WO2016035286A1 - 二次電池電極用バインダー組成物、二次電池電極用スラリー組成物、二次電池用電極および二次電池 - Google Patents
二次電池電極用バインダー組成物、二次電池電極用スラリー組成物、二次電池用電極および二次電池 Download PDFInfo
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- C08J2333/00—Characterised by the use of homopolymers or copolymers of compounds having one or more unsaturated aliphatic radicals, each having only one carbon-to-carbon double bond, and only one being terminated by only one carboxyl radical, or of salts, anhydrides, esters, amides, imides, or nitriles thereof; Derivatives of such polymers
- C08J2333/04—Characterised by the use of homopolymers or copolymers of compounds having one or more unsaturated aliphatic radicals, each having only one carbon-to-carbon double bond, and only one being terminated by only one carboxyl radical, or of salts, anhydrides, esters, amides, imides, or nitriles thereof; Derivatives of such polymers esters
- C08J2333/06—Characterised by the use of homopolymers or copolymers of compounds having one or more unsaturated aliphatic radicals, each having only one carbon-to-carbon double bond, and only one being terminated by only one carboxyl radical, or of salts, anhydrides, esters, amides, imides, or nitriles thereof; Derivatives of such polymers esters of esters containing only carbon, hydrogen, and oxygen, the oxygen atom being present only as part of the carboxyl radical
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Definitions
- the present invention relates to a secondary battery electrode binder composition, a secondary battery electrode slurry composition, a secondary battery electrode, and a secondary battery.
- Secondary batteries such as 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. Therefore, in recent years, improvement of battery members such as electrodes has been studied for the purpose of further improving the performance of secondary batteries.
- the electrode for secondary batteries such as a lithium ion secondary battery
- the electrode mixture layer is formed by, for example, applying a slurry composition obtained by dispersing an electrode active material and a binder composition containing a binder in a dispersion medium on a current collector, and applying the slurry composition. It is formed by drying. Therefore, in recent years, in order to achieve further performance improvement of the secondary battery, the binder component in the binder composition used for forming the electrode mixture layer has been actively improved.
- Patent Document 1 an aliphatic conjugated diene monomer, a (meth) acrylic acid alkyl ester monomer having an alkyl group having 1 to 3 carbon atoms, an ethylenically unsaturated carboxylic acid monomer, and an aromatic Obtained by emulsion polymerization of a monomer composition containing an aromatic vinyl monomer and / or vinyl cyanide monomer at a specific ratio, and the number average particle diameter and degree of swelling with respect to diethyl carbonate are specified. It has been reported that a copolymer latex in the range of is excellent in binding properties. And in patent document 1, the technique which improves the high rate discharge characteristic and charging / discharging cycle characteristic of a secondary battery is proposed by forming an electrode using the binder composition containing the said copolymer latex.
- an aliphatic conjugated diene monomer, an ethylenically unsaturated carboxylic acid monomer, and another monomer copolymerizable with these monomers in a specific ratio respectively.
- a copolymer latex obtained by emulsion polymerization of a body composition, wherein the filtration residue remaining on a 400 mesh sieve is 0.01% by weight or less with respect to 100% by weight of the solid content of the copolymer latex There has been proposed a technique for forming an electrode mixture layer having high uniformity and excellent binding properties to a current collector by using a combined latex as a binder for an electrode.
- the binder composition using the above-mentioned conventional binder has room for improvement in terms of further improving the electrical characteristics such as rate characteristics and cycle characteristics of the secondary battery formed using the binder composition. was there.
- an object of this invention is to provide the binder composition for secondary battery electrodes which can exhibit the rate characteristic and cycling characteristic which were excellent in the secondary battery.
- Another object of the present invention is to provide a slurry composition for a secondary battery electrode capable of forming an electrode mixture layer capable of exhibiting excellent rate characteristics and cycle characteristics in a secondary battery.
- an object of the present invention is to provide a secondary battery electrode capable of exhibiting excellent rate characteristics and cycle characteristics in a secondary battery.
- An object of the present invention is to provide a secondary battery excellent in rate characteristics and cycle characteristics.
- the present inventor has intensively studied for the purpose of solving the above problems.
- the inventor of the present invention has sufficient ion conductivity such as lithium ions in the electrode mixture layer formed with the binder composition using the conventional technique, particularly when the electrode mixture layer is densified. In view of this, it was noted that sufficient electrical characteristics were not exhibited.
- the present inventor uses a particulate polymer having a core-shell structure including a core part and a shell part partially covering the outer surface of the core part as a binder for an electrode.
- the present invention has been completed by finding that the secondary battery can exhibit excellent rate characteristics and cycle characteristics while ensuring the strength of the electrode mixture layer.
- the binder composition for secondary battery electrodes of this invention partially covers the core part and the outer surface of the said core part.
- a first particulate polymer having a core-shell structure including a shell portion is included.
- the binder composition is used.
- the formed secondary battery can exhibit excellent rate characteristics and cycle characteristics.
- the polymer solution constituting the core part has an electrolyte solution swelling degree of 300% by mass or more and 900% by mass or less, and the polymer constituting the shell part.
- the degree of swelling of the electrolytic solution is preferably more than 100% by mass and 200% by mass or less.
- the rate characteristics and cycle characteristics of the secondary battery are further improved. This is because it can be improved.
- the “electrolyte swelling degree” of the polymer constituting the core part and the polymer constituting the shell part can be measured using the measuring method described in the examples of the present specification.
- the binder composition for a secondary battery electrode of the present invention is such that the polymer constituting the core part has a glass transition temperature of ⁇ 60 ° C. or more and ⁇ 15 ° C. or less, and the polymer constituting the shell part has a glass transition temperature. It is preferable that temperature is 40 degreeC or more and 200 degrees C or less.
- the peel between the electrode mixture layer and the current collector This is because an electrode having excellent strength can be obtained and the rate characteristics of the secondary battery can be further improved.
- the “glass transition temperature” of the polymer constituting the core part and the polymer constituting the shell part can be measured using the measuring method described in the examples of the present specification.
- the mass ratio of the shell portion in the first particulate polymer is preferably 3% by mass or more and 35% by mass or less. If the ratio of the mass of the shell portion to the mass of the first particulate polymer is within the above range, an electrode having excellent peel strength between the electrode mixture layer and the current collector can be obtained, and the secondary battery This is because rate characteristics and cycle characteristics can be achieved at a high level.
- the “mass ratio of the shell portion” in the first particulate polymer can be calculated using the calculation method described in the examples of the present specification.
- the polymer constituting the core part may contain 50% by mass or more and 99.5% by mass or less of a (meth) acrylic acid ester monomer unit. preferable. This is because the rate characteristics of the secondary battery can be further improved if the polymer constituting the core part of the first particulate polymer contains the (meth) acrylic acid ester monomer unit in the above proportion. .
- (meth) acryl means acryl and / or methacryl.
- the binder composition for a secondary battery electrode of the present invention further includes a second particulate polymer, and the second particulate polymer has an electrolyte swelling degree of more than 100% by mass and less than 200% by mass.
- the glass transition temperature is preferably ⁇ 10 ° C. or higher and 40 ° C. or lower.
- the binder composition for a secondary battery electrode of the present invention is the first particulate polymer per 100 parts by mass in total of the first particulate polymer and the second particulate polymer in terms of solid content. It is preferable that 30 mass parts or more and 95 mass parts or less are included. This is because, if the content ratio of the first particulate polymer is within the above range, the rate characteristics and cycle characteristics of the secondary battery can be achieved at a high level.
- the second particulate polymer contains 5% by mass to 70% by mass of a conjugated diene monomer unit, and the aromatic vinyl monomer. It is preferable to contain 10 mass% or more and 90 mass% or less of a unit. This is because if the second particulate polymer contains the conjugated diene monomer unit and the aromatic vinyl monomer unit in the above ratio, the cycle characteristics of the secondary battery can be further improved.
- the number average particle diameter of the first particulate polymer is 1 to 5 times the number average particle diameter of the second particulate polymer. Preferably there is. This is because, if the ratio of the number average particle diameter of the first particulate polymer and the second particulate polymer is within the above range, the rate characteristics and cycle characteristics of the secondary battery can be achieved at a high level. .
- the “number average particle diameter” of the particulate polymer can be measured using the measuring method described in the examples of the present specification.
- the slurry composition for secondary battery electrodes of this invention is either of the binder composition for secondary battery electrodes mentioned above, and an electrode. And an active material.
- the binder composition containing the first particulate polymer and the second particulate polymer is used, an electrode mixture layer capable of exhibiting excellent rate characteristics and cycle characteristics in the secondary battery is formed.
- a possible slurry composition for a secondary battery electrode is obtained.
- the electrode for secondary batteries of this invention is the electrode compound-material layer obtained using the said slurry composition for secondary battery electrodes It is characterized by having.
- excellent rate characteristics and cycle characteristics can be exhibited in the secondary battery.
- the porosity of the said electrode compound material layer is 10.7% or more and 24.1% or less. This is because if the porosity of the electrode mixture layer is within the above range, the secondary battery can exhibit excellent rate characteristics and cycle characteristics while achieving high density of the electrode mixture layer.
- the secondary battery of this invention is equipped with a positive electrode, a negative electrode, a separator, and electrolyte solution, and at least one of the said positive electrode and the said negative electrode is It is one of the secondary battery electrodes described above.
- the electrode mentioned above is used as a positive electrode and / or a negative electrode, a secondary battery having excellent rate characteristics and cycle characteristics can be obtained.
- the binder composition for secondary battery electrodes which can exhibit the rate characteristic and cycling characteristics which were excellent in the secondary battery can be provided.
- the slurry composition for secondary battery electrodes which can form the electrode compound-material layer which can exhibit the rate characteristic and cycling characteristics excellent in the secondary battery can be provided.
- a secondary battery excellent in rate characteristics and cycle characteristics can be provided.
- the binder composition for secondary battery electrodes of the present invention can be used when preparing a slurry composition for secondary battery electrodes.
- the slurry composition for secondary battery electrodes prepared using the binder composition for secondary battery electrodes of this invention can be used when forming the electrode of a secondary battery.
- the secondary battery of the present invention is characterized by using the secondary battery electrode of the present invention.
- the binder composition for a secondary battery electrode of the present invention is an aqueous binder composition using an aqueous medium as a dispersion medium, and includes a particulate binder and water, and is generally used in the field of secondary batteries. It further contains other components used.
- the binder composition for secondary battery electrodes of the present invention is a particulate weight having a core-shell structure comprising a core part and a shell part partially covering the outer surface of the core part as a particulate binder. It is characterized by using a coalescence (first particulate polymer).
- the binder is a secondary battery electrode produced by forming an electrode mixture layer on a current collector using a slurry composition for a secondary battery electrode containing the binder composition of the present invention and an electrode active material.
- the component included in the electrode mixture layer is a component that can be held so as not to be detached from the electrode mixture layer.
- the particulate binder in the electrode mixture layer is immersed in the electrolytic solution, the particulate binder maintains the particulate shape while absorbing and swelling the electrolytic solution.
- the electrode active material and the current collector are bound to prevent the electrode active material from falling off the current collector.
- the binder also serves to bind particles other than the electrode active material contained in the electrode mixture layer and maintain the strength of the electrode mixture layer.
- the binder composition of the present invention in order to enable an electrode including an electrode mixture layer formed using the binder composition to exhibit excellent rate characteristics and cycle characteristics in a secondary battery,
- the first particulate polymer having the above-described specific core-shell structure is used as the binder.
- the binder composition of this invention may contain particulate polymers other than 1st particulate polymer like the 2nd particulate polymer mentioned later as a particulate binder.
- the first particulate polymer has a core-shell structure including a core part and a shell part that covers the outer surface of the core part.
- the shell portion partially covers the outer surface of the core portion. That is, the shell part of the first particulate polymer covers the outer surface of the core part, but does not cover the entire outer surface of the core part. Even if it appears that the outer surface of the core part is completely covered by the shell part, the shell part is outside the core part as long as a hole that communicates the inside and outside of the shell part is formed.
- the reason why the electrical characteristics of the secondary battery can be improved by using the first particulate polymer having such a core-shell structure is not clear, but is presumed to be as follows. Yes. That is, since the first particulate polymer has a structure in which the shell part partially covers the outer surface of the core part, the first particulate polymer is covered with the shell part in the obtained electrode mixture layer. The strength of the electrode mixture layer is maintained by adhering to an electrode active material or a current collector through the outer surface of the core portion that is not present. On the other hand, in the electrode mixture layer, a void capable of ion conduction derived from a portion not covered by the shell portion on the surface of the particulate polymer is secured. Therefore, according to the first particulate polymer, ion conductivity is secured while constraining the electrode active material in the electrode mixture layer, so that the secondary battery can exhibit excellent rate characteristics and cycle characteristics. It is assumed that it is possible.
- the shell part of the first particulate polymer is preferably composed of a plurality of shell part structures.
- the first particulate polymer 100 includes a shell portion including a core portion 110 and a plurality of shell portion structures 120. It preferably has a core-shell structure.
- the core part 110 is a part inside the shell part structure 120 in the first particulate polymer 100.
- the shell part structure 120 covers the outer surface 110S of the core part 110, and the shell part composed of the plurality of shell part structures 120 is usually the outermost part of the first particulate polymer 100.
- the shell part which consists of several shell part structure 120 does not cover the whole outer surface 110S of the core part 110, but has covered the outer surface 110S of the core part 110 partially.
- the 1st particulate polymer may be equipped with arbitrary components other than the core part and shell part which were mentioned above, unless the expected effect is impaired remarkably.
- the first particulate polymer may have a portion formed of a polymer different from the core portion inside the core portion.
- the seed particles used when the first particulate polymer is produced by the seed polymerization method may remain inside the core portion.
- the first particulate polymer includes only the core part and the shell part from the viewpoint of remarkably exhibiting the intended effect.
- the degree of swelling of the electrolyte solution of the polymer constituting the core part is preferably 300% by mass or more, more preferably 400% by mass or more, and still more preferably. It is 500% by mass or more, preferably 900% by mass or less, more preferably 800% by mass or less, and still more preferably 700% by mass or less.
- the electrolyte swelling degree of the polymer in the core part 300% by mass or more, ion conductivity is ensured and electrical characteristics such as rate characteristics of the secondary battery can be improved.
- the electrode active material can be sufficiently restrained, and the cycle characteristics of the secondary battery can be improved.
- the glass transition temperature of the polymer constituting the core part is preferably ⁇ 60 ° C. or higher, more preferably ⁇ 55 ° C. or higher, further preferably ⁇ 50 ° C. or higher, particularly preferably ⁇ 40 ° C. or higher, preferably It is ⁇ 15 ° C. or lower, more preferably ⁇ 25 ° C. or lower, and further preferably ⁇ 30 ° C. or lower. Peeling of an electrode having an electrode mixture layer formed using a binder composition by increasing the binding property of the first particulate polymer by setting the glass transition temperature of the polymer in the core part to ⁇ 60 ° C. or higher. Strength can be improved. Further, by setting the glass transition temperature of the polymer in the core part to ⁇ 15 ° C. or lower, deformation of the electrode active material due to press working when forming the electrode mixture layer is suppressed, and as a result, the rate of the secondary battery Characteristics can be improved.
- the electrolyte swelling degree and glass transition temperature of the core polymer are not particularly limited, and the type and amount of monomers used for forming the core polymer, and the core polymer It can be adjusted by changing the molecular weight and the cross-linking density.
- a copolymer (A) containing a (meth) acrylic acid ester monomer unit will be described as an example of the polymer of the core part of the first particulate polymer.
- “including a monomer unit” means that “a monomer-derived structural unit (repeating unit) is included in a polymer obtained using the monomer”. means.
- the (meth) acrylic acid ester monomer that can form the (meth) acrylic acid ester monomer unit of the copolymer (A) is not particularly limited, and is not limited to methyl acrylate, ethyl acrylate, n -Propyl acrylate, isopropyl acrylate, n-butyl acrylate, t-butyl acrylate, pentyl acrylate, hexyl acrylate, heptyl acrylate, octyl acrylate, 2-ethylhexyl acrylate, nonyl acrylate, decyl acrylate, lauryl acrylate, n-tetradecyl acrylate, stearyl Acrylic acid alkyl esters such as acrylate; methyl methacrylate, ethyl methacrylate, n-propyl methacrylate, isopropyl methacrylate, n-butyl methacrylate Methacrylate
- acrylic acid alkyl esters are preferred, ethyl acrylate, n-butyl acrylate, 2-ethylhexyl acrylate are more preferred, and n-butyl acrylate is even more preferred.
- acrylic acid alkyl esters are preferred, ethyl acrylate, n-butyl acrylate, 2-ethylhexyl acrylate are more preferred, and n-butyl acrylate is even more preferred.
- these may be used individually by 1 type and may be used combining two or more types by arbitrary ratios.
- the proportion of the (meth) acrylic acid ester monomer unit in the copolymer (A) is preferably 20% by mass or more, more preferably 50% by mass or more, still more preferably 70% by mass or more, and particularly preferably 90%. It is at least 9 mass%, preferably at most 99.5 mass%, more preferably at most 99 mass%, still more preferably at most 98 mass%. If the ratio of the (meth) acrylic acid ester monomer unit in the polymer of the core part is within the above range, the secondary battery can exhibit excellent rate characteristics.
- the copolymer (A) is optionally composed of an ethylenically unsaturated carboxylic acid monomer unit, a vinyl cyanide monomer unit, a conjugated diene. System monomer units and other monomer units may be included.
- the ethylenically unsaturated carboxylic acid monomer that can form the ethylenically unsaturated carboxylic acid monomer unit of the copolymer (A) is not particularly limited, and acrylic acid, methacrylic acid, croton Examples thereof include monocarboxylic acids and dicarboxylic acids such as acid, maleic acid, fumaric acid and itaconic acid, and anhydrides thereof.
- an ethylenically unsaturated monocarboxylic acid monomer is preferable, acrylic acid and methacrylic acid are more preferable, and methacrylic acid is still more preferable.
- an ethylenically unsaturated carboxylic acid monomer may be used individually by 1 type, and may be used combining two or more types by arbitrary ratios.
- the ratio of the ethylenically unsaturated carboxylic acid monomer unit in the copolymer (A) is preferably 0.1% by mass or more, more preferably 0.2% by mass or more, and further preferably 0.3% by mass. It is above, Preferably it is 10 mass% or less, More preferably, it is 8 mass% or less, More preferably, it is 5 mass% or less.
- the vinyl cyanide monomer capable of forming the vinyl cyanide monomer unit of the copolymer (A) is not particularly limited, and is not limited to acrylonitrile, methacrylonitrile, ⁇ -chloroacrylonitrile, ⁇ -Ethylacrylonitrile and the like. Of these, acrylonitrile is preferred.
- a vinyl cyanide-type monomer may be used individually by 1 type, and may be used combining two or more types by arbitrary ratios.
- the proportion of the vinyl cyanide monomer unit in the copolymer (A) is preferably 0.1% by mass or more, more preferably 0.2% by mass or more, and preferably 10% by mass or less. Preferably it is 5 mass% or less.
- the conjugated diene monomer capable of forming the conjugated diene monomer unit of the copolymer (A) is not particularly limited, and 1,3-butadiene, 2-methyl-1,3- Examples include butadiene, 2,3-dimethyl-1,3-butadiene, 2-chloro-1,3-butadiene, substituted linear conjugated pentadienes, and aliphatic conjugated diene monomers of substituted and side chain conjugated hexadienes. . Of these, 1,3-butadiene is preferred.
- a conjugated diene monomer may be used individually by 1 type, and may be used combining two or more types by arbitrary ratios.
- the copolymer (A) may contain a conjugated diene monomer unit when the proportion of the (meth) acrylate monomer unit is small (for example, 15% by mass or more and 25% by mass or less).
- the ratio of the conjugated diene monomer unit in the copolymer (A) is preferably 5% by mass or more, more preferably 15% by mass or more, and further preferably 20% by mass or more.
- it is 70 mass% or less, More preferably, it is 55 mass% or less, More preferably, it is 50 mass% or less.
- Examples of other monomer units include monomer units obtained by polymerizing the following arbitrary monomers.
- arbitrary monomers may be used individually by 1 type, and may be used combining two or more types by arbitrary ratios.
- the optional monomer include crosslinkable monomers such as allyl methacrylate and N-methylolacrylamide, styrene, chlorostyrene, vinyltoluene, t-butylstyrene, methyl vinylbenzoate, vinylnaphthalene, chloromethylstyrene, Styrene monomers such as hydroxymethylstyrene and ⁇ -methylstyrene; unsaturated carboxylic acid amide monomers such as acrylamide and methacrylamide; vinyl sulfonic acid, styrene sulfonic acid, allyl sulfonic acid, sulfoethyl methacrylate, sulfopropyl methacrylate And sulfonic acid group-containing mono
- the proportion of other monomer units in the copolymer (A) is preferably 0% by mass to 30% by mass, and more preferably 0% by mass to 25% by mass.
- the polymer of the core part which consists of the copolymer (A) mentioned above is manufactured by superposing
- the content rate of each monomer in the monomer composition for core parts is usually made the same as the content ratio of each corresponding repeating unit (monomer unit) in the polymer of the desired core part. .
- the electrolyte solution swelling degree of the polymer constituting the shell part is preferably more than 100% by mass, more preferably 105% by mass or more, and still more preferably. It is 110 mass% or more, preferably 200 mass% or less, more preferably 170 mass% or less, and still more preferably 140 mass% or less.
- the electrolyte solution swelling degree of the polymer in the shell part exceed 100% by mass, ion conductivity can be ensured and electrical characteristics such as rate characteristics of the secondary battery can be improved.
- the electrode active material can be sufficiently restrained, and the cycle characteristics of the secondary battery can be improved.
- the glass transition temperature of the polymer constituting the shell part is preferably 40 ° C. or higher, more preferably 60 ° C. or higher, still more preferably 80 ° C. or higher, preferably 200 ° C. or lower, more preferably 160 ° C. or lower, More preferably, it is 140 degrees C or less. Peeling of an electrode having an electrode mixture layer formed using a binder composition by increasing the binding property of the first particulate polymer by making the glass transition temperature of the polymer of the shell part within the above-mentioned range Strength can be improved.
- a copolymer (B) containing an aromatic vinyl monomer unit and an ethylenically unsaturated carboxylic acid monomer unit can be used, for example. Therefore, hereinafter, a copolymer (B) containing an aromatic vinyl monomer unit and an ethylenically unsaturated carboxylic acid monomer unit will be described as an example of the polymer of the shell portion of the first particulate polymer. .
- the aromatic vinyl monomer capable of forming the aromatic vinyl monomer unit of the copolymer (B) is not particularly limited, and styrene, ⁇ -methylstyrene, vinyl toluene, divinylbenzene, etc. Is mentioned. Of these, styrene is preferred.
- an aromatic vinyl monomer may be used individually by 1 type, and may be used combining two or more types by arbitrary ratios.
- the ratio of the aromatic vinyl monomer unit in a copolymer (B) becomes like this.
- it is 50 mass% or more, More preferably, it is 70 mass% or more, More preferably, it is 80 mass% or more, Preferably it is 99. It is not more than mass%, more preferably not more than 98 mass%, still more preferably not more than 97 mass%.
- the ethylenically unsaturated carboxylic acid monomer that can form the ethylenically unsaturated carboxylic acid monomer unit of the copolymer (B) is not particularly limited, Those mentioned in the section can be used. Of these, acrylic acid and methacrylic acid are more preferable, and methacrylic acid is more preferable.
- the ratio of the ethylenically unsaturated carboxylic acid monomer unit in the copolymer (B) is preferably 0.1% by mass or more, more preferably 1% by mass or more, and further preferably 3% by mass or more. Preferably it is 15 mass% or less, More preferably, it is 10 mass% or less, More preferably, it is 8 mass% or less.
- the copolymer (B) may contain other monomer units in addition to the above-described aromatic vinyl monomer units and ethylenically unsaturated carboxylic acid monomer units.
- examples of other monomer units in the copolymer (B) include all of the monomer units enumerated in "Composition of core part", aromatic vinyl monomer units, and ethylenically unsaturated carboxylic acid monomers. The thing except a unit is mentioned.
- the ratio of other monomer units other than the aromatic vinyl monomer unit and the ethylenically unsaturated carboxylic acid monomer unit in the copolymer (B) is preferably 0% by mass to 30% by mass, More preferably, it is 0 mass% or more and 25 mass% or less.
- the polymer of the shell part which consists of a copolymer (B) mentioned above superposes
- the content rate of each monomer in the monomer composition for shell parts is usually made the same as the content ratio of each corresponding repeating unit (monomer unit) in the polymer of the desired shell part. .
- the mass ratio of the shell portion in the first particulate polymer is preferably 3% by mass or more, more preferably 5% by mass or more, still more preferably 10% by mass or more, preferably 35% by mass or less, more preferably It is 30 mass% or less, More preferably, it is 25 mass% or less.
- the ratio of the mass of the shell part to the mass of the first particulate polymer is 3% by mass or more, the cycle characteristics of the secondary battery can be improved.
- the ratio of the mass of the shell portion to the mass of the first particulate polymer is 35% by mass or less, the ionic conductivity of the electrode mixture layer obtained using the binder composition is ensured, and rate characteristics The electrical characteristics such as can be improved. Further, the peel strength between the electrode mixture layer and the current collector can be improved.
- the number average particle diameter of the first particulate polymer is preferably 100 nm or more, more preferably 150 nm or more, further preferably 200 nm or more, and particularly preferably 300 nm or more. , Preferably 1000 nm or less, more preferably 800 nm or less, still more preferably 600 nm or less, and particularly preferably 400 nm or less. This is because when the number average particle diameter of the first particulate polymer is within the above range, it is possible to satisfactorily achieve suppression of expansion and contraction of the electrode active material and reduction of the resistance of the electrode mixture layer. .
- the number average particle diameter of the first particulate polymer can be appropriately adjusted by adjusting the amount of the emulsifier, the amount of the monomer, and the like.
- the first particulate polymer having the core-shell structure described above uses, for example, a polymer monomer for the core part and a monomer for the polymer for the shell part. It can manufacture by changing in ratio and polymerizing in steps. Specifically, the first particulate polymer is produced by a continuous multi-stage emulsion polymerization method and a multi-stage suspension polymerization method in which the polymer of the previous stage is sequentially coated with the polymer of the subsequent stage. be able to.
- an emulsifier for example, an anionic surfactant such as sodium dodecylbenzenesulfonate and sodium dodecylsulfate, a nonionic surfactant such as polyoxyethylene nonylphenyl ether and sorbitan monolaurate, or Cationic surfactants such as octadecylamine acetate can be used.
- anionic surfactant such as sodium dodecylbenzenesulfonate and sodium dodecylsulfate
- a nonionic surfactant such as polyoxyethylene nonylphenyl ether and sorbitan monolaurate
- Cationic surfactants such as octadecylamine acetate
- polymerization initiator examples include peroxides such as t-butylperoxy-2-ethylhexanoate, potassium persulfate, cumene peroxide, 2,2′-azobis (2-methyl-N- (2 An azo compound such as -hydroxyethyl) -propionamide) or 2,2'-azobis (2-amidinopropane) hydrochloride can be used.
- a step of polymerizing a monomer composition for a core part to form a polymer of the core part (core part forming step), and a shell part in a polymerization system including the polymer of the core part Adding a monomer composition and polymerizing the monomer composition for a shell part to form a polymer of a shell part partially covering the outer surface of the core part (shell part forming step) It is possible to adopt a method that goes through.
- a specific polymerization procedure first, in the core part forming step, polymerization is performed on the monomer composition for the core part, which is obtained by mixing the monomer that forms the core part and the emulsifier in a polymerization solvent such as water.
- the above-described core shell is polymerized by polymerizing a shell part monomer composition containing a monomer that forms the shell part in the presence of the particulate polymer constituting the core part.
- a first particulate polymer having a structure can be obtained.
- the monomer composition for the shell part is preferably supplied to the polymerization system in a plurality of times or continuously.
- the addition of the monomer composition for the shell portion into the polymerization system is preferably performed in a short time.
- the time from the start of addition of the monomer composition for shell part to the end of addition (addition time) varies depending on the production scale, etc., but is preferably 1 hour or less, more preferably 40 minutes or less, still more preferably 20 minutes or less, More preferably, it is 10 minutes or less, particularly preferably 5 minutes or less, and most preferably 3 minutes or less (substantially collectively added).
- the polymer constituting the shell part is formed in a particle shape to form a shell part structure, and this shell part structure is bonded to the core part to partially cover the core part.
- a shell portion can be formed.
- the monomer that forms the polymer of the shell part preferably includes a hydrophobic monomer, and particularly preferably includes an aromatic vinyl monomer.
- the amount of the emulsifier used for the polymerization of the shell part is reduced or the temperature during the polymerization of the shell part is increased, a shell part that partially covers the core part tends to be easily formed. Therefore, a shell part that partially covers the core part can also be formed by appropriately adjusting the amount of emulsifier and the polymerization temperature.
- the binder composition of the present invention has an electrolyte swelling degree of more than 100% by mass and 200% by mass and a glass transition temperature of ⁇ 10 ° C. or more and 40 ° C. or less. It is preferable to contain a second particulate polymer. And when the 2nd particulate polymer forms an electrode compound-material layer using the binder composition of this invention, while mainly exhibiting favorable binding property, it expands and shrinks the electrode active material. Fully restrains and exerts the function of suppressing the swelling of the electrode. In the present invention, the polymer included in the first particulate polymer is not included in the second particulate polymer.
- the degree of swelling of the electrolyte solution of the second particulate polymer needs to be more than 100% by mass and 200% by mass or less, preferably 120% by mass or more, and more preferably 140% by mass or more. More preferably, it is preferably 180% by mass or less, and more preferably 160% by mass or less.
- the electrolyte solution swelling degree of the second particulate polymer more than 100% by mass, it is possible to suppress the decrease in ionic conductivity and the deterioration of the electrical characteristics such as the rate characteristics of the secondary battery. it can.
- an electrode active material can fully be restrained, As a result, the cycling characteristics of a secondary battery improve.
- the glass transition temperature of the second particulate polymer needs to be ⁇ 10 ° C. or higher and 40 ° C. or lower, preferably ⁇ 5 ° C. or higher, more preferably 0 ° C. or higher, 30 It is preferably at most 0 ° C, more preferably at most 20 ° C, still more preferably at most 15 ° C.
- the electrolyte solution swelling degree and the glass transition temperature of the second particulate polymer are not particularly limited, and the type and amount of the monomer used for forming the second particulate polymer, and the second particles It can be adjusted by changing the molecular weight and crosslink density of the polymer.
- the second particulate polymer preferably has a number average particle diameter of 100 nm or more, more preferably 120 nm or more, preferably 200 nm or less, and more preferably 170 nm or less.
- the number average particle diameter is within the above range, it is possible to satisfactorily achieve suppression of expansion and contraction of the electrode active material and reduction of resistance of the electrode mixture layer.
- the number average particle diameter of the first particulate polymer and the second particulate polymer described above is the same as the number average particle diameter of the second particulate polymer. It is preferably 1 times or more, more preferably 1.5 times or more, further preferably 2 times or more, preferably 5 times or less, more preferably 4 times or less, More preferably, it is 3 times or less.
- the particle diameter ratio of the number average particle diameter of the first particulate polymer and the second particulate polymer is within the above range, the first particulate weight
- first particulate weight can exhibit the desired function satisfactorily, and both the rate characteristics and the cycle characteristics of the secondary battery can be achieved at a high level.
- any polymer may be used as long as it has the above-described properties and is a polymer that exists in a particulate state in an aqueous medium as a dispersion medium. can do.
- the polymer constituting the second particulate polymer is not particularly limited.
- a copolymer having a conjugated diene monomer unit and an aromatic vinyl monomer unit (C ) Can be used. Therefore, hereinafter, a copolymer (C) having a conjugated diene monomer unit and an aromatic vinyl monomer unit will be described as an example of a polymer that can constitute the second particulate polymer.
- the conjugated diene monomer capable of forming the conjugated diene monomer unit of the copolymer (C) is not particularly limited, and is mentioned in the section of “First particulate polymer”. Things can be used. Of these, 1,3-butadiene is preferred.
- a conjugated diene monomer may be used individually by 1 type, and may be used combining two or more types by arbitrary ratios.
- the proportion of the conjugated diene monomer unit in the copolymer (C) is preferably 5% by mass or more, more preferably 15% by mass or more, still more preferably 20% by mass or more, and preferably 70%. It is at most mass%, more preferably at most 55 mass%, still more preferably at most 40 mass%. If the ratio of the conjugated diene monomer unit in the second particulate polymer is within the above range, the cycle characteristics excellent in the secondary battery can be exhibited.
- aromatic vinyl monomer that can form the aromatic vinyl monomer unit of the copolymer (C) is not particularly limited, and those listed in the section of “First particulate polymer” Can be used. Of these, styrene is preferred.
- an aromatic vinyl monomer may be used individually by 1 type, and may be used combining two or more types by arbitrary ratios.
- the proportion of the aromatic vinyl monomer unit in the copolymer (C) is preferably 10% by mass or more, more preferably 30% by mass or more, still more preferably 50% by mass or more, and preferably 90%. It is not more than mass%, more preferably not more than 80 mass%, still more preferably not more than 70 mass%. If the ratio of the aromatic vinyl monomer unit in the second particulate polymer is within the above range, the cycle characteristics excellent in the secondary battery can be exhibited.
- the copolymer (C) may have a monomer unit other than the conjugated diene monomer unit and the aromatic vinyl monomer unit described above.
- the copolymer (C) is composed of an ethylenically unsaturated carboxylic acid monomer unit, a vinyl cyanide monomer unit in addition to a conjugated diene monomer unit and an aromatic vinyl monomer unit.
- the ethylenically unsaturated carboxylic acid monomer capable of forming the ethylenically unsaturated carboxylic acid monomer unit of the copolymer (C) is not particularly limited, and “the first particulate polymer” Can be used. Of these, acrylic acid, methacrylic acid and itaconic acid are preferred. In addition, an ethylenically unsaturated carboxylic acid monomer may be used individually by 1 type, and may be used combining two or more types by arbitrary ratios.
- the proportion of the ethylenically unsaturated carboxylic acid monomer unit in the copolymer (C) is preferably 0.1% by mass or more, more preferably 0.2% by mass or more, and further preferably 0.3% by mass. It is above, Preferably it is 10 mass% or less, More preferably, it is 8 mass% or less, More preferably, it is 5 mass% or less.
- the vinyl cyanide monomer that can form the vinyl cyanide monomer unit of the copolymer (C) is not particularly limited, and is mentioned in the section of “First particulate polymer”. Can be used. Of these, acrylonitrile and methacrylonitrile are preferable.
- a vinyl cyanide-type monomer may be used individually by 1 type, and may be used combining two or more types by arbitrary ratios.
- the proportion of the vinyl cyanide monomer units in the copolymer (C) is preferably 0.1% by mass or more, more preferably 1% by mass or more, preferably 10% by mass or less, more preferably It is 8 mass% or less.
- the (meth) acrylic acid ester monomer that can form the (meth) acrylic acid ester monomer unit of the copolymer (C) is not particularly limited, and “first particulate polymer” Those mentioned in the section can be used.
- a (meth) acrylic acid ester monomer may be used individually by 1 type, and may be used combining two or more types by arbitrary ratios.
- the ratio of the (meth) acrylic acid ester monomer unit in the copolymer (C) is preferably 10% by mass or less, more preferably 8% by mass or less, and further preferably 5% by mass or less.
- Examples of the unsaturated monomer containing a hydroxyalkyl group that can form an unsaturated monomer unit containing a hydroxyalkyl group of the copolymer (C) include ⁇ -hydroxyethyl acrylate, ⁇ -hydroxy Ethyl methacrylate, hydroxypropyl acrylate, hydroxypropyl methacrylate, hydroxybutyl acrylate, hydroxybutyl methacrylate, 3-chloro-2-hydroxypropyl methacrylate, di- (ethylene glycol) maleate, di- (ethylene glycol) itaconate, 2-hydroxyethyl maleate And bis (2-hydroxyethyl) maleate and 2-hydroxyethyl methyl fumarate.
- ⁇ -hydroxyethyl acrylate is preferred.
- these may be used individually by 1 type and may be used combining two or more types by arbitrary ratios.
- the ratio of the unsaturated monomer unit containing the hydroxyalkyl group in a copolymer (C) becomes like this.
- it is 0.1 mass% or more, More preferably, it is 0.2 mass% or more, More preferably, it is 0.3. It is at least 10 mass%, preferably at most 10 mass%, more preferably at most 5 mass%, still more preferably at most 3 mass%.
- examples of the unsaturated carboxylic acid amide monomer that can form the unsaturated carboxylic amide monomer unit of the copolymer (C) include acrylamide, methacrylamide, N-methylol acrylamide, and N-methylol methacrylamide. , N, N-dimethylacrylamide and the like. Of these, acrylamide and methacrylamide are preferable. In addition, these may be used individually by 1 type and may be used combining two or more types by arbitrary ratios.
- the proportion of the unsaturated carboxylic acid amide monomer unit in the copolymer (C) is preferably 10% by mass or less, more preferably 8% by mass or less, and further preferably 5% by mass or less.
- the 2nd particulate polymer which consists of a copolymer (C) mentioned above is polymerizing the monomer composition for 2nd particulate polymers containing the monomer mentioned above in an aqueous solvent, for example.
- the content ratio of each monomer in the monomer composition for the second particulate polymer is usually the corresponding repeating unit (monomer unit) in the desired second particulate polymer. Same as the content ratio.
- the polymerization mode is not particularly limited, and any method such as a solution polymerization method, a suspension polymerization method, a bulk polymerization method, and an emulsion polymerization method can be used.
- the polymerization reaction for example, any method such as ionic polymerization, radical polymerization, and living radical polymerization can be used.
- Production efficiency such as being easy to obtain a high molecular weight, and being able to be used in the production of a binder composition as it is, since the polymer is obtained in a state of being dispersed in water, so that redispersion treatment is unnecessary.
- the emulsion polymerization method is particularly preferable.
- the emulsion polymerization can be performed according to a conventional method.
- seed polymerization may be performed using seed particles.
- the polymerization conditions can also be arbitrarily selected depending on the polymerization method and the type of polymerization initiator.
- the aqueous dispersion of polymer particles obtained by the above-described polymerization method is, for example, an alkali metal (eg, Li, Na, K, Rb, Cs) hydroxide, ammonia, an inorganic ammonium compound (eg, NH 4 Cl, etc.). ),
- an organic amine compound eg, ethanolamine, diethylamine, etc.
- pH is usually in the range of 5 to 10, preferably 5 to 9.
- the content of the first particulate polymer is 30 mass per 100 mass parts in total of the first particulate polymer and the second particulate polymer in terms of solid content. Part or more, preferably 50 parts by weight or more, more preferably 70 parts by weight or more, further preferably 95 parts by weight or less, and more preferably 90 parts by weight or less. More preferably, it is 85 parts by mass or less. If the content of the first particulate polymer is within the above range, the rate characteristics and cycle characteristics of the secondary battery can be achieved at a high level.
- the binder composition of the present invention includes a water-soluble polymer, a conductive auxiliary agent, a reinforcing material, a leveling agent, a viscosity, in addition to the particulate binder (the first particulate polymer and the second particulate polymer).
- You may contain components, such as a regulator and electrolyte solution additive. These are not particularly limited as long as they do not affect the battery reaction, and known ones such as those described in International Publication No. 2012/115096 can be used. Moreover, these components may be used individually by 1 type, and may be used combining two or more types by arbitrary ratios.
- the binder 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-mentioned components and an aqueous medium such as water are mixed using a mixing machine such as a ball mill, a sand mill, a bead mill, a pigment disperser, a grinder, an ultrasonic disperser, a homogenizer, a planetary mixer, and a fill mix. By mixing, a binder composition can be prepared. Each particulate polymer can be mixed as it is in the form of an aqueous dispersion when it is prepared by polymerizing the monomer composition in an aqueous solvent. Moreover, when mixing a particulate polymer in the state of an aqueous dispersion, you may use the water in an aqueous dispersion as said aqueous medium.
- a mixing machine such as a ball mill, a sand mill, a bead mill, a pigment
- the slurry composition for secondary battery electrodes of the present invention is an aqueous slurry composition using an aqueous medium as a dispersion medium, and includes an electrode active material and the binder composition described above. That is, the slurry composition for a secondary battery electrode of the present invention includes at least an electrode active material, the above-described first particulate polymer, and a dispersion medium such as water. Optionally, the second particulate polymer and It further contains other components. And since the slurry composition for secondary battery electrodes of this invention contains the binder composition mentioned above, the electrode which has the electrode compound-material layer formed using the said slurry composition used the said electrode. The secondary battery can exhibit excellent rate characteristics and cycle characteristics. In addition, although the case where the slurry composition for secondary battery electrodes is a slurry composition for lithium ion secondary battery electrodes is demonstrated as an example below, this invention is not limited to the following example.
- the electrode active material is a substance that transfers electrons in the electrodes (positive electrode and negative electrode) of the lithium ion secondary battery.
- As the electrode active material (positive electrode active material, negative electrode active material) of the lithium ion secondary battery a material that can occlude and release lithium is usually used.
- a compound containing a transition metal for example, a transition metal oxide, a transition metal sulfide, a composite metal oxide of lithium and a transition metal, or the like can be used.
- a transition metal Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Mo etc. are mentioned, for example.
- transition metal oxide for example, 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 , amorphous Examples include MoO 3 , amorphous V 2 O 5 , and amorphous V 6 O 13 .
- the composite metal oxide of lithium and transition metal 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. It is done.
- lithium-containing composite metal oxide having a layered structure examples include lithium-containing cobalt oxide (LiCoO 2 ), lithium-containing nickel oxide (LiNiO 2 ), and Co—Ni—Mn lithium-containing composite oxide (Li (Co Mn Ni) O 2 ), Ni—Mn—Al lithium-containing composite oxide, Ni—Co—Al lithium-containing composite oxide, and solid solution of LiMaO 2 and Li 2 MbO 3 .
- examples of the Co—Ni—Mn lithium-containing composite oxide include Li [Ni 0.5 Co 0.2 Mn 0.3 ] O 2 and Li [Ni 1/3 Co 1/3 Mn 1/3 ] O 2 .
- 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+.
- Examples of such a solid solution include Li [Ni 0.17 Li 0.2 Co 0.07 Mn 0.56 ] O 2 .
- the “average oxidation state” indicates an average oxidation state of the “one or more transition metals”, and is calculated from the molar amount and valence of the transition metal.
- lithium-containing composite metal oxide having a spinel structure examples include lithium manganate (LiMn 2 O 4 ) and 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
- Specific examples include Li s [Mn 2-t Mc t] O 4 , such as LiNi 0.5 Mn 1.5 O 4.
- Mc represents one or more transition metals having an average oxidation state of 4+.
- Mc 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.
- Examples of the lithium-containing composite metal oxide having an olivine type structure include olivine type phosphorus represented by Li y MdPO 4 such as olivine type lithium iron phosphate (LiFePO 4 ) and olivine type lithium manganese phosphate (LiMnPO 4 ).
- An acid lithium compound is mentioned.
- Md 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.
- Md may be partially substituted with another metal. Examples of the metal that can be substituted include Cu, Mg, Zn, V, Ca, Sr, Ba, Ti, Al, Si, B, and Mo.
- examples of the negative electrode active material 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.
- the carbon-based negative electrode active material refers to an active material having carbon as a main skeleton capable of inserting lithium (also referred to as “dope”).
- examples of the carbon-based negative electrode active material include carbonaceous materials and graphite. Quality materials.
- 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.
- 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.
- 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 includes an element capable of inserting lithium in the structure, and the theoretical electric capacity per unit mass when lithium is inserted is 500 mAh /
- the active material which is more than g.
- 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.
- an active material containing silicon is preferable as the metal-based negative electrode active material. 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 containing silicon, SiO, SiO x , and a composite of a Si-containing material obtained by coating or combining a Si-containing material with conductive carbon and conductive carbon. Etc.
- silicon type negative electrode active materials may be used individually by 1 type, and may be used in combination of 2 types.
- the alloy containing silicon examples include an alloy composition containing silicon, aluminum, and a transition metal such as iron, and further containing a rare earth element such as tin and yttrium.
- SiO x 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. Then, SiO x, for example, can be formed by using a disproportionation reaction of silicon monoxide (SiO). Specifically, SiO x 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.
- SiO x 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
- 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.
- an organic gas and / or steam can be mentioned.
- a method of coating the surface of the SiO particles by a chemical vapor deposition method using an organic gas a method of forming composite particles (granulation) of the SiO particles and graphite or artificial graphite by a mechanochemical method, etc. It can also be obtained by a known method.
- the binder composition that can be blended in the slurry composition for a lithium ion secondary battery electrode includes the water, the first particulate polymer, and optionally the second particulate polymer, for the secondary battery electrode of the present invention.
- a binder composition can be used.
- the compounding quantity of a binder composition is not specifically limited, For example, per 100 mass parts of electrode active materials, 1st particulate polymer and 2nd particulate polymer are 0.5 in total in conversion of solid content. It can be made into the quantity used as the mass part or more and 3.0 mass parts or less.
- Other components that can be blended in the slurry composition are not particularly limited, and examples thereof include those similar to other components that can be blended in the binder composition of the present invention.
- the other component may be used individually by 1 type, and may be used combining two or more types by arbitrary ratios.
- the slurry composition described above 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.
- the mixing of each of the above components and the aqueous medium can usually be carried out in the range of room temperature to 80 ° C. for 10 minutes to several hours.
- water is usually used as the aqueous medium, 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 water used as the aqueous medium may include water contained in the binder composition.
- the secondary battery electrode slurry composition (negative electrode slurry composition and positive electrode slurry composition) prepared using the secondary battery electrode binder composition of the present invention is a secondary battery electrode (negative electrode and positive electrode).
- the electrode for a secondary battery 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 the first particles described above. Polymer.
- each component contained in the electrode mixture layer is contained in the slurry composition for secondary battery electrodes, and a suitable abundance ratio of each component is in the slurry composition. It is the same as the preferred abundance ratio of each component.
- the said secondary battery electrode uses the binder composition for secondary battery electrodes of this invention, the rate characteristic and cycling characteristics which were excellent in the secondary battery can be exhibited.
- the secondary battery electrode of the present invention includes, for example, a step of applying the above-described slurry composition for a secondary battery electrode on a current collector (application step), and a secondary battery applied on the current collector.
- the electrode slurry composition is dried to produce an electrode mixture layer on the current collector (drying step). That is, the electrode mixture layer in the secondary battery electrode of the present invention is formed of a dried product of the secondary battery electrode slurry composition of the present invention.
- the polymer contained in the first particulate polymer and / or the second particulate polymer described above contains a monomer unit derived from a crosslinkable monomer (crosslinkable monomer unit).
- the polymer containing the crosslinkable monomer unit may be crosslinked at the time of drying the slurry composition for secondary battery electrodes or at the time of heat treatment optionally performed after drying (that is, the two of the present invention).
- the electrode mixture layer in the secondary battery electrode may contain a cross-linked product of the first particulate polymer and / or the second particulate polymer described above).
- 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.
- an electrically conductive and electrochemically durable material is used as the current collector to which the slurry composition is applied.
- a current collector for example, a current collector made of iron, copper, aluminum, nickel, stainless steel, titanium, tantalum, gold, platinum, or the like can be used.
- a collector used for a negative electrode copper foil is especially preferable.
- the current collector used for the positive electrode is particularly preferably an aluminum foil.
- the said material may be used individually by 1 type, and may be used combining two or more types by arbitrary ratios.
- a method for drying the slurry composition on the current collector is not particularly limited, and a known method can be used. A drying method is mentioned. Thus, by drying the electrode slurry composition on the current collector, an electrode mixture layer is formed on the current collector to obtain a secondary battery electrode including the current collector and the electrode mixture layer. be able to.
- press process a pressurizing process
- the adhesion between the electrode mixture layer and the current collector can be improved.
- the electrode mixture layer can be densified and the secondary battery can be miniaturized.
- the electrode mixture layer includes a curable polymer, it is preferable to cure the polymer after the electrode mixture layer is formed.
- the porosity of the electrode mixture layer is preferably 10.7% or more, more preferably 15.2% or more, More preferably, press working should be performed so as to be 17.4% or more, and preferably 24.1% or less, more preferably 22.8% or less, and further preferably 21.9% or less. It is good to press.
- the binder composition for secondary battery electrodes of the present invention is used, particularly when the electrode mixture layer is densified with a porosity of 24.1% or less, compared to the case of using other binder compositions. Therefore, the secondary battery can exhibit excellent rate characteristics and cycle characteristics. When the porosity is less than 10.7%, the rate characteristics and the cycle characteristics may be deteriorated even when the secondary battery electrode binder composition of the present invention is used.
- the bulk density of the negative electrode mixture layer is preferably 1.70 g / cm 3 or more, and preferably 1.73 g / cm 3 or more. more preferably, more preferably to 1.75 g / cm 3 or more, preferably to 2.00 g / cm 3 or less, more preferably, to 1.90 g / cm 3 or less, 1.85 g / cm 3 More preferably, it is as follows. If the binder composition for secondary battery electrodes of the present invention is used, even when the negative electrode mixture layer is densified with a bulk density of 1.70 g / cm 3 or more, another binder composition was used. Compared to the case, the secondary battery can exhibit excellent rate characteristics and cycle characteristics. In addition, when it is densified until the bulk density exceeds 2.00 g / cm 3 , even when the binder composition for secondary battery electrodes of the present invention is used, rate characteristics and cycle characteristics are deteriorated. There is a risk of doing.
- 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. And since the secondary battery of this invention is equipped with the electrode for secondary batteries of this invention, it is excellent in a rate characteristic and cycling characteristics.
- the secondary battery is a lithium ion secondary battery will be described as an example, but the present invention is not limited to the following example.
- the secondary battery electrode of the present invention is used as at least one of a positive electrode and a negative electrode. That is, the positive electrode of the lithium ion secondary battery may be an electrode of the present invention and the negative electrode may be another known negative electrode, and the negative electrode of the lithium ion secondary battery is an electrode of the present invention and the positive electrode is another known positive electrode. In addition, both the positive electrode and the negative electrode of the lithium ion secondary battery may be the electrode of the present invention.
- an electrolytic solution in which an electrolyte is dissolved in a solvent can be used.
- the solvent an organic solvent capable of dissolving the electrolyte can be used.
- the solvent include alkyl carbonate solvents such as ethylene carbonate, propylene carbonate, and ⁇ -butyrolactone, 2,5-dimethyltetrahydrofuran, tetrahydrofuran, diethyl carbonate, ethyl methyl carbonate, dimethyl carbonate, methyl acetate, dimethoxyethane. , Dioxolane, methyl propionate, methyl formate and the like can be used.
- a lithium salt can be used as the electrolyte.
- the lithium salt for example, those described in JP 2012-204303 A can be used.
- LiPF 6 , LiClO 4 , and CF 3 SO 3 Li are preferable as the electrolyte because they are easily dissolved in an organic solvent and exhibit a high degree of dissociation.
- ⁇ Separator> As the separator, for example, those described in JP 2012-204303 A can be used. Among these, the thickness of the separator as a whole can be reduced, thereby increasing the ratio of the electrode active material in the lithium ion secondary battery and increasing the capacity per volume.
- a microporous film made of a series resin polyethylene, polypropylene, polybutene, polyvinyl chloride is preferred.
- a lithium ion secondary battery is formed by stacking a positive electrode and a negative electrode through a separator, and winding or folding the positive electrode and a negative electrode according to the shape of the battery as necessary. Can be manufactured by injecting and sealing.
- an overcurrent prevention element such as a fuse or a PTC element, an expanded metal, a lead plate, etc. may be provided as necessary.
- the shape of the lithium ion secondary battery may be any of, for example, a coin shape, a button shape, a sheet shape, a cylindrical shape, a square shape, a flat shape, and the like.
- the polymer (core part of the core part) is a measurement sample under the same polymerization conditions as those for the core part and the shell part.
- Polymer and shell dispersion polymers were prepared.
- the core part polymer, the shell part polymer, and the aqueous dispersion of the second particulate polymer described above were each dried in an environment of 50% humidity and 23 to 25 ° C. for 3 days to obtain a thickness of 3 ⁇ 0.
- the film was formed to 3 mm.
- the film formed was cut into a diameter of 12 mm and precisely weighed. Let the mass of the film piece obtained by cutting be W0.
- Swelling degree (% by mass) (W1 / W0) ⁇ 100 ⁇ Glass transition temperature>
- the core portion polymer, the shell portion polymer, and the aqueous dispersion of the second particulate polymer were dried to prepare measurement samples.
- the glass transition temperature was measured using the differential thermal-analysis measuring apparatus (The SII nanotechnology company make, product name "EXSTAR DSC6220"). Specifically, 10 mg of a measurement sample is weighed into an aluminum pan, and an empty aluminum pan is used as a reference. The measurement temperature is between ⁇ 100 ° C. and 500 ° C., the heating rate is 10 ° C./min, and the temperature is normal. DSC curve was measured.
- the particle diameter-number cumulative distribution of the particulate polymer was measured using a laser diffraction / scattering particle size distribution measuring device, and the cumulative distribution value was 50%.
- the particle diameter was determined as the number average particle diameter.
- Mass ratio of shell part in first particulate polymer is the sum M1 of the masses of all monomers contained in the core part-forming monomer composition and the total mass contained in the shell part-forming monomer composition. It calculated with the following formula
- Mass ratio of shell part (mass%) ⁇ M2 / (M1 + M2) ⁇ ⁇ 100 ⁇ Peel strength of electrode>
- the produced negative electrode for a lithium ion secondary battery was cut into a rectangular shape having a width of 1.0 cm and a length of 10 cm to form a test piece, and fixed with the surface on the negative electrode mixture layer side facing up. And the cellophane tape was affixed on the surface by the side of the negative mix layer of a test piece. At this time, the cellophane tape defined in JIS Z1522 was used.
- the stress was measured when the cellophane tape was peeled from the one end of the test piece in the 180 ° direction (the other end side of the test piece) at a speed of 50 mm / min.
- the measurement was performed 10 times, the average value of the stress was determined, and this was taken as the peel strength (N / m), and evaluated according to the following criteria. It shows that the binding property of the negative mix layer with respect to a collector is excellent, so that peel strength is large.
- the porosity and bulk density of the negative electrode mixture layer of the produced negative electrode for a lithium ion secondary battery were calculated based on the following equations.
- the true density of the negative electrode composite material layer was calculated from the density (theoretical value) of the solid content contained in the slurry composition for negative electrode.
- the ratio of the battery capacity at 3.0C to the battery capacity at 1.0C was calculated as a percentage to obtain charge / discharge rate characteristics, and evaluated according to the following criteria.
- the prepared pouch-type lithium ion secondary battery was allowed to stand for 24 hours, and then charged to 4.4 V and discharged to 3.0 V at a charge / discharge rate of 0.2 C, and the initial capacity C0 was measured. . Further, in a 45 ° C. environment, a charge / discharge cycle of charging to 4.4 V at a charge / discharge rate of 1.0 C and discharging to 3.0 V was repeated, and the capacity C1 after 300 cycles was measured.
- Capacity maintenance ratio ⁇ C (C1 / C0) ⁇ 100 (%). The higher the capacity retention rate, the lower the discharge capacity and the better the high-temperature cycle characteristics.
- Example 1 ⁇ Preparation of first particulate polymer>
- a reactor equipped with a stirrer 76.8 parts of butyl acrylate (96.0% in the core part) as a (meth) acrylic acid ester monomer, 1.6 parts of methacrylic acid as an ethylenically unsaturated carboxylic acid monomer (2.0% in the core part), 1.6 parts of acrylonitrile (2.0% in the core part) as the vinyl cyanide monomer, 0.3 parts of sodium dodecylbenzenesulfonate as the emulsifier, and excess as the polymerization initiator.
- the first particulate polymer in which the shell part partially covers the outer surface of the core part (a plurality of shell part structures are present on the outer surface of the core part) by a scanning electron microscope (SEM) It was confirmed that an aqueous dispersion was obtained. Further, the number average particle diameter of the first particulate polymer, the electrolyte solution swelling degree and the glass transition temperature of the polymer in the core part and the polymer in the shell part were measured by the method described above. The results are shown in Table 2.
- the prepared slurry composition for negative electrode was applied on a copper foil (current collector) having a thickness of 15 ⁇ m with a comma coater so that the amount applied was 13.5 to 14.5 mg / cm 2 and dried. In addition, drying was performed by conveying copper foil over 2 minutes in the 70 degreeC oven at the speed
- the basis weight of the negative electrode mixture layer after pressing was 14.0 mg / cm 2 , and the porosity was 18.8%. And the peel strength was evaluated about the produced negative electrode.
- the results are shown in Table 2.
- ⁇ Preparation of positive electrode for lithium ion secondary battery> 96.0 parts of LiCoO 2 as a positive electrode active material, 2.0 parts of acetylene black (manufactured by Denki Kagaku Kogyo Co., Ltd., HS-100) as a conductive auxiliary agent, PVDF (polyvinylidene fluoride, 2.0 parts of Kureha Chemical Co., Ltd.
- the obtained slurry composition for positive electrodes was apply
- a single-layer polypropylene separator (width 65 mm, length 500 mm, thickness 25 ⁇ m; manufactured by a dry method; porosity 55%) was prepared and cut into a 5 cm ⁇ 5 cm square.
- the aluminum packaging material exterior was prepared as a battery exterior.
- the produced positive electrode was cut out into a 4 cm x 4 cm square, and it has arrange
- a square separator was disposed on the surface of the positive electrode mixture layer side of the positive electrode.
- Examples 2 to 5, 13, 14 A first particulate polymer was prepared in the same manner as in Example 1 except that the monomers shown in Table 1 were used in the amounts shown in Table 1. In addition, although these 1st particulate polymers differ in the mass ratio of a shell part from what was used in Example 1, the composition itself of a core part and a shell part is the same as Example 1.
- FIG. By means of a scanning electron microscope (SEM), the shell part of the first particulate polymer partially covers the outer surface of the core part (a plurality of shell part structures are present on the outer surface of the core part). Confirmed).
- SEM scanning electron microscope
- the number average particle diameter of the first particulate polymer, the electrolyte solution swelling degree and the glass transition temperature of the polymer in the core part and the polymer in the shell part were measured by the method described above. The results are shown in Table 2. And except having used the said 1st particulate polymer, it carried out similarly to Example 1, and produced a binder composition, the slurry composition for negative electrodes, a negative electrode, a positive electrode, and a secondary battery, and carried out similarly to Example 1. Evaluation was performed. The results are shown in Table 2.
- Example 6 The binder composition and the negative electrode slurry composition were the same as in Example 1 except that the aqueous dispersion of the first particulate polymer and the aqueous dispersion of the second particulate polymer were used in the ratios shown in Table 2.
- a negative electrode, a positive electrode, and a secondary battery were prepared and evaluated in the same manner as in Example 1. The results are shown in Table 2.
- Example 10 A first particulate polymer was prepared in the same manner as in Example 1 except that the monomers shown in Table 1 were used in the amounts shown in Table 1.
- SEM scanning electron microscope
- the shell part of the first particulate polymer partially covers the outer surface of the core part (a plurality of shell part structures are present on the outer surface of the core part). )It was confirmed. Further, the number average particle diameter of the first particulate polymer, the electrolyte solution swelling degree and the glass transition temperature of the polymer in the core part and the polymer in the shell part were measured by the method described above. The results are shown in Table 2.
- Example 2 Except having used the said 1st particulate polymer, it carried out similarly to Example 1, and produced a binder composition, the slurry composition for negative electrodes, a negative electrode, a positive electrode, and a secondary battery, and carried out similarly to Example 1. Evaluation was performed. The results are shown in Table 2.
- Example 11 When producing a negative electrode for a lithium ion secondary battery, the same procedure as in Example 1 was conducted except that the negative electrode raw material was pressed with a roll press so that the bulk density of the negative electrode mixture layer was 1.65 g / cm 3. , Binder composition, negative electrode slurry composition, negative electrode, positive electrode, and secondary battery were prepared. The porosity of the negative electrode mixture layer was 26.4%. Then, evaluation was performed in the same manner as in Example 1. The results are shown in Table 2.
- Example 12 A first particulate polymer was prepared in the same manner as in Example 1 except that the addition time of the monomer composition for shell portion was changed to 30 minutes. By the scanning electron microscope (SEM), the shell part of the first particulate polymer partially covers the outer surface of the core part (a plurality of shell part structures are present on the outer surface of the core part). )It was confirmed. Further, the number average particle diameter of the first particulate polymer, the electrolyte solution swelling degree and the glass transition temperature of the polymer in the core part and the polymer in the shell part were measured by the method described above. The results are shown in Table 2.
- Example 2 Except having used the said 1st particulate polymer, it carried out similarly to Example 1, and produced a binder composition, the slurry composition for negative electrodes, a negative electrode, a positive electrode, and a secondary battery, and carried out similarly to Example 1. Evaluation was performed. The results are shown in Table 2.
- Example 17 The binder composition, the negative electrode slurry composition, the negative electrode, the positive electrode, and the two were the same as in Example 1 except that the second particulate polymer was not used and the amount of the first particulate polymer was 1 part.
- a secondary battery was prepared and evaluated in the same manner as in Example 1. The results are shown in Table 2.
- Example 1 A first particulate polymer was prepared in the same manner as in Example 1 except that the addition time of the monomer composition for shell portion was changed to 180 minutes. It was confirmed by a scanning electron microscope (SEM) that the shell part of the first particulate polymer covered the entire outer surface of the core part. Further, the number average particle diameter of the first particulate polymer, the electrolyte solution swelling degree and the glass transition temperature of the polymer in the core part and the polymer in the shell part were measured by the method described above. The results are shown in Table 2.
- Example 2 Except having used the said 1st particulate polymer, it carried out similarly to Example 1, and produced a binder composition, the slurry composition for negative electrodes, a negative electrode, a positive electrode, and a secondary battery, and carried out similarly to Example 1. Evaluation was performed. The results are shown in Table 2.
- Comparative Example 2 When producing a negative electrode for a lithium ion secondary battery, the same procedure as in Comparative Example 1 was conducted except that the negative electrode raw material was pressed with a roll press so that the bulk density of the negative electrode mixture layer was 1.65 g / cm 3. , Binder composition, negative electrode slurry composition, negative electrode, positive electrode, and secondary battery were prepared. The porosity of the negative electrode mixture layer was 26.4%. Then, evaluation was performed in the same manner as in Example 1. The results are shown in Table 2.
- BA Butyl acrylate (acrylate monomer)
- MAA Methacrylic acid (ethylenically unsaturated carboxylic acid monomer)
- AN Acrylonitrile (vinyl cyanide monomer)
- BD 1,3-butadiene (conjugated diene monomer)
- ST Styrene (aromatic vinyl monomer)
- CMC-Na Carboxymethylcellulose sodium salt
- Example 1 in Table 2 exhibits the same performance as Example 11, whereas Comparative Example 1 is inferior to Comparative Example 2 in peel strength, rate characteristics, and cycle characteristics.
- Comparative Example 1 is inferior to Comparative Example 2 in peel strength, rate characteristics, and cycle characteristics.
- the binder composition for secondary battery electrodes which can exhibit the rate characteristic and cycling characteristics which were excellent in the secondary battery can be provided.
- the slurry composition for secondary battery electrodes which can form the electrode compound-material layer which can exhibit the rate characteristic and cycling characteristics excellent in the secondary battery can be provided.
- a secondary battery excellent in rate characteristics and cycle characteristics 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、電極合材層の形成に用いられるバインダー組成物中の結着材成分の改良が盛んに行われている。
また、本発明は、二次電池に優れたレート特性およびサイクル特性を発揮させることができる電極合材層を形成可能な二次電池電極用スラリー組成物を提供することを目的とする。
更に、本発明は、二次電池に優れたレート特性およびサイクル特性を発揮させることが可能な二次電池用電極を提供することを目的とする。
そして、本発明は、レート特性およびサイクル特性に優れる二次電池を提供することを目的とする。
なお、本発明において、コア部を構成する重合体、シェル部を構成する重合体の「電解液膨潤度」は、本明細書の実施例に記載の測定方法を用いて測定することができる。
なお、本発明において、コア部を構成する重合体、シェル部を構成する重合体の「ガラス転移温度」は、本明細書の実施例に記載の測定方法を用いて測定することができる。
なお、本発明において、第1粒子状重合体中の「シェル部の質量比率」は、本明細書の実施例に記載の算出方法を用いて算出することができる。
なお、本発明において「(メタ)アクリル」とは、アクリルおよび/またはメタクリルを意味する。
なお、本発明において、第2粒子状重合体の「電解液膨潤度」および「ガラス転移温度」は、本明細書の実施例に記載の測定方法を用いて測定することができる。
なお、本発明において、粒子状重合体の「個数平均粒子径」は、本明細書の実施例に記載の測定方法を用いて測定することができる。
なお、本発明において、「気孔率」とは、電極合材層の真密度に対する、電極合材層の真密度とかさ密度との差の割合を百分率で表した値であり、例えば下記の式を用いて算出することができる。
気孔率(%)=〔1-{(電極合材層の目付量/電極合材層の厚み)/電極合材層の真密度}〕×100
また、本発明によれば、二次電池に優れたレート特性およびサイクル特性を発揮させることができる電極合材層を形成可能な二次電池電極用スラリー組成物を提供することができる。
更に、本発明によれば、二次電池に優れたレート特性およびサイクル特性を発揮させることが可能な二次電池用電極を提供することができる。
また、本発明によれば、レート特性およびサイクル特性に優れる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、二次電池電極用スラリー組成物を調製する際に用いることができる。そして、本発明の二次電池電極用バインダー組成物を用いて調製した二次電池電極用スラリー組成物は、二次電池の電極を形成する際に用いることができる。更に、本発明の二次電池は、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、水系媒体を分散媒とした水系バインダー組成物であり、粒子状の結着材と、水とを含み、任意に、二次電池の分野において一般に使用されるその他の成分を更に含有する。そして、本発明の二次電池電極用バインダー組成物は、粒子状の結着材として、コア部と、前記コア部の外表面を部分的に覆うシェル部とを備えるコアシェル構造を有する粒子状重合体(第1粒子状重合体)を使用することを特徴とする。
結着材は、本発明のバインダー組成物と電極活物質とを含む二次電池電極用スラリー組成物を用いて集電体上に電極合材層を形成することにより製造した二次電池用電極において、電極合材層に含まれる成分が電極合材層から脱離しないように保持しうる成分である。一般的に、電極合材層中の粒子状の結着材は、電解液に浸漬された際に、電解液を吸収して膨潤しながらも粒子状の形状を維持し、電極活物質同士または電極活物質と集電体とを結着させ、電極活物質が集電体から脱落するのを防ぐ。また、結着材は、電極合材層に含まれる電極活物質以外の粒子をも結着し、電極合材層の強度を維持する役割も果たしている。
上述した通り第1粒子状重合体は、コア部と、コア部の外表面を覆うシェル部とを備えるコアシェル構造を有している。また、シェル部は、コア部の外表面を部分的に覆っている。即ち、第1粒子状重合体のシェル部は、コア部の外表面を覆っているが、コア部の外表面の全体を覆ってはいない。外観上、コア部の外表面がシェル部によって完全に覆われているように見える場合であっても、シェル部の内外を連通する孔が形成されていれば、そのシェル部はコア部の外表面を部分的に覆うシェル部である。したがって、例えば、シェル部の外表面(即ち、粒子状重合体の周面)からコア部の外表面まで連通する細孔を有するシェル部を備える粒子状重合体は、上記第1粒子状重合体に含まれる。
すなわち、第1粒子状重合体は、シェル部がコア部外表面を部分的に覆う構造を備えているため、得られる電極合材層において、第1粒子状重合体はシェル部および被覆されていないコア部外表面を介して電極活物質や集電体などと接着して電極合材層の強度を保持する。一方で、当該電極合材層には、粒子状重合体表面のシェル部により覆われていない箇所に由来する、イオン伝導可能な空隙が確保されることとなる。よって、上記第1粒子状重合体によれば、電極合材層において電極活物質を拘束しつつイオン伝導性が確保されるため、二次電池に優れたレート特性およびサイクル特性を発揮させることができると推察される。
具体的には、第1粒子状重合体の一例の断面構造を図1に示すように、第1粒子状重合体100は、コア部110および複数のシェル部構造体120からなるシェル部を備えるコアシェル構造を有することが好ましい。ここで、コア部110は、この第1粒子状重合体100においてシェル部構造体120よりも内側にある部分である。また、シェル部構造体120は、コア部110の外表面110Sを覆い、複数のシェル部構造体120からなるシェル部は、通常は第1粒子状重合体100において最も外側にある部分である。そして、複数のシェル部構造体120からなるシェル部は、コア部110の外表面110Sの全体を覆っているのではなく、コア部110の外表面110Sを部分的に覆っている。
-コア部の性状-
コア部を構成する重合体(以下、「コア部の重合体」と略記する場合がある。)の電解液膨潤度は、好ましくは300質量%以上、より好ましくは400質量%以上、更に好ましくは500質量%以上であり、好ましくは900質量%以下、より好ましくは800質量%以下、更に好ましくは700質量%以下である。コア部の重合体の電解液膨潤度を300質量%以上にすることにより、イオン伝導性が確保され、二次電池のレート特性などの電気的特性を向上させることができる。一方、コア部の重合体の電解液膨潤度を900質量%以下にすることにより、電極活物質を十分に拘束することができ、二次電池のサイクル特性を向上させることができる。
なお、第1粒子状重合体のコア部の重合体としては、任意の重合体を使用することができる。そして、コア部の重合体としては、例えば、(メタ)アクリル酸エステル単量体単位を含む共重合体(A)を使用することができる。そこで、以下では、第1粒子状重合体のコア部の重合体の一例として、(メタ)アクリル酸エステル単量体単位を含む共重合体(A)について説明する。
なお、本明細書において「単量体単位を含む」とは、「その単量体を用いて得た重合体中に単量体由来の構造単位(繰り返し単位)が含まれている」ことを意味する。
なお、これらは、1種類を単独で用いてもよく、2種類以上を任意の比率で組み合わせて用いてもよい。
なお、エチレン性不飽和カルボン酸単量体は、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。
なお、シアン化ビニル系単量体は、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。
なお、共役ジエン系単量体は、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。
任意の単量体としては、例えば、アリルメタクリレート、N-メチロールアクリルアミドなどの架橋性単量体、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-メチルスチレン等のスチレン系単量体;アクリルアミド、メタクリルアミドなどの不飽和カルボン酸アミド単量体;ビニルスルホン酸、スチレンスルホン酸、アリルスルホン酸、スルホエチルメタクリレート、スルホプロピルメタクリレート、スルホブチルメタクリレートなどのスルホン酸基含有単量体およびそのアルカリ金属塩;フッ素含有(メタ)アクリル酸エステル単量体が挙げられる。
-シェル部の性状-
シェル部を構成する重合体(以下、「シェル部の重合体」と略記する場合がある。)の電解液膨潤度は、好ましくは100質量%超、より好ましくは105質量%以上、更に好ましくは110質量%以上であり、好ましくは200質量%以下、より好ましくは170質量%以下、更に好ましくは140質量%以下である。シェル部の重合体の電解液膨潤度を100質量%超にすることにより、イオン伝導性が確保され、二次電池のレート特性などの電気的特性を向上させることができる。一方、シェル部の重合体の電解液膨潤度を200質量%以下にすることにより、電極活物質を十分に拘束することができ、二次電池のサイクル特性を向上させることができる。
なお、第1粒子状重合体のシェル部の重合体としては、任意の重合体を使用することができる。そして、シェル部の重合体としては、例えば、芳香族ビニル単量体単位と、エチレン性不飽和カルボン酸単量体単位とを含む共重合体(B)を使用することができる。そこで、以下では、第1粒子状重合体のシェル部の重合体の一例として、芳香族ビニル単量体単位およびエチレン性不飽和カルボン酸単量体単位を含む共重合体(B)について説明する。
なお、芳香族ビニル単量体は、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。
共重合体(B)におけるその他の単量体単位としては、「コア部の組成」で列挙された全ての単量体単位から芳香族ビニル単量体単位およびエチレン性不飽和カルボン酸単量体単位を除いたものが挙げられる。
-シェル部の質量比率-
第1粒子状重合体中のシェル部の質量比率は、好ましくは3質量%以上、より好ましくは5質量%以上、更に好ましくは10質量%以上であり、好ましくは35質量%以下、より好ましくは30質量%以下、更に好ましくは25質量%以下である。第1粒子状重合体の質量に占めるシェル部の質量の比率が3質量%以上であることで、二次電池のサイクル特性を向上させることができる。一方、第1粒子状重合体の質量に占めるシェル部の質量の比率が35質量%以下であることで、バインダー組成物を用いて得られる電極合材層のイオン伝導性が確保され、レート特性などの電気的特性を向上させることができる。また、電極合材層と集電体の間のピール強度を向上させることができる。
また、第1粒子状重合体は、個数平均粒子径が、100nm以上であることが好ましく、150nm以上であることがより好ましく、200nm以上であることが更に好ましく、300nm以上であることが特に好ましく、1000nm以下であることが好ましく、800nm以下であることがより好ましく、600nm以下であることが更に好ましく、400nm以下であることが特に好ましい。第1粒子状重合体の個数平均粒子径が上記範囲内にあることで、電極活物質の膨張および収縮の抑制と電極合材層の抵抗の低減とを良好に達成することができるからである。
なお、第1粒子状重合体の個数平均粒子径は、例えば、乳化剤の量、単量体の量などを調整することで、適宜調整することができる。
そして、上述したコアシェル構造を有する第1粒子状重合体は、例えば、コア部の重合体の単量体と、シェル部の重合体の単量体とを用い、経時的にそれらの単量体の比率を変えて段階的に重合することにより、製造することができる。具体的には、第1粒子状重合体は、先の段階の重合体を後の段階の重合体が順次に被覆するような連続した多段階乳化重合法および多段階懸濁重合法によって製造することができる。
そして、具体的な重合手順としては、まずコア部形成工程において、水などの重合溶媒に、コア部を形成する単量体および乳化剤を混合してなるコア部用単量体組成物に、重合開始剤を入れ、一括で乳化重合することによってコア部を構成する粒子状の重合体を得る。さらにシェル部形成工程において、このコア部を構成する粒子状の重合体の存在下にシェル部を形成する単量体を含むシェル部用単量体組成物の重合を行うことによって、上述したコアシェル構造を有する第1粒子状重合体を得ることができる。
このような手法を採用することにより、シェル部を構成する重合体が粒子状に形成されシェル部構造体となり、このシェル部構造体がコア部と結合することで、コア部を部分的に覆うシェル部を形成することができる。
更に、シェル部の重合に用いる乳化剤量を少なくしたり、シェル部の重合の際の温度を上昇させたりすると、コア部を部分的に覆うシェル部を形成し易くなる傾向がある。従って、適宜乳化剤量や重合温度を調整することによっても、コア部を部分的に覆うシェル部を形成することができる。
本発明のバインダー組成物は、上述の第1粒子状重合体に加え、電解液膨潤度が100質量%超200質量%以下であり、且つ、ガラス転移温度が-10℃以上40℃以下である第2粒子状重合体を含むことが好ましい。そして、第2粒子状重合体は、本発明のバインダー組成物を用いて電極合材層を形成した際に、主に、良好な結着性を発揮すると共に、膨張および収縮する電極活物質を十分に拘束して電極の膨れを抑制する機能を発揮する。
なお、本発明において第1粒子状重合体に含まれる重合体は、第2粒子状重合体には含まれないものとする。
ここで、第2粒子状重合体の電解液膨潤度は、100質量%超200質量%以下であることが必要であり、120質量%以上であることが好ましく、140質量%以上であることがより好ましく、180質量%以下であることが好ましく、160質量%以下であることがより好ましい。第2粒子状重合体の電解液膨潤度を100質量%超にすることにより、イオン伝導性の低下を抑制して二次電池のレート特性などの電気的特性が低下するのを抑制することができる。また、第2粒子状重合体の電解液膨潤度を200質量%以下にすることにより、電極活物質が十分に拘束でき、その結果、二次電池のサイクル特性が向上する。
また、第2粒子状重合体のガラス転移温度は、-10℃以上40℃以下であることが必要であり、-5℃以上であることが好ましく、0℃以上であることがより好ましく、30℃以下であることが好ましく、20℃以下であることがより好ましく、15℃以下であることが更に好ましい。第2粒子状重合体のガラス転移温度を上述の範囲内にすることにより、結着性が十分に向上し、その結果、バインダー組成物を用いて形成した電極合材層を有する電極のピール強度が向上する。また、第2粒子状重合体のガラス転移温度を40℃以下とすることにより、プレス加工時の電極活物質の変形を抑制することができ、その結果、二次電池のレート特性が向上する。
また、第2粒子状重合体は、個数平均粒子径が、100nm以上であることが好ましく、120nm以上であることがより好ましく、200nm以下であることが好ましく、170nm以下であることがより好ましい。個数平均粒子径が上記範囲内にあることで、電極活物質の膨張および収縮の抑制と電極合材層の抵抗の低減とを良好に達成することができる。
なお、第2粒子状重合体を構成する重合体としては、上述した性状を有し、且つ、分散媒としての水系媒体中において粒子状態で存在する重合体であれば、任意の重合体を使用することができる。具体的には、第2粒子状重合体を構成する重合体としては、特に限定されることなく、例えば、共役ジエン系単量体単位および芳香族ビニル単量体単位を有する共重合体(C)を使用することができる。そこで、以下では、第2粒子状重合体を構成し得る重合体の一例として、共役ジエン系単量体単位および芳香族ビニル単量体単位を有する共重合体(C)について説明する。
なお、共役ジエン系単量体は、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。
なお、芳香族ビニル単量体は、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。
具体的には、共重合体(C)は、共役ジエン系単量体単位および芳香族ビニル単量体単位に加え、エチレン性不飽和カルボン酸単量体単位、シアン化ビニル系単量体単位、(メタ)アクリル酸エステル単量体単位、ヒドロキシアルキル基を含有する不飽和単量体単位、不飽和カルボン酸アミド単量体単位等を含むことができる。
なお、エチレン性不飽和カルボン酸単量体は、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。
なお、シアン化ビニル系単量体は、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。
なお、(メタ)アクリル酸エステル単量体は、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。
なお、これらは、1種類を単独で用いてもよく、2種類以上を任意の比率で組み合わせて用いてもよい。
なお、これらは、1種類を単独で用いてもよく、2種類以上を任意の比率で組み合わせて用いてもよい。
ここで、本発明のバインダー組成物は、第1粒子状重合体の含有量が、固形分換算で、第1粒子状重合体と第2粒子状重合体との合計100質量部当たり、30質量部以上であることが好ましく、50質量部以上であることがより好ましく、70質量部以上であることが更に好ましく、95質量部以下であることが好ましく、90質量部以下であることがより好ましく、85質量部以下であることが更に好ましい。第1粒子状重合体の含有量を上記範囲内とすれば、二次電池のレート特性とサイクル特性とを高い次元で両立させることができる。
本発明のバインダー組成物は、上記粒子状の結着材(第1粒子状重合体および第2粒子状重合体)の他に、水溶性重合体、導電助剤、補強材、レベリング剤、粘度調整剤、電解液添加剤等の成分を含有していてもよい。これらは、電池反応に影響を及ぼさないものであれば特に限られず、公知のもの、例えば国際公開第2012/115096号に記載のものを使用することができる。また、これらの成分は、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。
本発明のバインダー組成物は、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などの水系媒体とを混合することにより、バインダー組成物を調製することができる。
なお、各粒子状重合体は、水系溶媒中で単量体組成物を重合して調製した場合には、水分散液の状態でそのまま混合することができる。また、粒子状重合体を水分散液の状態で混合する場合には、水分散液中の水を上記水系媒体として使用してもよい。
本発明の二次電池電極用スラリー組成物は、水系媒体を分散媒とした水系スラリー組成物であり、電極活物質と、上述したバインダー組成物とを含む。即ち、本発明の二次電池電極用スラリー組成物は、電極活物質と、上述した第1粒子状重合体と、水などの分散媒とを少なくとも含み、任意に、第2粒子状重合体およびその他の成分を更に含有する。そして、本発明の二次電池電極用スラリー組成物は、上述したバインダー組成物を含んでいるので、当該スラリー組成物を用いて形成された電極合材層を有する電極は、当該電極を用いた二次電池に優れたレート特性およびサイクル特性を発揮させることができる。
なお、以下では、一例として二次電池電極用スラリー組成物がリチウムイオン二次電池電極用スラリー組成物である場合について説明するが、本発明は下記の一例に限定されるものではない。
電極活物質は、リチウムイオン二次電池の電極(正極、負極)において電子の受け渡しをする物質である。そして、リチウムイオン二次電池の電極活物質(正極活物質、負極活物質)としては、通常は、リチウムを吸蔵および放出し得る物質を用いる。
具体的には、正極活物質としては、遷移金属を含有する化合物、例えば、遷移金属酸化物、遷移金属硫化物、リチウムと遷移金属との複合金属酸化物などを用いることができる。なお、遷移金属としては、例えば、Ti、V、Cr、Mn、Fe、Co、Ni、Cu、Mo等が挙げられる。
遷移金属硫化物としては、TiS2、TiS3、非晶質MoS2、FeSなどが挙げられる。
リチウムと遷移金属との複合金属酸化物としては、層状構造を有するリチウム含有複合金属酸化物、スピネル型構造を有するリチウム含有複合金属酸化物、オリビン型構造を有するリチウム含有複合金属酸化物などが挙げられる。
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。
また、負極活物質としては、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。
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。
リチウムイオン二次電池電極用スラリー組成物に配合し得るバインダー組成物としては、水と、第1粒子状重合体と、任意に第2粒子状重合体とを含む本発明の二次電池電極用バインダー組成物を用いることができる。
なお、バインダー組成物の配合量は、特に限定されることなく、例えば電極活物質100質量部当たり、固形分換算で、第1粒子状重合体および第2粒子状重合体が合計で0.5質量部以上3.0質量部以下となる量とすることができる。
スラリー組成物に配合し得るその他の成分としては、特に限定することなく、本発明のバインダー組成物に配合し得るその他の成分と同様のものが挙げられる。また、その他の成分は、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。
上述したスラリー組成物は、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、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。
ここで、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。なお、水系媒体として使用される水には、バインダー組成物が含有していた水も含まれ得る。
本発明の二次電池電極用バインダー組成物を用いて調製した上記二次電池電極用スラリー組成物(負極用スラリー組成物および正極用スラリー組成物)は、二次電池用電極(負極および正極)の製造に用いることができる。
ここで、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、上述した第1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各成分は、上記二次電池電極用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、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。
そして、上記二次電池用電極は、本発明の二次電池電極用バインダー組成物を使用しているので、二次電池に優れたレート特性およびサイクル特性を発揮させることができる。
なお、本発明の二次電池用電極は、例えば、上述した二次電池電極用スラリー組成物を集電体上に塗布する工程(塗布工程)と、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集電体上に電極合材層を形成する工程(乾燥工程)とを経て製造される。即ち、本発明の二次電池用電極中の電極合材層は、本発明の二次電池電極用スラリー組成物の乾燥物よりなる。なお、上述した第1粒子状重合体および/または第2粒子状重合体に含まれる重合体が、架橋性単量体由来の単量体単位(架橋性単量体単位)を含む場合には、当該架橋性単量体単位を含む重合体は、二次電池電極用スラリー組成物の乾燥時、または、乾燥後に任意に実施される熱処理時に架橋されていてもよい(即ち、本発明の二次電池用電極中の電極合材層は、上述した第1粒子状重合体および/または第2粒子状重合体の架橋物を含んでいてもよい)。
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。
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電極用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二次電池用電極を得ることができる。
さらに、電極合材層が硬化性の重合体を含む場合は、電極合材層の形成後に前記重合体を硬化させることが好ましい。
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。そして、本発明の二次電池は、本発明の二次電池用電極を備えているので、レート特性およびサイクル特性に優れている。
なお、以下では、一例として二次電池がリチウムイオン二次電池である場合について説明するが、本発明は下記の一例に限定されるものではない。
上述のように、本発明の二次電池用電極が、正極および負極の少なくとも一方として用いられる。即ち、リチウムイオン二次電池の正極が本発明の電極であり負極が他の既知の負極であってもよく、リチウムイオン二次電池の負極が本発明の電極であり正極が他の既知の正極であってもよく、そして、リチウムイオン二次電池の正極および負極の両方が本発明の電極であってもよい。
電解液としては、溶媒に電解質を溶解した電解液を用いることができる。
ここで、溶媒としては、電解質を溶解可能な有機溶媒を用いることができる。具体的には、溶媒としては、エチレンカーボネート、プロピレンカーボネート、γ-ブチロラクトン等のアルキルカーボネート系溶媒に、2,5-ジメチルテトラヒドロフラン、テトラヒドロフラン、ジエチルカーボネート、エチルメチルカーボネート、ジメチルカーボネート、酢酸メチル、ジメトキシエタン、ジオキソラン、プロピオン酸メチル、ギ酸メチル等の粘度調整溶媒を添加したものを用いることができる。
電解質としては、リチウム塩を用いることができる。リチウム塩としては、例えば、特開2012-204303号公報に記載のものを用いることができる。これらのリチウム塩の中でも、有機溶媒に溶解しやすく、高い解離度を示すという点より、電解質としてはLiPF6、LiClO4、CF3SO3Liが好ましい。
セパレータとしては、例えば特開2012-204303号公報に記載のものを用いることができる。これらの中でも、セパレータ全体の膜厚を薄くすることができ、これにより、リチウムイオン二次電池内の電極活物質の比率を高くして体積あたりの容量を高くすることができるという点より、ポリオレフィン系の樹脂(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)からなる微多孔膜が好ましい。
リチウムイオン二次電池は、例えば、正極と、負極とを、セパレータを介して重ね合わせ、これを必要に応じて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ことにより製造することができる。リチウムイオン二次電池の内部の圧力上昇、過充放電などの発生を防止するために、必要に応じて、ヒューズ、PTC素子などの過電流防止素子、エキスパンドメタル、リード板などを設けてもよい。リチウムイオン二次電池の形状は、例えば、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、第1粒子状重合体のコア部を構成する重合体およびシェル部を構成する重合体、並びに第2粒子状重合体の電解液膨潤度、ガラス転移温度、第1粒子状重合体および第2粒子状重合体の個数平均粒子径、第1粒子状重合体中のシェル部の質量比率、電極のピール強度、気孔率およびかさ密度、二次電池のレート特性およびサイクル特性は、それぞれ以下の方法を使用して評価した。
第1粒子状重合体のコア部およびシェル部の調製に使用した単量体組成物を使用し、コア部およびシェル部の重合条件と同様の重合条件で測定試料となる重合体(コア部の重合体、シェル部の重合体)の水分散液をそれぞれ作製した。
上述のコア部の重合体およびシェル部の重合体、並びに第2粒子状重合体の水分散液をそれぞれ50%湿度、23~25℃の環境下で3日間乾燥させて、厚み3±0.3mmに成膜した。成膜したフィルムを直径12mmに裁断し、精秤した。
裁断により得られたフィルム片の質量をW0とする。このフィルム片を、50gの電解液(組成:濃度1.0MのLiPF6溶液(溶媒はエチレンカーボネート/エチルメチルカーボネート=3/7(重量比)の混合溶媒、添加剤としてビニレンカーボネート2体積%(溶媒比)を添加))に、60℃の環境下で72時間浸漬し、膨潤させた。その後、引き揚げたフィルム片(膨潤後)を軽く拭いた後、質量W1を計測した。
そして、下記式にしたがって膨潤度(質量%)を算出した。
膨潤度(質量%)=(W1/W0)×100
<ガラス転移温度>
コア部の重合体およびシェル部の重合体、並びに第2粒子状重合体の水分散液をそれぞれ乾燥させて測定試料を準備した。そして、示差熱分析測定装置(エスアイアイ・ナノテクノロジー社製、製品名「EXSTAR DSC6220」)を用いてガラス転移温度を測定した。
具体的には、測定試料10mgをアルミパンに計量し、リファレンスとして空のアルミパンを用い、測定温度範囲-100℃~500℃の間で、昇温速度10℃/分、常温常湿下で、DSC曲線を測定した。この昇温過程で、微分信号(DDSC)が0.05mW/分/mg以上となるDSC曲線の吸熱ピークが出る直前のベースラインと、吸熱ピーク後に最初に現れる変曲点でのDSC曲線の接線との交点から、ガラス転移温度を求めた。
<個数平均粒子径>
粒子状重合体(第1粒子状重合体、第2粒子状重合体)の個数平均粒子径は、レーザー回折・散乱式粒度分布測定装置(ベックマン・コールター社製、LS230)を用いて測定した。
具体的には、粒子状重合体を含む水分散液について、レーザー回折・散乱式粒度分布測定装置を用いて粒子状重合体の粒子径-個数積算分布を測定し、積算分布の値が50%となる粒子径を個数平均粒子径とした。
<第1粒子状重合体中のシェル部の質量比率>
第1粒子状重合体中のシェル部の質量比率は、コア部形成用単量体組成物に含まれる全単量体の質量の合計M1とシェル部形成用単量体組成物に含まれる全単量体の質量の合計M2とから以下の式で算出した。
シェル部の質量比率(質量%)={M2/(M1+M2)}×100
<電極のピール強度>
作製したリチウムイオン二次電池用負極を、幅1.0cm×長さ10cmの矩形に切って試験片とし、負極合材層側の表面を上にして固定した。そして、試験片の負極合材層側の表面にセロハンテープを貼り付けた。この際、セロハンテープはJIS Z1522に規定されるものを用いた。その後、試験片の一端からセロハンテープを50mm/分の速度で180°方向(試験片の他端側)に引き剥がしたときの応力を測定した。測定を10回行い、応力の平均値を求めて、これをピール強度(N/m)とし、以下の基準で評価した。ピール強度が大きいほど、集電体に対する負極合材層の結着性が優れていることを示す。
A:ピール強度が8N/m以上
B:ピール強度が5N/m以上8N/m未満
C:ピール強度が3N/m以上5N/m未満
D:ピール強度が3N/m未満
<気孔率およびかさ密度>
作製したリチウムイオン二次電池用負極の負極合材層の気孔率およびかさ密度は、以下の式に基づいて算出した。なお、負極合材層の真密度は、負極用スラリー組成物中に含まれている固形分の密度(理論値)より算出した。
かさ密度(g/cm3)=負極合材層の目付量/負極合材層の厚み
気孔率(%)={1-(負極合材層のかさ密度/負極合材層の真密度)}×100
<二次電池のレート特性>
作製したパウチ型のリチウムイオン二次電池を、24時間静置した後に、0.2Cの充放電レートにて4.4Vまで充電し3.0Vまで放電する操作を行った。その後、25℃で0.2Cの充電レートで4.4Vまで充電し、1.0Cの放電レートで3.0Vまで放電する充放電サイクルと、3.0Cの放電レートで3.0Vまで放電する充放電サイクルとを、それぞれ行った。1.0Cにおける電池容量に対する3.0Cにおける電池容量の割合を百分率で算出して充放電レート特性とし、下記の基準で評価した。充放電レート特性の値が高いほど、内部抵抗が小さく、高速充放電が可能であり、レート特性に優れていることを示す。
A:充放電レート特性が70%以上
B:充放電レート特性が65%以上70%未満
C:充放電レート特性が60%以上65%未満
D:充放電レート特性が60%未満
<二次電池のサイクル特性>
作製したパウチ型のリチウムイオン二次電池を、24時間静置した後に、0.2Cの充放電レートにて4.4Vまで充電し3.0Vまで放電する操作を行い、初期容量C0を測定した。さらに、45℃環境下で、1.0Cの充放電レートで4.4Vまで充電し、3.0Vまで放電する充放電サイクルを繰り返し、300サイクル後の容量C1を測定した。そして、高温サイクル特性を、ΔC=(C1/C0)×100(%)で示す容量維持率にて評価した。この容量維持率の値が高いほど、放電容量の低下が少なく、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ΔCが80%以上
B:容量維持率ΔCが75%以上80%未満
C:容量維持率ΔCが70%以上75%未満
D:容量維持率ΔCが70%未満
<第1粒子状重合体の調製>
攪拌機を備えた反応器に、(メタ)アクリル酸エステル単量体としてブチルアクリレート76.8部(コア部中96.0%)、エチレン性不飽和カルボン酸単量体としてメタクリル酸1.6部(コア部中2.0%)、シアン化ビニル系単量体としてアクリロニトリル1.6部(コア部中2.0%)、乳化剤としてドデシルベンゼンスルホン酸ナトリウム0.3部、重合開始剤として過硫酸アンモニウム0.3部、イオン交換水300部を入れ、十分に攪拌した後、70℃に加温して4時間反応を進行させた(コア部形成工程)。次いで、芳香族ビニル単量体としてスチレン19.0部(シェル部中95%)、エチレン性不飽和カルボン酸単量体としてメタクリル酸1.0部(シェル部中5%)を、添加時間2分以内に重合系内に添加した。添加終了後、80℃に加温して3時間反応を進行させた(シェル部形成工程)。こうして得られた重合体を含んだ水分散液を30℃以下まで冷却した。
走査型電子顕微鏡(SEM)により、シェル部がコア部の外表面を部分的に覆っている(複数のシェル部構造体がコア部の外表面に存在している)第1粒子状重合体の水分散液が得られたことを確認した。また、上述した方法で、第1粒子状重合体の個数平均粒子径、並びに、コア部の重合体とシェル部の重合体の電解液膨潤度およびガラス転移温度を測定した。結果を表2に示す。
<第2粒子状重合体の製造>
攪拌機を備えた5MPa耐圧容器に、共役ジエン系単量体として1,3-ブタジエン33.2部、エチレン性不飽和カルボン酸単量体としてイタコン酸3.8部、芳香族ビニル単量体としてスチレン62.0部、ヒドロキシアルキル基を含有する不飽和単量体として2-ヒドロキシエチルアクリレート1部、分子量調整剤としてt-ドデシルメルカプタン0.3部、乳化剤としてドデシルベンゼンスルホン酸ナトリウム0.3部、イオン交換水150部、および、重合開始剤として過硫酸カリウム1.0部を入れ、十分に攪拌した後、50℃に加温して重合を開始した。重合転化率が96%になった時点で冷却して反応を停止した。得られた共重合体を含んだ水分散液に、5%水酸化ナトリウム水溶液を添加して、pH8に調整した。その後、加熱減圧蒸留によって未反応単量体の除去を行った。更にその後、30℃以下まで冷却した。これにより、第2粒子状重合体の水分散液を得た。そして、上述した方法で、第2粒子状重合体の個数平均粒子径、電解液膨潤度およびガラス転移温度を測定した。結果を表2に示す。
<リチウムイオン二次電池負極用スラリー組成物の調製>
ディスパー付きのプラネタリーミキサーに、負極活物質として人造黒鉛(比表面積:3.6m2/g、体積平均粒子径:20μm)98.0部と、粘度調整剤としてのカルボキシメチルセルロースナトリウム塩(CMC-Na)の1%水溶液を固形分相当で1部とを加えた。そして、これらの混合物をイオン交換水で固形分濃度60%に調整した後、25℃で60分混合した。
次に、イオン交換水で固形分濃度52%に調整した後、さらに25℃で15分混合し混合液を得た。
次いで、上記の混合液に、第1粒子状重合体の水分散液および第2粒子状重合体の水分散液を表2に示す比率(第1粒子状重合体:第2粒子状重合体(質量比)=80:20)で混合してなるバインダー組成物を固形分相当で1部添加すると共にイオン交換水を添加し、最終固形分濃度が50%となるように調整して、さらに10分間混合した。これを減圧下で脱泡処理して、負極用スラリー組成物を得た。
<リチウムイオン二次電池用負極の作製>
調製した負極用スラリー組成物を厚さ15μmの銅箔(集電体)の上にコンマコーターで塗付量が13.5~14.5mg/cm2となるように塗布し、乾燥させた。なお、乾燥は、70℃のオーブン内で銅箔を0.5m/分の速度で2分間かけて搬送することにより行った。その後、120℃にて2分間加熱処理して負極原反を得た。次に、得られた負極原反をロールプレス機にて負極合材層のかさ密度が1.82g/cm3となるようプレスし、負極とした。なお、プレス後の負極合材層の目付量は14.0mg/cm2であり、気孔率は18.8%であった。
そして、作製した負極について、ピール強度を評価した。結果を表2に示す。
<リチウムイオン二次電池用正極の作製>
プラネタリーミキサーに、正極活物質としてLiCoO296.0部、導電助剤としてアセチレンブラック2.0部(電気化学工業(株)製、HS-100)、結着材としてPVDF(ポリフッ化ビニリデン、(株)クレハ化学製KF-1100)2.0部を投入し、さらに全固形分濃度が67%となるようにN-メチルピロリドンを加えて混合して、正極用スラリー組成物を得た。
そして、得られた正極用スラリー組成物を厚さ20μmのアルミ箔(集電体)の上にコンマコーターで塗布し、乾燥させた。なお、乾燥は、60℃のオーブン内でアルミ箔を0.5m/分の速度で2分間かけて搬送することにより行った。その後、120℃にて2分間加熱処理して正極原反を得た。次に、得られた正極原反をロールプレス機にて正極合材層のかさ密度が3.5g/cm3となるようにプレスし、正極を得た。
<リチウムイオン二次電池の作製>
単層のポリプロピレン製セパレータ(幅65mm、長さ500mm、厚さ25μm;乾式法により製造;気孔率55%)を用意し、5cm×5cmの正方形に切り抜いた。また、電池の外装として、アルミ包材外装を用意した。
そして、作製した正極を、4cm×4cmの正方形に切り出し、集電体側の表面がアルミ包材外装に接するように配置した。次に、正極の正極合材層側の表面上に、正方形のセパレータを配置した。更に、作製した負極を、4.2cm×4.2cmの正方形に切り出し、セパレータ上に、負極合材層側の表面がセパレータに向かい合うよう配置した。その後、電解液として濃度1.0MのLiPF6溶液(溶媒はエチレンカーボネート/エチルメチルカーボネート=3/7(重量比)の混合溶媒、添加剤としてビニレンカーボネート2体積%(溶媒比)を添加)を充填した。更に、アルミ包材外装の開口を密封するために、150℃のヒートシールをしてアルミ包材外装を閉口し、リチウムイオン二次電池を製造した。
作製したリチウムイオン二次電池について、レート特性およびサイクル特性を評価した。結果を表2に示す。
表1に示す単量体を表1に示す量で使用した以外は実施例1と同様にして、第1粒子状重合体を調製した。なお、これらの第1粒子状重合体は、実施例1で使用したものとシェル部の質量比率が異なるが、コア部、シェル部の組成自体は実施例1と同じである。走査型電子顕微鏡(SEM)により、これらの第1粒子状重合体のシェル部がコア部の外表面を部分的に覆っている(複数のシェル部構造体がコア部の外表面に存在している)ことを確認した。また、上述した方法で、第1粒子状重合体の個数平均粒子径、並びに、コア部の重合体とシェル部の重合体の電解液膨潤度およびガラス転移温度を測定した。結果を表2に示す。
そして、当該第1粒子状重合体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、負極用スラリー組成物、負極、正極および二次電池を作製し、実施例1と同様にして評価を行った。結果を表2に示す。
第1粒子状重合体の水分散液および第2粒子状重合体の水分散液を表2に示す比率で使用した以外は、実施例1と同様にして、バインダー組成物、負極用スラリー組成物、負極、正極および二次電池を作製し、実施例1と同様にして評価を行った。結果を表2に示す。
表1に示す単量体を表1に示す量で使用した以外は実施例1と同様にして、第1粒子状重合体を調製した。走査型電子顕微鏡(SEM)により、この第1粒子状重合体のシェル部がコア部の外表面を部分的に覆っている(複数のシェル部構造体がコア部の外表面に存在している)ことを確認した。また、上述した方法で、第1粒子状重合体の個数平均粒子径、並びに、コア部の重合体とシェル部の重合体の電解液膨潤度およびガラス転移温度を測定した。結果を表2に示す。
そして、当該第1粒子状重合体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、負極用スラリー組成物、負極、正極および二次電池を作製し、実施例1と同様にして評価を行った。結果を表2に示す。
リチウムイオン二次電池用負極を作製する際に、負極原反をロールプレス機にて負極合材層のかさ密度が1.65g/cm3となるようプレスした以外は実施例1と同様にして、バインダー組成物、負極用スラリー組成物、負極、正極および二次電池を作製した。なお、負極合材層の気孔率は26.4%であった。
そして、実施例1と同様にして評価を行った。結果を表2に示す。
シェル部用単量体組成物の添加時間を30分に変更した以外は、実施例1と同様にして、第1粒子状重合体を調製した。走査型電子顕微鏡(SEM)により、この第1粒子状重合体のシェル部がコア部の外表面を部分的に覆っている(複数のシェル部構造体がコア部の外表面に存在している)ことを確認した。また、上述した方法で、第1粒子状重合体の個数平均粒子径、並びに、コア部の重合体とシェル部の重合体の電解液膨潤度およびガラス転移温度を測定した。結果を表2に示す。
そして、当該第1粒子状重合体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、負極用スラリー組成物、負極、正極および二次電池を作製し、実施例1と同様にして評価を行った。結果を表2に示す。
第2粒子状重合体を使用せず、第1粒子状重合体の量を1部とした以外は、実施例1と同様にして、バインダー組成物、負極用スラリー組成物、負極、正極および二次電池を作製し、実施例1と同様にして評価を行った。結果を表2に示す。
シェル部用単量体組成物の添加時間を180分に変更した以外は、実施例1と同様にして、第1粒子状重合体を調製した。走査型電子顕微鏡(SEM)により、この第1粒子状重合体のシェル部がコア部の外表面の全体を覆っていることを確認した。また、上述した方法で、第1粒子状重合体の個数平均粒子径、並びに、コア部の重合体とシェル部の重合体の電解液膨潤度およびガラス転移温度を測定した。結果を表2に示す。
そして、当該第1粒子状重合体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、負極用スラリー組成物、負極、正極および二次電池を作製し、実施例1と同様にして評価を行った。結果を表2に示す。
リチウムイオン二次電池用負極を作製する際に、負極原反をロールプレス機にて負極合材層のかさ密度が1.65g/cm3となるようプレスした以外は比較例1と同様にして、バインダー組成物、負極用スラリー組成物、負極、正極および二次電池を作製した。なお、負極合材層の気孔率は26.4%であった。
そして、実施例1と同様にして評価を行った。結果を表2に示す。
BA:ブチルアクリレート(アクリル酸エステル単量体)
MAA:メタクリル酸(エチレン性不飽和カルボン酸単量体)
AN:アクリロニトリル(シアン化ビニル系単量体)
BD:1,3-ブタジエン(共役ジエン系単量体)
ST:スチレン(芳香族ビニル単量体)
CMC-Na:カルボキシメチルセルロースナトリウム塩
また、表2の比較例1、2よりシェル部がコア部の外表面の全体を覆うコアシェル構造を有する第1粒子状重合体を用いると、電極合材層の密度を高めた場合には特に、電極のピール強度の向上と、二次電池のレート特性およびサイクル特性の向上とを並立させることができないことが分かる。
また、表2の実施例1、6~9、15~17より、第1粒子状重合体と第2粒子状重合体の量比を調整することにより、電極のピール強度、並びに二次電池のレート特性およびサイクル特性を更に向上させうることがわかる。
加えて、表2の実施例1、10より、第1粒子状重合体のコア部の組成を変更することにより、二次電池のレート特性を更に向上させうることがわかる。
更に、表2の実施例1は実施例11と同等の性能を発揮しているのに対し、比較例1は比較例2よりもピール強度、レート特性およびサイクル特性に劣っていることから、シェル部がコア部の外表面を部分的に覆うコアシェル構造を有する第1粒子状重合体を用いれば、電極合材層の密度を高めた場合には特に、電極のピール強度の向上と、二次電池のレート特性およびサイクル特性の向上とを良好に並立させ得ることが分かる。
また、本発明によれば、二次電池に優れたレート特性およびサイクル特性を発揮させることができる電極合材層を形成可能な二次電池電極用スラリー組成物を提供することができる。
更に、本発明によれば、二次電池に優れたレート特性およびサイクル特性を発揮させることが可能な二次電池用電極を提供することができる。
また、本発明によれば、レート特性およびサイクル特性に優れる二次電池を提供することができる。
110 コア部
110S コア部の外表面
120 シェル部構造体
Claims (13)
- コア部と、前記コア部の外表面を部分的に覆うシェル部とを備えるコアシェル構造を有する第1粒子状重合体を含む、二次電池電極用バインダー組成物。
- 前記コア部を構成する重合体の電解液膨潤度が300質量%以上900質量%以下であり、
前記シェル部を構成する重合体の電解液膨潤度が100質量%超200質量%以下である、請求項1に記載の二次電池電極用バインダー組成物。 - 前記コア部を構成する重合体のガラス転移温度が-60℃以上-15℃以下であり、
前記シェル部を構成する重合体のガラス転移温度が40℃以上200℃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。 - 前記第1粒子状重合体中のシェル部の質量比率が3質量%以上35質量%以下である、請求項1~3の何れかに記載の二次電池電極用バインダー組成物。
- 前記コア部を構成する重合体が、(メタ)アクリル酸エステル単量体単位を50質量%以上99.5質量%以下含む、請求項1~4の何れかに記載の二次電池電極用バインダー組成物。
- さらに第2粒子状重合体を含み、
前記第2粒子状重合体が、電解液膨潤度が100質量%超200質量%以下であり、かつガラス転移温度が-10℃以上40℃以下である、請求項1~5の何れかに記載の二次電池電極用バインダー組成物。 - 固形分換算で、前記第1粒子状重合体と前記第2粒子状重合体との合計100質量部当たり、前記第1粒子状重合体を30質量部以上95質量部以下含む、請求項6に記載の二次電池電極用バインダー組成物。
- 前記第2粒子状重合体が、共役ジエン系単量体単位を5質量%以上70質量%以下含み、且つ、芳香族ビニル単量体単位を10質量%以上90質量%以下含む、請求項6又は7に記載の二次電池電極用バインダー組成物。
- 前記第1粒子状重合体の個数平均粒子径が、前記第2粒子状重合体の個数平均粒子径の1倍以上5倍以下である、請求項6~8の何れかに記載の二次電池電極用バインダー組成物。
- 請求項1~9の何れかに記載の二次電池電極用バインダー組成物と、電極活物質とを含む、二次電池電極用スラリー組成物。
- 請求項10に記載の二次電池電極用スラリー組成物を用いて得られる電極合材層を有する、二次電池用電極。
- 前記電極合材層の気孔率が10.7%以上24.1%以下である、請求項11に記載の二次電池用電極。
- 正極、負極、セパレータおよび電解液を備え、
前記正極および前記負極の少なくとも一方が、請求項11または12に記載の二次電池用電極である、二次電池。
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WO2020004332A1 (ja) | 2018-06-29 | 2020-01-02 | 日本ゼオン株式会社 | 電気化学素子電極用バインダー組成物、電気化学素子電極用スラリー組成物、電気化学素子用電極、および電気化学素子 |
WO2021241599A1 (ja) * | 2020-05-29 | 2021-12-02 | 日本ゼオン株式会社 | 全固体二次電池用スラリー組成物、固体電解質含有層及び全固体二次電池 |
WO2024024912A1 (ja) * | 2022-07-29 | 2024-02-01 | 日本ゼオン株式会社 | 非水系二次電池電極用バインダー組成物、非水系二次電池電極用スラリー組成物、非水系二次電池用電極、および非水系二次電池 |
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US20170256800A1 (en) | 2017-09-07 |
JPWO2016035286A1 (ja) | 2017-06-15 |
JP6627763B2 (ja) | 2020-01-08 |
US10290873B2 (en) | 2019-05-14 |
KR20170053615A (ko) | 2017-05-16 |
CN106663813B (zh) | 2020-12-08 |
CN106663813A (zh) | 2017-05-10 |
KR102418499B1 (ko) | 2022-07-06 |
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