WO2014156195A1 - 二次電池電極用バインダー組成物及びその製造方法、二次電池電極用スラリー組成物、二次電池用電極、並びに、二次電池 - Google Patents
二次電池電極用バインダー組成物及びその製造方法、二次電池電極用スラリー組成物、二次電池用電極、並びに、二次電池 Download PDFInfo
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- WO2014156195A1 WO2014156195A1 PCT/JP2014/001852 JP2014001852W WO2014156195A1 WO 2014156195 A1 WO2014156195 A1 WO 2014156195A1 JP 2014001852 W JP2014001852 W JP 2014001852W WO 2014156195 A1 WO2014156195 A1 WO 2014156195A1
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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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- 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
- Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
- Y02P70/00—Climate change mitigation technologies in the production process for final industrial or consumer products
- Y02P70/50—Manufacturing or production processes characterised by the final manufactured product
Definitions
- the present invention relates to a binder composition for a secondary battery electrode and a manufacturing method thereof, a slurry composition for a secondary battery electrode, an electrode for a secondary battery, and a secondary battery.
- Secondary batteries especially lithium ion secondary batteries, are small and light, have high energy density, and can be repeatedly charged and discharged, and are used in a wide range of applications.
- the battery members such as the electrodes (positive electrode and negative electrode) of the secondary battery are obtained by binding the components contained in these battery members or the components and a substrate (for example, a current collector) with a binder.
- a substrate for example, a current collector
- a binder for example, a current collector
- an electrode of a secondary battery usually includes a current collector and an electrode mixture layer formed on the current collector.
- the electrode mixture layer is formed by, for example, applying an electrode slurry composition in which an electrode active material and a binder composition are dispersed in a dispersion medium on a current collector, and drying the electrode active material and the like with a binder. It is formed by binding.
- attempts have been made to improve the binder composition and slurry composition used for forming these battery members in order to achieve further performance improvement of the secondary battery.
- Patent Documents 1 to 3 it has been proposed to use a binder composition containing a specific copolymer for the production of a secondary battery electrode (see, for example, Patent Documents 1 to 3).
- the binder composition described in Patent Document 1 it contains a copolymer having specific properties obtained by emulsion polymerization of styrene, an ethylenically unsaturated carboxylic acid ester, an ethylenically unsaturated carboxylic acid, and an internal crosslinking agent. Therefore, the active material and the active material and the current collector can be satisfactorily bound in an aqueous dispersion, and by using such a binder composition, a lithium ion secondary battery excellent in charge / discharge high temperature cycle characteristics Can be provided.
- the binder composition described in Patent Document 2 contains a copolymer of an acrylate ester or a methacrylic ester and an ⁇ , ⁇ -unsaturated nitrile compound.
- An excellent positive electrode can be provided.
- a predetermined ⁇ , ⁇ -ethylenically unsaturated nitrile compound such as acrylonitrile is polymerized, and then a predetermined ethylenically unsaturated carboxylic acid such as 2-ethylhexyl acrylate is used. Since a copolymer (multistage polymer) obtained by adding and polymerizing an ester or the like is contained, a positive electrode excellent in binding power and flexibility can be provided.
- JP 2011-243464 A International Publication No. 2006/038652 Japanese Patent No. 4736804
- the present invention can provide a secondary battery that has excellent high-temperature storage characteristics and high-temperature cycle characteristics, in which generation of gas due to residual monomers is suppressed, and is excellent in stability over time.
- An object is to provide a binder composition.
- an object of this invention is to provide the slurry composition for secondary battery electrodes using the said binder composition.
- an object of this invention is to provide the electrode for secondary batteries using the said slurry composition for secondary battery electrodes.
- Another object of the present invention is to provide a secondary battery using the secondary battery electrode, which has excellent high-temperature storage characteristics and high-temperature cycle characteristics and reduced gas generation.
- a polymer is prepared once, and then a polymerization initiator such as persulfate is further added to further advance the polymerization reaction (that is, the residual monomer is polymerized). It is also conceivable to reduce the amount of residual monomer containing a (meth) acrylic acid ester monomer at or above.
- a polymerization initiator was further added and the polymerization reaction proceeded, a large amount of the residue of the polymerization initiator such as persulfate was present in the binder composition or the secondary. It became clear that the electrical characteristics of the secondary battery might be deteriorated due to inclusion in the battery.
- the present inventors conducted further intensive studies for the purpose of solving the above problems. And the present inventors polymerize the residual monomer by reducing the amount of residual monomer without using a large amount of peroxide by performing redox polymerization using a reductone compound after preparing a polymer once. I was inspired by that. Furthermore, the inventors of the present invention also provide a secondary battery electrode binder composition containing a particulate polymer prepared using a (meth) acrylate monomer, and a (meth) acrylate ester having a boiling point of 145 ° C. or higher.
- the present invention aims to advantageously solve the above problems, and the binder composition for a secondary battery electrode of the present invention comprises at least one of a particulate polymer, a reductone compound and an oxidant thereof.
- the water-containing particulate polymer is obtained by polymerizing a monomer mixture containing a (meth) acrylic acid ester monomer having a boiling point of 145 ° C. or higher, and the (meth) acrylic acid having a boiling point of 145 ° C. or higher.
- the content ratio of the ester monomer is 1 ⁇ 10 ⁇ 6 parts by mass to 1500 ⁇ 10 ⁇ 6 parts by mass with respect to 100 parts by mass of the particulate polymer.
- the binder composition for secondary battery electrodes is formed by using a reductone compound and limiting the content ratio of the (meth) acrylic acid ester monomer having a boiling point of 145 ° C. or higher to a predetermined range.
- the secondary battery obtained by using such a binder composition for a secondary battery electrode it is possible to suppress the generation of gas due to the residual monomer.
- the said binder composition for secondary battery electrodes is used, the secondary battery which has a favorable high temperature storage characteristic and a high temperature cycling characteristic can be provided.
- At least one of the reductone compound and its oxidant is selected from (iso) ascorbic acid and its salt, and their oxidant. It is preferable that This is because (iso) ascorbic acid and salts thereof, and oxidants thereof have little influence on battery characteristics and lead to improvement of slurry stability.
- the content ratio of at least one of the reductone compound and its oxidant is 0.05 parts by mass or more and 5 parts by mass per 100 parts by mass of the particulate polymer.
- the following is preferable.
- the content of at least one of a reductone compound and its oxidant while sufficiently reducing the amount of residual monomer such as a (meth) acrylate monomer having a boiling point of 145 ° C. or higher in the binder composition for secondary battery electrodes As a result, the electrical characteristics of the secondary battery produced using the binder composition for secondary battery electrodes deteriorated, and at least one of the reductone compound and its oxidant remaining in the binder composition was decomposed. This is because the generation of gas in the secondary battery can be suppressed.
- the monomer mixture preferably further contains a crosslinkable monomer.
- the degree of swelling of the electrode formed using the binder composition for a secondary battery electrode with respect to the electrolyte solution is set to an appropriate level, and the high-temperature storage characteristics and high temperature of a lithium ion secondary battery produced using such a binder composition This is because the cycle characteristics can be improved.
- the monomer mixture further contains 5 to 35% by mass of a (meth) acrylonitrile monomer. This is because the peel strength of the electrode mixture layer formed using the binder composition for secondary battery electrodes can be improved, and the degree of swelling with respect to the electrolytic solution can be made moderate.
- the (meth) acrylic acid ester monomer having a boiling point of 145 ° C. or higher is preferably 2-ethylhexyl acrylate. Since a polymer obtained using 2-ethylhexyl acrylate has high electrochemical stability and flexibility, current collection of an electrode mixture layer obtained using a binder composition for a secondary battery electrode blended with such a polymer. This is because the adhesion strength to the body can be improved, and as a result, the battery characteristics of the secondary battery having such an electrode mixture layer can be improved.
- the manufacturing method of the binder composition for secondary battery electrodes of this invention is (meth) acrylic acid ester single-piece
- a step (2) of 1 ⁇ 10 ⁇ 6 parts by mass to 1500 ⁇ 10 ⁇ 6 parts by mass with respect to 100 parts by mass of the combined product a step (2) of 1 ⁇ 10 ⁇ 6 parts by mass to 1500 ⁇ 10 ⁇ 6 parts by mass with respect to 100 parts by mass of the combined product .
- the slurry composition for secondary battery electrodes of this invention is the binder composition and electrode for any of the above-mentioned secondary battery electrodes. It is characterized by containing an active material.
- Such a slurry composition for a secondary battery electrode is excellent in stability over time, and when such a slurry composition for a secondary battery electrode is used, generation of gas due to residual monomers is suppressed, and good high-temperature storage characteristics and A secondary battery having high-temperature cycle characteristics can be provided.
- the electrode for secondary batteries of this invention apply
- the secondary battery of this invention is equipped with a positive electrode, a negative electrode, electrolyte solution, and a separator, and at least one of the said positive electrode and the said negative electrode is The electrode for a secondary battery described above.
- Such a secondary battery has excellent high-temperature storage characteristics and high-temperature cycle characteristics, and gas generation is suppressed.
- ADVANTAGE OF THE INVENTION According to this invention, generation
- a binder composition can be provided.
- the slurry composition for secondary battery electrodes using the said binder composition can be provided.
- the electrode for secondary batteries using the said slurry composition for secondary battery electrodes can be provided.
- the binder composition for secondary battery electrodes of the present invention is used when forming an electrode of a secondary battery.
- the slurry composition for secondary battery electrodes of this invention is prepared including the binder composition for secondary battery electrodes of this invention, and an electrode active material.
- the manufacturing method of the binder composition for secondary battery electrodes of this invention can be used when manufacturing the binder composition for secondary battery electrodes of this invention.
- the secondary battery electrode of the present invention can be produced using the slurry composition for the secondary battery electrode of the present invention, and the lithium ion secondary battery of the present invention is the secondary battery electrode of the present invention. It is characterized by using.
- the binder composition for secondary battery electrodes of the present invention is an aqueous binder composition using water as a dispersion medium, and includes at least one of a particulate polymer, a reductone compound, and an oxidant thereof.
- the particulate polymer is a particulate polymer produced by polymerizing a monomer mixture containing a (meth) acrylic acid ester monomer having a boiling point of 145 ° C. or higher.
- the content ratio of the (meth) acrylic acid ester monomer having a boiling point of 145 ° C. or higher is 1 ⁇ 10 ⁇ 6 mass relative to 100 mass parts of the particulate polymer.
- the (meth) acrylic acid ester monomer having a boiling point of 145 ° C. or higher contained in the binder composition of the present invention is a polymerization of a monomer mixture containing a (meth) acrylic acid ester monomer having a boiling point of 145 ° C. or higher.
- “(meth) acryl” refers to acryl and / or methacryl.
- the component contained in the electrode mixture layer (for example, the electrode active material) is produced from the electrode in the manufactured electrode. It is a component that can be held so as not to be detached.
- a particulate polymer to be blended in the binder composition a particulate polymer produced by polymerizing a monomer mixture containing a (meth) acrylic acid ester monomer having a boiling point of 145 ° C. or higher described below is used. be able to.
- a particulate polymer is produced
- the binder composition of the present invention is used for forming a positive electrode (secondary battery positive electrode binder composition)
- the particulate polymer is an ⁇ , ⁇ -unsaturated nitrile monomer such as a (meth) acrylonitrile monomer.
- (meth) acrylonitrile refers to acrylonitrile and / or methacrylonitrile.
- the monomer mixture used when polymerizing the particulate polymer contains a (meth) acrylic acid ester monomer having a boiling point of 145 ° C. or higher.
- the electrochemical stability and flexibility of the particulate polymer obtained by polymerizing the monomer mixture can be enhanced.
- the adhesive strength of the electrode compound layer using a binder composition can be raised and the peel strength of an electrode can be improved, the battery characteristic of a secondary battery can be improved.
- examples of the (meth) acrylic acid ester monomer include compounds represented by the formula (I): CH 2 ⁇ CR 1 —COOR 2 .
- R 1 represents a hydrogen atom or a methyl group
- R 2 represents an alkyl group, a cycloalkyl group, or a functional group in which a part thereof is substituted.
- Examples of (meth) acrylate monomers having a boiling point of 145 ° C. or higher include n-butyl acrylate (BA), n-amyl acrylate, isoamyl acrylate, n-hexyl acrylate, 2-ethylhexyl acrylate.
- acrylates such as 2-methoxyethyl acrylate, 2-ethoxyethyl acrylate, hexyl acrylate, nonyl acrylate, lauryl acrylate, stearyl acrylate, and benzyl acrylate; n-butyl methacrylate, methacrylic acid Isobutyl, n-amyl methacrylate, isoamyl methacrylate, n-hexyl methacrylate, 2-ethylhexyl methacrylate, octyl methacrylate, isodecyl methacrylate, lauryl methacrylate, tridecyl methacrylate, stear methacrylate Le, and the like methacrylates such as benzyl methacrylate.
- These monomers may be used individually by 1 type, and may be used in combination of 2 or more types.
- acrylate is preferable, 2-ethylhexyl acrylate and n-butyl acrylate are preferable in terms of improving the peel strength of the secondary battery electrode, and 2-ethylhexyl acrylate is particularly preferable.
- 2-ethylhexyl acrylate has a long side chain and reduces Tg of a polymer having monomer units derived from 2-ethylhexyl acrylate to improve flexibility and to improve electrochemical stability
- a polymer Improved the adhesion strength of the electrode mixture layer obtained by using the binder composition for secondary battery electrodes blended with the current collector, and thus the battery characteristics of the secondary battery using the electrode having the electrode mixture layer It is because it can improve.
- the boiling point of the (meth) acrylic acid ester monomer can be measured according to JIS K2254.
- the content ratio of the (meth) acrylic acid ester monomer having a boiling point of 145 ° C. or higher is preferably 10% by mass or more, more preferably 15% by mass in the monomer mixture. % Or more, still more preferably 30% by mass or more, particularly preferably 50% by mass or more, and preferably 70% by mass or less, more preferably 60% by mass or less.
- the content ratio of the (meth) acrylic acid ester monomer having a boiling point of 145 ° C. or higher in the monomer mixture 10% by mass or higher, flexibility is provided to the negative electrode mixture layer formed using the binder composition. be able to.
- the peel strength of the negative electrode mixture layer formed using the binder composition can be improved by setting the content ratio of the (meth) acrylic acid ester monomer having a boiling point of 145 ° C. or higher to 70% by mass or less. .
- the content rate of the (meth) acrylic acid ester monomer with a boiling point of 145 degreeC or more mentioned above in a monomer mixture Preferably it is 60 mass% or more, More preferably 65% by weight or more, still more preferably 70% by weight or more, particularly preferably 75% by weight or more, and preferably 95% by weight or less, more preferably 90% by weight or less, still more preferably 85% by weight or less, particularly preferably Is 83 mass% or less.
- the polymer is prevented from excessive swelling with respect to the electrolyte solution, and the binder
- the peel strength of the positive electrode mixture layer formed using the composition can be improved, and the flexibility of the positive electrode mixture layer can be improved to make the positive electrode having the positive electrode mixture layer difficult to break.
- the content ratio of the (meth) acrylic acid ester monomer having a boiling point of 145 ° C. or higher to 95% by mass or less the polymer is appropriately swollen with respect to the electrolytic solution, and the binder composition is used. An increase in electrical resistance of a secondary battery including the positive electrode formed in this manner can be suppressed, and the mechanical strength of the particulate polymer can be maintained and adhesion can be maintained.
- the boiling point of the (meth) acrylic acid ester monomer having a boiling point of 145 ° C. or higher is preferably 150 ° C. or higher and 250 ° C. or lower.
- High-boiling point (meth) acrylate monomer units generally have long side chains, and polymers comprising such monomer units are rich in flexibility and have good binding properties. This is because the battery characteristics of a secondary battery produced using a binder composition containing such a polymer can be improved.
- Crosslinking monomer When using a binder composition for formation of a negative electrode, it is preferable that the monomer mixture used when superposing
- the crosslinkable monomer is a monomer having the following crosslinkable group.
- the crosslinkable group a heat crosslinkable group that usually causes a crosslinking reaction by heat is used.
- the crosslinkable group include an epoxy group, an N-methylolamide group, an oxazoline group, and an allyl group.
- the crosslinkable group and the crosslinking density can be easily adjusted.
- a methylolamide group, an epoxy group, and an allyl group are preferred.
- the higher the crosslinking density the lower the degree of swelling of the particulate polymer with respect to the electrolytic solution. Therefore, the degree of swelling of the particulate polymer can be controlled by adjusting the crosslinking density.
- the kind of crosslinkable group may be one kind, and may be two or more kinds.
- a crosslinkable monomer shall not be contained in the (meth) acrylic acid ester monomer mentioned above.
- the monomer containing an epoxy group include a monomer containing a carbon-carbon double bond and an epoxy group, a monomer containing a halogen atom and an epoxy group, and the like.
- Examples of the monomer containing a carbon-carbon double bond and an epoxy group include unsaturated glycidyl ethers such as vinyl glycidyl ether, allyl glycidyl ether, butenyl glycidyl ether, o-allylphenyl glycidyl ether; butadiene monoepoxide, Diene or polyene monoepoxides such as chloroprene monoepoxide, 4,5-epoxy-2-pentene, 3,4-epoxy-1-vinylcyclohexene, 1,2-epoxy-5,9-cyclododecadiene; Alkenyl epoxides such as epoxy-1-butene, 1,2-epoxy-5-hexene, 1,2-epoxy-9-decene; glycidyl acrylate, glycidyl methacrylate, glycidyl crotonate, glycidyl-4-he
- Examples of the monomer having a halogen atom and an epoxy group include epihalohydrin such as epichlorohydrin, epibromohydrin, epiiodohydrin, epifluorohydrin, ⁇ -methylepichlorohydrin; p-chlorostyrene oxide; dibromo Phenyl glycidyl ether; and the like.
- Examples of the monomer containing an N-methylolamide group include (meth) acrylamides having a methylol group such as N-methylol (meth) acrylamide.
- Examples of the monomer containing an oxazoline group include 2-vinyl-2-oxazoline, 2-vinyl-4-methyl-2-oxazoline, 2-vinyl-5-methyl-2-oxazoline, 2-isopropenyl- Examples include 2-oxazoline, 2-isopropenyl-4-methyl-2-oxazoline, 2-isopropenyl-5-methyl-2-oxazoline, 2-isopropenyl-5-ethyl-2-oxazoline and the like.
- Examples of the monomer containing an allyl group include allyl acrylate and allyl methacrylate.
- crosslinkable monomer allyl acrylate or allyl methacrylate is preferable from the viewpoint of improvement in crosslink density and high copolymerizability.
- the content of the crosslinkable monomer is preferably 0.5% by mass or more, more preferably 0.8% by mass or more, particularly preferably 1% by mass or more, preferably 5% by mass in the monomer mixture. Hereinafter, it is more preferably 3% by mass or less, particularly preferably 2% by mass or less.
- the degree of swelling in the electrolytic solution is set to an appropriate level, and the binder composition obtained using such a monomer mixture is used. This is because the high-temperature storage characteristics and high-temperature cycle characteristics of the secondary battery can be improved. Moreover, it is because the softness
- Aromatic vinyl monomer When the binder composition is used for forming the negative electrode, the monomer mixture used for polymerizing the particulate polymer is styrene, chlorostyrene, vinyl toluene, t-butyl styrene, vinyl benzoic acid, methyl vinyl benzoate, It is preferable to further contain a styrene monomer such as vinyl naphthalene, chloromethyl styrene, hydroxymethyl styrene, ⁇ -methyl styrene, divinyl benzene and the like. Of these, styrene is preferably used as the aromatic vinyl monomer.
- these may be used individually by 1 type and may be used in combination of 2 or more types.
- an aromatic vinyl monomer such as a styrene monomer
- the Tg of the polymer having monomer units derived from the aromatic vinyl monomer is increased, and the polymer strength of the particulate polymer is increased, As a result, the peel strength of the negative electrode mixture layer can be improved.
- an aromatic vinyl monomer such as a styrene monomer
- the ⁇ electron of the aromatic ring introduced into the particulate polymer contained in the binder composition and the aromatic of the carbon-based negative electrode active material. This is because the dispersibility of the conductive material can be improved by the interaction with the ⁇ electrons of the group ring.
- the blending amount of the aromatic vinyl monomer such as the styrene monomer is preferably 20% by mass or more, more preferably 30% by mass or more, and preferably 80% by mass in the monomer mixture.
- it is more preferably 70% by mass or less, particularly preferably 60% by mass or less.
- the monomer mixture used when polymerizing the particulate polymer preferably further contains a monomer containing an acidic group (hereinafter sometimes referred to as “acidic group-containing monomer”).
- a monomer unit containing an acidic group (hereinafter sometimes referred to as an “acidic group-containing monomer unit”) is particulate. It can be introduced into the polymer.
- the acidic group include a carboxylic acid group (—COOH), a sulfonic acid group (—SO 3 H), and a phosphoric acid group (—PO 3 H 2 ).
- the acidic group-containing monomer may have one acidic group or two or more acidic groups.
- the number of acidic groups which the monomer containing an acidic group has may be one, and two or more may be sufficient as it.
- the monomer containing a carboxylic acid group a monomer having a carboxylic acid group and a polymerizable group is usually used.
- the monomer containing a carboxylic acid group include an unsaturated carboxylic acid monomer.
- the unsaturated carboxylic acid monomer is a monomer having a carbon-carbon unsaturated bond and having a carboxylic acid group.
- Examples of the unsaturated carboxylic acid monomer include unsaturated monocarboxylic acid and derivatives thereof; unsaturated dicarboxylic acid and acid anhydrides and derivatives thereof; and the like.
- unsaturated monocarboxylic acids include ethylenically unsaturated monocarboxylic acids such as acrylic acid, methacrylic acid, and crotonic acid.
- unsaturated monocarboxylic acid derivatives include 2-ethylacrylic acid, isocrotonic acid, ⁇ -acetoxyacrylic acid, ⁇ -trans-aryloxyacrylic acid, ⁇ -chloro- ⁇ -E-methoxyacrylic acid, and ⁇ -Derivatives of ethylenically unsaturated monocarboxylic acids, such as diaminoacrylic acid.
- unsaturated dicarboxylic acids include ethylenically unsaturated dicarboxylic acids such as maleic acid, fumaric acid, and itaconic acid.
- unsaturated dicarboxylic acid anhydrides include ethylenically unsaturated dicarboxylic acid anhydrides such as maleic anhydride, acrylic anhydride, methyl maleic anhydride, and dimethyl maleic anhydride.
- Examples of derivatives of unsaturated dicarboxylic acids include methylmaleic acid, dimethylmaleic acid, phenylmaleic acid, chloromaleic acid, dichloromaleic acid, fluoromaleic acid, methylallyl maleate; and diphenyl maleate, nonyl maleate, maleate Examples thereof include maleate esters such as decyl acid, dodecyl maleate, octadecyl maleate, and fluoroalkyl maleate.
- Examples of monomers having a sulfonic acid group include vinyl sulfonic acid, methyl vinyl sulfonic acid, (meth) allyl sulfonic acid, styrene sulfonic acid, (meth) acrylic acid-2-ethyl sulfonate, 2-acrylamido-2-methyl. Examples thereof include propanesulfonic acid and 3-allyloxy-2-hydroxypropanesulfonic acid. In the present specification, “(meth) allyl” means allyl and / or methallyl.
- Monomers having a phosphoric acid group include phosphoric acid-2- (meth) acryloyloxyethyl phosphate, methyl-2- (meth) acryloyloxyethyl phosphate, ethyl phosphate- (meth) acryloyloxyethyl phosphate, ethylene phosphate And methacrylate.
- acrylic acid, methacrylic acid, itaconic acid, 2-acrylamido-2-methylpropanesulfonic acid (AMPS), and ethylene methacrylate phosphate are preferable.
- unsaturated monocarboxylic acids such as acrylic acid and methacrylic acid, and ethylenically unsaturated dicarboxylic acids such as itaconic acid are preferred. This is because the storage stability of the binder composition can be improved by further increasing the dispersibility of the particulate polymer in water.
- the compounding amount of the acidic group-containing monomer is preferably 0.5% by mass or more, more preferably 1% by mass or more, particularly preferably 1.5% by mass or more, preferably 8% by mass in the monomer mixture. % Or less, more preferably 5% by mass or less, and particularly preferably 4% by mass or less.
- the monomer mixture used for polymerizing the particulate polymer in the present invention is not limited to those described above unless the present invention is significantly impaired. May contain body.
- These arbitrary monomers are monomers copolymerizable with the above-mentioned monomers.
- Examples of monomers copolymerizable with the above-mentioned monomers include amide monomers such as acrylamide; olefins such as ethylene and propylene; diene monomers such as butadiene and isoprene; vinyl chloride; Monomers containing halogen atoms such as vinylidene chloride; vinyl esters such as vinyl acetate, vinyl propionate, vinyl butyrate and vinyl benzoate; vinyl ethers such as methyl vinyl ether, ethyl vinyl ether and butyl vinyl ether; methyl vinyl ketone and ethyl vinyl Vinyl ketones such as ketone, butyl vinyl ketone, hexyl vinyl ketone and isopropenyl vinyl ketone; heterocyclic compounds containing a heterocyclic ring such as N-vinyl pyrrolidone, vinyl pyridine and vinyl imidazole; methyl (meth) acrylate, ethyl (meth) acrylate, propy
- an ⁇ , ⁇ -unsaturated nitrile monomer and an acidic group-containing monomer can be suitably blended in the monomer mixture used for the polymerization of the particulate polymer.
- the body is described in detail.
- the monomer mixture used for polymerizing the particulate polymer further contains an ⁇ , ⁇ -unsaturated nitrile monomer such as a (meth) acrylonitrile monomer. It is preferable. This is because the use of an ⁇ , ⁇ -unsaturated nitrile monomer such as a (meth) acrylonitrile monomer can increase the binding strength of the binder composition and can significantly improve the strength of the positive electrode.
- Examples of the ⁇ , ⁇ -unsaturated nitrile monomer include acrylonitrile, methacrylonitrile, ⁇ -chloroacrylonitrile, ⁇ -ethylacrylonitrile and the like. Among these, acrylonitrile and methacrylonitrile are preferable and acrylonitrile is particularly preferable from the viewpoint of improving mechanical strength and binding properties. In addition, these may be used individually by 1 type and may be used in combination of 2 or more types.
- the blending amount of the ⁇ , ⁇ -unsaturated nitrile monomer such as (meth) acrylonitrile monomer is preferably 1 to 50% by mass, more preferably 5 to 35% by mass in the monomer mixture. is there.
- the content of an ⁇ , ⁇ -unsaturated nitrile monomer such as a (meth) acrylonitrile monomer is less than 1% by mass, the Tg of the particulate polymer is lowered, and the binder composition containing such a particulate polymer There is a possibility that the peel strength of the positive electrode mixture layer formed using the product may be lowered. Furthermore, in this case, the particulate polymer is likely to swell excessively with respect to the electrolytic solution, which may cause a decrease in peel strength.
- a secondary material produced using such a binder composition with an appropriate value for the degree of swelling of the particulate polymer with respect to the electrolyte while improving the peel strength of the positive electrode produced using the binder composition containing the particulate polymer An increase in the internal resistance of the battery can be suppressed.
- polymerizing a particulate polymer further contains an acidic group containing monomer.
- the acidic group-containing monomer those described above in the section of “Monomer used for polymerization of particulate polymer in binder composition for negative electrode” can be used.
- the blending amount in the monomer mixture is the same.
- the monomer mixture used when polymerizing the particulate polymer in the present invention is not limited to those described above unless the present invention is significantly impaired. May contain body.
- These arbitrary monomers are monomers copolymerizable with the above-mentioned monomers. Examples of monomers copolymerizable with the above-described monomers include styrene, chlorostyrene, vinyl toluene, t-butyl styrene, vinyl benzoic acid, methyl vinyl benzoate, vinyl naphthalene, chloromethyl styrene, hydroxymethyl.
- Styrene monomers such as styrene, ⁇ -methylstyrene and divinylbenzene; Amide monomers such as acrylamide; Olefins such as ethylene and propylene; Diene monomers such as butadiene and isoprene; Vinyl chloride and vinylidene chloride Halogen atom-containing monomers such as vinyl acetate, vinyl propionate, vinyl butyrate, vinyl benzoate, etc .; vinyl ethers such as methyl vinyl ether, ethyl vinyl ether, butyl vinyl ether; methyl vinyl ketone, ethyl vinyl ketone, Butyl vinyl keto , Vinyl ketones such as hexyl vinyl ketone and isopropenyl vinyl ketone; heterocycle-containing vinyl compounds such as N-vinyl pyrrolidone, vinyl pyridine and vinyl imidazole; methyl (meth) acrylate, ethyl (meth) acrylate, propyl (
- the particulate polymer has a crosslinked structure.
- the cross-linked structure can be formed by introducing the cross-linked monomer into the particulate polymer.
- the particulate polymer may have a crosslinked structure.
- a method for introducing a cross-linked structure for example, a method of incorporating a cross-linkable group by polymerizing a polymer from a monomer composition containing the above cross-linkable monomer, a combination of a polymer and a cross-linking agent And the method used.
- the polymer can be crosslinked by irradiation with heat or energy rays.
- the degree of crosslinking can be adjusted by the intensity of heating or irradiation with energy rays. Since the degree of swelling decreases as the degree of crosslinking increases, the degree of swelling of the particulate polymer can be controlled by adjusting the degree of crosslinking.
- the weight average molecular weight of the particulate polymer is preferably 10,000 or more, more preferably 20,000 or more, preferably 1,000,000 or less, more preferably 500,000 or less.
- the weight average molecular weight of the particulate polymer can be determined as a value in terms of polystyrene using tetrahydrofuran as a developing solvent by gel permeation chromatography (GPC).
- the glass transition temperature (Tg) of the particulate polymer is preferably ⁇ 50 ° C. or higher, more preferably ⁇ 45 ° C.
- the glass transition temperature of the particulate polymer is in the above range, a secondary battery electrode having excellent strength and flexibility can be obtained, and a secondary battery having high output characteristics can be obtained.
- the glass transition temperature of the particulate polymer can be adjusted by combining various monomers.
- the particulate polymer is insoluble in water. Therefore, the particulate polymer is usually in the form of particles in the binder composition and the slurry composition for secondary battery electrodes containing the binder composition, and is included in the secondary battery electrode while maintaining the particle shape. It is.
- the particulate polymer being “water-insoluble” means that when 0.5 g of the compound is dissolved in 100 g of water at 25 ° C., the insoluble content becomes 90% by mass or more.
- the volume average particle diameter of the particulate polymer is usually 0.001 ⁇ m or more, preferably 0.01 ⁇ m or more, more preferably 0.05 ⁇ m or more, and usually 100 ⁇ m or less, preferably 10 ⁇ m or less, more preferably 1 ⁇ m or less. is there.
- the binder composition can exhibit an excellent binding force even when used in a small amount.
- the volume average particle diameter is measured using a light scattering particle diameter measuring instrument.
- the shape of the particles may be either spherical or irregular.
- a particulate polymer may be used individually by 1 type, and may be used in combination of 2 or more types.
- the proportion of the particulate polymer in the solid content of the binder composition is usually 50% by mass or more, and preferably 70% by mass or more.
- the “reductone compound” contained in the binder composition for a secondary battery electrode of the present invention refers to a compound having a R 3 C (OH) ⁇ C (OH) C ( ⁇ O) R 4 structure and a salt thereof. .
- R 3 and R 4 may each be an independent organic group, or may together form a ring structure.
- Examples of the compound having the R 3 C (OH) ⁇ C (OH) C ( ⁇ O) R 4 structure include glucic acid and its derivatives, reductic acid and its derivatives, ascorbic acid and its isomers, derivatives and the like. It is done.
- the reductone compound is included in the form of an oxidant (deprotonated reductone; a compound having a R 3 C ( ⁇ O) C ( ⁇ O) C ( ⁇ O) R 4 structure and a salt thereof). Also good. Among these, at least one selected from ascorbic acid and its isomers, derivatives, salts thereof, and oxidants thereof is preferable because of low cost, toxicity, and environmental burden and high human safety.
- Ascorbic acid (vitamin C) and its isomers and derivatives include, for example, D- or L-ascorbic acid and its sugar derivatives (for example, ⁇ -lactoascorbic acid, glucoascorbic acid, fucoscorbic acid, glucoheptascorbic acid, Maltoascorbic acid), isoascorbic acid (or L-erythroascorbic acid), also called erythorbic acid, enediol type ascorbic acid, enaminol type ascorbic acid, thioenolic type ascorbic acid, and ascorbyl palmitate.
- D- or L-ascorbic acid and its sugar derivatives for example, ⁇ -lactoascorbic acid, glucoascorbic acid, fucoscorbic acid, glucoheptascorbic acid, Maltoascorbic acid
- isoascorbic acid (or L-erythroascorbic acid) also called erythorbic acid,
- ascorbic acid (vitamin C) and isomers and derivatives thereof include salts of the above-described compounds (for example, alkali metal salts, ammonium salts or salts known in the art), such as sodium ascorbic acid.
- salts of the above-described compounds for example, alkali metal salts, ammonium salts or salts known in the art
- sodium ascorbic acid such as sodium ascorbic acid.
- potassium salt of ascorbic acid L-ascorbyl magnesium phosphate may be mentioned.
- at least one of reductone compound and its oxidant is at least 1 sort (s) selected from (iso) ascorbic acid and its salt, and those oxidants.
- the salt of (iso) ascorbic acid is preferably an alkali metal salt, more preferably a sodium salt. A mixture of these reductone compounds can be used as necessary.
- the binder composition is preferably 0.05 parts by mass or more, more preferably 0.1 parts by mass or more, particularly preferably at least one of the reductone compound and its oxidant with respect to 100 parts by mass of the particulate polymer. 0.3 parts by mass or more, preferably 5 parts by mass or less, more preferably 2 parts by mass or less, particularly preferably 1 part by mass or less.
- residual monomers such as (meth) acrylic acid ester monomers having a boiling point of 145 ° C. or more are sufficiently reduced. can do.
- the content of at least one of the reductone compound and its oxidant is 5 parts by mass or less with respect to 100 parts by mass of the particulate polymer.
- the amount of gas generated by decomposition of the remaining reductone compound can be reduced by preventing at least one of the compound and its oxidant from remaining in large amounts, and the initial capacity of the secondary battery can be improved. be able to.
- the binder composition may contain known arbitrary components that can be blended in the binder composition in addition to the components described above. Moreover, residues, such as a polymerization initiator used for superposition
- the binder composition has a solid content of usually 10% by mass or more, preferably 20% by mass or more, usually 70% by mass or less, preferably 60% by mass or less.
- content of the (meth) acrylic acid ester monomer with a boiling point of 145 degreeC or more in the binder composition containing the particulate polymer polymerized using the monomer mixture mentioned above is particulate polymer 100 mass. 1500 ⁇ 10 ⁇ 6 parts by mass or less, preferably 1000 ⁇ 10 ⁇ 6 parts by mass or less, more preferably 500 ⁇ 10 ⁇ 6 parts by mass or less, and 300 ⁇ 10 ⁇ 6 parts by mass. It is particularly preferred that the amount is not more than parts.
- the binder composition has a boiling point of 145 ° C.
- the (meth) acrylate monomer is often contained in an amount of 1 ⁇ 10 ⁇ 6 parts by mass or more with respect to 100 parts by mass of the particulate polymer.
- binder is made into the binder composition by making the content rate of the (meth) acrylic acid ester monomer of boiling point 145 degreeC or more in a binder composition into 1500x10-6 mass parts or less with respect to 100 mass parts of particulate polymers.
- the amount of gas generated in the secondary battery formed using the composition can be reduced, the high-temperature storage characteristics and the high-temperature cycle characteristics can be improved, and the excellent temporal stability of the binder composition can be obtained.
- a monomer mixture containing a (meth) acrylic acid ester monomer having a boiling point of 145 ° C. or higher is polymerized in water until the polymerization conversion becomes 90% by mass or more.
- Step (1) to obtain a mixture containing a monomer and an unreacted monomer, and after step (1), a reductone compound and a peroxide are added to the mixture to polymerize the unreacted monomer,
- step (2) of setting the content ratio of the (meth) acrylic acid ester monomer having a boiling point of 145 ° C.
- the method for producing the binder composition of the present invention comprises at least two steps: a step of polymerizing the majority of the monomer mixture and a step of further polymerizing the remaining unreacted residual monomer to reduce the amount of residual monomer. And a step of polymerizing the particulate polymer by polymerization.
- the monomer mixture is preferably emulsified in advance and then added to the reactor for polymerization.
- step (1) a monomer mixture containing a (meth) acrylic acid ester monomer having a boiling point of 145 ° C. or higher that can be used for the production of the particulate polymer described above is polymerized in water.
- the polymerization in the step (1) may be performed in one stage or may be performed in multiple stages.
- the polymerization conversion rate can be controlled by adjusting the reaction temperature, reaction time, and the like.
- the polymerization conversion rate in the step (1) is 90% by mass or more, preferably 95% by mass or more is to obtain a particulate polymer having desired properties.
- the polymerization conversion rate in the step (1) is 90% by mass or more, preferably 95% by mass or more, so that it can be newly generated by redox polymerization in the step (2). It is possible to reduce the amount of the particulate polymer having a property different from that of the particulate polymer.
- any method such as a suspension polymerization method or an emulsion polymerization method may be used.
- any of ions, radicals, and living radicals may be used as the reactive species.
- it is easy to obtain a high molecular weight polymer, and since the polymer is obtained in the form of particles dispersed in water, there is no need for redispersion treatment, and for producing a secondary battery electrode as it is.
- an emulsion polymerization method is particularly preferable.
- the emulsion polymerization method can be carried out according to a conventional method (see, for example, “Experimental Chemistry Course” Vol. 28 (Publisher: Maruzen Co., Ltd., edited by The Chemical Society of Japan)).
- a sealed container equipped with a stirrer and a heating device
- water an additive such as a dispersant, an emulsifier, a crosslinking agent, a polymerization initiator, and the monomer mixture described above are added so as to have a predetermined composition.
- a method may be used in which the composition in the container is stirred to emulsify the monomer or the like in water, and the temperature is increased while stirring to initiate polymerization. Or after emulsifying the said composition, it can put into an airtight container, and the method of starting reaction similarly can be used.
- Examples of the emulsifier used when carrying out the emulsion polymerization in the step (1) include nonionic emulsifiers such as polyoxyethylene alkylene ether, polyoxyethylene alkylphenol ether, polyoxyethylene alkyl ester, polyoxyethylene sorbitan alkyl ester and the like.
- Anionic emulsifiers such as fatty acids such as myristic acid, palmitic acid, oleic acid, linolenic acid and salts thereof, alkylallyl sulfonates, higher alcohol sulfates, alkyl sulfosuccinic acids; ammonium chlorides such as trimethylammonium chloride and dialkylammonium chloride And cationic emulsifiers such as benzylammonium salts and quaternary ammonium salts; sulfoesters of ⁇ , ⁇ -unsaturated carboxylic acids, ⁇ , ⁇ -unsaturated carbo Sulphate esters of acids, such as copolymerizable emulsifiers containing a double bond such as sulfoalkyl aryl ether.
- fatty acids such as myristic acid, palmitic acid, oleic acid, linolenic acid and salts thereof, alkylallyl s
- emulsifiers can be used alone or in combination of two or more.
- the amount of the emulsifier used is 0.1 to 10% by mass relative to the monomer mixture. If it is less than 0.1% by mass, aggregates are produced during polymerization. On the other hand, if it exceeds 10% by mass, the average particle size of the obtained particulate polymer becomes small.
- step (1) water-soluble initiators such as potassium persulfate, ammonium persulfate, and hydrogen peroxide as radical polymerization initiators; benzoyl peroxide, di-t-butyl peroxide, 2,2-azobis-2,4 Oil-soluble initiators such as dimethylvaleronitrile;
- the addition amount of the polymerization initiator varies depending on each initiator, but in the case of a water-soluble initiator, it is 0.1% by mass or more and 5% by mass or less, and in the case of an oil-soluble initiator, 0.1% by mass or more and 3% by mass % Or less.
- the reaction temperature in step (1) is usually 0 ° C. or higher, preferably 40 ° C. or higher, usually 150 ° C.
- the polymerization time is usually 1 hour or more and 20 hours or less. If the polymerization temperature is too low, the reaction rate is too slow and the efficiency is poor, and if the polymerization temperature is too high, the aqueous medium tends to evaporate, making the polymerization difficult.
- the reaction pressure may be normal pressure. Although the reaction can be carried out in air, it is preferably in the presence of an inert gas such as nitrogen or argon.
- the dispersant those used in ordinary synthesis may be used.
- the dispersant include benzenesulfonates such as sodium dodecylbenzenesulfonate and sodium dodecylphenylethersulfonate; alkyl sulfates such as sodium lauryl sulfate and sodium tetradodecylsulfate; sodium dioctylsulfosuccinate and sodium dihexylsulfosuccinate Sulfosuccinates such as; fatty acid salts such as sodium laurate; ethoxy sulfate salts such as polyoxyethylene lauryl ether sulfate sodium salt, polyoxyethylene nonylphenyl ether sulfate sodium salt; alkane sulfonates; alkyl ether phosphates Sodium salt; polyoxyethylene nonylphenyl ether, polyoxyethylene sorbitan lauryl ester, polyoxyethylene- Nonionic
- benzenesulfonates such as sodium dodecylbenzenesulfonate and sodium dodecylphenylethersulfonate; alkyl sulfates such as sodium lauryl sulfate and sodium tetradodecylsulfate can be preferably used. More preferably, benzenesulfonates such as sodium dodecylbenzenesulfonate and sodium dodecylphenylethersulfonate can be used from the viewpoint of excellent oxidation resistance. In addition, these may be used individually by 1 type and may be used combining two or more types by arbitrary ratios. The amount of the dispersant is usually 0.01 parts by mass to 10 parts by mass with respect to 100 parts by mass of the total amount of the monomer mixture.
- seed polymerization may be performed using seed particles.
- step (2) Here, in the step (1) described above, it is difficult to set the polymerization conversion rate to 100% by mass, and an unreacted monomer containing a (meth) acrylic acid ester monomer having a boiling point of 145 ° C. or higher remains. To do. Therefore, in the step (2), an unreacted monomer (residual monomer) remaining after the step (1) is polymerized using a redox initiator that uses a reductone compound and a peroxide in combination, and a binder composition is obtained. Reduce the amount of residual monomer in it.
- step (2) a reductone compound and a peroxide are added to the polymerization system containing a mixture of a polymer and an unreacted monomer obtained through step (1).
- the redox polymerization is carried out until the content of the (meth) acrylic acid ester monomer having a boiling point of 145 ° C. or higher is 1 ⁇ 10 ⁇ 6 parts by mass to 1500 ⁇ 10 ⁇ 6 parts by mass with respect to 100 parts by mass of the polymer.
- the content ratio of the (meth) acrylic acid ester monomer having a boiling point of 145 ° C. or higher should be adjusted by changing the amount of the reductone compound and peroxide used in step (2) and the redox polymerization conditions. Can do.
- the reaction temperature of the step (2) is preferably 0 ° C. or higher, more preferably 10 ° C. or higher, preferably 80 ° C. or lower, more preferably 60 ° C. or lower. If the polymerization temperature is too low, the reaction rate is too slow, so the efficiency is poor, and if the polymerization temperature is too high, the decomposition rate of the redox initiator (reductone compounds and peroxides) is too high, making the polymerization difficult. is there. By performing redox polymerization, the polymerization reaction of the residual monomer can be performed even under such a mild temperature condition.
- the polymerization time in step (2) is preferably 1 hour or more and 6 hours or less.
- the temperature of the polymerization system is set to a temperature higher than the polymerization temperature at the time of redox polymerization (for example, more than 80 ° C ), It is preferable to remove or decompose the peroxide.
- the pressure of the reaction in the step (2) may be a normal pressure.
- the reaction can be carried out in air, it is preferably in the presence of an inert gas such as nitrogen or argon.
- the amount of the reductone compound is appropriately changed according to the kind of the peroxide, but is preferably 0.05% by mass or more, preferably with respect to the amount of the monomer mixture used in the step (1). 0.1 mass% or more, usually 5 mass% or less, preferably 1 mass% or less.
- the amount of the reductone compound used is too small, redox polymerization does not proceed sufficiently, and the residual monomer reducing effect may be insufficient.
- the reductone compounds may be added all at once or dividedly, but in terms of improving the residual monomer reduction efficiency, divided addition is preferred.
- the blending amount of the polymerization initiator is reduced as compared with the case where other polymerization methods are employed to reduce the residual monomer, and polymerization is started in the binder composition. It is possible to prevent the residue of the agent from remaining in large quantities.
- the reductone compounds used in the step (2) can include the reductone compounds exemplified in the section of the binder composition, and as mentioned above, ascorbic acid, its isomers and derivatives, and salts thereof. Is preferred.
- the peroxide include water-soluble peroxides exemplified below. Since the binder composition of this invention is a binder composition used for manufacture of a secondary battery, it is preferable to use the peroxide which does not contain a transition metal element. In addition, that the peroxide is “water-soluble” means that it has a water solubility of 5% by mass or more in water at 25 ° C.
- peroxide that can be used in the present invention, hydrogen peroxide, peroxy acid salt, peroxodisulfuric acid and its salt, peroxy ester salt, peroxide ammonium or alkali metal salt, perborate, persulfate And tert-butyl hydroperoxide (t-BuOOH), benzoyl peroxide.
- tert-butyl hydroperoxide, hydrogen peroxide, and peracetic acid are preferable, and tert-butyl hydroperoxide is particularly preferable. This is because radical species generated from peroxides that are water-soluble and have a low molecular weight have high molecular mobility and can reduce residual monomers effectively because of their low molecular weight.
- the amount of peroxide used is preferably 0.05% by mass or more, more preferably 0.1% by mass or more, preferably 5% by mass or less, based on the amount of the monomer mixture used in step (1). More preferably, it is 1 mass% or less.
- the slurry composition for secondary battery electrodes of the present invention is characterized by containing the above-mentioned binder composition and electrode active material (positive electrode active material or negative electrode active material).
- electrode active material positive electrode active material or negative electrode active material.
- the slurry composition for secondary battery electrodes of the present invention may contain a conductive material, a viscosity modifier, a surfactant, a dispersant and the like in addition to the binder composition and the electrode active material.
- the ratio of the binder composition contained in the slurry composition of the present invention can be adjusted as appropriate so that the performance of the obtained battery is satisfactorily exhibited.
- the ratio of the solid content of the binder composition to 100 parts by mass of the electrode active material is usually 0.1 parts by mass or more, preferably 0.5 parts by mass or more, more preferably 0.8 parts by mass or more, and usually 50 parts by mass.
- Part or less preferably 20 parts by weight or less, more preferably 10 parts by weight or less, and particularly preferably 3 parts by weight or less.
- a negative electrode active material When using a slurry composition for formation of a negative electrode, a negative electrode active material is used as an electrode active material.
- the negative electrode active material is an electrode active material used in the negative electrode, and is a material that transfers electrons in the negative electrode of the secondary battery.
- a material that can occlude and release lithium is usually used as the negative electrode active material. Examples of the material that can occlude and release lithium include a carbon-based negative electrode active material, a metal-based negative electrode active material, and a negative electrode active material obtained by combining these materials.
- the carbon-based negative electrode active material refers to an active material having carbon as a main skeleton into which lithium can be inserted (also referred to as “dope”).
- Examples of the carbon-based negative electrode active material include a carbonaceous material and a graphite material. Is mentioned.
- the carbonaceous material is a material having a low degree of graphitization (ie, low crystallinity) obtained by carbonizing a carbon precursor by heat treatment at 2000 ° C. or lower.
- the minimum of the heat processing temperature at the time of carbonizing is not specifically limited, For example, it can be 500 degreeC or more.
- Examples of the carbonaceous material include graphitizable carbon that easily changes the carbon structure depending on the heat treatment temperature, and non-graphitizable carbon having a structure close to an amorphous structure typified by glassy carbon.
- the graphitizable carbon for example, a carbon material using tar pitch obtained from petroleum or coal as a raw material can be mentioned. Specific examples include coke, mesocarbon microbeads (MCMB), mesophase pitch carbon fibers, pyrolytic vapor grown carbon fibers, and the like.
- examples of the non-graphitizable carbon include a phenol resin fired body, polyacrylonitrile-based carbon fiber, pseudo-isotropic carbon, furfuryl alcohol resin fired body (PFA), and hard carbon.
- the graphite material is a material having high crystallinity close to that of graphite obtained by heat-treating graphitizable carbon at 2000 ° C. or higher.
- the upper limit of heat processing temperature is not specifically limited, For example, it can be 5000 degrees C or less.
- the graphite material include natural graphite and artificial graphite.
- the artificial graphite for example, artificial graphite obtained by heat-treating carbon containing graphitizable carbon mainly at 2800 ° C. or higher, graphitized MCMB heat-treated at 2000 ° C. or higher, and mesophase pitch-based carbon fiber at 2000 ° C. Examples thereof include graphitized mesophase pitch-based carbon fibers that have been heat-treated.
- the metal-based negative electrode active material is an active material containing a metal, and usually contains an element capable of inserting lithium in the structure, and the theoretical electric capacity per unit mass when lithium is inserted is 500 mAh / g or more.
- the metal active material include lithium metal and a single metal capable of forming a lithium alloy (for example, Ag, Al, Ba, Bi, Cu, Ga, Ge, In, Ni, P, Pb, Sb, Si, Sn). , Sr, Zn, Ti, etc.) and alloys thereof, and oxides, sulfides, nitrides, silicides, carbides, phosphides, and the like thereof.
- active materials containing silicon are preferable. This is because the capacity of the lithium ion secondary battery can be increased by using the silicon-based negative electrode active material.
- silicon-based negative electrode active materials examples include silicon (Si), alloys of silicon and cobalt, nickel, iron, etc., SiO, SiOx, mixtures of Si-containing materials and carbon materials, and Si-containing materials coated with conductive carbon.
- Si silicon
- alloys of silicon and cobalt nickel, iron, etc.
- SiO, SiOx mixtures of Si-containing materials and carbon materials
- Si-containing materials coated with conductive carbon examples include silicon (Si), alloys of silicon and cobalt, nickel, iron, etc., SiO, SiOx, mixtures of Si-containing materials and carbon materials, and Si-containing materials coated with conductive carbon.
- Si-containing materials coated with conductive carbon.
- a composite of a Si-containing material and conductive carbon formed into a composite can be used.
- SiOx is a compound containing at least one of SiO and SiO 2 and Si, and x is usually 0.01 or more and less than 2.
- SiOx can be formed using the disproportionation reaction of a silicon monoxide (SiO), for example.
- SiOx can be prepared by heat-treating SiO, optionally in the presence of a polymer such as polyvinyl alcohol, to produce silicon and silicon dioxide. The heat treatment can be performed at a temperature of 900 ° C. or higher, preferably 1000 ° C. or higher, in an atmosphere containing an organic gas and / or steam after grinding and mixing SiO and optionally a polymer.
- Si-containing material such as silicon and SiOx
- carbon material such as carbonaceous material and graphite material
- a polymer such as polyvinyl alcohol. Things.
- carbonaceous material and a graphite material the material which can be used as a carbon-type negative electrode active material can be used.
- a composite of Si-containing material and conductive carbon for example, a pulverized mixture of SiO, a polymer such as polyvinyl alcohol, and optionally a carbon material is heat-treated in an atmosphere containing, for example, an organic gas and / or steam.
- a known method such as a method of coating the surface of SiO particles with an organic gas or the like by chemical vapor deposition, or a composite particle (granulation) of SiO particles and graphite or artificial graphite by a mechanochemical method. Can be used.
- the negative electrode active material when a carbon-based negative electrode active material or a metal-based negative electrode active material is used as the negative electrode active material, these negative electrode active materials expand and contract as the lithium ion secondary battery is charged and discharged. Therefore, when these negative electrode active materials are used, normally, the negative electrode gradually expands due to repeated expansion and contraction of the negative electrode active material, the secondary battery is deformed, and electrical characteristics such as cycle characteristics are obtained. May be reduced.
- the negative electrode bulge caused by expansion and contraction of the negative electrode active material is suppressed by the cross-linked structure formed due to the cross-linkable monomer described above. Electrical characteristics such as cycle characteristics can be improved.
- capacitance of a lithium ion secondary battery can be increased if the said silicon-type negative electrode active material is used, generally a silicon-type negative electrode active material expand
- the negative electrode active material when a mixture of a carbon-based negative electrode active material and a silicon-based negative electrode active material is used as the negative electrode active material, from the viewpoint of sufficiently increasing the capacity of the lithium ion secondary battery while sufficiently suppressing the occurrence of swelling of the negative electrode. It is preferable to use artificial graphite as the carbon-based negative electrode active material, and as the silicon-based negative electrode active material, Si, SiOx, a mixture of a Si-containing material and a carbon material, and a composite of a Si-containing material and conductive carbon.
- a composite of a Si-containing material and conductive carbon as the silicon-based negative electrode active material, and a composite in which SiOx is dispersed in a conductive carbon matrix. It is particularly preferable to use (Si—SiOx—C composite). While these negative electrode active materials can occlude and release a relatively large amount of lithium, the volume change when lithium is occluded and released is relatively small. Therefore, if these negative electrode active materials are used, a lithium ion secondary using a negative electrode for a lithium ion secondary battery formed using the slurry composition while suppressing an increase in volume change of the negative electrode active material during charge / discharge. The capacity of the battery can be sufficiently increased.
- the particle size and specific surface area of the negative electrode active material are not particularly limited, and can be the same as those of conventionally used negative electrode active materials.
- the content ratio of the negative electrode active material in the slurry composition of the present invention is preferably 90% by mass or more, more preferably 95% by mass or more, preferably 99.9% by mass or less, more preferably 99% by mass or less. .
- the capacity of the secondary battery of the present invention can be increased, and the flexibility of the negative electrode and the binding property between the current collector and the negative electrode mixture layer are improved. Can be made.
- a positive electrode active material is used as the electrode active material.
- the positive electrode active material is an electrode active material used in the positive electrode, and is a material that transfers electrons in the positive electrode of the secondary battery.
- the secondary battery of the present invention is a lithium ion secondary battery
- a material capable of inserting and extracting lithium ions is usually used as the positive electrode active material.
- Such positive electrode active materials are roughly classified into those made of inorganic compounds and those made of organic compounds.
- Examples of the positive electrode active material made of an inorganic compound include transition metal oxides, transition metal sulfides, lithium-containing composite metal oxides of lithium and transition metals, and the like.
- Examples of the transition metal include Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Mo.
- Examples of the transition metal oxide include MnO, MnO 2 , V 2 O 5 , V 6 O 13 , TiO 2 , Cu 2 V 2 O 3 , amorphous V 2 O—P 2 O 5 , MoO 3 , V 2 O 5 , V 6 O 13 and the like are mentioned, and among them, MnO, V 2 O 5 , V 6 O 13 and TiO 2 are preferable from the viewpoint of cycle stability and capacity of the secondary battery.
- transition metal sulfide examples include TiS 2 , TiS 3 , amorphous MoS 2 , and FeS.
- lithium-containing composite metal oxide examples include a lithium-containing composite metal oxide having a layered structure, a lithium-containing composite metal oxide having a spinel structure, and a lithium-containing composite metal oxide having an olivine structure.
- lithium-containing composite metal oxide having a layered structure examples include lithium-containing cobalt oxide (LiCoO 2 ), lithium-containing nickel oxide (LiNiO 2 ), Co—Ni—Mn lithium composite oxide, Ni—Mn— Examples thereof include a lithium composite oxide of Al, a lithium composite oxide of Ni—Co—Al, and a solid solution of LiMaO 2 and Li 2 MbO 3 . Examples of the solid solution of LiMaO 2 and Li 2 MbO 3 include xLiMaO 2. (1-x) Li 2 MbO 3 .
- x represents a number satisfying 0 ⁇ x ⁇ 1
- Ma represents one or more transition metals having an average oxidation state of 3+
- Mb represents one or more transition metals having an average oxidation state of 4+.
- LiCoO 2 is preferably used from the viewpoint of improving the cycle characteristics of the secondary battery
- LiMaO is used from the viewpoint of improving the energy density of the secondary battery.
- a solid solution of 2 and Li 2 MbO 3 is preferred.
- xLiMaO 2 ⁇ (1-x) Li 2 MbO 3 (x represents a number satisfying 0 ⁇ x ⁇ 1, Ma is Ni, Co, Mn Mb represents one or more selected from the group consisting of Mn, Zr and Ti).
- xLiMaO 2 ⁇ (1-x) Li 2 MnO 3 (x represents a number satisfying 0 ⁇ x ⁇ 1, and Ma is one or more selected from the group consisting of Ni, Co, Mn, Fe, and Ti. Is preferred).
- An example of such a solid solution is Li [Ni 0.17 Li 0.2 Co 0.07 Mn 0.56 ] O 2 .
- Examples of the lithium-containing composite metal oxide having a spinel structure include compounds in which a part of Mn of lithium manganate (LiMn 2 O 4 ) is substituted with another transition metal.
- LiMn 2 O 4 lithium manganate
- a specific example is Li s [Mn 2 ⁇ t Md t ] O 4 .
- Md represents one or more transition metals having an average oxidation state of 4+.
- Specific examples of Md include Ni, Co, Fe, Cu, and Cr.
- T represents a number satisfying 0 ⁇ t ⁇ 1, and s represents a number satisfying 0 ⁇ s ⁇ 1.
- a lithium-excess spinel compound represented by Li 1 + x Mn 2 ⁇ x O 4 (0 ⁇ X ⁇ 2) can also be used.
- s represents a number satisfying 0 ⁇ s ⁇ 1
- t represents a number satisfying 0 ⁇ t ⁇ 1
- z represents a number satisfying 0 ⁇ z ⁇ 0.1.
- LiNi 0.5 Mn 1.5 O 4 in which Mn of lithium manganate is substituted with Ni is also preferable.
- LiNi 0.5 Mn 1.5 O 4 and the like can replace all of Mn 3+ considered to be a factor of structural deterioration.
- LiNi 0.5 Mn 1.5 O 4 and the like undergo an electrochemical reaction from Ni 2+ to Ni 4+ , a secondary battery having a high operating voltage and a high capacity can be realized.
- Examples of the lithium-containing composite metal oxide having an olivine type structure include an olivine type lithium phosphate compound represented by Li y McPO 4 .
- Mc represents one or more transition metals having an average oxidation state of 3+, and examples thereof include Mn, Fe, and Co.
- Y represents a number satisfying 0 ⁇ y ⁇ 2.
- Mn or Co may be partially substituted with another metal.
- the metal that can be substituted include Fe, Cu, Mg, Zn, V, Ca, Sr, Ba, Ti, Al, Si, B, and Mo.
- examples of the positive electrode active material made of an inorganic compound include a positive electrode active material having a polyanion structure such as Li 2 MeSiO 4 , LiFeF 3 having a perovskite structure, and Li 2 Cu 2 O 4 having an orthorhombic structure. Can be mentioned.
- Me represents Fe or Mn.
- examples of the positive electrode active material made of an organic compound include conductive polymers such as polyacetylene and poly-p-phenylene.
- a composite material covered with a carbon material may be produced by reducing and firing an iron-based oxide in the presence of a carbon source material, and this composite material may be used as a positive electrode active material.
- Iron-based oxides tend to have poor electrical conductivity, but can be used as a high-performance positive electrode active material by using a composite material as described above.
- the positive electrode active material is preferably a lithium-containing composite metal oxide because it has a high energy density.
- Many lithium-containing composite metal oxides have a hydrophilic group as a surface functional group. Therefore, by using the lithium-containing composite metal oxide, a slurry composition having high dispersion stability can be obtained, and the binding between the positive electrode active materials in the electrode can be kept strong.
- the surface state of the positive electrode active material can be determined by measuring the contact angle between the positive electrode active material and the solvent. For example, it can be confirmed by pressure-molding only the positive electrode active material to produce pellets and determining the contact angle of the pellets with a polar solvent (for example, N-methylpyrrolidone). A lower contact angle indicates that the positive electrode active material is more hydrophilic.
- examples of the positive electrode active material include nickel hydroxide particles.
- the nickel hydroxide particles may be dissolved in cobalt, zinc, cadmium, or the like, or may be coated with a cobalt compound whose surface is subjected to an alkali heat treatment.
- the positive electrode active material may be partially element-substituted.
- an inorganic compound and an organic compound may be used in combination.
- one type of positive electrode active material may be used alone, or two or more types may be used in combination at any ratio.
- the particle size of the positive electrode active material particles is usually selected as appropriate in consideration of other constituent requirements of the secondary battery.
- the 50% volume cumulative diameter of the positive electrode active material particles is usually 0.1 ⁇ m or more, preferably 0.4 ⁇ m or more, more preferably 1 ⁇ m or more, from the viewpoint of improving battery characteristics such as load characteristics and cycle characteristics. It is 50 ⁇ m or less, preferably 30 ⁇ m or less, more preferably 20 ⁇ m or less. When the 50% volume cumulative diameter is within this range, a secondary battery having excellent output characteristics and a large charge / discharge capacity can be obtained. Moreover, the handling at the time of manufacturing the slurry composition for manufacturing a positive electrode compound material layer and manufacturing a positive electrode is easy.
- the 50% volume cumulative diameter can be determined by measuring the particle size distribution by laser diffraction.
- lithium-containing nickel oxide (LiNiO 2 ), lithium-containing cobalt oxide (LiCoO). 2 ), Co—Ni—Mn lithium composite oxide (Li (Co 2 Mn Ni) O 2 ), Li [Ni 0.17 Li 0.2 Co 0.07 Mn 0.56 ] O 2 , LiNi 0.5 Mn 1.5 O 4 and the like are preferable.
- Co-Ni-Mn lithium composite oxide (Li (Co Mn Ni) O 2 ) and lithium-containing cobalt oxide (LiCoO 2 ) are preferable.
- the content ratio of the positive electrode active material in the slurry composition of the present invention is preferably 50% by mass or more, more preferably 60% by mass or more, and preferably 95% by mass or less, more preferably 90% by mass or less. is there. By setting the content ratio of the positive electrode active material in the slurry composition within the above range, a good slurry composition and positive electrode can be produced.
- the said slurry composition contains a electrically conductive material.
- the conductive material include particles made of carbon allotrope having conductivity.
- the conductive material include conductive carbon such as acetylene black, ketjen black, carbon black, graphite, vapor grown carbon fiber, and carbon nanotube. Further, for example, carbon powder such as graphite, fibers and foils of various metals, and the like are also included.
- a conductive material may be used individually by 1 type, and may be used combining two or more types by arbitrary ratios.
- acetylene black is used as the conductive material from the viewpoint of improving the electrical contact between the positive electrode active materials and improving the electrical characteristics of the secondary battery using the positive electrode formed using the slurry composition. It is preferable.
- the 50% volume cumulative diameter of the conductive material is preferably smaller than the 50% volume cumulative diameter of the positive electrode active material.
- the specific range of the 50% volume cumulative diameter of the conductive material is usually 0.001 ⁇ m or more, preferably 0.05 ⁇ m or more, more preferably 0.01 ⁇ m or more, and usually 10 ⁇ m or less, preferably 5 ⁇ m or less, more preferably 1 ⁇ m. It is as follows. When the 50% volume cumulative diameter of the conductive material is within this range, high conductivity can be obtained with a smaller amount of use.
- the amount of the conductive material is usually 0.01 parts by mass or more, preferably 1 part by mass or more, and usually 20 parts by mass or less, preferably 10 parts by mass or less with respect to 100 parts by mass of the positive electrode active material.
- the amount of the conductive material When the amount of the conductive material is within this range, the capacity of the secondary battery can be increased and high load characteristics can be exhibited. If the amount of the conductive material is too small, the conductive path formed by electrical contact between the positive electrode active materials becomes insufficient, and the output of the secondary battery may be reduced. On the other hand, when the blending amount of the conductive material is too large, the stability of the slurry composition is lowered and the density of the positive electrode mixture layer in the positive electrode is lowered, so that the capacity of the lithium ion secondary battery cannot be sufficiently increased. .
- the viscosity modifier is for adjusting the viscosity of the slurry composition and facilitating the application of the slurry composition on the current collector.
- a water-soluble polymer can be used as the viscosity modifier.
- examples of the viscosity modifier include cellulose polymers such as carboxymethyl cellulose, methyl cellulose, and hydroxypropyl cellulose, and ammonium salts and alkali metal salts of these cellulose polymers; modified or unmodified poly (meth) acrylic Acids and ammonium and alkali metal salts of these poly (meth) acrylic acids; modified or unmodified polyvinyl alcohol, copolymers of acrylic acid or acrylates with vinyl alcohol, maleic anhydride, maleic acid or Polyvinyl alcohols such as copolymers of fumaric acid and vinyl alcohol; polyethylene glycol, polyvinyl pyrrolidone, modified polyacrylic acid, oxidized starch, phosphate starch, casein, various modified starches, acrylonitrile Butadiene copolymer hydrides can be used.
- any polymer that is water-soluble and has a thiol group at the terminal water-soluble polymer having a thiol group at the terminal
- the compounding quantity of a viscosity modifier is 0.1 mass part or more normally per 100 mass parts of electrode active materials, Preferably, it is 0.3 mass part or more, Usually 2 mass parts or less, Preferably it is 1 mass part or less.
- the blending amount of the viscosity modifier is within the above range, a slurry composition having a good viscosity can be obtained. Therefore, the slurry composition can be satisfactorily applied onto the current collector during electrode formation, and as a result, the product life of the obtained electrode can be extended.
- the slurry composition of the present invention may further contain a surfactant.
- the surfactant is effective for preventing the repelling that occurs when applying the slurry composition to the current collector and improving the smoothness of the electrode.
- examples of the surfactant include alkyl surfactants, silicon surfactants, fluorine surfactants, metal surfactants, and the like.
- the compounding amount of the surfactant is preferably 0.01 to 10 parts by mass with respect to 100 parts by mass of the electrode active material. When the surfactant is in the above range, productivity and smoothness during electrode production are improved, and the battery characteristics of the secondary battery are excellent.
- the slurry composition of the present invention may further contain a dispersant.
- the dispersant include an anionic compound, a cationic compound, a nonionic compound, and a polymer compound.
- a specific dispersing agent is selected according to the electrode active material and conductive material to be used. By using the dispersant, the stability of the slurry composition is improved and a smooth electrode is obtained, so that the battery capacity of the secondary battery can be increased.
- the amount of the dispersing agent is usually 0.1 parts by mass or more, preferably 0.5 parts by mass or more, more preferably 0.8 parts by mass or more, and usually 10 parts by mass with respect to 100 parts by mass of the electrode active material. Part or less, preferably 5 parts by weight or less, more preferably 2 parts by weight or less.
- the slurry composition of the present invention may contain components such as a reinforcing material, an antioxidant, and an electrolytic solution additive having a function of suppressing decomposition of the electrolytic solution.
- components such as a reinforcing material, an antioxidant, and an electrolytic solution additive having a function of suppressing decomposition of the electrolytic solution.
- known ones can be used. For example, those described in International Publication No. 2012/115096, International Publication No. 2012/036260, and Japanese Patent Application Laid-Open No. 2012-204303 can be used. it can.
- the slurry composition of the present invention can be prepared by dispersing each of the above components in an aqueous medium as a dispersion medium. Specifically, the above components and the aqueous medium are mixed using a mixer such as a ball mill, a sand mill, a bead mill, a pigment disperser, a grinder, an ultrasonic disperser, a homogenizer, a planetary mixer, or a fill mix. Thus, a slurry composition can be prepared.
- a slurry composition can be prepared.
- water is usually used, but an aqueous solution of an arbitrary compound or a mixed solution of a small amount of an organic medium and water may be used.
- the solid content concentration of the slurry composition can be set to a concentration at which each component can be uniformly dispersed, for example, 10 to 80% by mass. Further, the mixing of each of the above components and the aqueous medium can be usually carried out in the range of room temperature to 80 ° C. for 10 minutes to several hours.
- the viscosity of the slurry composition is preferably 10 mPa ⁇ s or more, more preferably 100 mPa ⁇ s or more, preferably 100,000 mPa ⁇ s or less, more preferably from the viewpoint of the temporal stability and coating properties of the slurry composition. Is 50,000 mPa ⁇ s or less.
- the viscosity is a value measured using a B-type viscometer at 25 ° C. and a rotation speed of 60 rpm.
- the pH of the said slurry composition is 7 or more normally, Preferably it is 8 or more, Usually, 12 or less, Preferably it is 11.5 or less.
- a method of adjusting the pH of the slurry composition for example, a method of adjusting the pH of the slurry composition by washing the positive electrode active material before preparing the slurry composition, bubbling carbon dioxide gas to the prepared slurry composition Examples thereof include a method for adjusting pH and a method for adjusting using a pH adjusting agent.
- a pH adjuster is a water-soluble substance which shows acidity. Either a strong acid or a weak acid may be used.
- water-soluble substances that exhibit weak acidity include organic compounds having an acid group such as a carboxylic acid group, a phosphoric acid group, and a sulfonic acid group.
- an organic compound having a carboxylic acid group is particularly preferably used.
- Specific examples of the compound having a carboxylic acid group include succinic acid, phthalic acid, maleic acid, succinic anhydride, phthalic anhydride, maleic anhydride and the like. These compounds can be made into acid anhydrides having little influence in the secondary battery by drying.
- water-soluble substances that exhibit strong acidity include hydrochloric acid, nitric acid, sulfuric acid, and acetic acid.
- pH adjusting agents described above it is preferable that they are decomposed or volatilized in the drying step of the slurry composition. In this case, no pH adjuster remains in the obtained positive electrode.
- examples of such a pH adjuster include acetic acid and hydrochloric acid.
- a pH adjuster may be used individually by 1 type, and may be used combining two or more types by arbitrary ratios.
- the pH of the slurry composition may be adjusted at any time as long as it is during the production process of the slurry composition. Especially, after adjusting a slurry composition to desired solid content concentration, it is preferable to adjust pH with a pH adjuster. By adjusting the pH after adjusting the slurry composition to a predetermined solid content concentration, it is possible to easily adjust the pH while preventing dissolution of the positive electrode active material.
- the slurry composition of the present invention is prepared by preparing the binder composition of the present invention and then mixing the binder composition and the electrode active material, or dispersing them in an aqueous medium as a dispersion medium. Also good.
- the secondary battery electrode of the present invention is obtained by applying the slurry composition for a secondary battery electrode obtained as described above on a current collector, and applying the slurry for the secondary battery electrode on the current collector. It is obtained by drying the composition.
- the electrode for a secondary battery of the present invention includes a current collector and an electrode mixture layer formed on the current collector.
- the electrode mixture layer includes at least an electrode active material and a particulate weight. The merger is included.
- each component such as an electrode active material contained in the electrode mixture layer is contained in the slurry composition of the present invention, and a suitable abundance ratio of these components is determined according to the present invention. It is the same as the preferred abundance ratio of each component in the slurry composition.
- a method for applying the slurry composition for a secondary battery electrode on the current collector is not particularly limited, and a known method can be used. Specifically, as a coating method, a doctor blade method, a dip method, a reverse roll method, a direct roll method, a gravure method, an extrusion method, a brush coating method, or the like can be used. At this time, the slurry composition may be applied to only one side of the current collector or may be applied to both sides. The thickness of the slurry film on the current collector after application and before drying can be appropriately set according to the thickness of the electrode mixture layer obtained by drying.
- the current collector to which the slurry composition is applied is not particularly limited as long as it is a material having electrical conductivity and electrochemical durability.
- the current collector is preferably made of metal, such as iron, copper, aluminum, nickel, stainless steel, titanium, tantalum, gold, or platinum.
- a copper foil is particularly preferable as the current collector used for the negative electrode
- aluminum is particularly preferable as the current collector used for the positive electrode.
- One type of current collector material may be used alone, or two or more types may be used in combination at any ratio.
- the shape of the current collector is not particularly limited, but a sheet shape having a thickness of about 0.001 mm to 0.5 mm is preferable.
- the current collector is used after roughening the surface in advance in order to increase the adhesive strength of the electrode mixture layer.
- the roughening method include a mechanical polishing method, an electrolytic polishing method, and a chemical polishing method.
- a mechanical polishing method usually, a polishing cloth with a fixed abrasive particle, a grindstone, an emery buff, a wire brush provided with a steel wire or the like is used.
- a method for drying the slurry composition on the current collector is not particularly limited, and a known method can be used. For example, drying with warm air, hot air, low-humidity air, vacuum drying, irradiation with infrared rays, electron beams, or the like. A drying method is mentioned. Thus, by drying the slurry composition on the current collector, an electrode mixture layer is formed on the current collector, and a lithium ion secondary battery electrode including the current collector and the electrode mixture layer is obtained. be able to.
- the electrode mixture layer may be subjected to pressure treatment using a die press or a roll press.
- the pressure treatment can improve the adhesion between the electrode mixture layer and the current collector and reduce the porosity of the electrode.
- the porosity is preferably 5% or more, more preferably 7% or more, preferably 15% or less, more preferably 13% or less.
- the thickness of the electrode mixture layer is usually 5 ⁇ m or more, preferably 10 ⁇ m or more, and usually 300 ⁇ m or less, preferably 250 ⁇ m or less. When the thickness of the electrode mixture layer is in the above range, both load characteristics and energy density are high.
- the water content in the electrode mixture layer is preferably 1000 ppm or less, and more preferably 500 ppm or less. By setting the water content of the electrode mixture layer within the above range, a secondary battery electrode having excellent durability can be realized. The amount of water can be measured by a known method such as the Karl Fischer method.
- the powder molding method refers to preparing a slurry composition for producing an electrode for a secondary battery, preparing composite particles including an electrode active material from the slurry composition, and placing the composite particles on a current collector. It is a manufacturing method which supplies the electrode for secondary batteries by forming an electrode compound-material layer by supplying and roll-pressing as needed and shaping
- the secondary battery of the present invention includes a positive electrode, a negative electrode, an electrolytic solution, and a separator, and uses the secondary battery electrode of the present invention as at least one of the positive electrode and the negative electrode.
- the secondary battery of the present invention is excellent in high temperature storage characteristics and high temperature cycle characteristics, and gas generation is suppressed.
- the secondary battery of the present invention may be any of a lithium ion secondary battery, a nickel hydride secondary battery, and the like. Among these, lithium ion secondary batteries are preferable because performance improvement effects such as improvement of cycle characteristics and output characteristics are particularly remarkable.
- the secondary battery of the present invention is a lithium ion secondary battery will be described.
- At least one of the positive electrode and the negative electrode may be the electrode for the secondary battery of the present invention. That is, a known electrode may be used as either the positive electrode or the negative electrode.
- Examples of known positive electrodes include those usually comprising a current collector and a positive electrode mixture layer formed on the surface of the current collector.
- the positive electrode mixture layer comprises a positive electrode active material, a conductive material and a binder.
- the positive electrode active material, and the conductive material those described above in the sections of “secondary battery electrode” and “secondary battery electrode slurry composition” can be used.
- the binder any known binder can be used as long as the present invention is not significantly impaired.
- the positive electrode mixture layer may contain components other than the positive electrode active material, the conductive material, and the binder as necessary, for example, the components listed as “other components” in the section of “Slurry composition for secondary battery electrode”. It may be included. One of these may be used alone, or two or more of these may be used in combination at any ratio.
- the negative electrode there is usually one having a current collector and a negative electrode mixture layer formed on the surface of the current collector, and the negative electrode mixture layer contains a negative electrode active material and a binder.
- the current collector and the negative electrode active material those described above in the sections of “secondary battery electrode” and “secondary battery electrode slurry composition” can be used.
- the binder any known binder can be used as long as the present invention is not significantly impaired. Examples of such binders include polyethylene, polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), polyacrylic acid derivatives, and polyacrylonitrile derivatives.
- a soft polymer such as an acrylic soft polymer, a diene soft polymer, an olefin soft polymer, or a vinyl soft polymer may be used. Moreover, these may be used individually by 1 type and may be used combining two or more types by arbitrary ratios.
- the negative electrode mixture layer contains components other than the negative electrode active material and the binder, for example, the components listed as “other components” in the section of “slurry composition for secondary battery electrode” as necessary. May be. One of these may be used alone, or two or more of these may be used in combination at any ratio.
- the total thickness of these electrodes is usually 5 ⁇ m or more, preferably 10 ⁇ m or more, and usually 300 ⁇ m or less, preferably 250 ⁇ m or less. When the thickness of the electrode is in the above range, both load characteristics and energy density can be improved.
- Electrodes prepare the slurry composition for electrodes containing an electrode active material, a binder, and water similarly to the electrode for secondary batteries of this invention, for example,
- the layer of the slurry composition is used as a collector. It may be formed and the layer dried.
- Electrode As an electrolytic solution for a lithium ion secondary battery, for example, a nonaqueous electrolytic solution in which a supporting electrolyte is dissolved in a nonaqueous solvent is used.
- a lithium salt is usually used.
- the lithium salt include LiPF 6 , LiAsF 6 , LiBF 4 , LiSbF 6 , LiAlCl 4 , LiClO 4 , CF 3 SO 3 Li, C 4 F 9 SO 3 Li, CF 3 COOLi, (CF 3 CO) 2 NLi , (CF 3 SO 2 ) 2 NLi, (C 2 F 5 SO 2 ) NLi, and the like.
- LiPF 6 , LiClO 4 , and CF 3 SO 3 Li that are easily soluble in a solvent and exhibit a high degree of dissociation are preferable.
- One of these may be used alone, or two or more of these may be used in combination at any ratio. Since the lithium ion conductivity increases as the supporting electrolyte having a higher degree of dissociation is used, the lithium ion conductivity can be adjusted depending on the type of the supporting electrolyte.
- the concentration of the supporting electrolyte in the electrolytic solution is usually 1% by mass or more, preferably 5% by mass or more, and usually 30% by mass or less, preferably 20% by mass or less. Depending on the type of supporting electrolyte, it may be used usually at a concentration of 0.5 mol / L to 2.5 mol / L. If the concentration of the supporting electrolyte is too low or too high, the ionic conductivity may decrease.
- the non-aqueous solvent is not particularly limited as long as it can dissolve the supporting electrolyte.
- non-aqueous solvents include carbonates such as dimethyl carbonate (DMC), ethylene carbonate (EC), diethyl carbonate (DEC), propylene carbonate (PC), butylene carbonate (BC), methyl ethyl carbonate (MEC); and esters such as ⁇ -butyrolactone and methyl formate; ethers such as 1,2-dimethoxyethane and tetrahydrofuran; sulfur-containing compounds such as sulfolane and dimethyl sulfoxide; Among these, carbonates are preferable because they have a high dielectric constant and a wide stable potential region.
- a non-aqueous solvent may be used individually by 1 type, and may be used combining two or more types by arbitrary ratios.
- an additive in electrolyte solution examples include carbonate compounds such as vinylene carbonate (VC).
- An additive may be used individually by 1 type and may be used combining two or more types by arbitrary ratios.
- an electrolytic solution other than the above for example, a polymer electrolyte such as polyethylene oxide or polyacrylonitrile; a gel polymer electrolyte obtained by impregnating the polymer electrolyte with an electrolytic solution; an inorganic solid electrolyte such as LiI or Li 3 N; Also good.
- a polyolefin resin such as polyethylene or polypropylene, or a microporous film or nonwoven fabric containing an aromatic polyamide resin; a porous resin coat containing an inorganic ceramic powder;
- microporous membranes made of polyolefin resins (polyethylene, polypropylene, polybutene, polyvinyl chloride), and resins such as mixtures or copolymers thereof; polyethylene terephthalate, polycycloolefin, polyether sulfone, polyamide, Examples thereof include a microporous film made of a resin such as polyimide, polyimide amide, polyaramid, polycycloolefin, nylon, and polytetrafluoroethylene; a polyolefin fiber woven or non-woven fabric thereof; an aggregate of insulating substance particles, and the like.
- a microporous film made of a polyolefin-based resin is preferable
- the thickness of the separator is usually 0.5 ⁇ m or more, preferably 1 ⁇ m or more, and usually 40 ⁇ m or less, preferably 30 ⁇ m or less, more preferably 10 ⁇ m or less. Within this range, the resistance due to the separator in the secondary battery is reduced, and the workability when manufacturing the secondary battery is excellent.
- a positive electrode and a negative electrode are overlapped via a separator, and this is wound into a battery container according to the shape of the battery.
- the method of injecting and sealing is mentioned.
- an expanded metal; an overcurrent prevention element such as a fuse or a PTC element; a lead plate or the like may be inserted to prevent an increase in pressure inside the battery or overcharge / discharge.
- the shape of the secondary battery may be any of a coin shape, a button shape, a sheet shape, a cylindrical shape, a square shape, a flat shape, and the like.
- the manufactured laminated cell type lithium ion secondary battery was charged to 4.3 V by a constant current method of 0.1 C in a 25 ° C. environment, and then stored at 80 ° C. for 50 hours.
- the cell of the laminated cell type lithium ion secondary battery was sandwiched between glass plates, and the thickness of the cell was measured with a micro gauge.
- the thickness of the cell before storage at 80 ° C. is a
- the thickness of the cell after storage at 80 ° C. for 50 hours is b
- the thickness ratio (b / a) before and after storage at 80 ° C. is determined. did. It shows that the smaller the calculated ratio (b / a), the smaller the gas generation amount and the better the gas generation suppression effect.
- Ratio (b / a) is 1.00 times or more and 1.05 times or less
- B: Ratio (b / a) is more than 1.05 times and 1.10 times or less
- D: Ratio (b / a) is more than 1.15 times and less than 1.20 times
- the manufactured laminated cell type lithium ion secondary battery was charged to 4.3 V by a constant current method of 0.1 C in a 25 ° C. environment, and then stored at 80 ° C. for 100 hours.
- the open circuit voltage (Open circuit voltage, hereinafter referred to as “OCV”) before the start of storage at 80 ° C. and the OCV of the cell after storage for 100 hours at 80 ° C. were measured.
- the OCV ratio after storage for 100 hours is defined as the OCV maintenance rate, and is determined according to the following criteria. It shows that it is excellent in a high temperature storage characteristic, ie, a lifetime characteristic, so that an OCV maintenance factor is large.
- OCV maintenance rate is 99.0% or more
- B OCV maintenance rate is 98.8% or more and less than 99.0%
- C OCV maintenance rate is 98.6% or more and less than 98.8%
- D OCV maintenance rate is 98 0.4% or more and less than 98.6%
- E OCV maintenance rate is 98.2% or more and less than 98.4%
- F OCV maintenance rate is 98.0% or more and less than 98.2%
- Capacity maintenance rate is 90% or more
- ⁇ Peel strength (adhesiveness between negative electrode mixture layer and current collector)>
- the produced negative electrode for a secondary battery was cut into a rectangular shape having a length of 100 mm and a width of 10 mm to obtain a test piece.
- a stress was measured when one end of the current collector was pulled in a vertical direction at a pulling speed of 50 mm / min and peeled off (the cellophane tape was fixed to the test stand). The measurement was performed three times, the average value was obtained and this was taken as the peel peel strength, and evaluated according to the following criteria. It shows that it is excellent in the adhesiveness of a negative mix layer and a collector, so that the value of peeling peel strength is large.
- Peel peel strength is 30 N / m or more
- B Peel peel strength is 25 N / m or more and less than 30 N / m
- C Peel peel strength is 20 N / m or more and less than 25 N / m
- D Peel peel strength is less than 20 N / m
- the amount of (meth) acrylic acid ester monomer (residual monomer) in the binder composition was measured by gas chromatography. Specifically, first, a sample for measurement was prepared by diluting with acetone so that the solid content concentration of the particulate polymer in the binder composition was 1% by mass and filtering using 5C filter paper. And about the prepared sample, the gas chromatography was implemented on condition of the following.
- Apparatus Agilent 6850A (manufactured by Agilent Technologies) Column: HP-1 Average linear velocity: 15 cm / s Injection volume: 1ml Inlet temperature: 250 ° C Split ratio: 20: 1 Detector: Flame ionization detector (FID) Detector temperature: 280 ° C Oven: Hold at 40 ° C for 3 minutes, then heat at 10 ° C / minute and hold at 280 ° C for 5 minutes
- volume average particle size of the particulate polymer was measured using a light scattering particle size measuring device (Coulter LS230, manufactured by Coulter Inc.).
- the binder composition for secondary battery electrodes of the present invention was used for forming a negative electrode, the case where it was used for forming a positive electrode was evaluated.
- the binder composition of the present invention was used for forming a negative electrode.
- styrene as an aromatic vinyl monomer
- acidic An emulsion was prepared by adding 8.0 parts of itaconic acid (IA) as a group-containing monomer and 4.8 parts of sodium lauryl sulfate and 192 parts of ion-exchanged water as a dispersant. This emulsion was sequentially added from polymerization vessel B to polymerization vessel A over about 180 minutes. Thereafter, the mixture was stirred for about 120 minutes, and when the polymerization conversion rate (monomer consumption amount) reached 93%, it was cooled to 40 ° C.
- IA itaconic acid
- the obtained particulate polymer had a glass transition temperature of ⁇ 6.2 ° C. and a volume average particle size of 0.167 ⁇ m.
- the content of the (meth) acrylic acid ester monomer unit having a boiling point of 145 ° C. or higher in the particulate polymer is 50%, the content of the acidic group-containing monomer unit is 2%, and the aromatic vinyl monomer unit. was 47%, and the crosslinkable monomer unit content was 1%.
- the content of the (meth) acrylic acid ester monomer (2-EHA) having a boiling point of 145 ° C. or higher with respect to 100 parts by mass of the particulate polymer was 208 ⁇ 10 ⁇ 6 parts by mass.
- This negative electrode slurry composition was applied to one side of a 10 ⁇ m thick copper foil, dried at 110 ° C. for 3 hours, and then roll pressed to obtain a negative electrode having a negative electrode mixture layer having a thickness of 60 ⁇ m. About the obtained negative electrode, peel strength was measured. The results are shown in Table 1.
- Co-Ni-Mn lithium composite oxide based active material product name: Cellseed NMC613, manufactured by Nippon Chemical Industry Co., Ltd .; hereinafter sometimes referred to as “NMC”
- NMC Cellseed NMC613, manufactured by Nippon Chemical Industry Co., Ltd .
- CMC sodium salt
- BM-610B binder solid content concentration 40%
- the obtained positive electrode slurry composition was applied to an aluminum foil having a thickness of 18 ⁇ m, dried at 120 ° C. for 3 hours, and then roll-pressed to obtain a positive electrode having a positive electrode mixture layer having a thickness of 50 ⁇ m.
- a battery container was prepared using a laminate film made of an aluminum sheet and a polypropylene resin covering both surfaces thereof. Subsequently, the mixture layer was removed from the end portions of the positive electrode and the negative electrode obtained above to form a portion where the copper foil or the aluminum foil was exposed. A Ni tab was welded to the portion where the aluminum foil of the positive electrode was exposed, and a Cu tab was welded to the portion where the copper foil of the negative electrode was exposed. The obtained tabbed positive electrode and tabbed negative electrode were stacked with a separator made of a polyethylene microporous film interposed therebetween.
- the direction of the surface of the electrode was such that the surface on the positive electrode mixture layer side and the surface of the negative electrode mixture layer side face each other.
- the stacked electrode and separator were wound and stored in the battery container.
- the electrolytic solution a solution prepared by dissolving LiPF 6 to a concentration of 1 mol / L in a mixed solvent in which ethylene carbonate and diethyl carbonate were mixed at a volume ratio of 1: 2 at 25 ° C. was used. .
- the laminate film was sealed to produce a laminate cell type lithium ion secondary battery which is the lithium ion secondary battery of the present invention.
- the amount of gas generation, high temperature storage characteristics, and high temperature cycle characteristics of the obtained laminated cell type lithium ion secondary battery were evaluated. The results are shown in Table 1.
- Example 2 Table 1 shows the blending amount of 2-ethylhexyl acrylate, which is a (meth) acrylic acid ester monomer having a boiling point of 145 ° C. or higher, blending amount of styrene, which is an aromatic vinyl monomer, and the polymerization conversion rate in step (1).
- a binder composition was prepared in the same manner as in Example 1 except that the composition was changed as shown in FIG. In addition, even if it was a case where the compounding quantity of each component was changed, the ratio between the compounding quantities of each component contained in superposition
- the laminated cell type lithium ion secondary battery which has a negative electrode and this negative electrode was manufactured like Example 1 using the prepared binder composition.
- the peel strength was evaluated for the negative electrode, and the gas generation amount, high-temperature storage characteristics, and high-temperature cycle characteristics were evaluated for the laminated cell type lithium ion secondary battery. The results are shown in Table 1.
- Example 3 Example 1 except that the blending amount of styrene as an aromatic vinyl monomer and the polymerization conversion rate in the step (1) were changed as shown in Table 1 and allyl methacrylate as a crosslinkable monomer was not blended.
- a binder composition was prepared, and stability over time was evaluated.
- polymerization can A and B was made to be the same as that of Example 1.
- the laminated cell type lithium ion secondary battery which has a negative electrode and this negative electrode was manufactured like Example 1 using the prepared binder composition. The peel strength was evaluated for the negative electrode, and the amount of gas generation, high-temperature storage characteristics, and high-temperature cycle characteristics were evaluated for the laminated cell type lithium ion secondary battery. The results are shown in Table 1.
- Example 4 Table 1 shows the blending amount of 2-ethylhexyl acrylate, which is a (meth) acrylic acid ester monomer having a boiling point of 145 ° C. or higher, blending amount of styrene, which is an aromatic vinyl monomer, and the polymerization conversion rate in step (1).
- the binder composition was prepared in the same manner as in Example 1 except that 20 parts of butadiene was added as another monomer, and the stability over time was evaluated. In addition, even if it was a case where the compounding quantity of each component was changed, the ratio between the compounding quantities of each component contained in superposition
- the laminated cell type lithium ion secondary battery which has a negative electrode and this negative electrode was manufactured like Example 1 using the prepared binder composition.
- the peel strength was evaluated for the negative electrode, and the gas generation amount, high-temperature storage characteristics, and high-temperature cycle characteristics were evaluated for the laminated cell type lithium ion secondary battery. The results are shown in Table 1.
- Table 1 shows the blending amount of 2-ethylhexyl acrylate, which is a (meth) acrylic acid ester monomer having a boiling point of 145 ° C. or more, blending amount of styrene, which is an aromatic vinyl monomer, and polymerization conversion rate in polymerization step 1. Except for changing as shown, a binder composition was prepared in the same manner as in Example 1, and stability over time was evaluated. In addition, even if it was a case where the compounding quantity of each component was changed, the ratio between the compounding quantities of each component contained in superposition
- the laminated cell type lithium ion secondary battery which has a negative electrode and this negative electrode was manufactured like Example 1 using the prepared binder composition.
- the peel strength was evaluated for the negative electrode, and the gas generation amount, high-temperature storage characteristics, and high-temperature cycle characteristics were evaluated for the laminated cell type lithium ion secondary battery. The results are shown in Table 1.
- Example 6 A binder composition was prepared in the same manner as in Example 1 except that butyl acrylate was used in place of 2-ethylhexyl acrylate as a (meth) acrylic acid ester monomer having a boiling point of 145 ° C. or higher. evaluated. In addition, even if it was a case where the compounding quantity of each component was changed, the ratio between the compounding quantities of each component contained in superposition
- Example 7 to 10 A binder composition was prepared in the same manner as in Example 1 except that sodium ascorbate, which is a reductone compound, was changed to the reductone compound shown in Table 1, and the stability over time was evaluated. Moreover, the laminated cell type lithium ion secondary battery which has a negative electrode and this negative electrode was manufactured like Example 1 using the prepared binder composition. The peel strength was evaluated for the negative electrode, and the gas generation amount, high-temperature storage characteristics, and high-temperature cycle characteristics were evaluated for the lithium ion secondary battery. The results are shown in Table 1.
- Example 1 except that the blending ratio of sodium ascorbate as a reductone compound was changed as shown in Table 1, and the blending ratio of tert-butyl hydroperoxide as a peroxide was changed as shown in Table 1.
- a binder composition was prepared, and stability over time was evaluated.
- the laminated cell battery type lithium ion secondary battery which has a negative electrode and this negative electrode was manufactured like Example 1 using the prepared binder composition. The peel strength was evaluated for the negative electrode, and the gas generation amount, high-temperature storage characteristics, and high-temperature cycle characteristics were evaluated for the laminated cell type lithium ion secondary battery. The results are shown in Table 1.
- Example 15 A binder composition was prepared in the same manner as in Example 1 except that the peroxide tert-butyl hydroperoxide was changed to benzoyl peroxide (BPO), and the stability over time was evaluated. Moreover, the laminated cell type lithium ion secondary battery which has a negative electrode and this negative electrode was manufactured like Example 1 using the prepared binder composition. The peel strength was evaluated for the negative electrode, and the gas generation amount, high-temperature storage characteristics, and high-temperature cycle characteristics were evaluated for the laminated cell type lithium ion secondary battery. The results are shown in Table 1.
- Example 1 Binder composition in the same manner as in Example 1 except that sodium ascorbate which is a reductone compound is not blended and the reaction is performed at 40 ° C. for 2 hours and then at 80 ° C. for 2 hours in Step (2). Were prepared and the stability over time was evaluated. Moreover, the laminated cell type lithium ion secondary battery which has a negative electrode and this negative electrode was manufactured like Example 1 using the prepared binder composition. The peel strength was evaluated for the negative electrode, and the gas generation amount, high-temperature storage characteristics, and high-temperature cycle characteristics were evaluated for the laminated cell type lithium ion secondary battery. The results are shown in Table 1.
- Example 2 A binder composition was prepared in the same manner as in Example 1 except that ethyl acrylate (EA) having a boiling point of 100 ° C. was used instead of 2-ethylhexyl acrylate which is a (meth) acrylic acid ester monomer having a boiling point of 145 ° C. or higher. Prepared and evaluated for stability over time. Moreover, the laminated cell type lithium ion secondary battery which has a negative electrode and this negative electrode was manufactured like Example 1 using the prepared binder composition. The peel strength was evaluated for the negative electrode, and the gas generation amount, high-temperature storage characteristics, and high-temperature cycle characteristics were evaluated for the laminated cell type lithium ion secondary battery. The results are shown in Table 1.
- the mixture was stirred for about 120 minutes, and when the polymerization conversion rate (monomer consumption) reached 93%, distillation was performed under reduced pressure over 8 hours using an evaporator at 80 ° C. and 100 hPa (0.1 atm). It was. Thereafter, the mixture was cooled, a 5% aqueous sodium hydroxide solution was added to adjust the pH to 8, and an aqueous dispersion (binder composition) containing a desired particulate polymer was obtained. The obtained binder composition was evaluated for stability over time. Moreover, the laminated cell type lithium ion secondary battery which has a negative electrode and this negative electrode was manufactured like Example 1 using the prepared binder composition. The peel strength was evaluated for the negative electrode, and the gas generation amount, high-temperature storage characteristics, and high-temperature cycle characteristics were evaluated for the lithium ion secondary battery. The results are shown in Table 1.
- Example 4 A binder composition was prepared in the same manner as in Example 1 except that succinic acid was added instead of sodium ascorbate, which is a reductone compound, and stability over time was evaluated. Moreover, the laminated cell type lithium ion secondary battery which has a negative electrode and this negative electrode was manufactured like Example 1 using the prepared binder composition. The peel strength was evaluated for the negative electrode, and the gas generation amount, high-temperature storage characteristics, and high-temperature cycle characteristics were evaluated for the laminated cell type lithium ion secondary battery. The results are shown in Table 1.
- Example 5 A binder composition was prepared in the same manner as in Example 1 except that the step (2) was carried out at a polymerization conversion rate of 81% in the step (1), and the temporal stability was evaluated. Moreover, the laminated cell type lithium ion secondary battery which has a negative electrode and this negative electrode was manufactured like Example 1 using the prepared binder composition. The peel strength was evaluated for the negative electrode, and the gas generation amount, high-temperature storage characteristics, and high-temperature cycle characteristics were evaluated for the laminated cell type lithium ion secondary battery. The results are shown in Table 1.
- step (2) was not performed.
- step (1) the polymerization conversion rate in step (1) reached 93%
- the mixture was cooled to 40 ° C., 0.3% by mass of ammonium persulfate (APS), which is a peroxide, was added, and the mixture was further reacted for 4 hours.
- APS ammonium persulfate
- 5% sodium hydroxide aqueous solution was added with respect to this aqueous dispersion, and it adjusted to pH8, and obtained the aqueous dispersion (binder composition) containing a desired particulate polymer.
- the obtained binder composition was evaluated for stability over time.
- the laminated cell type lithium ion secondary battery which has a negative electrode and this negative electrode was manufactured like Example 1 using the prepared binder composition.
- the peel strength was evaluated for the negative electrode, and the gas generation amount, high-temperature storage characteristics, and high-temperature cycle characteristics were evaluated for the laminated cell type lithium ion secondary battery. The results are shown in Table 1.
- the binder compositions of Examples 1 to 15 have good stability over time, and the negative electrode produced using such a binder composition has good peel strength. It can be seen that the secondary battery manufactured in this manner has a reduced gas generation amount and good high-temperature storage characteristics and high-temperature cycle characteristics.
- Comparative Example 1 in which sodium ascorbate, which is a reductone compound, was not blended the residual amount of 2-ethylhexyl acrylate monomer in the obtained binder composition was large, and the temporal stability of the binder composition was low.
- the stability of the binder composition is good, Although the peel strength of the negative electrode produced using the obtained binder is good and the amount of gas generated in the secondary battery is small, it can be seen that the high-temperature storage characteristics and the high-temperature cycle characteristics deteriorate. This is considered to be because the flexibility of the particulate polymer in the binder composition was lowered, and the binding property was lowered as the binder composition, which adversely affected the battery characteristics.
- Example 1 and 3 it can be seen that the high-temperature storage characteristics and the high-temperature cycle characteristics of the secondary battery can be improved by using a crosslinkable monomer when forming the particulate polymer.
- the peel strength of the negative electrode can be improved by adjusting the blending amount of the (meth) acrylic acid ester monomer having a boiling point of 145 ° C. or more when forming the particulate polymer. I understand that.
- Example 1 and Examples 7 to 10 it can be seen that all characteristics can be arranged in a high dimension by using ascorbic acid and its isomers, derivatives, and salts as reductone compounds.
- Example 9 using ascorbyl palmitate, the gas generation amount, high-temperature storage characteristics, high-temperature cycle characteristics, and stability over time of the binder composition were inferior to Examples 1, 7, 8, and 10, but this This is because the molecular weight of ascorbyl palmitate is relatively large, and if it is added in the same amount as other ascorbic acid-based reductone compounds, the amount in terms of moles is reduced and the residual monomer reduction effect cannot be fully exhibited. It is thought that.
- the amount of reductone compound and peroxide by adjusting the amount of reductone compound and peroxide, the amount of residual monomers can be sufficiently reduced, the amount of gas generated can be reduced, and high temperature storage characteristics and high temperature cycles can be achieved. It can be seen that the characteristics can be further improved.
- the amount of sodium ascorbate, which is a reductone compound is relatively large, and it is considered that ascorbic acid remaining in the binder composition increases the gas generation amount.
- the binder composition of the present invention was used for forming a positive electrode.
- acrylonitrile as an ⁇ , ⁇ -unsaturated nitrile monomer 74.4 parts
- 4.8 parts of sodium lauryl sulfate and 192 parts of ion-exchanged water as dispersants were added and stirred to prepare an emulsion.
- This emulsion was sequentially added from polymerization vessel B to polymerization vessel A over about 180 minutes. Thereafter, the mixture was stirred for about 120 minutes, and when the polymerization conversion rate (monomer consumption amount) reached 95%, it was cooled to 40 ° C.
- the obtained particulate polymer had a glass transition temperature of ⁇ 38.0 ° C. and a volume average particle size of 0.11 ⁇ m.
- the content ratio of (meth) acrylic acid ester monomer units having a boiling point of 145 ° C. or higher is 78%
- the content ratio of acidic group-containing monomer units is 2%
- ⁇ , ⁇ -unsaturated nitrile was 20%.
- the content of the (meth) acrylic acid ester monomer (2-EHA) having a boiling point of 145 ° C. or higher with respect to 100 parts by mass of the particulate polymer was 208 ⁇ 10 ⁇ 6 parts by mass.
- Co-Ni-Mn lithium composite oxide based active material product name: Cellseed NMC613, manufactured by Nippon Chemical Industry Co., Ltd .; hereinafter sometimes referred to as “NMC”
- NMC Cellseed NMC613, manufactured by Nippon Chemical Industry Co., Ltd .
- CMC sodium salt
- aqueous solution of carboxymethyl cellulose as a viscosity modifier
- the positive electrode slurry composition was applied to an aluminum foil having a thickness of 18 ⁇ m, dried at 120 ° C. for 3 hours, and then roll pressed to obtain a positive electrode having a positive electrode mixture layer having a thickness of 50 ⁇ m.
- slurry composition for negative electrode and negative electrode > 98 parts of graphite having a volume average particle diameter of 20 ⁇ m and a specific surface area of 4.2 m 2 / g as a negative electrode active material, and negative electrode binder BM-400B (40 of styrene-butadiene copolymer manufactured by Nippon Zeon Co., Ltd.) as a binder. (Mass% aqueous dispersion) is mixed with 1.0 part in solid content and 0.8 part in CMC with solid content. Further, water is added and mixed with a planetary mixer to obtain a slurry composition for negative electrode. Prepared. This negative electrode slurry composition was applied to one side of a 10 ⁇ m thick copper foil, dried at 110 ° C. for 3 hours, and then roll pressed to obtain a negative electrode having a negative electrode mixture layer having a thickness of 60 ⁇ m.
- BM-400B 40 of styrene-butadiene copolymer manufactured by Nippon Zeon Co
- a battery container was prepared using a laminate film made of an aluminum sheet and a polypropylene resin covering both surfaces thereof. Subsequently, the mixture layer was removed from the end portions of the positive electrode and the negative electrode obtained above to form a portion where the copper foil or the aluminum foil was exposed. A Ni tab was welded to the portion where the aluminum foil of the positive electrode was exposed, and a Cu tab was welded to the portion where the copper foil of the negative electrode was exposed. The obtained tabbed positive electrode and tabbed negative electrode were stacked with a separator made of a polyethylene microporous film interposed therebetween.
- the direction of the surface of the electrode was such that the surface on the positive electrode mixture layer side and the surface of the negative electrode mixture layer side face each other.
- the stacked electrode and separator were wound and stored in the battery container.
- the electrolytic solution a solution prepared by dissolving LiPF 6 to a concentration of 1 mol / L in a mixed solvent in which ethylene carbonate and diethyl carbonate were mixed at a volume ratio of 1: 2 at 25 ° C. was used. .
- the laminate film was sealed to produce a laminate cell type lithium ion secondary battery which is the lithium ion secondary battery of the present invention.
- the amount of gas generation and high-temperature storage characteristics of the obtained laminated cell type lithium ion secondary battery were evaluated. The results are shown in Table 2.
- the positive electrode obtained above was cut into a circle having a diameter of 13 mm.
- the negative electrode obtained above was cut into a circle having a diameter of 14 mm.
- a single-layer polypropylene separator (porosity 55%) produced by a dry method having a thickness of 25 ⁇ m was cut into a circle having a diameter of 18 mm. These were housed in a stainless steel coin-type outer container (diameter 20 mm, height 1.8 mm, stainless steel thickness 0.25 mm) provided with polypropylene packing.
- the arrangement of the circular electrodes and separators in the outer container was as follows. The circular positive electrode was disposed so that the aluminum foil was in contact with the bottom surface of the outer container.
- the circular separator was disposed so as to be interposed between the circular positive electrode and the circular negative electrode.
- a full coin cell type lithium ion battery which is a lithium ion secondary battery of the present invention having a diameter of 20 mm and a thickness of about 3.2 mm, is sealed by covering with a stainless steel cap having a thickness of 0.2 mm and sealing the outer container.
- a secondary battery (coin cell CR2032) was produced. The high-temperature cycle characteristics of the obtained full coin cell type lithium ion secondary battery were evaluated. The results are shown in Table 2.
- Example 17 to 18 The blending amount of 2-ethylhexyl acrylate which is a (meth) acrylic acid ester monomer having a boiling point of 145 ° C. or higher and the blending amount of acrylonitrile which is an ⁇ , ⁇ -unsaturated nitrile monomer were changed as shown in Table 2, respectively. Except for this, a binder composition was prepared in the same manner as in Example 16, and the stability over time was evaluated. In addition, even if it was a case where the compounding quantity of each component was changed, the ratio between the compounding quantities of each component contained in superposition
- Example 16 volume average particle diameter of the obtained particulate polymer was 0.11 ⁇ m as in Example 16. Further, using the prepared binder composition, a laminated cell type lithium ion secondary battery and a full coin cell type lithium ion secondary battery were produced in the same manner as in Example 16, and the gas generation amount and the high temperature storage characteristics were respectively obtained. As well as high temperature cycle characteristics. The results are shown in Table 2.
- Example 19 A binder composition was prepared in the same manner as in Example 16 except that n-butyl acrylate (boiling point 148 ° C.) was used as a (meth) acrylic acid ester monomer having a boiling point of 145 ° C. or higher, and the stability over time was evaluated. Further, using the prepared binder composition, a laminated cell type lithium ion secondary battery and a full coin cell type lithium ion secondary battery were produced in the same manner as in Example 16, and the gas generation amount and the high temperature storage characteristics were respectively obtained. As well as high temperature cycle characteristics. The results are shown in Table 2.
- Example 20 to 23 A binder composition was prepared in the same manner as in Example 16 except that sodium ascorbate, which is a reductone compound, was changed to a reductone compound shown in Table 2, and the temporal stability was evaluated. Further, using the prepared binder composition, a laminated cell type lithium ion secondary battery and a full coin cell type lithium ion secondary battery were produced in the same manner as in Example 16, and the gas generation amount and the high temperature storage characteristics were respectively obtained. As well as high temperature cycle characteristics. The results are shown in Table 2.
- Example 1 except that the blending ratio of sodium ascorbate as a reductone compound was changed as shown in Table 2, and the blending ratio of tert-butyl hydroperoxide as a peroxide was changed as shown in Table 2.
- a binder composition was prepared in the same manner as in No. 16, and the stability over time was evaluated. Further, using the prepared binder composition, a laminated cell battery type lithium ion secondary battery and a full coin cell type lithium ion secondary battery were produced in the same manner as in Example 16, and the gas generation amount and high temperature storage characteristics were respectively obtained. In addition, the high-temperature cycle characteristics were evaluated. The results are shown in Table 2.
- Example 28 A binder composition was prepared in the same manner as in Example 16 except that the peroxide tert-butyl hydroperoxide was changed to benzoyl peroxide (BPO), and the stability over time was evaluated. Further, using the prepared binder composition, a laminated cell type lithium ion secondary battery and a full coin cell type lithium ion secondary battery were produced in the same manner as in Example 16, and the gas generation amount and the high temperature storage characteristics were respectively obtained. As well as high temperature cycle characteristics. The results are shown in Table 2.
- Example 7 Binder composition in the same manner as in Example 16 except that sodium ascorbate which is a reductone compound is not blended, and the reaction is performed at 40 ° C. for 2 hours and then at 80 ° C. for 2 hours in Step (2). Were prepared and the stability over time was evaluated. Further, using the prepared binder composition, a laminated cell type lithium ion secondary battery and a full coin cell type lithium ion secondary battery were produced in the same manner as in Example 16, and the gas generation amount and the high temperature storage characteristics were respectively obtained. As well as high temperature cycle characteristics. The results are shown in Table 2.
- a binder composition was prepared in the same manner as in Example 16 except that ethyl acrylate (EA) having a boiling point of 100 ° C. was used in place of 2-ethylhexyl acrylate which is a (meth) acrylic acid ester monomer, and was stable over time. Sex was evaluated. Further, using the prepared binder composition, a laminated cell type lithium ion secondary battery and a full coin cell type lithium ion secondary battery were produced in the same manner as in Example 16, and the gas generation amount and the high temperature storage characteristics were respectively obtained. As well as high temperature cycle characteristics. The results are shown in Table 2.
- acrylonitrile as an ⁇ , ⁇ -unsaturated nitrile monomer 74.4 parts
- 4.8 parts of sodium lauryl sulfate and 192 parts of ion-exchanged water as dispersants were added and stirred to prepare an emulsion.
- This emulsion was sequentially added from polymerization vessel B to polymerization vessel A over about 180 minutes. Thereafter, the mixture was stirred for about 120 minutes, and when the polymerization conversion rate (monomer consumption) reached 95%, distillation was performed under reduced pressure over 8 hours using an evaporator at 80 ° C. and 100 hPa (0.1 atm). .
- binder composition aqueous dispersion (binder composition) containing a desired particulate polymer was obtained.
- the obtained binder composition was evaluated for stability over time. Further, using the prepared binder composition, a laminated cell type lithium ion secondary battery and a full coin cell type lithium ion secondary battery were produced in the same manner as in Example 16, and the gas generation amount and the high temperature storage characteristics were respectively obtained. As well as high temperature cycle characteristics. The results are shown in Table 2.
- Example 10 A binder composition was prepared in the same manner as in Example 16 except that succinic acid was added instead of sodium ascorbate, which is a reductone compound, and stability over time was evaluated. Further, using the prepared binder composition, a laminated cell type lithium ion secondary battery and a full coin cell type lithium ion secondary battery were produced in the same manner as in Example 16, and the gas generation amount and the high temperature storage characteristics were respectively obtained. As well as high temperature cycle characteristics. The results are shown in Table 2.
- Example 11 A binder composition was prepared in the same manner as in Example 16 except that the step (2) was carried out at a polymerization conversion rate of 80% in the step (1), and the temporal stability was evaluated. Further, using the prepared binder composition, a laminated cell type lithium ion secondary battery and a full coin cell type lithium ion secondary battery were produced in the same manner as in Example 16, and the gas generation amount and the high temperature storage characteristics were respectively obtained. As well as high temperature cycle characteristics. The results are shown in Table 2.
- step (2) was not performed.
- step (2) was not performed.
- the polymerization conversion rate in step (1) reached 95%
- the mixture was cooled to 40 ° C., 0.3% by mass of ammonium persulfate (APS) as a peroxide was added, and the reaction was further continued for 4 hours.
- APS ammonium persulfate
- 5% sodium hydroxide aqueous solution was added with respect to this aqueous dispersion, and it adjusted to pH8, and obtained the aqueous dispersion (binder composition) containing a desired particulate polymer.
- the obtained binder composition was evaluated for stability over time.
- the binder compositions of Examples 16 to 28 have good temporal stability, and the secondary battery produced using such a binder composition has a reduced amount of gas generation, high temperature storage characteristics and It can be seen that the high-temperature cycle characteristics are good.
- Comparative Example 7 in which sodium ascorbate as a reductone compound was not blended, the residual amount of 2-ethylhexyl acrylate monomer in the obtained binder composition was large, and the stability of the binder composition with time was low. And it turns out that the gas generation amount of the secondary battery manufactured using this binder composition increases, and a high temperature storage characteristic and a high temperature cycling characteristic deteriorate remarkably.
- the stability of the binder composition is good, Although the amount of gas generated in the secondary battery produced using the obtained binder is small, it can be seen that the high-temperature storage characteristics and the high-temperature cycle characteristics deteriorate. This is considered to be because the flexibility of the particulate polymer in the binder composition was lowered, and the binding property was lowered as the binder composition, which adversely affected the battery characteristics.
- the compounding amounts of the redox initiator sodium ascorbate and Tert-butyl hydroperoxide
- the unconverted monomer cannot be sufficiently polymerized in the redox polymerization, and the result It is considered that the residual monomer amount increases and the high-temperature storage characteristics and the high-temperature cycle characteristics deteriorate.
- the temporal stability of the binder composition was good. The gas generation of the secondary battery is not a little, but the residual monomer amount in the binder composition cannot be sufficiently reduced, and the battery characteristics deteriorate.
- ADVANTAGE OF THE INVENTION generation
- a binder composition can be provided.
- the slurry composition for secondary battery electrodes using the said binder composition can be provided.
- the electrode for secondary batteries using the said slurry composition for secondary battery electrodes can be provided.
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Abstract
Description
そこで、近年では、二次電池の更なる性能向上を達成すべく、これら電池部材の形成に用いられるバインダー組成物やスラリー組成物の改良が試みられている。
また、本発明は、当該二次電池用電極を用いた、高温保存特性及び高温サイクル特性に優れると共に、ガス発生が低減された二次電池を提供することを目的とする。
また、上述した残留モノマーの問題に対し、重合体を一度調製した後に過硫酸塩などの重合開始剤を更に添加して重合反応を更に進める(即ち、残留モノマーを重合させる)ことで、沸点145℃以上の(メタ)アクリル酸エステル単量体を含む残留モノマーの量を低減することも考えられる。しかし、本発明者らが鋭意研究を重ねたところ、重合開始剤を更に添加して重合反応を進めた場合には、過硫酸塩などの重合開始剤の残渣が大量にバインダー組成物や二次電池中に含まれることとなり、二次電池の電気的特性が低下する虞があることが明らかとなった。
また、本発明によれば、当該二次電池用電極を用いた、高温保存特性及び高温サイクル特性に優れると共に、ガス発生が低減された二次電池を提供することができる。
ここで、本発明の二次電池電極用バインダー組成物は、二次電池の電極を形成する際に用いられる。そして、本発明の二次電池電極用スラリー組成物は、本発明の二次電池電極用バインダー組成物及び電極活物質を含んで調製される。また、本発明の二次電池電極用バインダー組成物の製造方法は、本発明の二次電池電極用バインダー組成物を製造する際に用いることができる。
さらに、本発明の二次電池用電極は、本発明の二次電池電極用スラリー組成物を用いて製造することができ、本発明のリチウムイオン二次電池は、本発明の二次電池用電極を用いたことを特徴とする。
本発明の二次電池電極用バインダー組成物は、水を分散媒とした水系バインダー組成物であり、粒子状重合体及びレダクトン類化合物およびその酸化体の少なくとも一方を含む。ここで、粒子状重合体は沸点145℃以上の(メタ)アクリル酸エステル単量体を含む単量体混合物を重合して生成された粒子状重合体である。さらに、本発明の二次電池電極用バインダー組成物は、沸点145℃以上の(メタ)アクリル酸エステル単量体の含有割合が、粒子状重合体100質量部に対して1×10-6質量部以上1500×10-6質量部以下であることを特徴とする。ここで、本発明のバインダー組成物に含まれる沸点145℃以上の(メタ)アクリル酸エステル単量体は、沸点145℃以上の(メタ)アクリル酸エステル単量体を含む単量体混合物を重合させて粒子状重合体を生成させた後に、重合していない、単量体のままの状態(いわゆる、遊離状態)で残った(メタ)アクリル酸エステル単量体である。
なお、本発明において、「(メタ)アクリル」とは、アクリル及び/又はメタクリルを指す。
粒子状重合体は、本発明の二次電池電極用バインダー組成物を用いて電極を形成した際に、製造した電極において、電極合材層に含まれる成分(例えば、電極活物質)が電極から脱離しないように保持しうる成分である。
バインダー組成物に配合する粒子状重合体としては、以下に説明する沸点145℃以上の(メタ)アクリル酸エステル単量体を含む単量体混合物を重合して生成される粒子状重合体を用いることができる。
なお、本発明のバインダー組成物を負極の形成に用いる場合(二次電池負極用バインダー組成物)、粒子状重合体は、架橋性単量体を更に含む単量体混合物を重合して生成されることが好ましく、スチレンなどの芳香族ビニル単量体や、酸性基を含有する単量体を更に含む単量体混合物を重合して生成されることが更に好ましい。
また、本発明のバインダー組成物を正極の形成に用いる場合(二次電池正極用バインダー組成物)、粒子状重合体は、(メタ)アクリロニトリル単量体などのα,β-不飽和ニトリル単量体を更に含む単量体混合物を重合して生成されることが好ましく、酸性基を含有する単量体を更に含む単量体混合物を重合して生成されることが更に好ましい。なお、本発明において、「(メタ)アクリロニトリル」とは、アクリロニトリル及び/又はメタクリロニトリルを指す。
以下、本発明における粒子状重合体を生成するための単量体混合物に配合されうる各単量体について詳述する。
粒子状重合体を重合する際に使用する単量体混合物は、沸点145℃以上の(メタ)アクリル酸エステル単量体を含有する。沸点145℃以上の(メタ)アクリル酸エステル単量体を含有することで、単量体混合物を重合して得られる粒子状重合体の電気化学的安定性及び柔軟性を高めることができる。そして、バインダー組成物を用いた電極合材層の密着強度を高めて電極のピール強度を向上させることができると共に、二次電池の電池特性を向上させることができる。
ここで、(メタ)アクリル酸エステル単量体としては、例えば、式(I):CH2=CR1-COOR2で表される化合物が挙げられる。式(I)において、R1は水素原子またはメチル基を表し、R2はアルキル基またはシクロアルキル基、或いは、それらの一部を置換した官能基を表す。
そして、沸点145℃以上の(メタ)アクリル酸エステル単量体の例を挙げると、n-ブチルアクリレート(BA)、アクリル酸n-アミル、アクリル酸イソアミル、アクリル酸n-ヘキシル、2-エチルヘキシルアクリレート(2EHA)、アクリル酸-2-メトキシエチル、アクリル酸-2-エトキシエチル、アクリル酸ヘキシル、アクリル酸ノニル、アクリル酸ラウリル、アクリル酸ステアリル、ベンジルアクリレートなどのアクリレート;メタクリル酸n-ブチル、メタクリル酸イソブチル、メタクリル酸n-アミル、メタクリル酸イソアミル、メタクリル酸n-ヘキシル、メタクリル酸2-エチルヘキシル、メタクリル酸オクチル、メタクリル酸イソデシル、メタクリル酸ラウリル、メタクリル酸トリデシル、メタクリル酸ステアリル、ベンジルメタクリレートなどのメタアクリレート等が挙げられる。これらの単量体は、1種類を単独で用いてもよく、2種類以上を組み合わせて用いてもよい。これらの中でも、アクリレートが好ましく、2-エチルヘキシルアクリレート及びn-ブチルアクリレートが、二次電池用電極のピール強度を向上できる点で好ましく、さらに、2-エチルヘキシルアクリレートが特に好ましい。2-エチルヘキシルアクリレートは側鎖が長く、2-エチルヘキシルアクリレート由来の単量体単位を有する重合体のTgを低減させて柔軟性を向上させると共に、電気化学的安定性を向上させるため、かかる重合体を配合した二次電池電極用バインダー組成物を用いて得た電極合材層の集電体に対する密着強度を向上し、ひいては、かかる電極合材層を有する電極を用いた二次電池の電池特性を向上できるからである。
ここで、(メタ)アクリル酸エステル単量体の沸点は、JIS K2254に従って測定することができる。
以下、バインダー組成物を負極の形成に用いる場合に、粒子状重合体の重合に使用する単量体混合物に好適に配合されうる、架橋性単量体、芳香族ビニル単量体、および酸性基を含有する単量体について詳述する。
バインダー組成物を負極の形成に用いる場合、粒子状重合体を重合する際に使用する単量体混合物は、架橋性単量体を更に含むことが好ましい。かかる粒子状重合体を含むバインダー組成物を用いて製造したリチウムイオン二次電池の高温保存特性や高温サイクル特性を向上させることができるからである。
架橋性基としては、通常は熱により架橋反応を生じる熱架橋性基を用いる。架橋性基の例を挙げると、エポキシ基、N-メチロールアミド基、オキサゾリン基、アリル基などが挙げられ、なかでも架橋および架橋密度の調節が容易であるので、架橋性基としては、N-メチロールアミド基、エポキシ基、アリル基が好ましい。架橋密度が高いほど、粒子状重合体の電解液に対する膨潤度が低くなるため、架橋密度を調節することにより、粒子状重合体の膨潤度を制御することが可能である。なお、架橋性基の種類は、1種類であってもよく、2種類以上であってもよい。
架橋性単量体は、前述した(メタ)アクリル酸エステル単量体には含まれないものとする。
エポキシ基を含有する単量体としては、例えば、炭素-炭素二重結合およびエポキシ基を含有する単量体、ハロゲン原子およびエポキシ基を含有する単量体、などが挙げられる。
炭素-炭素二重結合およびエポキシ基を含有する単量体としては、例えば、ビニルグリシジルエーテル、アリルグリシジルエーテル、ブテニルグリシジルエーテル、o-アリルフェニルグリシジルエーテル等の不飽和グリシジルエーテル;ブタジエンモノエポキシド、クロロプレンモノエポキシド、4,5-エポキシ-2-ペンテン、3,4-エポキシ-1-ビニルシクロヘキセン、1,2-エポキシ-5,9-シクロドデカジエン等のジエンまたはポリエンのモノエポキシド;3,4-エポキシ-1-ブテン、1,2-エポキシ-5-ヘキセン、1,2-エポキシ-9-デセン等のアルケニルエポキシド;グリシジルアクリレート、グリシジルメタクリレート、グリシジルクロトネート、グリシジル-4-ヘプテノエート、グリシジルソルベート、グリシジルリノレート、グリシジル-4-メチル-3-ペンテノエート、3-シクロヘキセンカルボン酸のグリシジルエステル、4-メチル-3-シクロヘキセンカルボン酸のグリシジルエステル等の不飽和カルボン酸のグリシジルエステル類;などが挙げられる。
ハロゲン原子およびエポキシ基を有する単量体としては、例えば、エピクロロヒドリン、エピブロモヒドリン、エピヨードヒドリン、エピフルオロヒドリン、β-メチルエピクロルヒドリン等のエピハロヒドリン;p-クロロスチレンオキシド;ジブロモフェニルグリシジルエーテル;などが挙げられる。
N-メチロールアミド基を含有する単量体としては、例えば、N-メチロール(メタ)アクリルアミド等のメチロール基を有する(メタ)アクリルアミド類などが挙げられる。
オキサゾリン基を含有する単量体としては、例えば、2-ビニル-2-オキサゾリン、2-ビニル-4-メチル-2-オキサゾリン、2-ビニル-5-メチル-2-オキサゾリン、2-イソプロペニル-2-オキサゾリン、2-イソプロペニル-4-メチル-2-オキサゾリン、2-イソプロペニル-5-メチル-2-オキサゾリン、2-イソプロペニル-5-エチル-2-オキサゾリン等が挙げられる。
アリル基を含有する単量体としては、例えば、アリルアクリレート、アリルメタクリレートなどが挙げられる。
バインダー組成物を負極の形成に用いる場合、粒子状重合体を重合する際に使用する単量体混合物は、スチレン、クロロスチレン、ビニルトルエン、t-ブチルスチレン、ビニル安息香酸、ビニル安息香酸メチル、ビニルナフタレン、クロロメチルスチレン、ヒドロキシメチルスチレン、α-メチルスチレン、ジビニルベンゼン等のスチレン系単量体を更に含むことが好ましい。中でも、芳香族ビニル単量体として、スチレンを使用することが好ましい。なお、これらは1種類を単独で用いてもよく、2種類以上を組み合わせて用いてもよい。
スチレン系単量体のような芳香族ビニル単量体の使用により、芳香族ビニル単量体由来の単量体単位を有する重合体のTgを高め、粒子状重合体のポリマー強度を強くし、ひいては負極合材層のピール強度を向上させることができるからである。さらに、スチレン系単量体のような芳香族ビニル単量体の使用により、バインダー組成物に含有される粒子状重合体に導入された芳香族環のπ電子と、炭素系負極活物質の芳香族環のπ電子との相互作用により、導電材の分散性を向上させることができるからである。
ここで、スチレン系単量体のような芳香族ビニル単量体の配合量は、単量体混合物中、好ましくは20質量%以上、より好ましくは30質量%以上であり、好ましくは80質量%以下、より好ましくは70質量%以下、特に好ましくは60質量%以下である。芳香族ビニル単量体の配合量を20質量%以上とすることで、上述したような負極合材層のピール強度の向上や、導電材の分散性の向上といった効果を得ることができ、配合量を80質量%以下とすることで、負極合材層の柔軟性を維持することができるからである。
粒子状重合体を重合する際に使用する単量体混合物は、酸性基を含有する単量体(以下、「酸性基含有単量体」と称することがある)を更に含むことが好ましい。酸性基を含有する単量体を含む単量体混合物を重合することで、酸性基を含有する単量体単位(以下、「酸性基含有単量体単位」と称することがある)を粒子状重合体に導入することができる。酸性基としては、例えば、カルボン酸基(-COOH)、スルホン酸基(-SO3H)、リン酸基(-PO3H2)などが挙げられる。ただし、酸性基含有単量体が有する酸性基は、1種類でもよく、2種類以上でもよい。また、酸性基を含有する単量体が有する酸性基の数は、1つでもよく、2つ以上でもよい。
不飽和カルボン酸単量体の例としては、不飽和モノカルボン酸及びその誘導体;不飽和ジカルボン酸及びその酸無水物並びにそれらの誘導体;などが挙げられる。
不飽和モノカルボン酸の例としては、アクリル酸、メタクリル酸、及びクロトン酸等の、エチレン性不飽和モノカルボン酸が挙げられる。
不飽和モノカルボン酸の誘導体の例としては、2-エチルアクリル酸、イソクロトン酸、α-アセトキシアクリル酸、β-trans-アリールオキシアクリル酸、α-クロロ-β-E-メトキシアクリル酸、及びβ-ジアミノアクリル酸等の、エチレン性不飽和モノカルボン酸の誘導体が挙げられる。
不飽和ジカルボン酸の例としては、マレイン酸、フマル酸、及びイタコン酸等の、エチレン性不飽和ジカルボン酸が挙げられる。
不飽和ジカルボン酸の酸無水物の例としては、無水マレイン酸、アクリル酸無水物、メチル無水マレイン酸、及びジメチル無水マレイン酸等の、エチレン性不飽和ジカルボン酸の無水物が挙げられる。
不飽和ジカルボン酸の誘導体の例としては、メチルマレイン酸、ジメチルマレイン酸、フェニルマレイン酸、クロロマレイン酸、ジクロロマレイン酸、フルオロマレイン酸等のマレイン酸メチルアリル;並びにマレイン酸ジフェニル、マレイン酸ノニル、マレイン酸デシル、マレイン酸ドデシル、マレイン酸オクタデシル、マレイン酸フルオロアルキル等のマレイン酸エステルが挙げられる。
なお、本明細書において、「(メタ)アリル」とは、アリルおよび/またはメタアリルを意味する。
さらに、バインダー組成物を負極の形成に用いる場合、本発明における粒子状重合体を重合する際に使用する単量体混合物は、本発明を著しく損なわない限り、上述したもの以外に任意の単量体を含んでいてもよい。これらの任意の単量体は、上述した単量体と共重合可能な単量体である。上述した単量体と共重合可能な単量体の例を挙げると、アクリルアミドなどのアミド系単量体;エチレン、プロピレン等のオレフィン類;ブタジエン、イソプレン等のジエン系単量体;塩化ビニル、塩化ビニリデン等のハロゲン原子含有単量体;酢酸ビニル、プロピオン酸ビニル、酪酸ビニル、安息香酸ビニル等のビニルエステル類;メチルビニルエーテル、エチルビニルエーテル、ブチルビエルエーテル等のビニルエーテル類;メチルビニルケトン、エチルビニルケトン、ブチルビニルケトン、ヘキシルビニルケトン、イソプロペニルビニルケトン等のビニルケトン類;N-ビニルピロリドン、ビニルピリジン、ビニルイミダゾール等の複素環含有ビニル化合物;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)アクリレートなどの沸点145℃未満の(メタ)アクリル酸エステル単量体;後述するα,β-不飽和ニトリル単量体などが挙げられる。これらは、1種類を単独で用いてもよく、2種類以上を組み合わせて用いてもよい。
以下、バインダー組成物を正極の形成に用いる場合に、粒子状重合体の重合に使用する単量体混合物に好適に配合されうる、α,β-不飽和ニトリル単量体および酸性基含有単量体について詳述する。
バインダー組成物を正極の形成に用いる場合、粒子状重合体を重合する際に使用する単量体混合物は、(メタ)アクリロニトリル単量体などのα,β-不飽和ニトリル単量体を更に含むことが好ましい。(メタ)アクリロニトリル単量体などのα,β-不飽和ニトリル単量体の使用により、バインダー組成物の結着力を高めて正極の強度を顕著に向上させることができるからである。
α,β-不飽和ニトリル単量体としては、アクリロニトリル、メタクリロニトリル、α-クロロアクリロニトリル、α-エチルアクリロニトリルなどが挙げられる。これらの中でも、機械的強度および結着性向上の観点からは、アクリロニトリルおよびメタクリロニトリルが好ましく、アクリロニトリルが特に好ましい。なお、これらは1種類を単独で用いてもよく、2種類以上を組み合わせて用いてもよい。
ここで、(メタ)アクリロニトリル単量体などのα,β-不飽和ニトリル単量体の配合量は、単量体混合物中、好ましくは1~50質量%、より好ましくは5~35質量%である。(メタ)アクリロニトリル単量体などのα,β-不飽和ニトリル単量体の含有量が1質量%未満の場合、粒子状重合体のTgが低くなり、かかる粒子状重合体を含有するバインダー組成物を用いて形成した正極合材層のピール強度が低下する虞がある。さらにこの場合、粒子状重合体が電解液に対して過剰に膨潤し易くなり、このことによってもピール強度が低下する虞がある。他方、(メタ)アクリロニトリル単量体などのα,β-不飽和ニトリル単量体の含有量が50質量%超の場合、粒子状重合体のTgが高くなり、かかる粒子状重合体を含有するバインダー組成物を用いて形成した正極合材層の柔軟性が低下する虞がある。さらにこの場合、粒子状重合体が電解液に対してより膨潤しにくくなり、かかる正極合材層を用いて製造した電極の抵抗が上昇する虞がある。すなわち、アクリロニトリル単量体の単量体混合物中の含有量を、好ましくは1~50質量%、より好ましくは5~35質量%とすることで、かかる単量体混合物を重合して得た粒子状重合体を含有するバインダー組成物を用いて製造した正極のピール強度を向上させると共に、粒子状重合体の電解液に対する膨潤度を適切な値として、かかるバインダー組成物を用いて製造した二次電池の内部抵抗の増加を抑制することができる。
バインダー組成物を正極の形成に用いる場合、粒子状重合体を重合する際に使用する単量体混合物は、酸性基含有単量体を更に含むことが好ましい。ここで、酸性基含有単量体は、「負極用バインダー組成物中の粒子状重合体の重合に用いる単量体」の項で上述したものを用いることができ、好適な例および好適な単量体混合物中の配合量も同様である。
さらに、バインダー組成物を正極の形成に用いる場合、本発明における粒子状重合体を重合する際に使用する単量体混合物は、本発明を著しく損なわない限り、上述したもの以外に任意の単量体を含んでいてもよい。これらの任意の単量体は、上述した単量体と共重合可能な単量体である。上述した単量体と共重合可能な単量体の例を挙げると、スチレン、クロロスチレン、ビニルトルエン、t-ブチルスチレン、ビニル安息香酸、ビニル安息香酸メチル、ビニルナフタレン、クロロメチルスチレン、ヒドロキシメチルスチレン、α-メチルスチレン、ジビニルベンゼン等のスチレン系単量体;アクリルアミドなどのアミド系単量体;エチレン、プロピレン等のオレフィン類;ブタジエン、イソプレン等のジエン系単量体;塩化ビニル、塩化ビニリデン等のハロゲン原子含有単量体;酢酸ビニル、プロピオン酸ビニル、酪酸ビニル、安息香酸ビニル等のビニルエステル類;メチルビニルエーテル、エチルビニルエーテル、ブチルビエルエーテル等のビニルエーテル類;メチルビニルケトン、エチルビニルケトン、ブチルビニルケトン、ヘキシルビニルケトン、イソプロペニルビニルケトン等のビニルケトン類;N-ビニルピロリドン、ビニルピリジン、ビニルイミダゾール等の複素環含有ビニル化合物;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)アクリレートなどの沸点145℃未満の(メタ)アクリル酸エステル単量体;上述した架橋性単量体などが挙げられる。これらは、1種類を単独で用いてもよく、2種類以上を組み合わせて用いてもよい。
バインダー組成物を負極の形成に用いる場合、粒子状重合体は、架橋構造を有していることが好ましい。架橋構造は、上述したように、架橋性単量体により粒子状重合体中に導入することにより形成することができる。
そして、バインダー組成物を正極の形成に用いる場合も、粒子状重合体は、架橋構造を有していてもよい。架橋構造を導入する方法としては、例えば、重合体を、上述の架橋性単量体を含む単量体組成物から重合することで架橋性基を含有させる方法、重合体と架橋剤とを組み合わせて用いる方法が挙げられる。この場合、加熱又はエネルギー線を照射することにより、重合体を架橋させることができる。架橋度は、加熱又はエネルギー線の照射の強度により調節しうる。架橋度が高いほど膨潤度が小さくなるので、架橋度を調整することにより、粒子状重合体の膨潤度を制御することが可能である。
また、粒子状重合体のガラス転移温度(Tg)は、好ましくは-50℃以上、より好ましくは-45℃以上、特に好ましくは-40℃以上であり、好ましくは25℃以下、より好ましくは15℃以下、特に好ましくは5℃以下である。粒子状重合体のガラス転移温度が上記範囲にあることにより、優れた強度と柔軟性を有する二次電池用電極を得ることができ、出力特性の高い二次電池を得ることができる。なお、粒子状重合体のガラス転移温度は、様々な単量体を組み合わせることによって調整可能である。
粒子状重合体の体積平均粒子径は、通常は0.001μm以上、好ましくは0.01μm以上、より好ましくは0.05μm以上であり、通常100μm以下、好ましくは10μm以下、より好ましくは1μm以下である。バインダー組成物に含有される粒子状重合体の体積平均粒子径がこの範囲であることにより、かかるバインダー組成物は、少量の使用でも優れた結着力を発現しうる。ここで、体積平均粒子径は、光散乱粒子径測定器を用いて測定したものである。粒子の形状は、球形及び異形のどちらでもかまわない。
また、粒子状重合体は、1種類を単独で用いてもよく、2種類以上を組み合わせて用いてもよい。
バインダー組成物の固形分における、粒子状重合体の割合は、通常50質量%以上であり、好ましくは70質量%以上である。
本発明の二次電池電極用バインダー組成物に含有される「レダクトン類化合物」とは、R3C(OH)=C(OH)C(=O)R4構造を有する化合物及びその塩を指す。ただし、R3及びR4は、それぞれ独立の任意の有機基であってもよいし、共に環構造を形成していても良い。R3C(OH)=C(OH)C(=O)R4構造を有する化合物としては、例えば、グルシン酸とその誘導体、レダクチン酸とその誘導体、アスコルビン酸とその異性体、誘導体等が挙げられる。また、前記レダクトン類化合物は、酸化体(脱プロトン化レダクトン;R3C(=O)C(=O)C(=O)R4構造を有する化合物及びその塩)の形態で含まれていてもよい。これらの中でも、コスト、毒性及び環境負荷が低く、さらに人体安全性が高いため、アスコルビン酸とその異性体、誘導体及びそれらの塩並びにそれらの酸化体から選択される少なくとも1種が好ましい。
なかでも、レダクトン類化合物およびその酸化体の少なくとも一方が、(イソ)アスコルビン酸及びその塩、並びに、それらの酸化体から選択される少なくとも1種であることが好ましい。(イソ)アスコルビン酸の塩としては、アルカリ金属塩が好ましく、ナトリウム塩がより好ましい。必要に応じてこれらのレダクトン類化合物の混合物を用いることができる。
バインダー組成物は、上述した成分に加え、バインダー組成物に配合し得る既知の任意成分を含有していても良い。また、粒子状重合体の重合に使用した重合開始剤などの残渣を含んでいてもよい。
本発明のバインダー組成物の製造方法は、沸点145℃以上の(メタ)アクリル酸エステル単量体を含む単量体混合物を重合転化率が90質量%以上となるまで水中で重合し、重合体と未反応の単量体とを含む混合物を得る工程(1)と、工程(1)の後、混合物にレダクトン類化合物および過酸化物を添加して未反応の単量体を重合し、前記沸点145℃以上の(メタ)アクリル酸エステル単量体の含有割合を、重合体100質量部に対して1×10-6質量部以上1500×10-6質量部以下にする工程(2)を含む。すなわち、本発明のバインダー組成物の製造方法は、単量体混合物の大部分を重合させる工程と、残留した未反応の残留モノマーを更に重合させて残留モノマー量を減少させる工程との少なくとも2段階の重合により粒子状重合体を重合する工程を含む。さらに、本発明のバインダー組成物の製造方法では、工程(1)において、単量体混合物を予めエマルジョン化した後、反応器に添加して重合することが好ましい。
工程(1)では、上述した粒子状重合体の製造に使用し得る、沸点145℃以上の(メタ)アクリル酸エステル単量体を含む単量体混合物を水中で重合する。そして、工程(1)では、重合転化率(=(得られた重合体量/単量体混合物の量)×100%)が90質量%以上、好ましくは95質量%以上になるまで単量体混合物を重合する。ここで、工程(1)における重合は、1段階で行っても良いし、多段階に分けて行っても良い。
なお、重合転化率は、反応温度や反応時間等を調整することにより制御することができる。そして、工程(1)の重合転化率を90質量%以上、好ましくは95質量%以上としたのは、所望の性状を有する粒子状重合体を得るためである。このように、工程(1)における重合転化率を90質量%以上、好ましくは95質量%以上とすることで、工程(2)においてレドックス重合で新たに生成されうる、既に工程(1)で生成されている粒子状重合体とは性状の異なる粒子状重合体の量を低減することができる。
なお、乳化重合法は、常法に従って実施し得る(例えば、「実験化学講座」第28巻(発行元:丸善(株)、日本化学会編)参照)。すなわち、攪拌機及び加熱装置付きの密閉容器に、水と、分散剤、乳化剤、架橋剤などの添加剤と、重合開始剤と、上述した単量体混合物とを所定の組成になるように加え、容器中の組成物を攪拌して単量体等を水に乳化させ、攪拌しながら温度を上昇させて重合を開始する方法を用いうる。あるいは、上記組成物を乳化させた後に密閉容器に入れ、同様に反応を開始させる方法を用いうる。
さらに、工程(1)の反応温度は、通常0℃以上、好ましくは40℃以上、通常150℃以下、好ましくは95℃以下である。また、重合時間は通常1時間以上、20時間以下である。重合温度が低すぎると反応速度が遅すぎ効率が悪く、重合温度が高すぎると水性媒体が蒸発しやすいため重合が困難になる。反応の圧力は常圧でもよい。反応は空気中でも可能であるが、窒素、アルゴン等の不活性ガスの存在下が好ましい。
ここで、上述した工程(1)では、重合転化率を100質量%とすることは困難であり、沸点145℃以上の(メタ)アクリル酸エステル単量体を含む未反応の単量体が残留する。
そこで、工程(2)では、レダクトン類化合物および過酸化物を併用するレドックス系開始剤を用いて工程(1)の後に残った未反応の単量体(残留モノマー)を重合させ、バインダー組成物中の残留モノマー量を低減させる。具体的には、工程(2)では、工程(1)を経て得られた、重合体と未反応の単量体との混合物を含む重合系に対し、レダクトン類化合物および過酸化物を添加して、沸点145℃以上の(メタ)アクリル酸エステル単量体の含有割合が重合体100質量部に対して1×10-6質量部以上1500×10-6質量部以下になるまでレドックス重合を行う。
なお、沸点145℃以上の(メタ)アクリル酸エステル単量体の含有割合は、工程(2)で使用するレダクトン類化合物および過酸化物の量や、レドックス重合条件を変更することにより調整することができる。
なお、工程(2)では、上述の反応温度で、好ましくは1時間以上、6時間以下レドックス重合を実施した後に、重合系の温度をレドックス重合時の重合温度より高い温度(例えば、80℃超)として、過酸化物を除去又は分解させることが好ましい。これにより、得られたバインダー組成物中において、過酸化物の含有量が低減され、かかるバインダー組成物を用いて二次電池を製造した場合に、二次電池の電池性能や安全性を向上させることができる。
また、工程(2)における反応の圧力は常圧でもよい。反応は空気中でも可能であるが、窒素、アルゴン等の不活性ガスの存在下が好ましい。レダクトン類化合物の使用量は過酸化物の種類に応じて適宜変更するが、好ましくは、工程(1)で使用した単量体混合物の量に対して、通常0.05質量%以上、好ましくは0.1質量%以上、通常5質量%以下、好ましくは1質量%以下である。なお、残留モノマーの含有率によってレダクトン類化合物の使用量を調整してもよい。レダクトン類化合物の使用量が多すぎると、バインダー組成物中で大量に残留するため、当該バインダー組成物を用いた二次電池のガス発生量が増加して、初期容量に影響する。レダクトン類化合物の使用量が少なすぎると、レドックス重合が十分に進行せず、残留モノマーの低減効果が不十分となる虞がある。レダクトン類化合物は一括添加しても分割添加してもよいが、残留モノマー低減効率を向上するという観点では、分割添加が好ましい。
なお、工程(2)でレドックス重合を実施することにより、他の重合方法を採用して残留モノマーの低減を図った場合よりも重合開始剤の配合量を低減し、バインダー組成物中に重合開始剤の残渣が大量に残留するのを防止することができる。
なお、上述した工程(1)及び工程(2)の間に、例えば、蒸留工程などの追加工程を実施することも可能である。
本発明の二次電池電極用スラリー組成物は、上述のバインダー組成物及び電極活物質(正極活物質又は負極活物質)を含むことを特徴とする。このような二次電池電極用スラリー組成物を用いれば、得られる二次電池において残留モノマーに起因したガスの発生が抑制され、良好な高温保存特性及び高温サイクル特性を有する二次電池を提供することができる。
なお、本発明の二次電池電極用スラリー組成物は、バインダー組成物及び電極活物質に加え、導電材、粘度調整剤、界面活性剤、分散剤などを含んでいても良い。
本発明のスラリー組成物が含有するバインダー組成物の割合は、得られる電池の性能が良好に発現されるよう適宜調整することができる。例えば、電極活物質100質量部に対するバインダー組成物固形分の割合として、通常0.1質量部以上、好ましくは0.5質量部以上、より好ましくは0.8質量部以上であり、通常50質量部以下、好ましくは20質量部以下、より好ましくは10質量部以下、特に好ましくは3質量部以下とすることができる。バインダー組成物の量をこの範囲にすることにより、密着性を充分に確保でき、二次電池の容量を高くでき、且つ、二次電池用電極の内部抵抗を低くすることができる。
スラリー組成物を負極の形成に用いる場合、電極活物質として負極活物質を用いる。負極活物質は、負極において用いられる電極活物質であり、二次電池の負極において電子の受け渡しをする物質である。例えば、本発明の二次電池がリチウムイオン二次電池である場合、負極活物質としては、通常、リチウムを吸蔵および放出し得る物質が用いられる。リチウムを吸蔵および放出し得る物質としては、例えば、炭素系負極活物質、金属系負極活物質、およびこれらを組み合わせた負極活物質などが挙げられる。
炭素質材料は、炭素前駆体を2000℃以下で熱処理して炭素化させることによって得られる、黒鉛化度の低い(即ち、結晶性の低い)材料である。なお、炭素化させる際の熱処理温度の下限は特に限定されないが、例えば500℃以上とすることができる。
そして、炭素質材料としては、例えば、熱処理温度によって炭素の構造を容易に変える易黒鉛性炭素や、ガラス状炭素に代表される非晶質構造に近い構造を持つ難黒鉛性炭素などが挙げられる。
ここで、易黒鉛性炭素としては、例えば、石油または石炭から得られるタールピッチを原料とした炭素材料が挙げられる。具体例を挙げると、コークス、メソカーボンマイクロビーズ(MCMB)、メソフェーズピッチ系炭素繊維、熱分解気相成長炭素繊維などが挙げられる。
また、難黒鉛性炭素としては、例えば、フェノール樹脂焼成体、ポリアクリロニトリル系炭素繊維、擬等方性炭素、フルフリルアルコール樹脂焼成体(PFA)、ハードカーボンなどが挙げられる。
そして、黒鉛質材料としては、例えば、天然黒鉛、人造黒鉛などが挙げられる。
ここで、人造黒鉛としては、例えば、易黒鉛性炭素を含んだ炭素を主に2800℃以上で熱処理した人造黒鉛、MCMBを2000℃以上で熱処理した黒鉛化MCMB、メソフェーズピッチ系炭素繊維を2000℃以上で熱処理した黒鉛化メソフェーズピッチ系炭素繊維などが挙げられる。
スラリー組成物を正極の形成に用いる場合、電極活物質として正極活物質を用いる。正極活物質は、正極において用いられる電極活物質であり、二次電池の正極において電子の受け渡しをする物質である。例えば、本発明の二次電池がリチウムイオン二次電池である場合、正極活物質としては、通常、リチウムイオンの挿入及び脱離が可能な物質が用いられる。このような正極活物質は、無機化合物からなるものと有機化合物からなるものとに大別される。
なお、本明細書において、「平均酸化状態」とは、前記「1種類以上の遷移金属」の平均の酸化状態を示し、遷移金属のモル量と原子価とから算出される。例えば、「1種類以上の遷移金属」が、50mol%のNi2+と50mol%のMn4+から構成される場合には、「1種類以上の遷移金属」の平均酸化状態は、(0.5)×(2+)+(0.5)×(4+)=3+となる。
ここで、正極活物質の表面状態は、正極活物質と溶媒との接触角を測ることにより求めることができる。例えば、正極活物質のみを加圧成型してペレットを作製し、極性溶媒(例えば、N-メチルピロリドン)に対する前記ペレットの接触角を求めることで、確認できる。接触角が低いほど、その正極活物質は親水性であることを示す。
スラリー組成物を正極の形成に用いる場合、当該スラリー組成物は導電材を含むことが好ましい。導電材としては、例えば、導電性を有する、炭素の同素体からなる粒子が挙げられる。導電材を用いることにより、正極活物質同士の電気的接触を向上させることができ、特にリチウムイオン二次電池に用いる場合に放電負荷特性を改善することができる。
導電材の具体例を挙げると、アセチレンブラック、ケッチェンブラック、カーボンブラック、グラファイト、気相成長カーボン繊維、カーボンナノチューブ等の導電性カーボンが挙げられる。また、例えば、黒鉛等の炭素粉末、各種金属のファイバー及び箔なども挙げられる。ここで、導電材は、1種類を単独で用いてもよく、2種類以上を任意の比率で組み合わせて用いてもよい。これらの中でも、正極活物質同士の電気的接触を向上させ、スラリー組成物を用いて形成した正極を使用した二次電池の電気的特性を向上させる観点からは、導電材として、アセチレンブラックを用いることが好ましい。
導電材の量は、正極活物質100質量部に対して、通常0.01質量部以上、好ましくは1質量部以上であり、通常20質量部以下、好ましくは10質量部以下である。導電材の量がこの範囲にあることにより、二次電池の容量を高くでき、また、高い負荷特性を示すことができる。導電材の配合量が少なすぎると、正極活物質同士の電気的接触により形成される導電パスが不十分となり、二次電池の出力が低下する虞がある。一方、導電材の配合量が多すぎると、スラリー組成物の安定性が低下すると共に正極中の正極合材層の密度が低下し、リチウムイオン二次電池を十分に高容量化することができない。
粘度調整剤は、スラリー組成物の粘度を調整し、集電体上へのスラリー組成物の塗布を容易にするためのものである。そして、粘度調整剤としては、例えば水溶性高分子を用いることができる。具体的には、粘度調整剤としては、カルボキシメチルセルロース、メチルセルロース、ヒドロキシプロピルセルロースなどのセルロース系ポリマー、並びに、これらのセルロース系ポリマーのアンモニウム塩およびアルカリ金属塩;変性または未変性のポリ(メタ)アクリル酸、並びに、これらのポリ(メタ)アクリル酸のアンモニウム塩およびアルカリ金属塩;変性または未変性のポリビニルアルコール、アクリル酸またはアクリル酸塩とビニルアルコールとの共重合体、無水マレイン酸、マレイン酸またはフマル酸とビニルアルコールとの共重合体などのポリビニルアルコール類;ポリエチレングリコール、ポリビニルピロリドン、変性ポリアクリル酸、酸化スターチ、リン酸スターチ、カゼイン、各種変性デンプン、アクリロニトリル-ブタジエン共重合体水素化物などを用いることができる。さらに、粘度調整剤として、水溶性で、末端にチオール基を有する任意の高分子(末端にチオール基を有する水溶性高分子)を用いることができる。
本発明のスラリー組成物は、更に界面活性剤を含有してもよい。界面活性剤は、スラリー組成物を集電体に塗工する時に発生するはじきを防止し、電極の平滑性を向上させるために有効である。界面活性剤としては、例えば、アルキル系界面活性剤、シリコン系界面活性剤、フッ素系界面活性剤、金属系界面活性剤などが挙げられる。界面活性剤の配合量は、電極活物質100質量部に対して、好ましくは0.01質量部~10質量部である。界面活性剤が上記範囲であることにより電極作製時の生産性、平滑性が良好になり、二次電池の電池特性が優れたものになる。
本発明のスラリー組成物は、更に分散剤を含有してもよい。分散剤としては、例えば、アニオン性化合物、カチオン性化合物、非イオン性化合物、高分子化合物などが例示される。具体的な分散剤は、用いる電極活物質及び導電材に応じて選択される。分散剤を用いることにより、スラリー組成物の安定性が向上し、平滑な電極が得られるので、二次電池の電池容量を高めることができる。
分散剤の配合量は、電極活物質100質量部に対して、通常は0.1質量部以上、好ましくは0.5質量部以上、より好ましくは0.8質量部以上であり、通常10質量部以下、好ましくは5質量部以下、より好ましくは2質量部以下である。
本発明のスラリー組成物は、上記成分の他に、例えば、補強材、酸化防止剤、電解液の分解を抑制する機能を有する電解液添加剤などの成分を含有していてもよい。これらの他の成分は、公知のものを使用することができ、例えば国際公開第2012/115096号、国際公開第2012/036260号、特開2012-204303号公報に記載のものを使用することができる。
本発明のスラリー組成物は、上記各成分を分散媒としての水系媒体中に分散させることにより調製することができる。具体的には、ボールミル、サンドミル、ビーズミル、顔料分散機、らい潰機、超音波分散機、ホモジナイザー、プラネタリーミキサー、フィルミックスなどの混合機を用いて上記各成分と水系媒体とを混合することにより、スラリー組成物を調製することができる。
なお、水系媒体としては、通常は水を用いるが、任意の化合物の水溶液や、少量の有機媒体と水との混合溶液などを用いてもよい。また、スラリー組成物の固形分濃度は、各成分を均一に分散させることができる濃度、例えば、10~80質量%とすることができる。更に、上記各成分と水系媒体との混合は、通常、室温~80℃の範囲で、10分~数時間行うことができる。
スラリー組成物のpHを調整する方法としては、例えば、スラリー組成物の調製前に正極活物質を洗浄してスラリー組成物のpHを調整する方法、作製したスラリー組成物に炭酸ガスをバブリングしてpHを調整する方法、pH調整剤を使って調整する方法などが挙げられる。なかでも、pH調整剤を用いることが好ましい。pH調整剤の種類は特に限定されないが、酸性を示す水溶性物質であることが好ましい。強酸及び弱酸のいずれを使用してもよい。弱酸性を示す水溶性物質の例としては、カルボン酸基、リン酸基、スルホン酸基など酸基を有する有機化合物が挙げられる。これらの中でも、特にカルボン酸基を有する有機化合物が好ましく用いられる。カルボン酸基を有する化合物の具体例としては、コハク酸、フタル酸、マレイン酸、無水コハク酸、無水フタル酸、無水マレイン酸などが挙げられる。こられの化合物は、乾燥することにより二次電池内において影響が少ない酸無水物にすることができる。また、強酸性を示す水溶性物質の例としては、塩酸、硝酸、硫酸、酢酸などが挙げられる。
本発明の二次電池用電極は、集電体上に、上述のようにして得られた二次電池電極用スラリー組成物を塗布し、集電体上に塗布された二次電池電極用スラリー組成物を乾燥して得られることを特徴とする。
そして、本発明の二次電池用電極は、集電体と、集電体上に形成された電極合材層とを備え、電極合材層には、少なくとも、電極活物質と、粒子状重合体とが含まれている。なお、電極合材層中に含まれている電極活物質などの各成分は、本発明のスラリー組成物中に含まれていたものであり、それら各成分の好適な存在比は、本発明のスラリー組成物中の各成分の好適な存在比と同じである。
上記二次電池電極用スラリー組成物を集電体上に塗布する方法としては、特に限定されず公知の方法を用いることができる。具体的には、塗布方法としては、ドクターブレード法、ディップ法、リバースロール法、ダイレクトロール法、グラビア法、エクストルージョン法、ハケ塗り法などを用いることができる。この際、スラリー組成物を集電体の片面だけに塗布してもよいし、両面に塗布してもよい。塗布後乾燥前の集電体上のスラリー膜の厚みは、乾燥して得られる電極合材層の厚みに応じて適宜に設定しうる。
また、集電体の形状は特に制限されないが、厚さ0.001mm~0.5mm程度のシート状のものが好ましい。さらに、集電体は、電極合材層の接着強度を高めるため、表面に予め粗面化処理して使用することが好ましい。粗面化方法としては、例えば、機械的研磨法、電解研磨法、化学研磨法などが挙げられる。機械的研磨法においては、通常、研磨剤粒子を固着した研磨布紙、砥石、エメリーバフ、鋼線などを備えたワイヤーブラシ等が使用される。また、電極合材層の接着強度や導電性を高めるために、集電体の表面に中間層を形成してもよい。
集電体上のスラリー組成物を乾燥する方法としては、特に限定されず公知の方法を用いることができ、例えば温風、熱風、低湿風による乾燥、真空乾燥、赤外線や電子線などの照射による乾燥法が挙げられる。このように集電体上のスラリー組成物を乾燥することで、集電体上に電極合材層を形成し、集電体と電極合材層とを備えるリチウムイオン二次電池用電極を得ることができる。
また、電極合材層における水分量は、1000ppm以下であることが好ましく、500ppm以下であることがより好ましい。電極合材層の水分量を上記範囲内とすることにより、耐久性に優れる二次電池用電極を実現できる。水分量は、カールフィッシャー法等の既知の方法により測定しうる。
本発明の二次電池は、正極と、負極と、電解液と、セパレータとを備え、正極および負極の少なくとも一方として、本発明の二次電池用電極を用いたものである。本発明の二次電池は、高温保存特性及び高温サイクル特性に優れると共に、ガス発生が抑制されている。
本発明の二次電池は、上述のように、正極および負極の少なくとも一方が、本発明の二次電池用電極であればよい。すなわち、正極および負極の何れか一方として、既知の電極を用いてもよい。
リチウムイオン二次電池用の電解液としては、例えば、非水溶媒に支持電解質を溶解した非水電解液が用いられる。支持電解質としては、通常、リチウム塩が用いられる。リチウム塩としては、例えば、LiPF6、LiAsF6、LiBF4、LiSbF6、LiAlCl4、LiClO4、CF3SO3Li、C4F9SO3Li、CF3COOLi、(CF3CO)2NLi、(CF3SO2)2NLi、(C2F5SO2)NLiなどが挙げられる。なかでも、溶媒に溶けやすく高い解離度を示すLiPF6、LiClO4、CF3SO3Liが好ましい。これらは1種類を単独で用いてもよく、2種類以上を任意の比率で組み合わせて用いてもよい。解離度の高い支持電解質を用いるほど、リチウムイオン伝導度が高くなるので、支持電解質の種類によりリチウムイオン伝導度を調節することができる。
セパレータとしては、例えば、ポリエチレン、ポリプロピレンなどのポリオレフィン樹脂や、芳香族ポリアミド樹脂を含んでなる微孔膜または不織布;無機セラミック粉末を含む多孔質の樹脂コート;などを用いてもよい。具体例を挙げると、ポリオレフィン系(ポリエチレン、ポリプロピレン、ポリブテン、ポリ塩化ビニル)、及びこれらの混合物あるいは共重合体等の樹脂からなる微多孔膜;ポリエチレンテレフタレート、ポリシクロオレフィン、ポリエーテルスルフォン、ポリアミド、ポリイミド、ポリイミドアミド、ポリアラミド、ポリシクロオレフィン、ナイロン、ポリテトラフルオロエチレン等の樹脂からなる微多孔膜;ポリオレフィン系の繊維を織ったもの又はその不織布;絶縁性物質粒子の集合体等が挙げられる。これらの中でも、セパレータ全体の膜厚を薄くし二次電池内の活物質比率を上げて体積あたりの容量を上げることができるため、ポリオレフィン系の樹脂からなる微多孔膜が好ましい。
二次電池の具体的な製造方法としては、例えば、正極と負極とをセパレータを介して重ね合わせ、これを電池形状に応じて巻く、折るなどして電池容器に入れ、電池容器に電解液を注入して封口する方法が挙げられる。さらに、必要に応じてエキスパンドメタル;ヒューズ、PTC素子などの過電流防止素子;リード板などを入れ、電池内部の圧力上昇、過充放電を防止してもよい。二次電池の形状は、コイン型、ボタン型、シート型、円筒型、角形、扁平型など、何れであってもよい。
実施例および比較例において、バインダー組成物の経時安定性、並びに、二次電池におけるガス発生量、高温保存特性、高温サイクル特性、及びピール強度は、それぞれ以下の方法を使用して評価した。さらに、残留モノマー量、体積平均粒子径は以下の方法により測定した。
調製したバインダー組成物を40℃で3ヶ月保存した後、200メッシュのスクリーンを通過させた。そして、スクリーンを通過せずにスクリーン上に残る残渣の固形分の質量を測定し、かかる質量がバインダー組成物の全固形分量に対して占める比率を算出し、以下の基準にて評価した。値が小さいほど、バインダー組成物が経時安定性に富むことを意味する。
A:残渣の比率が100質量ppm未満
B:残渣の比率が100質量ppm以上500質量ppm未満
C:残渣の比率が500質量ppm以上
作製したラミネートセル型のリチウムイオン二次電池について、25℃環境下で、0.1Cの定電流法によって4.3Vまで充電した後、80℃で50時間保存した。ラミネートセル型リチウムイオンニ次電池のセルをガラス板にて挟み、マイクロゲージにてセルの厚さを計測した。80℃保存前のセルの厚さをa、80℃で50時間保存後のセルの厚さをbとし、80℃保存前後の厚さの比(b/a)を求め、以下の基準により判定した。算出した比(b/a)が小さいほど、ガス発生量が少なく、ガス発生抑制効果に優れることを示す。
A:比(b/a)が1.00倍以上1.05倍以下
B:比(b/a)が1.05倍超1.10倍以下
C:比(b/a)が1.10倍超1.15倍以下
D:比(b/a)が1.15倍超1.20倍以下
E:比(b/a)が1.20倍超
作製したラミネートセル型のリチウムイオン二次電池について、25℃環境下で、0.1Cの定電流法によって4.3Vまで充電した後、80℃で100時間保存した。80℃保存開始前の開路電圧(Open circuit voltage,以下、「OCV」と表記する。)と80℃で100時間保存後のセルのOCVを測定し、80℃保存開始前のOCVに対する80℃で100時間保存後のOCVの比をOCV維持率とし、以下の基準により判定する。OCV維持率が大きいほど、高温保存特性に優れる、すなわち寿命特性に優れることを示す。
A:OCV維持率が99.0%以上
B:OCV維持率が98.8%以上99.0%未満
C:OCV維持率が98.6%以上98.8%未満
D:OCV維持率が98.4%以上98.6%未満
E:OCV維持率が98.2%以上98.4%未満
F:OCV維持率が98.0%以上98.2%未満
G:OCV維持率が98.0%未満
作製したラミネートセル型のリチウムイオン二次電池またはフルコインセル型のリチウムイオン二次電池について、それぞれ60℃環境下で、0.2Cの定電流で4.3Vまで充電し、0.2Cの定電流で3.0Vまで放電する充放電サイクルを行った。充放電サイクルは100サイクルまで行い、初期放電容量(1サイクル目の放電容量)に対する100サイクル目の放電容量の比を容量維持率とし、下記の基準で判定した。容量維持率が大きいほど繰り返し充放電による容量減が少なく、高温サイクル特性に優れていることを示す。
A:容量維持率が90%以上
B:容量維持率が85%以上90%未満
C:容量維持率が80%以上85%未満
D:容量維持率が75%以上80%未満
E:容量維持率が70%以上75%未満
F:容量維持率が70%未満
作製した二次電池用負極を長さ100mm、幅10mmの長方形状に切り出して試験片とし、負極合材層を有する面を下にし、負極合材層表面にセロハンテープ(JIS Z1522に規定されるもの)を貼り付け、集電体の一端を垂直方向に引張り速度50mm/分で引っ張って剥がしたときの応力を測定した(なお、セロハンテープは試験台に固定されている)。測定を3回行い、その平均値を求めてこれを剥離ピール強度とし、以下の基準により評価した。剥離ピール強度の値が大きいほど、負極合材層と集電体との密着性に優れることを示す。
A:剥離ピール強度が30N/m以上
B:剥離ピール強度が25N/m以上30N/m未満
C:剥離ピール強度が20N/m以上25N/m未満
D:剥離ピール強度が20N/m未満
バインダー組成物中の(メタ)アクリル酸エステル単量体(残留モノマー)量は、ガスクロマトグラフィーにて測定した。具体的には、まず、バインダー組成物中の粒子状重合体の固形分濃度が1質量%になるよう、アセトンで希釈し、5Cろ紙を用いて濾過して測定用サンプルを準備した。そして、準備したサンプルについて、以下の条件でガスクロマトグラフィーを実施した。
装置:Agilent 6850A(アジレント・テクノロジー株式会社製)
カラム:HP-1
平均線速度:15cm/s
注入量:1ml
注入口温度:250℃
スプリット比: 20:1
検出器:水素炎イオン化型検出器(FID:Flame Ionization Detector)
検出器温度:280℃
オーブン:40℃で3分保持した後、10℃/分で加熱し、280℃で5分間保持
粒子状重合体の体積平均粒子径は、光散乱粒子径測定器(コールター社製、コールターLS230)を用いて測定した。
まず、本発明のバインダー組成物を負極の形成に使用した。
<バインダー組成物の製造>
[重合工程1]
攪拌機付き重合缶Aに、沸点145℃以上の(メタ)アクリル酸エステル単量体として2-エチルヘキシルアクリレート(2-EHA)20部、芳香族ビニル単量体としてスチレン37.6部、分散剤としてラウリル硫酸ナトリウム(LAS)1.2部、イオン交換水320部を加えた。この重合缶Aに、さらに重合開始剤として過硫酸アンモニウム(APS)1.0部及びイオン交換水40部を加え、70℃に加温し、90分攪拌した。
また、別の重合缶Bに、沸点145℃以上の(メタ)アクリル酸エステル単量体として2-エチルヘキシルアクリレート(沸点215℃)180部、芳香族ビニル単量体としてスチレン150.4部、酸性基含有単量体としてイタコン酸(IA)8.0部、分散剤としてラウリル硫酸ナトリウム4.8部及びイオン交換水192部を加えて攪拌して、エマルジョンを作製した。このエマルジョンを、約180分かけて重合缶Bから重合缶Aに逐次添加した。その後、約120分攪拌して、重合転化率(モノマー消費量)が93%になったところで、40℃に冷却した。
工程1で得られた粒子状重合体を含有する水分散液に対して、レダクトン類化合物としてアスコルビン酸ナトリウム1.2部と、過酸化物としてtert-ブチルヒドロペルオキシド (t-BuOOH)1.2部とを添加して、更に40℃で4時間反応させた。その後、かかる水分散液に対して5%水酸化ナトリウム水溶液を添加して、pH8に調整し、所望の粒子状重合体を含む水分散液(バインダー組成物)を得た。得られたバインダー組成物について、経時安定性を評価した。結果を表1に示す。
負極活物質として体積平均粒子径20μm、比表面積4.2m2/gのグラファイト98部と、結着剤として上述のバインダー組成物を固形分相当量で1.2部と、CMCを固形分相当量で0.8部とを混合し、更に水を加えてプラネタリーミキサーで混合して負極用スラリー組成物を調製した。
この負極用スラリー組成物を厚さ10μmの銅箔の片面に塗布し、110℃で3時間乾燥した後、ロールプレスして厚さ60μmの負極合材層を有する負極を得た。
得られた負極について、ピール強度を測定した。結果を表1に示す。
正極活物質としてCo-Ni-Mnのリチウム複合酸化物系の活物質(製品名:セルシードNMC613、日本化学工業社製。以下、「NMC」と記載することがある。)100部、導電材としてアセチレンブラック2部、及び粘度調整剤としてカルボキシメチルセルロースのナトリウム塩(CMC)水溶液を固形分相当量で0.8部となる量を加え、60分混合した。更に水を加えて希釈した後に、BM-610Bバインダー(固形分濃度40%)を粒子状重合体として1.0部となるように添加し、10分混合した。これを脱泡処理して艶のある流動性の良い正極用スラリー組成物を得た。
得られた正極用スラリー組成物を厚さ18μmのアルミニウム箔に塗布し、120℃で3時間乾燥した後、ロールプレスして厚さ50μmの正極合材層を有する正極を得た。
アルミニウムシートと、その両面を被覆するポリプロピレン樹脂とからなるラミネートフィルムを用いて電池容器を作成した。次いで、上記で得た正極および負極それぞれの端部から合材層を除去し、銅箔又はアルミニウム箔が露出した箇所を形成した。正極のアルミニウム箔が露出した箇所にはNiタブを、負極の銅箔が露出した箇所にはCuタブを溶接した。得られたタブ付きの正極及びタブ付きの負極を、ポリエチレン製の微多孔膜からなるセパレータを挟んで重ねた。電極の面の向きは、正極の合材層側の面と負極の合材層側の面とが対向する向きとした。重ねた電極及びセパレータを、捲回して上記の電池容器に収納した。続いてここに、電解液(EC/DEC=1/2、1M LiPF6)を注入した。電解液としては、エチレンカーボネートとジエチルカーボネートを25℃の下、体積比1:2で混合した混合溶媒に、LiPF6を1モル/Lの濃度になるように溶解させて調製したものを用いた。
次いで、ラミネートフィルムを封止させて本発明のリチウムイオン二次電池であるラミネートセル型リチウムイオン二次電池を作製した。得られたラミネートセル型リチウムイオン二次電池のガス発生量、高温保存特性及び高温サイクル特性を評価した。結果を表1に示す。
沸点145℃以上の(メタ)アクリル酸エステル単量体である2-エチルヘキシルアクリレートの配合量、芳香族ビニル単量体であるスチレンの配合量、及び工程(1)における重合転化率をそれぞれ表1に示す通りに変更した以外は実施例1と同様にしてバインダー組成物を調製し、経時安定性を評価した。なお、各成分の配合量を変更した場合であっても、重合缶A及びBに含まれる各成分の配合量間の比率は実施例1と同様にした。また、調製したバインダー組成物を用いて、実施例1と同様にして負極及びかかる負極を有するラミネートセル型リチウムイオン二次電池を製造した。負極についてはピール強度を評価し、ラミネートセル型リチウムイオン二次電池については、ガス発生量、高温保存特性、及び高温サイクル特性を評価した。結果を表1に示す。
芳香族ビニル単量体であるスチレンの配合量及び工程(1)における重合転化率を表1に示す通りに変更し、架橋性単量体であるアリルメタクリレートを配合しなかった以外は実施例1と同様にしてバインダー組成物を調製し、経時安定性を評価した。なお、各成分の配合量を変更した場合であっても、重合缶A及びBに含まれる各成分の配合量間の比率は実施例1と同様にした。また、調製したバインダー組成物を用いて、実施例1と同様にして負極及びかかる負極を有するラミネートセル型リチウムイオン二次電池を製造した。負極についてはピール強度を評価し、ラミネートセル型リチウムイオン二次電池については、ガス発生量、及び高温保存特性、並びに高温サイクル特性を評価した。結果を表1に示す。
沸点145℃以上の(メタ)アクリル酸エステル単量体である2-エチルヘキシルアクリレートの配合量、芳香族ビニル単量体であるスチレンの配合量、及び工程(1)における重合転化率をそれぞれ表1に示す通りに変更し、さらに、その他の単量体としてブタジエンを20部配合した以外は実施例1と同様にしてバインダー組成物を調製し、経時安定性を評価した。なお、各成分の配合量を変更した場合であっても、重合缶A及びBに含まれる各成分の配合量間の比率は実施例1と同様にした。また、調製したバインダー組成物を用いて、実施例1と同様にして負極及びかかる負極を有するラミネートセル型リチウムイオン二次電池を製造した。負極についてはピール強度を評価し、ラミネートセル型リチウムイオン二次電池については、ガス発生量、高温保存特性、及び高温サイクル特性を評価した。結果を表1に示す。
沸点145℃以上の(メタ)アクリル酸エステル単量体である2-エチルヘキシルアクリレートの配合量、芳香族ビニル単量体であるスチレンの配合量、及び重合工程1における重合転化率をそれぞれ表1に示す通りに変更した以外は実施例1と同様にしてバインダー組成物を調製し、経時安定性を評価した。なお、各成分の配合量を変更した場合であっても、重合缶A及びBに含まれる各成分の配合量間の比率は実施例1と同様にした。また、調製したバインダー組成物を用いて、実施例1と同様にして負極及びかかる負極を有するラミネートセル型リチウムイオン二次電池を製造した。負極についてはピール強度を評価し、ラミネートセル型リチウムイオン二次電池については、ガス発生量、高温保存特性、及び高温サイクル特性を評価した。結果を表1に示す。
沸点145℃以上の(メタ)アクリル酸エステル単量体として、2-エチルヘキシルアクリレートに代えて、ブチルアクリレートを使用した以外は、実施例1と同様にしてバインダー組成物を調製し、経時安定性を評価した。なお、各成分の配合量を変更した場合であっても、重合缶A及びBに含まれる各成分の配合量間の比率は実施例1と同様にした。また、調製したバインダー組成物を用いて、実施例1と同様にして負極及びかかる負極を有するラミネートセル型リチウムイオン二次電池を製造した。負極についてはピール強度を評価し、ラミネートセル型リチウムイオン二次電池については、ガス発生量、高温保存特性、及び高温サイクル特性を評価した。結果を表1に示す。
レダクトン類化合物であるアスコルビン酸ナトリウムを、表1に示すレダクトン類化合物にそれぞれ変更した以外は実施例1と同様にしてバインダー組成物を調製し、経時安定性を評価した。また、調製したバインダー組成物を用いて、実施例1と同様にして負極及びかかる負極を有するラミネートセル型リチウムイオン二次電池を製造した。負極についてはピール強度を評価し、リチウムイオン二次電池については、ガス発生量、高温保存特性、及び高温サイクル特性を評価した。結果を表1に示す。
レダクトン類化合物であるアスコルビン酸ナトリウムの配合割合をそれぞれ表1に示す通りに変更し、また、過酸化物であるtert-ブチルヒドロペルオキシドの配合割合を表1に示す通りに変更した以外は実施例1と同様にしてバインダー組成物を調製し、経時安定性を評価した。また、調製したバインダー組成物を用いて、実施例1と同様にして負極及びかかる負極を有するラミネートセル電池型リチウムイオン二次電池を製造した。負極についてはピール強度を評価し、ラミネートセル型リチウムイオン二次電池については、ガス発生量、高温保存特性、及び高温サイクル特性を評価した。結果を表1に示す。
過酸化物であるtert-ブチルヒドロペルオキシドを過酸化ベンゾイル(BPO)に変更した以外は実施例1と同様にしてバインダー組成物を調製し、経時安定性を評価した。また、調製したバインダー組成物を用いて、実施例1と同様にして負極及びかかる負極を有するラミネートセル型リチウムイオン二次電池を製造した。負極についてはピール強度を評価し、ラミネートセル型リチウムイオン二次電池については、ガス発生量、高温保存特性、及び高温サイクル特性を評価した。結果を表1に示す。
レダクトン類化合物であるアスコルビン酸ナトリウムを配合せず、工程(2)において、40℃で2時間反応させた後、80℃で2時間反応させた以外は、実施例1と同様にしてバインダー組成物を調製し、経時安定性を評価した。また、調製したバインダー組成物を用いて、実施例1と同様にして負極及びかかる負極を有するラミネートセル型リチウムイオン二次電池を製造した。負極についてはピール強度を評価し、ラミネートセル型リチウムイオン二次電池については、ガス発生量、高温保存特性、及び高温サイクル特性を評価した。結果を表1に示す。
沸点145℃以上の(メタ)アクリル酸エステル単量体である2-エチルヘキシルアクリレートに代えて、沸点が100℃のエチルアクリレート(EA)を使用した以外は実施例1と同様にしてバインダー組成物を調製し、経時安定性を評価した。また、調製したバインダー組成物を用いて、実施例1と同様にして負極及びかかる負極を有するラミネートセル型リチウムイオン二次電池を製造した。負極についてはピール強度を評価し、ラミネートセル型リチウムイオン二次電池については、ガス発生量、高温保存特性、及び高温サイクル特性を評価した。結果を表1に示す。
<バインダー組成物の製造>
攪拌機付き重合缶Aに、沸点145℃以上の(メタ)アクリル酸エステル単量体として2-エチルヘキシルアクリレート(2-EHA)20部、芳香族ビニル単量体としてスチレン37.6部、分散剤としてラウリル硫酸ナトリウム(LAS)1.2部、イオン交換水320部を加えた。この重合缶Aに、さらに重合開始剤として過硫酸アンモニウム(APS)1.0部及びイオン交換水40部を加え、70℃に加温し、90分攪拌した。
また、別の重合缶Bに、沸点145℃以上の(メタ)アクリル酸エステル単量体として2-エチルヘキシルアクリレート(沸点215℃)180部、芳香族ビニル単量体としてスチレン150.4部、酸性基含有単量体としてイタコン酸(IA)8.0部、分散剤としてラウリル硫酸ナトリウム4.8部及びイオン交換水192部を加えて攪拌して、エマルジョンを作製した。このエマルジョンを、約180分かけて重合缶Bから重合缶Aに逐次添加した。
その後、約120分攪拌して、重合転化率(モノマー消費量)が93%になったところで、エバポレータを用い、80℃、100hPa(0.1気圧)の条件で8時間かけて減圧蒸留を行った。その後冷却し、5%水酸化ナトリウム水溶液を添加して、pH8に調整し、所望の粒子状重合体を含む水分散液(バインダー組成物)を得た。得られたバインダー組成物について、経時安定性を評価した。また、調製したバインダー組成物を用いて、実施例1と同様にして負極及びかかる負極を有するラミネートセル型リチウムイオン二次電池を製造した。負極についてはピール強度を評価し、リチウムイオン二次電池については、ガス発生量、高温保存特性、及び高温サイクル特性を評価した。結果を表1に示す。
レダクトン類化合物であるアスコルビン酸ナトリウムに代えてコハク酸を配合した以外は、実施例1と同様にしてバインダー組成物を調製し、経時安定性を評価した。また、調製したバインダー組成物を用いて、実施例1と同様にして負極及びかかる負極を有するラミネートセル型リチウムイオン二次電池を製造した。負極についてはピール強度を評価し、ラミネートセル型リチウムイオン二次電池については、ガス発生量、高温保存特性、及び高温サイクル特性を評価した。結果を表1に示す。
工程(2)の実施タイミングを、工程(1)における重合転化率が81%のときとした以外は、実施例1と同様にしてバインダー組成物を調製し、経時安定性を評価した。また、調製したバインダー組成物を用いて、実施例1と同様にして負極及びかかる負極を有するラミネートセル型リチウムイオン二次電池を製造した。負極についてはピール強度を評価し、ラミネートセル型リチウムイオン二次電池については、ガス発生量、高温保存特性、及び高温サイクル特性を評価した。結果を表1に示す。
比較例6では、工程(2)を実施しなかった。工程(1)の重合転化率が93%となったところで40℃に冷却して、過酸化物である過硫酸アンモニウム(APS)0.3質量%を添加して更に4時間反応させた。その後、かかる水分散液に対して5%水酸化ナトリウム水溶液を添加して、pH8に調整し、所望の粒子状重合体を含む水分散液(バインダー組成物)を得た。得られたバインダー組成物について、経時安定性を評価した。また、調製したバインダー組成物を用いて、実施例1と同様にして負極及びかかる負極を有するラミネートセル型リチウムイオン二次電池を製造した。負極についてはピール強度を評価し、ラミネートセル型リチウムイオン二次電池については、ガス発生量、高温保存特性、及び高温サイクル特性を評価した。結果を表1に示す。
<バインダー組成物の製造>
[重合工程1]
重合缶Aに、沸点145℃以上の(メタ)アクリル酸エステル単量体として2-エチルヘキシルアクリレート(2-EHA)43部、α,β-不飽和ニトリル単量体としてアクリロニトリル(AN)5部、分散剤としてラウリル硫酸ナトリウム(LAS)1.2部、イオン交換水320部を加えた。この重合缶Aに、さらに重合開始剤として過硫酸アンモニウム(APS)1.0部及びイオン交換水40部を加え、70℃に加温し、90分攪拌した。
また、別の重合缶Bに、沸点145℃以上の(メタ)アクリル酸エステル単量体として2-エチルヘキシルアクリレート(沸点215℃)268.6部、α,β-不飽和ニトリル単量体としてアクリロニトリル74.4部、酸性基含有単量体としてイタコン酸(IA)8.0部、分散剤としてラウリル硫酸ナトリウム4.8部及びイオン交換水192部を加えて攪拌して、エマルジョンを作製した。このエマルジョンを、約180分かけて重合缶Bから重合缶Aに逐次添加した。その後、約120分攪拌して、重合転化率(モノマー消費量)が95%になったところで、40℃に冷却した。
工程1で得られた粒子状重合体を含有する水分散液に対して、レダクトン類化合物としてアスコルビン酸ナトリウム1.2部と、過酸化物としてtert-ブチルヒドロペルオキシド (t-BuOOH)1.2部とを添加して、更に40℃で4時間反応させた。その後、かかる水分散液に対して5%水酸化ナトリウム水溶液を添加して、pH8に調整し、所望の粒子状重合体を含む水分散液(バインダー組成物)を得た。得られたバインダー組成物について、経時安定性を評価した。結果を表2に示す。
正極活物質としてCo-Ni-Mnのリチウム複合酸化物系の活物質(製品名:セルシードNMC613、日本化学工業社製。以下、「NMC」と記載することがある。)100部、導電材としてアセチレンブラック2部、及び粘度調整剤としてカルボキシメチルセルロースのナトリウム塩(CMC)水溶液を固形分相当量で0.8部となる量を加え、60分混合した。更に水を加えて希釈した後に、前記のバインダー組成物(固形分濃度40%)を粒子状重合体として1.0部となるように添加し、10分混合した。これを脱泡処理して艶のある流動性の良い正極用スラリー組成物を得た。
上記正極用スラリー組成物を厚さ18μmのアルミニウム箔に塗布し、120℃で3時間乾燥した後、ロールプレスして厚さ50μmの正極合材層を有する正極を得た。
負極活物質として体積平均粒子径20μm、比表面積4.2m2/gのグラファイト98部と、結着剤として日本ゼオン(株)製の負極用バインダーBM-400B(スチレン-ブタジエン共重合体の40質量%水性分散液)を固形分相当で1.0部と、CMCを固形分相当で0.8部とを混合し、更に水を加えてプラネタリーミキサーで混合して負極用スラリー組成物を調製した。この負極用スラリー組成物を厚さ10μmの銅箔の片面に塗布し、110℃で3時間乾燥した後、ロールプレスして厚さ60μmの負極合材層を有する負極を得た。
アルミニウムシートと、その両面を被覆するポリプロピレン樹脂とからなるラミネートフィルムを用いて電池容器を作成した。次いで、上記で得た正極および負極それぞれの端部から合材層を除去し、銅箔又はアルミニウム箔が露出した箇所を形成した。正極のアルミニウム箔が露出した箇所にはNiタブを、負極の銅箔が露出した箇所にはCuタブを溶接した。得られたタブ付きの正極及びタブ付きの負極を、ポリエチレン製の微多孔膜からなるセパレータを挟んで重ねた。電極の面の向きは、正極の合材層側の面と負極の合材層側の面とが対向する向きとした。重ねた電極及びセパレータを、捲回して上記の電池容器に収納した。続いてここに、電解液(EC/DEC=1/2、1M LiPF6)を注入した。電解液としては、エチレンカーボネートとジエチルカーボネートを25℃の下、体積比1:2で混合した混合溶媒に、LiPF6を1モル/Lの濃度になるように溶解させて調製したものを用いた。
次いで、ラミネートフィルムを封止させて本発明のリチウムイオン二次電池であるラミネートセル型リチウムイオン二次電池を作製した。得られたラミネートセル型リチウムイオン二次電池のガス発生量、及び高温保存特性を評価した。結果を表2に示す。
上記で得られた正極を直径13mmの円形に切り抜いた。上記で得られた負極を直径14mmの円形に切り抜いた。厚さ25μmの乾式法により製造された単層のポリプロピレン製セパレータ(気孔率55%)を直径18mmの円形に切り抜いた。これらを、ポリプロピレン製パッキンを設置したステンレス鋼製のコイン型外装容器(直径20mm、高さ1.8mm、ステンレス鋼厚さ0.25mm)中に収納した。外装容器内の円形の電極及びセパレータの配置は、下記の通りとした。円形の正極は、そのアルミニウム箔が外装容器底面に接触するよう配置した。円形のセパレータは、円形の正極と円形の負極との間に介在するよう配置した。円形の負極は、その負極合材層側の面が、円形のセパレータを介して円形の正極の正極合材層側の面に対向するよう配置した。更に負極の上にエキスパンドメタルを載置し、この容器中に電解液(EC/DEC=1/2、1M LiPF6)を空気が残らないように注入し、ポリプロピレン製パッキンを介して外装容器に厚さ0.2mmのステンレス鋼のキャップをかぶせて固定し、外装容器を封止して、直径20mm、厚さ約3.2mmの本発明のリチウムイオン二次電池であるフルコインセル型リチウムイオン二次電池(コインセルCR2032)を作製した。得られたフルコインセル型リチウムイオン二次電池について高温サイクル特性を評価した。結果を表2に示す。
沸点145℃以上の(メタ)アクリル酸エステル単量体である2-エチルヘキシルアクリレートの配合量及びα,β-不飽和ニトリル単量体であるアクリロニトリルの配合量をそれぞれ表2に示す通りに変更した以外は実施例16と同様にしてバインダー組成物を調製し、経時安定性を評価した。なお、各成分の配合量を変更した場合であっても、重合缶A及びBに含まれる各成分の配合量間の比率は実施例16と同様にした。そして、得られた粒子状重合体の体積平均粒子径は、実施例16と同様に0.11μmであった。また、調製したバインダー組成物を用いて、実施例16と同様にしてラミネートセル型リチウムイオン二次電池及びフルコインセル型リチウムイオン二次電池を製造して、それぞれ、ガス発生量、及び高温保存特性、並びに高温サイクル特性を評価した。結果を表2に示す。
沸点145℃以上の(メタ)アクリル酸エステル単量体としてn-ブチルアクリレート(沸点148℃)を使用した以外は実施例16と同様にしてバインダー組成物を調製し、経時安定性を評価した。また、調製したバインダー組成物を用いて、実施例16と同様にしてラミネートセル型リチウムイオン二次電池及びフルコインセル型リチウムイオン二次電池を製造して、それぞれ、ガス発生量、及び高温保存特性、並びに高温サイクル特性を評価した。結果を表2に示す。
レダクトン類化合物であるアスコルビン酸ナトリウムを、表2に示すレダクトン類化合物にそれぞれ変更した以外は実施例16と同様にしてバインダー組成物を調製し、経時安定性を評価した。また、調製したバインダー組成物を用いて、実施例16と同様にしてラミネートセル型リチウムイオン二次電池及びフルコインセル型リチウムイオン二次電池を製造して、それぞれ、ガス発生量、及び高温保存特性、並びに高温サイクル特性を評価した。結果を表2に示す。
レダクトン類化合物であるアスコルビン酸ナトリウムの配合割合をそれぞれ表2に示す通りに変更し、また、過酸化物であるtert-ブチルヒドロペルオキシドの配合割合を表2に示す通りに変更した以外は実施例16と同様にしてバインダー組成物を調製し、経時安定性を評価した。また、調製したバインダー組成物を用いて、実施例16と同様にしてラミネートセル電池型リチウムイオン二次及びフルコインセル型リチウムイオン二次電池を製造して、それぞれ、ガス発生量、及び高温保存特性、並びに高温サイクル特性を評価した。結果を表2に示す。
過酸化物であるtert-ブチルヒドロペルオキシドを過酸化ベンゾイル(BPO)に変更した以外は実施例16と同様にしてバインダー組成物を調製し、経時安定性を評価した。また、調製したバインダー組成物を用いて、実施例16と同様にしてラミネートセル型リチウムイオン二次電池及びフルコインセル型リチウムイオン二次電池を製造して、それぞれ、ガス発生量、及び高温保存特性、並びに高温サイクル特性を評価した。結果を表2に示す。
レダクトン類化合物であるアスコルビン酸ナトリウムを配合せず、工程(2)において、40℃で2時間反応させた後、80℃で2時間反応させた以外は、実施例16と同様にしてバインダー組成物を調製し、経時安定性を評価した。また、調製したバインダー組成物を用いて、実施例16と同様にしてラミネートセル型リチウムイオン二次電池及びフルコインセル型リチウムイオン二次電池を製造して、それぞれ、ガス発生量、及び高温保存特性、並びに高温サイクル特性を評価した。結果を表2に示す。
(メタ)アクリル酸エステル単量体である2-エチルヘキシルアクリレートに代えて、沸点が100℃のエチルアクリレート(EA)を使用した以外は実施例16と同様にしてバインダー組成物を調製し、経時安定性を評価した。また、調製したバインダー組成物を用いて、実施例16と同様にしてラミネートセル型リチウムイオン二次電池及びフルコインセル型リチウムイオン二次電池を製造して、それぞれ、ガス発生量、及び高温保存特性、並びに高温サイクル特性を評価した。結果を表2に示す。
<バインダー組成物の製造>
重合缶Aに、沸点145℃以上の(メタ)アクリル酸エステル単量体として2-エチルヘキシルアクリレート(2-EHA)43部、α,β-不飽和ニトリル単量体としてアクリロニトリル(AN)5部、分散剤としてラウリル硫酸ナトリウム(LAS)1.2部、イオン交換水320部を加えた。この重合缶Aに、さらに重合開始剤として過硫酸アンモニウム(APS)1.0部及びイオン交換水40部を加え、70℃に加温し、90分攪拌した。
また、別の重合缶Bに、沸点145℃以上の(メタ)アクリル酸エステル単量体として2-エチルヘキシルアクリレート(沸点215℃)268.6部、α,β-不飽和ニトリル単量体としてアクリロニトリル74.4部、酸性基含有単量体としてイタコン酸(IA)8.0部、分散剤としてラウリル硫酸ナトリウム4.8部及びイオン交換水192部を加えて攪拌して、エマルジョンを作製した。このエマルジョンを、約180分かけて重合缶Bから重合缶Aに逐次添加した。その後、約120分攪拌し、重合転化率(モノマー消費量)が95%になったところで、エバポレータを用い、80℃、100hPa(0.1気圧)の条件で8時間かけて減圧蒸留を行った。その後冷却し、5%水酸化ナトリウム水溶液を添加して、pH8に調整し、所望の粒子状重合体を含む水分散液(バインダー組成物)を得た。得られたバインダー組成物について、経時安定性を評価した。また、調製したバインダー組成物を用いて、実施例16と同様にしてラミネートセル型リチウムイオン二次電池及びフルコインセル型リチウムイオン二次電池を製造して、それぞれ、ガス発生量、及び高温保存特性、並びに高温サイクル特性を評価した。結果を表2に示す。
レダクトン類化合物であるアスコルビン酸ナトリウムに代えてコハク酸を配合した以外は、実施例16と同様にしてバインダー組成物を調製し、経時安定性を評価した。また、調製したバインダー組成物を用いて、実施例16と同様にしてラミネートセル型リチウムイオン二次電池及びフルコインセル型リチウムイオン二次電池を製造して、それぞれ、ガス発生量、及び高温保存特性、並びに高温サイクル特性を評価した。結果を表2に示す。
工程(2)の実施タイミングを、工程(1)における重合転化率が80%のときとした以外は、実施例16と同様にしてバインダー組成物を調製し、経時安定性を評価した。また、調製したバインダー組成物を用いて、実施例16と同様にしてラミネートセル型リチウムイオン二次電池及びフルコインセル型リチウムイオン二次電池を製造して、それぞれ、ガス発生量、及び高温保存特性、並びに高温サイクル特性を評価した。結果を表2に示す。
比較例12では、工程(2)を実施しなかった。工程(1)の重合転化率が95%となったところで40℃に冷却して、過酸化物である過硫酸アンモニウム(APS)0.3質量%を添加して更に4時間反応させた。その後、かかる水分散液に対して5%水酸化ナトリウム水溶液を添加して、pH8に調整し、所望の粒子状重合体を含む水分散液(バインダー組成物)を得た。得られたバインダー組成物について、経時安定性を評価した。また、調製したバインダー組成物を用いて、実施例16と同様にしてラミネートセル型リチウムイオン二次電池及びフルコインセル型リチウムイオン二次電池を製造して、それぞれ、ガス発生量、及び高温保存特性、並びに高温サイクル特性を評価した。結果を表2に示す。
また、本発明によれば、当該二次電池用電極を用いた、残留モノマーに起因したガスの発生が抑制され、良好な高温保存特性及び高温サイクル特性を有する二次電池を提供することができる。
Claims (10)
- 粒子状重合体、レダクトン類化合物およびその酸化体の少なくとも一方、及び水を含み、
前記粒子状重合体は、沸点145℃以上の(メタ)アクリル酸エステル単量体を含む単量体混合物を重合してなり、
前記沸点145℃以上の(メタ)アクリル酸エステル単量体の含有割合は、前記粒子状重合体100質量部に対して1×10-6質量部以上1500×10-6質量部以下である、二次電池電極用バインダー組成物。 - 前記レダクトン類化合物およびその酸化体の少なくとも一方が、(イソ)アスコルビン酸及びその塩、並びに、それらの酸化体から選択される少なくとも1種である、請求項1に記載の二次電池電極用バインダー組成物。
- 前記レダクトン類化合物およびその酸化体の少なくとも一方の含有割合が、前記粒子状重合体100質量部あたり、0.05質量部以上5質量部以下である、請求項1又は2に記載の二次電池電極用バインダー組成物。
- 前記単量体混合物は、さらに架橋性単量体を含む、請求項1~3のいずれか一項に記載の二次電池電極用バインダー組成物。
- 前記単量体混合物は、さらに(メタ)アクリロニトリル単量体を5~35質量%含む、請求項1~3のいずれか一項に記載の二次電池電極用バインダー組成物。
- 前記沸点が145℃以上の(メタ)アクリル酸エステル単量体が、2-エチルヘキシルアクリレートである、請求項1~5のいずれか一項に記載の二次電池電極用バインダー組成物。
- 沸点145℃以上の(メタ)アクリル酸エステル単量体を含む単量体混合物を重合転化率が90質量%以上となるまで水中で重合し、重合体と未反応の単量体とを含む混合物を得る工程(1)と、
前記工程(1)の後、前記混合物にレダクトン類化合物および過酸化物を添加して前記未反応の単量体を重合し、前記沸点145℃以上の(メタ)アクリル酸エステル単量体の含有割合を、重合体100質量部に対して1×10-6質量部以上1500×10-6質量部以下にする工程(2)とを含む、
請求項1~6のいずれか一項に記載の二次電池電極用バインダー組成物の製造方法。 - 請求項1~6のいずれか一項に記載の二次電池電極用バインダー組成物及び電極活物質を含む、二次電池電極用スラリー組成物。
- 集電体上に、請求項8に記載の二次電池電極用スラリー組成物を塗布し、前記集電体上に塗布された二次電池電極用スラリー組成物を乾燥して得られる、二次電池用電極。
- 正極、負極、電解液及びセパレータを備え、前記正極および前記負極の少なくとも一方が、請求項9に記載の二次電池用電極である、二次電池。
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