Paste for secondary battery, slurry for positive electrode of secondary battery, positive electrode for secondary battery, secondary battery, and method for manufacturing paste for secondary battery

Abstract

The purpose of the present invention is to provide: a paste for secondary batteries, which can reduce the internal resistance of a secondary battery and enables the manufacturing of an electrode that allows the secondary battery to exhibit excellent cycle properties; and a method for producing the paste. The paste for secondary batteries of the present invention comprises a conductive additive, a polymer and a dispersion medium, wherein the conductive additive comprises a carbon nanotube having a surface base amount of 0.01 to 0.10 mmol / g inclusive and a ratio of a surface acid amount to the surface base amount of 0.1 to 1.0 inclusive.

Description

Technical Field

[0001] The present invention relates to a paste for a secondary battery, a slurry for a positive electrode of a secondary battery, a positive electrode for a secondary battery, a secondary battery, and a method for manufacturing the paste for a secondary battery.

Background Art

[0002] Secondary batteries such as lithium-ion secondary batteries are small, lightweight, have a high energy density, and can be repeatedly charged and discharged, and are used in a wide range of applications. In particular, in recent years, lithium-ion secondary batteries have attracted attention as an energy source for electric vehicles (EVs) and hybrid electric vehicles (HEVs), and further performance improvement is required. Therefore, in recent years, improvements in battery components such as electrodes have been studied for the purpose of further improving the performance of secondary batteries such as lithium-ion secondary batteries.

[0003] An electrode for a secondary battery usually includes a current collector and an electrode mixture layer (positive electrode mixture layer, negative electrode mixture layer) formed on the current collector. Here, when forming the electrode mixture layer, a paste for a secondary battery obtained by dispersing a conductive auxiliary material in a dispersion medium using a dispersion material made of a polymer has been conventionally prepared (for example, see Patent Document 1). By mixing such a paste for a secondary battery with an electrode active material to prepare a slurry for a secondary battery electrode, and then removing the dispersion medium from the slurry for a secondary battery electrode, an electrode mixture layer can be formed.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] However, there has been a demand for the conventional paste for secondary batteries to further reduce the internal resistance of the secondary battery while further improving the cycle characteristics.

[0006] Therefore, an object of the present invention is to provide a paste for secondary batteries and a method for manufacturing the same, which can reduce the internal resistance of a secondary battery and produce an electrode capable of exhibiting excellent cycle characteristics in the secondary battery. Another object of the present invention is to provide a slurry for a positive electrode of a secondary battery that can reduce the internal resistance of the secondary battery and produce a positive electrode capable of exhibiting excellent cycle characteristics in the secondary battery. And an object of the present invention is to provide a positive electrode for a secondary battery that can reduce the internal resistance of the secondary battery and exhibit excellent cycle characteristics in the secondary battery. Furthermore, an object of the present invention is to provide a secondary battery having a reduced internal resistance and excellent cycle characteristics.

Means for Solving the Problems

[0007] The present inventor has intensively studied for the purpose of solving the above problems. And the present inventor newly found that according to a paste for secondary batteries obtained by using carbon nanotubes (hereinafter, may be abbreviated as "CNT") having a predetermined property as a conductive auxiliary material, it is possible to reduce the internal resistance of the secondary battery and exhibit excellent cycle characteristics in the secondary battery, and thus completed the present invention.

[0008] That is, this invention aims to advantageously solve the above problems, and the paste for secondary batteries of the present invention is a paste for secondary batteries containing a conductive auxiliary material, a polymer, and a dispersion medium, wherein the conductive auxiliary material contains surface-treated carbon nanotubes having a surface base amount of 0.01 mmol / g or more and 0.10 mmol / g or less, and a ratio of surface acid amount to the surface base amount of 0.1 or more and 1.0 or less. According to an electrode provided with an electrode composite layer obtained by using the paste containing the above-described CNT, it is possible to reduce the internal resistance of the secondary battery and exhibit excellent cycle characteristics in the secondary battery. In the present invention, the "surface base amount" and "surface acid amount" of the carbon nanotubes can be measured using the methods described in the examples.

[0009] Here, for the paste for secondary batteries of the present invention, it is preferable that the specific surface area of the carbon nanotubes is 150 m 2 / g or more. If CNTs with a specific surface area of the above-described value or more are used, even if the amount of CNTs is small, the internal resistance of the secondary battery can be sufficiently reduced. In the present invention, the "specific surface area" refers to the BET specific surface area by the nitrogen adsorption method, and can be measured using, for example, Belsorp-mini (manufactured by MicrotracBEL Corp., compliant with ASTM D3037-81).

[0010] Also, for the paste for secondary batteries of the present invention, it is preferable that the surface acid amount of the carbon nanotubes is 0.01 mmol / g or more and 0.10 mmol / g or less. If the surface acid amount of the CNTs is within the above-described range, while ensuring the stability of the paste for secondary batteries, the internal resistance of the secondary battery can be further reduced.

[0011] And, the paste for secondary batteries of the present invention is characterized in that the polymer contains a nitrile group-containing monomer unit in a proportion of 10% by mass or more and 40% by mass or less, and contains a conjugated diene monomer unit in a proportion of 15% by mass or more and 55% by mass or less. If the polymer has the above-described composition, the internal resistance of the secondary battery can be further reduced while further improving the cycle characteristics. In the present invention, that the polymer "contains a monomer unit" means that a structural unit derived from the monomer is contained in the polymer obtained using the monomer. Also, in the present invention, the content ratio of each monomer unit in the polymer can be measured by a nuclear magnetic resonance method such as 1 H-NMR.

[0012] Here, in the paste for a secondary battery of the present invention, it is preferable that the polymer has a hydrophilic group. If the polymer has a hydrophilic group, the adhesiveness of the electrode composite layer formed from the electrode slurry containing the paste can be improved, and the internal resistance of the secondary battery can be further reduced.

[0013] Further, in the paste for a secondary battery of the present invention, it is preferable that the iodine value of the polymer is 3 mg / 100 mg or more and 50 mg / 100 mg or less. If the iodine value of the polymer is within the above-described range, while ensuring the flexibility of the electrode provided with the electrode composite layer formed from the electrode slurry containing the paste, the internal resistance of the secondary battery can be further reduced. In the present invention, the "iodine value" can be measured according to JIS K6235 (2006) using the method described in the examples.

[0014] And, in the paste for a secondary battery of the present invention, it is preferable that the content ratio of the conductive auxiliary material is 2% by mass or more and 20% by mass or less, and the content ratio of the polymer is 0.1% by mass or more and 6% by mass or less. A paste in which the content ratios of the conductive auxiliary material and the polymer are respectively within the above-described ranges is excellent in handleability, and the productivity when forming an electrode composite layer from an electrode slurry prepared using the paste can be enhanced.

[0015] Further, this invention aims to advantageously solve the above problems, and the slurry for a positive electrode of a secondary battery of the present invention is characterized by containing a positive electrode active material and any one of the above-described pastes for a secondary battery. If a positive electrode composite layer is formed from a positive electrode slurry containing a positive electrode active material and any one of the above-described pastes, the internal resistance of the secondary battery can be reduced by the positive electrode provided with the positive electrode composite layer, and excellent cycle characteristics can be exhibited in the secondary battery.

[0016] Here, in the slurry for the positive electrode of the secondary battery of the present invention, the ratio of nickel in the transition metal in the positive electrode active material is preferably 60.0 mol% or more and 100.0 mol% or less. If a positive electrode active material in which the ratio of nickel in the transition metal is within the above-described range is used, the secondary battery can be made to have a higher capacity. And while ensuring the stability of the slurry for the positive electrode, it is possible to sufficiently achieve reduction of the internal resistance of the secondary battery and improvement of the cycle characteristics. In the present invention, the "ratio of nickel in the transition metal in the positive electrode active material" can be measured using inductively coupled plasma atomic emission spectrometry (ICP-AES method).

[0017] Further, this invention aims to advantageously solve the above problems, and the positive electrode for a secondary battery of the present invention is characterized by including a positive electrode composite layer formed using any of the above-described slurries for the positive electrode of the secondary battery. According to the positive electrode including the positive electrode composite layer formed from any of the above-described slurries for the positive electrode, the internal resistance of the secondary battery can be reduced, and the secondary battery can exhibit excellent cycle characteristics.

[0018] And this invention aims to advantageously solve the above problems, and the secondary battery of the present invention is characterized by including the above-described positive electrode for a secondary battery. The secondary battery including the above-described positive electrode has a reduced internal resistance and excellent cycle characteristics.

[0019] Furthermore, the present invention aims to advantageously solve the above problems. The method for manufacturing a paste for a secondary battery according to the present invention is a method for manufacturing any of the above-described pastes for a secondary battery, and includes a step of subjecting raw material carbon nanotubes to an acid treatment, a step of subjecting the raw material carbon nanotubes subjected to the acid treatment to a base treatment, a step of washing the raw material carbon nanotubes subjected to the base treatment to obtain the carbon nanotubes, and a step of mixing a conductive auxiliary material containing the carbon nanotubes with the polymer and the dispersion medium. According to the method for manufacturing a paste for a secondary battery of the present invention including the above-described steps, a paste for a secondary battery of the present invention containing CNTs having predetermined properties can be efficiently prepared.

Advantages of the Invention

[0020] According to the present invention, it is possible to provide a paste for a secondary battery and a method for manufacturing the same, which can reduce the internal resistance of the secondary battery and produce an electrode capable of exhibiting excellent cycle characteristics in the secondary battery. Also, according to the present invention, it is possible to provide a slurry for a positive electrode of a secondary battery, which can reduce the internal resistance of the secondary battery and produce a positive electrode capable of exhibiting excellent cycle characteristics in the secondary battery. And, according to the present invention, it is possible to provide a positive electrode for a secondary battery, which can reduce the internal resistance of the secondary battery and produce a positive electrode capable of exhibiting excellent cycle characteristics in the secondary battery. Furthermore, according to the present invention, it is possible to provide a secondary battery having a reduced internal resistance and excellent cycle characteristics.

Embodiments for Carrying Out the Invention

[0021] Hereinafter, embodiments of the present invention will be described in detail. Here, the paste for a secondary battery of the present invention can be used as a material for preparing a slurry for a secondary battery electrode (preferably, a slurry for a positive electrode of a secondary battery). Note that the paste for a secondary battery of the present invention can be manufactured by the method for manufacturing the paste for a secondary battery of the present invention. Further, the slurry for a positive electrode of a secondary battery of the present invention is prepared using the paste for a secondary battery of the present invention. In addition, the positive electrode for a secondary battery of the present invention includes a positive electrode composite layer formed from the slurry for a positive electrode of a secondary battery of the present invention. And, the secondary battery of the present invention includes the positive electrode for a secondary battery of the present invention.

[0022] (Paste for secondary battery) The paste of the present invention contains a conductive auxiliary material, a polymer, and a dispersion medium, and optionally contains other components. Here, the paste of the present invention contains, as the above conductive auxiliary material, a CNT having a surface base amount of 0.01 mmol / g or more and 0.10 mmol / g or less, and a ratio of the surface acid amount to the surface base amount of 0.1 or more and 1.0 or less. And, since the paste of the present invention contains a CNT having the above-described properties, if an electrode composite layer is produced using the paste, the internal resistance of the secondary battery can be reduced, and excellent cycle characteristics can be exhibited in the secondary battery.

[0023] <Conductive auxiliary material> The paste of the present invention contains, as a conductive auxiliary material, a CNT having the above-described predetermined properties. Note that the paste of the present invention may optionally contain a conductive auxiliary material other than the CNT (other conductive auxiliary materials).

[0024] <<Carbon nanotube>> Here, the surface base amount of the CNT needs to be 0.01 mmol / g or more and 0.10 mmol / g or less, preferably 0.02 mmol / g or more, more preferably 0.03 mmol / g or more, and still more preferably 0.04 mmol / g or more. When the surface base amount is less than 0.01 mmol / g, the residual acid components adhering to the surface of the CNT become excessive. It is presumed that side reactions occur in the secondary battery due to this residual acid component, resulting in an increase in the internal resistance of the secondary battery. On the other hand, when the surface base amount exceeds 0.10 mmol / g, it is presumed to be due to the reaction with the acid components contained in the electrode slurry prepared using the paste, but the CNT is likely to aggregate, increasing the internal resistance of the secondary battery and deteriorating the cycle characteristics. In addition, when the surface base amount is 0.01 mmol / g or more and 0.10 mmol / g or less, the CNT can be well dispersed in the dispersion medium, ensuring the stability of the paste.

[0025] Also, the surface acid amount of the CNT is preferably 0.01 mmol / g or more, preferably 0.10 mmol / g or less, more preferably 0.03 mmol / g or less, and still more preferably 0.02 mmol / g or less. If the surface acid amount is 0.01 mmol / g or more, the adhesiveness of the electrode composite layer obtained using the paste is sufficiently ensured, and the electrode composite layer can adhere firmly to the current collector. And the internal resistance of the secondary battery can be further reduced. On the other hand, if the surface acid amount is 0.10 mmol / g or less, it is presumed that the amount of residual acid components adhering to the surface of the CNT is reduced, and side reactions in the secondary battery are sufficiently suppressed, and the internal resistance of the secondary battery can be further reduced. In addition, when the surface acid amount is 0.01 mmol / g or more and 0.10 mmol / g or less, the CNT can be well dispersed in the dispersion medium, ensuring the stability of the paste.

[0026] And the CNT is required to have a ratio of the surface acid amount to the surface base amount (surface acid amount / surface base amount) of 0.1 or more and 1.0 or less, preferably 0.2 or more, more preferably 0.27 or more, preferably 0.8 or less, more preferably 0.6 or less, and still more preferably 0.5 or less. When the surface acid amount / surface base amount is less than 0.1, the adhesiveness of the electrode composite layer obtained using the paste decreases, and the electrode composite layer cannot adhere firmly to the current collector. And the internal resistance of the secondary battery increases. On the other hand, when the surface acid amount / surface base amount exceeds 1.0, the residual acid components adhering to the surface of the CNT become excessive. Although it is presumed that side reactions occur in the secondary battery due to this residual acid component, the internal resistance of the secondary battery increases. In addition, when the surface acid amount / surface base amount is 0.1 or more and 1.0 or less, the CNT can be well dispersed in the dispersion medium, and the stability of the paste is ensured.

[0027] Also, the surface-treated CNT preferably has a specific surface area of 150 m 2 / g or more, more preferably 200 m 2 / g or more, still more preferably 250 m 2 / g or more, particularly preferably 300 m 2 / g or more, and preferably 1200 m 2 / g or less, more preferably 1000 m 2 / g or less, and still more preferably 500 m 2 / g or less. If the specific surface area is 150 m 2 / g or more, even if the amount of CNT is small, the internal resistance of the secondary battery can be sufficiently reduced. On the other hand, if the specific surface area is 1200 m 2 / g or less, the polymer can be well bound to the CNT, and the adhesiveness of the electrode composite layer can be sufficiently ensured. Also, a conductive network by the CNT is sufficiently formed inside the electrode composite layer obtained using the paste. Therefore, the internal resistance of the secondary battery can be further reduced.

[0028] Note that the CNT may be a single-walled CNT or a multi-walled CNT. Further, the CNT having the predetermined properties described above can be prepared (as a surface-treated carbon nanotube) by performing a surface treatment on the raw material CNT by, for example, the method described later in the section "Method for producing paste for secondary battery". Also, the CNT preferably has an average diameter of, for example, 0.5 nm or more and 200 nm or less. Further, the CNT preferably has an average length of 1 μm or more and 1000 μm or less. Note that the average diameter and average length of the CNT can be determined as the arithmetic mean values of the measured values by observing the CNT with a transmission electron microscope (TEM) and measuring the diameters (outer diameters) and lengths of 50 CNTs from the obtained TEM image.

[0029] <<Other conductive aids>> As other conductive aids, known conductive aids that can be incorporated into the electrodes of secondary batteries can be used. Examples of such conductive aids include carbon blacks (e.g., acetylene black, Ketjenblack (registered trademark), furnace black, etc.), carbon nanohorns, vapor-grown carbon fibers, mild carbon fibers obtained by crushing polymer fibers after firing, single-layer or multi-layer graphene, carbon non-woven fabric sheets obtained by firing non-woven fabrics made of polymer fibers, and various metal fibers or foils. These can be used alone or in combination of two or more.

[0030] When such other conductive aids are used in combination with the above-described CNT, a conductive path can be formed better in the electrode composite layer obtained using the paste, increasing the conductivity, and in some cases, further reducing the internal resistance of the secondary battery. Note that the proportion of other conductive aids contained in the conductive aid is, for example, 0 mass% or more and 50 mass% or less, with the total mass of the conductive aid (i.e., the total mass of the above-described CNT and other conductive aids) being 100 mass%.

[0031] <<Content ratio of conductive aid>> And the content ratio of the conductive auxiliary material in the paste is preferably 2% by mass or more, more preferably 3% by mass or more, still more preferably 4% by mass or more, preferably 20% by mass or less, more preferably 15% by mass or less, still more preferably 10% by mass or less, and particularly preferably 8% by mass or less, with the total mass of the paste being 100% by mass. If the content ratio of the conductive auxiliary material is 2% by mass or more, the solid content concentration of the electrode slurry prepared using the paste can be increased, and the productivity when forming the electrode composite layer from the electrode slurry can be enhanced. On the other hand, if the content ratio of the conductive auxiliary material is 20% by mass or less, the viscosity of the paste will not increase excessively, and sufficient handleability can be ensured. Furthermore, the content ratio of the above-described CNT in the paste is preferably 2% by mass or more, more preferably 3% by mass or more, still more preferably 4% by mass or more, preferably 20% by mass or less, more preferably 15% by mass or less, still more preferably 10% by mass or less, and particularly preferably 8% by mass or less, with the total mass of the paste being 100% by mass. If the content ratio of the CNT is 2% by mass or more, the solid content concentration of the electrode slurry prepared using the paste can be increased, and the productivity when forming the electrode composite layer from the electrode slurry can be enhanced. On the other hand, if the content ratio of the CNT is 20% by mass or less, the viscosity of the paste will not increase excessively, and sufficient handleability can be ensured.

[0032] <Polymer> The polymer is not particularly limited as long as it is a polymer (dispersant) having a function of dispersing the above-described conductive auxiliary material in the dispersion medium in the paste for the secondary battery of the present invention. Note that the polymer can also function as a component (i.e., binder) that can hold the components contained in the electrode composite layer so that the components contained in the electrode composite layer do not detach from the electrode composite layer in the electrode composite layer formed on the current collector using the electrode slurry containing the paste of the present invention.

[0033] Examples of the polymer include acrylic rubber (ACM), polyvinylpyrrolidone (PVP), polyvinylidene fluoride (PVDF), nitrile rubber (NBR), and hydrogenated nitrile rubber (HNBR). These can be used alone or in combination of two or more. Among these, from the viewpoint of improving the stability of the paste and further improving the cycle characteristics while further reducing the internal resistance of the secondary battery, polyvinylpyrrolidone, polyvinylidene fluoride, nitrile rubber, and hydrogenated nitrile rubber are preferred, nitrile rubber and hydrogenated nitrile rubber are more preferred, and hydrogenated nitrile rubber is even more preferred.

[0034] In addition, from the viewpoint of improving the adhesiveness of the electrode composite layer formed from the electrode slurry containing the paste and further reducing the internal resistance of the secondary battery, the polymer preferably has a hydrophilic group. Here, examples of the hydrophilic group include a carboxylic acid group, a sulfonic acid group, a phosphoric acid group, and a hydroxyl group. The polymer may have only one of these hydrophilic groups or two or more thereof. Among these, from the viewpoint of further improving the adhesiveness of the electrode composite layer and further reducing the internal resistance of the secondary battery, a carboxylic acid group is preferred.

[0035] [Example of Polymer] An example of a suitable polymer is given below, but the present invention is not limited thereto. For example, the polymer preferably contains at least one selected from the group consisting of a nitrile group-containing monomer unit, a conjugated diene monomer unit, and a hydrophilic group-containing monomer unit, and more preferably contains all of the nitrile group-containing monomer unit, the conjugated diene monomer unit, and the hydrophilic group-containing monomer unit. And the polymer may contain a structural unit other than the nitrile group-containing monomer unit, the conjugated diene monomer unit, and the hydrophilic group-containing monomer unit (other structural units).

[0036] [Nitrile Group-Containing Monomer Unit] Examples of the nitrile group-containing monomer capable of forming a nitrile group-containing monomer unit include α,β-ethylenically unsaturated nitrile monomers. Specifically, the α,β-ethylenically unsaturated nitrile monomer is not particularly limited as long as it is an α,β-ethylenically unsaturated compound having a nitrile group, and examples thereof include acrylonitrile, methacrylonitrile, and α-alkylacrylonitrile (such as α-ethylacrylonitrile). These can be used alone or in combination of two or more. Among these, acrylonitrile is preferred.

[0037] Here, among all the structural units contained in the polymer, the proportion of the nitrile group-containing monomer unit preferably accounts for 10% by mass or more, more preferably 13% by mass or more, still more preferably 18% by mass or more, preferably 40% by mass or less, more preferably 33% by mass or less, and still more preferably 28% by mass or less, with respect to 100% by mass of all the structural units. If the proportion of the nitrile group-containing monomer unit in all the structural units is 10% by mass or more, the polymer can be well dissolved in a dispersion medium such as N-methylpyrrolidone, and the dispersibility of the polymer is enhanced. Therefore, the conductive auxiliary material can be well dispersed to improve the stability of the paste. Furthermore, an electrode composite material layer in which the conductive auxiliary material is well dispersed can be formed, and the internal resistance of the secondary battery can be further reduced. On the other hand, if the proportion of the nitrile group-containing monomer unit in all the structural units is 40% by mass or less, excessive swelling of the polymer by the electrolytic solution can be suppressed, and the cycle characteristics of the secondary battery can be further improved.

[0038] [Conjugated diene monomer unit] Examples of the conjugated diene monomer capable of forming a conjugated diene monomer unit include 1,3-butadiene, isoprene, 2,3-dimethyl-1,3-butadiene, and 1,3-pentadiene. These can be used alone or in combination of two or more. In the present invention, the "conjugated diene monomer unit" is also intended to include a structural unit (hydride unit) obtained by further hydrogenating a monomer unit contained in a polymer obtained using a conjugated diene monomer. Among the conjugated diene monomers described above, 1,3-butadiene and isoprene are preferred. In other words, as the conjugated diene monomer unit, a 1,3-butadiene unit, an isoprene unit, a 1,3-butadiene hydride unit, and an isoprene hydride unit are preferred.

[0039] Here, among all the structural units contained in the polymer, the proportion of the conjugated diene monomer unit is preferably 15% by mass or more, more preferably 20% by mass or more, still more preferably 25% by mass or more, particularly preferably 30% by mass or more, preferably 55% by mass or less, more preferably 50% by mass or less, and still more preferably 45% by mass or less, with respect to 100% by mass of all the structural units. If the proportion of the conjugated diene monomer unit in all the structural units is 15% by mass or more, the flexibility of the electrode provided with an electrode composite layer formed from an electrode paste-containing slurry is ensured. On the other hand, if the proportion of the conjugated diene monomer unit in all the structural units is 50% by mass or less, the polymer dissolves well in a dispersion medium such as N-methylpyrrolidone, and the dispersibility of the polymer is enhanced. Therefore, the conductive auxiliary material can be well dispersed to improve the stability of the paste. And if the proportion of the conjugated diene monomer unit in all the structural units is 15% by mass or more and 55% by mass or less, the internal resistance of the secondary battery can be further reduced while further improving the cycle characteristics.

[0040] [Monomer unit containing a hydrophilic group] Examples of the hydrophilic group-containing monomer capable of forming a hydrophilic group-containing monomer unit include polymerizable monomers having a hydrophilic group. Specifically, examples of the hydrophilic group-containing monomer include monomers having a carboxylic acid group, monomers having a sulfonic acid group, monomers having a phosphoric acid group, and monomers having a hydroxyl group.

[0041] Examples of the monomer having a carboxylic acid group include monocarboxylic acids and their derivatives, dicarboxylic acids and their acid anhydrides, and their derivatives. Examples of the monocarboxylic acid include acrylic acid, methacrylic acid, crotonic acid, and the like. Examples of the monocarboxylic acid derivative include 2-ethylacrylic acid, isocrotonic acid, α-acetoxyacrylic acid, β-trans-aryloxyacrylic acid, α-chloro-β-E-methoxyacrylic acid, β-diaminoacrylic acid, and the like. Examples of the dicarboxylic acid include maleic acid, fumaric acid, itaconic acid, and the like. Examples of the dicarboxylic acid derivative include methylmaleic acid, dimethylmaleic acid, phenylmaleic acid, chloromaleic acid, dichloromaleic acid, fluoromaleic acid, and maleic acid esters such as methylallyl maleate, diphenyl maleate, nonyl maleate, decyl maleate, dodecyl maleate, octadecyl maleate, and fluoroalkyl maleate. Examples of the acid anhydride of the dicarboxylic acid include maleic anhydride, acrylic anhydride, methyl maleic anhydride, dimethyl maleic anhydride, and the like. In addition, as the monomer having a carboxylic acid group, an acid anhydride that generates a carboxylic acid group by hydrolysis can also be used. In addition, monoesters and diesters of α,β-ethylenically unsaturated polyvalent carboxylic acids such as monoethyl maleate, diethyl maleate, monobutyl maleate, dibutyl maleate, monoethyl fumarate, diethyl fumarate, monobutyl fumarate, dibutyl fumarate, monocyclohexyl fumarate, dicyclohexyl fumarate, monoethyl itaconate, diethyl itaconate, monobutyl itaconate, and dibutyl itaconate are also included.

[0042] Examples of the monomer having a sulfonic acid group include vinylsulfonic acid, methylvinylsulfonic acid, (meth)allylsulfonic acid, styrenesulfonic acid, 2-sulfoethyl (meth)acrylate, 2-acrylamido-2-methylpropanesulfonic acid, 3-allyloxy-2-hydroxypropanesulfonic acid, and the like. In the present invention, “(meth)acryl” means acrylic and / or methacrylic. Further, in the present invention, “(meth)allyl” means allyl and / or methallyl.

[0043] Examples of the monomer having a phosphate group include 2-(meth)acryloyloxyethyl phosphate, methyl 2-(meth)acryloyloxyethyl phosphate, ethyl (meth)acryloyloxyethyl phosphate, and the like. In the present invention, “(meth)acryloyl” means acryloyl and / or methacryloyl.

[0044] Examples of the monomer having a hydroxyl group include ethylenically unsaturated alcohols such as (meth)allyl alcohol, 3-buten-1-ol, 5-hexen-1-ol; alkanol esters of ethylenically unsaturated carboxylic acids such as 2-hydroxyethyl acrylate, 2-hydroxypropyl acrylate, 2-hydroxyethyl methacrylate, 2-hydroxypropyl methacrylate, di-2-hydroxyethyl maleate, di-4-hydroxybutyl maleate, di-2-hydroxypropyl itaconate; general formula: CH2=CR 1 -COO-(C q H 2q O) p -H (wherein p is an integer from 2 to 9, q is an integer from 2 to 4, R 1esters of polyalkylene glycols represented by (wherein represents a hydrogen atom or a methyl group) and (meth)acrylic acid; mono(meth)acrylic esters of dihydroxy esters of dicarboxylic acids such as 2-hydroxyethyl-2'-(meth)acryloyloxy phthalate and 2-hydroxyethyl-2'-(meth)acryloyloxy succinate; vinyl ethers such as 2-hydroxyethyl vinyl ether and 2-hydroxypropyl vinyl ether; mono(meth)allyl ethers of alkylene glycols such as (meth)allyl-2-hydroxyethyl ether, (meth)allyl-2-hydroxypropyl ether, (meth)allyl-3-hydroxypropyl ether, (meth)allyl-2-hydroxybutyl ether, (meth)allyl-3-hydroxybutyl ether, (meth)allyl-4-hydroxybutyl ether, (meth)allyl-6-hydroxyhexyl ether; mono(meth)allyl ethers of polyoxyalkylene glycols such as diethylene glycol mono(meth)allyl ether and dipropylene glycol mono(meth)allyl ether; mono(meth)allyl ethers of (poly)alkylene glycols substituted with halogen and hydroxy such as glycerin mono(meth)allyl ether, (meth)allyl-2-chloro-3-hydroxypropyl ether, and (meth)allyl-2-hydroxy-3-chloropropyl ether; mono(meth)allyl ethers of polyhydric phenols such as eugenol and isoeugenol and halogen-substituted products thereof; (meth)allyl-2-hydroxyethyl thioether, (meth)allyl-2-hydroxypropyl thioether, and other (meth)allyl thioethers of alkylene glycols; and the like.

[0045] These hydrophilic group-containing monomers can be used alone or in combination of two or more. Among these, from the viewpoint of improving the adhesiveness of the electrode binder layer and further reducing the internal resistance of the secondary battery, monomers having a carboxylic acid group are preferred, and acrylic acid and methacrylic acid are more preferred.

[0046] Here, among all the structural units contained in the polymer, the proportion of the hydrophilic group-containing monomer unit preferably accounts for 0.05% by mass or more, more preferably 0.1% by mass or more, still more preferably 1% by mass or more, particularly preferably 3% by mass or more, preferably 10% by mass or less, and more preferably 8% by mass or less, with respect to 100% by mass of all the structural units. If the proportion of the hydrophilic group-containing monomer unit in all the structural units is 0.05% by mass or more, the adhesiveness of the electrode mixture layer obtained using the paste is improved, and the electrode mixture layer can adhere firmly to the current collector. And the internal resistance of the secondary battery can be further reduced. On the other hand, if the proportion of the hydrophilic group-containing monomer unit in all the structural units is 10% by mass or less, the polymer dissolves well in a dispersion medium such as N-methylpyrrolidone, and the dispersibility of the polymer is enhanced. Therefore, the conductive auxiliary material can be dispersed well to improve the stability of the paste.

[0047] [Other structural units] The other structural units are not particularly limited, and examples thereof include structural units derived from known monomers copolymerizable with the above-described nitrile group-containing monomer, conjugated diene monomer, and hydrophilic group-containing monomer. Specifically, the other structural units are not particularly limited, and preferably include, for example, aromatic vinyl monomer units and (meth)acrylate monomer units.

[0048] Examples of the aromatic vinyl monomer that can form the aromatic vinyl monomer unit include styrene, styrene sulfonic acid and its salts, α-methylstyrene, butoxystyrene, and vinylnaphthalene. These can be used alone or in combination of two or more. Among these, styrene is preferred. Here, among all the structural units contained in the polymer, the proportion of the aromatic vinyl monomer units preferably accounts for 15% by mass or more, more preferably 20% by mass or more, still more preferably 24% by mass or more, preferably 55% by mass or less, more preferably 50% by mass or less, and still more preferably 45% by mass or less, with respect to 100% by mass of all the structural units. If the proportion of the aromatic vinyl monomer units in all the structural units is 15% by mass or more, an electrode composite layer in which the conductive auxiliary material is well dispersed can be formed, and the internal resistance of the secondary battery can be further reduced. On the other hand, if the proportion of the aromatic vinyl monomer units in all the structural units is 55% by mass or less, the flexibility of the electrode including the electrode composite layer formed from the electrode slurry containing the paste is ensured.

[0049] Examples of the (meth)acrylic acid ester monomer capable of forming the (meth)acrylic acid ester monomer unit include (meth)acrylic acid alkyl esters and (meth)acrylic acid perfluoroalkyl esters. Examples of the (meth)acrylic acid alkyl ester include acrylic acid alkyl esters such as methyl acrylate, ethyl acrylate, n-propyl acrylate, isopropyl acrylate, n-butyl acrylate, t-butyl acrylate, isobutyl acrylate, n-pentyl acrylate, isopentyl acrylate, hexyl acrylate, heptyl acrylate, octyl acrylate, 2-ethylhexyl acrylate, nonyl acrylate, decyl acrylate, lauryl acrylate, n-tetradecyl acrylate, stearyl acrylate; alkyl methacrylates such as methyl methacrylate, ethyl methacrylate, n-propyl methacrylate, isopropyl methacrylate, n-butyl methacrylate, t-butyl methacrylate, isobutyl methacrylate, n-pentyl methacrylate, isopentyl methacrylate, hexyl methacrylate, heptyl methacrylate, octyl methacrylate, 2-ethylhexyl methacrylate, nonyl methacrylate, decyl methacrylate, lauryl methacrylate, n-tetradecyl methacrylate, stearyl methacrylate, glycidyl methacrylate; and the like. (Meth)acrylic acid perfluoroalkyl esters include 2-(perfluorobutyl)ethyl acrylate, 2-(perfluoropentyl)ethyl acrylate, 2-(perfluorohexyl)ethyl acrylate, 2-(perfluorooctyl)ethyl acrylate, 2-(perfluorononyl)ethyl acrylate, 2-(perfluorodecyl)ethyl acrylate, 2-(perfluorododecyl)ethyl acrylate, 2-(perfluorotetradecyl)ethyl acrylate, 2-(perfluorohexadecyl)ethyl acrylate and other 2-(perfluoroalkyl)ethyl acrylates; 2-(perfluorobutyl)ethyl methacrylate, 2-(perfluoropentyl)ethyl methacrylate, 2-(perfluorohexyl)ethyl methacrylate, 2-(perfluorooctyl)ethyl methacrylate, 2-(perfluorononyl)ethyl methacrylate, 2-(perfluorodecyl)ethyl methacrylate, 2-(perfluorododecyl)ethyl methacrylate, 2-(perfluorotetradecyl)ethyl methacrylate, 2-(perfluorohexadecyl)ethyl methacrylate and other 2-(perfluoroalkyl)ethyl methacrylates; etc. These can be used alone or in combination of two or more. Here, when the polymer contains (meth)acrylic acid ester monomer units, among all the structural units contained in the polymer, the proportion occupied by the (meth)acrylic acid ester monomer units is preferably 15% by mass or more, more preferably 20% by mass or more, still more preferably 24% by mass or more, preferably 55% by mass or less, more preferably 50% by mass or less, and still more preferably 45% by mass or less, with the total structural units being 100% by mass.

[0050] [[Properties]] The polymer preferably has an iodine value of 3 mg / 100 mg or more, preferably 50 mg / 100 mg or less, more preferably 25 mg / 100 mg or less, and still more preferably 8 mg / 100 mg or less. If the iodine value is 3 mg / 100 mg or more, the flexibility of the electrode provided with the electrode composite layer formed from the electrode slurry containing the paste is ensured. On the other hand, if the iodine value is 50 mg / 100 mg or less, an electrode composite layer in which the conductive auxiliary material is well dispersed can be formed, and the internal resistance of the secondary battery can be further reduced.

[0051] <<Preparation method>> The method for preparing the polymer is not particularly limited. For example, a polymer can be prepared by polymerizing a monomer composition containing the above-described monomers and optionally performing hydrogenation. Here, in the present invention, the content ratio of each monomer in the monomer composition can be determined according to the content ratio of each monomer unit in the polymer. The polymerization mode is not particularly limited, and any method such as a solution polymerization method, a suspension polymerization method, a bulk polymerization method, or an emulsion polymerization method can be used. In each polymerization method, known emulsifiers and polymerization initiators can be used as necessary. The method for hydrogenation is not particularly limited, and a general method using a catalyst (see, for example, International Publication No. 2012 / 165120, International Publication No. 2013 / 080989, and Japanese Patent Application Laid-Open No. 2013-8485) can be used.

[0052] <<Content ratio of polymer>> And the content ratio of the polymer in the paste is preferably 0.1% by mass or more, more preferably 0.3% by mass or more, still more preferably 0.4% by mass or more, particularly preferably 0.7% by mass or more, preferably 6% by mass or less, more preferably 3% by mass or less, and still more preferably 2.4% by mass or less, with the total mass of the paste being 100% by mass. If the content ratio of the polymer is 0.1% by mass or more, the conductive auxiliary material can be well dispersed to improve the stability of the paste. On the other hand, if the content ratio of the polymer is 6% by mass or less, the internal resistance of the secondary battery can be sufficiently reduced.

[0053] <Dispersion medium> The dispersion medium is not particularly limited, but an organic solvent having polarity capable of dissolving the above-described polymer can be preferably used. Examples of such organic solvents include acetonitrile, N-methylpyrrolidone, cyclopentanone, N,N-dimethylacetamide, dimethylformamide, dimethyl sulfoxide, methylformamide, methyl ethyl ketone, furfural, etc. Among these, N-methylpyrrolidone is preferable from the viewpoints of ease of handling and safety. The dispersion medium can be used alone or in combination of two or more.

[0054] <Other components> As other components, for example, components such as a viscosity modifier, a reinforcing material, an antioxidant, a surfactant, and an electrolyte additive having a function of suppressing the decomposition of the electrolyte can be mixed. These other components can be those known in the art. These other components can be used alone or in combination of two or more. Note that the paste of the present invention usually does not contain an electrode active material (a positive electrode active material, a negative electrode active material).

[0055] (Method for manufacturing a paste for a secondary battery) And the paste for a secondary battery of the present invention described above can be obtained, for example, by using the method for manufacturing a paste for a secondary battery of the present invention. The manufacturing method of the paste for secondary batteries of the present invention includes at least a step of subjecting raw material CNT to acid treatment (acid treatment step), a step of subjecting the acid-treated raw material CNT to base treatment (base treatment step), a step of washing the base-treated raw material CNT to obtain CNT having predetermined properties (surface-treated CNT) (washing step), and a step of mixing a conductive auxiliary material containing the surface-treated CNT with a polymer and the dispersion medium (mixing step).

[0056] And by using the manufacturing method of the paste for secondary batteries of the present invention, the paste for secondary batteries of the present invention containing CNT having predetermined properties can be efficiently prepared.

[0057] <Acid treatment step> In the acid treatment step, the raw material CNT is subjected to acid treatment. The raw material CNT is not particularly limited and can be appropriately selected from known CNTs according to the properties (number of layers, specific surface area, etc.) of the desired surface-treated CNT.

[0058] Here, the method of acid treatment is not particularly limited as long as the acid can be brought into contact with the raw material CNT, but the method of immersing the raw material CNT in an acid treatment solution (aqueous solution of acid) is preferred. The acid contained in the acid treatment solution is not particularly limited, and examples thereof include nitric acid, sulfuric acid, and hydrochloric acid. These can be used alone or in combination of two or more. Among these, nitric acid and sulfuric acid are preferred.

[0059] The time for immersing the raw material CNT in the acid treatment solution (immersion time) is preferably 1 minute or more, more preferably 10 minutes or more, still more preferably 30 minutes or more, particularly preferably 50 minutes or more, preferably 120 minutes or less, more preferably 100 minutes or less, and still more preferably 80 minutes or less. If the immersion time is 1 minute or more, the surface acid amount of the surface-treated CNT can be increased. If it is 120 minutes or less, the surface acid amount of the surface-treated CNT will not increase excessively, and the production efficiency of the paste is sufficiently ensured.

[0060] When immersing the raw CNT in the acid treatment solution, the temperature (immersion temperature) is preferably 20°C or higher, more preferably 40°C or higher, preferably 80°C or lower, and more preferably 70°C or lower. If the immersion temperature is within the above-described range, the surface acid amount of the obtained surface-treated CNT can be moderately increased.

[0061] After the above immersion, the acid-treated CNT can be recovered from the mixture of the CNT (acid-treated CNT) that has undergone the acid treatment process and the acid treatment solution by a known method such as filtration. The recovered acid-treated CNT may be washed with water as necessary.

[0062] <Base treatment step> In the base treatment step, the acid-treated CNT obtained through the above-described acid treatment step is subjected to base treatment.

[0063] Here, the method of base treatment is not particularly limited as long as the base can be brought into contact with the acid-treated CNT, but a method of immersing the acid-treated CNT in a base treatment solution (aqueous solution of base) is preferred. The base contained in the base treatment solution is not particularly limited, and examples include lithium hydroxide, ammonium chloride, sodium bicarbonate, and sodium hydroxide. These can be used alone or in combination of two or more. Among these, lithium hydroxide and ammonium chloride are preferred, and lithium hydroxide is more preferred.

[0064] The time (immersion time) for immersing the acid-treated CNT in the base treatment solution is preferably 10 minutes or longer, more preferably 60 minutes or longer, further preferably 80 minutes or longer, particularly preferably 90 minutes or longer, preferably 240 minutes or shorter, more preferably 200 minutes or shorter, and further preferably 150 minutes or shorter. If the immersion time is 10 minutes or longer, the surface base amount of the surface-treated CNT can be increased. If it is 240 minutes or shorter, the surface base amount of the surface-treated CNT will not increase excessively, and the production efficiency of the paste can be sufficiently ensured.

[0065] When immersing the acid-treated CNT in the base treatment solution, the temperature (immersion temperature) is preferably 10°C or higher, more preferably 20°C or higher, preferably 40°C or lower, and more preferably 27°C or lower. If the immersion temperature is within the above-mentioned range, the surface base amount of the obtained surface-treated CNT can be appropriately increased.

[0066] <Washing step> In the washing step, the raw material CNT (acid-base treated CNT) obtained through the above-mentioned acid treatment step and base treatment step is washed. By this washing, excess acid components and base components (especially base components) adhering to the surface of the acid-base treated CNT can be removed, and a surface-treated CNT having predetermined properties can be obtained.

[0067] Also, the method for washing the acid-base treated CNT is not particularly limited, but water washing is preferred. For example, the acid-base treated CNT is recovered from the mixture of the acid-base treated CNT and the base treatment solution by a known method such as filtration, and the acid-base treated CNT is washed with water. At this time, by measuring the electrical conductivity of the water (washing water) used for washing the acid-base treated CNT, it is possible to estimate the degree to which the acid components and base components have been removed. After the above-mentioned washing step, if necessary, the water adhering to the surface can be removed by drying or the like to obtain the surface-treated CNT.

[0068] Note that the surface acid amount and surface base amount of the surface-treated CNT can be adjusted by changing the conditions of the above-mentioned acid treatment step, base treatment step, and washing step. For example, the surface acid amount and surface base amount of the surface-treated CNT can be adjusted by changing the types of acid and base contained in the acid treatment solution and base treatment solution used in the acid treatment step and base treatment step, respectively, and their concentrations. Also, by increasing the immersion time of the acid treatment step, the surface acid amount of the surface-treated CNT can be increased, and by increasing the immersion time of the base treatment step, the surface base amount of the surface-treated CNT can be increased. Furthermore, in the washing step, the surface acid amount and surface base amount (especially the surface base amount) can be adjusted by changing the degree of washing.

[0069] <Hybrid process> In the hybrid process, the surface-treated CNT obtained as described above is mixed with a polymer, a dispersion medium, and other conductive aids and / or other components used as necessary. The mixing method in the hybrid process is not particularly limited, and for example, general mixing devices such as a disper, a mill, and a kneader can be used.

[0070] (Slurry for positive electrode of secondary battery) The slurry for the positive electrode of the secondary battery of the present invention contains a positive electrode active material and the paste for the secondary battery of the present invention described above. In other words, the slurry for the positive electrode of the present invention contains a positive electrode active material, CNT having a predetermined property, a polymer, a dispersion medium, and optionally further contains other conductive aids and / or other components. And since the slurry for the positive electrode of the present invention is prepared using the paste of the present invention, if a positive electrode composite layer is formed from the slurry for the positive electrode, the internal resistance of the secondary battery can be reduced, and a positive electrode capable of exhibiting excellent cycle characteristics in the secondary battery can be obtained.

[0071] <Positive electrode active material> Here, the positive electrode active material is a substance that transfers electrons at the positive electrode of the secondary battery. And when the secondary battery is a lithium-ion secondary battery, for example, as the positive electrode active material, a substance that can occlude and release lithium is usually used. In the following, the case where the slurry for the positive electrode of the secondary battery is the slurry for the positive electrode of the lithium-ion secondary battery will be described as an example, but the present invention is not limited to the following example.

[0072] And as the positive electrode active material for the lithium ion secondary battery, without particular limitation, lithium-containing cobalt oxide (LiCoO2), lithium manganate (LiMn2O4), lithium-containing nickel oxide (LiNiO2), lithium-containing composite oxide of Co-Ni-Mn (for example, Li[CoαMnβNiγ]O2, α + β + γ = 1), lithium-containing composite oxide of Ni-Mn-Al, lithium-containing composite oxide of Ni-Co-Al, olivine-type lithium iron phosphate (LiFePO4), olivine-type lithium manganese phosphate (LiMnPO4), Li2MnO3-LiNiO2-based solid solution, Li 1+x Mn 2-x O4 (0 < X < 2) lithium-excess spinel compound represented by, Li[Ni 0.17 Li 0.2 Co 0.07 Mn 0.56 O2, LiNi 0.5 Mn 1.5 O4 and other known positive electrode active materials can be mentioned. And as the positive electrode active material, from the viewpoint of increasing the capacity of the secondary battery, a positive electrode active material in which the proportion of nickel in the transition metal is 60.0 mol% or more and 100.0 mol% or less is preferable. Here, the positive electrode slurry prepared using a positive electrode active material with a large proportion of nickel in the transition metal may be inferior in stability, and therefore may cause an increase in the internal resistance of the secondary battery and a decrease in cycle characteristics. However, by preparing a positive electrode slurry using such a positive electrode active material with a large proportion of nickel in the transition metal and the paste of the present invention, it is possible to sufficiently achieve an improvement in the internal resistance and cycle characteristics of the secondary battery while ensuring the stability of the positive electrode slurry. In addition, examples of transition metals other than nickel contained in the positive electrode active material include cobalt, manganese, iron, and titanium. And as the positive electrode active material in which the proportion of nickel in the transition metal is 60.0 mol% or more and 100.0 mol% or less, for example, LiNi 0.6 Co 0.2 Mn 0.2 O2, LiNi 0.8 Co 0.1 Mn 0.1 O2 can be mentioned. Note that the particle size of the positive electrode active material is not particularly limited and can be the same as that of the conventionally used positive electrode active materials. Further, the positive electrode active material can be used alone or in combination of two or more kinds. The content ratio of the positive electrode active material in the positive electrode slurry is not particularly limited, but it is preferably 90% by mass or more and 99% by mass or less, assuming that the total solid content in the positive electrode slurry is 100% by mass.

[0073] <Paste for secondary battery> As the paste for secondary battery, the paste for secondary battery of the present invention described above, which contains a conductive auxiliary material, a polymer, and a dispersion medium and optionally contains other components, is used.

[0074] Note that the content ratio of the conductive auxiliary material in the positive electrode slurry is not particularly limited, but it is preferably 0.4% by mass or more and 5% by mass or less, assuming that the total solid content in the positive electrode slurry is 100% by mass. Further, the content ratio of the polymer in the positive electrode slurry is not particularly limited, but it is preferably 0.01% by mass or more and 2% by mass or less, assuming that the total solid content in the positive electrode slurry is 100% by mass.

[0075] <Method for preparing positive electrode slurry> The positive electrode slurry of the present invention can be prepared by mixing the above-described positive electrode active material and the above-described paste for secondary battery. The mixing method is not particularly limited, and for example, general mixing devices such as a disperser, a mill, and a kneader can be used.

[0076] (Positive electrode for secondary battery) The positive electrode for secondary battery of the present invention includes a positive electrode composite layer formed using the above-described positive electrode slurry for secondary battery of the present invention. For example, the positive electrode of the present invention can be formed by applying the above-described positive electrode slurry for the present invention onto the surface of a current collector to form a coating film, and then drying the formed coating film. That is, the positive electrode composite layer included in the positive electrode of the present invention is composed of the dried product of the above-described positive electrode slurry for the present invention, and usually includes a positive electrode active material, CNT having a predetermined property, and a polymer, and optionally further includes other conductive aids and / or other components. Note that each component included in the positive electrode composite layer was included in the above-described positive electrode slurry, and the content ratio of these components is usually equal to the content ratio in the above-described positive electrode slurry.

[0077] And since the positive electrode of the present invention includes a positive electrode composite layer formed from the positive electrode slurry for the present invention, by using the positive electrode of the present invention, it is possible to reduce the internal resistance of the secondary battery while exhibiting excellent cycle characteristics in the secondary battery.

[0078] <Method for manufacturing positive electrode> Here, the positive electrode of the present invention can be formed on a current collector through, for example, a step of applying the above-described positive electrode slurry onto the current collector (coating step) and a step of drying the positive electrode slurry applied onto the current collector to form a positive electrode composite layer on the current collector (drying step).

[0079] <<Coating step>> And as a method for applying the above-described positive electrode slurry onto the current collector, there is no particular limitation, and a known method can be used. Specifically, as the coating method, a doctor blade method, a dip method, a reverse roll method, a direct roll method, a gravure method, an extrusion method, a brush coating method, or the like can be used. At this time, the slurry may be applied only to one side of the current collector or to both sides. The thickness of the slurry film on the current collector before drying can be appropriately set according to the thickness of the positive electrode composite layer obtained after drying.

[0080] Here, as the current collector for applying the positive electrode slurry, a material having electrical conductivity and being electrochemically durable is used. Specifically, as the current collector, for example, a current collector made of iron, copper, aluminum, nickel, stainless steel, titanium, tantalum, gold, platinum, etc. can be used. Note that the above materials may be used alone or in combination of two or more in any ratio.

[0081] <<Drying Process>> The method for drying the positive electrode slurry on the current collector is not particularly limited, and known methods can be used. For example, drying methods using warm air, hot air, low humidity air, vacuum drying method, drying methods by irradiation with infrared rays, electron beams, etc. can be mentioned. By drying the slurry on the current collector in this way, a positive electrode composite layer can be formed on the current collector, and a positive electrode including the current collector and the positive electrode composite layer can be obtained.

[0082] Note that after the drying process, a pressure treatment may be applied to the positive electrode composite layer using a die press or a roll press, etc. The pressure treatment can improve the adhesion between the positive electrode composite layer and the current collector. Further, when the positive electrode composite layer contains a curable polymer, it is preferable to cure the polymer after the formation of the positive electrode composite layer.

[0083] (Secondary Battery) The secondary battery of the present invention includes the positive electrode for the secondary battery of the present invention described above. More specifically, the secondary battery of the present invention usually includes a positive electrode, a negative electrode, an electrolytic solution, and a separator, and the positive electrode is the positive electrode of the present invention described above. And since the secondary battery of the present invention uses the positive electrode of the present invention described above, the internal resistance is reduced and the cycle characteristics are excellent.

[0084] <Negative Electrode> Here, as the negative electrode for the secondary battery that can be used in the secondary battery of the present invention, there is no particular limitation, and known negative electrodes used in the manufacture of secondary batteries can be used. For example, such a negative electrode can be a negative electrode formed by forming a negative electrode composite layer on a current collector using a known manufacturing method.

[0085] <Electrolyte solution> As the electrolyte solution, an organic electrolyte solution in which a supporting electrolyte is dissolved in an organic solvent is usually used. For example, a lithium salt is used as the supporting electrolyte of a lithium-ion secondary battery. Examples of the lithium salt include LiPF6, LiAsF6, LiBF4, LiSbF6, LiAlCl4, LiClO4, CF3SO3Li, C4F9SO3Li, CF3COOLi, (CF3CO)2NLi, (CF3SO2)2NLi, (C2F5SO2)NLi, etc. Among them, LiPF6, LiClO4, and CF3SO3Li are preferable because they are easily soluble in the solvent and show a high degree of dissociation, and LiPF6 is particularly preferable. Note that the electrolyte may be used alone or in combination of two or more in any ratio. Usually, the higher the degree of dissociation of the supporting electrolyte used, the higher the lithium ion conductivity tends to be, so the lithium ion conductivity can be adjusted according to the type of the supporting electrolyte.

[0086] The organic solvent used in the electrolyte solution is not particularly limited as long as it can dissolve the supporting electrolyte. For example, carbonates such as dimethyl carbonate (DMC), ethylene carbonate (EC), diethyl carbonate (DEC), propylene carbonate (PC), butylene carbonate (BC), and ethyl methyl carbonate (EMC); esters such as γ-butyrolactone and methyl formate; ethers such as 1,2-dimethoxyethane and tetrahydrofuran; sulfur-containing compounds such as sulfolane and dimethyl sulfoxide; etc. are preferably used. Also, a mixture of these solvents may be used. Among them, it is preferable to use carbonates because they have a high dielectric constant and a wide stable potential region. Note that the concentration of the electrolyte in the electrolyte solution can be adjusted as appropriate. Also, known additives can be added to the electrolyte solution.

[0087] <Separator> As the separator, without being particularly limited, for example, those described in JP-A-2012-204303 can be used. Among these, a microporous membrane made of a polyolefin resin (polyethylene, polypropylene, polybutene, polyvinyl chloride) is preferable because the film thickness of the entire separator can be reduced, and thereby the ratio of the electrode active material in the secondary battery can be increased and the capacity per volume can be increased.

[0088] And the secondary battery can be manufactured, for example, by laminating a positive electrode and a negative electrode with a separator interposed therebetween, winding or folding this according to the battery shape as necessary, placing it in a battery container, and injecting an electrolytic solution into the battery container and sealing it. Here, in the electrochemical element of the present invention, the positive electrode for an electrochemical element described above is used. In addition, in the electrochemical element of the present invention, in order to prevent an increase in the internal pressure of the secondary battery, the occurrence of overcharge / discharge, etc., an overcurrent prevention element such as a fuse, a PTC element, an expandable metal, a lead plate, etc. may be provided as necessary. The shape of the secondary battery may be, for example, a coin type, a button type, a sheet type, a cylindrical type, a rectangular type, a flat type, etc.

Examples

[0089] Hereinafter, the present invention will be specifically described based on examples, but the present invention is not limited to these examples. In the following description, “%” and “parts” representing amounts are on a mass basis unless otherwise specified. In the examples and comparative examples, the surface acid amount, surface base amount, specific surface area of the carbon nanotube, the stability of the paste for secondary batteries, and the internal resistance and cycle characteristics of the secondary batteries were evaluated using the following methods, respectively.

[0090] <Surface acid amount> Precisely weigh about 1 g of the CNT to be measured, and 0.01 mol dm -3It was immersed in 100 ml of tetrabutyl hydride (TBA OH) / 4-methyl-2-pentanone (MIBK) solution and stirred with a stirrer for 1 hour. Then, centrifugation was performed and the supernatant was filtered through a filter. The TBA OH remaining in 50 mL of the obtained filtrate was quantitatively analyzed by non-aqueous coulometric titration with perchloric acid (HClO4) / MIBK solution, and the acid amount (mmol / g) per 1 g of CNT was specified from the obtained value. For the analysis, an automatic coulometric titrator (manufactured by Kyoto Electronics Co., Ltd., product name "AT-700") was used. Also, the series of operations were carried out at room temperature under an argon stream. -3 The surface base amount <Surface base amount> Approximately 1 g of CNT to be measured was precisely weighed and immersed in 100 ml of 0.01 mol dm -3 HClO4 / MIBK solution and stirred with a stirrer for 1 hour. Then, centrifugation was performed and the supernatant was filtered through a filter. The HClO4 remaining in 50 mL of the obtained filtrate was quantitatively analyzed by non-aqueous coulometric titration with 0.01 mol dm -3 TBA OH / MIBK solution, and the base amount (mmol / g) per 1 g of CNT was specified from the obtained value. For the analysis, an automatic coulometric titrator (manufactured by Kyoto Electronics Co., Ltd., product name "AT-700") was used. Also, the series of operations were carried out at room temperature under an argon stream. <Specific surface area> The specific surface area (BET specific surface area) of CNT was measured using Belsorp-mini (manufactured by Microtrac·BEL Co., Ltd., conforming to ASTM D3037-81). <Stability> The viscosity of the paste for secondary battery immediately after preparation (initial viscosity) was measured using a rheometer (manufactured by Anton Paar Co., Ltd., "MCR302") under the conditions of a temperature of 25°C and a shear rate of 0.1 s -1 . Then, the paste for secondary battery was stored at room temperature after 7 days, and the viscosity (viscosity after storage) was measured under the same conditions as the initial viscosity. And the value (%) of the viscosity after storage when the initial viscosity was set to 100% was evaluated according to the following criteria. The smaller the viscosity change, the better the stability of the paste. A: The viscosity after storage is 100% or more and less than 150% B: Viscosity after storage is 150% or more and less than 200% C: Viscosity after storage is 200% or more and less than 250% D: Viscosity after storage is 250% or more <Internal Resistance> The secondary battery was charged to 50% of SOC (State Of Charge, depth of charge) at 1C (C is the value represented by the rated capacity (mA) / 1 hour (h)) in an atmosphere of 25°C. Then, in an environment of 25°C, charging and discharging were each performed for 20 seconds at 0.2C, 0.5C, 1.0C, 2.0C, and 3.0C with 50% of SOC as the center. The battery voltage 20 seconds after each case (charging side and discharging side) was plotted against the current value, and the slope was determined as the IV resistance (Ω) (charging-time IV resistance and discharging-time IV resistance). For the obtained values of the IV resistance (Ω), the rate of change (%) based on the value of Comparative Example 5 was calculated (Comparative Example 5 is 0%), and evaluation was performed according to the following criteria. The smaller the IV resistance (compared to Comparative Example 5), the less the internal resistance and the better the output characteristics of the secondary battery. A: The IV resistance decreased by 11.0% or more B: The IV resistance decreased by 6.0% or more and less than 11.0% C: The IV resistance decreased by 1.0% or more and less than 6.0% D: The IV resistance decreased by less than 1.0%, was the same as Comparative Example 5, or increased from Comparative Example 5 <Cycle Characteristics> The operation of charging the secondary battery to 4.2V at 0.2C (C is the value represented by the rated capacity (mA) / 1 hour (h)) and discharging it to 3.0V in an atmosphere of 25°C was repeated 3 times. Then, in an environment of 45°C, the operation of charging at 1C until the battery voltage reached 4.2V and discharging at 1C until the battery voltage reached 3.0V was repeated 200 times. And from the first discharge capacity (C0) and the 200th discharge capacity (C1), the capacity retention rate ΔC = (C1 / C0) × 100 (%) was calculated, and evaluation was performed according to the following criteria. The higher the value of this capacity retention rate, the less the decrease in the discharge capacity and the better the cycle characteristics of the secondary battery. A: The capacity retention rate ΔC is 90.0% or more B: The capacity retention rate ΔC is 88.5% or more and less than 90.0% C: Capacity retention rate ΔC is 87.0% or more and less than 88.5% D: Capacity retention rate ΔC is less than 87.0%

[0091] (Example 1) <Preparation of surface-treated CNT>[[]] 1 g of weighed multi-walled carbon nanotubes (specific surface area: 300 m 2 / g) was added to a mixed solution of 40 mL of concentrated nitric acid and 40 mL of 2 M sulfuric acid, and stirred for 1 hour while maintaining at 60 °C (acid treatment). Then, filtration was performed using filter paper (Toyo Roshi Kaisha, Filter Paper No. 2, 125 mm) for solid-liquid separation. The solid matter on the filter paper was washed with 200 ml of purified water, and then the CNT solid matter (acid-treated CNT) was recovered. Further, this CNT solid matter was put into 200 ml of an aqueous lithium hydroxide solution with a concentration of 2.5 mol / liter, and stirred for 2 hours while maintaining at 25 °C in a water bath (base treatment). Then, suction filtration was performed using a membrane filter with a pore size of 10 μm for solid-liquid separation. The CNT solid matter (acid-base treated CNT) on the membrane filter was repeatedly washed with purified water. When the electrical conductivity of the washing water became 50 μs / m or less, the CNT solid matter was separated by solid-liquid separation in the same manner as above. The obtained CNT solid matter was dried under reduced pressure at 50 °C for 8 hours to prepare surface-treated CNT. For this surface-treated CNT, the surface acid amount, surface base amount, and specific surface area were measured. Also, the ratio of the surface acid amount to the surface base amount was calculated. The results are all shown in Table 1. <Preparation of polymer>[[]] Into a reactor with an internal volume of 10 liters, 100 parts of ion-exchanged water, 23 parts of acrylonitrile as a monomer, 30 parts of 1,3-butadiene, 4 parts of methacrylic acid, and 43 parts of styrene were charged, 2 parts of potassium oleate as an emulsifier, 0.1 part of potassium phosphate as a stabilizer, and further, 0.5 part of 2,2′,4,6,6′-pentamethylheptane-4-thiol (TIBM) as a molecular weight regulator were added, and emulsion polymerization was carried out at 30 °C in the presence of 0.35 part of potassium persulfate as a polymerization initiator to copolymerize the above-mentioned monomers. When the coincidence conversion rate reached 90%, 0.2 parts of hydroxylamine sulfate per 100 parts of the monomer was added to stop the polymerization. Subsequently, the mixture was heated and steam distilled at about 70 °C under reduced pressure to recover the residual monomer. Then, 2 parts of alkylated phenol was added as an antioxidant to obtain an aqueous dispersion of the polymer. Next, 400 mL (total solid content: 48 g) of the obtained aqueous dispersion of the polymer was charged into a 1-liter autoclave equipped with a stirrer, and nitrogen gas was passed through for 10 minutes to remove dissolved oxygen in the aqueous dispersion. Then, as a hydrogenation reaction catalyst, 50 mg of palladium acetate was dissolved in 180 mL of water to which nitric acid 4 times the molar equivalent of Pd was added, and then added. After replacing the inside of the system with hydrogen gas twice, the contents of the autoclave were heated to 50 °C under a pressure of hydrogen gas up to 3 MPa (gauge pressure), and a hydrogenation reaction was carried out for 6 hours. Thereafter, the contents were returned to room temperature, and after making the inside of the system a nitrogen atmosphere, it was concentrated using an evaporator until the solid content concentration reached 40% to obtain a hydrogenated nitrile rubber as a polymer. Then, N-methylpyrrolidone was added to an aqueous solution of the hydrogenated nitrile rubber having a solid content concentration of 40% as a polymer, and then vacuum distillation was carried out to remove water and excess N-methylpyrrolidone to obtain an N-methylpyrrolidone solution of the hydrogenated nitrile rubber having a solid content concentration of 8%. <Preparation of Paste for Secondary Battery> 4 parts of the surface-treated CNT obtained as described above, 0.8 part (equivalent amount of solid content) of the N-methylpyrrolidone solution of the hydrogenated nitrile rubber obtained as described above, and an appropriate amount of N-methylpyrrolidone as a dispersion medium were added, and stirred with a disper (3000 rpm, 60 minutes). Then, a bead mill using zirconia beads with a diameter of 1 mm was used, and the mixture was mixed at a peripheral speed of 8 m / s for 1 hour to produce a paste for a secondary battery. The paste had a viscosity of 49,000 mPa·s at a temperature of 25 °C and a shear rate of 0.1 s -1 and a solid content concentration value of 4.8%. The stability of this paste for a secondary battery was evaluated. The results are shown in Table 1. <Preparation of Slurry for Positive Electrode of Secondary Battery> Into the paste for secondary battery obtained as described above, 100 parts of a ternary active material (LiNi 0.6 Co 0.2 Mn 0.2 O2, ratio of nickel in transition metals: 60.0 mol%) (volume average particle diameter: 10 μm) having a layered structure as a positive electrode active material and an appropriate amount of N-methylpyrrolidone as a dispersion medium were added and stirred with a disper (3000 rpm, 20 minutes) to prepare a slurry for the positive electrode of a secondary battery. The addition amount of N-methylpyrrolidone was adjusted so that the viscosity of the obtained positive electrode slurry at 60 rpm was in the range of 3000 to 4000 mPa·s. <Fabrication of Positive Electrode for Secondary Battery> As a current collector, an aluminum foil with a thickness of 20 μm was prepared. The positive electrode slurry obtained as described above was applied to one side of the aluminum foil with a dry basis weight of 20 mg / cm 2 by a comma coater, dried at 90 °C for 20 minutes and at 120 °C for 20 minutes, and then heat-treated at 60 °C for 10 hours to obtain a positive electrode precursor. This positive electrode precursor was rolled by a roll press to fabricate a sheet-shaped positive electrode composed of a positive electrode composite layer (density: 3.2 g / cm 3 ) and an aluminum foil. Then, the sheet-shaped positive electrode was cut into a width of 48.0 mm and a length of 47 cm to obtain a positive electrode for a lithium-ion secondary battery. <Fabrication of Negative Electrode for Secondary Battery> 90 parts of spherical artificial graphite (volume average particle diameter: 12 μm) as a negative electrode active material and 10 parts of SiO x (volume average particle diameter: 10 μm), 1 part of styrene-butadiene polymer as a binder for the negative electrode, 1 part of carboxymethyl cellulose as a thickener, and an appropriate amount of water as a dispersion medium were stirred with a planetary mixer to prepare a slurry for the negative electrode of a secondary battery. Next, as a current collector, a copper foil with a thickness of 15 μm was prepared. The slurry for the negative electrode of a secondary battery obtained as described above was applied to one side of the copper foil with a dry coating amount of 10 mg / cm 2 and dried at 60 °C for 20 minutes and at 120 °C for 20 minutes. Then, it was heat-treated at 150 °C for 2 hours to obtain a negative electrode precursor. This negative electrode precursor was rolled by a roll press to a density of 1.6 g / cm3 A sheet-like negative electrode composed of the negative electrode composite layer and a copper foil was fabricated. Then, the sheet-like negative electrode was cut into a width of 50.0 mm and a length of 52 cm to be used as a negative electrode for a lithium-ion secondary battery. <Manufacture of Secondary Battery> The positive electrode for a lithium-ion secondary battery and the negative electrode for a lithium-ion secondary battery fabricated as described above were arranged such that the electrode composite layers faced each other, with a separator (a microporous membrane made of polypropylene) having a thickness of 15 μm interposed therebetween, and wound using a core with a diameter of 20 mm to obtain a wound body. Then, the obtained wound body was compressed from one direction at a speed of 10 mm / second until it had a thickness of 4.5 mm. Note that the wound body after compression had an elliptical shape in plan view, and the ratio of its major axis to minor axis (major axis / minor axis) was 7.7. Also, an electrolytic solution (a 1.0 M LiPF6 solution (the solvent is a mixed solution of ethylene carbonate / ethyl methyl carbonate = 3 / 7 (mass ratio) with 5 mass% of fluoroethylene carbonate added, and 2 volume% of vinylene carbonate is added as an additive)) was prepared. Thereafter, the wound body after compression was housed in an aluminum laminate case together with 3.2 g of the electrolytic solution. Then, a nickel lead wire was connected to a predetermined location of the negative electrode for a lithium-ion secondary battery, and an aluminum lead wire was connected to a predetermined location of the positive electrode for a lithium-ion secondary battery. After that, the opening of the case was heat-sealed to obtain a lithium-ion secondary battery. This lithium-ion secondary battery had a pouch shape with a width of 35 mm, a height of 60 mm, and a thickness of 5 mm, and the nominal capacity of the battery was 700 mAh. The obtained lithium-ion secondary battery was evaluated for its internal resistance and cycle characteristics. The results are shown in Table 1.

[0092] (Example 2) When preparing the surface-treated CNT, except that an aqueous ammonium chloride solution was used instead of an aqueous lithium hydroxide solution, in the same manner as in Example 1, surface-treated CNT, polymer, paste for secondary battery, slurry for secondary battery positive electrode, positive electrode for secondary battery, negative electrode for secondary battery, and secondary battery were fabricated and various evaluations were performed. The results are shown in Table 1.

[0093] (Example 3) In the preparation of the surface-treated CNT, except that the time of the base treatment was changed from 2 hours to 4 hours, the surface-treated CNT, polymer, paste for secondary battery, slurry for secondary battery positive electrode, positive electrode for secondary battery, negative electrode for secondary battery, and secondary battery were produced in the same manner as in Example 1, and various evaluations were carried out. The results are shown in Table 1.

[0094] (Example 4) In the preparation of the surface-treated CNT, the raw material CNT was changed to another multi-walled CNT (specific surface area: 165 m 2 / g), and in the preparation of the paste for secondary battery, except that the amount of the surface-treated CNT was changed from 4 parts to 7 parts and the amount of the polymer was changed from 0.8 part to 0.7 part, the surface-treated CNT, polymer, paste for secondary battery, slurry for secondary battery positive electrode, positive electrode for secondary battery, negative electrode for secondary battery, and secondary battery were produced in the same manner as in Example 1, and various evaluations were carried out. The results are shown in Table 1.

[0095] (Example 5) In the preparation of the surface-treated CNT, the raw material CNT was further changed to another multi-walled CNT (specific surface area: 100 m 2 / g), and in the preparation of the paste for secondary battery, except that the amount of the surface-treated CNT was changed from 4 parts to 15 parts and the amount of the polymer was changed from 0.8 part to 1.5 part, the surface-treated CNT, polymer, paste for secondary battery, slurry for secondary battery positive electrode, positive electrode for secondary battery, negative electrode for secondary battery, and secondary battery were produced in the same manner as in Example 1, and various evaluations were carried out. The results are shown in Table 1.

[0096] (Example 6) In the preparation of the polymer, except that 18 parts of acrylonitrile, 41 parts of 1,3-butadiene, 4 parts of methacrylic acid, and 37 parts of styrene were used as monomers, the surface-treated CNT, polymer, paste for secondary battery, slurry for secondary battery positive electrode, positive electrode for secondary battery, negative electrode for secondary battery, and secondary battery were produced in the same manner as in Example 1, and various evaluations were carried out. The results are shown in Table 1.

[0097] (Example 7) In the preparation of the polymer, except that 28 parts of acrylonitrile, 44 parts of 1,3-butadiene, 4 parts of methacrylic acid, and 24 parts of styrene were used as monomers, in the same manner as in Example 1, surface-treated CNT, polymer, paste for secondary battery, slurry for secondary battery positive electrode, positive electrode for secondary battery, negative electrode for secondary battery, and secondary battery were prepared, and various evaluations were performed. The results are shown in Table 1.

[0098] (Examples 8 - 9) Without preparing the polymer, and except that the following polymers were used as dispersants respectively, in the same manner as in Example 1, surface-treated CNT, paste for secondary battery, slurry for secondary battery positive electrode, positive electrode for secondary battery, negative electrode for secondary battery, and secondary battery were prepared, and various evaluations were performed. The results are shown in Table 1 or 2. Example 8: PVDF (manufactured by Solvay, "Solef (registered trademark) 5130") Example 9: PVP (manufactured by Junsei Chemical Co., Ltd., "Polyvinylpyrrolidone K - 15")

[0099] (Comparative Example 1) In the preparation of the surface-treated CNT, without performing the base treatment, and except that the acid-treated CNT was washed until the electrical conductivity of the washing water was 50 μs / m or less, in the same manner as in Example 1, surface-treated CNT, polymer, paste for secondary battery, slurry for secondary battery positive electrode, positive electrode for secondary battery, negative electrode for secondary battery, and secondary battery were prepared, and various evaluations were performed. The results are shown in Table 2.

[0100] (Comparative Example 2) In the preparation of the surface-treated CNT, except that the acid-base-treated CNT was washed until the electrical conductivity of the washing water was 300 μs / m or less, in the same manner as in Example 1, surface-treated CNT, polymer, paste for secondary battery, slurry for secondary battery positive electrode, positive electrode for secondary battery, negative electrode for secondary battery, and secondary battery were prepared, and various evaluations were performed. The results are shown in Table 2.

[0101] (Comparative Example 3) When preparing the surface-treated CNT, except that no base treatment was performed and the acid-treated CNT was washed until the electrical conductivity of the washing water became 300 μs / m or less, in the same manner as in Example 1, a surface-treated CNT, a polymer, a paste for a secondary battery, a slurry for a secondary battery positive electrode, a positive electrode for a secondary battery, a negative electrode for a secondary battery, and a secondary battery were produced, and various evaluations were conducted. The results are shown in Table 2.

[0102] (Comparative Example 4) When preparing the surface-treated CNT, except that the acid treatment time was changed from 1 hour to 3 hours, in the same manner as in Example 1, a surface-treated CNT, a polymer, a paste for a secondary battery, a slurry for a secondary battery positive electrode, a positive electrode for a secondary battery, a negative electrode for a secondary battery, and a secondary battery were produced, and various evaluations were conducted. The results are shown in Table 2.

[0103] (Comparative Example 5) Except for not preparing the surface-treated CNT and using the raw material CNT instead of the surface-treated CNT, in the same manner as in Example 1, a polymer, a paste for a secondary battery, a slurry for a secondary battery positive electrode, a positive electrode for a secondary battery, a negative electrode for a secondary battery, and a secondary battery were produced, and various evaluations were conducted. The results are shown in Table 2.

[0104] In Tables 1 and 2 shown below, "AN" represents an acrylonitrile unit, "BD" represents a 1,3-butadiene unit (or a 1,3-butadiene hydride unit), "MAA" represents a methacrylic acid unit, "ST" represents a styrene unit.

[0105] [Table 1]

[0106] [Table 2]

[0107] From Tables 1 and 2, it can be seen that in Examples 1 to 9 where the positive electrode was fabricated using the paste for secondary batteries containing CNTs having a predetermined property, it is possible to reduce the internal resistance of the secondary battery while improving the cycle characteristics. Also, from Examples 1 to 9, it can be seen that the paste for secondary batteries is excellent in stability. On the other hand, from Table 2, in Comparative Example 1 where the paste for secondary batteries containing CNTs in which the surface base amount and the ratio of the surface acid amount to the surface base amount are each outside the predetermined range, it can be seen that the internal resistance of the secondary battery increases and the cycle characteristics deteriorate. Also, from Table 2, it can be seen that in Comparative Example 2 where the paste for secondary batteries containing CNTs with a surface base amount outside the predetermined range, the stability of the paste for secondary batteries and the cycle characteristics of the secondary battery deteriorate. And from Table 2, it can be seen that in Comparative Examples 3 to 5 where the paste for secondary batteries containing CNTs with a ratio of the surface acid amount to the surface base amount outside the predetermined range, the stability of the paste for secondary batteries and the cycle characteristics of the secondary battery deteriorate, and the internal resistance of the secondary battery increases.

Industrial Applicability

[0108] According to the present invention, it is possible to provide a paste for secondary batteries and a method for manufacturing the same that can fabricate an electrode capable of reducing the internal resistance of a secondary battery and exhibiting excellent cycle characteristics in the secondary battery. Also, according to the present invention, it is possible to provide a slurry for a positive electrode of a secondary battery that can fabricate a positive electrode capable of reducing the internal resistance of a secondary battery and exhibiting excellent cycle characteristics in the secondary battery. And according to the present invention, it is possible to provide a positive electrode for a secondary battery that can reduce the internal resistance of a secondary battery and exhibit excellent cycle characteristics in the secondary battery. Furthermore, according to the present invention, it is possible to provide a secondary battery with a reduced internal resistance and excellent cycle characteristics.

Claims

1. A paste for a secondary battery containing a conductive auxiliary material, a polymer, and a dispersion medium, wherein the conductive auxiliary material contains carbon nanotubes having a surface base amount of 0.01 mmol / g or more and 0.10 mmol / g or less, and a ratio of surface acid amount to the surface base amount of 0.1 or more and 1.0 or less, the carbon nanotubes are obtained by contacting an acid-treated raw material carbon nanotube with at least one base selected from the group consisting of lithium hydroxide, ammonium chloride, sodium bicarbonate, and sodium hydroxide, and then washing, A paste for a secondary battery.

2. The specific surface area of the carbon nanotube is 150 m ２ / g or more, the paste for secondary battery according to claim 1.

3. The paste for a secondary battery according to claim 1 or 2, wherein the surface acid amount of the carbon nanotubes is 0.01 mmol / g or more and 0.10 mmol / g or less.

4. The paste for a secondary battery according to any one of claims 1 to 3, wherein the polymer contains a nitrile group-containing monomer unit in a proportion of 10% by mass or more and 40% by mass or less, and a conjugated diene monomer unit in a proportion of 15% by mass or more and 55% by mass or less.

5. The paste for a secondary battery according to any one of claims 1 to 4, wherein the polymer has a hydrophilic group.

6. The paste for a secondary battery according to any one of claims 1 to 5, wherein the iodine value of the polymer is 3 mg / 100 mg or more and 50 mg / 100 mg or less.

7. The paste for a secondary battery according to any one of claims 1 to 6, wherein the content ratio of the conductive auxiliary material is 2% by mass or more and 20% by mass or less, and the content ratio of the polymer is 0.1% by mass or more and 6% by mass or less.

8. A slurry for a positive electrode of a secondary battery, comprising a positive electrode active material and the paste for a secondary battery according to any one of claims 1 to 7.

9. The slurry for a positive electrode of a secondary battery according to claim 8, wherein the ratio of nickel in the transition metal in the positive electrode active material is 60.0 mol% or more and 100.0 mol% or less.

10. A positive electrode for a secondary battery, comprising a positive electrode composite layer formed using the slurry for a positive electrode of a secondary battery according to claim 8 or 9.

11. A secondary battery, comprising the positive electrode for a secondary battery according to claim 10.

12. A method for manufacturing the paste for a secondary battery according to any one of claims 1 to 7, comprising: a step of subjecting a raw material carbon nanotube to an acid treatment; and a step of subjecting the acid-treated raw material carbon nanotube to a base treatment. A step of washing the base-treated raw material carbon nanotubes to obtain the carbon nanotubes; A step of mixing the conductive auxiliary material containing the carbon nanotubes with the polymer and the dispersion medium; A method for manufacturing a paste for a secondary battery, comprising the above steps.

Images