JP5552865B2 - Composite polymer impregnated active material for non-aqueous secondary battery electrode - Google Patents

Description

  The present invention relates to an active material used for an electrode for a non-aqueous secondary battery, and more particularly, a composite material characterized in that a specific composite polymer is attached to at least a part of the surface of the active material. The present invention relates to a polymer impregnated active material. The present invention also relates to a non-aqueous secondary battery electrode provided with an active material layer containing a composite polymer-impregnated active material, and a lithium ion secondary battery provided with this electrode as a negative electrode.

  With recent reduction in weight and size of electrical products, development of lithium ion secondary batteries, which are non-aqueous secondary batteries having a high energy density, is being promoted. As the application field of lithium ion secondary batteries expands, further improvements in battery characteristics are required, and among them, there is a strong demand for improving input / output characteristics and cycle characteristics at low temperatures in a balanced manner.

  Until now, efforts to improve the characteristics of lithium ion secondary batteries have been centered on the development of non-aqueous electrolytes used and the development of active materials. In recent years, research on binders used in the production of electrodes has been conducted. It also extends to the surface treatment of active materials.

  For example, as a binder development, a binder composed of an aqueous dispersion of a composite polymer of a specific fluoropolymer and a functional group-containing polymer has been proposed (Patent Document 1).

  As a surface treatment of the active material, a treatment of covering the active material particles with a porous lithium ion conductive polymer film has been proposed (Patent Document 2). Moreover, the process which adsorb | sucks or coat | covers surface active effect materials, such as a starch derivative and viscous polysaccharide, to the graphite powder for negative electrodes is proposed (patent document 3).

JP-A-10-302799 JP-A-9-219197 WO1999 / 001904

  However, in the method of Patent Document 1, as a result of using a composite polymer containing a fluoropolymer having poor binding properties as a binder, there has been a problem that cycle characteristics are not sufficient. In addition, since the method of Patent Document 2 requires an organic solvent for dissolving the polymer during the coating process, it has a large environmental load and further requires equipment for recovering the organic solvent during drying. Is very bad. In the method of Patent Document 3, there is a problem that the ionic conductivity of starch derivatives and viscous polysaccharides is low and the input / output characteristics are inferior. Thus, the present condition is that the further improvement is requested | required with respect to the input / output characteristic and cycling characteristics in the low temperature of a lithium ion secondary battery.

  As a result of intensive studies to solve the above-mentioned problems, the present inventors have found that a composite polymer in which a composite polymer of a fluorine-containing polymer and an oxygen-containing functional group-containing polymer is attached to at least a part of the surface of the active material. When the molecularly adsorbed active material is used for an electrode of a non-aqueous secondary battery, particularly a lithium ion secondary battery, it has been found that input / output characteristics and cycle characteristics at low temperatures can be improved in a balanced manner, and the present invention has been completed. I let you.

The present invention is characterized in that a composite polymer (A) of a fluorine-based polymer (a1) and a polymer (a2) having an oxygen-containing functional group is attached to at least a part of the surface of the active material. And a composite polymer impregnated active material for a non-aqueous secondary battery electrode.
The present invention relates to any one of the above composite polymer-impregnated active materials obtained by mixing the composite polymer (A) and the active material in an aqueous solvent and drying the mixture.
The present invention relates to a composite polymer-impregnated active material in which the fluorine-based polymer (a1) is a homopolymer or copolymer of a fluorine-containing monomer.
The present invention further relates to any one of the above composite polymer-impregnated active materials having the conductive additive (B) on at least a part of the surface of the active material.

  The present invention also relates to an electrode for a non-aqueous secondary battery, in particular, an electrode for a lithium ion secondary battery, comprising an active material layer containing the composite polymer-impregnated active material on a current collector. The present invention relates to a negative electrode for a non-aqueous secondary battery, particularly a negative electrode for a lithium ion secondary battery, wherein the material layer is a negative electrode active material layer and further contains a cellulose polymer and a rubber polymer. The present invention relates to a non-aqueous secondary battery including the above-described negative electrode, positive electrode, and electrolyte, and more particularly to a lithium ion secondary battery.

  The composite polymer-impregnated active material of the present invention can improve the input / output characteristics and cycle characteristics at low temperatures in a well-balanced manner when used for an electrode of a non-aqueous secondary battery, particularly a lithium ion secondary battery. Since the electrode using the composite polymer impregnated active material of the present invention can be prepared using an aqueous solvent, the load on the environment is small and the cost performance is excellent.

  The present invention is characterized in that a composite polymer (A) of a fluorine-based polymer (a1) and a polymer (a2) having an oxygen-containing functional group is attached to at least a part of the surface of the active material. The present invention also relates to a composite polymer-impregnated active material for non-aqueous secondary battery electrodes.

  Here, the term “adhesion” refers to a state in which the composite polymer is adsorbed on at least a part of the surface of the active material without using an adhesive or the like, and is simply mixed with the active material. It is different from the mixed state. Adsorption may be chemical adsorption by hydrogen bonding or physical adsorption by van der Waals force, but is not limited thereto. In addition, the surface of the active material refers to a site where the aqueous dispersion of the composite polymer can approach and can be adsorbed. That is, it also includes an inner wall surface of an aperture that is inside the active material and into which an aqueous dispersion of the composite polymer can enter.

  The composite polymer (A) is a polymer obtained by combining a fluorine-based polymer (a1) and a polymer (a2) having an oxygen-containing functional group, and includes the fluorine-based polymer (a1). It includes a form in which the polymer (a2) having an oxygen functional group forms an interpenetrating structure (Inter Penetrating Network structure (IPN structure)). However, the form of the composite is not limited to the IPN structure, and the fluorine-based polymer (a1) and the polymer (a2) having an oxygen-containing functional group form a core-shell structure, a graft copolymer, a block copolymer, or the like. Including structures.

  When the composite polymer-attached active material in which the composite polymer (A) is attached to at least a part of the surface of the active material is used as an active material for an electrode of a non-aqueous secondary battery, particularly a lithium ion secondary battery, It is considered that the fluorine polymer (a1) in the composite polymer (A) swells in the electrolyte, thereby functioning as a path for ions and improving the input / output characteristics of the battery. In addition, since the polymer (a2) having an oxygen-containing functional group is present in the composite polymer (A), the composite polymer (A) is firmly attached to the active material, and the fluorine-based polymer It is considered that the path of ions composed of the molecule (a1) functions reliably. In addition, the presence of the oxygen-containing functional group enhances the dispersibility of the composite polymer additive active material in water and facilitates the production of the electrode using an aqueous solvent.

  Examples of the fluorine-based polymer (a1) include polymers containing fluorine-containing monomer units. The fluorine-based polymer (a1) may be a polymer composed of one or more fluorine-containing monomers, or a polymer composed of one or more fluorine-containing monomers and one or more other monomers. In the latter case, the fluorine-containing monomer unit is preferably 60% by mass or more, more preferably 70% by mass or more from the viewpoint of ion conductivity.

  Examples of the fluorine-containing monomer include vinylidene fluoride (VDF), chlorotrifluoroethylene (CTFE), tetrafluoroethylene (TFE), hexafluoropropylene (HFP), trifluoroethylene (TrFE), and vinyl fluoride (VF). And the like. Among these, vinylidene fluoride, tetrafluoroethylene, hexafluoropropylene, trifluoroethylene, and vinyl fluoride are preferable, and vinylidene fluoride, hexafluoropropylene, and trifluoroethylene are particularly preferable from the viewpoint of ion conductivity.

  Other monomers include (meth) acrylic acid; methyl (meth) acrylate, ethyl (meth) acrylate, n-propyl (meth) acrylate, i-propyl (meth) acrylate, n-butyl (meth) acrylate, i- Butyl (meth) acrylate, n-pentyl (meth) alkyl (meth) acrylate, i-pentyl (meth) alkyl (meth) acrylate, n-hexyl (meth) acrylate, 2-ethylhexyl (meth) acrylate, n-octyl ( (Meth) acrylate, n-nonyl (meth) acrylate, n-decyl (meth) acrylate, ethylene glycol di (meth) acrylate, propyleneglycone di (meth) acrylate, trimethylolpropane tri (meth) acrylate, pentaerythritol tetra Alkyl) (meth) acrylates such as meth) acrylate and dipentaerythritol hexa (meth) acrylate; aromatic vinyl compounds such as styrene, α-methylstyrene and divinylbenzene; conjugated dienes such as butadiene, isoprene and chloroprene; ethylene and propylene Is mentioned.

  As the fluorine-based polymer (a1), vinylidene fluoride homopolymer (that is, polyvinylidene fluoride (PVDF)) or copolymer (for example, vinylidene fluoride) is used because of its low ion conductivity and low reactivity with lithium. Ride and hexafluoropropylene copolymers) are preferred. The fluorine-based polymer (a1) may be used alone or in combination of two or more.

  As the oxygen-containing functional group in the polymer (a2) having an oxygen-containing functional group, a hydroxy group, a peroxy group, a carboxyl group, a carbonyl group, an acetyl group, an aldehyde group, a sulfo group, a nitro group, a nitroso group, an ether bond, Examples thereof include an ester bond and an amide bond. Among these, a hydroxy group, a carboxyl group, a carbonyl group, and an ester bond are particularly preferable in terms of water dispersibility and electrochemical stability.

  Examples of the polymer (a2) having an oxygen-containing functional group include a polymer including a monomer unit having an oxygen-containing functional group. Even if the polymer (a2) having an oxygen-containing functional group is a polymer composed of one or more monomers having an oxygen-containing functional group, one or more monomers having an oxygen-containing functional group and one kind of other monomers The polymer which consists of the above may be sufficient. In the latter case, the monomer having an oxygen-containing functional group is preferably 60% by mass or more, and more preferably 70% by mass or more.

  Examples of the monomer having an oxygen-containing functional group include an ethylenically unsaturated monomer having an oxygen-containing functional group. For example, (meth) acrylic acid; methyl (meth) acrylate, ethyl (meth) acrylate, n-propyl (meth) ) Acrylate, i-propyl (meth) acrylate, n-butyl (meth) acrylate, i-butyl (meth) acrylate, n-pentyl (meth) alkyl (meth) acrylate, i-pentyl (meth) alkyl (meth) acrylate , N-hexyl (meth) acrylate, 2-ethylhexyl (meth) acrylate, n-octyl (meth) acrylate, n-nonyl (meth) acrylate, n-decyl (meth) acrylate, ethylene glycol di (meth) acrylate, propylene Glycorn di (meth) acrylate Trimethylolpropane tri (meth) acrylate, pentaerythritol tetra (meth) acrylate, dipentaerythritol hexa (meth) acrylate.

  Other monomers include conjugated dienes such as butadiene, isoprene and chloroprene; ethylene, propylene and the like.

  Specific examples of the polymer (a2) having an oxygen-containing functional group include cellulose polymers, acrylic polymers, ether polymers, alcohol polymers, ester polymers, and the like.

  Examples of the cellulose polymer include methylcellulose, ethylcellulose, acetylcellulose, hydroxyethylcellulose, carboxymethylcellulose, starch, carrageenan, pullulan, guar gum, xanthan gum (xanthan gum), hydroxypropylcellulose, hydroxypropylmethylcellulose and the like.

  Examples of the acrylic polymer include polyacrylic acid, polymethacrylic acid, polymethyl acrylate, polyalkyl acrylate including polymethyl methacrylate, and polyalkyl methacrylate.

  Examples of ether polymers include polyethylene oxide, polypropylene oxide, polyphenylene oxide, and polyetherimide.

  Examples of the alcohol polymer include polyvinyl alcohol, polybutyl alcohol, polyalkylene glycol and the like.

  Examples of the ester polymer include polyethylene terephthalate, polybutylene terephthalate, polyethylene-2,6-naphthalate, and polyarylate.

  Among these, from the viewpoint of affinity with the fluorine-based polymer (a1), the polymer (a2) having an oxygen-containing functional group is preferably an acrylic polymer, a cellulose polymer, or an ether polymer, and more preferably It is an acrylic polymer, and a polymer of a lower alkyl ester of (meth) acrylic acid such as polymethyl (meth) acrylate and polybutyl (meth) acrylate is particularly preferable. The polymer (a2) having an oxygen-containing functional group may be used alone or in combination of two or more.

  The mass ratio of the fluorine-based polymer (a1) and the polymer (a2) having an oxygen-containing functional group is such that the content of the fluorine-based polymer (a1) is 1 to 2 when the total of both is 100% by mass. It can be 99 mass%. If it is this range, the uniform dispersibility in an aqueous solvent and the favorable binding property to an active material are compatible, and it is easy to ensure favorable ionic conductivity. The content of the fluorine-based polymer (a1) is preferably 10% by mass or more, more preferably 20% by mass or more, and preferably 90% by mass or less, more preferably 80% by mass or less.

  The composite polymer (A) is, for example, a monomer constituting the polymer (a2) having an oxygen-containing functional group in an aqueous solvent in the presence of the fluorine-based polymer (a1) (for example, polyvinylidene fluoride). It can be obtained by polymerizing (for example, methyl (meth) acrylic acid). At that time, an emulsifier, a polymerization initiator, a pH adjuster and the like may be present. Further, the monomer constituting the fluorine-based polymer (a1) is emulsion-polymerized in the presence of water and an emulsifier to obtain primary particles, and then the polymer (a2 having an oxygen-containing functional group in the presence of the primary particles). The monomer (for example, methyl (meth) acrylic acid) which comprises) is polymerized, and composite polymer (A) can be obtained. In these methods, the composite polymer (A) is usually obtained as an aqueous dispersion. The average particle size of the composite polymer (A) in the aqueous dispersion can be 10 to 1000 nm, and preferably 50 to 500 nm. The composite polymer (A) preferably has a glass transition point (Tg) of −120 to 80 ° C., more preferably −100 to 60 ° C. from the viewpoint of ion conductivity. The average particle diameter is a median diameter value measured by a dynamic light scattering method.

  The active material is not particularly limited, but a negative electrode active material is preferable, and a carbonaceous material is particularly preferable.

  The carbon material as the negative electrode active material is not particularly limited, and examples thereof include artificial graphite, graphite such as natural graphite (graphite), and thermal decomposition products of organic substances under various thermal decomposition conditions. Organic pyrolysis products include coal-based coke, petroleum-based coke, coal-based pitch carbide, petroleum-based pitch carbide, or carbides obtained by oxidizing these pitches, needle coke, pitch coke, phenolic resin, crystalline cellulose And carbon materials obtained by partially graphitizing these, furnace black, acetylene black, pitch-based carbon fibers, and the like. Of these, graphite is preferable, and artificial graphite, purified natural graphite, or graphite materials containing pitch in these graphites, which are manufactured by subjecting easy-graphite pitches obtained from various raw materials to high-temperature heat treatment, are more preferable. The surface treatment may be performed. These carbon materials may be used alone or in combination of two or more.

  In the case of using a graphite material, the d value (interlayer distance) of the lattice plane (002 plane) obtained by X-ray diffraction by the Gakushin method is preferably 0.335 nm or more, and preferably 0.34 nm or less. Preferably it is 0.337 nm or less.

  The ash content of the graphite material is preferably 1% by mass or less, more preferably 0.5% by mass or less, and particularly preferably 0.1% by mass or less with respect to the mass of the graphite material.

  The crystallite size (Lc) of the graphite material determined by X-ray diffraction by the Gakushin method is preferably 30 nm or more, more preferably 50 nm or more, and particularly preferably 100 nm or more.

  The median diameter of the graphite material obtained by the laser diffraction / scattering method is preferably 1 μm or more, more preferably 3 μm or more, further preferably 5 μm or more, particularly preferably 7 μm or more, and preferably 100 μm or less, more Preferably it is 50 micrometers or less, More preferably, it is 40 micrometers or less, Most preferably, it is 30 micrometers or less.

The BET specific surface area of the graphite material is preferably 0.5 m ² / g or more, more preferably 0.7 m ² / g or more, still more preferably 1.0 m ² / g or more, and particularly preferably 1.5 m ² / g. or more, also preferably 25.0 m ² / g or less, more preferably 20.0 m ² / g or less, more preferably 15.0 m ² / g or less, particularly preferably not more than 10.0 m ² / g .

Graphite material, when subjected to Raman spectrum analysis using argon laser beam, the intensity _{I A} of the peak _{P A} is detected in the range of 1580～1620Cm ^-1, is detected in the range of 1350 ^-1 intensity ratio _I a / _{I B} of the intensity _{I B} of a peak _{P B} is what is preferably 0 to 0.5. Further, the half width of the peak _{P A} is preferably 26cm ^-1 or less, 25 cm ^-1 or less is more preferable.

  In addition to the carbon material described above, other materials capable of occluding and releasing lithium known as a negative electrode active material can also be used. Examples include metal oxides such as tin oxide and silicon oxide, sulfides and nitrides, lithium alloys such as lithium alone and lithium aluminum alloys, and the like. These materials other than carbon materials may be used alone or in combination of two or more. Moreover, you may use in combination with the above-mentioned carbon material.

  The amount of the composite polymer (A) with respect to 100% by mass of the active material before attachment is preferably 0.01 to 5% by mass. Within this range, it is easy to ensure good cycle characteristics and input / output characteristics. The amount is more preferably 0.05% by mass or more, and more preferably 3% by mass or less.

  The impregnation is performed by adsorbing the composite polymer (A) on the active material surface in an aqueous solvent. Specifically, the active material is put into an aqueous dispersion of the composite polymer (A), mixed and stirred, and then preferably dried in an inert gas (for example, nitrogen gas) to thereby obtain an active material. A composite polymer-attached active material having the composite polymer (A) attached to the surface and / or inside thereof can be obtained. Examples of the aqueous solvent include water or a mixed solvent of water and alcohol (for example, a lower alcohol such as ethanol).

The composite polymer-attached active material is usually obtained in a form in which the particulate composite polymer (A) is attached to the surface of the active material. Even if each of these particles independently covers a part of the surface, even if the particles enlarged by aggregation cover a part of the surface, a film that covers the entire surface by overlapping the particles several times It may be. The coating ratio of the composite polymer (A) is preferably 0.1% or more, more preferably 1% or more with respect to the entire surface of the active material from the viewpoint of improving input / output. The coating ratio is a numerical value obtained by substituting the specific surface area of the active material before and after attachment measured by the BET method into the following formula.
Covering ratio (%) = [(active material specific surface area before attachment−active material specific surface area after attachment) / active material specific surface area before attachment] × 100

  In addition to the composite polymer (A), the composite polymer-impregnated active material may have a conductive additive (B) on at least a part of the surface. Here, “having” may be an existence state adhered to the surface of the active material through the composite polymer (A), a simple mixture state with the composite polymer-attached active material, etc., but is not limited thereto. . As the conductive additive (B), vapor grown carbon fiber (VGCF), carbon nanotube, carbon nanofiber, graphite, carbon black, Cu, Ni, Si, or an alloy powder thereof (preferably an average particle size of 10 μm or less) ) And the like. A conductive support agent (B) may be individual, or may use 2 or more types together. From the viewpoint of cycle characteristics, the content of the conductive auxiliary agent (B) is preferably 0.01 to 10% by mass, more preferably 0.1 to 5% by mass with respect to the active material.

  An electrode can be formed by forming an active material layer on a current collector using the composite polymer-impregnated active material. The method for producing the electrode is not particularly limited.For example, a composite polymer-impregnated active material, a conductive aid, if desired, a binder, etc. are mixed in a dry form to form a sheet, and this is pressure-bonded to a current collector, Examples include a method in which a solvent is added to a composite polymer adsorbing active material, if necessary, a conductive additive, a binder, and the like to form a slurry, which is applied to a current collector substrate and dried. In addition, the composite polymer-impregnated active material and other active materials may be used in combination as long as the effects of the present invention are not impaired. Since the composite polymer impregnated active material is excellent in water dispersibility, it is suitable for preparing an electrode as a slurry using an aqueous solvent. Hereinafter, although the case where the negative electrode of a lithium ion secondary battery is prepared will be described as an example, the composite polymer-impregnated active material of the present invention can also be used for the preparation of a positive electrode.

<Negative electrode>
The material of the negative electrode current collector is not particularly limited, and examples thereof include metal materials such as copper, nickel, stainless steel, and nickel-plated steel, and carbon materials such as carbon cloth and carbon paper. Among these, a metal material is preferable, and copper is more preferable.

  The shape of the negative electrode current collector is not particularly limited, and examples thereof include a thin film shape, a columnar shape, and a plate shape. In the case of a thin film, although thickness is not specifically limited, Usually, it is 5-30 micrometers. The thickness is preferably 9 μm or more, and preferably 20 μm or less. Among these, a metal thin film is preferable, and copper foil is particularly preferable. The thin film can be appropriately meshed.

  The amount of the negative electrode active material is usually 70 to 99.99% by mass in the negative electrode active material layer. The amount is preferably 75% by mass or more, more preferably 80% by mass or more, and preferably 99.9% by mass or less. A composite polymer-impregnated active material is used as the negative electrode active material, but other known negative electrode active materials may be used as long as the effects of the present invention are not impaired.

  The binder is not particularly limited as long as it can bind the negative electrode active material, and is a resin-based polymer such as polyethylene, polypropylene, polyethylene terephthalate, polymethyl methacrylate, aromatic polyamide, cellulose, nitrocellulose; SBR (styrene Butadiene rubber), NBR (acrylonitrile butadiene rubber), fluorine rubber, isoprene rubber, butadiene rubber, rubber polymer such as ethylene propylene rubber; styrene butadiene styrene block copolymer or hydrogenated product thereof, EPDM (ethylene・ Propylene / diene terpolymers, styrene / ethylene / butadiene / ethylene copolymers, styrene / isoprene / styrene block copolymers or hydrogenated products thereof; thermoplastic elastomer polymers; syndiotactic Soft resin polymers such as 1,2-polybutadiene, polyvinyl acetate, ethylene / vinyl acetate copolymer, propylene / α-olefin copolymer; polyvinylidene fluoride (PVDF), polytetrafluoroethylene, fluorinated poly Fluorine polymers such as vinylidene fluoride and polytetrafluoroethylene / ethylene copolymer; polymer compositions having ion conductivity of alkali metal ions (especially lithium ions), among which the weight average molecular weight is 10,000- 3 million can be used. The weight average molecular weight is preferably 100,000 or more, and preferably 1,000,000 or less. Here, the weight average molecular weight is a value determined by gel permeation chromatography (polystyrene conversion). A binder may be individual or may use 2 or more types together.

The amount of the binder is not particularly limited, but is generally 0.05 to 20% by mass with respect to the negative electrode active material, from the viewpoint of securing battery performance such as retention and mechanical strength of the negative electrode active material, and cycle characteristics. Preferably it is 0.1-10 mass%, More preferably, it is 0.5-5 mass%.

  When the negative electrode active material layer is formed using the slurry, the solvent is not particularly limited as long as it can dissolve or disperse the negative electrode active material and the like, and either an aqueous solvent or an organic solvent is used. be able to. Since the composite polymer impregnated active material is excellent in water dispersibility, it is suitable for preparing a negative electrode as a slurry using an aqueous solvent. Aqueous solvents are also preferred from the viewpoint of reducing environmental burden.

  Examples of the aqueous solvent include water, alcohol (for example, lower alcohol such as ethanol), and examples of the organic solvent include N-methylpyrrolidone (NMP), dimethylformamide, dimethylacetamide, methyl ethyl ketone, cyclohexanone, methyl acetate, acrylic acid. Methyl, diethyltriamine, N, N-dimethylaminopropylamine, ethylene oxide, tetrahydrofuran (THF), toluene, acetone, dimethyl ether, dimethylacetamide, hexamethylphosphamide, dimethyl sulfoxide, benzene, xylene, quinoline, pyridine, methyl Naphthalene, hexane, etc. are mentioned. A solvent may be individual or may use 2 or more types together.

  In particular, when an aqueous solvent is used, it is preferable to use a thickener together. In particular, it is preferable to use a thickener in combination with a rubber-based polymer such as SBR. The thickener is not particularly limited, and examples thereof include water-soluble cellulose polymers such as carboxymethyl cellulose, methyl cellulose, hydroxymethyl cellulose, and ethyl cellulose, polyvinyl alcohol, oxidized starch, phosphorylated starch, casein, and metal salts thereof. Thickeners may be used alone or in combination of two or more.

  When using a thickener, the amount ensures good coatability, keeps the proportion of the active material in the negative electrode active material layer properly, reduces the battery capacity, and reduces the electric resistance between the negative electrode active materials. In order to avoid the problem of increasing, it is usually 0.05 to 20% by mass, preferably 0.1 to 10% by mass, more preferably 0.5 to 5% by mass, based on the negative electrode active material. It is.

  After apply | coating a slurry on a collector, it can dry in dry air or inert atmosphere, a negative electrode active material layer can be formed, and a negative electrode can be obtained. The drying temperature is 60 ° C to 200 ° C, preferably 70 ° C or higher, and preferably 190 ° C or lower.

  The thickness of the negative electrode active material layer is usually 5 to 200 μm in order to ensure practicality as a negative electrode and sufficient lithium occlusion / release function for high-density current values. The thickness is preferably 20 μm or more, more preferably 30 μm or more, and preferably 100 μm or less, more preferably 75 μm or less.

  The present invention also relates to a lithium ion secondary battery including the negative electrode. The basic structure of a lithium ion secondary battery is the same as that of a well-known thing, Usually, the positive electrode and electrolyte which can occlude / release lithium ion other than the said negative electrode are provided.

<Positive electrode>
As the positive electrode, one in which an active material layer containing a positive electrode active material is usually formed on a current collector is used. The method for producing the positive electrode is not particularly limited. For example, a positive electrode active material, a binder, and the like are mixed in a dry form to form a sheet, which is pressure-bonded to the positive electrode current collector, a positive electrode active material, a binder, and the like with a solvent. In addition, a method of forming a slurry, applying this to a substrate of a positive electrode current collector, and drying the slurry can be mentioned.

  The material of the positive electrode current collector is not particularly limited, and examples thereof include metal materials such as aluminum, stainless steel, nickel plating, titanium, and tantalum, and carbon materials such as carbon cloth and carbon paper. Among these, a metal material is preferable, and aluminum is more preferable.

  The shape of the positive electrode current collector is not particularly limited, and examples thereof include a thin film shape, a columnar shape, and a plate shape. In the case of a thin film, the thickness is not particularly limited, but is usually 1 μm to 100 mm from the viewpoint of strength required for a positive electrode current collector and handleability. The thickness is preferably 3 μm or more, more preferably 5 μm or more, and preferably 1 mm or less, more preferably 50 μm or less. Among these, a metal thin film is preferable, and can be appropriately meshed.

The positive electrode active material is not particularly limited as long as it is a material that can occlude and release lithium ions during charge and discharge, and examples thereof include metal chalcogen compounds. Examples of metal chalcogen compounds include vanadium oxide, molybdenum oxide, manganese oxide, chromium oxide, titanium oxide, tungsten oxide and other transition metal oxides, vanadium sulfide, molybdenum sulfide. , Transition metal sulfides such as CuS, transition metal sulfides such as NiPS ₃ and FePS ₃ , selenium compounds of transition metals such as VSe ₂ and NbSe ₃ , Fe _0.25 V _0. Examples thereof include composite oxides of transition metals such as ₇₅ S ₂ and Na _0.1 CrS ₂ and composite sulfides of transition metals such as LiCoS ₂ and LiNiS ₂ . Among them, V ₂ O ₅ , V ₅ O ₁₃ , VO ₂ , Cr ₂ O ₅ , MnO ₂ , TiO, MoV ₂ O ₈ , LiCoO ₂ , LiNiO ₂ , LiMn ₂ O ₄ , TiS ₂ , V ₂ S ₅ , Cr _0.25 V _0.75 S ₂ , Cr _0.5 V _0.5 S _{2 and the} like are preferable, and LiCoO ₂ , LiNiO ₂ , LiMn ₂ O ₄ , and a part of these transition metals are preferably used as other metals. Lithium transition metal composite oxide substituted with The positive electrode active material may be used alone or in combination of two or more.

  The amount of the positive electrode active material is usually 10 to 99.9% by mass in the positive electrode active material layer. The amount is preferably 30% by mass or more, more preferably 50% by mass or more, and preferably 99% by mass or less.

  The binder is not particularly limited as long as it can bind the positive electrode active material. Resin-based polymers such as polyethylene, polypropylene, polyethylene terephthalate, polymethyl methacrylate, aromatic polyamide, cellulose, nitrocellulose; SBR (styrene Butadiene rubber), NBR (acrylonitrile butadiene rubber), fluorine rubber, isoprene rubber, butadiene rubber, rubber polymer such as ethylene propylene rubber; styrene butadiene styrene block copolymer or hydrogenated product thereof, EPDM (ethylene・ Propylene / diene terpolymers, styrene / ethylene / butadiene / ethylene copolymers, styrene / isoprene / styrene block copolymers or hydrogenated products thereof; thermoplastic elastomer polymers; syndiotactic Soft resin polymers such as 1,2-polybutadiene, polyvinyl acetate, ethylene / vinyl acetate copolymer, propylene / α-olefin copolymer; polyvinylidene fluoride (PVDF), polytetrafluoroethylene, fluorinated poly Fluorine polymers such as vinylidene fluoride and polytetrafluoroethylene / ethylene copolymer; polymer compositions having ion conductivity of alkali metal ions (especially lithium ions), among which the weight average molecular weight is 10,000- 3 million can be used. The weight average molecular weight is preferably 100,000 or more, and preferably 1,000,000 or less. Here, the weight average molecular weight is a value determined by gel permeation chromatography (polystyrene conversion). A binder may be individual or may use 2 or more types together.

  The amount of the binder is not particularly limited, but is generally 0.1 to 80 mass in the positive electrode active material from the viewpoint of securing the positive electrode active material and mechanical strength, and battery performance such as cycle characteristics, capacity, and conductivity. %. The amount is preferably 1% by mass or more, more preferably 5% by mass or more, and preferably 60% by mass or less, more preferably 40% by mass or less, and particularly preferably 10% by mass or less.

  In order to improve the conductivity of the positive electrode, the positive electrode active material layer can contain a conductive material. The conductive material is not particularly limited, and examples thereof include carbon powder such as acetylene black, carbon black, and graphite, metal fiber, metal powder, and metal foil. The conductive material may be used alone or in combination of two or more.

  When the positive electrode active material layer is formed using the slurry, the solvent is not particularly limited as long as the positive electrode active material or the like can be dissolved or dispersed, and either an aqueous solvent or an organic solvent is used. be able to. Aqueous solvents are preferred from the viewpoint of reducing environmental burden.

  Examples of the aqueous solvent include water, alcohol (for example, ethanol), and examples of the organic solvent include N-methylpyrrolidone (NMP), dimethylformamide, dimethylacetamide, methyl ethyl ketone, cyclohexanone, methyl acetate, methyl acrylate, diethyltriamine, N, N-dimethylaminopropylamine, ethylene oxide, tetrahydrofuran (THF), toluene, acetone, dimethyl ether, dimethylacetamide, hexamethylphosphalamide, dimethyl sulfoxide, benzene, xylene, quinoline, pyridine, methylnaphthalene, hexane, etc. Can be mentioned. A solvent may be individual or may use 2 or more types together.

  In particular, when an aqueous solvent is used, it is preferable to use a thickener together. In particular, it is preferable to use a thickener in combination with a rubber-based polymer such as SBR. The thickener is not particularly limited, and examples thereof include water-soluble cellulose polymers such as carboxymethyl cellulose, methyl cellulose, hydroxymethyl cellulose, and ethyl cellulose, polyvinyl alcohol, oxidized starch, phosphorylated starch, casein, and salts thereof. Thickeners may be used alone or in combination of two or more.

  When using a thickener, the amount ensures good coatability, keeps the proportion of the active material in the positive electrode active material layer appropriate, reduces the battery capacity, and reduces the resistance between the positive electrode active materials. In order to avoid the problem of increasing, it is usually 0.1 to 5% by mass in the positive electrode active material layer. The amount is preferably 0.5% by mass or more, more preferably 0.6% by mass or more, and preferably 3% by mass or less, more preferably 2% by mass or less.

  The thickness of the positive electrode active material layer is usually 10 to 200 μm. The positive electrode active material layer obtained by applying the slurry to the positive electrode current collector and drying is preferably consolidated by a roller press or the like in order to increase the packing density of the positive electrode active material.

<Electrolyte>
As the electrolyte, a nonaqueous electrolytic solution in which a lithium salt is dissolved in a nonaqueous solvent, a gel electrolyte, a rubber electrolyte, a solid sheet electrolyte, or the like is usually used.

  The non-aqueous solvent used for the non-aqueous electrolyte is not particularly limited, and a non-aqueous solvent known in the art can be used. For example, chain carbonates such as diethyl carbonate, dimethyl carbonate and ethyl methyl carbonate; cyclic carbonates such as ethylene carbonate, propylene carbonate and butylene carbonate; chain ethers such as 1,2-dimethoxyethane; tetrahydrofuran, 2-methyl Examples include cyclic ethers such as tetrahydrofuran, sulfolane and 1,3-dioxolane; chain esters such as methyl formate, methyl acetate and methyl propionate; cyclic esters such as γ-butyrolactone and γ-valerolactone. These non-aqueous solvents may be used alone or in combination of two or more. Among these, a mixed solvent containing a cyclic carbonate and a chain carbonate is preferable.

The lithium salt used for the non-aqueous electrolyte is not particularly limited, and a lithium salt known in the art can be used. For example, halides such as LiCl and LiBr, perhalogenates such as LiClO ₄ , LiBrO ₄ and LiClO ₄ , inorganic fluoride salts such as LiPF ₆ , LiBF ₄ and LiAsF ₆ , lithium bis (oxalatoborate) LiBC ₄ O Inorganic lithium salts such as ₈ , perfluoroalkane sulfonates such as LiCF ₃ SO ₃ and LiC ₄ F ₉ SO ₃ , perfluoroalkane sulfonic imides such as Li trifluorosulfonimide ((CF ₃ SO ₂ ) ₂ NLi) Examples thereof include fluorine-containing organic lithium salts such as salts. Lithium salts may be used alone or in combination of two or more. The concentration of the lithium salt in the non-aqueous electrolyte is usually 0.5M or more and 2.0M or less.

  The non-aqueous electrolyte solution may contain an organic polymer compound to form a gel electrolyte, rubber electrolyte, or solid sheet electrolyte. In this case, examples of the organic polymer compound include polyether polymer compounds such as polyethylene oxide and polypropylene oxide; crosslinked polymers of polyether polymer compounds; vinyl alcohol polymer compounds such as polyvinyl alcohol and polyvinyl butyral; vinyl Insoluble matter of alcohol polymer compound; polyepichlorohydrin; polyphosphazene; polysiloxane; vinyl polymer compound such as polyvinylpyrrolidone, polyvinylidene carbonate, polyacrylonitrile; poly (ω-methoxyoligooxyethylene methacrylate), poly (ω Polymer copolymers such as -methoxyoligooxyethylene methacrylate-co-methyl methacrylate) and poly (hexafluoropropylene-vinylidene fluoride).

  The non-aqueous electrolyte can further contain a film forming agent. Examples of the film forming agent include vinylene carbonate, vinyl ethylene carbonate, vinyl ethyl carbonate, methyl phenyl carbonate, fluoroethylene carbonate, difluoroethylene carbonate, and other carbonate compounds, ethylene sulfide, propylene sulfide, and other alkene sulfides; 1,3-propane sultone, Examples include sultone compounds such as 1,4-butane sultone; acid anhydrides such as maleic anhydride and succinic anhydride.

  When a film forming agent is used, the amount is usually 10% by mass or less, preferably 8%, from the viewpoint of securing an appropriate initial irreversible capacity and avoiding deterioration of battery characteristics such as low temperature characteristics and rate characteristics. It is 5 mass% or less, More preferably, it is 5 mass% or less, Most preferably, it is 2 mass% or less.

  As the electrolyte, a polymer solid electrolyte that is a conductor of an alkali metal cation such as lithium ion can also be used. Examples of the polymer solid electrolyte include those obtained by dissolving a lithium salt in the aforementioned polyether polymer compound, and polymers in which the terminal hydroxyl group of the polyether is substituted with an alkoxide.

<Separator>
Usually, a separator is interposed between the positive electrode and the negative electrode in order to prevent a short circuit. A non-aqueous electrolyte is usually used by impregnating this separator. The material of the separator is not particularly limited, and materials known in the art can be used. Examples thereof include polyolefins such as polyethylene and polypropylene, polyethersulfone, and the like, preferably polyolefin.

  The form of the lithium ion secondary battery of this invention is not specifically limited, It can be set as a well-known form in the said field | area. For example, a cylinder type in which a sheet electrode and a separator are spirally formed, a cylinder type having an inside-out structure in which a pellet electrode and a separator are combined, a coin type in which a pellet electrode and a separator are stacked, and the like can be given. In addition, by storing batteries of these forms in an optional outer case, the battery can be used in an arbitrary shape such as a coin shape, a cylindrical shape, or a square shape.

  The procedure for assembling the lithium ion secondary battery of the present invention is not particularly limited, and can be assembled by an appropriate procedure according to the structure of the battery. For example, a negative electrode can be placed on an outer case, an electrolyte and a separator can be provided thereon, and a positive electrode can be placed so as to face the negative electrode, and can be caulked together with a gasket and a sealing plate to form a battery.

  Next, specific embodiments of the present invention will be described in more detail with reference to examples, but the present invention is not limited to these examples.

(Manufacture of negative electrode active material)
Naturally produced graphite with a 002 plane spacing (d002) of 3.36 mm by L-ray wide angle diffraction method, Lc of 1000 mm or more, tap density of 0.46 g / cm ³ , and 1580 cm ⁻¹ in an argon ion laser Raman spectrum. The scale-like graphite particles having a Raman R value which is a peak intensity ratio around 1360 cm ^{−1 to} a peak intensity around 0.13, an average particle diameter of 28.7 μm and a true density of 2.27 g / cm ³ are Damage to the graphite particle surface by continuously processing the scaly graphite particles at a processing speed of 20 kg / hr under conditions of a rotor peripheral speed of 60 m / second and 10 minutes using a machine system manufactured by Kikai Seisakusho. The spheroidization treatment was performed while fine powder was removed by further classification treatment. The obtained spheroidized graphitic carbon has an 002 plane spacing (d002) of 3.36 mm, an Lc of 1000 mm or more, a tap density of 0.83 g / cm ³ by an X-ray wide angle diffraction method, and an argon ion laser Raman spectrum. 1580 cm ^-1 Raman R value is the peak intensity ratio in the vicinity of 1360 cm ^-1 to the peak intensity near 0.24, an average particle diameter of 11.6, BET method specific surface area of 7.7 m ² / g, a true density of 2.27 g / cm ³ and the average circularity was 0.909.
Next, 100 parts by mass of this spheroidized graphitic carbon and 9.4 parts by mass of coal-derived pitch were mixed by heating (160 ° C.) with a compounding machine, and then fired to 1000 ° C. in a non-oxidizing atmosphere over 2 weeks. After cooling to room temperature, pulverization and classification were performed to obtain a multi-layer structure spheroidized carbon material. This multi-layered spheroidized carbon material has a 002 plane spacing (d002) of 3.36 mm, an Lc of 1000 mm or more, a tap density of 0.98 g / cm ³ by an X-ray wide angle diffraction method, and an argon ion laser Raman spectrum of 1580 cm. ^The Raman R value, which is the peak intensity ratio near 1360 cm ^{−1 to} the peak intensity near ⁻¹ , is 0.31, the average particle size is 11.6 μm, the d10 particle size is 7.6 μm, the d90 particle size is 17.5 μm, and the BET specific surface area Is 3.5 m ² / g, the coverage is 5.0%, and the ratio of rhombohedral 3R to hexagonal 2H by the X-ray wide angle diffraction method is 3R / 2H of 0.26, 10 nm to 100,000 nm. Was 0.74 ml / g. In addition, the pitch derived from coal used alone was fired in a nitrogen atmosphere to 1300 ° C., then cooled to room temperature, and pulverized to obtain a 002 surface by an X-ray wide angle diffraction method of an amorphous carbon single material. The surface separation (d002) was 3.45 mm and Lc was 24 mm.

(Preparation of composite polymer impregnated active material)
10 g of the negative electrode active material, 10 g of pure water, an emulsion in which a composite polymer in which polyvinylidene fluoride (PVDF) and polymethyl methacrylate (PMMA) form an IPN structure is dispersed in water (Arkema, trade name: Kynar Aquatec, Concentration: 50% by mass, PVDF: PMMA (mass ratio) = 70: 30, Tg of composite polymer = 0 ° C., dispersion average particle size: 120 nm (dynamic scattering method: median size)) 0.02 g were mixed. Thereafter, it was dried at 110 ° C. in nitrogen gas to obtain a composite polymer-impregnated active material. The coverage ratio of the active material by the composite polymer was 8%. The particle size of the attached composite polymer that was not overlapped or condensed was 150 nm. The particle diameter of the composite polymer attached to the active material surface was determined by observation using a scanning electron microscope.

(Preparation of negative electrode)
10 g of the composite polymer adsorbing active material, 10 g of an aqueous solution of carboxymethyl cellulose (concentration of carboxymethyl cellulose: 1% by mass) as a thickener, and styrene-butadiene rubber (SBR, unsaturation: 75%, weight average molecular weight as a binder) 120,000) of emulsion (concentration: 50% by mass) was mixed using a high speed mixer to form a slurry. This slurry was applied onto a copper foil (current collector) by a doctor blade method and dried at 110 ° C. This was pressed at a linear density of 20 to 300 kg / cm by a roll press to form an active material layer. The mass of the active material layer after drying was 10 mg / cm ² , the density was 1.6 g / cm ³ , and the average electrode thickness was 68 μm. The negative electrode (negative electrode for lithium ion secondary battery) produced by the above procedure was used as the negative electrode of Example 1.

  A negative electrode of Example 2 was produced in the same manner as in Example 1 except that the amount of the emulsion in which the composite polymer was dispersed in water was 0.2 g. The coverage of the active material with the composite polymer was 30%.

  Example 3 was carried out in the same manner as in Example 1 except that the amount of the emulsion in which the composite polymer was dispersed in water was 0.2 g, and 0.1 g of VGCF was added as a conductive aid to the slurry during the production of the negative electrode. A negative electrode was prepared. The coverage of the active material with the composite polymer was 30%.

  A negative electrode of Comparative Example 1 was produced in the same manner as in Example 1 except that a negative electrode active material to which no composite polymer was added was used.

  Using a negative electrode active material to which no composite polymer was attached, instead of using an SBR emulsion, 0.2 g of an aqueous dispersion of the composite polymer was used as a binder. A negative electrode of Comparative Example 2 was produced.

(Preparation of positive electrode)
41.7 parts by mass of N-methylpyrrolidone solution (polyvinylidene fluoride concentration: 12% by mass) in which 10 parts by mass of carbon black and polyvinylidene fluoride are dissolved in 85 parts by mass of lithium nickel manganese cobalt based composite oxide powder And an appropriate amount of N-methylpyrrolidone were added and kneaded to prepare a slurry. This slurry was applied to an aluminum foil with a basis weight of 8.8 mg / cm ² by a doctor blade method. The film was dried at 110 ° C. and further consolidated by a roll press so that the positive electrode layer had a density of 2.45 g / cm ³ . This was cut into a 30 mm × 40 mm square and dried at 140 ° C. to obtain a positive electrode.

(Production of battery for performance evaluation)
The positive electrode described above and the negative electrode described in Examples and Comparative Examples were stacked through a separator impregnated with an electrolytic solution to prepare a battery for a charge / discharge test. As the electrolytic solution, a solution obtained by dissolving LiPF _{6 in a} mixed solution of ethylene carbonate: dimethyl carbonate: ethyl methyl carbonate = 3: 3: 4 (mass ratio) to 1 mol / liter was used.
The battery was first charged to 4.1V at 0.2C, further charged to 0.1mA at 4.1V, then discharged to 3.0V at 0.2C, and then 4.2V at 0.2C. Then, the battery was further charged to 4.2 mA at 0.1 mA, and then discharged at 0.2 C to 3.0 V twice to obtain initial adjustment.

(Evaluation of low temperature output characteristics)
The battery for evaluation produced as described above was charged at a constant current of 0.2 C for 150 minutes in a 25 ° C. environment, and then stored in a thermostatic bath at −30 ° C. for 3 hours or more. The battery was discharged at .50 C, 0.75 C, 1.00, 1.25 C, 1.50 C, 1.75 C, and 2.00 C for 2 seconds, and the voltage at the second second was measured. The area of the triangle surrounded by the current-voltage line and the lower limit voltage (3 V) was defined as output (W). The results are shown in Table 1.

(Evaluation of cycle maintenance rate)
The evaluation battery produced as described above was charged to 4.2 V at 0.2 C, charged to 4.2 mA at 4.2 V, and then precharged / discharged to 3.0 V at 0.2 C. It was. Next, the battery was charged to 0.7 V at 0.7 C, charged to 4.2 mA at 4.2 V, and then subjected to 300 cycles of charge / discharge to discharge to 3.0 V at 1 C. The ratio of the discharge capacity at the 300th time to the discharge capacity at the first time was determined, and this was used as the cycle retention rate. The results are shown in Table 1.

From the results in Table 1, the following is clear.
Compared to Comparative Example 1 using the negative electrode active material to which the composite polymer was not attached, the batteries of Examples 1 to 3 using the composite polymer attached active material had improved low-temperature output characteristics, It can be seen that good cycle characteristics are maintained.
On the other hand, in Comparative Example 2 in which the composite polymer was added as a binder to the slurry at the time of preparing the negative electrode instead of the SBR emulsion without attaching the composite polymer to the active material surface, Although the low-temperature output characteristics were improved, the cycle characteristics were extremely inferior. Further, the improvement range of the low-temperature output characteristics was not as good as in Examples 2 and 3. Therefore, it can be said that only the composite polymer-impregnated active material for the non-aqueous secondary battery electrode of the present invention, particularly for the lithium ion secondary battery electrode, achieves both low temperature output and cycle characteristics at a high level.

  The composite polymer-containing active material of the present invention can improve the input / output characteristics and cycle characteristics at a low temperature in a balanced manner when used for an electrode of a non-aqueous secondary battery, particularly a lithium ion secondary battery. An electrode using the composite polymer-impregnated active material of the present invention can be prepared with an aqueous solvent and has a low environmental impact.

Claims (7)

The composite polymer (A) in which the fluorine polymer (a1) and the polymer (a2) having an oxygen-containing functional group form an interpenetrating structure is attached to at least a part of the surface of the active material. A composite polymer-impregnated active material for a non-aqueous secondary battery electrode. The composite polymer impregnated active material according to claim 1, which is obtained by mixing the composite polymer (A) and the active material in an aqueous solvent and then drying the mixture. The composite polymer-impregnated active material according to claim 1 or 2, wherein the fluorine-based polymer (a1) is a homopolymer or copolymer of a fluorine-containing monomer. Furthermore, the composite polymer adhesion active material of any one of Claims 1-3 which has a conductive support agent (B) in at least one part of the surface of an active material. An electrode for a non-aqueous secondary battery, comprising an active material layer containing the composite polymer-impregnated active material according to any one of claims 1 to 4 on a current collector. The negative electrode for a non-aqueous secondary battery according to claim 5, wherein the active material layer is a negative electrode active material layer and further contains a cellulose polymer and a rubber polymer. A non-aqueous secondary battery comprising a positive electrode capable of inserting and extracting lithium ions, a negative electrode, and an electrolyte solution, wherein the negative electrode is the electrode according to claim 5 or the negative electrode according to claim 6. .