 1000 mm or more.

[0077] <Electrode sheet resistance> The obtained dispersion for positive electrode was applied to one side of a PET film (Lumirror #100-T60, Toray Industries, Inc.) with an applicator so that the liquid film was 50 μm, and then dried at room temperature for 30 minutes and then further dried at 80° C. for 5 minutes to obtain an electrode layer. The sheet resistance of the produced electrode layer was measured using an apparatus consisting of a four-point probe with a probe spacing of 10 mm and a Milliohm HiTester 3227 (Hioki Electric Corporation).

[0078] <Capacity retention rate after 10C charge / discharge> The obtained dispersion for positive electrode was coated on aluminum foil with a wet film thickness of 100 μm using a comma coater with a coating width of 200 mm, and dried in a drying oven at 100° C. for 5 minutes to prepare five coin secondary batteries as the positive electrodes. The capacity retention rate after charging and discharging at 10 C was evaluated by calculating the average value of the five coin secondary batteries from the discharge capacity (A) when the coin secondary batteries were charged and discharged once at 1 C and the discharge capacity (B) when the coin secondary batteries were subsequently charged and discharged once at a charge and discharge rate of 10 C according to the formula "B / A×100."

[0079] In this coin secondary battery, a carbonate-based solvent was used as the electrolyte, a polypropylene separator was used, and metallic lithium was used as the counter electrode.

[0080] <100 cycles maintenance rate> Five coin secondary batteries were produced under the same conditions as above, and the initial discharge capacity (A) and the 100th discharge capacity (B) after 100 charge / discharge cycles at a charge / discharge rate of 1C were used to calculate the average value of the five coin secondary batteries using the formula "B / A x 100." The 100 cycle retention rate was evaluated according to the following criteria. Evaluation criteria: ◎: 98% or more ○: 95% or more △: 85% or more ×: Less than 85%

[0081] Table 2 shows the configurations and evaluation results of the examples and comparative examples.

[0082] [Table 2]

[0083] From Table 2, the CNT dispersion of the example and the dispersion for the positive electrode using the same were found to have a melting point of 0.1s -1 Viscosity / 1000s -1 It can be seen that the viscosity ratio is kept low, which indicates that good stability of the CNTs is achieved.

[0084] It can also be seen that the electrode layer obtained from the CNT dispersion of the example has good coating stability, electrode sheet resistance, capacity retention rate, and cycle retention rate.

Claims

1. The composition contains at least carbon nanotubes, a solvent, and a polyvinyl acetal represented by the following formula (I): The polyvinyl acetal has a peak at 1650 cm in the FT-IR spectrum obtained by the ATR method. ―１ The intensity of the peak at 2950 cm －１ is 50 or more, and The polyvinyl acetal has a weight average molecular weight of 100,000 or less as measured by gel permeation chromatography. Carbon nanotube dispersion. 【Chemical 1】 (In the formula, l, m, and n are positive integers, and R is an alkyl group having 1 to 20 carbon atoms.)

2. 2. The carbon nanotube dispersion according to claim 1, wherein a 10% by mass solution of the polyvinyl acetal in NMP has an absorbance at 320 nm of 0.50 or more.

3. 3. The carbon nanotube dispersion according to claim 1, wherein a 10 mass % solution of the polyvinyl acetal in NMP has a viscosity of 21.0 mPa·s or more and 31.0 mPa·s or less at a shear rate of 76.6 / s.

4. 3. The carbon nanotube dispersion according to claim 1, wherein the polyvinyl acetal has a mass average molecular weight of 30,000 to 50,000 as measured by gel permeation chromatography.

5. A battery comprising the carbon nanotube dispersion according to claim 1 or 2 and a positive electrode active material, The mass of the carbon nanotubes relative to 100 parts by mass of the positive electrode active material is 0.1 to 6.0 parts by mass. Dispersion for positive electrodes.

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