Composite for secondary battery electrode

Abstract

To provide a composite for a secondary battery electrode, having a good charge / discharge reaction by controlling a type of fibrous carbons on an active material and a composite state.SOLUTION: The present invention provide a composite for a secondary battery electrode, including an electrode active material and a fibrous carbon. The fibrous carbon contains at least two types of fibrous carbons of which an average fiber diameter is different, at least the two types of fibrous carbons includes: a first fibrous carbon of which the average fiber diameter is 1 nm or more and is less than 9 nm; and a second fibrous carbon of which the average fiber diameter is 9 nm or more and is 150 nm or less.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a composite for a secondary battery electrode, an electrode using the same, a secondary battery, and a device.

Background Art

[0002] With the spread of electric vehicles and the miniaturization, light weight, and high performance of portable devices, there is a demand for secondary batteries having a high energy density, and further, an increase in the capacity of such secondary batteries. Under such circumstances, non-aqueous electrolyte secondary batteries using a non-aqueous electrolyte, particularly lithium-ion secondary batteries, have come to be used in many devices due to their characteristics of high energy density and high voltage.

[0003] The electrode of a secondary battery is manufactured by coating a composite slurry containing a positive electrode active material or a negative electrode active material, a conductive material, a binder resin, etc. on a current collector. By preparing a conductive material dispersion liquid in which a conductive material is dispersed in a dispersion medium and adding an active material and a binder resin to the conductive material dispersion liquid to prepare a composite slurry, the conductive material is uniformly dispersed and contained in the electrode film, and the conductivity of the electrode film can be improved. In order to increase the capacity of the secondary battery, an increase in the ratio of the active material in the electrode is desired. Therefore, carbon nanotubes that can form an efficient conductive network even in a small amount as a conductive agent and can reduce the electrode resistance are expected. On the other hand, in order to reduce the resistance with a smaller addition amount, it is effective to use carbon nanotubes having a small average outer diameter and a large fiber length. However, these carbon nanotubes have a strong cohesive force and it is difficult to uniformly distribute them in the electrode. In order to efficiently form a conductive network in the electrode and ensure a conductive path to the active material, a method of using a composite in which a conductive material such as carbon nanotubes is adsorbed on the surface of the active material in advance has been proposed.

[0004] Patent Document 1 discloses an example of producing a positive electrode active material having high electron conductivity and high mechanical strength by mechanically chemically treating a positive electrode active material and acetylene black in a nitrogen atmosphere to form a composite. In Patent Document 2, an example is disclosed in which a slurry containing a positive electrode active material, carbon black, and cellulose nanofibers is spray granulated to produce composite particles excellent in particle strength. Further, in Patent Document 3, an example is disclosed in which a positive electrode active material and carbon nanotubes are compounded by mechanochemical treatment in a nitrogen atmosphere to produce a positive electrode active material having high electron conductivity and high mechanical strength. Further, in Patent Document 4, an example is disclosed in which a slurry in which positive electrode active material particles and fibrous carbon are dispersed using an ultrasonic disperser in a solvent is spray granulated to produce composite particles in which the materials are uniformly mixed.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Summary of the Invention

Problems to be Solved by the Invention

[0006] In the methods disclosed in Patent Documents 1 and 2, since a dispersion of acetylene black is used as a conductive aid, it is difficult to efficiently form a conductive network with a small amount of addition compared to the case where fibrous carbon is used as a conductive aid. In the method disclosed in Patent Document 3, it is difficult to unravel the fibrous and intertwined carbon nanotubes during the compounding process, and uniform adsorption of the carbon nanotubes onto the surface of the active material cannot be achieved, easily resulting in a non-uniform composite, and it is difficult to form a uniform conductive network in the electrode. In the method disclosed in Patent Document 4, since carbon nanotubes are present in the composite particles in a state where they are unraveled in a network form, the formation of a conductive path between the composite particles when used as an electrode is not sufficient.

[0007] An object of the present invention is to provide a composite for a secondary battery electrode capable of obtaining high conductivity even in an electrode by a composite method using an active material and fibrous carbon in order to solve the above problems.

Means for Solving the Problems

[0008] The present invention is a composite for a secondary battery electrode containing an electrode active material and fibrous carbon, wherein the fibrous carbon contains at least two types of fibrous carbon having different average fiber diameters, The present invention relates to a composite for a secondary battery electrode, characterized in that the at least two types of fibrous carbon include a first fibrous carbon having an average fiber diameter of 1 nm or more and less than 9 nm and a second fibrous carbon having an average fiber diameter of 9 nm or more and 150 nm or less.

[0009] The present invention relates to a composite for a secondary battery electrode, characterized in that a part of the fibrous carbon has a crushed form and adheres to the surface of the electrode active material, and a part of the fibrous carbon has an aggregate form.

[0010] The present invention relates to the composite for a secondary battery electrode, wherein the average particle diameter of the aggregate is 0.2 μm or more and 50 μm or less.

[0011] Furthermore, the present invention relates to the above composite for a secondary battery electrode containing a binder resin.

[0012] The present invention relates to an electrode for a secondary battery containing the composite for a secondary battery electrode.

[0013] The present invention relates to a secondary battery containing the electrode for a secondary battery.

[0014] The present invention relates to a vehicle or a device equipped with the secondary battery.

[0015] The present invention relates to a method for manufacturing the composite for a secondary battery electrode, comprising: a step of compounding a fibrous carbon dispersion and an electrode active material using a dry mixing device, and a step of removing a solvent and drying, the method for manufacturing the composite for a secondary battery electrode being characterized by including these steps.

Effects of the Invention

[0016] According to an embodiment of the present invention, by finely controlling the dispersion state of fibrous carbon on the active material, a composite for a secondary battery electrode with excellent conductivity can be provided. According to still another embodiment of the present invention, a non-aqueous electrolyte secondary battery with high output and long life and an electrode film used therefor can be provided.

Modes for Carrying Out the Invention

[0017] Hereinafter, a method for manufacturing a composite for a secondary battery electrode according to an embodiment of the present invention will be described in detail. The present invention is not limited to the following embodiments, and embodiments implemented within a range not changing the gist of the present invention are also included.

[0018] In this specification, carbon nanotubes may be denoted as "CNT". Hydrogenated nitrile rubber may be denoted as "H-NBR", and N-methyl-2-pyrrolidone may be denoted as "NMP". Note that in this specification, a carbon nanotube dispersion may sometimes be simply referred to as a "CNT dispersion" or a "dispersion liquid". Further, the composite for a secondary battery electrode may sometimes be referred to as a "composite for an electrode" or a "composite".

[0019] <Composite for Secondary Battery Electrode> The composite for a secondary battery electrode contains an electrode active material and fibrous carbon, and may further optionally contain a dispersant and a binder resin. The composite for an electrode may further contain optional components.

[0020] contains at least two types of fibrous carbon having different average fiber diameters, The at least two types of fibrous carbon include first fibrous carbon with an average fiber diameter of 1 nm or more and less than 9 nm, and second fibrous carbon with an average fiber diameter of 9 nm or more and 150 nm or less.

[0021] In the present invention, the inclusion of at least two types of fibrous carbon with different average fiber diameters is important for exhibiting excellent battery characteristics. If there is only the first fibrous carbon with an average fiber diameter of 1 nm or more and less than 9 nm, the number per unit weight increases, which is advantageous for electron conduction. However, since it is adsorbed densely on the surface of the active material, it works disadvantageously for ion conduction to the active material. On the other hand, if there is only the second fibrous carbon with an average fiber diameter of 9 nm or more and 150 nm or less, the number per unit weight relatively decreases. Therefore, it is necessary to increase the content of fibrous carbon in the composite to ensure electron conduction, which works disadvantageously for the energy density. By containing at least two types of fibrous carbon with different fiber diameters, it is possible to ensure the number per unit weight and suppress the over-dense structure on the surface of the active material. Therefore, it is considered that the suppression of the decrease in energy density and the balance between electron and ion conduction can be achieved, leading to the manifestation of good battery characteristics.

[0022] The average fiber diameter of the fibrous carbon can be calculated by observing and imaging the fibrous carbon with a transmission electron microscope, and in the observation photograph, selecting any 300 fibrous carbons and measuring their respective outer diameters. In this specification, different average fiber diameters mean that there is a difference of 2 nm or more between the average fiber diameters of the two, preferably 3 to 30 nm, and more preferably 3 to 6 nm.

[0023] The average fiber diameter of the first fibrous carbon is preferably 2 nm or more, more preferably 3 nm or more. Also, it is preferably 8 nm or less, more preferably 7 nm or less. The average fiber diameter of the second fibrous carbon is preferably 10 nm or more. Also, it is preferably 100 nm or less, more preferably 50 nm or less. Within the above range, it can be more uniformly compounded with the composite, and the battery characteristics are also improved.

[0024] In the present invention, the content ratio of at least two kinds of fibrous carbons having different average fiber diameters is preferably such that the first fibrous carbon is contained in an amount of 5 to 90% by weight, more preferably 10 to 60% by weight, and even more preferably 20 to 50% by weight, based on the total content of the fibrous carbons.

[0025] Moreover, it is preferable that the fibrous carbon contained in the composite for an electrode of the present invention has a partially crushed form and adheres to the surface of the active material, and a part thereof has an aggregate form.

[0026] When the fibrous carbon is in a crushed form on the surface of the active material, electrons required for the battery reaction can be efficiently supplied to the active material. In addition, when the fibrous carbon is contained in the composite in the form of an aggregate, it can contribute to reducing the contact resistance between the active materials when the electrode is fabricated. Therefore, when the fibrous carbon is adsorbed uniformly and without unevenness on the surface of the active material in a crushed form, and an aggregate of fibrous carbons having an appropriate particle size and number is included, when used as an electrode for a secondary battery, the electrode resistance can be lowered, and the formation of an electron conduction path to the active material during the battery reaction is facilitated, and the battery rate characteristics and cycle characteristics can be improved. In addition, since an efficient conductive path can be formed with a small amount of conductive assistant, the amount of the conductive assistant in the electrode can be reduced, and the battery capacity can be improved. However, the above is based on scientific considerations, and the present invention is not limited only to the above effects.

[0027] The form in which the fibrous carbon is crushed refers to a form in which secondary particles (aggregates) of the fibrous carbon are dissociated into primary particles or a bundle state in which a plurality of primary particles are bundled. Examples of the state in which the fibrous carbon is crushed and adsorbed on the surface of the active material include a state in which primary particles and / or bundles are isolated, and a state in which primary particles and / or bundles overlap. From the viewpoint of electron supply to the active material and the conductive path between the composites, a state in which primary particles and / or bundles overlap and are adsorbed in a network form is preferable.

[0028] The state where the fibrous carbon in the present invention is adsorbed on the surface of the active material in a fragmented form means that the area occupied by the fibrous carbon adsorbed on the active material in the fragmented state of the fiber is 10% or more of the entire surface of the active material, preferably 20% or more, and more preferably 50% or more.

[0029] The area occupied by the fibrous carbon adsorbed on the active material in the fragmented state of the above fiber can be measured from the observation images at magnifications of 5,000 to 50,000 times taken by scanning electron microscopy (SEM) or scanning electron microscope - energy dispersive X-ray spectroscopy (SEM-EDX) based on the contrast difference between the active material and the fibrous carbon, and the average value of the above area calculated from 20 active materials in the composite was used.

[0030] The aggregate of fibrous carbon in the present invention is an aggregate having a form in which the primary particles of fibrous carbon and / or bundles formed by a plurality of primary particles are intertwined.

[0031] The particle size of the aggregate of fibrous carbon in the present invention can be measured from the observation images at magnifications of 5,000 to 50,000 times taken by scanning electron microscopy (SEM) or scanning electron microscope - energy dispersive X-ray spectroscopy (SEM-EDX) based on the length of the major axis of the aggregate of fibrous carbon, and the average value of the particle sizes of 10 aggregates of fibrous carbon was taken as the average particle size of the aggregate.

[0032] The aggregate of fibrous carbon in the present invention refers to those having an average particle size of 0.1 to 100 μm, preferably 0.2 to 10 μm, more preferably 0.2 to 5 μm, and still more preferably 0.2 to 3 μm. If there are aggregates of fibrous carbon with an average particle size exceeding 100 μm, it may be difficult to form a uniform composite.

[0033] The number of aggregates of fibrous carbon in the present invention can be measured, for example, from the average number of aggregates of fibrous carbon measured from 10 observation images (observation field area: 20 μm × 15 μm) at a magnification of 5,000 taken by a scanning electron microscope (SEM) or a scanning electron microscope - energy dispersive X-ray spectroscopy (SEM-EDX). The average number of aggregates of fibrous carbon in the present invention is preferably 1 or more.

[0034] When the combustion temperature of fibrous carbon in an oxygen-containing atmosphere of the composite for an electrode of the present invention is 550°C or lower, the composite is in a state where the fibrous carbon adsorbed on the surface of the active material is released and in a uniform state.

[0035] When the volume resistivity of the composite for an electrode of a secondary battery under a load of 20 kN is 500 Ω·cm or less, uneven distribution of fibrous carbon in the composite does not occur, and it is adsorbed on the active material in a more uniform state, so that the electrode resistance can be reduced when used as an electrode for a secondary battery. It is preferably 100 Ω·cm or less, and more preferably 50 Ω·cm or less.

[0036] The volume resistivity of the composite for an electrode of a secondary battery can be determined by performing resistivity measurement based on the four-terminal four-probe method. Specifically, the composite for an electrode of a secondary battery to be measured is brought into contact with four needle-shaped probes (electrodes), and the resistance of the test piece is obtained from the current flowing between the two outer probes and the potential difference generated between the two inner probes. Then, the volume resistivity of the CNT can be calculated from the measured resistance and the thickness of the test piece. The measurement is performed using a volume resistivity measuring device (for example, an automatic powder resistivity measuring system low resistance version MCP-PD600 manufactured by Nitto Seiko Analytic Co., Ltd.) under the condition that the load is 20 kN.

[0037] The composite for an electrode of a secondary battery is preferably in a particulate form, and the average particle diameter is preferably 0.5 μm or more and 500 μm or less. More preferably, it is 1 μm or more and 200 μm or less.

[0038] <Active material>

[0039] The active material is a substance that causes a battery reaction necessary for extracting electrical energy and is not particularly limited. As the positive electrode active material, for example, for secondary battery applications, metal compounds such as metal oxides and metal sulfides capable of reversibly doping or intercalating lithium ions can be used. For example, lithium manganese composite oxide (e.g., Li x Mn 2 O 4 or Li x MnO 2 ), lithium nickel composite oxide (e.g., Li x NiO 2 ), lithium cobalt composite oxide (Li x CoO 2 ), lithium nickel cobalt composite oxide (e.g., Li x Ni 1-y Co y O 2 ), lithium manganese cobalt composite oxide (e.g., Li x Mn y Co 1-y O 2 ), lithium nickel manganese cobalt composite oxide (e.g., Li x Ni y Co z Mn 1-y-z O 2 ), spinel-type lithium manganese nickel composite oxide (e.g., Li x Mn 2-y Ni y O 4 ) and other composite oxide powders of lithium and transition metals, lithium phosphate powders having an olivine structure (e.g., Li x FePO 4 , Li x Fe 1-y Mn y PO 4 , Li x CoPO 4 etc.), manganese oxide, iron oxide, copper oxide, nickel oxide, vanadium oxides (e.g., V 2 O 5 , V 6 O 13 ), transition metal oxide powders such as titanium oxide, iron sulfate (Fe 2 (SO 4) 3 ) and transition metal sulfide powders such as TiS 2 and FeS, etc. Here, x, y, and z are numbers, where 0 < x < 1, 0 < y < 1, 0 < z < 1, and 0 < y + z < 1. These cathode active materials can be used alone or in combination of two or more. Among these active materials, in particular, active materials containing Ni and / or Mn (especially when the total amount of Ni and / or Mn in the transition metal is 50 mol% or more) tend to have a high basicity due to components derived from raw materials or elution of metal ions, and as a result, gelation of the binder resin and deterioration of the dispersion state are likely to occur. Therefore, in the case of a battery containing an active material containing Ni and / or Mn, the present embodiment is particularly effective.

[0040] As the anode active material, for example, metal Li capable of reversibly doping or intercalating lithium ions, or its alloy, tin alloy, silicon-based materials (such as metal Si, Si alloy, SiOx), metal oxide systems such as LiXTiO2, LiXFe2O3, LiXFe3O4, LiXWO2, conductive polymers such as polyacetylene and poly-p-phenylene, artificial graphite such as highly graphitized carbon materials, or carbonaceous powders such as natural graphite, and resin-fired carbon materials can be used. Here, x is a number, where 0 < x < 1. These anode active materials can be used alone or in combination of two or more. In particular, when using a silicon-based material, although the theoretical capacity is large, the volume expansion is extremely large. Therefore, it is preferably used in combination with artificial graphite such as highly graphitized carbon materials, or carbonaceous powders such as natural graphite, and resin-fired carbon materials.

[0041] <Fibrous carbon>

[0042] Fibrous carbon is a fibrous carbon material, and has a high aspect ratio in the long axis direction, making it easier to form a conductive network compared to particulate carbon such as carbon black. Examples of fibrous carbon include carbon fiber (CNF) and carbon nanotube (CNT). CNT has a shape formed by winding planar graphite into a cylindrical shape, including single-walled CNT, double-walled CNT, thin-layer (few-layer) CNT, multi-walled CNT, and these may be mixed. Single-walled CNT has a structure in which a single layer of graphite is wound. Multi-walled CNT has a structure in which two or more layers of graphite are wound. Also, the side wall of the CNT does not have to be a graphite structure. Further, for example, CNTs having side walls with an amorphous structure are also CNTs in this specification.

[0043] The shape of the CNT is not limited. Such shapes include various shapes such as needle-like, cylindrical tube-like, fishbone-like (fishbone or cup stacking type), trump-like (platelet), and coil-like. Among them, the shape of the CNT is preferably needle-like or cylindrical tube-like. The CNT may be in a single shape or a combination of two or more shapes.

[0044] The form of the CNT includes, for example, graphite whiskers, filamentous carbon, graphite fibers, ultra-fine carbon tubes, carbon tubes, carbon fibrils, carbon microtubes, and carbon nanofibers. The carbon nanotube may have these single forms or a form combined with two or more of them.

[0045] The content of fibrous carbon in the composite is preferably 0.01% by mass or more, more preferably 0.03% by mass or more, and even more preferably 0.05% or more, based on the mass of the composite for the secondary battery electrode. Also, it is preferably 20% by mass or less, more preferably 10% by mass or less, and even more preferably 5% by mass or less. If it exceeds the above range, the filling amount of the active material in the electrode decreases, leading to a reduction in the battery capacity. In particular, for high-capacity batteries, 10% by mass or less is preferred. Also, if it is below the above range, the conductivity of the electrode and the battery may be insufficient.

[0046] The composite may contain a conductive material other than fibrous carbon. Examples of other conductive materials include carbon materials such as carbon black, fullerenes, graphene, multi-layer graphene, graphite, etc. The carbon black may be neutral, acidic, or basic, and oxidized carbon black or graphitized carbon black may be used. These other conductive materials may be used alone or in combination of two or more.

[0047] <Binder resin> The composite of the present invention can contain a binder resin if necessary. The binder resin is not particularly limited as long as it is usually used as a binder resin, and can be appropriately selected according to the purpose. The binder resin used for the composite is preferably a resin that can bind substances such as active materials and fibrous carbon. Examples of the binder resin used for the composite include polymers or copolymers containing ethylene, propylene, vinyl chloride, vinyl acetate, maleic acid, acrylic acid, acrylic acid esters, methacrylic acid, methacrylic acid esters, styrene, etc. as structural units; polyurethane resins, polyester resins, phenolic resins, epoxy resins, phenoxy resins, urea resins, melamine resins, alkyd resins, acrylic resins, formaldehyde resins, silicone resins, fluorine resins; cellulose-based resins such as carboxymethyl cellulose; elastomers such as styrene-butadiene rubber and fluorine rubber; conductive resins such as polyaniline and polyacetylene. Further, modified products, mixtures, and copolymers of these resins may also be used. Particularly, fluorine resins are preferred, and examples include polytetrafluoroethylene (PTFE), polyvinyl fluoride (PVF), polyvinylidene fluoride (PVDF), and modified products thereof. These may be used alone or in combination of two or more. Among these, polymers or copolymers having a fluorine atom in the molecule from the viewpoint of resistance, such as polyvinylidene fluoride, polyvinyl fluoride, tetrafluoroethylene, etc., resins having these structural units, and modified products thereof are preferred.

[0048] The content of the binder resin in the composite is preferably 0.1% by mass or more, more preferably 0.3% by mass or more, based on the mass of the active material (assuming the mass of the active material is 100% by mass). Also, it is preferably 20% by mass or less, more preferably 10% by mass or less.

[0049] <Method for manufacturing composite for secondary battery electrode> The method of compounding the positive electrode active material with fibrous carbon or the like is not particularly limited. For example, there are a method of dry-mixing the active material and fibrous carbon or the like, a method of further adding a solvent and mixing, a method of mixing using a dispersion liquid in which the active material and fibrous carbon or the like are dispersed in a solvent, a method of adding the active material, fibrous carbon or the like and a solvent to prepare a slurry and mixing it wet, and the like. In order to produce a more uniform composite, a method of mixing using a dispersion liquid in which fibrous carbon or the like is previously dispersed in a solvent is preferable. Among them, the use of a fibrous carbon dispersion is preferable because an electrode composite in which the active material and fibrous carbon are uniformly combined can be produced. Even when a fibrous carbon dispersion is not used, the same effect may be obtained by dissociating the aggregation of fibrous carbon by a compounding machine with a high shearing force or the like, which is preferable. In particular, in order to produce an excellent composite in the present invention, a method of compounding the active material and the fibrous carbon dispersion with a compounding machine having a high shearing force under non-slurry conditions with a high concentration and no fluidity is preferable.

[0050] Examples of the dry mixing device include Roll mills such as two-roll mills and three-roll mills, high-speed stirrers such as Henschel mixers and Super mixers, fluid energy pulverizers such as micronizers and jet mills, attritors, particle compounding devices "Nano Cure", "Novilta", "Mekano Fusion" manufactured by Hosokawa Micron Corporation, powder surface modification devices "Hybridization System", "Mekano Micro S", "Mirror Lo" manufactured by Nara Machinery Co., Ltd., mixers "Planetary Mixer", "Trimix" manufactured by Inoue Manufacturing Co., Ltd., mixing and granulating machines "High Speed Mixer" manufactured by Earth Technica Co., Ltd., and the like.

[0051] When using a dry mixing device, other raw materials such as fibrous carbon may be directly added to the parent active material in powder form. However, in order to create a more uniform mixture, it is preferable to disperse or dissolve other raw materials such as fibrous carbon in a small amount of solvent in advance and then mix them. Among these, using a fibrous carbon dispersion is preferable because an electrode composite in which the active material and fibrous carbon are uniformly combined can be produced. Furthermore, in order to improve the processing efficiency, it may be preferable to heat. Among dry mixing devices, a planetary mixer, a trimix, or a mechanofusion capable of complexing by shear is preferable.

[0052] Examples of wet mixing devices include mixers such as a disper, a homomixer, or a planetary mixer; homogenizers such as "Creamix" manufactured by M Technique Co., Ltd. or "Filmix" manufactured by PRIMIX Corporation; mixers of the rotation - revolution type such as "Avatori Rentaro" manufactured by THINKY Corporation media type dispersers such as a paint conditioner (manufactured by Red Devil), a ball mill, a sand mill (such as "Dynomill" manufactured by Shinmaru Enterprises Co., Ltd.), an attritor, a pearl mill (such as "DCP Mill" manufactured by Eriez), or a colloid mill; media - less dispersers such as a wet jet mill (such as "Genius PY" manufactured by Genus, "Starburst" manufactured by Sugino Machine, or "Nanomizer" manufactured by Nanomizer), "Cream SS - 5" manufactured by M Technique Co., Ltd., or "Micros" manufactured by Nara Machinery Co., Ltd.; or other roll mills, kneaders, etc. can be mentioned, but it is not limited to these. Also, as the wet mixing device, it is preferable to use one that has been subjected to a process for preventing metal contamination from the device. When using a media type disperser such as a ball mill, there is a risk of breaking the structure of the active material by mixing, so a media - less disperser is preferable.

[0053] <Fibrous carbon dispersion> The fibrous carbon dispersion contains fibrous carbon and a dispersion medium, and optionally contains a dispersant. The fibrous carbon dispersion may, as needed, appropriately contain other additives such as a wetting agent, a surfactant, a pH adjuster, a wetting penetrant, a leveling agent, other conductive materials, other polymer components, etc., within a range that does not inhibit the object of the present invention. The optional components can be added at any timing, such as before preparing the dispersion, during dispersion, after dispersion, or a combination thereof.

[0054] The dispersibility of the fibrous carbon in the fibrous carbon dispersion can also be evaluated by the median diameter (μm) determined by a laser diffraction / scattering type particle size distribution analyzer. With the median diameter (μm) determined by a laser diffraction / scattering type particle size distribution analyzer, the particle size of the fibrous carbon agglomerated particles can be estimated from the scattered light intensity distribution by the particles. The median diameter (μm) is preferably 0.4 μm or more, preferably 5.0 μm or less, and more preferably 2.0 μm or less. By setting it within the above range, a fibrous carbon dispersion in an appropriate dispersed state can be obtained. If it is below the above range, fibrous carbon in an agglomerated state exists, and if it exceeds the above range, a large number of finely cut fibrous carbons are generated, making it difficult to form an efficient conductive network. The median diameter can be measured by the method described in the examples.

[0055] The viscosity of the fibrous carbon dispersion is preferably such that the viscosity measured at 60 rpm at 25°C using a B-type viscometer is 10 mPa·s or more and less than 10000 mPa·s, more preferably 10 mPa·s or more and less than 2000 mPa·s, and even more preferably 10 mPa·s or more and less than 1000 mPa·s.

[0056] The TI value of the fibrous carbon dispersion can be calculated from the value obtained by dividing the viscosity (mPa·s) at 6 rpm measured at 25°C with a B-type viscometer by the viscosity (mPa·s) at 60 rpm. The TI value is preferably 1.0 or more and less than 10.0, more preferably 1.0 or more and less than 5.0, and even more preferably 1.0 or more and less than 3.0. The higher the TI value, the greater the entanglement of the fibrous carbon, dispersant, and other resin components, or the structural viscosity due to intermolecular forces between these, and the lower the TI value, the smaller the structural viscosity. By setting the TI value within the above range, it is possible to suppress the entanglement of the fibrous carbon, dispersant, and other resin components while allowing these intermolecular forces to act appropriately.

[0057] The average fiber length of the fibrous carbon in the fibrous carbon dispersion is preferably 0.1 μm or more, more preferably 0.2 μm or more, and even more preferably 0.3 μm or more. Also, it is preferably 20 μm or less, more preferably 10 μm or less. The average fiber length of the fibrous carbon in the fibrous carbon dispersion can be calculated by observing a sample obtained by dropping a 50-fold dilution of the fibrous carbon dispersion with a non-aqueous solvent such as NMP onto a substrate and drying it with a scanning electron microscope, and selecting any 300 fibrous carbons in the observation photograph and measuring their respective fiber lengths.

[0058] The content of the fibrous carbon is preferably 0.1 mass% or more, more preferably 0.5 mass% or more, and even more preferably 0.8 mass% or more, based on the total amount of the fibrous carbon dispersion. Also, it is preferably 20 mass% or less, more preferably 10 mass% or less. By setting it within the above range, the fibrous carbon can be present well and stably without causing sedimentation or gelation. More preferably, it is 0.1 to 20 mass%, and even more preferably 0.5 to 10 mass%. Also, the content of the fibrous carbon is preferably adjusted appropriately so that a fibrous carbon dispersion with appropriate fluidity or viscosity can be obtained, depending on the specific surface area of the fibrous carbon, affinity for the dispersion medium, dispersing ability of the dispersant, etc.

[0059] The content of the dispersant is preferably 5 to 200 parts by mass, more preferably 10 to 100 parts by mass, and even more preferably 15 to 80 parts by mass with respect to 100 parts by mass of the fibrous carbon. The content of the dispersant is preferably 0.1 to 10% by mass, more preferably 0.5 to 5% by mass with respect to the total amount of the fibrous carbon dispersion.

[0060] The solid content of the fibrous carbon dispersion is preferably 0.2 to 40% by mass, more preferably 0.5 to 20% by mass, and even more preferably 1 to 10% by mass.

[0061] The fibrous carbon dispersion may be obtained by separately dispersing two or more types of fibrous carbon having different average fiber diameters and then mixing them.

[0062] The average fiber length of the fibrous carbon is preferably 0.5 μm or more, more preferably 0.8 μm or more, and even more preferably 1.0 μm or more. Also, it is preferably 20 μm or less, more preferably 10 μm or less. The average fiber length of the fibrous carbon can be calculated by first observing and imaging the fibrous carbon with a scanning electron microscope, and then selecting any 300 fibrous carbons in the observation photograph and measuring their respective fiber lengths.

[0063] The value obtained by dividing the fiber length of the fibrous carbon by the outer diameter is the aspect ratio. Using the values of the average fiber length and the average outer diameter, a representative aspect ratio can be obtained. The higher the aspect ratio of the conductive material, the higher the conductivity can be obtained when forming an electrode. The aspect ratio of the fibrous carbon is preferably 30 or more, more preferably 50 or more, and even more preferably 80 or more. Also, it is preferably 10,000 or less, more preferably 3,000 or less, and even more preferably 1,000 or less.

[0064] The specific surface area of the fibrous carbon is preferably 100 m 2 / g or more, more preferably 150 m 2 / g or more, and even more preferably 200 m 2It is more preferably at least / g. Also, 1200 m 2 It is preferably at most / g, and more preferably at most 1000 m 2 / g. The specific surface area of the fibrous carbon is calculated by the BET method using nitrogen adsorption measurement. When the average outer diameter, average fiber length, aspect ratio, and specific surface area of the fibrous carbon are within the above ranges, it becomes easier to form a developed conductive path in the electrode.

[0065] The carbon purity of the fibrous carbon is represented by the content rate (mass%) of carbon atoms in the fibrous carbon. The carbon purity is preferably 80 mass% or more with respect to 100 mass% of the fibrous carbon.

[0066] For the purpose of removing or reducing impurities such as metal catalysts and increasing the carbon purity, fibrous carbon subjected to a purification treatment may be used.

[0067] When the fibrous carbon is dispersed by a disperser by collision with media such as a bead mill, or when a treatment is performed such that the fibrous carbon is repeatedly passed through the disperser over a long period of time, the fibrous carbon may be damaged and short-piece carbonaceous substances may be generated. When short-piece carbonaceous substances are generated, the viscosity of the fibrous carbon dispersion decreases, and the gloss of the coating film obtained by coating and drying the fibrous carbon dispersion becomes high. Therefore, judging only from these evaluation results, the dispersion state seems to be good. However, the short-piece carbonaceous substances have a high contact resistance and it is difficult to form a conductive network, so the resistance of the electrode may be deteriorated. The degree to which short-piece carbonaceous substances are generated can be confirmed by methods such as diluting the dispersion liquid, dropping it onto a substrate with a smooth surface and good affinity with the dispersion medium, drying it, and observing the sample with a scanning electron microscope. By adjusting the dispersion conditions and the composition of the dispersion liquid so that carbonaceous substances of 0.1 μm or less are not generated, an electrode with high conductivity can be obtained.

[0068] The fibrous carbon may be produced by any method. Generally, fibrous carbon can be produced by laser ablation method, arc discharge method, thermal CVD method, plasma CVD method and combustion method, but is not limited thereto. For example, in an atmosphere with an oxygen concentration of 1% by volume or less, at 500 to 1000 °C, fibrous carbon can be produced by causing a carbon source to undergo a catalytic reaction. The carbon source may be at least one of hydrocarbons and alcohols.

[0069] <Dispersant> The dispersant of the fibrous carbon dispersion is preferably one that can disperse and stabilize the fibrous carbon in the fibrous carbon dispersion. As the dispersant, either a resin-type dispersant or a surfactant can be used, but a resin-type dispersant is preferred because it has a strong adsorption force to the fibrous carbon and good dispersion stability. Appropriate types of dispersants can be used in appropriate blending amounts according to the properties required for the dispersion of the fibrous carbon.

[0070] As the resin type dispersant, (meth)acrylic polymers, polymers derived from ethylenically unsaturated hydrocarbons, cellulose derivatives, copolymers thereof, etc. can be used. Examples of the polymers derived from ethylenically unsaturated hydrocarbons include polyvinyl alcohol resins, polyvinyl pyrrolidone resins, polyacrylonitrile resins, nitrile rubbers, etc. Examples of the polyvinyl alcohol resins include polyvinyl alcohol, modified polyvinyl alcohol having a functional group other than a hydroxyl group (for example, an acetyl group, a sulfo group, a carboxy group, a carbonyl group, an amino group), polyvinyl alcohol modified with various salts, other anion-modified or cation-modified polyvinyl alcohol, polyvinyl acetals (such as polyvinyl acetoacetal, polyvinyl butyral, etc.) acetal-modified (such as acetoacetal-modified or butyral-modified) with aldehydes. Examples of the polyacrylonitrile resins may be homopolymers of polyacrylonitrile, copolymers of polyacrylonitrile, modified products thereof, etc., and polyacrylonitrile resins having at least one selected from the group consisting of active hydrogen groups such as hydroxyl groups, carboxy groups, primary amino groups, secondary amino groups, and mercapto groups, basic groups, alkyl groups introduced from (meth)acrylic acid alkyl esters or α-olefins, etc. are preferable. For example, the acrylonitrile copolymer described in JP-A-2020-163362 can be used. Examples of the nitrile rubbers include acrylonitrile-butadiene rubber, hydrogenated acrylonitrile-butadiene rubber, etc. Examples of the cellulose derivatives include cellulose acetate, cellulose acetate butyrate, cellulose butyrate, cyanoethyl cellulose, ethyl hydroxyethyl cellulose, nitrocellulose, methyl cellulose, ethyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, hydroxypropyl methyl cellulose, carboxymethyl cellulose, etc., or copolymers thereof. Further, the dispersants described in WO2008 / 108360 pamphlet, JP-A-2018-192379, JP-A-2019-087304, Patent No. 6524479, JP-A-2009-026744 may be used, but are not limited thereto.Particularly preferred are methyl cellulose, ethyl cellulose, polyvinyl alcohol, polyvinyl butyral, polyvinyl pyrrolidone, homopolymers of polyacrylonitrile, copolymers of polyacrylonitrile, and hydrogenated acrylonitrile butadiene rubber. Polymers in which other substituents are introduced into a part of these polymers, modified polymers, etc. may also be used. From the viewpoint of the balance of affinity between the object to be dispersed and the dispersion medium and the resistance to the electrolyte, the weight average molecular weight of the resin type dispersant is preferably 500,000 or less, more preferably 300,000 or less, preferably 3,000 or more, and more preferably 5,000 or more. The resin type dispersant may be used alone or in combination of two or more.

[0071] Examples of commercially available polyvinyl alcohol-based resins include various grades available under trade names such as Kuraray Poval (polyvinyl alcohol resin manufactured by Kuraray), Gohsenol, and Gohsenex (polyvinyl alcohol resins manufactured by Nippon Synthetic Chemical Industry Co., Ltd.). Modified polyvinyl alcohols having various functional groups can also be obtained in the same manner. Specific examples of commercially available polyvinyl pyrrolidone-based resins include polyvinyl pyrrolidone K30, K90 (manufactured by Fujifilm Wako), and K120 (manufactured by DSP Gokyo & Chemical Co., Ltd.). Examples of commercially available nitrile rubbers include various grades with different nitrile ratios, hydrogenation rates, molecular weights, etc. under trade names such as Therban (hydrogenated nitrile rubber manufactured by Arlanxeo), Baymod (nitrile rubber manufactured by Arlanxeo), Zetpole (hydrogenated nitrile rubber manufactured by Nippon Zeon Co., Ltd.), and Nipole NBR (nitrile rubber manufactured by Nippon Zeon Co., Ltd.). Those synthesized by known synthesis methods may also be used. A surfactant may be used instead of or in addition to the above-described resin type dispersant. Surfactants are classified into anionic, cationic, amphoteric ionic surfactants, and nonionic surfactants.

[0072] As the resin-type dispersant, a polymer containing at least an aliphatic hydrocarbon structural unit and a nitrile group-containing structural unit may be used. The aliphatic hydrocarbon structural unit of the polymer may include an alkylene structural unit. This polymer may be hydrogenated.

[0073] The aliphatic hydrocarbon structural unit is a structural unit containing an aliphatic hydrocarbon structure, preferably a structural unit consisting only of an aliphatic hydrocarbon structure. The aliphatic hydrocarbon structure includes at least a saturated aliphatic hydrocarbon structure, and may further include an unsaturated aliphatic hydrocarbon structure. The aliphatic hydrocarbon structure preferably includes at least a linear aliphatic hydrocarbon structure, and may further include a branched aliphatic hydrocarbon structure.

[0074] Examples of the aliphatic hydrocarbon structural unit include an alkylene structural unit, an alkenylene structural unit, an alkyl structural unit, an alkanetriyl structural unit, an alkanetetrayl structural unit, etc. Structural units containing branch points such as an alkanetriyl structural unit and an alkanetetrayl structural unit are different structural units from the structural units containing a branched alkylene structure and the structural units containing a branched alkyl structure described later. The aliphatic hydrocarbon structural unit preferably includes at least an alkylene structural unit.

[0075] The alkylene structural unit is a structural unit containing an alkylene structure, preferably a structural unit consisting only of an alkylene structure. The alkylene structure is preferably a linear alkylene structure or a branched alkylene structure.

[0076] <Dispersion medium> The dispersion medium of the fibrous carbon dispersion is not particularly limited, but is preferably a high dielectric constant solvent, and preferably includes a solvent composed of any one of high dielectric constant solvents, or a mixed solvent composed of two or more of them. Further, one or two or more other solvents may be mixed with the high dielectric constant solvent and used.

[0077] As the high dielectric constant solvent, amide-based solvents (such as N-methyl-2-pyrrolidone (NMP), N-ethyl-2-pyrrolidone (NEP), N,N-dimethylformamide, N,N-dimethylacetamide, N,N-diethylacetamide, N-methylcaprolactam, etc.), heterocyclic-based solvents (such as cyclohexylpyrrolidone, 2-oxazolidone, 1,3-dimethyl-2-imidazolidinone, γ-butyrolactone, etc.), sulfoxide-based solvents (such as dimethyl sulfoxide, etc.), sulfone-based solvents (such as hexamethylphosphoramide, sulfolane, etc.), lower ketone-based solvents (such as acetone, methyl ethyl ketone, etc.), carbonate-based solvents (such as diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate, fluoroethylene carbonate, propylene carbonate, ethylene carbonate), water, and others such as tetrahydrofuran, urea, acetonitrile, etc. can be used. As the dispersion medium, it preferably contains an amide-based organic solvent or water, and the amide-based organic solvent more preferably contains at least one selected from the group consisting of N-methyl-2-pyrrolidone and N-ethyl-2-pyrrolidone. The relative dielectric constant of the high dielectric constant solvent can be the value described in a solvent handbook or the like, and it is preferably 2.5 or more at 20°C.

[0078] Examples of the method for removing and drying the solvent include drying methods such as hot air drying, vacuum drying, spray drying, and freeze drying. Among them, hot air drying and vacuum drying are preferable from the viewpoint of productivity.

[0079] <Composite Material Slurry for Secondary Battery Electrodes> A slurry for a secondary battery electrode can be produced using a composite for a secondary battery electrode. The composite slurry for a secondary battery electrode can be obtained by adding an electrode composite and a binder resin to a dispersion medium. If necessary, a fibrous carbon dispersion and other optional components may be appropriately included within a range that does not inhibit the object of the present invention. The optional components can be added at any timing, such as before the production of the composite slurry, during mixing, after mixing, or a combination thereof. The optional components may be those described for the fibrous carbon dispersion. The active material may be a positive electrode active material or a negative electrode active material. In this specification, the positive electrode active material and the negative electrode active material may sometimes be simply referred to as "active material".

[0080] The solid content in the composite slurry is preferably 30% by mass or more, more preferably 40% by mass or more, based on the mass of the composite slurry (assuming the mass of the composite slurry is 100% by mass). Also, it is preferably 90% by mass or less, more preferably 85% by mass or less.

[0081] The content of the electrode composite in the composite slurry is preferably 28% by mass or more, more preferably 38% by mass or more, and even more preferably 48% or more, based on the mass of the composite slurry (assuming the mass of the composite slurry is 100% by mass). Also, it is preferably 88% by mass or less, more preferably 80% by mass or less.

[0082] The content of the binder resin in the composite slurry is preferably 0.1% by mass or more, more preferably 0.3% by mass or more, based on the mass of the active material (assuming the mass of the active material is 100% by mass). Also, it is preferably 20% by mass or less, more preferably 10% by mass or less. When the electrode composite contains a binder resin, it is preferable to adjust so that the total amount of the binder resin in the composite slurry falls within the above range.

[0083] The content of fibrous carbon in the composite material slurry is preferably 0.01% by mass or more, more preferably 0.03% by mass or more, and even more preferably 0.05% or more, based on the mass of the active material (assuming the mass of the active material is 100% by mass). Also, it is preferably 20% by mass or less, more preferably 10% by mass or less, even more preferably 5% by mass or less, and even more preferably 3% by mass or less. Since the composite material for the electrode of the present invention contains fibrous carbon, it is preferable to adjust the total amount of fibrous carbon in the composite material slurry to be within the above range. If it exceeds the above range, the filling amount of the active material in the electrode will decrease, leading to a reduction in the battery capacity. On the other hand, if it is below the above range, the conductivity of the electrode and the battery may be insufficient.

[0084] <Method for manufacturing a composite material slurry for a secondary battery electrode>

[0085] In the method for preparing the composite material slurry, the order of adding the composite material for the electrode and the binder resin to the dispersion medium is not particularly limited. For example, a method of adding the binder resin to the dispersion medium to prepare a binder resin composition, and then adding the composite material for the electrode to the binder resin composition; a method of adding the composite material for the electrode to the dispersion medium and then adding the binder resin; a method of adding the binder resin and the composite material for the electrode to the dispersion medium all at once, etc. can be mentioned. As a method for preparing the composite material slurry, a method of adding the binder resin to the dispersion medium to prepare a binder resin composition, and then adding the composite material for the electrode to the binder resin composition and performing a stirring treatment is preferable. The stirring device used for stirring is not particularly limited. For the stirring device, a disper, a homogenizer, etc. can be used.

[0086] <Electrode for secondary battery> The electrode for a secondary battery in the present invention includes a composite for a secondary battery electrode. The electrode may further contain optional components such as a dispersant. The electrode can be formed by preparing the above-mentioned composite slurry, coating the composite slurry, or by adding a binder etc. to the composite for a secondary battery electrode without using the composite slurry and sheet-forming each component by pressing etc. Note that when not using the composite slurry, a pre-mixed dry mixture or a dried product after wet mixing may be used. When using the composite slurry, for example, the electrode film can be formed by coating the composite slurry on a current collector and removing the volatile components.

[0087] <Method for manufacturing a secondary battery electrode>

[0088] The material and shape of the current collector are not particularly limited, and those suitable for various secondary batteries can be appropriately selected. For example, examples of the material of the current collector include metals and alloys such as aluminum, copper, nickel, titanium, or stainless steel. Also, as the shape, generally a foil on a flat plate is used, but those with a roughened surface, perforated foil-like ones, and mesh-like current collectors can also be used. The thickness of the current collector is preferably about 0.5 to 30 μm.

[0089] As a method for coating the composite slurry on the current collector, there is no particular limitation and known methods can be used. Specifically, examples include the die coating method, dip coating method, roll coating method, doctor coating method, knife coating method, spray coating method, gravure coating method, screen printing method, or electrostatic coating method etc. As a drying method after coating, air drying, hot air drying, infrared heating, far-infrared heating etc. can be used, but it is not particularly limited to these.

[0090] As a method for adding a binder etc. to the composite for a secondary battery electrode and dry-mixing or wet-mixing each component without using the composite slurry, there is no particular limitation and known methods can be used. Specifically, dry and wet dispersers etc. described in the method for manufacturing the composite can be appropriately selected.

[0091] After applying the composite slurry, rolling treatment may be performed using an offset press, calender roll, or the like. The thickness of the electrode film is, for example, 1 μm or more and 500 μm or less, preferably 10 μm or more and 300 μm or less.

[0092] When forming an electrode by adding a binder or the like to the composite for a secondary battery electrode without using the composite slurry and sheet-forming each component by pressing or the like, each component obtained by adding a binder or the like to the composite for a secondary battery electrode, or the dry mixture or the dried product of the wet mixture may be directly supplied to the current collector and pressure-molded, or a self-supporting film composed of each component obtained by adding a binder or the like to the composite for a secondary battery electrode, or the dry mixture or the dried product of the wet mixture may be prepared in advance and then laminated on the current collector. Further, a current collector on which a resin thin film serving as a binder component has been previously formed may be used or produced.

[0093] Examples of the pressure-molding method include an offset press, calender roll, or the like. Among them, use of a calender roll that enables high productivity and uniform molding is preferred.

[0094] <Secondary battery> The secondary battery includes a positive electrode, a negative electrode, and an electrolyte, and at least one selected from the group consisting of the positive electrode and the negative electrode includes an electrode film made of the composite of the present invention.

[0095] As the positive electrode, one in which an electrode film containing a positive electrode active material is formed on a current collector can be used. As the negative electrode, one in which an electrode film containing a negative electrode active material is formed on a current collector can be used.

[0096] The electrolyte may be any of a liquid electrolyte, a gel electrolyte, and a solid electrolyte. For example, the liquid electrolyte may contain an electrolyte salt such as a lithium salt and a non-aqueous solvent. As the electrolyte salt, various conventionally known ones in which ions can move can be used. For example, LiBF 4 , LiClO 4 , LiPF 6 , LiAsF 6 , LiSbF 6, LiCF 3 SO 3 , Li(CF 3 SO 2 ) 2 , LiC 4 F 9 SO 3 , Li(CF 3 SO 2 ) 3 , LiI, LiBr, LiCl, LiAlCl, LiHF 2 , LiSCN, or LiBPh 4 (where Ph is a phenyl group), etc. Lithium salts such as these are included, but are not limited thereto. The electrolyte salt is preferably dissolved in a non-aqueous solvent and used as an electrolytic solution.

[0097] The non-aqueous solvent is not particularly limited. For example, carbonates such as ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate; lactones such as γ-butyrolactone, γ-valerolactone, and γ-octanoic lactone; glymes such as tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolane, 4-methyl-1,3-dioxolane, 1,2-methoxyethane, 1,2-ethoxyethane, and 1,2-dibutoxyethane; esters such as methyl formate, methyl acetate, and methyl propionate; sulfoxides such as dimethyl sulfoxide and sulfolane; and nitriles such as acetonitrile, etc. These solvents may be used alone or in combination of two or more.

[0098] The secondary battery preferably includes a separator. Examples of the separator include, but are not limited to, polyethylene non-woven fabric, polypropylene non-woven fabric, polyamide non-woven fabric, and non-woven fabrics subjected to hydrophilic treatment thereof.

[0099] The structure of the secondary battery is not particularly limited. Usually, it includes a positive electrode, a negative electrode, and a separator provided as needed, and can have various shapes according to the purpose of use, such as paper type, cylindrical type, button type, laminated type, etc.

[0100] Since the composite for the secondary battery electrode of the present invention is ideally composed of an active material and fibrous carbon, it can be suitably used not only for electrodes made from a composite material slurry but also for electrodes made by solid or semi-solid processes. These electrodes made by solid processes can be applied not only to lithium-ion secondary batteries using conventional electrolytes but also to all-solid-state batteries, semi-solid-state batteries, etc.

[0101] The secondary battery of the present invention can be suitably used as a battery for vehicles such as automobiles, and portable electronic devices such as personal computers and smartphones.

Examples

[0102] The present invention will be described more specifically with reference to the following examples. The present invention is not limited to the following examples as long as the gist thereof is not exceeded. Unless otherwise specified, "parts" represents "parts by mass" and "%" represents "% by mass".

[0103] <Production of Dispersant>

[0104] (Synthesis Example 1 Production of Polyacrylonitrile PAN) 100 parts of acetonitrile was charged into a reaction vessel equipped with a gas inlet tube, a thermometer, a condenser, and a stirrer, and replaced with nitrogen gas. The inside of the reaction vessel was heated to 70 °C, and a mixture of 90.0 parts of acrylonitrile, 10.0 parts of hydroxyethyl acrylate, and 5.0 parts of 2,2’-azobis(2,4-dimethylvaleronitrile) (manufactured by NOF Corporation; V-65) was added dropwise over 3 hours to conduct a polymerization reaction. After completion of the dropwise addition, the reaction was further carried out at 70 °C for 1 hour, then 0.5 part of Perbutyl O was added, and the reaction was continued at 70 °C for 1 hour. Thereafter, it was confirmed that the conversion rate exceeded 98% by non-volatile content measurement, and the dispersion medium was completely removed by concentration under reduced pressure to obtain a dispersant (polyacrylonitrile). The weight average molecular weight (Mw) of the polyacrylonitrile was 15,000.

[0105] In the examples and comparative examples, the following fibrous carbons were used. (First fibrous carbon) · 6A: JENOTUBE6A (manufactured by JEIO, multi-layer CNT, fiber diameter 5 nm) · 8S: JENOTUBE8S (manufactured by JEIO, multi-layer CNT, fiber diameter 7 nm) (Second fibrous carbon) · 10B: JENOTUBE10B (manufactured by JEIO Co., multi-layer CNT, fiber diameter 10 nm) · 100T: K-Nanos-100T (manufactured by KUMHO PETROCHEMICAL, multi-layer CNT, fiber diameter 14 nm) · Multi-layer CNT: (manufactured by FUJIFILM Wako Pure Chemical Corporation, multi-layer CNT, fiber diameter 30 nm)

[0106] In the examples and comparative examples, the following active materials were used. · NMC532: S740 (LiNi 0.5 Mn 0.3 Co 0.2 O 2 , manufactured by RONBAY) · SiOx: (SILICONMONOOXIDE, SiO 1.3C 5μm, manufactured by Osaka Titanium Technology Co., Ltd.,)

[0107] <Preparation of fibrous carbon dispersion> (Production Example 1) According to the materials and compositions shown in Table 1, a fibrous carbon dispersion was prepared as follows. First, NMP was placed in a stainless steel container and heated to 50 °C. After adding a dispersant while stirring with a disper, it was stirred for 1 hour to dissolve the dispersant. Subsequently, fibrous carbon was added while stirring with a disper, a square-hole high-shear screen was attached to a high-shear mixer (L5M-A, manufactured by SILVERSON), and batch dispersion was carried out at a speed of 8,000 rpm until the whole became uniform and the dispersion particle size became 250 μm or less as measured by a grind gauge with a maximum groove depth of 300 μm. At this time, the dispersion particle size confirmed by the grind gauge was 220 μm. Subsequently, the dispersion liquid was supplied from the stainless steel container to a high-pressure homogenizer (Starburst Labo HJP-17007, manufactured by Sugino Machine) through a pipe, and a cyclic dispersion treatment was performed. The dispersion treatment was carried out using a single-nozzle chamber at a nozzle diameter of 0.25 mm and a pressure of 100 MPa. After dispersing until the viscosity at 60 rpm measured with a B-type viscometer (manufactured by TOKISANGYO, VISCOMETER, MODEL: BL) of the dispersion liquid became 3,000 mPa·s or less, a 25-pass dispersion treatment was performed with a high-pressure homogenizer to obtain fibrous carbon dispersion 1.

[0108] (Production Examples 2 to 5) Fibrous carbon dispersions 2 to 5 were obtained in the same manner as in Production Example 1, except that the materials and compositions shown in Table 1 were changed.

[0109] (Production Examples 6, 7) 93.96 parts by mass of ion-exchanged water was added to a stainless steel container, and while stirring with a disper, 1 part by mass of PAN prepared in Synthesis Example 1 and 0.04 part of NaOH were added and dissolved. Then, 2.0 parts by mass of 100T (fibrous carbon) was taken, added while stirring with a disper, a square-hole high-shear screen was attached to a high-shear mixer (L5M-A, manufactured by SILVERSON), and the whole became uniform at a speed of 8,600 rpm. Batch dispersion was carried out until the dispersed particle size became 250 μm or less as confirmed by a grind gauge. At this time, the dispersed particle size confirmed by the grind gauge was 180 μm. Subsequently, the dispersion liquid was supplied from the stainless steel container to a high-pressure homogenizer (Starburst Turbo HJP-17007, manufactured by Sugino Machine) through a pipe, and a circulation-type dispersion treatment was carried out. The dispersion treatment was carried out using a single-nozzle chamber at a nozzle diameter of 0.25 mm and a pressure of 100 MPa. After dispersion until the viscosity at 60 rpm measured with a B-type viscometer (manufactured by TOKISANGYO, VISCOMETER, MODEL: BL) of the dispersion liquid became 3,000 mPa·s or less, while stirring with a disper, 0.5 part by mass of 100T was further added to the stainless steel container, and a circulation-type dispersion treatment was carried out again with a high-pressure homogenizer. After circulation-type dispersion with a high-pressure homogenizer until the viscosity became 3,000 mPa·s or less, the operation of adding 100T to the stainless steel container while stirring with a disper was repeated a total of 6 times (the total added amount of 100T was 5.0 parts by mass). Subsequently, a 10-pass type dispersion treatment was carried out with a high-pressure homogenizer to obtain a fibrous carbon dispersion 6 containing 5.0 parts by mass of fibrous carbon.

[0110] A fibrous carbon dispersion 7 was obtained in the same manner as in Production Example 6, except that the materials and compositions shown in Table 1 were changed.

[0111]

Table 1

[0112] <Electrode composite for secondary battery> (Example 1-1) According to the composition shown in Table 2, using a planetary mixer, NMC, fibrous carbon dispersion 1, and fibrous carbon dispersion 3 were stirred at 50 rpm for 45 minutes to prepare a mixture. Then, the mixture was dried in a hot air oven at 120°C for 2 hours to obtain Composite 1.

[0113] (Examples 1-2 to 1-7) According to the composition shown in Table 2, Composites 2 to 7 were obtained in the same manner as in Example 1-1.

[0114] (Example 1-8) According to the composition shown in Table 2, fibrous carbon dispersion 2, fibrous carbon dispersion 4, and solvent (NMP) were weighed into a stainless steel container, and while stirring at 1000 rpm using a Primix Corporation homomixer MARKII 2.5 type, NMC was added, and then stirred for 60 minutes to obtain a slurry. Using this slurry, a composite for a secondary battery electrode was prepared using a spray dryer Mini Spray Dryer B-290 manufactured by Buchi Japan. Under closed system conditions (nitrogen atmosphere) using an inert loop B-295, the slurry was spray dried at 220°C to obtain Composite 8.

[0115] (Example 1-9) According to the composition shown in Table 2, using a planetary mixer, SiOx, fibrous carbon dispersion 6, and fibrous carbon dispersion 7 were stirred at 50 rpm for 30 minutes to prepare a mixture. Then, the mixture was dried in an oven at 110°C for 2 hours to obtain Composite 9.

[0116] (Example 1-10) According to the composition shown in Table 2, the fibrous carbon dispersion 6, the fibrous carbon dispersion 7, and water were weighed into a stainless-steel container, and SiOx was added while stirring at 1000 rpm using a Primix homogenizer MARKII 2.5 type. Then, the mixture was stirred for 45 minutes to obtain a slurry. Using this slurry, a composite 10 was obtained by spray-drying the slurry at 120 °C using a Nihon Buchi spray dryer Mini Spray Dryer B-290.

[0117] (Comparative Examples 1-1, 1-2) According to the composition shown in Table 2, composites 11 and 12 were obtained in the same manner as in Example 1-8.

[0118] (Comparative Examples 1-3, 1-4) According to the composition shown in Table 2, composites 13 and 14 were obtained in the same manner as in Comparative Example 1-10.

[0119] (Evaluation of the state of fibrous carbon in the composite) The dispersion state of fibrous carbon in the composite was determined using a scanning electron microscope JSM-7800F manufactured by JEOL Ltd. · Criteria for determining aggregates of fibrous carbon [Average particle size of the aggregate] ◎: Aggregates of fibrous carbon are included, and the average particle size is 0.2 μm or more and 3 μm or less. ○: Aggregates of fibrous carbon are included, and the average particle size is less than 0.2 μm or more than 3 μm. ×: Aggregates of fibrous carbon are not included. [Average number of aggregates] ◎: Aggregates of fibrous carbon are included, and the average number of aggregates of fibrous carbon measured from 10 observation images at a magnification of 5,000 times is 2 or more. ○: Aggregates of fibrous carbon are included, and the average number of aggregates of fibrous carbon measured from 10 observation images at a magnification of 5,000 times is 0.5 or more and less than 2. ×: Aggregates of fibrous carbon are not included.

[0120] · Criteria for determining the state of fragmentation of fibrous carbon on the surface of the active material ◎: The area occupied by fibrous carbon adsorbed on the active material in a state where the fiber is crushed is 50% or more of the entire surface of the active material. ○: The area occupied by fibrous carbon adsorbed on the active material in a state where the fiber is crushed is more than 10% and less than 50% of the entire surface of the active material. ×: Not adsorbed on the active material in a state where the fiber is crushed.

[0121]

Table 2

[0122] <Preparation of positive electrode composite slurry and electrode film for positive electrode> In a plastic container with a volume of 150 cm 3 14.2 parts of an 8% binder solution (polyvinylidene fluoride resin: W#7200) previously dissolved in NMP and 11.1 of a solvent (NMP) were weighed, and then 74.7 parts of electrode composite 1 was added. Using a rotary-revolution mixer (Sinky's Awatori Renkijiro, ARE-310), it was stirred at 2,000 rpm for 150 seconds to obtain composite slurry 1. The non-volatile content of composite slurry 1 was 76 mass%.

[0123] Composites 2 to 8, 11, and 12 were processed in the same manner as composite slurry 1 to obtain composite slurries 2 to 8, 11, and 12.

[0124] Composite slurries 1 to 8, 11, and 12 were coated on an aluminum foil with a thickness of 20 μm using an applicator, and then dried in an electric oven at 120 °C ± 5 °C for 25 minutes to produce electrode films. Thereafter, the electrode films were subjected to rolling treatment using a roll press (manufactured by Sanku Metal, 3t hydraulic roll press) to obtain electrode films 1 to 8, 11, and 12. Note that the basis weight per unit of the composite layer is 20 mg / cm 2 and the density of the composite layer after rolling treatment was 3.2 g / cm 3 was.

[0125] <Preparation of negative electrode composite slurry and electrode film for negative electrode> An aqueous solution in which 2% by mass of sodium carboxymethyl cellulose (manufactured by Daicel Finechem Ltd., #1190) was dissolved in a plastic container was weighed at 12.50 parts by mass, and 12.34 parts by mass of ion-exchanged water was weighed. Then, using a rotation-revolution mixer (Sumiki Awatori Rentaro, ARE-310 manufactured by Sumiki Co., Ltd.), it was stirred at 2000 rpm for 1 minute. Then, 2.44 parts by mass of the composite for electrodes 9 was added, and using the same rotation-revolution mixer, it was stirred at 2000 rpm for 5 minutes. Further, 21.93 parts by mass of natural graphite (manufactured by Nippon Graphite Industry Co., Ltd., CGB-20) was added, and using the same rotation-revolution mixer, it was stirred at 2000 rpm for 5 minutes. Then, 0.78 parts by mass of an aqueous solution in which 48% by mass of styrene-butadiene emulsion was dispersed was added, and using a rotation-revolution mixer (Sumiki Awatori Rentaro, ARE-310 manufactured by Sumiki Co., Ltd.), it was stirred at 2000 rpm for 30 seconds to obtain the composite slurry 9. The non-volatile content of the composite slurry was set to 48% by mass.

[0126] Using the composites 10, 13, and 14, in the same manner as the composite slurry, composite slurries 10, 13, and 14 were obtained.

[0127] The composite slurries 9, 10, 13, and 14 were applied onto a copper foil with a thickness of 18 μm using an applicator, and then dried in an electric oven at 120 °C ± 5 °C for 25 minutes to produce electrode films. Then, the electrode films were subjected to rolling treatment using a roll press (manufactured by Sanku Metal, 3t hydraulic roll press) to obtain electrode films 9, 10, 13, and 14. The basis weight per unit of the composite layer was 8 mg / cm 2 and the density of the composite layer after rolling treatment was 1.6 g / cm 3 was obtained.

[0128] (Resistance of the positive electrode film) The volume resistivity of the electrode films 1 to 8, 11, and 12 was measured by the four-probe method using Loresta GP (manufactured by Nitto Seiko Analytic Co., Ltd.) in accordance with JIS-K7194. For the resistance measurement, a measurement electrode in which the coating base material at the time of manufacturing each electrode was changed from an aluminum foil to a PET base material was used. The relative value (%) based on the volume resistivity of the electrode films of Comparative Examples 1-2 was determined and evaluated according to the following criteria. Electrode Resistance Characteristic Judgment Criteria ◎: Less than 60% (excellent) ○: 60% or more and less than 100% (good) ×: 100% or more (defective)

[0129] (Resistance of the negative electrode electrode film) The volume resistivity of the electrode films 9, 10, 13, and 14 was measured by the four-probe method using Loresta GP (manufactured by Nitto Seiko Analytic Co., Ltd.) in accordance with JIS-K7194. In addition, for resistance measurement, a measurement electrode in which the coating base material at the time of manufacturing each electrode was changed from copper foil to PET base material was used. Relative values (%) based on the volume resistivity of the electrode films of Comparative Examples 1-4 were obtained and evaluated according to the following criteria. Electrode Resistance Characteristic Determination Criteria ◎: Less than 80% (excellent) ○: 80% or more and less than 100% (good) ×: 100% or more (defective)

[0130] <Assembly of Lithium Ion Secondary Battery Positive Electrode Evaluation Cell> The previously prepared electrode films 1 to 8, 11, and 12 were punched out to a diameter of φ16 mm to serve as the working electrode, and a metal lithium foil (thickness 0.15 mm) was used as the counter electrode. A separator (porous polypropylene film) was inserted and laminated between the working electrode and the counter electrode, and an electrolytic solution (a non-aqueous electrolytic solution in which LiPF 6 was dissolved at a concentration of 1 M in a mixed solvent of ethylene carbonate and diethyl carbonate mixed at a volume ratio of 1:1) was filled to assemble a two-electrode sealed metal cell (HS flat cell manufactured by Hozen Co., Ltd.). The assembly of the cell was carried out in a glove box purged with argon gas.

[0131] <Rate Characteristic Evaluation of Lithium Ion Secondary Battery Positive Electrode> The fabricated lithium-ion secondary battery positive electrode evaluation cell was placed in a thermostatic chamber at 25°C, and charge-discharge measurements were performed using a charge-discharge device (manufactured by Hokuto Denko Corporation, SM-8). After performing constant current and constant voltage charging (cutoff current: 0.02C current) at a charging rate of 0.2C with a charging end voltage of 4.2V, constant current discharging was performed at a discharging rate of 0.2C with a discharging end voltage of 2.5V. After repeating this operation three times, constant current and constant voltage charging (cutoff current: 0.02C current) was performed at a charging rate of 0.2C with a charging end voltage of 4.2V, and constant current discharging was performed at a discharging rate of 3C with a discharging end voltage of 2.5V. 1C was defined as the current value for charging or discharging the theoretical capacity of the positive electrode in one hour. The rate performance can be expressed by the ratio of the 0.2C discharge capacity to the 3C discharge capacity, as shown in Equation 1 below. (Equation 1) Rate performance = 3C discharge capacity / 0.2C discharge capacity × 100 (%) Relative values (%) based on the rate performance of Comparative Examples 1-2 were determined and evaluated according to the following criteria. Rate performance judgment criteria ◎: 130% or more (excellent) ○: Exceeding 100% and less than 130% (good) ×: 100% or less (poor)

[0132] <Evaluation of cycle performance of lithium-ion secondary battery positive electrode> The fabricated lithium-ion secondary battery positive electrode evaluation cell was placed in a thermostatic chamber at 25°C, and charge-discharge measurements were performed using a charge-discharge device (manufactured by Hokuto Denko Corporation, SM-8). After performing constant current and constant voltage charging (cutoff current: 0.02C current) at a charging rate of 0.2C with a charging end voltage of 4.2V, constant current discharging was performed at a discharging rate of 0.2C with a discharging end voltage of 2.5V. This operation was repeated 100 times. 1C was defined as the current value for charging or discharging the theoretical capacity of the positive electrode in one hour. The cycle performance can be expressed by the ratio of the 0.2C discharge capacity at the third cycle to the 0.2C discharge capacity at the 100th cycle, as shown in Equation 2 below. (Equation 2) Cycle performance = 0.2C discharge capacity at the third cycle / 0.2C discharge capacity at the 100th cycle × 100 (%) Relative values (%) based on the cycle performance of Comparative Examples 1-2 were determined and evaluated according to the following criteria. Cycle performance judgment criteria ◎: 150% or more (excellent) 〇: More than 100 and less than 150% (good) ×: 100% or less (poor)

[0133] <Assembly of Lithium Ion Secondary Battery Negative Electrode Evaluation Cell> The previously fabricated electrode films 9, 10, 13, and 14 were punched into φ16 mm to serve as the working electrode. Using a lithium metal foil (thickness 0.15 mm) as the counter electrode, a separator (porous polypropylene film) was inserted and laminated between the working electrode and the counter electrode. An electrolytic solution (a mixed solvent prepared by mixing ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate in a ratio of 3:5:2 (volume ratio)) was prepared. Further, as additives, VC (vinylene carbonate) and FEC (fluoroethylene carbonate) were added at 1 part by mass each per 100 parts by mass of the mixed solvent. After that, LiPF 6 was dissolved at a concentration of 1 M to fill the non-aqueous electrolytic solution, and a two-electrode sealed metal cell (HS flat cell manufactured by Baquan Co., Ltd.) was assembled. The assembly of the cell was carried out in a glove box replaced with argon gas.

[0134] <Rate Performance Evaluation of Lithium Ion Secondary Battery Negative Electrode> The fabricated lithium ion secondary battery negative electrode evaluation cell was placed in a constant temperature chamber at 25°C, and charge-discharge measurements were performed using a charge-discharge device (manufactured by Hokuto Denko Corporation, SM-8). After performing constant current constant voltage charging (cutoff current: 0.02C current) at a charging rate of 0.2C with a charging termination voltage of 0.05V, constant current discharge was performed at a discharge rate of 0.2C with a discharge termination voltage of 1.5V. After repeating this operation 3 times, constant current constant voltage charging (cutoff current: 0.02C current) was performed at a charging rate of 0.2C with a charging termination voltage of 0.05V, and constant current discharge was performed at a discharge rate of 3C with a discharge termination voltage of 1.5V. 1C was defined as the current value for charging or discharging the theoretical capacity of the negative electrode in 1 hour. The rate performance can be expressed by the ratio of the 0.2C discharge capacity to the 3C discharge capacity, as shown in the following formula 2. (Formula 2) Rate performance = 3C discharge capacity / 0.2C discharge capacity of the third cycle × 100 (%) Relative values (%) based on the rate performance of Comparative Examples 1-4 were determined and evaluated according to the following criteria. ◎: Above 150% (excellent) ○: Exceeding 100% and less than 150% (good) ×: 100% or less (defective)

[0135] <Evaluation of Cycle Characteristics of Lithium-Ion Secondary Battery Anode> The fabricated cell for evaluating the lithium-ion secondary battery anode was placed in a constant temperature chamber at 25°C, and charge and discharge measurements were performed using a charge and discharge device (manufactured by Hokuto Denko Corporation, SM-8). After performing constant current and constant voltage charging (cut-off current: 0.02C current) at a charging rate of 0.2C with a charging cut-off voltage of 0.05V, constant current discharge was performed at a discharge rate of 0.2C with a discharge cut-off voltage of 1.5V. This operation was repeated 10 times. 1C was defined as the current value for charging or discharging the theoretical capacity of the anode in 1 hour. The cycle characteristics can be represented by the ratio of the 0.2C discharge capacity at the third cycle to the 0.2C discharge capacity at the tenth cycle, as shown in the following formula 1. (Formula 1) Cycle characteristics = 0.2C discharge capacity at the third cycle / 0.2C discharge capacity at the tenth cycle × 100 (%) Relative values (%) based on the cycle characteristics of Comparative Examples 1-4 were determined and evaluated according to the following criteria. ◎: Above 150% (excellent) ○: Exceeding 100% and less than 150% (good) ×: 100% or less (defective)

Table 3

[0136] As shown in Table 3, the examples showed excellent battery characteristics compared to the comparative examples. It is presumed that this is because the mixed fibrous carbons with different thicknesses in the composite improved the ion and electron paths necessary for the charge and discharge reactions with respect to the active material in the composite, leading to an improvement in the rate characteristics and cycle characteristics. From the above, it was shown that excellent battery characteristics can be exhibited by the composite for electrodes within the scope of the present invention.

Claims

1. A composite for a secondary battery electrode comprising an electrode active material and fibrous carbon, wherein the fibrous carbon contains at least two types of fibrous carbon having different average fiber diameters, and the at least two types of fibrous carbon include first fibrous carbon having an average fiber diameter of 1 nm or more and less than 9 nm and second fibrous carbon having an average fiber diameter of 9 nm or more and 150 nm or less. A composite for a secondary battery electrode characterized by that.

2. The composite for a secondary battery electrode according to claim 1, wherein a part of the fibrous carbon has a crushed form and adheres to the surface of the electrode active material, and a part of the fibrous carbon has an aggregate form.

3. The composite for a secondary battery electrode according to claim 2, wherein the average particle diameter of the aggregate is 0.2 μm or more and 50 μm or less.

4. The composite for a secondary battery electrode according to claim 1 or 2, further comprising a binder resin.

5. An electrode for a secondary battery comprising the composite for a secondary battery electrode according to claim 1 or 2.

6. A secondary battery comprising the electrode for a secondary battery according to claim 5.

7. A vehicle or device equipped with the secondary battery according to claim 6.

8. A method for producing the composite for a secondary battery electrode according to claim 1, comprising: a step of compounding a fibrous carbon dispersion and an electrode active material using a dry mixing device, and a step of removing a solvent and drying. A method for producing a composite for a secondary battery electrode characterized by that.

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