Conductive material dispersion, slurry for secondary battery positive electrode, positive electrode for secondary battery and secondary battery

Abstract

The purpose of the present invention is to provide a conductive material dispersion liquid which is capable of ensuring good high-temperature storage characteristics of a secondary battery, while reducing the internal resistance of the secondary battery. A conductive material dispersion liquid according to the present invention contains a carbon material, a dispersant and a dispersion medium; and the HSP distance (Rd) between the Hansen solubility parameter (HSPc) of the carbon material and the Hansen solubility parameter (HSPd) of the dispersant is 10.0 MPa1 / 2 or less.

Description

[Technical Field]

[0001] The present invention relates to a conductive material dispersion, a slurry for a secondary battery positive electrode, a positive electrode for a secondary battery, and a secondary battery. [Background technology]

[0002] Secondary batteries, such as lithium-ion secondary batteries, are small, lightweight, have high energy density, and are capable of repeated charge and discharge, and are therefore used in a wide range of applications. For example, an electrode for a lithium-ion secondary battery typically includes a current collector and an electrode mixture layer formed on the current collector. The electrode mixture layer, e.g., a positive electrode mixture layer, is typically formed by applying a positive electrode slurry onto a current collector and drying the slurry. The positive electrode slurry contains, in a dispersion medium, a positive electrode active material, a conductive material for improving conductivity, a binder for binding these components, and the like.

[0003] To improve the performance of electrochemical devices, attempts have been made to improve electrode slurries, such as positive electrode slurries. For example, carbon materials used as conductive materials tend to aggregate. Therefore, in order to sufficiently disperse the carbon material and enable secondary batteries to exhibit excellent battery characteristics, a technique has been proposed in which a carbon material and a dispersant are premixed in a dispersion medium to form a conductive material dispersion, and the resulting conductive material dispersion is combined with an electrode active material, etc. to prepare an electrode slurry (see, for example, Patent Documents 1 to 5). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] International Publication No. 2016 / 103730 [Patent Document 2] International Publication No. 2019 / 181869 [Patent Document 3] Special Publication No. 2018-522803 [Patent Document 4] Japanese Patent Application Laid-Open No. 2015-13302 [Patent Document 5] JP 2018-45820 A Summary of the Invention [Problem to be solved by the invention]

[0005] However, in the above-mentioned conventional technology, there is a demand for further improvement in the battery characteristics of secondary batteries. Specifically, there is a demand for further reduction in the internal resistance of secondary batteries and for sufficient suppression of gas generation during high-temperature storage of secondary batteries (i.e., improvement in high-temperature storage characteristics).

[0006] Therefore, an object of the present invention is to provide a conductive material dispersion, a slurry for a secondary battery positive electrode, and a positive electrode for a secondary battery that can reduce the internal resistance of the secondary battery while ensuring good high-temperature storage characteristics of the secondary battery. Another object of the present invention is to provide a secondary battery having reduced internal resistance and excellent high-temperature storage characteristics. [Means for solving the problem]

[0007] The present inventors have conducted extensive research to solve the above problems, and have discovered a conductive material dispersion containing a carbon material and a dispersant in a dispersion medium, in which the Hansen solubility parameter (HSP) of the carbon material is c ) and the Hansen solubility parameter (HSP) of the dispersant d ) and the HSP distance (R d The present inventors have found that, if an electrode is produced using a conductive material dispersion liquid in which the value of (a) is a predetermined value or less, the internal resistance of the secondary battery can be reduced and the high-temperature storage characteristics of the secondary battery can be improved, and have completed the present invention.

[0008] That is, the present invention aims to advantageously solve the above-mentioned problems, and the conductive material dispersion of the present invention is a conductive material dispersion containing a carbon material, a dispersant, and a dispersion medium, and the carbon material has a Hansen solubility parameter (HSP) of 1.5. c ) and the Hansen solubility parameter (HSP) of the dispersant d ) and the HSP distance (R d ) is 10.0 MPa1 / 2 In this way, the HSP distance (R d When an electrode is produced using a conductive material dispersion liquid containing a carbon material and a dispersant in a dispersion medium, in which the value of σ is the above-mentioned value or less, the internal resistance of a secondary battery including the electrode can be reduced and good high-temperature storage characteristics can be ensured.

[0009] In the present invention, the "Hansen solubility parameter (HSP) of the carbon material" c )” is the polar term δ p1 , dispersion term δ d1 and the hydrogen bond term δ h1 It is composed of the Hansen Solubility Parameters (HSP) of the dispersant. d )” is the polar term δ p2 , dispersion term δ d2 , and the hydrogen bond term δ h2 It consists of: Here, in the present invention, "δ p1 ", "δ d1 " and "δ h1 " and "δ p2 ", "δ d2 " and "δ h2 " can be identified using the method described in the Examples. In the present invention, the "HSP distance (R d )" is expressed by the following formula (1): HSP distance (R d )={(δ p1 -δ p2 ) 2 +4×(δ d1 -δ d2 ) 2 +(δ h1 -δ h2 ) 2} 1 / 2 ···(1) It can be calculated using:

[0010] Here, the conductive material dispersion of the present invention is a dispersion liquid in which the Hansen solubility parameter (HSP) of the carbon material is c ) hydrogen bond term δ h1 is 12.0 MPa 1 / 2 It is preferable that the hydrogen bond term δ is equal to or less than δ.h1 When the value is equal to or less than the above-mentioned value, the viscosity stability of the conductive material dispersion liquid can be increased, and the internal resistance of the secondary battery can be further reduced while the high-temperature storage characteristics can be further improved.

[0011] In the conductive material dispersion of the present invention, the carbon material preferably contains a fibrous carbon material. If a fibrous carbon material is used as the carbon material, the capacity of the secondary battery can be increased. In the present invention, the "fibrous carbon material" has an aspect ratio (major axis / minor axis) of 5 or more. The aspect ratio of the fibrous carbon material is preferably greater than 10. The "aspect ratio" can be determined by observing any fibrous carbon material with a scanning electron microscope (SEM), measuring the maximum diameter (major axis) and the fiber diameter (minor axis) in the direction perpendicular to the maximum diameter, and calculating the ratio of the major axis to the minor axis (major axis / minor axis).

[0012] The present invention also aims to advantageously solve the above-mentioned problems, and provides a slurry for a positive electrode of a secondary battery, which is characterized by containing any of the above-mentioned conductive material dispersions and a positive electrode active material. When a positive electrode is produced using the positive electrode slurry containing any of the above-mentioned conductive material dispersions and a positive electrode active material, it is possible to reduce the internal resistance of a secondary battery including the positive electrode and ensure good high-temperature storage characteristics.

[0013] Here, the secondary battery positive electrode slurry of the present invention preferably further contains a binder, which can suppress sedimentation of the positive electrode active material and can ensure good adhesion of the positive electrode mixture layer formed using the positive electrode slurry to the current collector.

[0014] The slurry for a secondary battery positive electrode of the present invention has a Hansen solubility parameter (HSP) of the carbon material. c ) and the Hansen solubility parameter (HSP) of the binder b ) and the HSP distance (R b ) is the HSP distance (R d ) is preferable. b) is the HSP distance (R d ), the internal resistance of the secondary battery can be further reduced.

[0015] In the present invention, the "Hansen solubility parameter (HSP) of the binder" b )” is the polar term δ p3 , dispersion term δ d3 and the hydrogen bond term δ h3 It consists of: Here, in the present invention, "δ p3 ", "δ d3 " and "δ h3 " can be identified using the method described in the Examples. In the present invention, the "HSP distance (R b )" is expressed by the following formula (2): HSP distance (R b )={(δ p1 -δ p3 ) 2 +4×(δ d1 -δ d3 ) 2 +(δ h1 -δ h3 ) 2} 1 / 2 ···(2) It can be calculated using:

[0016] The present invention also aims to advantageously solve the above-mentioned problems, and provides a positive electrode for a secondary battery, characterized in that it includes a positive electrode composite layer formed using any of the above-mentioned slurries for a positive electrode for a secondary battery. A positive electrode including a positive electrode composite layer formed using any of the above-mentioned slurries for a positive electrode for a secondary battery can reduce the internal resistance of the secondary battery and ensure good high-temperature storage characteristics.

[0017] The present invention has an object to advantageously solve the above-mentioned problems, and provides a secondary battery comprising the above-mentioned positive electrode for a secondary battery. A secondary battery comprising the above-mentioned positive electrode for a secondary battery has reduced internal resistance and excellent high-temperature storage characteristics. [Effects of the Invention]

[0018] According to the present invention, it is possible to provide a conductive material dispersion, a slurry for a secondary battery positive electrode, and a positive electrode for a secondary battery, which can reduce the internal resistance of a secondary battery while ensuring good high-temperature storage characteristics of the secondary battery. Furthermore, according to the present invention, it is possible to provide a secondary battery having reduced internal resistance and excellent high-temperature storage characteristics. DETAILED DESCRIPTION OF THE INVENTION

[0019] Hereinafter, embodiments of the present invention will be described in detail. Here, the conductive material dispersion of the present invention is used as a material for producing a slurry for a secondary battery electrode, preferably a slurry for a secondary battery positive electrode. The slurry for a secondary battery positive electrode of the present invention is prepared using the conductive material dispersion of the present invention. In addition, the positive electrode for a secondary battery of the present invention is characterized by comprising a positive electrode mixture layer formed using the slurry for a secondary battery positive electrode of the present invention. Furthermore, the secondary battery of the present invention is characterized by comprising the positive electrode for a secondary battery of the present invention.

[0020] (Conductive material dispersion) The conductive material dispersion of the present invention contains a carbon material, a dispersant, and a dispersion medium, and optionally contains other components. Note that the conductive material dispersion usually does not contain an electrode active material (positive electrode active material, negative electrode active material). Here, the conductive material dispersion of the present invention is a carbon material having a Hansen solubility parameter (HSP c ) and Hansen Solubility Parameter (HSP) of the dispersant d ) and the HSP distance (R d ) is 10.0 MPa 1 / 2 The present invention is characterized by the following features: When an electrode mixture layer of an electrode is formed using an electrode slurry containing the conductive material dispersion of the present invention, it is possible to reduce the internal resistance of a secondary battery including the electrode and improve the high-temperature storage characteristics.

[0021] In this way, the HSP distance (R d ) is 10.0 MPa 1 / 2The reason why the conductive material dispersion liquid of the present invention provides the above-mentioned effects is not clear, but is presumed to be as follows. That is, the HSP distance (R d ) is 10.0 MPa 1 / 2 Since the dispersant has a high affinity with the carbon material, the dispersant can adequately coat the surface of the carbon material in the dispersion medium, thereby dispersing the carbon material sufficiently. Therefore, the carbon material as a conductive material is evenly distributed in the electrode mixture layer, forming an excellent conductive path and reducing the internal resistance of the secondary battery. Additionally, since the dispersant adequately coats the surface of the carbon material as described above, decomposition of the electrolyte on the surface of the carbon material inside the secondary battery can be suppressed. This effectively suppresses gas generation during high-temperature storage of the secondary battery.

[0022] <Carbon materials> The carbon material is not particularly limited as long as it functions as a conductive material that can ensure electrical contact between electrode active materials in the electrode mixture layer. Examples of such carbon materials include carbon black (e.g., acetylene black, Ketjen Black (registered trademark), furnace black, etc.); graphite; carbon flakes; and fibrous carbon materials such as carbon nanofibers, carbon nanotubes (hereinafter sometimes abbreviated as "CNTs"), and vapor-grown carbon fibers. These materials can be used alone or in combination of two or more. Among these, fibrous carbon materials are preferred, with carbon nanofibers and carbon nanotubes being more preferred, and carbon nanotubes being even more preferred, from the viewpoint that even a small amount can satisfactorily form a conductive path in the electrode mixture layer, thereby increasing the capacity of the secondary battery.

[0023] The above-mentioned carbon materials can be produced by known methods. For example, CNTs can be produced by a method such as the super-growth method (see International Publication No. 2006 / 011655). The carbon material may be subjected to a surface treatment. The surface treatment can be carried out by bringing the carbon material into contact with a treatment liquid such as an acid solution, an alkali solution, and / or ozone water, or a gas containing ozone, or by carrying out a plasma treatment. The method for bringing the treatment liquid into contact with the carbon material is not particularly limited, but it is preferable to immerse the carbon material in the treatment liquid.

[0024] << Hansen Solubility Parameter (HSP c )>> HSP c As mentioned above, the polar term δ p1 , dispersion term δ d1 and the hydrogen bond term δ h1 It consists of three parts:

[0025] where the polar term δ p1 is not particularly limited, but is preferably 3.0 MPa 1 / 2 Preferably, it is 5.0 MPa or more. 1 / 2 More preferably, it is 13.0 MPa or more. 1 / 2 Preferably, it is 11.0 MPa or less. 1 / 2 More preferably, it is: Also, the dispersion term δ d1 is 13.0 MPa 1 / 2 It is preferable that the pressure is 16.0 MPa or more. 1 / 2 More preferably, it is 22.0 MPa or more. 1 / 2 Preferably, it is 20.0 MPa or less. 1 / 2 More preferably, it is:

[0026] And the hydrogen bond term δ h1 is 12.0 MPa 1 / 2 Preferably, it is 10.0 MPa or less. 1 / 2 More preferably, it is 7.0 MPa or less. 1 / 2 More preferably, it is 5.0 MPa or less. 1 / 2 It is particularly preferred that the hydrogen bond term δ h1 is 12.0 MPa 1 / 2 If the carbon material and the dispersant have a high affinity, the hydrogen bond term δ h1This is presumably because the number of functional groups (which contribute to the increase in the viscosity) decreases, but it is possible to suppress the decomposition of the electrolyte on the surface of the carbon material and further improve the high-temperature storage characteristics. In addition, there is no excessive interaction between the carbon materials, and the viscosity stability of the conductive material dispersion can be sufficiently ensured. The hydrogen bond term δ of the carbon material h1 The lower limit is not particularly limited, but is, for example, 1.5 MPa. 1 / 2 It can be more than that.

[0027] The Hansen solubility parameter (HSP) of carbon materials c ) is presumed to be affected by the amount of defects and the amount of functional groups on the surface of the carbon material, and can be adjusted, for example, by changing the type of carbon material, the production conditions of the carbon material, the surface treatment conditions of the carbon material, etc.

[0028] The content of the carbon material in the conductive material dispersion is not particularly limited, but is preferably 1.0% by mass or more, more preferably 2.0% by mass or more, and even more preferably 3.0% by mass or more, and preferably 30.0% by mass or less, more preferably 25.0% by mass or less, and even more preferably 20.0% by mass or less, based on 100% by mass of the total mass of the conductive material dispersion. If the content of the carbon material in the conductive material dispersion is within the above-mentioned range, the carbon material can be well dispersed in the conductive material dispersion and the viscosity stability of the conductive material dispersion can be ensured. In addition, the internal resistance of the secondary battery can be further reduced and the high-temperature storage characteristics can be further improved.

[0029] <Dispersant> The dispersant is not particularly limited as long as it is a polymer that can disperse the carbon material in the dispersion medium. As such a polymer, a polymer containing at least a nitrile group-containing monomer unit and an alkylene structural unit and optionally containing other repeating units is preferred.

[0030] In the present invention, "containing a monomer unit" means that "a polymer obtained using the monomer contains a repeating unit derived from the monomer." In the present invention, the term "containing an alkylene structural unit" means "containing an alkylene structural unit of the general formula -C n H 2n - [where n is an integer of 2 or more].

[0031] In the present invention, the content ratio of the monomer unit and the structural unit in the polymer is 1 H-NMR and 13 It can be measured using nuclear magnetic resonance (NMR) techniques such as C-NMR.

[0032] <<Nitrile group-containing monomer unit>> Examples of nitrile group-containing monomers capable of forming nitrile group-containing monomer units 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. Examples include acrylonitrile; α-halogenoacrylonitriles such as α-chloroacrylonitrile and α-bromoacrylonitrile; and α-alkylacrylonitriles such as methacrylonitrile and α-ethylacrylonitrile. The nitrile group-containing monomers may be used alone or in combination of two or more in any ratio. Among these, acrylonitrile is preferred.

[0033] The content of the nitrile group-containing monomer units in the dispersant is preferably 3% by mass or more, more preferably 5% by mass or more, and even more preferably 7% by mass or more, and preferably 50% by mass or less, more preferably 40% by mass or less, even more preferably 30% by mass or less, and particularly preferably 28% by mass or less, based on 100% by mass of all repeating units in the dispersant. When the content of the nitrile group-containing monomer units in the dispersant is within the above-mentioned range, the solubility of the dispersant in a dispersion medium (e.g., N-methyl-2-pyrrolidone) is sufficiently ensured, and the resulting electrode mixture layer can be well adhered to the current collector. Therefore, the carbon material can be well dispersed in the conductive material dispersion, and the viscosity stability of the conductive material dispersion can be ensured. Furthermore, the internal resistance of the secondary battery can be further reduced, while the high-temperature storage characteristics can be further improved.

[0034] <<Alkylene structural unit>> The alkylene structural unit may be linear or branched. However, from the viewpoint of further reducing the internal resistance of the secondary battery and further improving the high-temperature storage characteristics, the alkylene structural unit is preferably linear, i.e., a linear alkylene structural unit.

[0035] The method for introducing alkylene structural units into a polymeric dispersant is not particularly limited, but may be, for example, the following methods (1) and (2): (1) A method of preparing a polymer from a monomer composition containing a conjugated diene monomer and converting the conjugated diene monomer unit into an alkylene structural unit by hydrogenating the polymer. (2) A method for preparing a polymer from a monomer composition containing a 1-olefin monomer Among these, method (1) is preferred because the dispersant can be easily produced.

[0036] Examples of conjugated diene monomers include conjugated diene compounds having 4 or more carbon atoms, such as 1,3-butadiene, isoprene, 2,3-dimethyl-1,3-butadiene, and 1,3-pentadiene. Among these, 1,3-butadiene is preferred. That is, the alkylene structural unit is preferably a structural unit (conjugated diene hydride unit) obtained by hydrogenating a conjugated diene monomer unit, and more preferably a structural unit (1,3-butadiene hydride unit) obtained by hydrogenating a 1,3-butadiene monomer unit. Examples of the 1-olefin monomer include ethylene, propylene, and 1-butene. These conjugated diene monomers and 1-olefin monomers may be used singly or in combination of two or more kinds in any ratio.

[0037] Here, the 1,3-butadiene hydride unit can be of the following two types depending on the bonding mode when the 1,3-butadiene unit is formed before hydrogenation. (A) A structural unit obtained by hydrogenating a 1,3-butadiene unit produced by 1,4-bonding of 1,3-butadiene (-CH-CH-CH-CH-. Hereinafter referred to as "1,4-bonded butadiene hydride unit") (B) A structural unit obtained by hydrogenating a 1,3-butadiene unit formed by 1,2-bonding of 1,3-butadiene (-CH2-CH(C2H5)-; hereinafter referred to as "1,2-bonded butadiene hydride unit"). When the dispersant contains 1,3-butadiene hydride units, the 1,2-bond butadiene hydride units preferably account for 15% by mass or less, more preferably 10% by mass or less, of the total 1,3-butadiene hydride units in the dispersant (i.e., the sum of 1,4-bond butadiene hydride units and 1,2-bond butadiene hydride units) as 100% by mass. When the proportion of 1,2-bond butadiene hydride units in the total 1,3-butadiene hydride units is 15% by mass or less, the number of side-chain ethyl groups (—CH) in the dispersant can be reduced, allowing the dispersant to be well dissolved in a dispersion medium (e.g., N-methyl-2-pyrrolidone). Therefore, the carbon material can be well dispersed in the conductive material dispersion, and the viscosity stability of the conductive material dispersion can be ensured. Furthermore, the internal resistance of the secondary battery can be further reduced, while the high-temperature storage characteristics can be further improved. The lower limit of the proportion of 1,2-bond butadiene hydride units in all 1,3-butadiene hydride units is not particularly limited, but it is 0% by mass or more, and can be, for example, 4.0% by mass or more. The proportion of 1,2-bond butadiene hydride units in all 1,3-butadiene hydride units can be adjusted by changing the preparation method of the dispersant (such as the type of polymerization initiator).

[0038] The content of the alkylene structural units in the dispersant is preferably 20% by mass or more, more preferably 25% by mass or more, and preferably 80% by mass or less, and more preferably 75% by mass or less, based on 100% by mass of all repeating units in the dispersant. It is presumed that the content of the alkylene structural units in the dispersant within the above-mentioned range is due to an increased affinity between the carbon material and the dispersant, which allows the carbon material to be well dispersed in the conductive material dispersion and ensures viscosity stability of the conductive material dispersion. Furthermore, the dispersant effectively coats the surface of the carbon material, thereby suppressing decomposition of the electrolyte on the surface of the carbon material and further improving high-temperature storage properties.

[0039] <<Other repeating units>> The other repeating units that can be contained in the dispersant are not particularly limited, but include aromatic vinyl monomer units, acidic group-containing monomer units, and (meth)acrylic acid ester monomer units. The dispersant may contain one type of other repeating unit, or may contain two or more types of other repeating units. In the present invention, "(meth)acrylic" means acrylic and / or methacrylic.

[0040] [Aromatic vinyl monomer unit] Examples of aromatic vinyl monomers that can form aromatic vinyl monomer units include styrene, α-methylstyrene, pt-butylstyrene, butoxystyrene, vinyltoluene, chlorostyrene, and vinylnaphthalene. The aromatic vinyl monomers may be used alone or in combination of two or more at any ratio. Among these, styrene is preferred.

[0041] When the dispersant contains aromatic vinyl monomer units, the content of the aromatic vinyl monomer units in the dispersant is, from the viewpoint of further reducing the internal resistance of the secondary battery while further improving the high-temperature storage characteristics, preferably 10% by mass or more, more preferably 20% by mass or more, more preferably 40% by mass or less, even more preferably 35% by mass or less, and particularly preferably 31% by mass or less, where the total amount of repeating units in the dispersant is 100% by mass.

[0042] [Acidic group-containing monomer unit] Examples of the acidic group-containing monomer that can form the acidic group-containing monomer unit include a carboxylic acid group-containing monomer, a sulfonic acid group-containing monomer, and a phosphoric acid group-containing monomer. The acidic group-containing monomer may be used alone or in combination of two or more kinds in any ratio.

[0043] Examples of the carboxylic acid group-containing monomer 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, and crotonic acid. Examples of the monocarboxylic acid derivatives include 2-ethylacrylic acid, isocrotonic acid, α-acetoxyacrylic acid, β-trans-aryloxyacrylic acid, and α-chloro-β-E-methoxyacrylic acid. Examples of dicarboxylic acids include maleic acid, fumaric acid, and itaconic acid. Examples of dicarboxylic acid derivatives include methylmaleic acid, dimethylmaleic acid, phenylmaleic acid, chloromaleic acid, dichloromaleic acid, fluoromaleic acid, and maleic acid monoesters such as nonyl maleate, decyl maleate, dodecyl maleate, octadecyl maleate, and fluoroalkyl maleate. Examples of the acid anhydrides of dicarboxylic acids include maleic anhydride, acrylic anhydride, methyl maleic anhydride, and dimethyl maleic anhydride. Furthermore, as the carboxylic acid group-containing monomer, an acid anhydride that generates a carboxylic acid group upon hydrolysis can also be used, among which acrylic acid and methacrylic acid are preferred as the carboxylic acid group-containing monomer.

[0044] Examples of sulfonic acid group-containing monomers include vinyl sulfonic acid, methyl vinyl sulfonic acid, (meth)allyl sulfonic acid, styrene sulfonic acid, (meth)acrylic acid-2-ethyl sulfonate, 2-acrylamido-2-methylpropanesulfonic acid, and 3-allyloxy-2-hydroxypropanesulfonic acid. In the present invention, "(meth)allyl" means allyl and / or methallyl.

[0045] Examples of the phosphate group-containing monomer include 2-(meth)acryloyloxyethyl phosphate, methyl-2-(meth)acryloyloxyethyl phosphate, and ethyl-(meth)acryloyloxyethyl phosphate. In the present invention, "(meth)acryloyl" means acryloyl and / or methacryloyl.

[0046] When the dispersant contains an acidic group-containing monomer unit, the content of the acidic group-containing monomer unit in the dispersant is, from the viewpoint of further reducing the internal resistance of the secondary battery while further improving the high-temperature storage characteristics, preferably 1% by mass or more, preferably 3% by mass or more, preferably 10% by mass or less, and more preferably 7% by mass or less, where the total amount of repeating units in the dispersant is 100% by mass.

[0047] [(Meth)acrylic acid ester monomer unit] Examples of (meth)acrylic acid ester monomers that can form (meth)acrylic acid ester monomer units include methyl acrylate, ethyl acrylate, n-propyl acrylate, isopropyl acrylate, n-butyl acrylate, t-butyl acrylate, pentyl acrylate, hexyl acrylate, heptyl acrylate, octyl acrylate, 2-ethylhexyl acrylate, nonyl acrylate, decyl acrylate, lauryl acrylate, n-tetradecyl acrylate, and stearyl acrylate. Acrylic acid alkyl esters include methacrylic acid alkyl esters such as methyl methacrylate, ethyl methacrylate, n-propyl methacrylate, isopropyl methacrylate, n-butyl methacrylate, t-butyl methacrylate, pentyl methacrylate, hexyl methacrylate, heptyl methacrylate, octyl methacrylate, 2-ethylhexyl methacrylate, nonyl methacrylate, decyl methacrylate, lauryl methacrylate, n-tetradecyl methacrylate, and stearyl methacrylate. The (meth)acrylic acid ester monomers may be used alone or in combination of two or more at any ratio.

[0048] As described above, the dispersant may contain (meth)acrylic acid ester monomer units. However, from the viewpoint of further reducing the internal resistance of the secondary battery and further improving the high-temperature storage characteristics, the content of the (meth)acrylic acid ester monomer units in the dispersant is preferably 30% by mass or less, more preferably 15% by mass or less, even more preferably 10% by mass or less, still more preferably 5% by mass or less, particularly preferably 1% by mass or less, and most preferably 0% by mass (i.e., the dispersant does not contain any (meth)acrylic acid ester monomer units), based on 100% by mass of all repeating units in the dispersant.

[0049] <<Preparation method>> The method for preparing the dispersant is not particularly limited. The dispersant is produced, for example, by polymerizing a monomer composition containing one or more types of monomers in an aqueous solvent, and optionally hydrogenating the polymer. The content of each monomer in the monomer composition can be determined based on the content of the desired repeating units (monomer units and / or structural units) in the polymer. The polymerization method is not particularly limited, and any of solution polymerization, suspension polymerization, bulk polymerization, emulsion polymerization, etc. can be used. The polymerization reaction can be any of ionic polymerization, radical polymerization, living radical polymerization, various condensation polymerizations, addition polymerization, etc. Known emulsifiers and polymerization initiators can be used during polymerization, if necessary. Hydrogenation can be carried out by known methods.

[0050] << Hansen Solubility Parameter (HSP d )>> HSP d As mentioned above, the polar term δ p2 , dispersion term δ d2 and the hydrogen bond term δ h2 It consists of three parts:

[0051] where the polar term δ p2 is not particularly limited, but is preferably 6.0 MPa 1 / 2 Preferably, it is 7.0 MPa or more.1 / 2 More preferably, it is 15.0 MPa or more. 1 / 2 Preferably, it is 10.0 MPa or less. 1 / 2 More preferably, it is: Also, the dispersion term δ d2 is not particularly limited, but is preferably 10.0 MPa 1 / 2 It is preferable that the pressure is 15.0 MPa or more. 1 / 2 More preferably, it is 25.0 MPa or more. 1 / 2 Preferably, it is 20.0 MPa or less. 1 / 2 More preferably, it is: And the hydrogen bond term δ h2 is not particularly limited, but is preferably 1.0 MPa 1 / 2 Preferably, it is 2.0 MPa or more. 1 / 2 More preferably, it is 4.0 MPa or more. 1 / 2 More preferably, it is 15.0 MPa or more. 1 / 2 Preferably, it is 10.0 MPa or less. 1 / 2 More preferably, it is 8.0 MPa or less. 1 / 2 It is more preferable that:

[0052] In addition, the Hansen solubility parameter (HSP) of the dispersant c ) can be adjusted by changing the type and ratio of monomers used in preparing the dispersant, the ratio of 1,2-bond butadiene hydride units to all 1,3-butadiene hydride units, and the preparation conditions of the dispersant.

[0053] <<Content>> The content of the dispersant in the conductive material dispersion is not particularly limited, but is preferably 1 part by mass or more, more preferably 5 parts by mass or more, and even more preferably 10 parts by mass or more, and is preferably 100 parts by mass or less, more preferably 70 parts by mass or less, even more preferably 50 parts by mass or less, and particularly preferably 30 parts by mass or less, per 100 parts by mass of the carbon material. If the content of the dispersant is within the above-mentioned range, the carbon material can be well dispersed in the conductive material dispersion and the viscosity stability of the conductive material dispersion can be ensured. Furthermore, the internal resistance of the secondary battery can be further reduced and the high-temperature storage characteristics can be further improved.

[0054] < <HSP c and HSP d HSP distance (R d )>> In the conductive material dispersion of the present invention, HSP c and HSP d HSP distance (R d ) is 10.0 MPa 1 / 2 It must be less than 8.0 MPa 1 / 2 Preferably, it is 6.0 MPa or less. 1 / 2 More preferably, it is 4.0 MPa or less. 1 / 2 More preferably, it is 3.8 MPa or less. 1 / 2 It is particularly preferable that the HSP distance (R d ) is 10.0 MPa 1 / 2 If the temperature exceeds this range, the internal resistance of the secondary battery increases and the high-temperature storage characteristics are impaired. Furthermore, the carbon material cannot be well dispersed in the conductive material dispersion, and the viscosity stability of the conductive material dispersion is impaired. In addition, the HSP distance (R d ) is not particularly limited, but is preferably 0 MPa 1 / 2 For example, 1.0 MPa 1 / 2 It can be more than that.

[0055] <Dispersion medium> As the dispersion medium, either water or an organic solvent can be used, but an organic solvent is preferred. The organic solvent is not particularly limited, and examples thereof include alcohols such as methanol, ethanol, n-propanol, isopropanol, n-butanol, isobutanol, t-butanol, pentanol, hexanol, heptanol, octanol, nonanol, decanol, and amyl alcohol; ketones such as acetone, methyl ethyl ketone, and cyclohexanone; esters such as ethyl acetate and butyl acetate; ethers such as diethyl ether, dioxane, and tetrahydrofuran; amide-based organic solvents such as N,N-dimethylformamide and N-methyl-2-pyrrolidone (NMP); and aromatic hydrocarbons such as toluene, xylene, chlorobenzene, orthodichlorobenzene, and paradichlorobenzene. One type of dispersion medium may be used alone, or two or more types may be used in combination at any ratio. From the viewpoint of favorably dispersing the carbon material in the conductive material dispersion liquid, an organic solvent is preferred, and NMP is more preferred.

[0056] <Other ingredients> Other components that the conductive material dispersion may contain are not particularly limited, and include components other than the positive electrode active material described later in the "Slurry for secondary battery positive electrode" section.

[0057] <Method for preparing conductive material dispersion> When the above-mentioned components are mixed to obtain a conductive material dispersion, the mixing method is not particularly limited, and for example, a general mixing device such as a disper, a mill, or a kneader can be used.

[0058] (Slurry for secondary battery positive electrodes) The positive electrode slurry of the present invention contains the above-described conductive material dispersion and positive electrode active material, and optionally contains optional components such as a binder, etc. In other words, the positive electrode slurry of the present invention contains a carbon material, a dispersant, a dispersion medium, and optionally contains optional components such as a binder, etc. According to the positive electrode provided with the positive electrode composite material layer formed from the positive electrode slurry containing the conductive material dispersion liquid described above, the internal resistance of the secondary battery can be reduced, and good high-temperature storage characteristics of the secondary battery can be ensured.

[0059] <Positive electrode active material> As the positive electrode active material to be blended in the positive electrode slurry, known positive electrode active materials can be used without particular limitation. For example, the positive electrode active material used in a lithium-ion secondary battery is not particularly limited, but includes metal oxides containing lithium (Li). And as the positive electrode active material, in addition to lithium (Li), a positive electrode active material containing at least one selected from the group consisting of cobalt (Co), nickel (Ni), and manganese (Mn) is preferable. Such positive electrode active materials include lithium-containing cobalt oxide (LiCoO2), lithium manganate (LiMn2O4), lithium-containing nickel oxide (LiNiO2), lithium-containing composite oxides of Co-Ni-Mn, lithium-containing composite oxides of Ni-Mn-Al, lithium-containing composite oxides of Ni-Co-Al, olivine-type lithium manganese phosphate (LiMnPO4), Li 1+x Mn 2-x O4 (0 <X <2) lithium-excess spinel compounds represented by, Li[Ni 0.17 Li 0.2 Co 0.07 Mn 0.56 O2, LiNi 0.5 Mn 1.5 O4 and the like. The positive electrode active material may be used alone or in combination of two or more in any ratio.

[0060] From the viewpoint of further increasing the capacity of the lithium-ion secondary battery, when the total amount of the transition metals contained is 100.0 mol%, it is preferable to use a positive electrode active material in which the ratio of nickel is 50.0 mol% or more and 100.0 mol% or less.

[0061] In addition, 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 material. The amount of the positive electrode active material in the positive electrode slurry is not particularly limited, and can be within the range of conventional use.

[0062] <Optional ingredients> Examples of optional components that can be contained in the positive electrode slurry include a binder, a viscosity modifier, a reinforcing material, an antioxidant, and an electrolyte additive that has the function of suppressing decomposition of the electrolyte. These optional components may be used alone or in combination of two or more in any ratio. Among the optional components described above, the positive electrode slurry preferably contains a binder from the viewpoint of preventing the positive electrode active material from settling and of adhering the resulting positive electrode mixture layer to the current collector well.

[0063] <<Binding material>> The binder is not particularly limited, but for example, fluorine-containing resins such as polyvinylidene fluoride, polyacrylonitrile (PAN), and polyvinyl alcohol (PVOH) are preferred, and fluorine-containing resins and PAN are more preferred.

[0064] [Hansen Solubility Parameter (HSP b )] HSP b As mentioned above, the polar term δ p3 , dispersion term δ d3 and the hydrogen bond term δ h3 It consists of three parts:

[0065] where the polar term δ p3 is not particularly limited, but is preferably 5.0 MPa 1 / 2 Preferably, it is 10.0 MPa or more. 1 / 2 More preferably, it is 25.0 MPa or more. 1 / 2 Preferably, it is 20.0 MPa or less. 1 / 2 More preferably, it is: Also, the dispersion term δ d3 is not particularly limited, but is preferably 10.0 MPa 1 / 2 It is preferable that the pressure is 15.0 MPa or more. 1 / 2 More preferably, it is 30.0 MPa or more. 1 / 2Preferably, it is 20.0 MPa or less. 1 / 2 More preferably, it is: And the hydrogen bond term δ h3 is not particularly limited, but is preferably 5.0 MPa 1 / 2 Preferably, it is 10.0 MPa or more. 1 / 2 More preferably, it is 25.0 MPa or more. 1 / 2 Preferably, it is 20.0 MPa or less. 1 / 2 More preferably, it is:

[0066] The Hansen solubility parameter (HSP) of the binder b ) can be adjusted by changing the type and ratio of the monomers used in preparing the binder, the manufacturing conditions of the binder, etc.

[0067] [HSP c and HSP b HSP distance (R b )] In the positive electrode slurry of the present invention, the HSP of the carbon material c and HSP of binder b HSP distance (R b ) is the HSP of carbon materials c and HSP of dispersant d HSP distance (R d ) is preferable. d ) is the HSP distance (R d ), the binder does not excessively interfere with the interaction between the dispersant and the carbon material in the positive electrode slurry, and these components can be maintained in a well-dispersed state. Therefore, by using the positive electrode slurry, the internal resistance of the secondary battery can be further reduced. Specifically, the HSP distance (R b ) is set to 4.0 MPa from the viewpoint of further reducing the internal resistance of the secondary battery. 1 / 2 Preferably, it is greater than 6.0 MPa 1 / 2 More preferably, it is greater than 8.0 MPa 1 / 2 More preferably, it is greater than 10.0 MPa. 1 / 2It is particularly preferable that the HSP distance (R b ) is not particularly limited to an upper limit, but is, for example, 40.0 MPa 1 / 2 It can be as follows:

[0068] <Method for preparing positive electrode slurry> When the above-mentioned components are mixed to obtain the positive electrode slurry, the mixing method is not particularly limited, and for example, a general mixing device such as a disper, a mill, or a kneader can be used.

[0069] (Positive electrode for secondary batteries) The positive electrode of the present invention includes a positive electrode mixture layer obtained using the above-described positive electrode slurry of the present invention. More specifically, the positive electrode of the present invention typically includes the above-described positive electrode mixture layer on a current collector. The positive electrode mixture layer contains a positive electrode active material, a conductive material, a dispersant, and optionally a binder, etc. The positive electrode active material, conductive material, dispersant, binder, etc. contained in the positive electrode mixture layer are the same as those contained in the conductive material dispersion liquid and the positive electrode slurry of the present invention, and the preferred abundance ratios of the respective components are the same as the preferred abundance ratios of the respective components in the conductive material dispersion liquid and the positive electrode slurry of the present invention.

[0070] The positive electrode of the present invention includes a positive electrode composite layer formed using the above-described positive electrode slurry of the present invention, and therefore, the internal resistance of the secondary battery can be reduced and good high-temperature storage characteristics of the secondary battery can be ensured.

[0071] <Current collector> The current collector is made of a material that is electrically conductive and electrochemically durable. Specifically, a current collector made of aluminum or an aluminum alloy can be used as the current collector. In this case, aluminum and an aluminum alloy may be used in combination, or different types of aluminum alloys may be used in combination. Aluminum and aluminum alloys are excellent current collector materials because they are heat resistant and electrochemically stable.

[0072] <Positive electrode manufacturing method> The method for producing the positive electrode of the present invention is not particularly limited. For example, the positive electrode of the present invention can be produced by applying the above-mentioned positive electrode slurry of the present invention to at least one surface of a current collector and drying the applied slurry to form a positive electrode composite layer. More specifically, the production method includes a step of applying the positive electrode slurry to at least one surface of the current collector (application step), and a step of drying the positive electrode slurry applied to at least one surface of the current collector to form a positive electrode composite layer on the current collector (drying step).

[0073] <<Coating process>> The method for applying the positive electrode slurry to the current collector is not particularly limited, and known methods can be used. Specifically, examples of the application method include a doctor blade method, a dipping method, a reverse roll method, a direct roll method, a gravure method, an extrusion method, and a brush coating method. In this case, the positive electrode slurry may be applied to only one side of the current collector, or may be applied to both sides. The thickness of the slurry film on the current collector after application and before drying can be appropriately set depending on the thickness of the positive electrode composite layer obtained by drying.

[0074] <<Drying process>> The method for drying the positive electrode slurry on the current collector is not particularly limited and any known method can be used, for example, drying with warm air, hot air, or low-humidity air, vacuum drying, or drying by irradiation with infrared rays or electron beams, etc. By drying the positive electrode slurry on the current collector in this manner, 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.

[0075] After the drying step, the positive electrode mixture layer may be subjected to pressure treatment using a mold press, a roll press, etc. Pressure treatment allows the positive electrode mixture layer to adhere well to the current collector. Furthermore, when the positive electrode mixture layer contains a curable polymer, the polymer may be cured after the positive electrode mixture layer is formed.

[0076] (Secondary battery) The secondary battery of the present invention includes the above-described positive electrode of the present invention. Furthermore, since the secondary battery of the present invention includes the positive electrode of the present invention, the internal resistance is reduced and the high-temperature storage characteristics are excellent. The secondary battery of the present invention is, for example, a nonaqueous secondary battery, and is preferably a lithium-ion secondary battery.

[0077] Hereinafter, the configuration of a lithium ion secondary battery will be described as an example of the secondary battery of the present invention. This lithium ion secondary battery usually includes, in addition to the positive electrode of the present invention, a negative electrode, an electrolyte, and a separator. Each of the above components will be described below.

[0078] <Negative electrode> The negative electrode of the lithium ion secondary battery can be a known negative electrode used as a negative electrode for lithium ion secondary batteries. Specifically, the negative electrode can be, for example, a negative electrode made of a thin plate of metallic lithium or a negative electrode formed by forming a negative electrode composite layer on a current collector. The current collector can be made of a metal material such as iron, copper, aluminum, nickel, stainless steel, titanium, tantalum, gold, or platinum. The negative electrode composite layer can be a layer containing a negative electrode active material and a binder. The binder is not particularly limited, and any known material can be used.

[0079] <Electrolyte> As the electrolyte, an organic electrolyte solution in which a supporting electrolyte is dissolved in an organic solvent is typically used. Examples of the supporting electrolyte include lithium salts. Examples of lithium salts include LiPF6, LiAsF6, LiBF4, LiSbF6, LiAlCl4, LiClO4, CF3SO3Li, CF4F9SO3Li, CF3COOLi, (CF3CO)2NLi, (CF3SO2)2NLi, and (C2F5SO2)NLi. Among these, LiPF6, LiClO4, and CF3SO3Li are preferred, with LiPF6 being particularly preferred, due to their high solubility in solvents and high dissociation. One type of electrolyte may be used alone, or two or more types may be used in combination at any ratio. Generally, the use of a supporting electrolyte with a higher dissociation degree tends to result in higher lithium ion conductivity, and the lithium ion conductivity can be adjusted by the type of supporting electrolyte.

[0080] The organic solvent used in the electrolyte 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 methyl ethyl carbonate (EMC) are preferred. Other suitable solvents include γ-butyrolactone, methyl formate, and other esters. Ethers such as 1,2-dimethoxyethane and tetrahydrofuran are also suitable. Sulfur-containing compounds such as sulfolane and dimethyl sulfoxide are also suitable. Mixtures of these solvents may also be used. Among these, carbonates are preferred because of their high dielectric constant and wide stable potential range. A mixture of ethylene carbonate and ethyl methyl carbonate is even more preferred. The concentration of the electrolyte in the electrolytic solution can be adjusted as appropriate, and is preferably 0.5 to 15 mass%, more preferably 2 to 13 mass%, and even more preferably 5 to 10 mass%. Known additives, such as fluoroethylene carbonate and ethyl methyl sulfone, may also be added to the electrolytic solution.

[0081] <Separator> The separator is not particularly limited, and for example, the one described in JP 2012-204303 A can be used. Among these, a microporous film made of a polyolefin resin (polyethylene, polypropylene, polybutene, polyvinyl chloride) is preferred because it allows the thickness of the entire separator to be thin, thereby increasing the ratio of electrode active material in the lithium ion secondary battery and increasing the capacity per volume.

[0082] <Method of manufacturing lithium-ion secondary batteries> The lithium ion secondary battery according to the present invention can be produced, for example, by stacking a positive electrode and a negative electrode with a separator interposed therebetween, rolling or folding the resulting structure as necessary according to the battery shape, placing the resultant structure in a battery container, injecting an electrolyte into the battery container, and sealing the container. To prevent internal pressure buildup, overcharging and overdischarging, and the like, a fuse, an overcurrent protection element such as a PTC element, an expanded metal, a lead plate, or the like may be provided as necessary. The shape of the secondary battery may be any of a coin type, a button type, a sheet type, a cylindrical type, a rectangular type, a flat type, and the like. [Example]

[0083] The present invention will be specifically described below based on examples, but the present invention is not limited to these examples. In the following description, "%" and "parts" representing amounts are based on mass unless otherwise specified. Furthermore, in a polymer produced by copolymerizing multiple types of monomers, the proportion of a monomer unit formed by polymerizing a certain monomer in the polymer usually coincides with the ratio (feed ratio) of the certain monomer to all the monomers used in the polymerization of the polymer, unless otherwise specified. In the examples and comparative examples, the Hansen solubility parameters of the carbon material, dispersant, and binder, the proportion of 1,2-bond butadiene hydride units in all 1,3-butadiene hydride units in the dispersant, the dispersion state and viscosity stability of the conductive material dispersion, and the internal resistance reduction and high-temperature storage characteristics of the secondary battery were evaluated using the following methods.

[0084] <Hansen Solubility Parameter> <<HSP of carbon materials c >> 0.1 g of carbon material was added to 10 ml of each of the 16 solvents shown in Table 1 below, and ultrasonically dispersed at 20 kHz, 200 W, for 10 minutes to prepare the test solution. Pulse NMR measurements were performed on the above-mentioned 16 solvents (pure solvents) and the test solution. From the results obtained, R as a function of the relaxation time T1 of the pure solvent and the relaxation time T2 of the solvent in the test solution was sp was calculated using the following formula: R sp =(T1 / T2)-1 The obtained R sp From the values, the affinity between each solvent and the carbon material was scored as follows: ·R sp ≦0.2 (poor solvent): 0 0.2 <R sp ≦0.5 (poor solvent): 2 0.5 <R sp (Good solvent): 1 According to the obtained scores, the HSPiP was calculated using the computer software "Hansen Solubility Parameters in Practice (HSPiP ver.5.2.05)". c The polar term δ p1 , dispersion term δ d1 and the hydrogen bond term δ h1 asked for. << HSP of dispersants d >> 0.5 g of dispersant was added to 10 ml of each of the 15 types of solvents shown in Table 2 below, and the mixture was left to stand at 25° C. for 24 hours to prepare evaluation liquids. These evaluation liquids were visually observed and scored as follows. Insoluble (poor solvent): 0 Turbid and / or shaky (poor solvent): 2 Completely soluble (good solvent): 1 According to the obtained score, HSPiP was used to d The polar term δ p2 , dispersion term δ d2 and the hydrogen bond term δ h2 asked for. <<HSP of binder b >> The above "HSP of dispersants" d " can be calculated in the same manner as ", but in this application the values ​​in the HSPiP database are used. <Proportion of 1,2-bond butadiene hydride units in all 1,3-butadiene hydride units in the dispersant> The aqueous dispersion of the polymer was vacuum dried at 60°C for 24 hours, and then 1 The amount of 1,4-bonded 1,3-butadiene units (1,4 bond amount) and the amount of 1,2-bonded 1,3-butadiene units (1,2 bond amount) were quantified by H-NMR, and calculated according to the following formula. Proportion of 1,2-bonded butadiene hydride units in all 1,3-butadiene hydride units in the dispersant = (1,2 bond amount) / (1,2 bond amount + 1,4 bond amount) × 100 (mass%) <Dispersed state> The shear rate dependency of viscosity of the conductive material dispersion was evaluated at a temperature of 25°C using a rheometer (manufactured by Anton Paar, product name "MCR302"). At this time, the shear rate was 10 s -1 Viscosity at time η 10 The viscosity was evaluated according to the following criteria: 10 The lower the value, the better the dispersion state of the conductive material dispersion. A:η 10 is 5 Pa·s or less B:η 10 is over 5 Pa·s and 10 Pa·s or less C:η 10 is over 10 Pa·s and 20 Pa·s or less D: Excessive viscosity, η 10 cannot be measured <Viscosity stability> Initial viscosity η of the conductive material dispersion immediately after preparation ini was measured using a Brookfield viscometer at a temperature of 25°C, a rotation speed of 60 rpm, and a rotation time of 60 seconds. Furthermore, the viscosity after storage of the conductive material dispersion liquid after being left to stand at room temperature for 7 days from the preparation, η 7d η ini The viscosity ratio was calculated using the following formula: Viscosity ratio=η 7d / η ini ×100(%) The obtained viscosity ratio was used to evaluate according to the following criteria: The closer the viscosity ratio value is to 100%, the smaller the change in viscosity of the conductive material dispersion due to long-term storage, indicating that the conductive material dispersion has excellent viscosity stability. A: Viscosity ratio is 70% or more and 130% or less B: Viscosity ratio is 50% or more but less than 70% or more than 130% but less than 150% C: Viscosity ratio is 0% or more but less than 50% or more than 150% but less than 200% <Reduced internal resistance> The lithium-ion secondary battery was charged at a constant current of 0.2 CmA until the battery voltage reached 4.2 V, and then charged at a constant voltage of 4.2 V until the charging current reached 0.02 CmA. Subsequently, the battery was discharged at a constant current of 0.2 CmA until the battery voltage reached 3.87 V (50% SOC; State of Charge), and then the voltage change after 30 seconds of discharge was measured at 0.2 CmA, 0.5 CmA, 1.0 CmA, 2.0 CmA, 2.5 CmA, and 3.0 CmA. The discharge current and the measured voltage change were plotted, and the slope was taken as the resistance value (Ω). The calculated resistance value was evaluated according to the following criteria: a lower resistance value indicates a lower internal resistance of the lithium-ion secondary battery. A: Resistance is less than 2Ω B: Resistance is 2Ω or more and less than 4Ω C: Resistance is 4Ω or more but less than 6Ω D: Resistance is 6Ω or more <High temperature storage characteristics> Lithium-ion secondary battery volume (V ini ) was measured by the Archimedes method. Next, this lithium ion secondary battery was charged at a constant current of 0.2 CmA in a 25°C environment until the battery voltage reached 4.2 V, and then constant voltage charged at 4.2 V until the charging current reached 0.02 CmA. After removing it from the charger and storing it in a 60°C environment for 7 days, it was cooled to 25°C and the volume (V 7d ) was measured by Archimedes' method, and the volume change was calculated using the following formula. Volume change = V 7d -Vini (ml) The obtained volume change was used to evaluate the battery according to the following criteria: The smaller the volume change value, the better the high-temperature storage characteristics of the lithium ion secondary battery. A: Volume change is less than 2 ml B: Volume change is 2 ml or more but less than 3 ml C: Volume change is 3 ml or more but less than 4 ml D: Volume change is 4 ml or more

[0085] Example 1 <Preparation of carbon materials> <<Preparing the substrate for carbon nanotube growth>> An Fe-Cr alloy SUS430 substrate (manufactured by JFE Steel Corporation, 50 cm x 50 cm, thickness: 0.3 mm, Cr: 18%, arithmetic mean roughness Ra ≈ 0.59 μm) was prepared as the substrate. 1.9 g of aluminum tri-sec-butoxide, an aluminum compound, was dissolved in 100 ml of 2-propanol, an organic solvent, and 0.9 g of triisopropanolamine, a stabilizer, was added and dissolved to prepare Coating Solution A. Furthermore, 174 mg of iron acetate, an iron compound, was dissolved in 100 ml of 2-propanol, an organic solvent, and 190 mg of triisopropanolamine, a stabilizer, was added and dissolved to prepare Coating Solution B. The prepared substrate was then coated with the above-mentioned Coating Solution A by dip coating at room temperature of 25°C and relative humidity of 50%. Specifically, the substrate was immersed in Coating Solution A, held there for 20 seconds, and then pulled up at a speed of 10 mm / sec. The substrate was then air-dried for 5 minutes, heated in an air environment at 300°C for 30 minutes, and cooled to room temperature, forming an alumina thin film with a thickness of 14 nm on the used substrate. Next, in an environment of room temperature 25°C and relative humidity 50%, the above coating liquid B was applied by dip coating on the alumina thin film provided on the substrate. Specifically, after dipping the substrate with the alumina thin film into the coating liquid B, it was held for 20 seconds, and then the substrate with the alumina thin film was pulled up at a pulling-up speed of 3 mm / second. Thereafter, by air drying for 5 minutes (drying temperature 45°C), an iron thin film with a thickness of 1 nm was formed, a catalyst layer with a thickness of 15 nm composed of an alumina-iron thin film was formed, and a substrate for carbon nanotube production was obtained. <<Synthesis of CNT>> Next, a CNT aligned aggregate was formed on the substrate for carbon nanotube production using a CVD apparatus. Specifically, the produced substrate for carbon nanotube production was placed in the reaction chamber of the CVD apparatus maintained at a furnace temperature of 750°C and a furnace pressure of 1.02×10 5 Pa, and He: 100 sccm and H2: 900 sccm were introduced into this reaction chamber for 6 minutes. As a result, the catalyst for CNT synthesis (iron) was reduced and the formation of fine particles was promoted, resulting in a state suitable for the growth of CNTs (a state in which a large number of catalyst fine particles with a nanometer size were formed) (formation step). The density of the catalyst fine particles at this time was adjusted to 1×10 12 ~1×10 14 pieces / cm 2 Then, in the reaction chamber maintained at a furnace temperature of 750°C and a furnace pressure of 1.02×10 5 Pa, He: 850 sccm, C2H4: 59 sccm, and an amount of H2O such that the H2O concentration became 300 ppm were supplied for 5 minutes. As a result, CNTs grew from each catalyst fine particle (CNT growth step). After the completion of the CNT growth step, only He: 1000 sccm was supplied into the reaction chamber to remove the remaining raw material gas and catalyst activating substance. As a result, a substrate on which a carbon nanotube aligned aggregate was formed on the surface of the catalyst layer was obtained. Thereafter, the CNT aligned aggregate grown on the catalyst layer was peeled off from the surface of the obtained substrate to obtain CNT-A as a carbon material. The affinity of this CNT-A with each solvent was scored by the method described above. The results are shown in Table 1. And the polar term δ c of HSP p1 and the dispersion term δd1 and the hydrogen bond term δ h1 The results are shown in Table 3. <Preparation of Dispersant> A reactor was charged with 180 parts of ion-exchanged water, 25 parts of an aqueous solution of sodium dodecylbenzenesulfonate (10% concentration) as an emulsifier, 28 parts of acrylonitrile as a nitrile group-containing monomer, and 0.8 parts of t-dodecyl mercaptan as a molecular weight modifier, in this order. The gas inside the reactor was then purged with nitrogen three times, after which 72 parts of 1,3-butadiene as a conjugated diene monomer was charged. The reactor was maintained at 10°C, and 0.1 parts of cumene hydroperoxide as a polymerization initiator was charged to initiate the polymerization reaction, which was then allowed to proceed with stirring. When the polymerization conversion reached 90%, 0.2 parts of hydroxylamine sulfate per 100 parts of monomer was added to terminate the polymerization. The mixture was then heated and steam distilled at approximately 70°C under reduced pressure to recover the residual monomer. Two parts of alkylated phenol were added as an antioxidant to obtain an aqueous dispersion of the polymer. Using this aqueous dispersion, the proportion of 1,2-bond butadiene hydride units relative to the total 1,3-butadiene hydride units in the dispersant was measured. The results are shown in Table 3. Next, 400 mL of the resulting aqueous dispersion of the polymer (total solids: 48 g) was placed in a 1-liter autoclave equipped with a stirrer, and nitrogen gas was passed through for 10 minutes to remove dissolved oxygen from the aqueous dispersion. Subsequently, 50 mg of palladium acetate was dissolved in 180 mL of water containing 4 times the molar amount of nitric acid relative to Pd as a hydrogenation catalyst and added. After the system was purged with hydrogen gas twice, the contents of the autoclave were heated to 50°C while pressurized with hydrogen gas to 3 MPa, and the hydrogenation reaction was carried out for 6 hours. Thereafter, the contents were returned to room temperature, the system was conditioned under a nitrogen atmosphere, and the contents were concentrated using an evaporator until the solid content reached 40%, to obtain hydrogenated nitrile rubber A as a dispersant. Then, NMP was added to an aqueous solution of hydrogenated nitrile rubber A as a dispersant having a solid content concentration of 40%, and then distillation under reduced pressure was carried out to remove water and excess NMP, thereby obtaining an NMP solution of hydrogenated nitrile rubber A having a solid content concentration of 8%. The obtained hydrogenated nitrile rubber A was scored for affinity with each solvent using the method described above. The results are shown in Table 2. d The polar term δ p2 , dispersion term δ d2 and the hydrogen bond term δ h2 The results are shown in Table 3. Furthermore, using the above formula (1), HSP c and HSP d HSP distance (R d The results are shown in Table 3. <Preparation of Conductive Material Dispersion> 5.0 parts of the CNT-A as the conductive material, 1.0 part of the hydrogenated nitrile rubber A as the dispersant (solids equivalent), and 94.0 parts of NMP as the dispersion medium were stirred using a disperser (3000 rpm, 10 minutes), and then dispersed for 1 hour at a peripheral speed of 8 m / s using a bead mill with 1 mm diameter zirconia beads to prepare a conductive material dispersion with a solids concentration of 6.0%. The dispersion state and storage stability of the obtained conductive material dispersion were evaluated. The results are shown in Table 3. <Preparation of positive electrode slurry> The positive electrode active material is a ternary active material (LiNi 0.5 Co 0.2 Mn 0.3 A positive electrode slurry was prepared by adding 98.0 parts of 02 (average particle size: 10 μm), 1.0 part of polyvinylidene fluoride as a binder, 1.0 part of the conductive material dispersion (solid content equivalent), and NMP, and mixing with a planetary mixer (60 rpm, 30 minutes). The amount of NMP added was adjusted so that the viscosity of the resulting positive electrode slurry composition (measured with a single cylindrical rotational viscometer in accordance with JIS Z8803:1991, temperature: 25°C, rotation speed: 60 rpm) was within the range of 4000 to 5000 mPa s. <Preparation of positive electrode> An aluminum foil having a thickness of 20 μm was prepared as a current collector. The positive electrode slurry was applied to the aluminum foil using a comma coater so that the coating weight after drying was 20 mg / cm. 2The coating was applied so that the density became 3.2 g / cm 3 , and the coating was 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 blank. This positive electrode blank was rolled using a roll press to obtain a positive electrode blank with a density of 3.2 g / cm 3 . 3 A sheet-shaped positive electrode was fabricated from the positive electrode composite layer and aluminum foil. The thickness of the sheet-shaped positive electrode was 70 μm. This sheet-shaped positive electrode was cut into a width of 4.8 mm and a length of 50 cm to prepare a positive electrode for a lithium-ion secondary battery. <Preparation of negative electrode> 90 parts of spherical artificial graphite (volume average particle diameter: 12 μm) as the negative electrode active material and SiO X A mixture of 10 parts of cellulose acetate (volume average particle diameter: 10 μm), 1 part of styrene-butadiene polymer as a binder, 1 part of carboxymethyl cellulose as a thickener, and an appropriate amount of water as a dispersion medium was stirred in a planetary mixer to prepare a slurry for the negative electrode. Next, a copper foil having a thickness of 15 μm was prepared as a current collector. The negative electrode slurry composition was applied to both sides of the copper foil in an amount of 10 mg / cm after drying. 2 The coating was applied so that the density was 1.8 g / cm 3 , and the coating was dried at 60°C for 20 minutes and at 120°C for 20 minutes. After that, the coating was heated at 150°C for 2 hours to obtain a negative electrode blank. This negative electrode blank was rolled using a roll press to a density of 1.8 g / cm 3 . 3 A sheet-shaped negative electrode was fabricated consisting of the negative electrode mixture layers (both sides) and copper foil, and the sheet-shaped negative electrode was cut into a width of 5.0 mm and a length of 52 cm to prepare a negative electrode for a lithium ion secondary battery. <Preparing the separator> A single-layer polypropylene separator (manufactured by Celgard, product name "Celgard 2500", thickness: 15 μm) was cut into a size of 120 cm x 5.5 cm. <Secondary battery manufacturing> The positive electrode and the negative electrode were wound around a core with a diameter of 20 mm, with a separator in between, to obtain a wound body. The wound body was compressed in one direction at a speed of 10 mm / sec until the thickness became 4.5 mm. The wound body after compression had an elliptical shape in a plan view, and the ratio of its major axis to its minor axis (major axis / minor axis) was 7.7. In addition, an electrolyte solution (composition: LiPF6 solution with a concentration of 1.0 M (the solvent is a mixed solution in which 5 mass% of fluoroethylene carbonate is added to a mixed solvent of ethylene carbonate / ethyl methyl carbonate = 3 / 7 (mass ratio) and 2 volume% of vinylene carbonate is added as an additive)) was prepared. The compressed wound body was then placed in an aluminum laminate case together with 3.2 g of nonaqueous electrolyte. A nickel lead wire was connected to a designated location on the negative electrode, and an aluminum lead wire was connected to a designated location on the positive electrode. The opening of the case was then thermally sealed to obtain a lithium-ion secondary battery. This lithium-ion secondary battery was a pouch-shaped battery measuring 35 mm wide, 48 mm high, and 5 mm thick, with a nominal capacity of 700 mAh. The resulting lithium-ion secondary battery was evaluated for internal resistance reduction and high-temperature storage characteristics. The results are shown in Table 3.

[0086] Example 2 In preparing the positive electrode slurry, a carbon material, a dispersant, a conductive material dispersion, a positive electrode slurry, a positive electrode, a negative electrode, a separator, and a secondary battery were prepared in the same manner as in Example 1, except that polyacrylonitrile was used instead of polyvinylidene fluoride as the binder. Various evaluations were then performed. The results are shown in Table 3.

[0087] Example 3 In preparing the conductive material dispersion, a dispersant, a conductive material dispersion, a positive electrode slurry, a positive electrode, a negative electrode, a separator, and a secondary battery were prepared in the same manner as in Example 1, except that CNT-B prepared as described below was used as the carbon material. Various evaluations were then performed. The results are shown in Tables 1 and 3. <Preparation of carbon material (CNT-B)> 10 g of CNT-A prepared in the same manner as in Example 1 was added to 1 L of ozone water with a dissolved ozone concentration of 5 mg / L and stirred for 5 hours at 20° C. Next, the solid content (CNT) was recovered by filtration and dried under reduced pressure at 100° C. to obtain CNT-B.

[0088] Example 4 In preparing the conductive material dispersion, a dispersant, a conductive material dispersion, a positive electrode slurry, a positive electrode, a negative electrode, a separator, and a secondary battery were prepared in the same manner as in Example 1, except that CNT-C prepared as described below was used as the carbon material. Various evaluations were then performed. The results are shown in Tables 1 and 3. <Preparation of carbon material (CNT-C)> 10 g of CNT-A prepared in the same manner as in Example 1 was added to 1 L of ozone water with a dissolved ozone concentration of 5 mg / L and stirred for 10 hours at 20° C. Next, the solid content (CNT) was recovered by filtration and dried under reduced pressure at 100° C. to obtain CNT-C.

[0089] Example 5 In preparing the conductive material dispersion, a carbon material, a conductive material dispersion, a positive electrode slurry, a positive electrode, a negative electrode, a separator, and a secondary battery were prepared in the same manner as in Example 1, except that hydrogenated nitrile rubber B prepared as described below was used as a dispersant. Various evaluations were then performed. The results are shown in Tables 2 and 3. <Preparation of Dispersant (Hydrogenated Nitrile Rubber B)> A reactor was charged with 180 parts of ion-exchanged water, 25 parts of an aqueous solution of sodium dodecylbenzenesulfonate (10% concentration) as an emulsifier, 28 parts of acrylonitrile as a nitrile group-containing monomer, and 0.8 parts of t-dodecyl mercaptan as a molecular weight modifier, in this order. The gas inside the reactor was then purged with nitrogen three times, after which 72 parts of 1,3-butadiene as a conjugated diene monomer were charged. 0.1 parts of potassium persulfate as a polymerization initiator was charged into the reactor, which was kept at 30°C, to initiate the polymerization reaction, which was then allowed to proceed with stirring. When the polymerization conversion rate reached 90%, 0.2 parts of hydroxylamine sulfate per 100 parts of monomer was added to terminate the polymerization. Subsequently, the mixture was heated and steam distilled at about 70°C under reduced pressure to recover the residual monomer, and then 2 parts of alkylated phenol were added as an antioxidant to obtain an aqueous dispersion of the polymer. The subsequent operations were the same as in Example 1 to obtain an NMP solution of hydrogenated nitrile rubber B having a solid content concentration of 8%.

[0090] Example 6 A conductive material dispersion and a positive electrode slurry were prepared as follows. A dispersant, a positive electrode, a negative electrode, a separator, and a secondary battery were prepared in the same manner as in Example 1. Various evaluations were then performed. The results are shown in Tables 1 and 3. <Preparation of Conductive Material Dispersion> 18.0 parts of carbon black (manufactured by Timacal, product name "Super-T", hereinafter referred to as "CB-A") as a conductive material, 1.8 parts (solid content equivalent) of hydrogenated nitrile rubber A as a dispersant obtained in the same manner as in Example 1, and 80.2 parts of NMP were stirred (3000 rpm, 10 minutes) using a disper, and then dispersed for 1 hour at a peripheral speed of 8 m / s using a bead mill using zirconia beads with a diameter of 1 mm, to prepare a conductive material dispersion liquid with a solid content concentration of 19.8%. <Preparation of positive electrode slurry> The positive electrode active material is a ternary active material (LiNi 0.5 Co 0.2 Mn 0.3 A positive electrode slurry was prepared by adding 96.0 parts of 02 (average particle size: 10 μm), 1.0 part of polyvinylidene fluoride as a binder, 3.0 parts of the conductive material dispersion (solid content equivalent), and NMP, and mixing with a planetary mixer (60 rpm, 30 minutes). The amount of NMP added was adjusted so that the viscosity of the resulting positive electrode slurry composition (measured with a single cylindrical rotational viscometer in accordance with JIS Z8803:1991, temperature: 25°C, rotation speed: 60 rpm) was within the range of 4000 to 5000 mPa s.

[0091] Example 7 In preparing the conductive material dispersion, a dispersant, a conductive material dispersion, a positive electrode slurry, a positive electrode, a negative electrode, a separator, and a secondary battery were prepared in the same manner as in Example 6, except that CB-B prepared as described below was used as the carbon material. Various evaluations were then performed. The results are shown in Tables 1 and 4. <Preparation of carbon material (CB-B)> 10 g of CB-A was added to 1 L of ozone water with a dissolved ozone concentration of 5 mg / L and stirred for 5 hours at 20° C. The solid content (carbon black) was then recovered by filtration and dried under reduced pressure at 100° C. to prepare CB-B.

[0092] Example 8 A carbon material, a conductive material dispersion, a positive electrode slurry, a positive electrode, a negative electrode, a separator, and a secondary battery were prepared in the same manner as in Example 1, except that a hydrogenated styrene-butadiene-nitrile rubber prepared as described below was used as a dispersant in preparing the conductive material dispersion. Various evaluations were then performed. The results are shown in Tables 2 and 4. <Preparation of Dispersant (Hydrogenated Styrene-Butadiene-Nitrile Rubber)> Into a reactor, 180 parts of ion-exchanged water, 25 parts of an aqueous sodium dodecylbenzenesulfonate solution (concentration 10%) as an emulsifier, 31 parts of styrene as an aromatic vinyl monomer, 7 parts of acrylonitrile as a nitrile group-containing monomer, 7 parts of methacrylic acid as an acidic group-containing monomer, and 2.0 parts of t-dodecyl mercaptan as a molecular weight modifier were charged in this order. Next, the gas inside the reactor was purged with nitrogen three times, and then 55 parts of 1,3-butadiene as a conjugated diene monomer was charged. The subsequent operations were the same as in Example 1 to obtain an NMP solution of hydrogenated styrene-butadiene-nitrile rubber with a solid content of 8%.

[0093] (Comparative Examples 1, 2, and 3) In preparing the conductive material dispersion, a carbon material, a conductive material dispersion, a positive electrode slurry, a positive electrode, a negative electrode, a separator, and a secondary battery were prepared in the same manner as in Examples 1, 3, and 6, respectively, except that polyvinylpyrrolidone was used instead of hydrogenated nitrile rubber A as the dispersant. Various evaluations were then performed. The results are shown in Table 4.

[0094] Comparative Example 4 In preparing the conductive material dispersion, a carbon material, a conductive material dispersion, a positive electrode slurry, a positive electrode, a negative electrode, a separator, and a secondary battery were prepared in the same manner as in Example 1, except that polyvinyl butyral was used as the dispersant instead of hydrogenated nitrile rubber A. Various evaluations were then performed. The results are shown in Table 4.

[0095] In addition, in Tables 2 to 4, "HNBR-A" indicates hydrogenated nitrile rubber A, "HNBR-B" indicates hydrogenated nitrile rubber B, "HSNBR" refers to hydrogenated styrene-butadiene-nitrile rubber; "AN unit" indicates an acrylonitrile unit, "ST unit" indicates a styrene unit, "H-BD unit" refers to a 1,3-butadiene hydride unit; "MAA unit" refers to a methacrylic acid unit; "1,2-bonded H-BD units" refers to 1,2-bonded butadiene hydride units; "PVP" indicates polyvinylpyrrolidone, "PVB" indicates polyvinyl butyral, "PVdF" stands for polyvinylidene fluoride, "PAN" indicates polyacrylonitrile; "NCM" stands for LiNi 0.5 Co 0.2 Mn 0.3 Indicates O2.

[0096] [Table 1]

[0097] [Table 2]

[0098] [Table 3]

[0099] [Table 4]

[0100] From Tables 3 and 4, the Hansen solubility parameters (HSP) of the carbon materials, including the carbon materials, dispersants, and dispersion media, are c ) and Hansen Solubility Parameter (HSP) of the dispersant d) and the HSP distance (R d It can be seen that in Examples 1 to 8, in which positive electrodes were produced using conductive material dispersions in which the value of (V) was a predetermined value or less, secondary batteries having reduced internal resistance and good high-temperature storage characteristics could be produced. It can also be seen that the conductive material dispersions of Examples 1 to 8 had good dispersion states and excellent viscosity stability. On the other hand, from Table 4, the HSP distance (R d In Comparative Examples 1 to 4, in which the positive electrode was prepared using a conductive material dispersion liquid in which the value of (a) exceeds a predetermined value, the internal resistance of the secondary battery was not sufficiently reduced and the high-temperature storage characteristics were deteriorated. In addition, in Comparative Examples 1 to 4, it was also found that the dispersion state of the conductive material dispersion liquid was deteriorated. [Industrial Applicability]

[0101] According to the present invention, it is possible to provide a conductive material dispersion, a slurry for a secondary battery positive electrode, and a positive electrode for a secondary battery, which can reduce the internal resistance of a secondary battery while ensuring good high-temperature storage characteristics of the secondary battery. Furthermore, according to the present invention, it is possible to provide a secondary battery having reduced internal resistance and excellent high-temperature storage characteristics.

Claims

1. A conductive material dispersion liquid containing a carbon material, a dispersant, and a dispersion medium, The Hansen solubility parameter (HSP) of the carbon material ｃ ) and the Hansen solubility parameter (HSP) of the dispersant ｄ ) and the HSP distance (R ｄ ) is 10.0 MPa １／２ is as follows: The conductive material dispersion, wherein the dispersant contains 1,3-butadiene hydride units, and the content of 1,2-bond butadiene hydride units is 4.0% by mass or more and 15% by mass or less, relative to 100% by mass of all 1,3-butadiene hydride units in the dispersant.

2. The Hansen solubility parameter (HSP) of the carbon material ｃ ) hydrogen bond term δ ｈ１ is 12.0 MPa １／２ The conductive material dispersion according to claim 1 , wherein:

3. The conductive material dispersion liquid according to claim 1 or 2, wherein the carbon material comprises a fibrous carbon material.

4. A slurry for a secondary battery positive electrode, comprising the conductive material dispersion liquid according to any one of claims 1 to 3 and a positive electrode active material.

5. The slurry for a secondary battery positive electrode according to claim 4 , further comprising a binder.

6. The Hansen solubility parameter (HSP) of the carbon material ｃ ) and the Hansen solubility parameter (HSP) of the binder ｂ ) and the HSP distance (R ｂ ) is the HSP distance (R ｄ 6. The slurry for a secondary battery positive electrode according to claim 5, wherein the average particle size is greater than 100 nm.

7. A positive electrode for a secondary battery, comprising a positive electrode mixture layer formed using the slurry for a secondary battery positive electrode according to any one of claims 4 to 6.

8. A secondary battery comprising the positive electrode for secondary batteries according to claim 7 .

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