WO2015053224A1 - 正極用バインダー組成物、正極用スラリー、正極及びリチウムイオン二次電池 - Google Patents
正極用バインダー組成物、正極用スラリー、正極及びリチウムイオン二次電池 Download PDFInfo
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- WO2015053224A1 WO2015053224A1 PCT/JP2014/076709 JP2014076709W WO2015053224A1 WO 2015053224 A1 WO2015053224 A1 WO 2015053224A1 JP 2014076709 W JP2014076709 W JP 2014076709W WO 2015053224 A1 WO2015053224 A1 WO 2015053224A1
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- H—ELECTRICITY
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- 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
- C08F—MACROMOLECULAR COMPOUNDS OBTAINED BY REACTIONS ONLY INVOLVING CARBON-TO-CARBON UNSATURATED BONDS
- C08F261/00—Macromolecular compounds obtained by polymerising monomers on to polymers of oxygen-containing monomers as defined in group C08F16/00
- C08F261/02—Macromolecular compounds obtained by polymerising monomers on to polymers of oxygen-containing monomers as defined in group C08F16/00 on to polymers of unsaturated alcohols
- C08F261/04—Macromolecular compounds obtained by polymerising monomers on to polymers of oxygen-containing monomers as defined in group C08F16/00 on to polymers of unsaturated alcohols on to polymers of vinyl alcohol
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- 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
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- 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
- H01M4/131—Electrodes based on mixed oxides or hydroxides, or on mixtures of oxides or hydroxides, e.g. LiCoOx
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- H—ELECTRICITY
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- 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
- H01M4/136—Electrodes based on inorganic compounds other than oxides or hydroxides, e.g. sulfides, selenides, tellurides, halogenides or LiCoFy
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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
- H01M4/48—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides
- H01M4/50—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides of manganese
- H01M4/505—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides of manganese of mixed oxides or hydroxides containing manganese for inserting or intercalating light metals, e.g. LiMn2O4 or LiMn2OxFy
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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
- H01M4/48—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides
- H01M4/52—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides of nickel, cobalt or iron
- H01M4/525—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides of nickel, cobalt or iron of mixed oxides or hydroxides containing iron, cobalt or nickel for inserting or intercalating light metals, e.g. LiNiO2, LiCoO2 or LiCoOxFy
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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
- H01M4/58—Selection of substances as active materials, active masses, active liquids of inorganic compounds other than oxides or hydroxides, e.g. sulfides, selenides, tellurides, halogenides or LiCoFy; of polyanionic structures, e.g. phosphates, silicates or borates
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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
- H01M4/58—Selection of substances as active materials, active masses, active liquids of inorganic compounds other than oxides or hydroxides, e.g. sulfides, selenides, tellurides, halogenides or LiCoFy; of polyanionic structures, e.g. phosphates, silicates or borates
- H01M4/5825—Oxygenated metallic salts or polyanionic structures, e.g. borates, phosphates, silicates, olivines
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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
- H01M4/624—Electric conductive fillers
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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
- H01M4/624—Electric conductive fillers
- H01M4/625—Carbon or graphite
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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
- H01M2004/028—Positive electrodes
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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
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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
- Y02T—CLIMATE CHANGE MITIGATION TECHNOLOGIES RELATED TO TRANSPORTATION
- Y02T10/00—Road transport of goods or passengers
- Y02T10/60—Other road transportation technologies with climate change mitigation effect
- Y02T10/70—Energy storage systems for electromobility, e.g. batteries
Definitions
- the present invention relates to a positive electrode binder composition, a positive electrode slurry using the binder composition, and a positive electrode and a lithium ion secondary battery using the positive electrode slurry.
- secondary batteries have been used as power sources for electronic devices such as notebook computers and mobile phones, and hybrid vehicles and electric vehicles using secondary batteries as power sources are being developed for the purpose of reducing environmental impact. .
- Secondary batteries having high energy density, high voltage, and high durability are required for these power sources.
- Lithium ion secondary batteries are attracting attention as secondary batteries that can achieve high voltage and high energy density.
- a lithium ion secondary battery is composed of a positive electrode, a negative electrode, an electrolyte, and a separator
- the positive electrode is composed of a positive electrode active material, a conductive additive, a current collector, and a binder.
- a fluorine resin such as polyvinylidene fluoride or polytetrafluoroethylene, a styrene-butadiene copolymer, or an acrylic copolymer is used (for example, see Patent Documents 1 to 3).
- conventional binders sometimes have poor binding properties with metal foils used for current collectors.
- a positive electrode active material having a high potential is required.
- conventional binders may have poor oxidation resistance, and thus repeatedly charge and discharge. In some cases, the binder is decomposed, the positive electrode active material is lost from the current collector, and the capacity of the battery is reduced.
- an object of the present invention is to provide a binder having good binding strength with a current collector and an active material, and further oxidation resistance. Furthermore, it aims at providing the slurry for positive electrodes manufactured using this binder, a positive electrode, and a lithium ion secondary battery.
- a polymer obtained by grafting a monomer having acrylonitrile as a main component onto polyvinyl alcohol is a binder having high oxidation resistance and good binding force. I found.
- this invention provides the binder composition for positive electrodes as described in the following [1].
- a positive electrode binder comprising a graft copolymer obtained by grafting a monomer mainly composed of acrylonitrile on polyvinyl alcohol having an average degree of polymerization of 300 to 3000 and a saponification degree of 70 to 100 mol%.
- This binder composition for positive electrodes may be a binder composition for positive electrodes described in [2] or [3] below.
- a positive electrode slurry comprising the binder composition according to any one of [1] to [3], a positive electrode active material, and a conductive additive.
- This positive electrode slurry may be the positive electrode slurry described in [5] or [6] below.
- the conductive assistant is at least one selected from the group consisting of (i) fibrous carbon, (ii) carbon black, and (iii) a carbon composite in which fibrous carbon and carbon black are interconnected.
- this invention provides the positive electrode as described in the following [7], and the lithium ion secondary battery as described in the following [8].
- [7] A positive electrode produced using the positive electrode slurry according to any one of [4] to [6].
- a lithium ion secondary battery comprising the positive electrode according to [7].
- this invention it is possible to provide a binder composition for a positive electrode that has good binding properties to an active material and a current collector and is excellent in oxidation resistance. Moreover, this invention can provide the battery which is excellent in the cycling characteristics which use the positive electrode active material of a high electric potential by this binder composition for positive electrodes.
- a positive electrode binder composition according to an embodiment of the present invention (hereinafter sometimes referred to as a “binder composition”) is a simple substance mainly composed of polyvinyl alcohol (hereinafter sometimes abbreviated as PVA) and acrylonitrile as a main component.
- the graft copolymer contains a graft copolymer.
- This graft copolymer is a copolymer in which a side branch of polyacrylonitrile (hereinafter sometimes abbreviated as PAN) is formed in the main chain of polyvinyl alcohol.
- the binder composition of this embodiment may contain a PAN homopolymer and / or a PVA homopolymer as a resin component (polymer component) in addition to the graft copolymer.
- the monomer grafted to PVA contains acrylonitrile as an essential component in terms of oxidation resistance.
- methyl methacrylate, methyl acrylate, ethyl acrylate, butyl acrylate, 2-ethylhexyl methacrylate and 2-acrylic acid 2-acrylate are used as long as they do not impair the oxidation resistance of the binder.
- An ethylenically unsaturated carboxylic acid ester such as ethylhexyl
- an ethylenically unsaturated carboxylic acid such as (meth) acrylic acid, maleic anhydride and itaconic acid, and styrene can be used in combination.
- ethylenically unsaturated carboxylic acid esters are preferred, acrylic acid esters and / or methacrylic acid esters are more preferred, and methyl methacrylate is more preferred.
- the monomer grafted to PVA is more preferably composed only of acrylonitrile, and a monomer such as methyl methacrylate may be used in combination with acrylonitrile as long as the effects of the present invention are not impaired.
- Acrylonitrile in the monomer grafted to PVA is the main component of the graft copolymerized monomer, preferably 50% by mass or more, and 90% by mass or more of the graft copolymerized monomer. It is more preferable.
- acrylonitrile is 50% by mass or more, preferably 90% by mass or more of the graft copolymerized monomer, the oxidation resistance of the binder composition can be improved.
- the upper limit of the ratio of acrylonitrile in the graft copolymerized monomer can be 100% by mass or less.
- the composition of the monomer grafted onto PVA can be determined by 1 H-NMR (proton nuclear magnetic resonance spectroscopy).
- the saponification degree of PVA is 70 to 100 mol% from the viewpoint of oxidation resistance, preferably 70 mol% or more and less than 100 mol%, and more preferably 80 to 90 mol%.
- the degree of saponification is less than 70 mol%, the oxidation resistance of the binder decreases. Even if the saponification degree is 100 mol%, there is no problem in the performance as a binder, but the production may take a long time.
- the saponification degree of PVA here is a value measured by a method according to JIS K 6726.
- the average degree of polymerization of PVA is 300 to 3000 from the viewpoints of solubility, binding power, and binder viscosity.
- the average degree of polymerization of PVA is preferably 320 to 2950, more preferably 500 to 2500, and even more preferably 500 to 1800.
- the average degree of polymerization of PVA is less than 300, the binding force between the binder, the active material, and the conductive auxiliary agent is lowered, and the durability may be lowered.
- the average degree of polymerization of PVA exceeds 3000, the solubility decreases and the viscosity increases, which may make it difficult to produce a positive electrode slurry.
- the average degree of polymerization of PVA here is a value measured by a method according to JIS K 6726.
- the graft copolymer preferably has a graft ratio within a specific range. Since a PAN homopolymer may be produced when a graft copolymer is produced (at the time of graft copolymerization), the calculation of the graft ratio involves the step of separating the PAN homopolymer from the graft copolymer. Necessary. A PAN homopolymer is soluble in dimethylformamide (hereinafter sometimes abbreviated as DMF), but PVA and graft-copolymerized PAN are not soluble in DMF. By utilizing this difference in solubility, the PAN homopolymer can be separated by an operation such as centrifugation.
- DMF dimethylformamide
- a graft copolymer having a known PAN content is immersed in a predetermined amount of DMF, and the PAN homopolymer is eluted in DMF. Next, the soaked liquid is separated into a DMF soluble part and a DMF insoluble part by centrifugation.
- the graft ratio of the graft copolymer obtained by the above formula (1) is preferably 20 to 150%, more preferably 30 to 140%, and further preferably 50 to 110%.
- the graft ratio of the graft copolymer is preferably 20% or more, more preferably 30% or more, and even more preferably 50% or more, oxidation resistance tends to increase.
- the graft ratio of the graft copolymer is preferably 150% or less, more preferably 140% or less, and even more preferably 110% or less, so that the binding property is easily increased.
- the binder composition of the present embodiment may contain a PAN homopolymer and a PVA homopolymer that can be produced when the graft copolymer is produced.
- the weight average molecular weight of the PAN homopolymer is preferably 30,000 to 250,000, more preferably 80000 to 150,000.
- the weight average molecular weight of the PAN homopolymer is preferably 250,000 or less, more preferably 200000 or less, so that the increase in viscosity of the PAN homopolymer can be suppressed and the positive electrode slurry can be easily produced. More preferably, it is as follows.
- the weight average molecular weight of the PAN homopolymer can be determined by GPC (gel permeation chromatography).
- the amount of PVA in the graft copolymer is preferably 40 to 80% by mass, and more preferably 50 to 65% by mass.
- the amount of PVA in the graft copolymer is preferably 40% by mass or more, and more preferably 50% by mass or more from the viewpoint of enhancing the binding property.
- the amount of PVA in the graft copolymer is preferably 80% by mass or less, and more preferably 65% by mass or less, from the viewpoint of enhancing oxidation resistance.
- the amount of PVA in the graft copolymer refers to the graft copolymer itself and acrylonitrile in the resin component including the PVA homopolymer and PAN homopolymer that can be formed during the copolymerization.
- the amount of PAN in the graft copolymer is preferably 20 to 60% by mass, more preferably 35 to 50% by mass.
- the amount of PAN in the graft copolymer is preferably 20% by mass or more, and more preferably 35% by mass or more, from the viewpoint of enhancing oxidation resistance.
- the amount of PAN in the graft copolymer is preferably 60% by mass or less, and more preferably 50% by mass or less from the viewpoint of enhancing the binding property.
- the amount of PAN in the graft copolymer refers to the graft copolymer itself and the graft copolymer to the PVA in the resin component including the PVA homopolymer and the PAN homopolymer that can be generated during the copolymerization. This refers to the total amount of the mass derived from polymerized acrylonitrile and the mass of the PAN homopolymer that can be produced during the graft copolymerization.
- composition ratio of the graft copolymer (composition ratio of the resin component in the binder composition) can be calculated from the reaction rate (polymerization rate) of acrylonitrile and the composition of the charged amount of each component used for the polymerization.
- the mass ratio of PAN produced at the time of copolymerization that is, the total amount of PAN grafted on PVA and PAN homopolymer can be calculated from the polymerization rate of acrylonitrile and the mass of charged acrylonitrile.
- the mass ratio of PVA and PAN is computable by taking ratio of the mass of this PAN, and the mass of the preparation of PVA.
- the mass% of PAN in the graft copolymer can be obtained from the following formula (2).
- Mass% of PAN in graft copolymer d ⁇ 0.01 ⁇ e / (f + d ⁇ 0.01 ⁇ e) ⁇ 100 (%) (2)
- d is the polymerization rate (%) of acrylonitrile
- e is the mass of acrylonitrile used in the graft copolymerization (preparation amount)
- f is the mass of the PVA used in the graft copolymerization (preparation amount).
- composition ratio of the graft copolymer can also be determined by 1 H-NMR.
- 1 H-NMR 1 H-NMR
- a monomer other than acrylonitrile is also used for graft copolymerization in addition to acrylonitrile, it is difficult to calculate by the above formula (2), and therefore it can be obtained by 1 H-NMR.
- the measurement of 1 H-NMR is performed using, for example, a trade name “ALPHA500” manufactured by JEOL Ltd., under the conditions of measurement solvent: dimethyl sulfoxide, measurement cell: 5 mm ⁇ , sample concentration: 50 mg / 1 ml, measurement temperature: 30 ° C. Can be done.
- the method for producing the binder composition of the present embodiment is not particularly limited, but after polymerization of polyvinyl acetate and saponification to obtain PVA, a method of graft copolymerizing a monomer mainly composed of acrylonitrile on PVA Is preferred.
- any known method such as bulk polymerization or solution polymerization can be used.
- Initiators used for polymerization of polyvinyl acetate include azo initiators such as azobisisobutyronitrile, and organic peroxides such as benzoyl peroxide and bis (4-t-butylcyclohexyl) peroxydicarbonate. Thing etc. are mentioned.
- the saponification reaction of polyvinyl acetate can be performed, for example, by a saponification method in an organic solvent in the presence of a saponification catalyst.
- organic solvent examples include methanol, ethanol, propanol, ethylene glycol, methyl acetate, ethyl acetate, acetone, methyl ethyl ketone, benzene, and toluene. These may be used alone or in combination of two or more. Of these, methanol is preferred.
- the saponification catalyst examples include basic catalysts such as sodium hydroxide, potassium hydroxide and sodium alkoxide, and acidic catalysts such as sulfuric acid and hydrochloric acid.
- basic catalysts such as sodium hydroxide, potassium hydroxide and sodium alkoxide
- acidic catalysts such as sulfuric acid and hydrochloric acid.
- sodium hydroxide is preferable from the viewpoint of saponification rate.
- the method of graft copolymerizing a monomer mainly composed of acrylonitrile with polyvinyl alcohol can be performed by solution polymerization.
- the solvent to be used include dimethyl sulfoxide, N-methylpyrrolidone and the like.
- Examples of the initiator used for graft copolymerization include organic peroxides such as benzoyl peroxide, azo compounds such as azobisisobutyronitrile, potassium peroxodisulfate, and ammonium peroxodisulfate.
- the binder composition of the present embodiment can be used after being dissolved in a solvent.
- the solvent include dimethyl sulfoxide and N-methylpyrrolidone. It is preferable that these solvents are contained in the binder composition, and these solvents may be contained in one kind or two or more kinds.
- the ratio of solid content (solid content concentration) including the graft copolymer, PVA homopolymer, PAN homopolymer, and the like in the binder composition of the present embodiment is not particularly limited, but is 1 mass from the viewpoint of increasing the binding property. % Or more, preferably 2% by mass or more, and more preferably 5% by mass or more. Further, from the viewpoint of workability such as easy application on the current collector, the ratio of the solid content in the binder composition is preferably 50% by mass or less, preferably 35% by mass or less, and preferably 20% by mass or less. Further preferred. Similarly, the content of the solvent in the binder composition is not particularly limited, but is preferably 50 to 99% by mass, preferably 65 to 98% by mass, and more preferably 80 to 95% by mass.
- the binder composition of the present embodiment described in detail above contains the graft copolymer described above, it has good binding properties with the positive electrode active material and the current collector, and is excellent in oxidation resistance. Therefore, a positive electrode slurry containing this binder composition is used to provide a lithium ion secondary battery excellent in cycle characteristics and rate characteristics using a high potential positive electrode active material, and an electrode from which such a lithium ion secondary battery can be obtained ( Positive electrode) can be obtained. Therefore, the binder composition of this embodiment is more suitable for a lithium ion secondary battery.
- the positive electrode slurry according to the embodiment of the present invention contains the binder composition described above, a positive electrode active material, and a conductive additive.
- LiCoO 2 LiNiO 2
- LiMn 2 O 4 and LiNi X Mn (2- X) O 4 (where 0 ⁇ X ⁇ 2) and other lithium transition metal complex oxides
- LiFePO 4 LiMnPO 4
- LiMn 2 O 4 LiNi X Mn (2-X) O 4 (where 0 ⁇ X ⁇ 2) is more preferable.
- the positive electrode slurry of the present embodiment can contain a conductive additive.
- the conductive auxiliary agent includes at least one selected from the group consisting of (i) fibrous carbon, (ii) carbon black, and (iii) a carbon composite in which fibrous carbon and carbon black are interconnected. It is preferable to use it.
- fibrous carbon include vapor growth carbon fiber, carbon nanotube, and carbon nanofiber.
- carbon black include acetylene black, furnace black, and ketjen black (registered trademark).
- These conductive assistants may be used alone or in combination of two or more. Among these, one or more selected from the group consisting of acetylene black, carbon nanotubes, and carbon nanofibers are preferable.
- the positive electrode slurry of the present embodiment may contain a carbon composite in which a plurality of conductive assistants and active materials are connected in order to improve the conductivity imparting ability and conductivity of the conductive assistant and the active material.
- a carbon composite in which fibrous carbon and carbon black are connected to each other, and a lithium-containing phosphate such as carbon-coated LiFePO 4 are added to the fibrous carbon and carbon.
- Examples include composites that are combined with black.
- a carbon composite in which fibrous carbon and carbon black are connected to each other can be obtained, for example, by firing a mixture of fibrous carbon and carbon black.
- what baked the mixture of this carbon composite and active materials, such as lithium containing phosphate can also be made into a carbon composite.
- the contents of the binder composition, the positive electrode active material, and the conductive additive are not particularly limited.
- the content of the above-mentioned binder composition is preferably 1 to 20% by mass, more preferably 2 to 15% by mass, and further preferably 3 to 10% by mass, based on the solid content in the binder composition.
- the content of the positive electrode active material is preferably 50 to 95% by mass, more preferably 60 to 95% by mass, and further preferably 70 to 90% by mass.
- the content of the conductive assistant is preferably 1 to 10% by mass, and more preferably 3 to 7% by mass.
- the content of the conductive auxiliary is preferably 1 to 10 parts by mass and more preferably 3 to 7 parts by mass in 100 parts by mass of the total amount of the binder, the active material and the conductive auxiliary.
- the content of the conductive additive is preferably 1 to 10 parts by mass and more preferably 3 to 7 parts by mass in 100 parts by mass of the total amount of the binder, the active material and the conductive auxiliary.
- the positive electrode which concerns on embodiment of this invention is manufactured using the above-mentioned slurry for positive electrodes.
- the positive electrode is preferably manufactured using a current collector and the above-described positive electrode slurry provided on the current collector. This positive electrode is preferably for a lithium ion secondary battery electrode.
- the positive electrode of the present embodiment is preferably produced by coating and drying the positive electrode slurry on the current collector.
- foil-like aluminum is preferably used as the current collector, and the thickness is preferably 5 to 30 ⁇ m from the viewpoint of workability.
- a known method can be used for coating the positive electrode slurry on the current collector. Examples thereof include a reverse roll method, a direct roll method, a blade method, a knife method, an extrusion method, a curtain method, a gravure method, a bar method, a dip method, and a squeeze method. Of these, blade method (comma roll or die cut), knife method and extrusion method are preferable. Under the present circumstances, the surface state of a favorable coating layer can be obtained by selecting a coating method according to the solution physical property and drying property of a binder.
- the application may be performed on one side or both sides, and in the case of both sides, each side may be sequentially or simultaneously.
- the application may be continuous, intermittent, or striped.
- the application thickness, length, and width of the positive electrode slurry may be appropriately determined according to the size of the battery.
- the coating thickness of the positive electrode slurry that is, the thickness of the positive electrode plate can be in the range of 10 ⁇ m to 500 ⁇ m.
- a method for drying the positive electrode slurry As a method for drying the positive electrode slurry, a generally adopted method can be used. In particular, it is preferable to use hot air, vacuum, infrared rays, far-infrared rays, electron beams and low-temperature air alone or in combination.
- the positive electrode can be pressed as necessary.
- a generally adopted method can be used, but a die pressing method and a calendar pressing method (cold or hot roll) are particularly preferable.
- the press pressure in the calendar press method is not particularly limited, but is preferably 0.2 to 3 ton / cm.
- a lithium ion secondary battery according to an embodiment of the present invention includes the above-described positive electrode, and is preferably manufactured using the positive electrode. More preferably, the above-described positive electrode, negative electrode, separator, and electrolyte solution (electrolyte) And an electrolytic solution).
- the negative electrode used for the lithium ion secondary battery of this embodiment is not specifically limited, It can manufacture using the slurry for negative electrodes containing a negative electrode active material.
- This negative electrode can be produced using, for example, a negative electrode current collector and a negative electrode slurry provided on the current collector.
- the negative electrode slurry preferably includes a negative electrode binder, a negative electrode active material, and the above-described conductive additive.
- the negative electrode binder is not particularly limited, and for example, polyvinylidene fluoride, polytetrafluoroethylene, styrene-butadiene copolymer, acrylic copolymer, and the like can be used.
- a fluorine-based resin is preferable, polyvinylidene fluoride and polytetrafluoroethylene are more preferable, and polyvinylidene fluoride is more preferable.
- Examples of the negative electrode active material used for the negative electrode include carbon materials such as graphite, polyacene, carbon nanotube, and carbon nanofiber, alloy materials such as tin and silicon, or tin oxide, silicon oxide, and lithium titanate. Examples thereof include oxide materials. These may be used alone or in combination of two or more.
- foil-shaped copper is preferably used as the current collector for the negative electrode, and the thickness is preferably 5 to 30 ⁇ m from the viewpoint of workability.
- the negative electrode can be produced using a negative electrode slurry and a negative electrode current collector by a method according to the above-described method for producing a positive electrode.
- separator Any separator can be used as long as it has sufficient strength, such as an electrically insulating porous film, a net, and a nonwoven fabric.
- a material that has low resistance to ion migration of the electrolytic solution and excellent in solution holding is not particularly limited, and examples thereof include inorganic fibers such as glass fibers or organic fibers, synthetic resins such as polyethylene, polypropylene, polyester, polytetrafluoroethylene, and polyflon, or layered composites thereof. From the viewpoint of adhesiveness and safety, polyethylene, polypropylene, or a layered composite film thereof is preferable.
- lithium salt is any conventionally available, LiClO 4, LiBF 4, LiBF 6, LiPF 6, LiCF 3 SO 3, LiCF 3 CO 2, LiAsF 6, LiSbF 6, LiB 10 Cl 10, LiAlCl 4 , LiCl, LiBr, LiI, LiB (C 2 H 5 ) 4 , LiCF 3 SO 3 , LiCH 3 SO 3 , LiCF 3 SO 3 , LiC 4 F 9 SO 3 , LiN (CF 3 SO 2 ) 2 , LiN ( Examples include C 2 F 5 SO 2 ) 2 , LiC (CF 3 SO 2 ) 3 and lower fatty acid lithium carboxylate.
- the electrolyte solution for dissolving the electrolyte is not particularly limited.
- the electrolyte include carbonates such as propylene carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate and methyl ethyl carbonate, lactones such as ⁇ -butyrolactone, trimethoxymethane, 1,2-dimethoxyethane, diethyl ether, 2 -Ethers such as ethoxyethane, tetrahydrofuran and 2-methyltetrahydrofuran, sulfoxides such as dimethyl sulfoxide, oxolanes such as 1,3-dioxolane and 4-methyl-1,3-dioxolane, acetonitrile, nitromethane and N-methyl- Nitrogen-containing compounds such as 2-pyrrolidone, esters such as methyl formate, methyl acetate, ethyl acetate, butyl acetate,
- an electrolyte solution in which LiPF 6 is dissolved in carbonates is preferable.
- concentration of the electrolyte in the solution varies depending on the electrode and the electrolyte used, but is 0.5 to 3 mol / L. Is preferred.
- Example 1 (Preparation of PVA) After adding 600 parts by mass of vinyl acetate and 400 parts by mass of methanol, degassing by bubbling nitrogen gas, 0.3 parts by mass of bis (4-tert-butylcyclohexyl) peroxydicarbonate was added as a polymerization initiator, Polymerization was carried out at 4 ° C. for 4 hours. The solid content concentration of the polymerization solution when the polymerization was stopped was 48%, and the polymerization rate of vinyl acetate determined from the solid content was 80%. Methanol vapor was blown into the obtained polymerization solution to remove unreacted vinyl acetate, and then diluted with methanol so that the concentration of polyvinyl acetate was 40% by mass.
- a binder means the graft copolymer by this invention. 15 parts by mass of the obtained PVA was added to 222.25 parts by mass of dimethyl sulfoxide and dissolved by stirring at 60 ° C. for 2 hours. Further, 0.124 parts by mass of ammonium peroxodisulfate dissolved in 50.63 parts by mass of acrylonitrile and 12 parts by mass of dimethyl sulfoxide was added at 55 ° C., and the graft copolymerization was performed while stirring at 60 ° C. Two hours after the start of polymerization, the polymerization was stopped by cooling to room temperature.
- composition ratio of the binder A was calculated from the composition of the reaction rate (polymerization rate) of acrylonitrile and the charged amount of each component used for the polymerization.
- the mass% of PAN produced at the time of copolymerization is the polymerization rate of acrylonitrile (%), the mass of acrylonitrile used for graft copolymerization (amount charged), and the graft copolymerization. It calculated using the formula (2) mentioned above from the mass (preparation amount) of the PVA used.
- the “mass ratio” in the table below is the mass ratio of the graft copolymer itself and the resin component including the PVA homopolymer and PAN homopolymer produced during the copolymerization.
- ⁇ Graft ratio> 1.00 g of binder A was weighed and added to 50 cc of special grade DMF (manufactured by Kokusan Chemical Co., Ltd.), and stirred at 80 ° C. for 24 hours. Next, this was centrifuged for 30 minutes at a rotational speed of 10,000 rpm with a centrifuge manufactured by Kokusan Co., Ltd. (model: H2000B, rotor: H). After carefully separating the filtrate (DMF soluble component), the pure water insoluble component was vacuum-dried at 100 ° C. for 24 hours, and the graft ratio was calculated using the above-described formula (1).
- acetylene black Denki Kagaku Kogyo Co., Ltd.
- Example 2 The charge during polymerization of polyvinyl acetate in Example 1 was changed to 500 parts by weight of vinyl acetate, 500 parts by weight of methanol, and 0.2 parts by weight of bis (4-tert-butylcyclohexyl) peroxydicarbonate, and at 60 ° C. for 5 hours. Polymerized. The polymerization rate was 75%. After removing unreacted vinyl acetate in the same manner as in Example 1, it was diluted with methanol so that the concentration of polyvinyl acetate was 30% by mass.
- Example 3 The charge during polymerization of polyvinyl acetate in Example 1 was changed to 0.15 parts by mass of bis (4-t-butylcyclohexyl) peroxydicarbonate, and polymerization was performed at 60 ° C. for 5 hours. The polymerization rate was 74%. After removing unreacted vinyl acetate in the same manner as in Example 1, saponification reaction was performed. It was diluted with methanol so that the concentration of polyvinyl acetate was 40% by mass. 20 parts by mass of a methanol solution of sodium hydroxide having a concentration of 10% by mass was added to 2000 parts by mass of this polyvinyl acetate solution, and a saponification reaction was carried out at 30 ° C. for 1 hour.
- Example 2 Neutralization, filtration and drying were carried out in the same manner as in Example 1 to obtain PVA having an average polymerization degree of 1170 and a saponification degree of 88.1 mol%.
- PAN was polymerized in the same manner as in Example 1 to prepare Binder C.
- the mass ratio of PVA and PAN in the binder C was 48:52, the graft ratio was 105%, and the weight average molecular weight of the PAN homopolymer was 103,000.
- Example 4 The charge during polymerization of polyvinyl acetate in Example 1 was changed to 800 parts by weight of vinyl acetate, 200 parts by weight of methanol, and 0.08 parts by weight of bis (4-tert-butylcyclohexyl) peroxydicarbonate, and 6 hours at 60 ° C. Polymerized. The polymerization rate was 70%. After removing unreacted vinyl acetate in the same manner as in Example 1, it was diluted with methanol so that the concentration of polyvinyl acetate was 20% by mass.
- Example 5 The charge during polymerization of polyvinyl acetate in Example 1 was changed to 1000 parts by mass of vinyl acetate and 0.01 parts by mass of bis (4-t-butylcyclohexyl) peroxydicarbonate, and polymerization was performed at 70 ° C. for 4 hours. The polymerization rate was 30%. After removing unreacted vinyl acetate in the same manner as in Example 1, it was diluted with methanol so that the concentration of polyvinyl acetate was 15% by mass. 20 parts by mass of a 10% strength by weight sodium hydroxide methanol solution was added to 1600 parts by mass of this polyvinyl acetate solution, and a saponification reaction was carried out at 30 ° C. for 2 hours.
- Example 2 Neutralization, filtration and drying were carried out in the same manner as in Example 1 to obtain PVA having an average polymerization degree of 3300 and a saponification degree of 85.2 mol%.
- PAN was polymerized in the same manner as in Example 1 to prepare Binder E.
- the mass ratio of PVA and PAN in Binder E was 53:47, the graft ratio was 84%, and the weight average molecular weight of the PAN homopolymer was 97,000.
- Binder F was prepared in the same manner as in Example 4 except that the saponification reaction time in Example 4 was 1.3 hours.
- the average degree of polymerization of the obtained PVA was 1800
- the degree of saponification was 71.2 mol%
- the mass ratio of PVA and PAN in the binder F was 48:52
- the graft ratio was 101%
- the weight average molecular weight of the PAN homopolymer. was 115,000.
- Binder G was prepared in the same manner as in Example 4 except that the saponification reaction time in Example 4 was 1.5 hours.
- the average degree of polymerization of the obtained PVA was 1790
- the degree of saponification was 80.3 mol%
- the mass ratio of PVA and PAN in the binder G was 53:47
- the graft ratio was 87%
- the weight average molecular weight of the PAN homopolymer. was 109000.
- Binder H was prepared in the same manner as in Example 4 except that the saponification reaction time in Example 4 was 3 hours.
- the average degree of polymerization of the obtained PVA was 1750
- the degree of saponification was 99.1 mol%
- the mass ratio of PVA and PAN in the binder H was 51:49
- the graft ratio was 92%
- the weight average molecular weight of the PAN homopolymer. was 99000.
- Binder I was prepared in the same manner as in Example 4 except that the polymerization time during PAN polymerization in Example 4 was 1.3 hours.
- the mass ratio of PVA and PAN in Binder I was 79:21, the graft ratio was 26%, and the weight average molecular weight of the PAN homopolymer was 110,000.
- Binder J was prepared in the same manner as in Example 4 except that the polymerization time during PAN polymerization in Example 4 was 1.5 hours.
- the mass ratio of PVA and PAN in the binder J was 63:37, the graft ratio was 55%, and the weight average molecular weight of the PAN homopolymer was 107,000.
- Binder K was prepared in the same manner as in Example 4 except that the polymerization time during PAN polymerization in Example 4 was 3 hours.
- the mass ratio of PVA and PAN in Binder K was 40:60, the graft ratio was 144%, and the weight average molecular weight of the PAN homopolymer was 105000.
- Binder L was prepared in the same manner as in Example 4 except that charging at the time of graft copolymerization in Example 4 was 50.5 parts by mass of acrylonitrile, 0.254 parts by mass of ammonium peroxodisulfate, and the polymerization time was 1 hour. It was.
- the mass ratio of PVA and PAN in the binder L was 54:46, the graft ratio was 110%, and the weight average molecular weight of the PAN homopolymer was 32,000.
- Example 13 The binder M was prepared in the same manner as in Example 4 except that the charging during graft copolymerization in Example 4 was 50.7 parts by mass of acrylonitrile, 0.054 parts by mass of ammonium peroxodisulfate, and the polymerization time was 5 hours. It was.
- the mass ratio of PVA and PAN in the binder M was 46:54, the graft ratio was 83%, and the weight average molecular weight of the PAN homopolymer was 245,000.
- Example 14 A binder N was prepared in the same manner as in Example 4 except that the charging at the time of graft copolymerization in Example 4 was changed to 47.84 parts by mass of acrylonitrile and 2.79 parts by mass of methyl methacrylate.
- the mass ratio of PVA, PAN and polymethyl methacrylate (PMMA) in binder N is 48: 48: 4, the graft ratio is 105%, the weight average molecular weight of the copolymer of PAN and PMMA is 121000, and graft copolymerization Acrylonitrile in the obtained monomer was 92% by mass in the total monomer.
- the graft ratio and the weight average molecular weight were measured by the same method as described in Example 1.
- composition ratio The mass ratio of PAN and PMMA in binder N was determined by 1 H-NMR. The 1 H-NMR measurement was performed under the following conditions.
- Example 1 The same procedure as in Example 1 was conducted except that the polymerization charge in polyvinyl acetate in Example 1 was changed to 500 parts by mass of vinyl acetate, 500 parts by mass of methanol, and 0.5 parts by mass of bis (4-t-butylcyclohexyl) peroxydicarbonate. Thus, PVA having an average polymerization degree of 110 and a saponification degree of 89.1 mol% was obtained. Using the obtained PVA, PAN was polymerized in the same manner as in Example 1 to prepare Binder O. The mass ratio of PVA and PAN in the binder O was 46:54, the graft ratio was 115%, and the weight average molecular weight of the PAN homopolymer was 99000. Further, the oxidative decomposition potential was measured in the same manner as in Example 1. The results are shown in Table 2.
- Example 2 The same polymerization as in Example 5 was carried out except that the charge during polymerization of polyvinyl acetate in Example 1 was 0.006 parts by mass of bis (4-t-butylcyclohexyl) peroxydicarbonate, and the average degree of polymerization was 5050, PVA having a saponification degree of 85.3 mol% was obtained. Using the obtained PVA, PAN was polymerized in the same manner as in Example 1 to prepare a binder P. The mass ratio of PVA and PAN in the binder P was 53:47, the graft ratio was 86%, and the weight average molecular weight of the PAN homopolymer was 119000.
- Example 3 Except for setting the saponification reaction time in Example 4 to 0.5 hours, the same operation as in Example 4 was performed to obtain PVA having an average polymerization degree of 1800 and a saponification degree of 51.2 mol%. Using the obtained PVA, PAN was polymerized in the same manner as in Example 1 to prepare Binder Q. The mass ratio of PVA and PAN in the binder Q was 47:53, the graft ratio was 107%, and the weight average molecular weight of the PAN homopolymer was 102,000.
- Example 4 During the PAN polymerization in Example 1, a PAN homopolymer was polymerized by performing polymerization without adding PVA, and a binder R was prepared. The weight average molecular weight of the PAN homopolymer was 115,000.
- Example 15 Using the binder A, a positive electrode slurry was prepared by the following method, and the peel adhesion strength was measured. Further, a positive electrode and a lithium ion secondary battery were prepared from the positive electrode slurry, and the discharge rate characteristics and the cycle characteristics were evaluated. The results are shown in Table 3.
- NMP N-methylpyrrolidone
- ⁇ Binding strength peeleling adhesive strength
- the obtained positive electrode slurry was coated on an aluminum foil so that the film thickness after drying was 100 ⁇ m, preliminarily dried at a temperature of 80 ° C. for 10 minutes, and then dried at 105 ° C. for 1 hour to obtain a positive electrode plate. Obtained.
- the obtained positive electrode plate was pressed with a roll press machine at a linear pressure of 0.2 to 3 ton / cm, and the thickness of the positive electrode plate was adjusted to 75 ⁇ m.
- the obtained positive electrode plate was cut into a width of 1.5 cm, an adhesive tape was attached to the surface of the positive electrode active material, and a stainless steel plate and a tape attached to the positive electrode plate were attached together with a double-sided tape.
- an adhesive tape was attached to an aluminum foil to obtain a test piece.
- the stress was measured when the adhesive tape affixed to the aluminum foil was peeled off at a speed of 50 mm / min in the direction of 180 ° in an atmosphere of 23 ° C. and 50% relative humidity. This measurement was repeated 5 times to obtain an average value, which was defined as peel adhesion strength.
- the prepared positive electrode slurry was applied to both sides of an aluminum foil having a thickness of 20 ⁇ m by an automatic coating machine so that each side was 140 g / m 2, and preliminarily dried at 80 ° C. for 10 minutes.
- the film was pressed with a roll press machine at a linear pressure of 0.2 to 3 ton / cm so that the thickness of the positive electrode current collector was 148 ⁇ m on both sides.
- the current collector was cut into a width of 54 mm to produce a strip-shaped current collector sheet.
- the positive electrode was obtained by drying at 105 ° C. for 1 hour in order to completely remove volatile components such as residual solvent and adsorbed moisture.
- the film was pressed with a roll press machine at a linear pressure of 0.2 to 3 ton / cm so that the thickness of the negative electrode current collector was 90 ⁇ m on both sides. Further, the negative electrode current collector was cut into a width of 54 mm to produce a short current collector sheet. A nickel current collector tab was ultrasonically welded to the end of the current collector sheet, and then dried at 105 ° C. for 1 hour to obtain a negative electrode in order to completely remove volatile components such as residual solvent and adsorbed moisture.
- Example 16 The binder A in Example 15 was changed to the binders shown in Table 3. Other than that, each evaluation was carried out in the same manner as in Example 15. The results are shown in Table 3.
- Example 16, 18, and 22 the high rate discharge capacity maintenance rate and the cycle capacity maintenance rate were measured.
- the high rate discharge capacity maintenance rate was 93%
- the cycle capacity maintenance rate was 79%
- the Examples 18, the high rate discharge capacity maintenance rate was 92%
- the cycle capacity maintenance rate was 81%
- Example 22 the high rate discharge capacity maintenance rate was 87% and the cycle capacity maintenance rate was 83%.
- Example 29 Except that Li (Ni 1/3 Mn 1/3 Co 1/3 ) O 2 is LiFePO 4 (“P2” manufactured by Clariant) and binder A is binder D, the method shown in Example 15 is used. A slurry, a positive electrode, a negative electrode, and a lithium ion secondary battery were prepared and subjected to various evaluations. The results are shown in Table 4. The high rate discharge capacity maintenance rate and the cycle capacity maintenance rate were evaluated with the voltage at the time of charging being 4.09 V and the voltage at the time of discharging being 2.09 V. As a result, the high rate discharge capacity retention rate was 90%, and the cycle capacity retention rate was 87%.
- Example 30 Except that Li (Ni 1/3 Mn 1/3 Co 1/3 ) O 2 was LiNi 0.5 Mn 1.5 O 4 and Binder A was Binder D, the positive electrode was prepared by the method shown in Example 15. Slurry, positive electrode, negative electrode and lithium ion secondary battery were prepared and subjected to various evaluations. The high rate discharge capacity maintenance rate and the cycle capacity maintenance rate were evaluated with the voltage at the time of charging being 4.99 V and the voltage at the time of discharging being 2.99 V. As a result, the high rate discharge capacity retention rate was 88%, and the cycle capacity retention rate was 75%.
- Examples 31 to 33 According to the formulation shown in Table 4, a positive electrode slurry, a positive electrode, a negative electrode, and a lithium ion secondary battery were prepared by the method shown in Example 15, and various evaluations were performed.
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Abstract
Description
[1] 平均重合度が300~3000で、かつ鹸化度が70~100モル%であるポリビニルアルコールに、アクリロニトリルを主成分とする単量体がグラフトしたグラフト共重合体を含有する、正極用バインダー組成物。
この正極用バインダー組成物は、以下の[2]又は[3]に記載の正極用バインダー組成物であってもよい。
[2] 前記グラフト共重合体のグラフト率が20~150%であり、該グラフト共重合時に生成するポリアクリロニトリルのホモポリマーの重量平均分子量が30000~250000である、[1]に記載の正極用バインダー組成物。
[3] 前記グラフト共重合体中の、ポリビニルアルコール量が40~80質量%であり、ポリアクリロニトリル量が60~20質量%である、[1]又は[2]に記載の正極用バインダー組成物。
また、本発明は、以下の[4]に記載の正極用スラリーを提供する。
[4] [1]~[3]のいずれか1つに記載のバインダー組成物と、正極活物質と、導電助剤とを含有する、正極用スラリー。
この正極用スラリーは、以下の[5]又は[6]に記載の正極用スラリーであってもよい。
[5] 上記正極活物質が、LiCoO2、LiNiO2、Li(CoXNiYMnZ)O2(但し、0<X<1、0<Y<1、0<Z<1、且つX+Y+Z=1)、Li(NiXAlYCoZ)O2(但し、0<X<1、0<Y<1、0<Z<1、且つX+Y+Z=1)、LiMn2O4、LiNiXMn(2-X)O4(但し、0<X<2)、LiFePO4、LiMnPO4、LiFeXMn(1-X)PO4(但し0<X<1)、LiCoPO4、Li3V2(PO4)3、及びLiNiPO4からなる群から選択される、少なくとも1種以上を含む、[4]に記載の正極用スラリー。
[6] 前記導電助剤が、(i)繊維状炭素、(ii)カーボンブラック、及び(iii)繊維状炭素とカーボンブラックとが相互に連結した炭素複合体からなる群から選択される少なくとも1種以上である、[4]又は[5]に記載の正極用スラリー。
さらに本発明は、以下の[7]に記載の正極及び以下の[8]に記載のリチウムイオン二次電池を提供する。
[7] [4]~[6]のいずれか一つに記載の正極用スラリーを用いて製造される正極。
[8] [7]に記載の正極を備えるリチウムイオン二次電池。
本発明の実施形態に係る正極用バインダー組成物(以下、「バインダー組成物」と称することがある。)は、ポリビニルアルコール(以下、PVAと略すことがある。)にアクリロニトリルを主成分とする単量体がグラフトしたグラフト共重合体を含有している。このグラフト共重合体は、ポリビニルアルコールの主鎖に、ポリアクリロニトリル(以下、PANと略すことがある。)の側枝が生成した共重合体である。バインダー組成物には、前記グラフト共重合体のほか、グラフト共重合に関与していない、PANのホモポリマー及び/又はPVAのホモポリマーが混在してもよい。したがって、本実施形態のバインダー組成物は、樹脂分(ポリマー分)として、グラフト共重合体のほか、PANホモポリマー及び/又はPVAホモポリマーを含有してもよい。
ここで、
a:測定に用いたグラフト共重合体の量、
b:測定に用いたグラフト共重合体中のPANの質量%、
c:DMF不溶分の量とすると、
グラフト率は、以下の式(1)により求めることができる。
グラフト率=[c-a×(100-b)×0.01]/[a×(100-b)×0.01]×100(%) ・・・(1)
共重合時に生成したPANの質量割合、すなわちPVAにグラフトしたPANとPANホモポリマーとの総量は、アクリロニトリルの重合率と仕込みのアクリロニトリルの質量とから、算出することができる。また、このPANの質量と、PVAの仕込みの質量との比を取ることで、PVAとPANの質量比を算出することができる。
具体的には、グラフト共重合体中のPANの質量%は、以下の式(2)から求めることができる。
グラフト共重合体中のPANの質量%=d×0.01×e/(f+d×0.01×e)×100(%)・・・(2)
ここで、上記式(2)中、dはアクリロニトリルの重合率(%)、eはグラフト共重合に使用したアクリロニトリルの質量(仕込み量)、fはグラフト共重合に使用したPVAの質量(仕込み量)を表す。
本発明の実施形態に係る正極用スラリーは、上述のバインダー組成物と、正極活物質と、導電助剤とを含有する。
正極に用いる正極活物質としては、特に限定されないが、リチウムと遷移金属からなる複合酸化物(リチウム遷移金属複合酸化物)、並びにリチウムと遷移金属のリン酸塩(リチウム遷移金属リン酸塩)からなる群から選択される少なくとも1種が好ましい。より具体的には、LiCoO2、LiNiO2、Li(CoXNiYMnZ)O2(但し、0<X<1、0<Y<1、0<Z<1、且つX+Y+Z=1)、Li(NiXAlYCoZ)O2(但し、0<X<1、0<Y<1、0<Z<1、且つX+Y+Z=1)、LiMn2O4、及びLiNiXMn(2-X)O4(但し、0<X<2)等のリチウム遷移金属複合酸化物、並びにLiFePO4、LiMnPO4、LiFeXMn(1-X)PO4(但し0<X<1)、LiCoPO4、Li3V2(PO4)3、及びLiNiPO4等のリチウム遷移金属リン酸塩を用いることができる。これらから選択される1種又は2種類以上の組み合わせの正極活物質を用いることが好ましい。これら正極活物質のうち、Li(CoXNiYMnZ)O2(但し、0<X<1、0<Y<1、0<Z<1、且つX+Y+Z=1)、Li(NiXAlYCoZ)O2(但し、0<X<1、0<Y<1、0<Z<1、且つX+Y+Z=1)、LiMn2O4、LiNiXMn(2-X)O4(但し、0<X<2)がより好ましい。
本実施形態の正極用スラリーには導電助剤を含有させることができる。導電助剤としては、(i)繊維状炭素、(ii)カーボンブラック、及び(iii)繊維状炭素とカーボンブラックとが相互に連結した炭素複合体からなる群から選択される少なくとも1種以上を用いることが好ましい。繊維状炭素としては、気相成長炭素繊維、カーボンナノチューブ及びカーボンナノファイバー等が挙げられる。カーボンブラックとしては、アセチレンブラック、ファーネスブラック及びケッチェンブラック(登録商標)等が挙げられる。これらの導電助剤は単体で用いてもよく、2種類以上を併用してもよい。これらの中ではアセチレンブラック、カーボンナノチューブ、及びカーボンナノファイバーからなる群から選択される1種又は2種以上が好ましい。
上述のバインダー組成物の含有量は、当該バインダー組成物中の固形分で、1~20質量%が好ましく、2~15質量%がより好ましく、3~10質量%がさらに好ましい。
上述の正極活物質の含有量は、50~95質量%が好ましく、60~95質量%がより好ましく、70~90質量%がさらに好ましい。
上述の導電助剤の含有量は、1~10質量%が好ましく、3~7質量%がより好ましい。
本発明の実施形態に係る正極は、上述の正極用スラリーを用いて製造される。この正極は、好ましくは集電体と、その集電体上に設けられる上述の正極用スラリーと、を用いて製造される。この正極は、好ましくはリチウムイオン二次電池電極用である。
本実施形態の正極は、好ましくは上述の正極用スラリーを集電体上に塗工及び乾燥することにより、製造される。例えば、集電体としては箔状のアルミニウムを用いることが好ましく、厚さは加工性の観点から5~30μmであることが好ましい。
正極用スラリーを集電体上に塗工する方法については、公知の方法を用いることができる。例えば、リバースロール法、ダイレクトロール法、ブレード法、ナイフ法、エクストルージョン法、カーテン法、グラビア法、バー法、ディップ法及びスクイーズ法を挙げることができる。そのなかでもブレード法(コンマロール又はダイカット)、ナイフ法及びエクストルージョン法が好ましい。この際、バインダーの溶液物性、乾燥性に合わせて塗布方法を選定することにより、良好な塗布層の表面状態を得ることができる。塗布は片面に施しても、両面に施してもよく、両面の場合、片面ずつ逐次でも両面同時でもよい。また、塗布は連続でも間欠でもストライプでもよい。正極用スラリーの塗布厚みや長さ、巾は、電池の大きさに合わせて適宜決定すればよい。例えば、正極用スラリーの塗布厚み、すなわち、正極板の厚さは、10μm~500μmの範囲とすることができる。
本発明の実施形態に係るリチウムイオン二次電池は、上述の正極を備え、好適には、その正極を用いて製造され、より好適には、上述の正極、負極、セパレーター、並びに電解質溶液(電解質及び電解液)を含んで構成される。
本実施形態のリチウムイオン二次電池に用いられる負極は、特に限定されないが、負極活物質を含む負極用スラリーを用いて製造することができる。この負極は、例えば、負極用集電体と、その集電体上に設けられる負極用スラリーとを用いて製造することができる。負極用スラリーは、負極用バインダーと、負極活物質と、前述の導電助剤とを含むことが好ましい。負極用バインダーとしては、特に限定されないが、例えば、ポリフッ化ビニリデン、ポリテトラフルオロエチレン、スチレン-ブタジエン系共重合体、及びアクリル系共重合体等を用いることができる。負極バインダーとしては、フッ素系樹脂が好ましく、ポリフッ化ビニリデン、ポリテトラフルオロエチレンがより好ましく、ポリフッ化ビニリデンがさらに好ましい。
例えば、負極用の集電体としては箔状の銅を用いることが好ましく、厚さは加工性の観点から5~30μmであることが好ましい。負極は、前述の正極の製造方法に準じた方法にて、負極用スラリー及び負極集電体を用いて製造することができる。
セパレーターには、電気絶縁性の多孔質膜、網、不織布等、充分な強度を有するものであればどのようなものでも使用可能である。特に、電解液のイオン移動に対して低抵抗であり、かつ、溶液保持に優れたものを使用するとよい。材質は特に限定しないが、ガラス繊維等の無機物繊維又は有機物繊維、ポリエチレン、ポリプロピレン、ポリエステル、ポリテトラフロオロエチレン、及びポリフロン等の合成樹脂若しくはこれらの層状複合体等を挙げることができる。接着性及び安全性の観点からポリエチレン、ポリプロピレン又はこれらの層状複合膜が好ましい。
電解質としては、従来より公知のリチウム塩がいずれも使用でき、LiClO4、LiBF4,LiBF6、LiPF6、LiCF3SO3、LiCF3CO2、LiAsF6、LiSbF6、LiB10Cl10、LiAlCl4、LiCl、LiBr、LiI、LiB(C2H5)4、LiCF3SO3、LiCH3SO3、LiCF3SO3、LiC4F9SO3、LiN(CF3SO2)2、LiN(C2F5SO2)2、LiC(CF3SO2)3、低級脂肪酸カルボン酸リチウム等が例として挙げられる。
上記電解質を溶解させる電解液は、特に限定されない。電解液としては、例えば、プロピレンカーボネート、エチレンカーボネート、ジメチルカーボネート、ジエチルカーボネート及びメチルエチルカーボネート等のカーボネート類、γ-ブチロラクトン等のラクトン類、トリメトキシメタン、1,2-ジメトキシエタン、ジエチルエーテル、2-エトキシエタン、テトラヒドロフラン及び2-メチルテトラヒドロフラン等のエーテル類、ジメチルスルホキシド等のスルホキシド類、1,3-ジオキソラン及び4-メチル-1,3-ジオキソラン等のオキソラン類、アセトニトリル、ニトロメタン及びN-メチル-2-ピロリドン等の含窒素化合物類、ギ酸メチル、酢酸メチル、酢酸エチル、酢酸ブチル、プロピオン酸メチル、プロピオン酸エチル及びリン酸トリエステル等のエステル類、硫酸エステル、硝酸エステル及び塩酸エステル等の無機酸エステル類、ジメチルホルムアミド及びジメチルアセトアミド等のアミド類、ジグライム、トリグライム及びテトラグライム等のグライム類、アセトン、ジエチルケトン、メチルエチルケトン及びメチルイソブチルケトン等のケトン類、スルホラン等のスルホラン類、3-メチル-2-オキサゾリジノン等のオキサゾリジノン類、並びに1,3-プロパンサルトン、4-ブタンスルトン及びナフタスルトン等のスルトン類等が挙げられる。これらの電解液の中から選択される1種類以上を使用することができる。
(PVAの調製)
酢酸ビニル600質量部及びメタノール400質量部を仕込み、窒素ガスをバブリングして脱酸素したのち、重合開始剤としてビス(4-t-ブチルシクロヘキシル)パーオキシジカーボネート0.3質量部を仕込み、60℃で4時間重合させた。重合停止時の重合溶液の固形分濃度は48%であり、固形分から求めた酢酸ビニルの重合率は80%であった。得られた重合溶液にメタノール蒸気を吹き込んで、未反応の酢酸ビニルを除去したのち、ポリ酢酸ビニルの濃度が40質量%になるようにメタノールで希釈した。
希釈したポリ酢酸ビニル溶液1200質量部に、濃度10質量%の水酸化ナトリウムのメタノール溶液20質量部を添加して、30℃で1.5時間鹸化反応を行った。
鹸化後の溶液を酢酸で中和し、濾過して100℃で2時間乾燥させてPVAを得た。得られたPVAの平均重合度は330、鹸化度は89.2モル%であった。
PVAの平均重合度及び鹸化度は、JIS K 6726に準ずる方法で測定した。
以下にバインダーAの調製方法を記す。尚、本[実施例]において、バインダーとは本発明によるグラフト共重合体を意味する。
得られたPVA15質量部を、ジメチルスルホキシド222.25質量部に添加し、60℃にて2時間撹拌して溶解させた。さらに、アクリロニトリル50.63質量部とジメチルスルホキシド12質量部に溶解させたペルオキソ二硫酸アンモニウム0.124質量部を55℃にて添加し、60℃で撹拌しながらグラフト共重合させた。重合開始より2時間後、室温まで冷却し重合を停止させた。
得られたバインダーAを含む反応液300質量部をメタノール3000質量部中に滴下し、バインダーAを析出させた。濾過してポリマーを分離して室温で2時間真空乾燥させ、更に80℃で2時間真空乾燥させた。固形分は10.2質量%で、アクリロニトリルの重合率は固形分より計算すると30.9%であった。
得られたバインダーA中のPANの質量は全ポリマーの51質量%であり、グラフト率は102%、PANのホモポリマーの重量平均分子量は105000であった。これらの測定方法は、後記の<組成比>、<グラフト率>及び<重量平均分子量>において説明する。また、後述する方法により求めた酸化分解電位は、6.6Vであった。結果を後記表1に示した。
バインダーAの組成比はアクリロニトリルの反応率(重合率)と重合に用いた各成分の仕込み量の組成から計算した。共重合時に生成したPANの質量%(グラフト共重合体中のPANの質量%)は、アクリロニトリルの重合率(%)、グラフト共重合に使用したアクリロニトリルの質量(仕込み量)、及びグラフト共重合に使用したPVAの質量(仕込み量)から、先述した式(2)を用いて算出した。なお、後記表中の「質量比」は、グラフト共重合体自体、並びにその共重合時に生成するPVAホモポリマー及びPANホモポリマーを含む樹脂分中の質量比である。
バインダーAを1.00g正秤し、これを特級DMF(国産化学株式会社製)50ccに添加し、80℃にて24時間撹拌した。次に、これを株式会社コクサン製の遠心分離機(型式:H2000B、ローター:H)にて回転数10000rpmで30分間遠心分離した。ろ液(DMF可溶分)を注意深く分離後、純水不溶分を100℃にて24時間真空乾燥し、先述した式(1)を用いグラフト率を計算した。
遠心分離の際のろ液(DMF可溶分)をメタノール1000mlに投入し、析出物を得た。析出物を80℃にて24時間真空乾燥し、GPCにて標準ポリスチレン換算の重量平均分子量を測定した。尚、GPCの測定は以下の条件にて行った。
カラム:GPC LF-804、φ8.0×300mm(昭和電工株式会社製)を2本直列に繋いで用いた。
カラム温度:40℃
溶媒:20mM-LiBr/DMF
バインダーA10質量部を、N-メチルピロリドン90質量部に溶解させ、得られたポリマー溶液100質量部にアセチレンブラック(電気化学工業株式会社製のデンカブラック(登録商標)「HS-100」)1質量部を加えて撹拌した。得られた溶液をアルミ箔上に乾燥後の厚さが20μmとなるように塗工し、80℃で10分間予備乾燥したのちに、105℃で1時間乾燥させて試験片とした。
作用極に得られた試験片、対極及び参照極にリチウム、電解液にLiPF6を電解質塩とするエチレンカーボネート/ジエチルカーボネート(=1/2(体積比))溶液(濃度1mol/L)を用いて東洋システム株式会社製の3極セルを組み立てた。ソーラートロン社製のポテンショ/ガルバノスタット(1287型)を用いてリニアースイープボルタンメトリー(以下LSVと略す)を25℃で10mV/secの走査速度にて行った。酸化分解電位を電流が0.1mA/cm2に達した時の電位と定めた。酸化分解電位が高い程、酸化分解しにくく耐酸化性が高いと判断される。
実施例1におけるポリ酢酸ビニル重合時の仕込みを酢酸ビニル500質量部、メタノール500質量部、ビス(4-t-ブチルシクロヘキシル)パーオキシジカーボネート0.2質量部へ変更し、60℃で5時間重合した。重合率は75%であった。実施例1と同様に未反応の酢酸ビニルを除去したのち、ポリ酢酸ビニルの濃度が30質量%となるようにメタノールで希釈した。このポリ酢酸ビニル溶液2000質量部に濃度10質量%の水酸化ナトリウムのメタノール溶液を20質量部添加して、30℃で1時間鹸化反応を行った。
実施例1と同様にして中和、濾過、乾燥を行い、平均重合度540、鹸化度88.8モル%のPVAを得た。
得られたPVAを用いて実施例1と同様にしてPANの重合を行い、バインダーBを調製した。バインダーBのPVAとPANの質量比は46:54であり、グラフト率は109%、PANのホモポリマーの重量平均分子量は110000であった。この組成比、グラフト率、及びPANホモポリマーの重量平均分子量については、実施例1で述べた方法と同様の方法により測定した。以下の実施例3~13も同様である。
実施例1におけるポリ酢酸ビニル重合時の仕込みをビス(4-t-ブチルシクロヘキシル)パーオキシジカーボネート0.15質量部へ変更し、60℃で5時間重合した。重合率は74%であった。実施例1と同様に未反応の酢酸ビニルを除去したのち、鹸化反応を行った。ポリ酢酸ビニルの濃度が40質量%となるようにメタノールで希釈した。このポリ酢酸ビニル溶液2000質量部に濃度10質量%の水酸化ナトリウムのメタノール溶液を20質量部添加して、30℃で1時間鹸化反応を行った。
実施例1と同様にして中和、濾過、乾燥を行い、平均重合度1170、鹸化度88.1モル%のPVAを得た。
得られたPVAを用いて実施例1と同様にしてPANの重合を行い、バインダーCを調製した。バインダーCのPVAとPANの質量比は48:52であり、グラフト率は105%、PANのホモポリマーの重量平均分子量は103000であった。
実施例1におけるポリ酢酸ビニル重合時の仕込みを酢酸ビニル800質量部、メタノール200質量部、ビス(4-t-ブチルシクロヘキシル)パーオキシジカーボネート0.08質量部へ変更し、60℃で6時間重合した。重合率は70%であった。実施例1と同様に未反応の酢酸ビニルを除去したのち、ポリ酢酸ビニルの濃度が20質量%となるようにメタノールで希釈した。このポリ酢酸ビニル溶液2800質量部に濃度10質量%の水酸化ナトリウムのメタノール溶液を20質量部添加して、30℃で2時間鹸化反応を行った。
実施例1と同様にして中和、濾過、乾燥を行い、平均重合度1760、鹸化度87.6モル%のPVAを得た。
得られたPVAを用いて実施例1と同様にしてPANの重合を行い、バインダーDを調製した。バインダーDのPVAとPANの質量比は50:50であり、グラフト率は98%、PANのホモポリマーの重量平均分子量は114000であった。
実施例1におけるポリ酢酸ビニル重合時の仕込みを酢酸ビニル1000質量部、ビス(4-t-ブチルシクロヘキシル)パーオキシジカーボネート0.01質量部へ変更し、70℃で4時間重合した。重合率は30%であった。実施例1と同様に未反応の酢酸ビニルを除去したのち、ポリ酢酸ビニルの濃度が15質量%となるようにメタノールで希釈した。このポリ酢酸ビニル溶液1600質量部に濃度10質量%の水酸化ナトリウムのメタノール溶液を20質量部添加して、30℃で2時間鹸化反応を行った。
実施例1と同様にして中和、濾過、乾燥を行い、平均重合度3300、鹸化度85.2モル%のPVAを得た。
得られたPVAを用いて実施例1と同様にしてPANの重合を行い、バインダーEを調製した。バインダーEのPVAとPANの質量比は53:47であり、グラフト率は84%、PANのホモポリマーの重量平均分子量は97000であった。
実施例4における鹸化反応時間を1.3時間とした以外は実施例4と同様にしてバインダーFの調製を行った。得られたPVAの平均重合度は1800、鹸化度は71.2モル%、バインダーFのPVAとPANの質量比は48:52であり、グラフト率は101%、PANのホモポリマーの重量平均分子量は115000であった。
実施例4における鹸化反応時間を1.5時間とした以外は実施例4と同様にしてバインダーGの調製を行った。得られたPVAの平均重合度は1790、鹸化度は80.3モル%、バインダーGのPVAとPANの質量比は53:47であり、グラフト率は87%、PANのホモポリマーの重量平均分子量は109000であった。
実施例4における鹸化反応時間を3時間とした以外は実施例4と同様にしてバインダーHの調製を行った。得られたPVAの平均重合度は1750、鹸化度は99.1モル%、バインダーHのPVAとPANの質量比は51:49であり、グラフト率は92%、PANのホモポリマーの重量平均分子量は99000であった。
実施例4におけるPAN重合時の重合時間を1.3時間とした以外は実施例4と同様にしてバインダーIの調製を行った。バインダーIのPVAとPANの質量比は79:21であり、グラフト率は26%、PANのホモポリマーの重量平均分子量は110000であった。
実施例4におけるPAN重合時の重合時間を1.5時間とした以外は実施例4と同様にしてバインダーJの調製を行った。バインダーJのPVAとPANの質量比は63:37であり、グラフト率は55%、PANのホモポリマーの重量平均分子量は107000であった。
実施例4におけるPAN重合時の重合時間を3時間とした以外は実施例4と同様にしてバインダーKの調製を行った。バインダーKのPVAとPANの質量比は40:60であり、グラフト率は144%、PANのホモポリマーの重量平均分子量は105000であった。
実施例4におけるグラフト共重合時の仕込みを、アクリロニトリル50.5質量部、ペルオキソ二硫酸アンモニウム0.254質量部、重合時間を1時間とした以外は実施例4と同様にしてバインダーLの調製を行った。バインダーLのPVAとPANの質量比は54:46であり、グラフト率は110%、PANのホモポリマーの重量平均分子量は32000であった。
実施例4におけるグラフト共重合時の仕込みを、アクリロニトリル50.7質量部、ペルオキソ二硫酸アンモニウム0.054質量部、重合時間を5時間とした以外は実施例4と同様にしてバインダーMの調製を行った。バインダーMのPVAとPANの質量比は46:54であり、グラフト率は83%、PANのホモポリマーの重量平均分子量は245000であった。
実施例4におけるグラフト共重合時の仕込みを、アクリロニトリル47.84質量部、メタクリル酸メチル2.79質量部とした以外は実施例4と同様にしてバインダーNの調製を行った。バインダーNのPVAとPANとポリメタクリル酸メチル(PMMA)との質量比は48:48:4であり、グラフト率は105%、PANとPMMAの共重合体の重量平均分子量は121000、グラフト共重合した単量体中のアクリロニトリルは全単量体中92質量%であった。このグラフト率及び重量平均分子量については、実施例1で述べた方法と同様の方法により測定した。
バインダーNのPANとPMMAの質量比は1H-NMRにより求めた。尚、1H-NMRの測定は以下の条件にて行った。
装置:ALPHA500(日本電子株式会社製)
測定溶媒:ジメチルスルホキシド
測定セル:5mmφ
試料濃度:50mg/1ml
測定温度:30℃
実施例1におけるポリ酢酸ビニル重合時の仕込みを酢酸ビニル500質量部、メタノール500質量部、ビス(4-t-ブチルシクロヘキシル)パーオキシジカーボネート0.5質量部とした以外は実施例1と同様の操作を行い、平均重合度110、鹸化度89.1モル%のPVAを得た。
得られたPVAを用いて実施例1と同様にしてPANの重合を行い、バインダーOを調製した。バインダーOのPVAとPANの質量比は46:54であり、グラフト率は115%、PANのホモポリマーの重量平均分子量は99000であった。さらに、実施例1と同様にして酸化分解電位を測定した。結果を表2に示す。
実施例1におけるポリ酢酸ビニル重合時の仕込みを、ビス(4-t-ブチルシクロヘキシル)パーオキシジカーボネート0.006質量部とした以外は実施例5と同様の操作を行い、平均重合度5050、鹸化度85.3モル%のPVAを得た。
得られたPVAを用いて実施例1と同様にしてPANの重合を行い、バインダーPを調製した。バインダーPのPVAとPANの質量比は53:47であり、グラフト率は86%、PANのホモポリマーの重量平均分子量は119000であった。
実施例4における鹸化反応の時間を0.5時間とした以外は実施例4と同様の操作を行い、平均重合度1800、鹸化度51.2モル%のPVAを得た。
得られたPVAを用いて実施例1と同様にしてPANの重合を行い、バインダーQを調製した。バインダーQのPVAとPANの質量比は47:53であり、グラフト率は107%、PANのホモポリマーの重量平均分子量は102000であった。
実施例1におけるPAN重合時、PVAを添加せずに重合行うことでPANのホモポリマーを重合し、バインダーRを調製した。PANのホモポリマーの重量平均分子量は115000であった。
実施例4で調製したPVAをバインダーSとした。
バインダーAを使用し、以下の方法にて正極用スラリーを調製し、剥離接着強さを測定した。さらに正極用スラリーより正極及びリチウムイオン二次電池を作製し、放電レート特性及びサイクル特性の評価を行なった。結果を表3に示す。
得られたバインダーA8質量部を、N-メチルピロリドン(以下、NMPと略す)92質量部に溶解させてバインダー溶液とした。さらに、アセチレンブラック(電気化学工業株式会社製のデンカブラック(登録商標)「HS-100」)3.72質量部、繊維状炭素としてカーボンナノファイバーのNMP分散液(エムディーナノテック株式会社製の「MDCNT-D 5%NMP分散液」)を固形分換算で1.86質量部、バインダー溶液を固形分換算で7質量部を撹拌混合した。混合後、Li(Ni1/3Mn1/3Co1/3)O2(日本化学工業株式会社製の「セルシード(登録商標)111」)87.42質量部を加えて撹拌混合し、正極用スラリーを得た。
得られた正極用スラリーをアルミ箔上に乾燥後の膜厚が100μmとなるように塗工し、温度80℃で10分間予備乾燥を行った後に、105℃で1時間乾燥させて正極板を得た。得られた正極板をロールプレス機にて線圧0.2~3ton/cmでプレスし、正極板の厚さが75μmとなるように調節した。得られた正極板を1.5cmの幅に切断し、正極活物質面に粘着テープを貼りつけ、更にステンレス製の板と正極板に張り付けたテープとを両面テープで貼り合せた。さらに粘着テープをアルミ箔に張り付け試験片とした。アルミ箔に貼り付けた粘着テープを、23℃、相対湿度50%の雰囲気にて、180°方向に50mm/minの速度で引きはがした時の応力を測定した。この測定を5回繰り返して平均値を求め、剥離接着強さとした。
厚み20μmのアルミニウム箔両面に、調製した正極用スラリーを、自動塗工機で片面ずつ140g/m2となるように塗布し、80℃で10分間予備乾燥した。次に、ロールプレス機にて0.2~3ton/cmの線圧でプレスし、正極集電体の厚さが両面で148μmになるように調製した。さらに集電体を54mm幅に切断して、短冊状の集電体シートを作製した。集電体シートの端部にアルミニウム製の集電体タブを超音波溶着した後、残留溶媒や吸着水分といった揮発成分を完全に除去するため、105℃で1時間乾燥して正極を得た。
負極活物質として黒鉛(株式会社クレハ製の「カーボトロン(登録商標)P」)96.6質量部、バインダーとしてポリフッ化ビニリデン(株式会社クレハ製の「KFポリマー(登録商標)#1120」)を固形分換算で3.4質量部、さらに全固形分が50質量%となるように適量のNMPを加えて撹拌混合して、負極用スラリーを得た。
厚み10μmの銅箔の両面に、調製した負極用スラリーを、自動塗工機で片面ずつ70g/m2となるように塗布し、80℃で10分間予備乾燥した。次に、ロールプレス機にて0.2~3ton/cmの線圧でプレスし、負極集電体の厚さが両面で90μmになるように調製した。さらに負極集電体を54mm幅に切断して短細状の集電体シートを作製した。集電体シートの端部にニッケル製の集電体タブを超音波溶着した後、残留溶媒や吸着水分といった揮発成分を完全に除去するため、105℃で1時間乾燥して負極を得た。
得られた正極と負極とを組合せ、厚み25μm、幅60mmのポリエチレン微多孔膜セパレーターを介して捲回し、スパイラル状の捲回群を作製した後、これを電池缶に挿入した。次いで、電解質としてLiPF6を1mol/Lの濃度で溶解した非水溶液系の電解液(エチレンカーボネート/メチルエチルカーボネート=30/70(質量比)混合液)を電池容器に5ml注入した後、注入口をかしめて密閉し、直径18mm、高さ65mmの円筒形のリチウム二次電池を作製した。作製したリチウムイオン二次電池について、以下の方法により電池性能を評価した。
作製したリチウムイオン二次電池を、25℃において4.29V、0.2ItA制限の定電流定電圧充電をした後、0.2ItAの定電流で2.69Vまで放電した。次いで、放電電流を0.2ItA、1ItAと変化させ、各放電電流に対する放電容量を測定した。各測定における回復充電は4.29V(1ItAカット)の定電流定電圧充電を行った。そして、二回目の0.2ItA放電時に対する1ItA放電時の高率放電容量維持率を計算した。
環境温度25℃にて、充電電圧4.29V、1ItAの定電流定電圧充電と、放電終止電圧2.69Vの1ItAの定電流放電を行った。充電及び放電のサイクルを繰り返し行い、1サイクル目の放電容量に対する500サイクル目の放電容量の比率を求めてサイクル容量維持率とした。
実施例15におけるバインダーAを、表3に示したバインダーに変更した。それ以外は実施例15と同様な方法で各評価を実施した。結果を表3に示す。尚、実施例16、18、22については高率放電容量維持率及びサイクル容量維持率を測定したが、実施例16では、高率放電容量維持率93%、サイクル容量維持率79%、実施例18では、高率放電容量維持率92%、サイクル容量維持率81%、実施例22では、高率放電容量維持率87%、サイクル容量維持率83%であった。
Li(Ni1/3Mn1/3Co1/3)O2をLiFePO4(クラリアント社製「P2」)とし、バインダーAをバインダーDとした以外は、実施例15に示した方法で正極用スラリー、正極、負極及びリチウムイオン二次電池を作製し、各種評価を実施した。結果を表4に示す。尚、高率放電容量維持率及びサイクル容量維持率は、充電時の電圧を4.09V、放電時の電圧を2.09Vとして評価を実施した。この結果、高率放電容量維持率は90%、サイクル容量維持率は87%であった。
Li(Ni1/3Mn1/3Co1/3)O2をLiNi0.5Mn1.5O4とし、バインダーAをバインダーDとした以外は、実施例15に示した方法で、正極用スラリー、正極、負極及びリチウムイオン二次電池を作製し、各種評価を実施した。尚、高率放電容量維持率及びサイクル容量維持率は、充電時の電圧を4.99V、放電時の電圧を2.99Vとして評価を実施した。この結果、高率放電容量維持率は88%、サイクル容量維持率は75%であった。
表4に示した配合に従い、実施例15に示した方法で、正極用スラリー、正極、負極及びリチウムイオン二次電池を作製し、各種評価を実施した。
表5に示したバインダーに変更し、実施例15に示した方法で、正極用スラリー、正極、負極及びリチウムイオン二次電池を作製し、各種評価を実施した。
Claims (8)
- 平均重合度が300~3000で、かつ鹸化度が70~100モル%であるポリビニルアルコールに、アクリロニトリルを主成分とする単量体がグラフトしたグラフト共重合体を含有する、正極用バインダー組成物。
- 前記グラフト共重合体のグラフト率が20~150%であり、該グラフト共重合時に生成するポリアクリロニトリルのホモポリマーの重量平均分子量が30000~250000である、請求項1に記載の正極用バインダー組成物。
- 前記グラフト共重合体中の、ポリビニルアルコール量が40~80質量%であり、ポリアクリロニトリル量が60~20質量%である、請求項1又は2に記載の正極用バインダー組成物。
- 請求項1~3のいずれか一項に記載のバインダー組成物と、正極活物質と、導電助剤とを含有する、正極用スラリー。
- 前記正極活物質が、LiCoO2、LiNiO2、Li(CoXNiYMnZ)O2(但し、0<X<1、0<Y<1、0<Z<1、且つX+Y+Z=1)、Li(NiXAlYCoZ)O2(但し、0<X<1、0<Y<1、0<Z<1、且つX+Y+Z=1)、LiMn2O4、LiNiXMn(2-X)O4(但し、0<X<2)、LiFePO4、LiMnPO4、LiFeXMn(1-X)PO4(但し0<X<1)、LiCoPO4、Li3V2(PO4)3、及びLiNiPO4からなる群から選択される少なくとも1種以上を含む、請求項4に記載の正極用スラリー。
- 前記導電助剤が、(i)繊維状炭素、(ii)カーボンブラック、及び(iii)繊維状炭素とカーボンブラックとが相互に連結した炭素複合体からなる群から選択される少なくとも1種以上である、請求項4又は5に記載の正極用スラリー。
- 請求項4~6のいずれか一項に記載の正極用スラリーを用いて製造される正極。
- 請求項7に記載の正極を備えるリチウムイオン二次電池。
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Also Published As
Publication number | Publication date |
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CN105637686B (zh) | 2018-05-08 |
KR20160068811A (ko) | 2016-06-15 |
US20160240854A1 (en) | 2016-08-18 |
JP6416103B2 (ja) | 2018-10-31 |
US9941518B2 (en) | 2018-04-10 |
CN105637686A (zh) | 2016-06-01 |
KR102280534B1 (ko) | 2021-07-21 |
JPWO2015053224A1 (ja) | 2017-03-09 |
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