JP2024030584A - carbon nanotube dispersion - Google Patents

Abstract

The present invention provides a novel carbon nanotube dispersion having good dispersibility. [Solution] The carbon nanotube dispersion of the present invention includes at least carbon nanotubes, a solvent, and polyvinyl acetal, and the intensity of the peak at 1650 cm-1 in the FT-IR spectrum of the polyvinyl acetal obtained by the ATR method. The carbon nanotube dispersion is a carbon nanotube dispersion in which the ratio of the peak intensity at 2950 cm-1 to [Selection diagram] None

Description

The present invention relates to a carbon nanotube dispersion, particularly a carbon nanotube dispersion for a positive electrode dispersion used in an electrode of a lithium ion secondary battery.

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

For example, Patent Document 1 describes 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 a method for producing an electrode slurry and an electrode using the carbon nanotube dispersion. An invention related to an electrode and a battery including the same is disclosed.

Further, 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.

Further, Patent Document 3 discloses a carbon nanotube dispersion and a carbon nanotube resin composition for obtaining an electrode film with high adhesiveness and conductivity by using specific carbon nanotubes and a vinyl alcohol skeleton-containing resin. There is.

Patent No. 6765685 Patent No. 2020-01960 Patent No. 2020-11873

However, both carbon nanotube dispersions and electrode dispersions still have high viscosity, so the yield in the manufacturing process of carbon nanotube dispersions and electrode dispersions is not sufficient, and the pressure loss during liquid transfer is large. In some cases, it was not possible to increase the solid content.

In addition, fluidity in the pipes and coating parts of the coating machine is still insufficient, and defects occur in the electrodes due to problems when coating the electrode dispersion onto the current collector foil, resulting in insufficient fluidity. In some cases, the yield of electrodes could not be obtained.

In addition, these conventional technologies cannot sufficiently lower the resistance value of electrodes using carbon nanotube dispersions because the dispersion of carbon nanotubes is still insufficient. It did not have any cycle characteristics.

Therefore, the present invention provides a novel carbon nanotube dispersion having good dispersibility.

（式中、ｌ、ｍ及びｎは正の整数であり、Ｒは、炭素数１～２０のアルキル基である。）
〈態様２〉前記ポリビニルアセタールの１０質量％ＮＭＰ溶液の３２０ｎｍにおける吸光度が０．５０以上である、態様１に記載のカーボンナノチューブ分散体。
〈態様３〉前記ポリビニルアセタールの１０質量％ＮＭＰ溶液の剪断速度７６．６／ｓにおける粘度が２１．０ｍＰａ・ｓ以上、３１．０ｍＰａ・ｓ以下である、態様１又は２に記載のカーボンナノチューブ分散体。
〈態様４〉前記ポリビニルアセタールの、ゲル浸透クロマトグラフィーにより測定した質量平均分子量が、３０，０００～５０，０００である、態様１～３のいずれか一項に記載のカーボンナノチューブ分散体。
〈態様５〉態様１～４のいずれか一項に記載のカーボンナノチューブ分散体、及び正極活物質を含有しており、かつ
前記正極活物質１００質量部に対する前記カーボンナノチューブの質量が、０．１～６．０質量部である、
正極電極用分散体。 After intensive study, the present inventors found that the above-mentioned problem could be solved by the following means, and completed the present invention. That is, the present invention is as follows:
<Aspect 1> At least contains 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, the ratio of the intensity of the peak at 2950 cm ⁻¹ to the intensity of the peak at 1650 cm ⁻¹ is 50 or more, and the gel permeation chromatography of the polyvinyl acetal The mass average molecular weight measured by is 100,000 or less,
Carbon nanotube dispersion.

(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 the absorbance of the 10% by mass NMP solution of the polyvinyl acetal at 320 nm is 0.50 or more.
<Aspect 3> The carbon nanotube dispersion according to aspect 1 or 2, wherein the viscosity of the 10% by mass NMP solution of the polyvinyl acetal at a shear rate of 76.6/s is 21.0 mPa·s or more and 31.0 mPa·s or less. body.
<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> Contains the carbon nanotube dispersion according to any one of aspects 1 to 4 and a positive electrode active material, and the mass of the carbon nanotubes relative to 100 parts by mass of the positive electrode active material is 0.1. ~6.0 parts by mass,
Dispersion for positive electrodes.

According to the present invention, a novel carbon nanotube dispersion having good dispersibility can be provided.

（式中、ｌ、ｍ及びｎは正の整数であり、Ｒは、炭素数１～２０のアルキル基である。） 《Carbon nanotube dispersion》
The carbon nanotube dispersion of the present invention is
Containing at least carbon nanotubes, a solvent, and a polyvinyl acetal represented by the following formula (I),
The ratio of the intensity of the peak at 2950 cm ⁻¹ to the intensity of the peak at 1650 cm ⁻¹ in the FT-IR (Fourier transform infrared spectroscopy) spectrum obtained by total reflection measurement (ATR method) of the polyvinyl acetal is 50 or more. and the mass average molecular weight of the polyvinyl acetal measured by gel permeation chromatography is 100,000 or less.
It is a carbon nanotube dispersion.

(In the formula, l, m and n are positive integers, and R is an alkyl group having 1 to 20 carbon atoms.)

According to the above configuration, a carbon nanotube dispersion having good dispersibility can be obtained.

In this specification, "the peak near 2950 cm ^-1 " refers to a region of 200 cm -1 before and after 2950 cm ^-1 , 150 cm ^-1 before and after, 100 cm ^-1 before and after, 50 cm ^-1 before and after 2950 cm ^-1 , or 30 cm -1 before and after 2950 cm ^-1 . It means the largest of the peaks. “Peak around 3400 cm ⁻¹ ” and “peak around 1650 cm ⁻¹ ” should be interpreted in the same way.

Furthermore, in the present invention, the ATR method was carried out using diamond as a crystal and using a sample that had been dried for three days in an environment with a dew point temperature of minus 30° C. before measurement.

Each component of the present invention will be explained below.

<carbon nanotube>
As the carbon nanotubes, known carbon nanotubes can be used.

The average particle diameter of 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 20 μm or less, 15 μm or less, 10 μm or less, or 7 μm. It can be less than or equal to: Here, the average particle diameter employed in this specification is appropriately selected depending on the size of the target carbon particles, and in the case of particles of approximately less than 1 μm, the histogram based on the scattering intensity distribution measured by the dynamic light scattering method. It is the value of the average particle diameter (D50), and in the case of particles of 1 μm or more, it is the value of the median diameter (D50) calculated on a volume basis in the laser diffraction method. Measurement by dynamic light scattering can be performed using, for example, DelsaMax CORE (Beckman Coulter). Measurement by laser diffraction can be performed using, for example, a particle size distribution analyzer MT3300II (Microtrac Bell Co., Ltd.). The average particle diameter of carbon nanotubes refers to secondary particles in which carbon nanotube single fibers form bundles or balls, and can be measured by suspending them in any solution.

The content of carbon nanotubes in the nonaqueous slurry for secondary battery positive electrodes is 0.1% by mass or more, 0.3% by mass or more, 0.5% by mass or more based on the total mass of the nonaqueous slurry for secondary battery positive electrodes. It may be at least 10.0 mass%, at most 8.0 mass%, at most 5.0 mass%, at most 3.0 mass%, and at most 2.5 mass%. The content may be 2.0% by mass or less, 1.5% by mass or less, 1.0% by mass or less, or 0.8% by mass or less.

<solvent>
As the solvent, any organic solvent can be used, such as protic polar solvents and aprotic polar solvents. These solvents may be used alone or in combination.

Protic polar liquid media are generally acidic hydrogen-containing solvents. As the protic polar liquid medium, for example, a phenolic liquid medium, a monohydric alcohol liquid medium, a polyhydric alcohol liquid medium, etc. can be used.

As the phenolic liquid medium, for example, cresols can be used.

Examples of the monohydric alcohol liquid medium include ethanol, n-propanol, isopropanol, n-butanol, isobutanol, tert-butyl alcohol, 1-pentanol, isoamyl alcohol, sec-amyl alcohol, 3-pentanol, and tert-amyl alcohol. , n-hexanol, etc. can be used.

As the polyhydric alcohol liquid medium, for example, a glycol liquid medium such as propylene glycol, triethylene glycol, tetraethylene glycol, diglyme, polyethylene glycol having a molecular weight of 600 or less, and glycerin can be used.

Aprotic polar liquid media are generally acidic hydrogen-free solvents. As the aprotic polar liquid medium, for example, N,N-dimethylformamide, N,N-dimethylacetamide, N-methyl-2-pyrrolidone, etc. can be used, and among them, N-methyl-2-pyrrolidone can be used. is preferable from the viewpoint of affinity with polyvinyl acetal.

（式中、ｌ、ｍ及びｎは正の整数であり、Ｒは、炭素数１～２０のアルキル基である。） <Polyvinyl acetal>
The polyvinyl acetal in the carbon nanotube dispersion of the present invention is constituted by the following formula.

(In the formula, l, m and n are positive integers, and R is an alkyl group having 1 to 20 carbon atoms.)

The ratio of the peak intensity at 2950 cm ⁻¹ to the peak intensity at 1650 cm ⁻¹ measured by FT-IR of the polyvinyl acetal in the present invention is 50 or more. This ratio may be 50 or more, 55 or more, 60 or more, 65 or more, 70 or more, 75 or more, 80 or more, 85 or more, 90 or more, 95 or more, or 100 or more.

Without wishing to be bound by theory, the intensity of this peak at 1650 cm ⁻¹ increases due to the proportion of acetyl groups, i.e. the value of m in formula (I) above, and the intensity of the peak at 2950 cm ⁻¹ increases. It is believed that the strength increases due to the proportion of the sum of acetal and acetyl groups, ie the sum of the values of l and m in formula (I) above. From this, it is considered that the ratio of the above-mentioned peak intensities indicates that the proportion of acetal groups is relatively large.

Furthermore, the ratio of the peak intensity at 2950 cm ⁻¹ to the peak intensity at 3400 cm ⁻¹ measured by FT-IR of the polyvinyl acetal in the present invention is 2.0 or more, 2.5 or more, or 3.0 or more. It may be. Without wishing to be bound by theory, it is believed that the intensity of this peak at 3400 cm ⁻¹ is increased due to the proportion of hydroxyl groups, ie, the value of n in formula (I) above. From this, it is considered that a large intensity ratio indicates a relatively large proportion of acetal groups.

It is thought that by relatively increasing the proportion of acetal groups, the polyvinyl acetal as a whole tends to be more hydrophobic, thereby obtaining high affinity with carbon nanotubes. As a result, the viscosity of the carbon nanotube dispersion at low shear rates, for example 0.1 s ⁻¹ can be reduced.

The ratio of the viscosity at 0.1 s ⁻¹ to the viscosity at 1000 s ⁻¹ of the carbon nanotube dispersion of the present invention having such a polyvinyl acetal is 1000 or less, 900 or less, 800 or less, 700 or less, 600 or less, or It can be 550 or less.

Here, in the measurement of the viscosity ratio mentioned above, the shear rate was changed from 0.1 s ^-1 to 1000 s ^-1 by a logarithmic slope in 20 seconds, and then the shear rate was changed from 1000 s ^-1 to 0.1 s ^-1 . The viscosity was changed over time using a logarithmic slope for 20 seconds, and the viscosity was measured over time for a total of 40 seconds. The viscosity at 20 seconds was taken as the measured value of the viscosity at a shear rate of 1000 s ^-1 , and the viscosity at 40 seconds was measured at a shear rate of 0.1 s. ^-1 was taken as the measured value of viscosity.

The content of the acetal groups in the polyvinyl acetal of the present invention is 70% by mass or more, 75% by mass or more, 80% by mass or more, 81% by mass or more, 82% by mass or more, or It can be 83% by mass or more.

The number of carbon atoms in the alkyl group constituting R in the above formula (I) is not particularly limited, but 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, It may be 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 alkyl group constituting R has 4 carbon atoms.

The absorbance at 320 nm of a 10% by mass NMP solution of polyvinyl acetal in the present invention is 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 Above, it may be 0.60 or more, or 0.65 or more, and 0.50 or more is particularly 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, 0.70 or less. In addition, in this absorbance measurement, the absorbance of NMP is used as a blank.

The viscosity of the 10% by mass NMP solution of polyvinyl acetal in the present invention at a shear rate of 76.6/s is 10.0 mPa. s or more, 15.0 mPa. s or more, 20.0 mPa. s or more, 21.0 mPa. s or more, and 70.0 mPa.s or more. 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. It may be less than or equal to s. This viscosity can be measured using a cone-plate viscometer for a measurement time of 60 seconds.

The weight average molecular weight of the polyvinyl acetal in the present invention measured by gel permeation chromatography is 100,000 or less. This weight average molecular weight is 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 It may be the following. According to such a weight average molecular weight, the viscosity of the polyvinyl acetal solution can be reduced, and as a result, aggregation of the resulting carbon nanotubes can be suppressed. The 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. The weight average molecular weight obtained by this gel permeation chromatography is a converted value obtained using polystyrene as a standard polymer.

The content of polyvinyl acetal is 0.1% by mass or more, 0.3% by mass or more, 0.5% by mass or more, 0.7% by mass or more, or 0.9% by mass with respect to the total mass of the carbon nanotube dispersion. % or more, and 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, It may be 1.5% by weight or less, 1.3% by weight or less, or 1.1% by weight or less.

<Other polymers>
As the polymer other than polyvinyl acetal, for example, a polymer that can be used as a binder can be used. As such a polymer, for example, various emulsion-type or dissolved-type polymers can be used, such as polyvinylidene fluoride (PVdF), tetrafluoroethylene/hexafluoropropylene polymer (FEP), and tetrafluoroethylene/perfluoroalkyl vinyl ether. Polymers (PFA), fluorine-based polymers such as polytetrafluoroethylene (PTFE), elastomers such as ethylene-propylene-diene copolymer (EPDM), nitrile-butadiene rubber (NBR), and styrene-butadiene rubber (SBR) Polymers, acrylic polymers, etc. can be used.

Further, as the polymer, for example, natural polymers such as polysaccharides and synthetic polymers can be used. Such polymers are sometimes used as thickeners.

Examples of polysaccharides include gum arabic, gum tragacanth, guar gum, locust bean gum, alginic acid, carrageenan, gelatin, xanthan gum, welan gum, succinoglycan, diutan gum, dextran, methylcellulose, ethylcellulose, hydroxyethylcellulose, carboxymethylcellulose, and starch glycolic acid. and its salts can be used. Among them, it is preferable to use carboxymethylcellulose from the viewpoint of dispersion stability.

Examples of synthetic polymers include polyvinylpyrrolidone, polyvinylmethyl ether, polyacrylic acid and its salts, polyethylene oxide, vinyl acetate-polyvinylpyrrolidone copolymer, styrene-acrylic acid copolymer and its salts, and isobutylene maleic anhydride copolymer. Water-soluble resins such as polymers and salts thereof can be used.

Furthermore, as the polymer, nonionic dispersants such as polyalkylene oxide, polyvinyl ether, chitins, chitosan, starch, etc. can be used. These polymers are sometimes used as dispersion aids.

Further, examples of the polymer include dispersants. Specifically, as the polymer which is a dispersant, for example, a nonionic or anionic dispersant or a polysaccharide can be used. As the nonionic dispersant, the above-mentioned dispersion aid or polyvinylpyrrolidone can be used, and as the anionic dispersant, acrylic resins such as styrene acrylic resins, urethane resins, polyester resins, polyvinyl chloride resins, and epoxy resin, etc. can be used.

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

<Proton concentration regulator>
Optional proton concentration regulators include, for example, ammonia, urea, monoethanolamine, diethanolamine, triethanolamine, aminomethylpropanol, alkali metal salts of carbonic acid or phosphoric acid, such as sodium tripophosphate, sodium carbonate, sodium hydroxide, etc. At least one kind of alkali metal hydroxides can be used.

<Preservative>
Optional preservatives include, for example, phenol, sodium omazine, sodium pentachlorophenol, 1,2-benzisothiazolin 3-one, 2,3,5,6-tetrachloro-4(methylphonyl)pyridine, benzoic acid, sorbin. At least one of alkali metal salts of acids or dehydroacetic acids, benzimidazole compounds, alcohols such as phenoxyethanol, and glycols such as 1,3-pentanediol can be used.

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

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

In the positive electrode dispersion of the present invention, the mass of carbon nanotubes relative to 100 parts by mass of the positive electrode active material is 0.1 part 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 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 It is not more than 3.0 parts by mass, not more than 2.5 parts by mass, not more than 2.0 parts by mass, or not more than 1.5 parts by mass.

The positive electrode dispersion may contain polymers other than polyvinyl acetal. As such a polymer, those mentioned in connection with the carbon nanotube dispersion can be used.

Further, the positive electrode dispersion may further contain a solvent. As the solvent, those mentioned regarding the carbon nanotube dispersion can be used.

Further, the positive electrode dispersion may contain other conductive substances. As the other conductive substance, for example, a carbon-based conductive material other than carbon nanotubes can be used.

As the conductive material, for example, a carbon-based conductive material can be used. The carbon-based conductive material may be carbon fibers and/or carbon particles.

Carbon fibers include, but are not limited to, milled fibers, chopped fibers, and the like. These may be used alone or in combination.

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.

Examples of the carbon particles include graphene, graphite, and carbon black such as acetylene black and Ketjen black. These may be used alone or in combination.

The shape of the carbon particles is not particularly limited, and may be, for example, flat, arrayed, spherical, or the like.

The average particle diameter 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 20 μm or less, 15 μm or less, 10 μm or less, or 7 μm. It can be less than or equal to: Here, the average particle diameter employed in this specification is appropriately selected depending on the size of the target carbon particles, and in the case of particles of approximately less than 1 μm, the histogram based on the scattering intensity distribution measured by the dynamic light scattering method. It is the value of the average particle diameter (D50), and in the case of particles of 1 μm or more, it is the value of the median diameter (D50) calculated on a volume basis in the laser diffraction method. Measurement by dynamic light scattering can be performed using, for example, DelsaMax CORE (Beckman Coulter). Measurement by laser diffraction can be performed using, for example, a particle size distribution analyzer MT3300II (Microtrac Bell Co., Ltd.).

The content of the carbonaceous conductive material in the nonaqueous dispersion for electrode layer formation is 1.0% by mass or more, 1.5% by mass or more, 2.0% by mass or more, based on the total mass of the nonaqueous dispersion for electrode layer formation. It may be 2.5% by mass or more, and 15.0% by mass or less, 12.0% by mass or less, 10.0% by mass or less, 8.0% by mass or less, 6.0% by mass. The content may be 5.0% by mass or less, 4.5% by mass or less, 4.0% by mass or less, 3.5% by mass or less, or 3.0% by mass or less.

<Cathode active material>
As the positive electrode active material, for example, metal compounds such as metal oxides and metal sulfides that can be doped or intercalated with lithium ions, conductive polymers, and the like can be used. Examples include oxides of transition metals such as Fe, Co, Ni, and Mn, composite oxides with lithium, and inorganic compounds such as transition metal sulfides. Specifically, transition metal oxide powders such as MnO, V ₂ O ₅ , V ₆ O ₁₃ , TiO _{2 ,} etc., lithium nickelate with a layered structure, lithium cobalt oxide, lithium manganate, lithium manganate with a spinel structure, (NCM) lithium materials, such as composite oxide powders of lithium and transition metals such as LiNi _0.8 Co _0.1 Mn _0.1 O ₂ , lithium iron phosphate materials that are phosphoric acid compounds with an olivine structure , TiS ₂ , FeS, and other transition metal sulfide powders. Furthermore, conductive polymers such as polyaniline, polyacetylene, polypyrrole, and polythiophene can also be used. Further, the above inorganic compounds and organic compounds may be used in combination.

The content of the positive electrode active material is 40% by mass or more, 45% by mass or more, 50% by mass or more, 55% by mass or more, 60% by mass or more, or 65% by mass or more with respect to the mass of the positive electrode dispersion. It may be 80% by mass or less, 75% by mass or less, or 70% by mass or less.

The present invention will be specifically explained with reference to Examples and Comparative Examples, but the present invention is not limited thereto.

《Preparation of carbon nanotube dispersion》
<Example 1>
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 were mixed using a high-pressure homogenizer (dispersion pressure of 100 MPa, 10Pass) to prepare 100 parts by mass of the carbon nanotube (CNT) dispersion of Example 1.

<Examples 2-3 and Comparative Examples 1-5>
The 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 the dispersant shown below and in Table 1 was used instead of PVB-A. Created.
PVB-B, PVB-C, PVB-D, PVB-E, PVB-F, PVB-G: Polyvinyl butyral PVP: Polyvinylpyrrolidone

The estimated proportions of functional groups in PVB-B and PVB-F are as follows:
PVB-B: Polyvinyl butyral with 83% by mass of butyral groups, 3% by mass of acetyl groups, and 14% by mass of vinyl alcohol groups PVB-F: Polyvinyl butyral with 78% by mass of butyral groups, 3% by mass of acetyl groups, and 19% by mass of vinyl alcohol groups

《Preparation of dispersion for positive electrode》
25 parts by mass of the prepared carbon nanotube dispersion, 2 parts by mass of acetylene black as a conductive material, 100 parts by mass of LiNi _0.8 Co _0.1 Mn _0.1 O ₂ as a positive electrode active material, 2 parts by mass Polyvinylidene fluoride as another polymer and 21 parts by mass of N-methyl-2-pyrrolidone as a solvent were dispersed using a planetary mixer to prepare 100 parts by mass of a positive electrode dispersion.

《Physical properties of dispersant》
<FT-IR intensity>
For each PVB, the FT-IR spectrum was measured to obtain peak intensity values around 3400 cm ^-1 , around 2950 cm ^-1 , and around 1650 cm ^-1 , and based on these values, a predetermined intensity ratio was calculated. The ATR method was adopted for the measurement, and diamond was used as the crystal. Furthermore, each PVB was dried for 3 days in an environment with a dew point temperature of minus 30° C. before measurement.

<Absorbance and viscosity>
A 10% by mass solution of N-methylpyrrolidone for each dispersant was prepared, and the absorbance of this solution at 320 nm was measured. At this time, the absorbance of N-methylpyrrolidone was used as a blank.

Further, the viscosity of this solution was measured using a cone-plate viscometer at a shear rate of 76.6/s and a measurement time of 60 seconds.

<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 converted value obtained using polystyrene as a standard polymer.

Table 1 shows the physical properties of the dispersant.

"evaluation"
<Viscosity of dispersion>
Each of the prepared CNT dispersions and the positive electrode dispersion obtained using the same was subjected to shearing at a sample temperature of 25°C using a rheometer (Modular Compact Rheometer MCR102 manufactured by Anton Paar, diameter 50 mm - 2° cone). The viscosity was measured at speeds of 0.1 s ⁻¹ and 1000 s ⁻¹ , respectively. The shear rate was varied from 0.1 s ^-1 to 1000 s ^-1 by a logarithmic ramp in 20 seconds, and then the shear rate was varied from 1000 s ^-1 to 0.1 s ^-1 by a logarithmic ramp in 20 seconds for a total of 40 seconds. The viscosity was measured over time. The viscosity at 20 seconds was taken as the measured value of viscosity at a shear rate of 1000 s ^-1 , and the viscosity at 40 seconds was taken as the measured value of viscosity at a shear rate of 0.1 s ^-1 .

<Coating stability>
The obtained positive electrode dispersion was coated on aluminum foil in a wet film thickness of 100 μm for 10 m using a comma coater with a coating width of 200 mm. Among the streaks that occurred on the obtained coating film, those with a length of 10 mm or more were evaluated according to the following criteria. If multiple streaks occurred, the total length of each streak was used as the standard.
Evaluation criteria:
◎: There were no streaks with a length of 10 mm or more.
Good: The total length of the streaks was 10 mm or more and less than 50 mm.
Δ: The total length of the streaks was 50 mm or more and less than 1000 mm.
×: The total length of the streaks was 1000 mm or more.

<Electrode sheet resistance>
The obtained positive electrode dispersion was applied to one side of a PET film (Lumirror #100-T60, Toray Industries, Inc.) using an applicator so that the liquid film was 50 μm, dried at room temperature for 30 minutes, and then heated at 80°C for 5 minutes. Further drying was performed for a minute to obtain an electrode layer. The sheet resistance of the prepared electrode layer was measured using an apparatus consisting of a 4-point probe with a probe interval of 10 mm and a milliohm high tester 3227 (manufactured by Hioki Electric Co., Ltd.).

<Capacity retention rate after 10C charge/discharge>
The obtained positive electrode dispersion 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 the electrode was dried in a drying oven at 100°C for 5 minutes. Five batteries were manufactured. The formula "B/A The capacity retention rate after charging and discharging at 10C was evaluated by calculating the average value of five coin secondary batteries using "x100".

In this coin secondary battery, a carbonate solvent was used as the electrolyte. As the separator, one made of polypropylene was used. Metallic lithium was used as the counter electrode.

<100 cycles maintenance rate>
Five coin secondary batteries were made under the same conditions as above, and from the initial discharge capacity (A) and the 100th discharge capacity (B) after repeated charging and discharging 100 times at a charge and discharge rate of 1C, By determining the average value of five coin secondary batteries using the formula "B/A x 100", the 100 cycle retention rate was evaluated based on the following criteria.
Evaluation criteria:
◎: 98% or more ○: 95% or more △: 85% or more ×: Less than 85%

Table 2 shows the configurations and evaluation results of Examples and Comparative Examples.

From Table 2, the CNT dispersion of the example and the positive electrode dispersion using the same have a low 0.1 s ^-1 viscosity/1000 s ^-1 viscosity ratio, and therefore have good stability of CNT. It can be seen that this is obtained.

Furthermore, it can 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 (5)

Containing at least carbon nanotubes, a solvent, and a polyvinyl acetal represented by the following formula (I),
The ratio of the intensity of the peak at 2950 cm ⁻¹ to the intensity of the peak at 1650 cm ⁻¹ in the FT-IR spectrum of the polyvinyl acetal obtained by the ATR method is 50 or more, and the gel permeation chromatography of the polyvinyl acetal The mass average molecular weight measured by graphography is 100,000 or less,
Carbon nanotube dispersion.

(In the formula, l, m and n are positive integers, and R is an alkyl group having 1 to 20 carbon atoms.) The carbon nanotube dispersion according to claim 1, wherein the 10% by mass NMP solution of the polyvinyl acetal has an absorbance at 320 nm of 0.50 or more. The carbon nanotube dispersion according to claim 1 or 2, wherein the viscosity of the 10% by mass NMP solution of the polyvinyl acetal at a shear rate of 76.6/s is 21.0 mPa·s or more and 31.0 mPa·s or less. The carbon nanotube dispersion according to claim 1 or 2, wherein the polyvinyl acetal has a mass average molecular weight of 30,000 to 50,000 as measured by gel permeation chromatography. It contains the carbon nanotube dispersion according to claim 1 or 2 and a positive electrode active material, and the mass of the carbon nanotubes is 0.1 to 6.0 parts by mass with respect to 100 parts by mass of the positive electrode active material. ,
Dispersion for positive electrodes.