WO2017104770A1 - 二次電池負極用バインダー組成物、二次電池負極及び二次電池 - Google Patents
二次電池負極用バインダー組成物、二次電池負極及び二次電池 Download PDFInfo
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- WO2017104770A1 WO2017104770A1 PCT/JP2016/087446 JP2016087446W WO2017104770A1 WO 2017104770 A1 WO2017104770 A1 WO 2017104770A1 JP 2016087446 W JP2016087446 W JP 2016087446W WO 2017104770 A1 WO2017104770 A1 WO 2017104770A1
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- 0 CC1(*)OC(*)(*C(*)=C(*)I)C(*)(*)O1 Chemical compound CC1(*)OC(*)(*C(*)=C(*)I)C(*)(*)O1 0.000 description 1
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M4/00—Electrodes
- H01M4/02—Electrodes composed of, or comprising, active material
- H01M4/62—Selection of inactive substances as ingredients for active masses, e.g. binders, fillers
- H01M4/621—Binders
- H01M4/622—Binders being polymers
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- C—CHEMISTRY; METALLURGY
- C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
- C08L—COMPOSITIONS OF MACROMOLECULAR COMPOUNDS
- C08L29/00—Compositions of homopolymers or copolymers of compounds having one or more unsaturated aliphatic radicals, each having only one carbon-to-carbon double bond, and at least one being terminated by an alcohol, ether, aldehydo, ketonic, acetal or ketal radical; Compositions of hydrolysed polymers of esters of unsaturated alcohols with saturated carboxylic acids; Compositions of derivatives of such polymers
- C08L29/02—Homopolymers or copolymers of unsaturated alcohols
- C08L29/04—Polyvinyl alcohol; Partially hydrolysed homopolymers or copolymers of esters of unsaturated alcohols with saturated carboxylic acids
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- C—CHEMISTRY; METALLURGY
- C09—DYES; PAINTS; POLISHES; NATURAL RESINS; ADHESIVES; COMPOSITIONS NOT OTHERWISE PROVIDED FOR; APPLICATIONS OF MATERIALS NOT OTHERWISE PROVIDED FOR
- C09D—COATING COMPOSITIONS, e.g. PAINTS, VARNISHES OR LACQUERS; FILLING PASTES; CHEMICAL PAINT OR INK REMOVERS; INKS; CORRECTING FLUIDS; WOODSTAINS; PASTES OR SOLIDS FOR COLOURING OR PRINTING; USE OF MATERIALS THEREFOR
- C09D129/00—Coating compositions based on homopolymers or copolymers of compounds having one or more unsaturated aliphatic radicals, each having only one carbon-to-carbon double bond, and at least one being terminated by an alcohol, ether, aldehydo, ketonic, acetal, or ketal radical; Coating compositions based on hydrolysed polymers of esters of unsaturated alcohols with saturated carboxylic acids; Coating compositions based on derivatives of such polymers
- C09D129/02—Homopolymers or copolymers of unsaturated alcohols
- C09D129/04—Polyvinyl alcohol; Partially hydrolysed homopolymers or copolymers of esters of unsaturated alcohols with saturated carboxylic acids
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M10/00—Secondary cells; Manufacture thereof
- H01M10/05—Accumulators with non-aqueous electrolyte
- H01M10/052—Li-accumulators
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M10/00—Secondary cells; Manufacture thereof
- H01M10/05—Accumulators with non-aqueous electrolyte
- H01M10/052—Li-accumulators
- H01M10/0525—Rocking-chair batteries, i.e. batteries with lithium insertion or intercalation in both electrodes; Lithium-ion batteries
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M4/00—Electrodes
- H01M4/02—Electrodes composed of, or comprising, active material
- H01M4/13—Electrodes for accumulators with non-aqueous electrolyte, e.g. for lithium-accumulators; Processes of manufacture thereof
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M4/00—Electrodes
- H01M4/02—Electrodes composed of, or comprising, active material
- H01M4/13—Electrodes for accumulators with non-aqueous electrolyte, e.g. for lithium-accumulators; Processes of manufacture thereof
- H01M4/134—Electrodes based on metals, Si or alloys
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M4/00—Electrodes
- H01M4/02—Electrodes composed of, or comprising, active material
- H01M4/13—Electrodes for accumulators with non-aqueous electrolyte, e.g. for lithium-accumulators; Processes of manufacture thereof
- H01M4/139—Processes of manufacture
- H01M4/1395—Processes of manufacture of electrodes based on metals, Si or alloys
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M4/00—Electrodes
- H01M4/02—Electrodes composed of, or comprising, active material
- H01M4/36—Selection of substances as active materials, active masses, active liquids
- H01M4/38—Selection of substances as active materials, active masses, active liquids of elements or alloys
- H01M4/381—Alkaline or alkaline earth metals elements
- H01M4/382—Lithium
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M4/00—Electrodes
- H01M4/02—Electrodes composed of, or comprising, active material
- H01M2004/021—Physical characteristics, e.g. porosity, surface area
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M4/00—Electrodes
- H01M4/02—Electrodes composed of, or comprising, active material
- H01M2004/026—Electrodes composed of, or comprising, active material characterised by the polarity
- H01M2004/027—Negative electrodes
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M4/00—Electrodes
- H01M4/02—Electrodes composed of, or comprising, active material
- H01M4/04—Processes of manufacture in general
- H01M4/0402—Methods of deposition of the material
- H01M4/0404—Methods of deposition of the material by coating on electrode collectors
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M4/00—Electrodes
- H01M4/02—Electrodes composed of, or comprising, active material
- H01M4/36—Selection of substances as active materials, active masses, active liquids
- H01M4/38—Selection of substances as active materials, active masses, active liquids of elements or alloys
- H01M4/40—Alloys based on alkali metals
- H01M4/405—Alloys based on lithium
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
- Y02E60/10—Energy storage using batteries
Definitions
- the present invention relates to a binder composition for a secondary battery negative electrode, a secondary battery negative electrode, and a secondary battery. Specifically, in order to constitute a negative electrode of a secondary battery, a binder composition used together with a negative electrode active material, a secondary battery negative electrode containing the binder composition, and a secondary battery having the secondary battery negative electrode About.
- a secondary battery generally includes a positive electrode in which a positive electrode active material layer including a positive electrode active material is formed on both sides of a positive electrode current collector, and a negative electrode active material layer including a negative electrode active material formed on both sides of the negative electrode current collector.
- the negative electrode is connected via an electrolyte layer and is housed in a battery case.
- Such an electrode is generally formed by applying and drying a mixed slurry of an active material and an electrode binder on a current collector surface.
- the binder for the electrode functions to bind the active materials to each other and to bind the metal foil as the current collector to the active material.
- the binder cannot bind a sufficient amount of the active material to the current collector or the active materials cannot be bound to each other, a battery having a large capacity cannot be obtained.
- the binding force of the binder is reduced due to volume fluctuations of the active material due to repeated charge and discharge, the active material may drop from the current collector and the battery capacity may be reduced.
- the battery capacity can be increased by repeatedly deforming the active material layer, dropping off the active material, or peeling from the current collector. It has been a problem to decrease. Therefore, there is a demand for a high-performance secondary battery electrode binder that sufficiently binds the active material and the current collector with a small amount of binder.
- active materials for battery negative electrodes capable of increasing capacity instead of the conventional graphite-based active materials, active materials of semimetals, metals and their alloys (hereinafter collectively referred to as “alloy-based active materials”).
- alloy-based active materials active materials of semimetals, metals and their alloys.
- an alloy active material such as silicon powder. It is disclosed.
- an alloy-based active material generally has a larger volume expansion than a graphite-based active material.
- a known silicon (Si) active material as an alloy-based active material undergoes a volume change of about 400% with charge / discharge. It is known to wake up. Because of this volume change, the active material is more easily peeled off from the electrode, so it is difficult to obtain a stable high charge capacity with a binder containing a conventional polyvinyl alcohol resin-containing emulsion used in a graphite-based active material. In addition, the charge / discharge cycle characteristics were not always satisfactory.
- the present invention has been made in view of such circumstances, and the object thereof is a binder composition for a secondary battery negative electrode used for an alloy-based active material, including repeated charge and discharge, etc.
- a secondary battery capable of forming a stable negative electrode active material layer capable of following the volume change of the negative electrode due to the above, thereby obtaining a high charge / discharge capacity and improving the charge / discharge cycle characteristics
- the present invention provides a secondary battery negative electrode binder composition, a secondary battery negative electrode containing the binder composition, and a secondary battery having the secondary battery negative electrode.
- the present inventors have found that in a binder composition containing an emulsion in which polymer particles derived from an ethylenically unsaturated monomer are dispersed in an aqueous polyvinyl alcohol resin solution, the polyvinyl alcohol resin / polymer
- the weight ratio of the particles within a specific range, a stable negative electrode active material layer that can follow the volume change of the negative electrode made of an alloy-based active material can be formed, thereby obtaining a high charge / discharge capacity.
- the secondary battery which can improve the cycling characteristics of charging / discharging is obtained.
- the negative electrode active material layer further follows the volume change of the negative electrode, and a secondary battery having a more stable negative electrode active material layer can be obtained.
- the present invention has been completed.
- the gist of the present invention is a binder composition for producing a secondary battery negative electrode containing an element capable of forming an alloy with lithium as an active material, and a polymer derived from an ethylenically unsaturated monomer
- the particles include an emulsion dispersed in a polyvinyl alcohol resin aqueous solution, and the ratio of the polyvinyl alcohol resin / polymer particles is 60/40 to 99/1 in terms of the weight ratio of the resin solids. It exists in the binder composition for secondary battery negative electrodes to do.
- the polymer particles preferably have a glass transition temperature of ⁇ 40 to 60 ° C.
- negative electrode binder composition may be simply abbreviated as “negative electrode binder composition”.
- the gist of the present invention also resides in a secondary battery negative electrode comprising the secondary battery negative electrode binder composition of the present invention and a secondary battery having the secondary battery negative electrode of the present invention.
- the binder composition for a secondary battery negative electrode of the present invention includes an emulsion containing a polyvinyl alcohol resin and polymer particles derived from an ethylenically unsaturated monomer at a specific weight ratio, the alloy active material and It has excellent binding properties with the current collector, and has a good balance between the elastic modulus and flexibility of the negative electrode active material layer (hereinafter also referred to as a film) containing the binder composition and the alloy-based active material. Therefore, the negative electrode active material layer containing the binder composition and the alloy-based active material is less prone to peeling and dropping off of the alloy-based active material due to the volume change of the negative electrode due to repeated charge / discharge, etc.
- the secondary battery negative electrode containing the binder composition Since it can follow the volume change, by using the secondary battery negative electrode containing the binder composition, a high charge / discharge capacity can be stably obtained, and the secondary battery having excellent charge / discharge cycle characteristics can be obtained. Manufacture is possible. Further, by adjusting the glass transition temperature of the polymer particles in the binder composition for secondary battery negative electrode of the present invention to a specific range, it can further follow the volume change of the negative electrode due to repeated charging and discharging, etc. Since a stable negative electrode active material layer can be formed, it is possible to manufacture a secondary battery with further excellent charge / discharge cycle characteristics.
- This binder composition is a resin composition containing an emulsion containing a polyvinyl alcohol resin and polymer particles, and the ratio of the polyvinyl alcohol resin component to the polymer particle component can be arbitrarily set within a specific range.
- the thickness By controlling the thickness, it is possible to control the elastic modulus and flexibility of the coating, and further the metal adhesion.
- the glass transition temperature of the polymer particles to a specific range, the elastic modulus and flexibility of the film, and further the metal adhesion can be controlled.
- a binder In general, as a binder, it has been considered that a film composition having a large amount of polymer particles with high flexibility can follow the volume change of an electrode. The reason for this is that the flexible binder resin relaxes the stress on the volume change associated with charge / discharge of the active material, thereby preventing the cracking of the binder and the structural change of the electrode, thereby improving the cycle characteristics. .
- the volume change due to charging / discharging is too large for the alloy-based active material as compared with the graphite-based active material, it is difficult to completely follow the electrode structure change with a flexible binder.
- the present invention has been completed by designing a resin having a suitable composition when using an alloy-based active material. From the above technical idea, unlike the conventional general binder design concept, when using an alloy-based active material for the negative electrode, surprisingly, the higher the proportion of the polyvinyl alcohol-based resin, the more stable and high the charge. It turned out that the battery which shows discharge capacity and is excellent in the cycling characteristics of charging / discharging comes to be obtained.
- the flexibility of the binder film is reduced and brittle fracture is likely to occur, and the negative electrode active material layer may be dropped due to the volume change associated with the stage of charging and discharging. Battery capacity may be reduced. Therefore, by adjusting the glass transition temperature of the polymer particles to a specific range, the film strength of the binder is increased, and the negative electrode active material layer can further follow the volume change of the negative electrode. It was found that a battery having a high charge / discharge capacity and excellent cycle characteristics of charge / discharge can be obtained.
- acrylic and methacryl are collectively referred to as (meth) acryl when not particularly distinguished, and acrylate and methacrylate are collectively referred to as (meth) acrylate when not particularly distinguished.
- the solid content means that obtained by subjecting an object to a drying loss method at 105 ° C. for 3 hours.
- the binder composition for secondary battery negative electrode of the present invention is a binder composition for producing a secondary battery negative electrode containing an element capable of forming an alloy with lithium as an active material, and comprising an ethylenically unsaturated monomer Is a binder composition containing an emulsion in which polymer particles derived from are dispersed in an aqueous polyvinyl alcohol resin solution.
- the polymer particles can be obtained by, for example, emulsion polymerization of an ethylenically unsaturated monomer in an aqueous dispersion medium using the PVA resin as a dispersant. Details will be described below.
- the polymer in the polymer particles is a polymer of an ethylenically unsaturated monomer.
- the ethylenically unsaturated monomer include the following (a) to (m). These may be used alone or in combination of two or more.
- D An epoxy group-containing ethylenically unsaturated monomer.
- monomers such as vinyl pyrrolidone, methyl vinyl ketone, butadiene, ethylene, propylene, vinyl chloride, and vinylidene chloride can be appropriately used as desired.
- Examples of the (meth) acrylic acid alkyl ester (a) include methyl (meth) acrylate, ethyl (meth) acrylate, propyl (meth) acrylate, isopropyl (meth) acrylate, butyl (meth) acrylate, and isobutyl (meth).
- Aliphatic (meth) acrylates such as acrylate, n-hexyl (meth) acrylate, cyclohexyl (meth) acrylate, 2-ethylhexyl (meth) acrylate, lauryl (meth) acrylate, preferably an alkyl group having 1 to 20 carbon atoms
- Aliphatic (meth) acrylates, aromatic (meth) acrylates such as benzyl (meth) acrylate, phenyl (meth) acrylate, and the like can be used, and these can be used alone or in combination of two or more.
- an aliphatic (meth) acrylate having an alkyl group of 1 to 10 carbon atoms such as butyl (meth) acrylate, 2-ethylhexyl (meth) acrylate, methyl (meth) acrylate, cyclohexyl (meth) acrylate, and the like is preferable. It is.
- hydroxyl group-containing ethylenically unsaturated monomer (b) examples include, for example, hydroxyalkyl (meth) acrylates such as hydroxyethyl (meth) acrylate and hydroxypropyl (meth) acrylate, and polymers such as polyethylene glycol (meth) acrylate.
- hydroxyalkyl (meth) acrylates such as hydroxyethyl (meth) acrylate and hydroxypropyl (meth) acrylate
- polymers such as polyethylene glycol (meth) acrylate.
- An alkylene glycol (meth) acrylate etc. are mentioned, These are used individually or in combination of 2 or more types.
- hydroxyalkyl (meth) acrylates having a hydroxyalkyl group having 2 to 4 carbon atoms
- polyalkylene glycol (meth) acrylates having an alkylene group having 2 to 4 carbon atoms
- hydroxyethyl ( (Meth) acrylate particularly preferred is hydroxyethyl ( (Meth) acrylate.
- Examples of the carboxyl group-containing ethylenically unsaturated monomer (c) include monocarboxylic acid monomers such as (meth) acrylic acid, crotonic acid, and undecylenic acid, dicarboxylic acids such as maleic acid, fumaric acid, and itaconic acid, and The anhydride monomer etc. are mentioned.
- (Meth) acrylic acid is preferred. These may be used alone or in combination of two or more. Among these, (meth) acrylic acid and itaconic acid are more preferable. In the case of dicarboxylic acids such as maleic acid, fumaric acid and itaconic acid, these monoesters and monoamides can also be used.
- Examples of the epoxy group-containing ethylenically unsaturated monomer (d) include glycidyl (meth) acrylate, allyl glycidyl ether, methyl glycidyl (meth) acrylate, and the like. These may be used alone or in combination of two or more. Used. Of these, glycidyl (meth) acrylate is preferred.
- methylol group-containing ethylenically unsaturated monomer (e) examples include N-methylol (meth) acrylamide, dimethylol (meth) acrylamide and the like, and these are used alone or in combination of two or more. .
- alkoxyalkyl group-containing ethylenically unsaturated monomer (f) examples include N-methoxymethyl (meth) acrylamide, N-butoxymethyl (meth) acrylamide, methoxyethyl (meth) acrylate, and methoxypropyl (meth).
- examples include acrylates, alkoxyalkyl (meth) acrylates such as ethoxyethyl (meth) acrylate, ethoxypropyl (meth) acrylate, and polyalkylene glycol monoalkoxy (meth) acrylates such as polyethylene glycol monomethoxy (meth) acrylate. It is used alone or in combination of two or more.
- cyano group-containing ethylenically unsaturated monomer (g) for example, an ⁇ , ⁇ -unsaturated nitrile compound is used.
- acrylonitrile monomers such as acrylonitrile, methacrylonitrile, ⁇ -chloroacrylonitrile, ⁇ -ethylacrylonitrile; cyano group disubstituted vinyl monomers such as vinylidene cyanide; methyl cyanoacrylate, ethyl cyanoacrylate, butyl cyanoacrylate, etc.
- unsaturated group-containing cyanoacrylate, tetracyanoquinodimethane, 2,2-diarylmalononitrile, and the like Among these, acrylonitrile monomers are preferable, (meth) acrylonitrile is particularly preferable, and acrylonitrile is more preferable.
- the nitrile monomers can be used alone or in combination of two or more.
- Examples of the ethylenically unsaturated monomer (h) having two or more radical polymerizable double bonds include di (meth) acrylate, tetra (meth) acrylate, and conjugated diene monomer.
- Examples of the di (meth) acrylate include divinylbenzene, polyoxyethylene di (meth) acrylate, polyoxypropylene di (meth) acrylate, neopentyl glycol di (meth) acrylate, and butanediol di (meth) acrylate. Can be mentioned.
- tetra (meth) acrylate examples include tri (meth) acrylate such as trimethylolpropane tri (meth) acrylate, pentaerythritol tetra (meth) acrylate, and the like.
- conjugated diene monomer examples include 1,3-butadiene, isoprene, 2,3-dimethyl-1,3-butadiene, 1,3-pentadiene, 1,3-hexadiene, 1,3-heptadiene, 2 Hydrocarbon conjugated diene monomers such as 1,3-dimethylbutadiene, 2-phenyl-1,3-butadiene, 3-methyl-1,3-pentadiene, 1,3-hexadiene; 2-chloro-1,3-butadiene, etc. And halogen-containing conjugated diene monomers; substituted linear conjugated pentadienes; substituted and side chain conjugated hexadienes, and the like.
- conjugated diene monomers having 4 to 6 carbon atoms are preferable, and 1,3-butadiene is particularly preferable.
- ethylenically unsaturated monomers (h) having two or more radical polymerizable double bonds can be used singly or in combination of two or more.
- Examples of the ethylenically unsaturated monomer (i) having an amino group include N such as (meth) acrylamide, N, N-dimethylaminoethyl (meth) acrylate, and N, N-diethylaminoethyl (meth) acrylate. , N-dialkylaminoalkyl (meth) acrylate and the like, and these may be used alone or in combination of two or more. Of these, (meth) acrylamide is preferred.
- Examples of the ethylenically unsaturated monomer (j) having a sulfonic acid group include vinyl sulfonic acid and vinyl styrene sulfonic acid (salt), and these are used alone or in combination of two or more. .
- Examples of the ethylenically unsaturated monomer (k) having a phosphoric acid group include vinylphosphonic acid, vinyl phosphate, acid phosphoxyethyl (meth) acrylate, acid phosphoxypropyl (meth) acrylate, and bis [(meta ) Acryloyloxyethyl] phosphate, diphenyl-2- (meth) acryloyloxyethyl phosphate, dibutyl-2- (meth) acryloyloxyethyl phosphate, dioctyl-2 (meth) acryloyloxyethyl phosphate, etc. These may be used alone or in combination of two or more.
- Examples of the aromatic ethylenically unsaturated monomer (l) include styrene, vinyltoluene, ⁇ -methylstyrene, and the like. These may be used alone or in combination of two or more. Of these, styrene is preferable.
- Examples of the fatty acid ester unsaturated monomer (m) include vinyl formate, vinyl acetate, vinyl propionate, vinyl valerate, vinyl butyrate, vinyl isobutyrate, vinyl pivalate, vinyl caprate, vinyl laurate, Examples include vinyl stearate, vinyl benzoate, and vinyl versatate. These may be used alone or in combination of two or more.
- Such content corresponds to the charged weight of the monomer charged in the production of the emulsion used in the present invention.
- the charged weight of such a monomer corresponds to the structure of the intended polymer particles, and is usually 5 to 100% by weight, preferably 20 to 100% by weight, particularly preferably more than 30 to 80% by weight of the polymer particles. .
- the polymer particles may contain a structural unit derived from a monomer other than the above (hereinafter, may be simply referred to as “other monomer”).
- the content of other monomers is usually 0 to 95% by weight, preferably 0 to 80% by weight, particularly preferably 0 to 70% by weight, based on the polymer particles.
- content of the structural unit derived from another monomer is proportional to the preparation weight of the other monomer prepared in the case of manufacture of the emulsion which the binder composition for secondary battery negative electrodes of this invention has.
- the amount of the other monomer charged corresponds to the structure of the target polymer particles, and is usually 0 to 95% by weight, preferably 0 to 80% by weight, particularly preferably 0 to 70%, based on the polymer particles. % By weight.
- the glass transition temperature (hereinafter sometimes referred to as Tg) of the polymer particles is preferably ⁇ 40 to 60 ° C., particularly preferably ⁇ 35 to 57 ° C., more preferably ⁇ 30 to 55. ° C, particularly preferably -25 to 50 ° C. If the glass transition temperature is too low, emulsion polymerization tends to become unstable depending on the composition of the monomer used. If the glass transition temperature is too high, the binder film tends to brittlely break, and the cycle characteristics during charge and discharge are reduced. It tends to be easy to do.
- the glass transition temperature of the polymer particles can be controlled by a known method such as selection of an ethylenically unsaturated monomer constituting the polymer particles and adjustment of the polymerization ratio.
- Tg of a copolymer may become such a temperature range.
- the Tg (° C.) of the copolymer obtained when two or more kinds of monomers are used can be determined, for example, by the thermal differential analysis method described below.
- the Tg (° C.) of the copolymer in the present invention the value of the reversible heat flow at the 2nd cycle measured in the modulation mode is adopted.
- the PVA resin in the PVA resin aqueous solution in which the polymer particles are dispersed is a known general water-soluble PVA resin.
- a PVA resin is obtained by polymerizing and saponifying a vinyl ester monomer.
- the vinyl ester monomer includes vinyl acetate.
- vinyl acetate for example, vinyl propionate, vinyl butyrate, vinyl caproate, vinyl caprylate, vinyl caprate, vinyl laurate, vinyl myristate, vinyl palmitate, vinyl stearate, vinyl cyclohexanecarboxylate, piperine
- vinyl acid vinyl octylate, vinyl monochloroacetate, vinyl adipate, vinyl methacrylate, vinyl crotonate, vinyl sorbate, vinyl benzoate, vinyl cinnamate, and vinyl trifluoroacetate.
- vinyl acetate is preferably used.
- the polymerization of the vinyl ester monomer can be performed by any known polymerization method, for example, solution polymerization, suspension polymerization, emulsion polymerization and the like. Especially, it is preferable to perform the solution polymerization which can remove reaction heat efficiently under recirculation
- the solvent for the solution polymerization an alcohol is usually used, and a lower alcohol having 1 to 3 carbon atoms is preferably used.
- a conventionally known saponification method can be employed. That is, it can be carried out using an alkali catalyst or an acid catalyst in a state where the polymer is dissolved in alcohol or water / alcohol solvent.
- the alkali catalyst include alkali metal hydroxides and alcoholates such as potassium hydroxide, sodium hydroxide, sodium methylate, sodium ethylate, potassium methylate, and lithium methylate.
- an ester exchange reaction using an alkali catalyst in an anhydrous alcohol solvent is suitably used in terms of reaction rate and reduction of impurities such as fatty acid salts.
- the reaction temperature of the saponification reaction is usually 20 to 60 ° C. If the reaction temperature is too low, the reaction rate tends to decrease and the reaction efficiency tends to decrease. If it is too high, the reaction solvent may have a boiling point or higher, and the safety in production tends to decrease.
- saponification is performed under high pressure using a tower type continuous saponification tower having high pressure resistance, it is possible to saponify at a higher temperature, for example, 80 to 150 ° C., and a small amount of saponification catalyst. It is also possible to obtain a high saponification degree for a short time.
- the degree of saponification (measured in accordance with JIS K 6726) of the PVA-based resin is usually 80 to 100 mol%, particularly 85 to 99.9 mol%, more preferably 90 to 99.5 mol%. preferable. If the degree of saponification is too low, the protective colloid property of the PVA resin becomes too high, so that the emulsion viscosity becomes too high, or a cloud point appears in the PVA resin, for example, polymerization stability during emulsion polymerization There is a tendency that the intended emulsion is difficult to obtain due to extremely low properties.
- Viscosity average degree of polymerization is usually 50 to 2500, preferably 100 to 1700, particularly preferably 100 to 1300. If the viscosity average polymerization degree is too low, the protective colloid function to acrylic monomers and the like tends to be reduced. Conversely, if it is too high, the viscosity of the polymerization reaction solution becomes too high, and stirring during polymerization becomes difficult. It tends to be difficult to polymerize.
- the protective colloid property becomes too high, for example, the dropped monomer at the time of emulsion polymerization is hardly absorbed in the polymerized particles, and the generation of new particles derived from the dropped monomer increases, resulting in coarse particles in the emulsion. The amount tends to increase.
- the PVA-based resin has a structural unit derived from another monomer other than the vinyl ester-based monomer within a range not inhibiting the effects of the present invention (for example, 10 mol% or less, preferably 5 mol% or less). May be.
- monomers having a vinyl group and an epoxy group such as glycidyl (meth) acrylate, glycidyl (meth) allyl ether, 3,4-epoxycyclohexyl (meth) acrylate, allyl glycidyl ether; triallyloxyethylene, diallyl maleate, tri
- Monomers having two or more allyl groups such as allyl cyanurate, triallyl isocyanurate, tetraallyloxyethane, diallyl phthalate, triallyl cyanurate, triallyl isocyanurate, tetraallyloxyethane, diallyl phthalate; allyl acetate, acetoacetate Allyl ester
- Halogenated olefin Olefin monomer such as ethylene sulfonic acid; Diene such as butadiene-1,3,2-methylbutadiene, 1,3 or 2,3-dimethylbutadiene-1,3, 2-chlorobutadiene-1,3 Monomer: 3-bute Hydroxy group-containing ⁇ -olefins such as -1-ol, 4-penten-1-ol, 5-hexene-1,2-diol, glycerol monoallyl ether, and derivatives thereof such as acylated products thereof; 1,3-diacetoxy -Hydroxymethylvinylidene diacetates such as 2-methylenepropane, 1,3-dipropionyloxy-2-methylenepropane, 1,3-dibutyronyloxy-2-methylenepropane; itaconic acid, maleic acid, acrylic acid, etc.
- Olefin monomer such as ethylene sulfonic acid
- Diene such
- ⁇ - (meth) acryloxypropylalkyldialkoxysilane vinyltris ( ⁇ -methoxyethoxy) silane, and hydroxymethylvinylidene diacetate.
- hydroxymethylvinylidene diacetate include 1,3-diacetoxy-2-methylenepropane, 1,3-dipropionyloxy-2-methylenepropane, and 1,3-dibutyronyloxy-2-methylenepropane. Etc. These monomers may be used alone or in combination of two or more.
- the PVA resin of the present invention may contain a PVA resin modified within a range not inhibiting the effects of the present invention (usually 15 mol% or less, preferably 10 mol% or less).
- PVA-based resin include formalized products, acetalized products, acetoacetylated products, butyralized products, urethanized products, esterified products with sulfonic acids, carboxylic acids, and the like.
- a modified PVA resin modified with a derivative such as a hydroxy group-containing ⁇ -olefin and an acylated product thereof is particularly preferable to use as the PVA resin contained in the emulsion.
- modified PVA resin containing a structural unit having a primary hydroxyl group in the side chain as the modified PVA resin.
- the number of primary hydroxyl groups in such a structural unit is usually 1 to 5, preferably 1 to 2, and particularly preferably 1.
- it preferably has a secondary hydroxyl group.
- modified PVA resin containing a structural unit having a primary hydroxyl group in the side chain examples include, for example, a modified PVA resin having a 1,2-diol structural unit in the side chain, and a hydroxyalkyl group structural unit in the side chain.
- modified PVA-based resins examples include modified PVA-based resins.
- a modified PVA resin containing a 1,2-diol structural unit in the side chain represented by the following general formula (1) hereinafter referred to as “modified PVA resin containing a side chain 1,2-diol structural unit”: Is preferably used).
- R 1 to R 6 each independently represents a hydrogen atom or an organic group, and X represents a single bond or a bond chain.
- R 1 to R 6 each independently represents a hydrogen atom or an organic group.
- R 1 to R 6 are preferably all hydrogen atoms, but may be organic groups as long as the resin properties are not significantly impaired.
- the organic group is not particularly limited.
- an alkyl group having 1 to 4 carbon atoms such as a methyl group, an ethyl group, an n-propyl group, an isopropyl group, an n-butyl group, an isobutyl group, and a tert-butyl group is preferable.
- it may have a substituent such as a halogen group, a hydroxyl group, an ester group, a carboxylic acid group, or a sulfonic acid group.
- X is a single bond or a bonded chain, and is from the viewpoint of the resistance to electrolytic solution (that is, the property that swelling by the electrolytic solution hardly occurs) due to the reduction of free volume (intermolecular void) in the amorphous part It is preferably a single bond.
- the bonding chain is not particularly limited, and examples thereof include hydrocarbons such as alkylene, alkenylene, alkynylene, phenylene, and naphthylene (these hydrocarbons may be substituted with halogen such as fluorine, chlorine, and bromine).
- —O—, — (CH 2 O) m—, — (OCH 2 ) m—, — (CH 2 O) mCH 2 —, —CO—, —COCO—, —CO (CH 2 ) mCO— , —CO (C 6 H 4 ) CO—, —S—, —CS—, —SO—, —SO 2 —, —NR—, —CONR—, —NRCO—, —CSNR—, —NRCS—, — NRNR -, - HPO 4 -, - Si (OR) 2 -, - OSi (OR) 2 -, - OSi (OR) 2 O -, - Ti (OR) 2 -, - OTi (OR) 2 -, - OTi (OR) 2 O—, —Al (OR) —, —OAl (OR) —, —O Al (OR) O— and the like can be mentioned.
- Each R is independently a hydrogen atom or an arbitrary substituent, and is preferably a hydrogen atom or an alkyl group (particularly a C 1 -C 4 alkyl group).
- M is a natural number, preferably 1-10.
- an alkylene group having 6 or less carbon atoms, particularly a methylene group, or —CH 2 OCH 2 — is preferable from the viewpoint of viscosity stability during production, heat resistance, and the like.
- Such modified side chain 1,2-diol structural unit-containing modified PVA resin can be produced by a known production method. For example, it can be produced by the methods described in JP-A Nos. 2002-284818, 2004-285143, and 2006-95825.
- R 1 to R 6 are all the same as in the general formula (1).
- R 7 and R 8 are each independently hydrogen or R 9 —CO— (wherein R 9 is an alkyl group having 1 to 4 carbon atoms), and R 10 and R 11 are each independently A hydrogen atom or an organic group, and the organic group is the same as in the case of the general formula (1).
- the method (i) is preferable in that it is excellent in copolymerization reactivity and industrial handling.
- R 1 to R 6 are hydrogen
- X is a single bond
- R 7 and R 8 are R 9-.
- Preferred is 3,4-diasiloxy-1-butene, which is CO— and R 9 is an alkyl group.
- 3,4-diacetoxy-1-butene in which R 9 is a methyl group is particularly preferred.
- the side chain 1,2-diol structural unit-containing modified PVA resin obtained by the method (ii) or (iii) has a low degree of saponification, or insufficient decarboxylation or deacetalization. In some cases, a carbonate ring or an acetal ring may remain in the side chain.
- a PVA resin is used as a dispersant, the resulting polymer particles tend to increase the proportion of coarse particles. For this reason, the PVA resin obtained by the method (i) is particularly suitable for this application.
- the content of the PVA-based resin is grafted to the ethylenically unsaturated polymer particles as a dispersoid during emulsion polymerization, electrolysis solution resistance, emulsion From the standpoint of standing stability, etc., it is usually 0.5 to 15 mol%, preferably 1 to 10 mol%, particularly preferably 1 to 8 mol%. If the content is too small, the grafting rate of the PVA resin to the ethylenically unsaturated polymer particles as the dispersoid tends to decrease, and the electrolytic solution resistance, the standing stability of the emulsion, etc. tend to decrease.
- the content of the 1,2-diol structural unit in the PVA resin is determined from the 1 H-NMR spectrum (solvent: DMSO-d6, internal standard: tetramethylsilane) of the PVA resin having a saponification degree of 100 mol%. Can be sought. Specifically, it can be calculated from the peak area derived from the hydroxyl proton, methine proton, and methylene proton in the 1,2-diol structural unit, the main chain methylene proton, the hydroxyl proton linked to the main chain, and the like.
- the solvent of the PVA-based resin aqueous solution is water.
- a solvent may contain an organic solvent miscible with water as long as the dissolution of the PVA-based resin is not inhibited (for example, 20% by weight or less, preferably 10% by weight or less of the solvent).
- the organic solvent include amide solvents such as N-methylpyrrolidone (NMP), dimethylformamide, dimethylacetamide, and methylformamide, sulfoxides such as dimethyl sulfoxide, lower ones having 1 to 3 carbon atoms such as methanol and ethanol.
- examples include alcohols and alcohol solvents such as 1,1,1,3,3,3-hexafluoro-2-propanol.
- the emulsion in which the polymer particles derived from the ethylenically unsaturated monomer in the present invention are dispersed in the PVA-based resin aqueous solution is obtained by dispersing the polymer particles in the PVA-based resin aqueous solution.
- such an emulsion is an emulsion in which the polymer particles are dispersed with good dispersibility in a dispersion medium when an ethylenically unsaturated monomer is used as a dispersoid and a known dispersant is used and emulsion polymerization is performed using the solvent as a dispersion medium. Can be obtained efficiently.
- the PVA resin to be added later in the prepared emulsion composition (for convenience, it may be referred to as a second PVA resin)
- a known PVA resin is used in the same manner as the PVA resin as the dispersant. It is possible.
- modified PVA-type resin similarly to the said PVA-type resin which is a dispersing agent also in 2nd PVA-type resin. It is preferable that the second PVA-based resin contains the above 1,2-diol structural unit from the viewpoint of improving the stability of the emulsion.
- PVA-type resin (it may be called 1st PVA-type resin for convenience) which is a dispersing agent of a polymer particle, and PVA-type resin (1st PVA-type resin aqueous solution of emulsion). It is also possible to use different types of PVA resins.
- these PVA resins become completely compatible with each other (sea-sea structure) to form a uniform phase, or to form a sea-island structure. Become.
- the domain size is usually 0 to 15 mol%, preferably 0 to 10 mol%.
- the saponification degree of the second PVA resin is preferably 85 to 100 mol%, particularly preferably 90 to 99.9 mol%. If the degree of saponification is too low, the second PVA-based resin tends to swell with respect to the electrolytic solution, and as a result, the binding property between the electrode members tends to decrease.
- the weight of the PVA resin of the present invention refers to the total weight of the first PVA resin and the second PVA resin.
- Examples of the method for carrying out the emulsion polymerization include: i) mixing the ethylenically unsaturated monomer as a dispersoid at once or continuously in the presence of water, a dispersant and a polymerization catalyst, and heating and stirring. And ii) a dispersion in which an ethylenically unsaturated monomer is mixed and dispersed in a dispersion medium, and the prepared dispersion is mixed with water, a dispersant, and a polymerization catalyst. Examples thereof include a method of emulsion polymerization by mixing at once or continuously, heating and stirring.
- Such a method using a dispersion prepared in advance is particularly referred to as a pre-emulsion method.
- Such a method is preferable because emulsion polymerization can be carried out while maintaining productivity even if the monomer composition to be polymerized is complex.
- the dispersion medium in the reaction solution used for the emulsion polymerization is usually water.
- an organic solvent that can be mixed with water mentioned in the above solvent can be used in combination with water.
- water alone is preferred.
- an emulsifying dispersant such as a surfactant or a water-soluble polymer protective colloid agent added during the preparation of the emulsion
- a method for controlling the addition conditions of the monomer and the polymerization catalyst (3) a method for controlling the design non-volatile content of the emulsion, and the like.
- a method of controlling the size, stirring speed, and stirring time of the mixing stirring blade of the polymerization apparatus can be employed.
- a membrane emulsification method in which the particle diameter is controlled by passing a monomer through a porous membrane, an ultrasonic emulsification method using ultrasonic waves as a stirring method, or the like can also be employed.
- the blending amount varies somewhat depending on the type of PVA resin used, the concentration of the emulsion to be synthesized, etc. It is 0.1 to 80% by weight, preferably 10 to 70% by weight, particularly preferably 20 to 60% by weight, and further preferably 20 to 57% by weight. If the blending amount of the PVA resin is too small, the emulsified state of the ethylenically unsaturated monomer becomes unstable, the polymerization reactivity is lowered, or the emulsion state stability of the particles in the emulsion obtained by polymerization Tend to decrease.
- a polymerization catalyst usually used in the field of emulsion polymerization can be used.
- water-soluble redox polymerization catalysts such as Rongalite-iron salt, and these may be used alone or in combination of two or more.
- a catalyst comprising an organic peroxide and a redox system such as “Kayabutyl B” manufactured by Kayaku Akzo Co., Ltd. or “Kayabutyl A-50C” manufactured by Kayaku Akzo may be used.
- the amount of the polymerization catalyst used is usually 0.01 to 10 parts by weight, preferably 0.05 to 5 parts by weight, particularly preferably 0.1 to 3 parts by weight, based on 100 parts by weight of the monomer used for the polymerization. Part. If the amount of the polymerization initiator used is too small, the polymerization rate tends to be slow, and conversely if too large, the polymerization stability tends to decrease. In addition, there is no restriction
- Emulsion polymerization may be performed in one stage or may be performed in two or more stages.
- the following two-stage polymerization is exemplified.
- First-stage polymerization step A part of the monomer to be polymerized is charged into a reaction vessel containing a dispersion medium and a dispersant, and the first-stage emulsion polymerization is performed.
- the amount of the monomer added to the first stage is not particularly limited, but is usually about 1 to 50% by weight, preferably 5 to 30% by weight of the monomer used for the polymerization. What is necessary is just to determine suitably the conditions of the emulsion polymerization process of the 1st step
- the temperature of the emulsion polymerization reaction is usually 30 to 90 ° C., preferably 40 to 80 ° C., and the polymerization time is preferably 1 to 4 hours.
- the polymerization conversion rate is preferably 30% or more, and particularly preferably 60% or more.
- the second-stage emulsion polymerization is carried out by introducing the remaining monomers into the reaction vessel after the first-stage polymerization.
- the charging is preferably performed while dropping.
- a polymerization catalyst may be added in the second stage polymerization.
- the second stage emulsion polymerization is carried out under conditions where the polymerization temperature is 40 to 80 ° C. and the polymerization time is 1 to 6 hours. It is also possible to use a power feed polymerization method in which the dropping monomer composition ratio is continuously changed.
- the polymerization may be performed while dropping a dispersion obtained by mixing and dispersing monomers in the presence of a PVA resin as a dispersant. If necessary, it is also possible to carry out a follow-up polymerization usually after this step for 1 to 6 hours.
- a polymerization catalyst may be added during such polymerization.
- a molecular weight regulator may be included as necessary.
- the molecular weight regulator include, for example, alkyl mercaptans such as n-hexyl mercaptan, n-octyl mercaptan, t-octyl mercaptan, n-dodecyl mercaptan, t-dodecyl mercaptan, n-stearyl mercaptan; dimethylxanthogen disulfide, Xanthogen compounds such as diisopropylxanthogen disulfide; thiuram compounds such as terpinolene, tetramethylthiuram disulfide, tetraethylthiuram disulfide, tetramethylthiuram monosulfide; 2,6-di-t-butyl-4-methylphenol, styrenated phenol, etc.
- Phenolic compounds such as allyl alcohol; halogenated hydrocarbon compounds such as dichloromethane, dibromomethane, carbon tetrabromide; ⁇ -benzyl Vinyl ethers such as xylstyrene, ⁇ -benzyloxyacrylonitrile, ⁇ -benzyloxyacrylamide; triphenylethane, pentaphenylethane, acrolein, methacrolein, thioglycolic acid, thiomalic acid, 2-ethylhexylthioglycolate, ⁇ -methylstyrene dimer, And carbon tetrachloride.
- these molecular weight regulators can be used alone or in combination of two or more.
- a surfactant such as a nonionic surfactant or an anionic surfactant is incorporated into the system, in addition to the dispersant previously contained, as long as the dispersion stabilizing effect of the PVA resin is not inhibited. You may coexist.
- the blending amount of such a surfactant is usually 10% by weight or less, preferably 5% by weight or less, based on the whole emulsion polymerization reaction system.
- Nonionic surfactants include, for example, polyoxyethylene-alkyl ether type, polyoxyethylene-alkylphenol type, polyoxyethylene-polyhydric alcohol ester type, ester of polyhydric alcohol and fatty acid, oxyethylene oxypropylene Examples include block polymers.
- anionic surfactant examples include higher alcohol sulfate, higher fatty acid alkali salt, polyoxyethylene alkylphenol ether sulfate, alkylbenzene sulfonate, naphthalene sulfonate formalin condensate, alkyl diphenyl ether sulfonate, dialkyl sulfosuccinic acid.
- examples thereof include salts and higher alcohol phosphate esters.
- plasticizers such as phthalate esters and phosphate esters, pH adjusters such as sodium carbonate, sodium acetate, and sodium phosphate may be used in combination.
- an emulsion in which polymer particles derived from an ethylenically unsaturated monomer are dispersed in an aqueous dispersion medium can be obtained.
- the resulting emulsion has a solid content (corresponding to the above-mentioned particle amount) of usually 10 to 60% by weight, preferably 20 to 58% by weight, particularly preferably 30 to 55% by weight, more preferably 35%. ⁇ 53 wt%.
- a value obtained by measuring the residue dried by heating at 105 ° C. for 3 hours in a dryer is adopted.
- the viscosity of the obtained emulsion is usually 100 to 20000 mPa ⁇ s, preferably 300 to 10000 mPa ⁇ s, particularly preferably 450 to 8000 mPa ⁇ s.
- the value measured with the B-type viscometer is employ
- the content of the polymer particles in the binder composition for a negative electrode of the secondary battery of the present invention is usually 1 to 40% by weight, preferably 5 to 35% by weight, particularly preferably 10 to 30% by weight in terms of solid content. If the content of polymer particles is too small, the internal resistance as a binder increases, the film is not flexible enough, and the capacity drops due to the active material falling off or cracking of the electrode with charge / discharge. Tend to. If the content of polymer particles is too high, it will be difficult to ensure a sufficient elastic modulus for the film. Therefore, particularly in lithium ion secondary batteries using alloy-based active materials, the initial discharge capacity and Coulomb efficiency will be low. There exists a tendency for it to fall or cycling characteristics to fall.
- the content of the PVA resin in the binder composition for a negative electrode of the secondary battery of the present invention is usually 60 to 99% by weight, preferably 65 to 90% by weight, particularly preferably 70 to 85% by weight, based on the solid content. . If the content of the PVA resin is too small, it will be difficult to ensure a sufficient elastic modulus for the film. Therefore, in the lithium ion secondary battery using an alloy-based active material, the initial discharge capacity and the Coulomb efficiency are low. There exists a tendency for it to fall or cycling characteristics to fall. If the PVA-based resin content is too high, the internal resistance as a binder increases, the film is not flexible enough, and the active material falls off and the electrode cracks with charge / discharge. Tends to decrease.
- the ratio of the PVA resin / polymer particles in the binder composition for a negative electrode of the secondary battery of the present invention is usually 60/40 to 99/1, preferably 65/35 to 95 / in terms of the weight ratio of the solid content. 5, particularly preferably 65/35 to 92/8, particularly preferably 65/35 to 85/15. If the ratio of the PVA resin / polymer particles is too low, it becomes difficult to ensure a sufficient elastic modulus for the film. Therefore, particularly in a lithium ion secondary battery using an alloy-based active material, Coulomb efficiency is lowered and cycle characteristics are lowered.
- the ratio is too high, the internal resistance as a binder increases, and the flexibility of the film is insufficient, and there is a tendency for the capacity to decrease due to the active material falling off or cracking of the electrode with charge and discharge, In addition, industrially, the viscosity of the binder becomes high and handling becomes difficult.
- the emulsion as described above may be directly used in the production of the negative electrode binder composition of the present invention, or in order to adjust the solid content of the emulsion contained in the binder composition, the viscosity of the obtained emulsion, etc.
- Water-soluble polymers other than resin may be added as appropriate.
- water-soluble polymers other than PVA resins include cellulose derivatives such as methyl cellulose, ethyl cellulose, hydroxymethyl cellulose, hydroxypropyl methyl cellulose, hydroxybutyl methyl cellulose, hydroxyethyl cellulose, carboxymethyl cellulose, aminomethyl hydroxypropyl cellulose, aminoethyl hydroxypropyl cellulose, and the like.
- Starch tragacanth, pectin, glue, alginic acid or salt thereof; gelatin; polyvinylpyrrolidone; polyacrylic acid or salt thereof polymethacrylic acid or salt thereof; polyacrylamide, acrylamide of polymethacrylamide sugar; vinyl acetate and maleic acid; With unsaturated acids such as maleic anhydride, acrylic acid, methacrylic acid, itaconic acid, fumaric acid, crotonic acid Polymer; Copolymer of styrene and the above unsaturated acid; Copolymer of vinyl ether and the above unsaturated acid; and Salt or ester of the unsaturated acid and each copolymer, Carrageenan, Xanthan gum, Sodium hyaluronate Natural polysaccharides such as locust bean gum, tara gum, guar gum and tamarind seed gum, and cellulose derivatives are preferred.
- unsaturated acids such as maleic anhydride, acrylic acid, methacrylic acid, itac
- the average particle diameter of the polymer particles in the present invention is 50 nm or more and 600 nm or less. Preferably, it is 200 to 500 nm.
- the average particle size of the particles is the average particle size of the volume distribution measured with a dynamic light scattering type particle size distribution measuring instrument for 1 minute after the ultrasonic irradiation treatment, for a measurement time of 3 minutes, and with a cumulative number of 5. adopt.
- the binder composition for secondary battery negative electrode of this invention contains the said emulsion.
- a PVA resin is blended separately from the PVA resin as a dispersant in the emulsion contained in the binder composition.
- a structural unit having a primary hydroxyl group in the side chain, particularly a modified PVA resin containing a side chain 1,2-diol structural unit has low crystallinity, so it is separately added to an aqueous solution of PVA resin in an emulsion and primary in the side chain.
- modified PVA resin containing a structural unit having a hydroxyl group, particularly a side chain 1,2-diol structural unit it is possible to impart viscosity stability of a water paste that is a binder composition for an electrode. , Work efficiency can be improved.
- a coating agent used for a coating film or a compounding agent used for a molding resin can be blended.
- the organic content of the compounding agent to contain is contained in solid content of a binder composition.
- the amount of the compounding agent is usually 10 parts by weight or less, preferably 5 parts by weight or less with respect to 100 parts by weight of the solid content of the emulsion in the binder composition.
- the above resin is a resin that can be charged and discharged even in a lithium ion secondary battery using an electrolyte solution that is difficult to form a stable SEI film such as PC (propylene carbonate), that is, a resin having a SEI complementary function.
- a lithium ion secondary battery using a PC-based electrolytic solution can be expected to have a great effect in terms of durability and safety of the negative electrode, use in a cold region, and the like by using this resin.
- the secondary battery negative electrode of the present invention contains at least the binder composition for secondary battery negative electrode of the present invention and an alloy-based active material for negative electrode.
- the secondary battery negative electrode of the present invention is usually produced by mixing a binder composition and an alloy-based active material to prepare a secondary battery negative electrode slurry, and applying and drying the slurry on a current collector. be able to.
- the secondary battery negative electrode of the present invention can be applied to various secondary batteries, such as lithium ion secondary batteries, lithium ion polymer secondary batteries, lead storage batteries, nickel / hydrogen storage batteries, nickel / cadmium storage batteries, nickel -It can be applied to iron storage batteries, nickel / zinc storage batteries, silver oxide / zinc storage batteries, sodium batteries, air aluminum batteries, and the like.
- a lithium ion secondary battery will be described as an example.
- the active material used for the secondary battery negative electrode is not particularly limited as long as it contains an element capable of forming an alloy with lithium. Specifically, tin, aluminum, silicon, bismuth, zinc, arsenic, antimony, And at least one element selected from the group consisting of lead and the like can be used.
- the negative electrode active material in the present invention can be used in a powder state, but may be composed of a single element, or may be an oxide or an alloy thereof. Among these, silicon and tin, and oxides and alloys containing these are preferable, and oxides and alloys containing silicon are particularly preferable because they can be alloyed with a large amount of lithium per unit weight.
- Examples of the alloy containing silicon or tin include a silicon-titanium alloy, a silicon-nickel alloy, a tin-iron alloy, a tin-nickel alloy, a tin-copper alloy, a tin-zinc alloy, and a tin-titanium alloy.
- the content of the active material in the slurry is 10 to 95% by weight, preferably 20 to 80% by weight, particularly preferably 35 to 65% by weight.
- the average particle diameter of the active material is usually 5 nm to 100 ⁇ m, preferably 20 nm to 50 ⁇ m, and particularly preferably 50 nm to 25 ⁇ m.
- the value measured by the laser diffraction type particle size distribution measurement (laser diffraction scattering method) shall be employ
- the content ratio of the active material and the binder composition in the negative electrode slurry is usually 0.1 to 20 parts by weight, preferably 0, in terms of solid content of the binder composition with respect to 100 parts by weight of the active material. .1 to 15 parts by weight, particularly preferably 0.1 to 10 parts by weight.
- the content of the negative electrode binder composition is too large, the internal resistance tends to increase.
- the amount is too small, the desired binding force between the active materials and the adhesive force to the current collector cannot be obtained, the electrode becomes unstable, and the charge / discharge cycle characteristics tend to deteriorate.
- the slurry for negative electrode may contain other materials in addition to the alloy-based active material and the binder composition for negative electrode.
- examples of other substances include a conductive additive, a supporting salt (lithium salt), and an ion conductive polymer.
- a polymerization initiator for polymerizing the polymer may be included.
- the compounding ratio of these components is a known general range. The blending ratio can also be adjusted by appropriately referring to known knowledge about lithium ion secondary batteries.
- “Conductive auxiliary agent” refers to a compound that is blended to improve conductivity.
- the conductive assistant include carbon powder such as graphite, and various carbon fibers such as acetylene black, ketjen black, vapor grown carbon fiber (VGCF (registered trademark)), and super gloss nanotube.
- VGCF vapor grown carbon fiber
- the blending amount of the conductive assistant is preferably 1 to 20% by weight, particularly preferably 1 to 10% by weight, based on the total mass of the active material layer.
- a slurry for the negative electrode may be prepared by adding a solvent for the purpose of adjusting the viscosity and adjusting the solid content of the binder composition.
- a solvent for the purpose of adjusting the viscosity and adjusting the solid content of the binder composition.
- the same organic solvent as described above can be used.
- a thickener is added separately from the binder composition for the purpose of improving the dispersibility of the active material, the negative electrode binder composition and the conductive auxiliary agent, or improving the leveling property during coating. Also good.
- the type of the thickening agent is not particularly limited, but mainly a water-soluble polymer is preferred because it is miscible with the PVA-based resin and water is preferably used as a dispersion medium for the emulsion composition. Used for.
- the water-soluble polymer in the present invention excludes not only PVA resins having a structural unit having a primary hydroxyl group in the side chain, but also other PVA resins.
- water-soluble polymers other than PVA resins include cellulose derivatives such as methyl cellulose, ethyl cellulose, hydroxymethyl cellulose, hydroxypropyl methyl cellulose, hydroxybutyl methyl cellulose, hydroxyethyl cellulose, carboxymethyl cellulose, aminomethyl hydroxypropyl cellulose, aminoethyl hydroxypropyl cellulose, and the like.
- Starch tragacanth, pectin, glue, alginic acid or salt thereof; gelatin; polyvinylpyrrolidone; polyacrylic acid or salt thereof, polymethacrylic acid or salt thereof; acrylamides of polyacrylamide or polymethacrylamide sugar; vinyl acetate and maleic acid , Maleic anhydride, acrylic acid, acrylic acid, methacrylic acid, itaconic acid, fumaric acid, crotonic acid, etc.
- the amount of the thickener used in the negative electrode slurry is usually 0.01 to 5% by weight, preferably 0.1 to 4% by weight, particularly preferably 0.5 to 4% by solid content of the negative electrode slurry. % By weight. If the amount used for the slurry is too small, the dispersion stability of the active material, the negative electrode binder, and the conductive auxiliary agent is lowered, and the electrode becomes non-uniform and stable charge / discharge tends to be difficult to obtain. . On the other hand, if the amount is too large, the viscosity of the slurry for the negative electrode becomes too high, which tends to make it difficult to uniformly coat the current collector when producing the negative electrode. There is a tendency that the internal resistance increases and the charge / discharge capacity decreases.
- the binder composition for the negative electrode For the mixing of the binder composition for the negative electrode, the active material, the compounding agent used as necessary, and the solvent, a stirrer, a defoamer, a bead mill, a high-pressure homogenizer, or the like can be used.
- the negative electrode slurry prepared as described above is applied onto a current collector and dried to produce the secondary battery negative electrode of the present invention (hereinafter sometimes abbreviated as “the negative electrode of the present invention”). can do. If necessary, the density can be adjusted by pressing after coating.
- those used as the negative electrode current collector of a lithium ion secondary battery can be used.
- a metal foil such as copper, SUS, or nickel, an etching metal foil, an expanded metal, or the like is used, and can be appropriately selected and used according to the type of the target power storage device.
- a negative electrode layer can be formed by applying and drying a negative electrode slurry on such a current collector.
- the method for applying the negative electrode slurry to the current collector include a doctor blade method, a reverse roll method, a comma bar method, a gravure method, and an air knife method.
- the treatment temperature is usually 20 to 180 ° C., preferably 50 to 150 ° C.
- the treatment time is usually 1 to 120 minutes, preferably 5 to 60 minutes.
- the thickness of the active material layer (the thickness of one surface of the coating layer) is usually 1 to 300 ⁇ m, preferably 2 to 100 ⁇ m, particularly preferably 2 to 30 ⁇ m.
- the negative electrode obtained using the binder composition for secondary battery negative electrode of the present invention is excellent in the dispersibility of the alloy active material and the binding property between the active material and the current collector. The effect that it becomes difficult to generate
- the lithium ion secondary battery has at least a positive electrode, a negative electrode, an electrolytic solution, and a separator.
- an aprotic polar solvent that dissolves a lithium salt is used.
- low-chain solvents such as dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate, which are low-viscosity solvents, are contained in cyclic carbonate high dielectric constant / high boiling point solvents such as ethylene carbonate and propylene carbonate. Used.
- lithium salt of the electrolyte examples include inorganic salts such as LiClO 4 , LiPF 6 , LiBF 4 , LiAsF 6 , LiCl, LiBr, LiCF 3 SO 3 , LiN (SO 2 CF 3 ) 2 , LiN (SO 2 C 2 F 5 ) 2 , organic salts such as LiC (SO 2 CF 3 ) 3 , LiN (SO 3 CF 3 ) 2, etc., which are commonly used as electrolytes for non-aqueous electrolytes may be used.
- LiPF 6 , LiBF 4 or LiClO 4 is preferably used.
- the separator is not particularly limited, and a nonwoven fabric of polyolefin, a porous film, a glass filter, a polyaramid film, a non-woven fabric or the like can be used from a PVA resin.
- a well-known general positive electrode can be combined.
- the positive electrode active material for example, olivine-type lithium iron phosphate, lithium cobaltate, lithium manganate, lithium nickelate, ternary nickel cobalt lithium manganate, lithium nickel cobalt aluminum composite oxide, or the like can be used.
- Examples of the positive electrode current collector include metal materials such as aluminum, copper, nickel, tantalum, stainless steel, and titanium, and can be appropriately selected and used according to the type of the target power storage device.
- the structure of the secondary battery is not particularly limited, and can be applied to any conventionally known form / structure such as a stacked (flat) battery or a wound (cylindrical) battery.
- the electrical connection form (electrode structure) in the lithium ion secondary battery can be applied to both non-bipolar (internal parallel connection type) batteries and bipolar (internal series connection type) batteries.
- the lithium ion secondary battery obtained as described above is based on the use of the negative electrode binder composition of the present invention, and the dispersibility of the alloy-based active material and the binding property between the alloy-based active material and the current collector. In addition, since the active material is less likely to be peeled off or dropped off, a high charge / discharge capacity can be stably obtained, and the charge / discharge cycle characteristics are excellent.
- the reaction temperature was lowered to 50 ° C., and the polymerization was conducted for 1 hour.
- 2.8 parts of a t-butyl hydroperoxy aqueous solution (10%) and 3.4 parts of an L-ascorbic acid aqueous solution (10%) were divided into two parts and blended every 30 minutes. Thereafter, it is cooled to room temperature, and a mixed composition of butyl acrylate (BA) and styrene (St) in a modified PVA resin aqueous solution containing a side chain 1,2-diol structural unit having the structure shown in the structural formula (1a).
- BA butyl acrylate
- St styrene
- An emulsion in which the polymer particles were dispersed was obtained.
- the solid content of the emulsion was 29.6%, and the weight ratio of the PVA resin to the polymer particles was 39/61.
- the second stage emulsion polymerization was performed. While maintaining the temperature of the reaction system in which the first stage emulsion polymerization was carried out in the range of 75 ° C. to 80 ° C., 180.0 parts of the mixed monomer having the same composition as the above was added as the second stage emulsion polymerization monomer for 3.5 hours. It was dripped over. During the dropping, 26.39 parts of ammonium persulfate aqueous solution (1%) was divided into 14 portions and blended every 15 minutes. Thereafter, the polymerization was continued for 90 minutes while maintaining the temperature at 75 ° C. During this time, 4.4 parts of an ammonium persulfate aqueous solution (1%) was divided into two portions and blended every 45 minutes.
- the reaction temperature was lowered to 50 ° C., and the polymerization was conducted for 1 hour.
- 2.0 parts of an aqueous t-butyl hydroperoxy solution (10%) and 2.4 parts of an L-ascorbic acid aqueous solution (10%) were divided into two parts and blended every 30 minutes. Thereafter, the mixture is cooled to room temperature, and mixed with butyl acrylate (BA) and methyl methacrylate (MMA) in a modified PVA resin aqueous solution containing a side chain 1,2-diol structural unit having the structure shown in the structural formula (1a).
- BA butyl acrylate
- MMA methyl methacrylate
- An emulsion in which polymer particles of the composition were dispersed was obtained.
- the solid content of the emulsion was 29.9%, and the weight ratio of the PVA resin to the polymer particles was 56/44.
- Polymerization Example 4 Polymerization of Base Emulsion
- the PVA resin used as the dispersant was changed (saponification degree: 99.1 mol%, viscosity average polymerization degree: 600, 1,2-diol structural unit content shown in the above structural formula (1a): 8 mol%).
- BA butyl acrylate
- St styrene
- a base emulsion was obtained in the same manner as above.
- the second stage emulsion polymerization was performed. While maintaining the temperature of the reaction system in which the first stage emulsion polymerization was carried out in the range of 75 ° C. to 80 ° C., 180.0 parts of the mixed monomer having the same composition as the above was added as the second stage emulsion polymerization monomer for 3.5 hours. It was dripped over. During the dropping, 26.39 parts of ammonium persulfate aqueous solution (1%) was divided into 14 portions and blended every 15 minutes. Thereafter, the polymerization was continued for 90 minutes while maintaining the temperature at 75 ° C. During this time, 4.4 parts of an ammonium persulfate aqueous solution (1%) was divided into two portions and blended every 45 minutes.
- the reaction temperature was lowered to 50 ° C., and the polymerization was conducted for 1 hour.
- 2.0 parts of an aqueous t-butyl hydroperoxy solution (10%) and 2.4 parts of an L-ascorbic acid aqueous solution (10%) were divided into two parts and blended every 30 minutes. Thereafter, it is cooled to room temperature, and a mixed composition of butyl acrylate (BA) and styrene (St) in a modified PVA resin aqueous solution containing a side chain 1,2-diol structural unit having the structure shown in the structural formula (1a).
- An emulsion in which the polymer particles were dispersed was obtained.
- the solid content of the emulsion was 29.7%, and the weight ratio of the PVA resin to the polymer particles was 56/44.
- a material diluted to 10% in advance was used.
- Example 1 Production of electrode
- ⁇ Preparation of battery negative electrode using silicon active material 65 parts of silicon powder (distributor: Alfa Aesar, average particle size: 50 nm) as an active material, 15 parts of acetylene black (“DENKA BLACK” manufactured by Denki Kagaku Kogyo Co., Ltd.) as a conductive auxiliary agent, and 1.72 as a dispersant.
- 3 parts of carboxymethylcellulose # 2260 manufactured in an aqueous solution, and after adding purified water in a timely manner, a planetary kneader (Shinky Co., Ltd.
- “Ryotaro Awatori”) was used to obtain a paste having a solid content concentration of 28.0% (mixed at 2000 rpm for 4.5 minutes and then defoamed at 2200 rpm for 0.5 minutes).
- 17 parts of the binder solution (10%) prepared in Production Example 1 as a negative electrode binder was added in terms of solid content, and purified water was added in a timely manner, followed by using a planetary kneader. By mixing under the conditions, an active material paste having a solid content concentration of 19.8% was obtained.
- a 25 ⁇ m applicator and a coating machine (“Control Coater (Coating Machine)” manufactured by Imoto Seisakusho Co., Ltd.) are applied to the surface of rolled copper foil (UACJ foil, thickness 18 ⁇ m) as a current collector.
- the active material paste was applied at a coating speed of 10 mm / second. This was dried at 80 ° C. for 2 hours, followed by vacuum drying at 120 ° C. for 4 hours to obtain a battery electrode.
- an electrolytic solution a solution in which ethylene carbonate and diethylene carbonate were mixed at a volume ratio of 3: 7, and LiPF6 as an electrolyte was dissolved at a concentration of 1 mol / liter was used.
- a stainless steel cap was put on and fixed to the outer layer container via a polypropylene packing, and a battery can was sealed to prepare a half cell, which was used as a battery for evaluation.
- Capacity retention rate (%) (discharge capacity at the fifth cycle) / (initial discharge capacity) ⁇ 100 The closer this value is to 100%, the less the electrode is deteriorated due to charging / discharging, indicating that the battery is operating stably.
- the battery performance evaluation results obtained are shown in Table 1.
- Example 2 A negative electrode was produced in the same manner as in Example 1 except that the binder solution prepared in Production Example 2 was used as the negative electrode binder, and battery performance was evaluated.
- Example 3 A negative electrode was produced in the same manner as in Example 1 except that the binder solution prepared in Production Example 3 was used as the negative electrode binder, and battery performance was evaluated.
- Example 2 A negative electrode was prepared in the same manner as in Example 1 except that the base emulsion prepared in Polymerization Example 1 was used as the binder solution for the electrode, and the battery performance was evaluated. At that time, the base emulsion diluted to 10% in advance was used as the binder solution.
- Example 4 Evaluation of film properties
- the binder solution obtained in Production Example 5 was cast on a polyethylene terephthalate film so as to have a film thickness of a designed value of 100 ⁇ m. Thereafter, the mixture was allowed to stand for 48 hours in an environment of 23 ° C. ⁇ 50% RH, and then dried for 1 hour with a blow dryer at 80 ° C. Further, vacuum drying was performed for 48 hours in the presence of phosphorus pentoxide in a vacuum dryer at 80 ° C.
- TGA thermogravimetric analyzer
- Example 2 In the same manner as in Example 1, a negative electrode for a battery was prepared using a silicon active material, and a battery performance was evaluated by performing a charge / discharge test. The obtained battery performance evaluation results are shown in Table 2.
- Example 5 A negative electrode was produced in the same manner as in Example 4 except that the binder solution prepared in Production Example 6 was used as the negative electrode binder, and battery performance was evaluated.
- Example 6 A negative electrode was prepared in the same manner as in Example 4 except that the binder solution prepared in Production Example 7 was used as the negative electrode binder, and battery performance was evaluated.
Abstract
Description
しかしながら、合金系活物質は一般に黒鉛系活物質に比べて体積膨張がより大きく、例えば、合金系活物質として公知のケイ素(Si)活物質は、充放電に伴って約400%もの体積変化を起こすことが知られている。このような体積変化の為、活物質がより電極から剥がれ易くなるので、黒鉛系活物質で使用されていた従来のポリビニルアルコール系樹脂含有エマルジョンを含むバインダーでは安定した高充電容量を得ることが困難であり、また充放電のサイクル特性も必ずしも満足できるものではなかった。
また、本発明の二次電池負極用バインダー組成物における前記重合体粒子のガラス転移温度を特定の範囲に調整することによって、充放電の繰り返し等に伴った負極の体積変化に更に追随でき、更に安定な負極活物質層を形成することができるので、充放電のサイクル特性が更に優れる二次電池の製造が可能となる。
本バインダー組成物は、ポリビニルアルコール系樹脂と重合体粒子とを含有するエマルジョンを含む樹脂組成物であるが、このポリビニルアルコール系樹脂の成分と重合体粒子の成分の比率を特定の範囲内で任意に制御することによって、皮膜の弾性率や柔軟性、さらには金属密着性などを制御することができる。例えば、ポリビニルアルコール系樹脂の成分を多くすることで高弾性皮膜に設計したり、一方で重合体粒子の成分を多くすることで柔軟性の高い皮膜を設計したりすることも可能である。
また、この重合体粒子のガラス転移温度を特定の範囲に調整することによって、皮膜の弾性率や柔軟性、さらには金属密着性などを制御することができる。
しかしながら一方で、合金系活物質は黒鉛系活物質と比べて充放電に伴う体積変化が大き過ぎる為、柔軟なバインダーでは電極の構造変化に完全には追随することが難しい。
そこで活物質が体積変化した場合に、そもそも電極構造を大きく変動させない程度の高い弾性率を担保しつつ、電極の変動に伴うバインダーの割れや活物質の脱落を抑制できるような柔軟性を併せ持った、合金系活物質を使用する際に好適な組成の樹脂設計を行うことで、本発明の完成に至った。
以上の技術思想より、従来の一般的なバインダー設計思想とは異なり、合金系活物質を負極に用いる際には、意外にも、ポリビニルアルコール系樹脂の比率の高い方がより安定的に高い充放電容量を示し、且つ充放電のサイクル特性に優れる電池が得られるようになることがわかった。
但し、ポリビニルアルコール系樹脂の比率を高くすることによってバインダー皮膜の柔軟性が低下して脆性破壊しやすくなり、電池を作製する段階や充放電に伴う体積変化によって負極活物質層の脱落が発生して電池容量が低下してしまうことが起こり得る。
そこで重合体粒子のガラス転移温度を特定の範囲に調整することによってバインダーの皮膜強度が高くなり、負極活物質層が負極の体積変化に対して更に追随できるようになる為、結果として更に安定的に高い充放電容量を示し、且つ充放電のサイクル特性に更に優れる電池が得られるようになることがわかった。
本発明において固形分とは、対象物を105℃、3時間の乾燥減量法に供することにより得られるものを意味する。
本発明の二次電池負極用バインダー組成物は、リチウムと合金を形成し得る元素を活物質として含有する二次電池負極を作製するためのバインダー組成物であって、エチレン性不飽和単量体に由来する重合体粒子がポリビニルアルコール系樹脂水溶液中に分散されたエマルジョンを含むバインダー組成物である。
重合体粒子における重合体は、エチレン性不飽和単量体の重合体である。エチレン性不飽和単量体としては、例えば、下記の(a)~(m)等が挙げられる。これらは単独で、もしくは2種以上併せて用いられる。
(a)(メタ)アクリル酸アルキルエステル。
(b)ヒドロキシル基含有エチレン性不飽和単量体。
(c)カルボキシル基含有エチレン性不飽和単量体。
(d)エポキシ基含有エチレン性不飽和単量体。
(e)メチロール基含有エチレン性不飽和単量体。
(f)アルコキシアルキル基含有エチレン性不飽和単量体。
(g)シアノ基含有エチレン性不飽和単量体。
(h)ラジカル重合性の二重結合を2個以上有しているエチレン性不飽和単量体。
(i)アミノ基を有するエチレン性不飽和単量体。
(j)スルホン酸基を有するエチレン性不飽和単量体。
(k)リン酸基を有するエチレン性不飽和単量体。
(l)芳香族エチレン性不飽和単量体。
(m)脂肪酸エステル系不飽和単量体。
上記(メタ)アクリル酸アルキルエステル(a)としては、例えば、メチル(メタ)アクリレート、エチル(メタ)アクリレート、プロピル(メタ)アクリレート、イソプロピル(メタ)アクリレート、ブチル(メタ)アクリレート、イソブチル(メタ)アクリレート、n-ヘキシル(メタ)アクリレート、シクロヘキシル(メタ)アクリレート、2-エチルヘキシル(メタ)アクリレート、ラウリル(メタ)アクリレート等の脂肪族(メタ)アクリレート、好ましくはアルキル基の炭素数が1~20の脂肪族(メタ)アクリレートや、ベンジル(メタ)アクリレート、フェニル(メタ)アクリレート等の芳香族(メタ)アクリレート等が挙げられ、これらは単独で、もしくは2種以上併せて用いられる。これらのなかでも好ましくは、ブチル(メタ)アクリレート、2-エチルヘキシル(メタ)アクリレート、メチル(メタ)アクリレート、シクロヘキシル(メタ)アクリレート等のアルキル基の炭素数が1~10の脂肪族(メタ)アクリレートである。
これらラジカル重合性の二重結合を2個以上有しているエチレン性不飽和単量体(h)は、一種単独で、または二種以上を組み合わせて用いることができる。
上記脂肪酸エステル系不飽和単量体(m)としては、例えば、ギ酸ビニル、酢酸ビニル、プロピオン酸ビニル、バレリン酸ビニル、酪酸ビニル、イソ酪酸ビニル、ピバリン酸ビニル、カプリン酸ビニル、ラウリン酸ビニル、ステアリン酸ビニル、安息香酸ビニル、バーサチック酸ビニル等が挙げられる。これらは単独で、もしくは2種以上併せて用いられる。
前記その他のモノマーの仕込み量は、目的とする重合体粒子の構成に対応し、重合体粒子に対して、通常、0~95重量%、好ましくは0~80重量%、特に好ましくは0~70重量%である。
なお、重合体粒子のガラス転移温度は、重合体粒子を構成するエチレン性不飽和単量体の選択やその重合比の調整等の公知の方法によりコントロールすることができる。
2種以上の単量体を用いた場合に得られる共重合体のTg(℃)は、例えば以下に記載の熱示差分析法によって求めることができる。なお本発明における共重合体のTg(℃)は、変調モードで測定した2ndサイクル目のリバーシブルヒートフローの値を採用する。
分析装置:ティー・エイ・インスツルメント社製「DSC Q2000」
測定範囲
1st: -30~215℃
2nd: -30~230℃
昇温速度:5℃/分
降温速度:10℃/分
変調周期:60秒毎
温度振幅:+/-0.80℃
前記重合体粒子が分散するPVA系樹脂水溶液におけるPVA系樹脂は、公知一般の水溶性のPVA系樹脂である。
かかるPVA系樹脂は、ビニルエステル系モノマーを重合し、ケン化することにより得られる。
前記アルカリ触媒としては、例えば、水酸化カリウム、水酸化ナトリウム、ナトリウムメチラート、ナトリウムエチラート、カリウムメチラート、リチウムメチラート等のアルカリ金属の水酸化物やアルコラートを用いることができる。
通常、無水アルコール系溶媒下、アルカリ触媒を用いたエステル交換反応が反応速度の点や脂肪酸塩等の不純物を低減できるなどの点で好適に用いられる。
例えば、グリシジル(メタ)アクリレート、グリシジル(メタ)アリルエーテル、3,4-エポキシシクロヘキシル(メタ)アクリレート、アリルグリシジルエーテル等のビニル基とエポキシ基を有するモノマー;トリアリルオキシエチレン、ジアリルマレアート、トリアリルシアヌレート、トリアリルイソシアヌレート、テトラアリルオキシエタン、ジアリルフタレート、トリアリルシアヌレート、トリアリルイソシアヌレート、テトラアリルオキシエタン、ジアリルフタレート等のアリル基を2個以上有するモノマー;酢酸アリル、アセト酢酸ビニルエステル、アセト酢酸アリルエステル、ジアセト酢酸アリルエステル等のアリルエステル系モノマー;アセトアセトキシエチル(メタ)アクリレート、アセトアセトキシプロピル(メタ)アクリレート等のアセトアセトキシアルキル(メタ)アクリレート;アセトアセトキシエチルクロトナート、アセトアセトキシプロピルクロトナート等のアセトアセトキシアルキルクロトナート;2-シアノアセトアセトキシエチル(メタ)アクリレート;ジビニルベンゼン;エチレングリコールジ(メタ)アクリレート、1,2-プロピレングリコールジ(メタ)アクリレート、1,3-プロピレングリコールジ(メタ)アクリレート、1,4-ブタンジオールジ(メタ)アクリレート、1,6-ヘキサンジオールジ(メタ)アクリレート、ネオペンチルグリコールジ(メタ)アクリレート等のアルキレングリコール(メタ)アクリレート;トリメチロールプロパントリ(メタ)アクリレート;アリル(メタ)アクリレート;2-ヒドロキシエチル(メタ)アクリレート、ヒドロキシプロピル(メタ)アクリレート、4-ヒドロキシブチル(メタ)アクリレート等のヒドロキシアルキル(メタ)アクリレート(アルキル部分がC1~C10アルキル基であり、好ましくはC1~C6アルキル基);(メタ)アクリロニトリルなどのニトリル系モノマー;スチレン、α-メチルスチレン等のスチレン系モノマー;エチレン、プロピレン、1-ブテン、イソブテン等のオレフィン;塩化ビニル、塩化ビニリデン、フッ化ビニル、フッ化ビニリデン等のハロゲン化オレフィン;エチレンスルホン酸等のオレフィン系モノマー;ブタジエン-1,3、2-メチルブタジエン、1,3又は2,3-ジメチルブタジエン-1,3、2-クロロブタジエン-1,3等のジエン系モノマー;3-ブテン-1-オール、4-ペンテン-1-オール、5-ヘキセン-1,2-ジオール、グリセリンモノアリルエーテル等のヒドロキシ基含有α-オレフィン類、およびそのアシル化物などの誘導体;1,3-ジアセトキシ-2-メチレンプロパン、1,3-ジプロピオニルオキシ-2-メチレンプロパン、1,3-ジブチロニルオキシ-2-メチレンプロパンなどのヒドロキシメチルビニリデンジアセテート類;イタコン酸、マレイン酸、アクリル酸等の不飽和酸類、その塩又はモノ若しくはジアルキルエステル;アクリロニトリル等のニトリル類、メタクリルアミド、ジアセトンアクリルアミド等のアミド類、エチレンスルホン酸、アリルスルホン酸、メタアリルスルホン酸、AMPS等のオレフィンスルホン酸あるいはその塩などの化合物、ビニルトリエトキシシラン、ビニルトリメトキシシラン、ビニルトリプロポキシシラン、ビニルトリブトキシシラン、ビニルメチルジメトキシシラン、ビニルメチルジエトキシシラン等のビニルアルキルジアルコキシシラン;γ-(メタ)アクリロキシプロピルトリメトキシシラン、γ-(メタ)アクリロキシプロピルトリエトキシシラン等のγ-(メタ)アクリロキシプロピルトリアルコキシシラン;γ-(メタ)アクリロキシプロピルメチルジメトキシシラン、γ-(メタ)アクリロキシプロピルメチルジエトキシシラン等のγ-(メタ)アクリロキシプロピルアルキルジアルコキシシラン;ビニルトリス(β-メトキシエトキシ)シラン、ヒドロキシメチルビニリデンジアセテートが挙げられる。ヒドロキシメチルビニリデンジアセテートの具体的な例としては、1,3-ジアセトキシ-2-メチレンプロパン、1,3-ジプロピオニルオキシ-2-メチレンプロパン、1,3-ジブチロニルオキシ-2-メチレンプロパン等が挙げられる。これらのモノマーは、単独で用いてもよく、又は2種以上を併用してもよい。
なかでも、製造時の粘度安定性や耐熱性等の点で、炭素数6以下のアルキレン基、特にメチレン基、あるいは-CH2OCH2-が好ましい。
上記方法のうち、共重合反応性及び工業的な取扱いにおいて優れるという点で(i)の方法が好ましく、特にR1~R6が水素、Xが単結合、R7、R8がR9-CO-であり、R9がアルキル基である3,4-ジアシロキシ-1-ブテンが好ましく、その中でも特にR9がメチル基である3,4-ジアセトキシ-1-ブテンが好ましく用いられる。
なお、PVA系樹脂中の1,2-ジオール構造単位の含有率は、ケン化度100モル%のPVA系樹脂の1H-NMRスペクトル(溶媒:DMSO-d6、内部標準:テトラメチルシラン)から求めることができる。具体的には1,2-ジオール構造単位中の水酸基プロトン、メチンプロトン、およびメチレンプロトン、主鎖のメチレンプロトン、主鎖に連結する水酸基のプロトンなどに由来するピーク面積から算出することができる。
かかる有機溶媒としては、例えば、N-メチルピロリドン(NMP)、ジメチルホルムアミド、ジメチルアセトアミド、メチルホルムアミドなどのアミド系溶媒、ジメチルスルホキサイドなどのスルホキサイド、メタノール、エタノール等の炭素数1~3の低級アルコール、1,1,1,3,3,3-ヘキサフルオロ-2-プロパノール等のアルコール系溶媒が挙げられる。
本発明におけるエチレン性不飽和単量体に由来する重合体粒子が上記PVA系樹脂水溶液中に分散しているエマルジョンは、上記重合体粒子を上記PVA系樹脂水溶液に分散させることで得られる。
特にかかるエマルジョンは、エチレン性不飽和単量体を分散質とし、公知の分散剤を用い、上記溶媒を分散媒として乳化重合する場合、上記重合体粒子が分散媒中に分散性良く分散したエマルジョンとして効率よく得られる。
この場合、負極におけるバインダーの連続層の強度の観点から、海島構造を形成する場合はドメインサイズを小さく(通常ドメイン径1.5μm以下)制御することが好ましい。かかる観点から、第1のPVA系樹脂と第2のPVA系樹脂のケン化度差は、通常0~15モル%、好ましくは0~10モル%である。
なお、本発明のPVA樹脂の重量とは、第1のPVA系樹脂と第2のPVA系樹脂との合計重量を指す。
前記乳化重合を実施する方法としては、例えば、i)水、分散剤及び重合触媒の存在下に、分散質たるエチレン性不飽和単量体を一度に又は連続的に配合して、加熱、撹拌することにより乳化重合する方法;ii)エチレン性不飽和単量体を分散媒に混合分散させた分散液を調製し、この調製した分散液を、水、分散剤及び重合触媒が配合された系内に、一度に又は連続的に配合して、加熱、撹拌して、乳化重合する方法などが挙げられる。このように予め調製した分散液を用いる方法は特にプレエマルジョン法と称される。かかる方法は、重合対象のモノマー組成がたとえ複雑であっても、生産性を維持して乳化重合を行なうことが可能であるので好ましい。
その他には、重合装置の混合攪拌翼の大きさや攪拌速度、攪拌時間を制御する方法などを採用することができる。さらには、モノマーを多孔質の膜中に通すことで粒子径を制御する膜乳化法や、攪拌方法に超音波を用いる超音波乳化法などを採用することもできる。
PVA系樹脂の配合量が少なすぎると、エチレン性不飽和単量体の乳化状態が不安定となって、重合反応性が低下したり、重合により得られるエマルジョン中での粒子の乳化状態安定性が低下する傾向にある。一方、PVA系樹脂の含有量が多すぎると、反応液の粘度が増大しすぎて均一分散性が低下し、重合率を高められなかったり、得られるエマルジョンの粘度が高くなりすぎて、製造上の歩留まりが低下する傾向にある。
なお、重合開始剤の配合方法としては、特に制限はなく、初期に一括して反応液中に配合してもよいし、重合の経過に伴って連続的に添加してもよい。
分散媒、分散剤を含有する反応容器に、重合しようとするモノマーの一部を仕込み、1段目の乳化重合を行う。1段目に投入するモノマーの量は、特に限定しないが、重合に使用するモノマーの通常1~50重量%程度であり、好ましくは5~30重量%である。1段目の乳化重合工程の条件は、用いるモノマーの種類、組成、重合開始剤の使用量等により適宜決定すればよい。
乳化重合反応の温度は、通常30~90℃であり、特に40~80℃が好ましく、重合時間は1~4時間とすることが好ましい。1段目の乳化重合工程においては、重合転化率が30%以上であることが好ましく、60%以上であることが特に好ましい。
2段目の乳化重合は、1段目の重合が終了した反応容器に、残りのモノマーを投入することにより行う。投入は、滴下しながら行うことが好ましい。また、2段目の重合に際して、重合触媒を投入してもよい。2段目の乳化重合は、重合温度が40~80℃、重合時間が1~6時間の条件で行う。
また、滴下するモノマー組成比を連続的に変えながら滴下するパワーフィード重合法を用いることも可能である。また、モノマーを分散剤たるPVA系樹脂の存在下で予め混合分散させた分散液を滴下しながら重合してもよい。
必要に応じて、かかる工程の後に通常1~6時間の追い込み重合をおこなうことも可能である。かかる重合中に重合触媒を投入してもよい。
更に、フタル酸エステル、リン酸エステル等の可塑剤、炭酸ナトリウム、酢酸ナトリウム、リン酸ナトリウム等のpH調整剤等も併用され得る。
本発明における重合体粒子の平均粒子径は、50nm以上600nm以下である。好ましくは200~500nmである。なお、前記粒子の平均粒子径は、動的光散乱式粒度分布測定器により、超音波照射処理の1分後に、測定時間3分間、積算回数5にて測定された体積分布の平均粒子径を採用する。
本発明の二次電池負極用バインダー組成物は上記エマルジョンを含む。
上記したように、本発明の負極用バインダー組成物においては、バインダー組成物に含まれる上記エマルジョン中の分散剤としてのPVA系樹脂とは別に、PVA系樹脂が配合されることが好ましい。側鎖に一級水酸基を有する構造単位、特に側鎖1,2-ジオール構造単位を含有する変性PVA系樹脂は低結晶性であることから、別途、エマルジョンのPVA系樹脂水溶液に、側鎖に一級水酸基を有する構造単位、特に側鎖1,2-ジオール構造単位を含有する変性PVA系樹脂が含まれることで、電極用バインダー組成物である水ペーストの粘度安定性を付与することが可能であり、作業効率を向上させることができる。
本発明のバインダー組成物には、通常、塗膜に用いられる塗料や成型用樹脂に用いられる配合剤等を配合することができる。例えば、光安定剤、紫外線吸収剤、増粘剤、レベリング剤、チクソ化剤、消泡剤、凍結安定剤、艶消し剤、架橋反応触媒、顔料、硬化触媒、架橋剤{ホウ酸、メチロール化メラミン、炭酸ジルコニウム、ジイソプロポキシチタンビス(トリエタノールアミネート)等}、皮張り防止剤、分散剤(上述の分散剤を除く。)、湿潤剤、酸化防止剤、紫外線吸収剤、レオロジーコントロール剤、成膜助剤、防錆剤、染料、可塑剤、潤滑剤、還元剤、防腐剤、防黴剤、消臭剤、黄変防止剤、静電防止剤又は帯電調整剤等が挙げられる。それぞれの目的に応じて選択したり、組み合わせたりして配合することができる。なお、バインダー組成物がこれらの配合剤を含有する場合、含有する配合剤の有機分は、バインダー組成物の固形分に含まれる。
上記配合剤の配合量は、バインダー組成物における上記エマルジョンの固形分100重量部に対して通常10重量部以下、好ましくは5重量部以下である。
本発明の二次電池負極は、本発明の二次電池負極用バインダー組成物、負極用の合金系活物質を少なくとも含有する。本発明の二次電池負極は、通常、バインダー組成物及び合金系活物質を混合して、二次電池負極用スラリーを調製し、このスラリーを集電体上に塗布、乾燥することによって製造することができる。
本発明の二次電池負極は、様々な二次電池に適用することができ、例えば、リチウムイオン二次電池、リチウムイオンポリマー二次電池、鉛蓄電池、ニッケル・水素蓄電池、ニッケル・カドミウム蓄電池、ニッケル・鉄蓄電池、ニッケル・亜鉛蓄電池、酸化銀・亜鉛蓄電池、ナトリウム電池、空気アルミニウム電池等に適用することができる。以下では、特にリチウムイオン二次電池を例示して説明する。
スラリー中の活物質の含有量は、10~95重量%、好ましくは20~80重量%、特に好ましくは35~65重量%である。
PVA系樹脂以外の水溶性高分子としては、例えば、メチルセルロース、エチルセルロース、ヒドロキシメチルセルロース、ヒドロキシプロピルメチルセルロース、ヒドロキシブチルメチルセルロース、ヒドロキシエチルセルロース、カルボキシメチルセルロース、アミノメチルヒドロキシプロピルセルロース、アミノエチルヒドロキシプロピルセルロース等のセルロース誘導体類;デンプン、トラガント、ペクチン、グルー、アルギン酸又はその塩;ゼラチン;ポリビニルピロリドン;ポリアクリル酸又はその塩、ポリメタクリル酸又はその塩;ポリアクリルアミド、ポリメタクリルアミド糖のアクリルアミド類;酢酸ビニルとマレイン酸、無水マレイン酸、アクリル酸、アクリル酸、メタクリル酸、イタコン酸、フマル酸、クロトン酸等の不飽和酸との共重合体;スチレンと上記不飽和酸との共重合体;ビニルエーテルと上記不飽和酸との共重合体;及び前記不飽和酸と各共重合体の塩類又はエステル類、カラギーナン、キサンタンガム、ヒアルロン酸ナトリウム、ローカストビーンガム、タラガム、グアーガム、タマリンドシードガム等の天然多糖類が挙げられ、好ましくはセルロース誘導体類である。
本発明の二次電池の例として、本発明の二次電池負極用バインダー組成物を用いて作製された負極を有するリチウムイオン二次電池について説明する。
リチウムイオン二次電池は、正極、負極、電解液、セパレータを少なくとも有する。
正極活物質としては、例えば、オリビン型リン酸鉄リチウム、コバルト酸リチウム、マンガン酸リチウム、ニッケル酸リチウム、三元系ニッケルコバルトマンガン酸リチウム、リチウムニッケルコバルトアルミニウム複合酸化物等を用いることができる。
下記実施例及び比較例において製造したPVA系樹脂は、以下の方法にて分析した。
残存酢酸ビニル及び3,4-ジアセトキシ-1-ブテンの加水分解に要するアルカリ消費量にて分析した。
JIS K 6726に準じて測定した。
BRUKER社製のAVANCEIIIHD 400を用いて、1H-NMR(400MHz、プロトンNMR、溶媒:重水溶液、温度:50℃)にて測定し、得られたNMRチャートに基づき、積分値より算出した。
還流冷却器、攪拌機、滴下漏斗、温度計を備え付けたセパラブルフラスコに、分散媒としての水1005.2部、分散剤として上記構造式(1a)に示す構造の側鎖1,2-ジオール構造単位を含有する変性PVA系樹脂(ケン化度:99.1モル%、粘度平均重合度:1200、上記構造式(1a)に示す1,2-ジオール構造単位含有率:6モル%)179.2部、酢酸ナトリウム0.59部を流し込み、95℃で撹拌しながら2時間溶解させた後、フラスコ内の温度を75℃に冷却した。
この温浴中に、1段目の乳化重合用モノマーとして、ブチルアクリレート(BA)とスチレン(St)との混合モノマー(混合重量比:BA/St=45/55)を28.0部、重合開始剤として亜硫酸水素ナトリウム水溶液(5%)5.60部及び過硫酸アンモニウム水溶液(1%)18.48部を加えて、1段目の乳化重合を開始した。反応温度を75℃~80℃に保持しながら、1時間重合を行った。
次に、2段目の乳化重合を行なった。1段階目の乳化重合を行った反応系の温度を75℃~80℃の範囲に保ちながら、2段目の乳化重合用モノマーとして、先ほどと同じ組成の混合モノマー252.0部を3時間半かけて滴下した。かかる滴下中に、過硫酸アンモニウム水溶液(1%)36.96部を14分割して15分毎に配合した。その後、温度を75℃に保ちながら、90分間重合を続けた。この間、過硫酸アンモニウム水溶液(1%)6.16部を2分割して45分毎に配合した。
2段目の乳化重合の後、反応温度を50℃まで低下させて、1時間追い込み重合を行った。かかる追い込み重合中はt-ブチルヒドロパーオキシ水溶液(10%)2.8部及びL-アスコルビン酸水溶液(10%)3.4部をそれぞれ2分割して30分毎に配合した。その後、室温まで冷却して、上記構造式(1a)に示す構造の側鎖1,2-ジオール構造単位を含有する変性PVA系樹脂水溶液中にブチルアクリレート(BA)及びスチレン(St)の混合組成の重合体粒子が分散するエマルジョンを得た。かかるエマルジョンにおける固形分は29.6%で、PVA系樹脂と重合体粒子の重量比は39/61であった。
還流冷却器、攪拌機、滴下漏斗、温度計を備え付けたセパラブルフラスコに、分散媒としての水1016.0部、分散剤として上記構造式(1a)に示す構造の側鎖1,2-ジオール構造単位を含有する変性PVA系樹脂(ケン化度:99.1モル%、粘度平均重合度:1200、上記構造式(1a)に示す1,2-ジオール構造単位含有率:6モル%)256.0部、酢酸ナトリウム0.42部を流し込み、95℃で撹拌しながら2時間溶解させた後、フラスコ内の温度を75℃に冷却した。
この温浴中に、1段目の乳化重合用モノマーとして、ブチルアクリレート(BA)とメチルメタクリレート(MMA)との混合モノマー(混合重量比:BA/MMA=71/29)を20.0部、重合開始剤として亜硫酸水素ナトリウム水溶液(5%)4.00部及び過硫酸アンモニウム水溶液(1%)13.20部を加えて、1段目の乳化重合を開始した。反応温度を75℃~80℃に保持しながら、1時間重合を行った。
次に、2段目の乳化重合を行なった。1段階目の乳化重合を行った反応系の温度を75℃~80℃の範囲に保ちながら、2段目の乳化重合用モノマーとして、先ほどと同じ組成の混合モノマー180.0部を3時間半かけて滴下した。かかる滴下中に、過硫酸アンモニウム水溶液(1%)26.39部を14分割して15分毎に配合した。その後、温度を75℃に保ちながら、90分間重合を続けた。この間、過硫酸アンモニウム水溶液(1%)4.4部を2分割して45分毎に配合した。
2段目の乳化重合の後、反応温度を50℃まで低下させて、1時間追い込み重合を行った。かかる追い込み重合中はt-ブチルヒドロパーオキシ水溶液(10%)2.0部及びL-アスコルビン酸水溶液(10%)2.4部をそれぞれ2分割して30分毎に配合した。その後、室温まで冷却して、上記構造式(1a)に示す構造の側鎖1,2-ジオール構造単位を含有する変性PVA系樹脂水溶液中にブチルアクリレート(BA)及びメチルメタクリレート(MMA)の混合組成の重合体粒子が分散するエマルジョンを得た。かかるエマルジョンにおける固形分は29.9%で、PVA系樹脂と重合体粒子の重量比は56/44であった。
乳化重合用のモノマーとして、ブチルアクリレート(BA)とスチレン(St)との混合モノマー(混合重量比:BA/St=45/55)を用いた以外は、重合例2と同様の方法でベースエマルジョンを得た。
分散剤として用いるPVA系樹脂を変更(ケン化度:99.1モル%、粘度平均重合度:600、上記構造式(1a)に示す1,2-ジオール構造単位含有率:8モル%)して、さらに乳化重合用のモノマーとして、ブチルアクリレート(BA)とスチレン(St)との混合モノマー(混合重量比:BA/St=27.5/72.5)を用いた以外は、重合例2と同様の方法でベースエマルジョンを得た。
前記重合例1で調製したベースエマルジョンを分取し、希釈用の精製水と、さらに上記構造式(1a)に示す構造の側鎖1,2-ジオール構造単位を含有する変性PVA系樹脂(ケン化度:99.1モル%、粘度平均重合度:1200、上記構造式(1a)に示す1,2-ジオール構造単位含有率:6モル%)の10%水溶液を添加することで、10%のバインダー溶液を調製した。この時、バインダーの固形分中の(1)PVA系樹脂と(2)ブチルアクリレート(BA)及びスチレン(St)の混合組成の重合体粒子の重量比が、(1)/(2)=90/10になるように調製した。
バインダーの固形分中の(1)PVA系樹脂と(2)ブチルアクリレート(BA)及びスチレン(St)の混合組成の重合体粒子の重量比が、(1)/(2)=80/20になるように調製した以外は、製造例1と同様にしてバインダー溶液を調製した。
バインダーの固形分中の(1)PVA系樹脂と(2)ブチルアクリレート(BA)及びスチレン(St)の混合組成の重合体粒子の重量比が、(1)/(2)=70/30になるように調製した以外は、製造例1と同様にしてバインダー溶液を調製した。
還流冷却器、攪拌機、滴下漏斗、温度計を備え付けたセパラブルフラスコに、分散媒としての水1016.0部、分散剤として上記構造式(1a)に示す構造の側鎖1,2-ジオール構造単位を含有するPVA系樹脂(ケン化度:99.1モル%、粘度平均重合度:1200、上記構造式(1a)に示す1,2-ジオール構造単位含有率:6モル%)256.0部、酢酸ナトリウム0.42部を流し込み、95℃で撹拌しながら2時間溶解させた後、フラスコ内の温度を75℃に冷却した。
この温浴中に、1段目の乳化重合用モノマーとして、ブチルアクリレート(BA)とスチレン(St)との混合モノマー(混合重量比:BA/St=45/55)を20.0部、重合開始剤として亜硫酸水素ナトリウム水溶液(5%)4.00部及び過硫酸アンモニウム水溶液(1%)13.20部を加えて、1段目の乳化重合を開始した。反応温度を75℃~80℃に保持しながら、1時間重合を行った。
次に、2段目の乳化重合を行なった。1段階目の乳化重合を行った反応系の温度を75℃~80℃の範囲に保ちながら、2段目の乳化重合用モノマーとして、先ほどと同じ組成の混合モノマー180.0部を3時間半かけて滴下した。かかる滴下中に、過硫酸アンモニウム水溶液(1%)26.39部を14分割して15分毎に配合した。その後、温度を75℃に保ちながら、90分間重合を続けた。この間、過硫酸アンモニウム水溶液(1%)4.4部を2分割して45分毎に配合した。
2段目の乳化重合の後、反応温度を50℃まで低下させて、1時間追い込み重合を行った。かかる追い込み重合中はt-ブチルヒドロパーオキシ水溶液(10%)2.0部及びL-アスコルビン酸水溶液(10%)2.4部をそれぞれ2分割して30分毎に配合した。その後、室温まで冷却して、上記構造式(1a)に示す構造の側鎖1,2-ジオール構造単位を含有する変性PVA系樹脂水溶液中にブチルアクリレート(BA)及びスチレン(St)の混合組成の重合体粒子が分散するエマルジョンを得た。かかるエマルジョンにおける固形分は29.7%で、PVA系樹脂と重合体粒子の重量比は56/44であった。
実際に電極用バインダーとして使用する際には、あらかじめ10%に希釈したものを使用した。
前記重合例2で調製したベースエマルジョンを分取し、希釈用の精製水と、さらに上記構造式(1a)に示す構造の側鎖1,2-ジオール構造単位を含有する変性PVA系樹脂(ケン化度:99.1モル%、粘度平均重合度:1200、上記構造式(1a)に示す1,2-ジオール構造単位含有率:6モル%)の10%水溶液を添加することで、10%のバインダー溶液を調製した。この時、バインダーの固形分中の(1)PVA系樹脂と(2)重合体粒子の重量比が、(1)/(2)=80/20になるように調製した。
前記重合例3で調製したベースエマルジョンを分取し、希釈用の精製水と、さらに上記構造式(1a)に示す構造の側鎖1,2-ジオール構造単位を含有する変性PVA系樹脂(ケン化度:99.1モル%、粘度平均重合度:1200、上記構造式(1a)に示す1,2-ジオール構造単位含有率:6モル%)の10%水溶液を添加することで、10%のバインダー溶液を調製した。この時、バインダーの固形分中の(1)PVA系樹脂と(2)重合体粒子の重量比が、(1)/(2)=80/20になるように調製した。
前記重合例4で調製したベースエマルジョンを分取し、希釈用の精製水と、さらに上記構造式(1a)に示す構造の側鎖1,2-ジオール構造単位を含有する変性PVA系樹脂(ケン化度:99.1モル%、粘度平均重合度:1200、上記構造式(1a)に示す1,2-ジオール構造単位含有率:6モル%)の10%水溶液を添加することで、10%のバインダー溶液を調製した。この時、バインダーの固形分中の(1)PVA系樹脂と(2)重合体粒子の重量比が、(1)/(2)=80/20になるように調製した。
<ケイ素活物質を用いた電池用負極の作製>
活物質としてケイ素粉末(販売元:AlfaAesar、平均粒子径:50nm)を65部、導電助剤としてアセチレンブラック(電気化学工業株式会社製「デンカブラック」)を15部、さらに分散剤として1.72%水溶液に調製したカルボキシメチルセルロース#2260(ダイセルファインケム株式会社製)を固形分換算で3部、また適時に精製水を加えた後、遊星式混練機(株式会社シンキー製「泡取り錬太郎」)を用いて混合して固形分濃度28.0%のペーストを得た(2000rpmで4.5分間混合した後、更に2200rpmで0.5分間脱泡した。)。
得られたペースト中に、負極用バインダーとして製造例1で作製したバインダー溶液(10%)を固形分換算で17部、また適時に精製水を加水した後、さらに遊星式混練機を用いて同様の条件で混合することで、固形分濃度が19.8%の活物質ペーストを得た。
次に、集電体として圧延銅箔(株式会社UACJ製箔、厚さ18μm)の表面に、25μmのアプリケータと塗工機(株式会社井元製作所製「コントロールコーター(塗工機)」)を用いて、塗工速度10mm/秒で上記活物質ペーストを塗工した。これを80℃で2時間乾燥させたのち、続いて120℃で4時間真空乾燥を行うことで、電池用電極を得た。
評価用の電池の外層としては、2032型のコイン型セルを使用した。得られた電池用電極を直径14mmの大きさに打ち抜き、更に80℃で12時間真空乾燥を行った後にグローブボックスへと仕込んだ。上記にて作製した電池用負極を作用極とし、金属リチウム(直径13mm)を対極として、厚さ16μmのポリプロピレン多孔膜から成るセパレータ(直径18mm)を介在させて、互いに電極が対向するように配置させた。電解液として、エチレンカーボネートとジエチレンカーボネートとを体積比で3:7に混合した溶媒に、電解質としてLiPF6を1mol/リットルの濃度に溶解したものを使用した。ポリプロピレン製パッキングを介して外層容器にステンレス鋼のキャップを被せて固定し、電池缶を封止することでハーフセルを作製し、評価用の電池とした。
作製した評価用の電池を50℃の環境下で24時間静置した。その後50℃を保ったまま、作用極に対して0.1C(1C=4200mA/g)の速度にて定電流条件で充放電を繰り返し行った。この時、電池性能の評価項目として、初回の放電容量とクーロン効率を採用した。
前述の条件で繰り返し充放電を行った後、5サイクル目の放電容量を測定して、下記式で求めた容量維持率をサイクル特性として採用した。
容量維持率(%)=(5サイクル目の放電容量)/(初回の放電容量)×100
この値が100%に近い程、充放電に伴う電極の劣化が少なく、電池が安定に作動していることを示す。
得られた電池性能の評価結果について、表1に記載した。
負極用バインダーとして製造例2で調製したバインダー溶液を使用する以外は、実施例1と同様の方法で負極を作製して電池性能を評価した。
負極用バインダーとして製造例3で調製したバインダー溶液を使用する以外は、実施例1と同様の方法で負極を作製して電池性能を評価した。
電極用バインダーとして製造例4で調製したバインダー溶液を使用する以外は、実施例1と同様の方法で電極を作製して電池性能を評価した。
電極用バインダーとして重合例1で調製したベースエマルジョンをバインダー溶液として使用する以外は、実施例1と同様の方法で負極を作製して電池性能を評価した。その際に、ベースエマルジョンは予め10%に希釈したものをバインダー溶液として使用した。
電極用バインダーとして、上記構造式(1a)に示す構造の側鎖1,2-ジオール構造単位を含有する変性PVA系樹脂(ケン化度:99.1モル%、粘度平均重合度:1200、上記構造式(1a)に示す1,2-ジオール構造単位含有率:6モル%)の10%水溶液を使用した以外は、実施例1と同様の方法で電極を作製して電池性能を評価した。
これに対して、比較例1及び2では、重合体粒子が多くなることで容量とクーロン効率の低下が見られる。一方でPVA系樹脂単独で重合体粒子を全く含まない比較例3のバインダー組成では、初回放電容量と初回クーロン効率は高いものの、容量維持率が低くなった。
よって、合金系活物質を用いた電極を作製する際には、バインダー中のPVA系樹脂と重合体粒子の重量比を好ましい範囲に設計することで、良好な充放電特性を発揮できることがわかる。
(皮膜物性の評価)
<バインダー皮膜の作製>
製造例5で得られたバインダー溶液を、設計値100μmの膜厚になるようにポリエチレンテレフタレートフィルム上にキャストした。その後23℃×50%RHの環境で48時間静置した後、80℃の送風乾燥機で1時間乾燥を行った。更に80℃の真空乾燥機にて、五酸化リンを共存させて48時間真空乾燥を行った。
得られた皮膜を熱重量分析装置(TGA)で揮発分測定を行ったところ、50~200℃の範囲で重量減少率が0.5%以下であることを確認した。測定条件を以下に示す。
測定範囲:50~300℃
測定モード:50℃で10分間静置した後、10℃/分で300℃まで定速昇温
揮発分(%)=(200℃でのサンプル重量)/(50℃でのサンプル重量)×100
上記方法で得られたバインダー皮膜について、下記の方法にて示差熱量分析を行った。なお重合体粒子のガラス転移温度は、変調モードで測定した2ndサイクル目のリバーシブルヒートフローの値を採用した。
分析装置:ティー・エイ・インスツルメント社製「DSC Q2000」
測定範囲
1st: -30~215℃
2nd: -30~230℃
昇温速度:5℃/分
降温速度:10℃/分
変調周期:60秒毎
温度振幅:+/-0.80℃
上記方法で得られたバインダー皮膜について、JIS K56005-1(塗膜の機械的性質 第一節:耐屈曲性(円筒形マンドレル法))に準じて耐屈曲性試験を行った。試験サンプルは3点用意し、いずれかが破断した際のマンドレル試験棒の値を採用した。最小の試験棒は2mmφ(直径)であり、3点とも破断しなかった場合「2mmφでも破断しない」と評価した。
負極用バインダーとして製造例6で調製したバインダー溶液を使用する以外は、実施例4と同様の方法で負極を作製して電池性能を評価した。
負極用バインダーとして製造例7で調製したバインダー溶液を使用する以外は、実施例4と同様の方法で負極を作製して電池性能を評価した。
よって、合金系活物質を用いた負極を作製する際には、PVA系樹脂と重合体粒子の重量比を好ましい範囲に設計し、更に重合体粒子のガラス転移温度を好ましい範囲に設計することで、体積変化に更に追随できる安定な負極活物質層が得られ、高い充放電特性を発揮できることがわかる。
Claims (6)
- リチウムと合金を形成し得る元素を活物質として含有する二次電池負極を作製するためのバインダー組成物であって、
エチレン性不飽和単量体に由来する重合体粒子がポリビニルアルコール系樹脂水溶液中に分散されたエマルジョンを含み、前記ポリビニルアルコール系樹脂/重合体粒子の比率が、樹脂固形分の重量比にて、60/40~99/1であること特徴とする二次電池負極用バインダー組成物。 - 前記重合体粒子のガラス転移温度が-40~60℃であることを特徴とする請求項1記載の二次電池負極用バインダー組成物。
- 前記ポリビニルアルコール系樹脂が、側鎖に一級水酸基を有する構造単位を含有する変性ポリビニルアルコール系樹脂を含むことを特徴とする請求項1又は2記載の二次電池負極用バインダー組成物。
- 請求項1~3のいずれか記載の二次電池負極用バインダー組成物を含有してなる二次電池負極。
- 増粘剤としてセルロース誘導体類を含有する請求項4に記載の二次電池負極。
- 請求項4又は5に記載の二次電池負極を有する二次電池。
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