阅读说明：本技术 组合物 (Composition comprising a metal oxide and a metal oxide ) 是由 中西崇一朗 成富拓也 铃木茂 渡边淳 于 2020-04-07 设计创作，主要内容包括：提供一种组合物,该组合物成为在高温下的循环容量维持率优异的粘合剂。根据本发明,提供一种组合物,含有接枝共聚物,其特征在于,上述接枝共聚物具有主干聚合物以及多个分支聚合物,上述主干聚合物具有聚乙烯醇结构,第1单体单元以及第2单体单元分别至少包含在上述多个分支聚合物中的任1个,上述第2单体单元与上述第1单体单元不同,玻璃化转变温度为260K～365K。(Provided is a composition which is an adhesive having excellent cycle capacity retention at high temperatures. According to the present invention, there is provided a composition comprising a graft copolymer, wherein the graft copolymer has a main polymer and a plurality of branched polymers, the main polymer has a polyvinyl alcohol structure, 1 st monomer unit and 2 nd monomer unit are respectively contained in at least 1 of the plurality of branched polymers, the 2 nd monomer unit is different from the 1 st monomer unit, and a glass transition temperature is 260K to 365K.)

1. A composition comprising a graft copolymer, characterized in that,

the graft copolymer has a backbone polymer and a plurality of branched polymers,

the backbone polymer has a polyvinyl alcohol structure,

the 1 st monomer unit and the 2 nd monomer unit are respectively contained in at least 1 of the plurality of branched polymers,

the 2 nd monomer unit is different from the 1 st monomer unit,

the glass transition temperature is 260-365K.

2. The composition of claim 1,

the composition further comprises a free polymer which is,

the free polymer has no covalent bonding with the graft copolymer,

the free polymer is at least one selected from the group consisting of a polymer having a polyvinyl alcohol structure and a polymer having a 1 st monomer unit and/or a 2 nd monomer unit.

3. The composition according to claim 1 or 2,

the 1 st monomer unit is a (meth) acrylonitrile monomer unit and/or a (meth) acrylic acid monomer unit.

4. The composition according to any one of claims 1 to 3,

in the composition, if the content of the polyvinyl alcohol structure in the composition is C_PVAMass%, the total content of the 1 st monomer unit and the 2 nd monomer unit in the composition is C_MThe mass percent of the raw material,

the mass ratio of the content of the polyvinyl alcohol structure to the total content of the polyvinyl alcohol structure, the 1 st monomer unit and the 2 nd monomer unit (C)_PVA/(C_M+C_PVA) ) is 0.05 to 0.7.

5. The composition according to any one of claims 1 to 4,

if the content of the 1 st monomer unit is PM₁Mole percent, the content of the 2 nd monomer unit being PM₂In a mole percent ratio,

a molar ratio (PM) of a molar amount of the 1 st monomer unit to a total molar amount of the 1 st monomer unit and the 2 nd monomer unit contained in the composition₁/(PM₁+PM₂) 0.1 to 0.9.

6. The composition according to any one of claims 1 to 5,

at least one of the plurality of branched polymers has a copolymerized structure of the 1 st monomer unit and the 2 nd monomer unit.

7. The composition according to any one of claims 1 to 6,

the average polymerization degree of a polyvinyl alcohol structure in the composition is 300-4000.

8. The composition according to any one of claims 1 to 7,

the saponification degree of the polyvinyl alcohol structure in the composition is 60 to 100 mol%.

9. The composition according to any one of claims 1 to 8,

the grafting rate of the graft copolymer is 40-3000%.

10. A slurry for a positive electrode, characterized by comprising the composition according to any one of claims 1 to 9, a positive electrode active material, and a conductive auxiliary agent.

11. The slurry for a positive electrode according to claim 10,

the solid content of the composition is 1-20% by mass relative to the total solid content of the positive electrode slurry.

12. The slurry for a positive electrode according to claim 10 or 11,

the positive electrode active material contains LiNi_XMn_(2－X)O₄(wherein 0 < X < 2), Li (Co)_XNi_YMn_Z)O₂(wherein 0 < X < 1, 0 < Y < 1, 0 < Z < 1, and X + Y + Z ═ 1), Li (Ni)_XCo_YAl_Z)O₂(wherein 0 < X < 1, 0 < Y < 1, 0 < Z < 1, and X + Y + Z ═ 1) is 1 or more.

13. The slurry for a positive electrode according to any one of claims 10 to 12,

the conductive additive is at least 1 or more selected from (i) fibrous carbon, (ii) carbon black, and (iii) a carbon composite in which fibrous carbon and carbon black are bonded to each other.

14. A positive electrode comprising a metal foil and a coating film of the slurry for a positive electrode according to any one of claims 10 to 13 formed on the metal foil.

15. A battery comprising the positive electrode according to claim 14.

16. An adhesive comprising the composition of any one of claims 1 to 9.

Technical Field

The invention relates to a composition, positive electrode slurry and a battery.

Background

In recent years, secondary batteries have been used as power sources for electronic devices such as notebook computers and cellular phones, and hybrid vehicles and electric vehicles using secondary batteries as power sources have been developed for the purpose of reducing environmental loads. These power sources require secondary batteries having high energy density, high voltage, and high durability. Lithium ion secondary batteries have attracted attention as secondary batteries capable of achieving high voltage and high energy density.

Lithium ion secondary batteries are composed of a positive electrode, a negative electrode, an electrolyte, and a separator, and the positive electrode is composed of a positive electrode active material, a conductive additive, a metal foil, and a binder (patent documents 1 to 3).

As a positive electrode binder for a lithium ion secondary battery, a binder (graft copolymer) containing polyvinyl alcohol and polyacrylonitrile as main components and having high adhesion and oxidation resistance is disclosed (patent document 4).

Documents of the prior art

Patent document

[ patent document 1] Japanese patent application laid-open No. 2013-98123

[ patent document 2] Japanese patent laid-open No. 2013-84351

[ patent document 3] Japanese patent laid-open No. Hei 6-172452

[ patent document 4] International publication No. 2015/053224

Disclosure of Invention

Problems to be solved by the invention

However, there is a further need for the development of a composition serving as a binder having excellent cycle capacity retention at high temperatures, and a positive electrode slurry and a secondary battery using the composition.

The present invention has been made in view of such circumstances, and provides a composition serving as a binder having an excellent cycle capacity retention rate at high temperatures, and a positive electrode slurry and a secondary battery using the composition.

Means for solving the problems

According to the present invention, there is provided a composition comprising a graft copolymer, wherein the graft copolymer has a main polymer and a plurality of branched polymers, the main polymer has a polyvinyl alcohol structure, a 1 st monomer unit and a 2 nd monomer unit are contained in at least one of the plurality of branched polymers, respectively, and the 2 nd monomer unit is different from the 1 st monomer unit and has a glass transition temperature of 260K to 365K.

The present inventors have conducted extensive studies and found that a binder having an excellent cycle capacity retention rate at high temperatures can be obtained by using, as a composition for a positive electrode, a composition having a glass transition temperature within a predetermined range, in which a graft copolymer having a structure in which a 1 st monomer unit and a 2 nd monomer unit (different from the 1 st monomer unit) are graft-copolymerized with a backbone polymer having a polyvinyl alcohol structure.

Various embodiments of the present invention are exemplified below. The embodiments shown below can be combined with each other.

Preferably, the composition further comprises a free polymer, the free polymer is not covalently bonded to the graft copolymer, and the free polymer is at least one selected from the group consisting of a polymer having a polyvinyl alcohol structure and a polymer having a 1 st monomer unit and/or a 2 nd monomer unit.

Preferably, the above-mentioned 1 st monomer unit is a (meth) acrylonitrile monomer unit and/or a (meth) acrylic acid monomer unit.

Preferably, in the composition, if the content of the polyvinyl alcohol structure in the composition is C_PVAThe total content of the 1 st monomer unit and the 2 nd monomer unit in the composition is C_MMass%, the mass ratio of the content of the polyvinyl alcohol structure to the total content of the polyvinyl alcohol structure, the 1 st monomer unit and the 2 nd monomer unit (C)_PVA/(C_M+C_PVA) ) is 0.05 to 0.7.

Preferably, the content of the 1 st monomer unit is PM₁Mol%, the content of the 2 nd monomer unit being PM₂The mole% is a molar ratio (PM) of a molar amount of the 1 st monomer unit to a total molar amount of the 1 st monomer unit and the 2 nd monomer unit contained in the composition₁/(PM₁+PM₂) 0.1 to 0.9.

Preferably, at least one of the plurality of branched polymers has a copolymerized structure of the 1 st monomer unit and the 2 nd monomer unit.

Preferably, the polyvinyl alcohol structure in the composition has an average degree of polymerization of 300 to 4000.

Preferably, the saponification degree of the polyvinyl alcohol structure in the composition is 60 to 100 mol%.

Preferably, the graft ratio of the graft copolymer is 40 to 3000%.

According to another aspect of the present invention, there is provided a positive electrode slurry containing the composition, a positive electrode active material, and a conductive auxiliary agent.

Preferably, the solid content of the composition is 1 to 20% by mass based on the total solid content of the positive electrode slurry.

Preferably, the positive electrode active material contains a material selected from the group consisting of LiNi_XMn_(2－X)O₄(wherein 0 < X < 2), Li (Co)_XNi_YMn_Z)O₂(wherein 0 < X < 1, 0 < Y < 1, 0 < Z < 1, and X + Y + Z ═ 1), Li (Ni)_XCo_YAl_Z)O₂(wherein 0 < X < 1, 0 < Y < 1, 0 < Z < 1, and X + Y + Z ═ 1) is 1 or more.

Preferably, the conductive additive is at least 1 or more selected from (i) fibrous carbon, (ii) carbon black, and (iii) a carbon composite in which fibrous carbon and carbon black are bonded to each other.

According to another aspect of the present invention, there is provided a positive electrode comprising a metal foil and a coating film of the positive electrode slurry formed on the metal foil.

According to another aspect of the present invention, there is provided a battery including the positive electrode.

According to another aspect of the present invention, there is provided an adhesive comprising the above composition.

Effects of the invention

The invention provides a composition having excellent cycle capacity maintenance rate at high temperature.

Detailed Description

The following describes embodiments of the present invention. Various feature items exemplified in the embodiments shown below may be combined with each other. Further, each feature independently establishes the invention.

1. Composition comprising a metal oxide and a metal oxide

The composition according to an embodiment of the present invention is a composition containing a graft copolymer, and the graft copolymer is a composition having a main polymer and a plurality of branched polymers.

The graft copolymer according to one embodiment of the present invention is synthesized by graft-copolymerizing the 1 st monomer and the 2 nd monomer with the trunk polymer. Here, the 1 st monomer is a different monomer from the 2 nd monomer. The branched polymers produced by polymerization are grafted, i.e., covalently bound, to the backbone polymer. In this case, it is possible to simultaneously form, as free polymers, polymers comprising ungrafted backbone polymers or polymers which are not grafted to the backbone polymers, i.e. the 1 st monomers and/or polymers comprising the 2 nd monomers which are not covalently bonded to the graft copolymer. Therefore, the composition according to one embodiment of the present invention is preferably substantially composed of the graft copolymer and the free polymer. Further, monomers other than the 1 st and 2 nd monomers may be polymerized within a range not impairing the effects of the present invention.

The graft ratio of the graft copolymer is preferably 40 to 3000%, more preferably 300 to 1500%. The graft ratio is preferably in the above range from the viewpoint of solubility. When the graft ratio is 40% or more, the solubility in NMP (N-methylpyrrolidone) is improved, and when the graft ratio is 3000% or less, the viscosity of the NMP solution is lowered, and the fluidity of the NMP solution is improved.

1-2. Trunk polymers

The backbone polymer has a polyvinyl alcohol structure. Here, the polyvinyl alcohol structure is, for example, a structure derived from polyvinyl alcohol synthesized by saponifying polyvinyl acetate obtained by polymerizing a vinyl acetate monomer. Preferably, the majority of the backbone polymer is comprised of polyvinyl alcohol structures. More preferably, the backbone polymer is polyvinyl alcohol.

The average polymerization degree of the polyvinyl alcohol structure in the composition is preferably 300-4000, and more preferably 500-2000. When the average polymerization degree is within the above range, the stability of the slurry is particularly high. The above range is also preferable from the viewpoint of solubility, adhesiveness, and adhesive viscosity. When the average polymerization degree is 300 or more, the adhesiveness among the binder, the active material, and the conductive auxiliary agent is improved, and the durability is improved. When the average polymerization degree is 4000 or less, the solubility is improved and the viscosity is lowered, so that the production of a slurry for a positive electrode is easy. The average polymerization degree mentioned here is a value measured by the method of JIS K6726.

The saponification degree of the polyvinyl alcohol structure in the composition is preferably 60 to 100 mol%, more preferably 80 to 100 mol%. When the saponification degree is within the above range, the stability of the slurry is particularly high. The degree of saponification mentioned here is a value measured by the method of JIS K6726.

1-3. Branched polymers

The 1 st monomer unit and the 2 nd monomer unit are contained in at least one of the plurality of branched polymers. That is, one of the plurality of branched polymers may contain only one of the 1 st monomer unit and the 2 nd monomer unit, or may contain both of the 1 st monomer unit and the 2 nd monomer unit. The monomer unit other than the 1 st monomer unit and the 2 nd monomer unit may be included within a range not impairing the effects of the present invention. Here, the 1 st monomer unit and the 2 nd monomer unit are monomer units derived from the 1 st monomer and the 2 nd monomer used for the synthesis of the graft copolymer, respectively. Preferably, at least one of the plurality of branched polymers has a copolymerized structure of the 1 st monomer unit and the 2 nd monomer unit. The branched polymer is preferably a copolymer substantially having the 1 st monomer unit and the 2 nd monomer unit, and more preferably a copolymer composed of only the 1 st monomer unit and the 2 nd monomer unit.

1-4. Free polymers

The composition according to an embodiment of the present invention may further include a free polymer. The free polymer is at least one selected from the group consisting of a polymer having a polyvinyl alcohol structure and a polymer having a 1 st monomer unit and/or a 2 nd monomer unit. The polymer having a polyvinyl alcohol structure means a trunk polymer which does not mainly participate in graft copolymerization. The polymer having the 1 st monomer unit and/or the 2 nd monomer unit means a homopolymer of the 1 st monomer, a homopolymer of the 2 nd monomer, a copolymer containing the 1 st monomer and the 2 nd monomer, and the like, and means that the polymer is not copolymerized with the graft copolymer (i.e., the trunk polymer).

Further, a homopolymer of a monomer other than the 1 st monomer unit and the 2 nd monomer unit and a copolymer of a monomer other than the 1 st monomer unit and the 2 nd monomer unit may be included within a range not impairing the effect of the present invention. The free polymer is preferably a copolymer substantially containing the 1 st monomer and the 2 nd monomer, and more preferably a copolymer composed of only the 1 st monomer and the 2 nd monomer.

The weight average molecular weight of the free polymer other than the main polymer, for example, a homopolymer of the 1 st monomer, a homopolymer of the 2 nd monomer, a copolymer containing the 1 st monomer and the 2 nd monomer, is preferably 30000 to 300000, more preferably 40000 to 200000, and further preferably 50000 to 150000. From the viewpoint of suppressing the increase in viscosity and facilitating the production of a slurry for a positive electrode, the weight average molecular weight of the free polymer other than the main polymer is preferably 300000 or less, more preferably 200000 or less, and still more preferably 150000 or less. The weight average molecular weight of the free polymer other than the main polymer can be determined by GPC (gel permeation chromatography).

1-5. 1 st monomer unit

The 1 st monomer unit is a (meth) acrylonitrile monomer unit and/or a (meth) acrylic acid monomer unit. The 1 st monomer unit is more preferably a (meth) acrylonitrile monomer unit, and still more preferably an acrylonitrile monomer unit.

That is, the 1 st monomer used in synthesizing the graft copolymer is preferably (meth) acrylonitrile and/or (meth) acrylic acid, more preferably (meth) acrylonitrile, and still more preferably acrylonitrile. Thus, the 1 st monomer unit has a structure derived from these monomer units.

1-6. 2 nd monomer unit

The 2 nd monomer unit is not particularly limited as long as it is a unit different from the 1 st monomer unit, and examples thereof include alkoxypolyethylene glycol (meth) acrylates such as (2- (2-ethoxy) ethyl (meth) acrylate, methoxypolyethylene glycol (meth) acrylate (poly: 23), methoxypropylene glycol (meth) acrylate, and 3, 6, 9, 12, 15-pentaoxa-1-heptadecene; alkyl (meth) acrylates such as dodecyl (meth) acrylate and 2, 2, 3, 3, 3-pentafluoropropyl (meth) acrylate; polyethylene glycol monovinyl ether (poly: 3) and the like. Among them, alkoxy polyethylene glycol (meth) acrylates are preferable.

1-7. Content and characteristics

The contents and properties of the respective components preferably satisfy the following requirements. When the content and the characteristics of each component are in the following ranges, a composition for a positive electrode that becomes a binder having an excellent cycle capacity retention rate at high temperatures can be provided.

The glass transition temperature of the composition is 260K to 365K, preferably 280K to 350K, and more preferably 300K to 340K. Within such a range, the cycle capacity retention rate at high temperatures is excellent. Specifically, the glass transition temperature is, for example, 260, 265, 270, 275, 280, 285, 290, 295, 300, 305, 310, 315, 320, 325, 330, 335, 340, 345, 350, 355, 360, 365K, and may be in a range of any 2 values. The glass transition here means a change in which the viscosity of a substance having fluidity at high temperature is rapidly increased within a certain temperature range due to a decrease in temperature, and the substance loses fluidity and becomes an amorphous solid. The method for measuring the glass transition temperature is not particularly limited, and means the glass transition temperature calculated by a thermogravimetry method, a differential scanning calorimetry method, a differential thermal analysis method, or a dynamic viscoelasticity measurement method. Among them, differential scanning calorimetry is preferable.

If the composition contains a polyvinyl alcohol structure and the content of the polyvinyl alcohol structure in the composition is C_PVAThe total content of the 1 st monomer unit and the 2 nd monomer unit in the composition is C_MMass% is the mass ratio of the content of the polyvinyl alcohol structure to the total content of the polyvinyl alcohol structure, the 1 st monomer unit and the 2 nd monomer unit (C)_PVA/(C_M+C_PVA) Preferably 0.05 to 0.7, more preferably 0.10 to 0.55. From the viewpoint of solubility, adhesiveness, and viscosity of the binder, the range is preferably within the above range. When the content is 0.05 or more, the adhesiveness between the binder and the active material and the conductive auxiliary agent is improved, and the durability is improved. If the amount is 0.7 or less, the solubility is improved, and a uniform resin solution is easily obtained, so that the production of the slurry for a positive electrode becomes easy.

The ratio (C)_PVA/(C_M+C_PVA) Specifically, for example, 0.05, 0.10, 0.15, 0.20, 0.25, 0.30, 0.35, 0.40, 0.45, 0.50, 0.55, 0.60, 0.65, 0.70, and a range between any two numerical values can be exemplified here.

If the content of the 1 st monomer unit in the composition is PM₁Mol%, the content of the 2 nd monomer unit being PM₂Mol% is a molar ratio (PM) of a molar amount of the 1 st monomer unit to a total molar amount of the 1 st monomer unit and the 2 nd monomer unit in the composition₁/(PM₁+PM₂) Preferably 0.1 to 0.9, more preferably 0.2 to 0.9. Within such a range, the cycle capacity retention rate at high temperatures is excellent. Specifically, the ratio is, for example, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, and may be in a range of any 2 values.

1-8. Various measuring/calculating methods

(Total content of the 1 st monomer unit and the 2 nd monomer unit)

The composition contains a PVA component, a 1 st monomer component and a 2 nd monomer component. Composition ratio in composition (PVA/PM)₁/PM₂) The amount of the polymerization initiator (g) and the polymerization ratios of the 1 st and 2 nd monomers can be determined. The polymerization rate (%) can be obtained by NMR.

If it is provided with

A: quality (input amount) of the 1 st monomer used for copolymerization

B: polymerization ratio of the 1 st monomer after reaction (%)

C: quality (input amount) of the 2 nd monomer used for copolymerization

D: polymerization ratio of the 2 nd monomer after reaction (%)

E: quality (input amount) of PVA for polymerization

The mass% of the total content of the 1 st monomer unit and the 2 nd monomer unit in the composition can be determined by the following formula (2). The amount of PVA charged is calculated based on the "total content of the 1 st monomer unit and the 2 nd monomer unit", and this is regarded as "content of polyvinyl alcohol structure".

((A×B/100+C×D/100)/(A×B/100+C×D/100+E))×100(％)···(2)

The mass% of the total content of the 1 st monomer unit and the 2 nd monomer unit can be calculated from the integral ratio of NMR. Let the integral value of protons per polyvinyl alcohol be S_PVAThe integral value of the proton of each of the 1 st and 2 nd monomers is S₁、S₂The total content of the 1 st monomer unit and the 2 nd monomer unit can be determined by the following formula (2-2).

((S₁+S₂)×100)/(S_PVA+S₁+S₂)···(2－2)

(ratio of the 1 st monomer unit to the 2 nd monomer unit)

The mass% of the 1 st monomer in the total amount of the 1 st monomer unit and the 2 nd monomer unit in the composition can be determined by the following formula (3).

((A×B/100)/(A×B/100+C×D/100))×100(％)···(3)

The mass% of the 1 st monomer in the total amount of the 1 st monomer unit and the 2 nd monomer unit in the composition can be determined by the following formula (3-2).

(S₁×100)/(S₁+S₂)···(3－2)

(graft ratio)

In the case of producing a graft copolymer (in the case of graft copolymerization), there is a possibility that homopolymers of the 1 st monomer and the 2 nd monomer are produced, and therefore, in the case of producing a graft copolymer, a step of separating the homopolymers from the graft copolymer is required. The homopolymer was dissolved in dimethylformamide (hereinafter sometimes abbreviated as DMF), but the PVA and the graft copolymer were not dissolved in DMF. By utilizing the difference in solubility, the homopolymer can be separated by an operation such as centrifugation.

Here, if provided

F: mass (g) of component dissolved in DMF

G: mass (g) of the composition used for the test

H: the total content (% by mass) of the 1 st monomer unit and the 2 nd monomer unit in the composition,

the graft ratio can be determined by the following formula (4).

((G－F)/(G×(100－H)/100))×100···(4)

(glass transition temperature)

In the specification, the glass transition point (Tg) is measured as follows.

According to JIS K7121: 1987 differential test calorie (DSC) measurement was performed. Further, Tg is defined as the intersection point of the tangent to the base line in the DSC curve and the tangent to the sharp drop position of the endothermic region due to glass transition.

1-9. Method for producing graft copolymer

The method for producing the graft copolymer according to one embodiment of the present invention is not particularly limited, and a method in which polyvinyl acetate is polymerized and then saponified to obtain polyvinyl alcohol, and then the 1 st monomer, the 2 nd monomer, and another monomer are graft-copolymerized to the polyvinyl alcohol is preferable.

As a method for polymerizing polyvinyl acetate, any known method such as bulk polymerization or solution polymerization can be used.

Examples of the initiator used for the polymerization of polyvinyl acetate include azo initiators such as azobisisobutyronitrile, and organic peroxides such as benzoyl peroxide and bis (4-t-butylcyclohexyl) peroxydicarbonate.

The saponification of polyvinyl acetate can be carried out, for example, by a method in which saponification is carried out in an organic solvent in the presence of a saponification catalyst.

Examples of the organic solvent include methanol, ethanol, propanol, ethylene glycol, methyl acetate, ethyl acetate, acetone, methyl ethyl ketone, benzene, and toluene. More than 1 of them may be used. Among them, methanol is preferred.

Examples of the saponification catalyst include alkaline catalysts such as sodium hydroxide, potassium hydroxide, and sodium alkoxide; acid catalysts such as sulfuric acid and hydrochloric acid. Among them, sodium hydroxide is preferable from the viewpoint of the saponification rate.

Examples of the method for graft-copolymerizing a monomer to polyvinyl alcohol include a solution polymerization method. Examples of the solvent used in the solution polymerization method include dimethyl sulfoxide and N-methylpyrrolidone.

As the initiator used for graft copolymerization, organic peroxides such as benzoyl peroxide, azo compounds such as azobisisobutyronitrile, potassium peroxodisulfate, ammonium peroxodisulfate, and the like can be used.

The graft copolymer according to one embodiment of the present invention may be dissolved in a solvent and used. Examples of the solvent include dimethyl sulfoxide and N-methylpyrrolidone. These solvents may be contained in the composition and the positive electrode slurry described later.

1-10. Others

The composition according to one embodiment of the present invention may contain other components, for example, a resin, within a range not to impair the effects of the present invention. Examples of such resins include fluorine-based resins such as polyvinylidene fluoride (PVDF) and polytetrafluoroethylene, styrene-butadiene-based copolymers (such as styrene-butadiene rubber), and (meth) acrylic copolymers. Among them, fluorine-based resins are preferable from the viewpoint of stability. Among the fluorine-based resins, polyvinylidene fluoride is preferable.

2. Slurry for positive electrode

The positive electrode slurry according to one embodiment of the present invention contains the above composition and has excellent stability. The positive electrode slurry according to an embodiment of the present invention contains the above composition, and has a low viscosity. The positive electrode slurry according to one embodiment of the present invention contains the above composition, and can produce a positive electrode having excellent rate characteristics. The positive electrode slurry may contain a composition and a conductive auxiliary agent, and may contain a composition, a positive electrode active material and a conductive auxiliary agent.

The viscosity of the slurry for a positive electrode according to an embodiment of the present invention is preferably 350mPa · s or less, and more preferably 300mPa · s or less. Using a conical flat plate type rotational viscometer in accordance with JIS Z8803: 2011 the viscosity of the slurry was measured (measuring machine: MCR302 manufactured by Beiabi corporation, measuring temperature: 25 ℃ and rotation speed: 1 s)^－1)。

The solid content of the positive electrode (binder) composition in the positive electrode slurry according to one embodiment of the present invention is preferably 0.1 to 20% by mass, and more preferably 1 to 10% by mass.

3. Lithium ion secondary battery

The battery according to one embodiment of the present invention is preferably a battery including a positive electrode. As the battery having a positive electrode, a secondary battery is preferable. The secondary battery is preferably 1 or more selected from a lithium ion secondary battery, a sodium ion secondary battery, a magnesium ion secondary battery, and a potassium ion secondary battery, and more preferably a lithium ion secondary battery.

The positive electrode according to one embodiment of the present invention and the lithium ion secondary battery including the positive electrode can be produced using the positive electrode slurry containing the composition. The positive electrode, the negative electrode, the separator, and an electrolyte solution (hereinafter, also referred to as an electrolyte or an electrolytic solution) are preferably contained.

[ Positive electrode ]

The positive electrode according to one embodiment of the present invention can be produced by applying a slurry for a positive electrode containing the composition, a conductive auxiliary agent, and a positive electrode active material to be used as needed onto a current collector such as an aluminum foil, heating the slurry to remove the solvent contained in the slurry, and pressing the current collector and an electrode material mixture layer by roll pressing or the like to adhere them. Thus, a positive electrode having a metal foil and a positive electrode slurry coating film formed on the metal foil was obtained.

As the current collector for the positive electrode, a metal foil is preferable. The metal foil for the positive electrode is preferably foil-shaped aluminum, and the thickness thereof is preferably 5 to 30 μm from the viewpoint of workability.

[ conductive auxiliary agent ]

The conductive auxiliary is preferably at least 1 or more selected from (i) fibrous carbon, (ii) carbon black, and (iii) a carbon composite in which fibrous carbon and carbon black are bonded to each other. Examples of the fibrous carbon include vapor grown carbon fibers, carbon nanotubes, and carbon nanofibers. Examples of the carbon black include acetylene black, furnace black, and ketjen black (registered trademark). These conductive aids may be used singly or in combination of 2 or more. Among them, from the viewpoint of high effect of improving dispersibility of the conductive auxiliary, 1 or more selected from acetylene black, carbon nanotubes and carbon nanofibers is preferable.

In the positive electrode slurry according to one embodiment of the present invention, the solid content of the conductive auxiliary agent is preferably 0.01 to 20% by mass, and more preferably 0.1 to 10% by mass, based on the total solid content of the positive electrode slurry.

[ Positive electrode active Material ]

[ Positive electrode active Material ]

The positive electrode active material may be used as needed. The positive electrode active material is preferably a positive electrode active material capable of reversibly occluding and releasing cations. The positive electrode active material preferably has a volume resistivity of 1X 10⁴A lithium-containing composite oxide or lithium-containing polyanion compound of Mn of not less than Ω · cm. For example, LiCoO can be mentioned₂、LiMn₂O₄、LiNiO₂、LiMPO₄、Li₂MSiO₄、LiNi_XMn_(2-X)O₄、Li(Co_XNi_YMn_Z)O₂、Li(Ni_XCo_YAl_Z)O₂Or xLi₂MnO₃-(1-X)LiMO₂And the like. Among them, LiNi is preferred_XMn_(2-X)O₄X in (1) satisfies 0<X<Relation of 2, Li (Co)_XNi_YMn_Z)O₂Or Li (Ni)_XCo_YAl_Z)O₂X, Y and Z in (1) satisfy the relationship of X + Y + Z ═ 1, and satisfy 0<X<1、0<Y<1、0<Z<Relation of 1, xLi₂MnO₃-(1-x)LiMO₂X in (2) satisfies 0<x<1, furthermore LiMPO₄、Li₂MSiO₄Or xLi₂MnO₃-(1-x)LiMO₂M in (2) is preferably 1 or more elements selected from Fe, Co, Ni and Mn.

Among the positive electrode active materials, it is preferably selected from LiNi_XMn_(2-X)O₄(wherein, 0<X<2)、Li(Co_XNi_YMn_Z)O₂(wherein, 0<X<1、0<Y<1、0<Z<1 and X + Y + Z ═ 1), Li (Ni)_XCo_YAl_Z)O₂(wherein, 0<X<1、0<Y<1、0<Z<1 and X + Y + Z ═ 1), more preferably selected from LiNi_XMn_(2-X)O₄(wherein, 0<X<2) And Li (Co)_XNi_YMn_Z)O₂(wherein, 0<X<1、0<Y<1、0<Z<1 and X +Y + Z ═ 1) or more.

In the positive electrode slurry according to one embodiment of the present invention, the solid content of the positive electrode active material is preferably 50 to 99.8 mass%, more preferably 80 to 99.5 mass%, and most preferably 95 to 99.0 mass%, based on the total solid content in the positive electrode slurry.

[ negative electrode ]

The negative electrode used in the lithium ion secondary battery according to the embodiment of the present invention is not particularly limited, and can be produced using a slurry for a negative electrode containing a negative electrode active material. The negative electrode can be produced, for example, using a metal foil for a negative electrode and a slurry for a negative electrode provided on the metal foil. The negative electrode slurry preferably contains a binder for a negative electrode (a negative electrode composition), a negative electrode active material, and the conductive assistant. The binder for the negative electrode is not particularly limited, and for example, polyvinylidene fluoride, polytetrafluoroethylene, a styrene-butadiene copolymer (styrene butadiene rubber or the like), an acrylic copolymer, or the like can be used. As the binder for the negative electrode, a fluorine-based resin is preferable. The fluorine-based resin is more preferably at least 1 selected from the group consisting of polyvinylidene fluoride and polytetrafluoroethylene, and polyvinylidene fluoride is more preferably used.

Examples of the negative electrode active material used for the negative electrode include carbon materials such as graphite, polyacene, carbon nanotube, and carbon nanofiber; alloy-based materials such as tin and silicon; or oxide materials such as tin oxide, silicon oxide, and lithium titanate. More than 1 of these materials can be used.

The metal foil for the negative electrode is preferably foil-like copper, and the thickness thereof is preferably 5 to 30 μm from the viewpoint of workability. The negative electrode can be produced by the above-described method for producing a positive electrode using the slurry for a negative electrode and the metal foil for a negative electrode.

[ separator ]

The separator may be any material having sufficient strength, such as an electrically insulating porous film, a net, a nonwoven fabric, and a fiber. The separator is particularly suitable for use, which has low resistance to ion migration of the electrolyte and is excellent in solution retention. The material is not particularly limited, and examples thereof include inorganic fibers such as glass fibers and organic fibers; synthetic resins such as polyethylene, polypropylene, polyester, polytetrafluoroethylene (Polyflon), and layered composites thereof. From the viewpoint of adhesiveness and stability, polyethylene, polypropylene, or a layered composite of these is preferable.

[ electrolyte ]

As the electrolyte, any known lithium salt can be used, and for example, LiClO can be mentioned₄、LiBF₄、LiBF₆、LiPF₆、LiCF₃SO₃、LiCF₃CO₂、LiAsF₆、LiSbF₆、LiB₁₀Cl₁₀、LiAlCl₄、LiCl、LiBr、LiI、LiB(C₂H₅)₄、LiCF₃SO₃、LiCH₃SO₃、LiCF₃SO₃、LiC₄F₉SO₃、LiN(CF₃SO₂)₂、LiN(C₂F₅SO₂)₂、LiC(CF₃SO₂)₃And lithium lower fatty acid carboxylates.

[ electrolyte ]

The electrolytic solution for dissolving the electrolyte is not particularly limited. Examples of the electrolyte solution include carbonates such as propylene carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate; lactones such as γ -butyrolactone; ethers such as trimethoxymethane, 1, 2-dimethoxyethane, diethyl ether, 2-ethoxyethane, tetrahydrofuran and 2-methyltetrahydrofuran; sulfoxides such as dimethyl sulfoxide; oxolanyls such as 1, 3-dioxolane and 4-methyl-1, 3-dioxolane; nitrogen-containing compounds such as acetonitrile, nitromethane and N-methyl-2-pyrrolidone; esters such as methyl formate, methyl acetate, ethyl acetate, butyl acetate, methyl propionate, ethyl propionate, and phosphoric triester; inorganic acid esters such as sulfuric acid esters, nitric acid esters, and hydrochloric acid esters; amides such as dimethylformamide and dimethylacetamide; glymes such as diglyme, triglyme and tetraglyme; ketones such as acetone, diethyl ketone, methyl ethyl ketone and methyl isobutyl ketone; sulfolanes such as sulfolane; oxazolidinones such as 3-methyl-2-oxazolidinone; and sultones such as 1, 3-propane sultone, 4-butane sultone, and naphthalene sultone. More than 1 kind selected from these electrolytic solutions can be used.

Among the above electrolytes and electrolytic solutions, LiPF is preferable₆Dissolved in an electrolyte solution of carbonates. The concentration of the electrolyte in the solution is preferably different depending on the electrode and the electrolyte to be used, and is preferably 0.5 to 3 mol/L.

Applications of the lithium ion secondary battery according to the embodiment of the present invention are not particularly limited, and examples thereof include various fields such as a digital camera, a video camera, a portable audio player, a portable AV device such as a portable liquid crystal television, a portable information terminal such as a notebook computer, a smartphone, and a mobile computer, a portable game machine, an electric power tool, an electric bicycle, a hybrid vehicle, an electric vehicle, and an electric storage system.

The present embodiment can provide a composition serving as a binder having excellent cycle capacity retention rate at high temperatures, and a positive electrode slurry and a battery using the composition.

Examples

The present invention will be described in further detail below with reference to examples. These examples are illustrative and do not limit the scope of the present invention.

[ example 1]

< preparation of polyvinyl alcohol (PVA) >

600 parts by mass of vinyl acetate and 400 parts by mass of methanol were added, nitrogen gas was bubbled and deoxygenated, and then 0.3 part by mass of bis (4-t-butylcyclohexyl) peroxydicarbonate was added as a polymerization initiator, and polymerization was carried out at 60 ℃ for 4 hours. The solid content concentration of the polymerization solution at the time of stopping the polymerization was 48% by mass, and the polymerization rate of vinyl acetate determined from the solid content was 80%. The obtained polymerization solution was purged with methanol vapor to remove unreacted vinyl acetate, and then diluted with methanol to a polyvinyl acetate concentration of 40 mass%.

To 1200 parts by mass of the diluted polyvinyl acetate solution, 20 parts by mass of a 10% by mass sodium hydroxide methanol solution was added, and a saponification reaction was performed at 30 ℃ for 2 hours.

The saponified solution was neutralized with acetic acid, filtered and dried at 100 ℃ for 2 hours to obtain PVA. The average polymerization degree and saponification degree of the PVA obtained are shown in table 1.

The average polymerization degree and saponification degree of PVA were measured according to JIS K6726.

< preparation of the composition >

1.65 parts by mass of the thus-obtained PVA was added to 8.63 parts by mass of dimethyl sulfoxide, and the mixture was stirred at 60 ℃ for 2 hours to dissolve the PVA. Further, 2.72 parts by mass of acrylonitrile, 0.47 part by mass of ethyl 2- (2-ethoxyethoxy) acrylate, and 0.45 part by mass of ammonium peroxodisulfate dissolved in 1.51 parts by mass of dimethyl sulfoxide were added at 60 ℃ and graft-copolymerized while stirring at 60 ℃.4 hours after the start of the polymerization, the reaction mixture was cooled to room temperature to stop the polymerization.

100 parts by mass of the obtained reaction solution was dropped into 300 parts by mass of methanol to precipitate a composition. The polymer was isolated by filtration and dried under vacuum at room temperature for 2 hours followed by 80 ℃ for 2 hours to give a composition (adhesive). The solid content was 9.76% by mass. If it passes through¹The polymerization rates of acrylonitrile (1 st monomer) and ethyl (2- (2-ethoxy) acrylate (2 nd monomer) were calculated by H-NMR and found to be 95%.

In the obtained composition, the mass% of the polyvinyl alcohol structure was 30 mass%, the mass% of the total content of the 1 st monomer unit and the 2 nd monomer unit was 70 mass%, the graft ratio was 222%, and the weight average molecular weight of the free polymer other than the main polymer among the free polymers was 76200. These measurement methods are described below in (total content of the 1 st monomer unit and the 2 nd monomer unit), (graft ratio), and (weight average molecular weight).

The composition and the like of the composition containing the obtained graft copolymer are shown in table 1.

(Total content of the 1 st monomer unit and the 2 nd monomer unit)

A: quality (input amount) of the 1 st monomer used for copolymerization

B: polymerization ratio of the 1 st monomer after reaction (%)

C: quality (input amount) of the 2 nd monomer used for copolymerization

D: polymerization ratio of the 2 nd monomer after reaction (%)

E: the mass (charge amount) of PVA used for polymerization,

the mass% of the total content of the 1 st monomer unit and the 2 nd monomer unit in the composition was determined by the following formula (2).

((A×B/100+C×D/100)/(A×B/100+C×D/100+E))×100(％)···(2)

And, the total content of the 1 st monomer unit and the 2 nd monomer unit can be calculated from the integral ratio of NMR. Assuming that the integral value of each proton of polyvinyl alcohol is S_PVAThe integral value of each proton of the 1 st and 2 nd monomers is S₁、S₂The total content of the 1 st monomer unit and the 2 nd monomer unit can be determined by the following formula (2-2).

((S₁+S₂)×100)/(S_PVA+S₁+S₂)···(2－2)

(ratio of the 1 st monomer unit to the 2 nd monomer unit)

The mass% of the 1 st monomer in the total amount of the 1 st monomer unit and the 2 nd monomer unit in the composition can be determined by the following formula (3).

((A×B/100)/(A×B/100+C×D/100))×100(％)···(3)

The mass% of the 1 st monomer in the total amount of the 1 st monomer unit and the 2 nd monomer unit in the composition can be determined by the following formula (3-2).

S₁＊100/(S₁+S₂)···(3－2)

(graft ratio)

1.00g of a binder was weighed, and this was added to 50cc of special DMF (produced by Kokai chemical Co., Ltd.) and stirred at 1000rpm for 24 hours at 80 ℃. Next, this was centrifuged at 10000rpm for 30 minutes using a centrifuge (model: H2000B, rotor: H) manufactured by KOKUSN corporation. The filtrate (DMF-soluble fraction) was carefully separated, and the DMF-insoluble fraction was dried under vacuum at 100 ℃ for 24 hours, and the graft ratio was calculated from the above formula (4).

(weight average molecular weight of free Polymer other than Main Polymer)

The filtrate (DMF-soluble fraction) obtained by the centrifugation was poured into 1000ml of methanol to obtain a precipitate. The precipitate was vacuum-dried at 80 ℃ for 24 hours, and the weight average molecular weight in terms of standard polystyrene was measured by GPC. The GPC measurement is performed under the following conditions.

Column: the number of 2 GPCs LF-804 was measured,(Showa Denko K.K.) was used in series.

Column temperature: 40 deg.C

Solvent: 20 mM-LiBr/DMF

(glass transition temperature)

About 5mg of the obtained sample was measured as a sample, and the measurement was carried out in accordance with JIS K7121: 1987 DSC (SEIKO electronics industry EXSTAR6000 DSC-6200) was determined. The intersection of the tangent to the base line in the DSC curve and the tangent to the sharp drop position of the endothermic region due to glass transition was read and made Tg.

< preparation of slurry >

The obtained binder 5 parts by mass was dissolved in 95 parts by mass of N-methylpyrrolidone (hereinafter abbreviated as NMP) to prepare a binder solution. Further, 1 part by mass of acetylene Black (DENKA Black (registered trademark) "HS-100" manufactured by DENKA corporation) and 1 part by mass of a binder solution in terms of solid content were mixed and stirred. After mixing, LiNi was added_0.5Mn_1.5O₄: 98 parts by mass were stirred and mixed to obtain a slurry for a positive electrode.

< production of Positive electrode >

On an aluminum foil having a thickness of 20 μm, 140g/m using an automatic coater²The prepared slurry for positive electrode was applied and pre-dried at 105 ℃ for 30 minutes. Then, the positive electrode plate was pressed by a roll press at a linear pressure of 0.1 to 3.0ton/cm to have a thickness of 75 μm. The positive electrode plate was cut into a width of 54mm, and a strip-shaped positive electrode plate was produced. After ultrasonically welding an aluminum current collecting sheet to the end of the positive electrode plate, the positive electrode plate was dried at 105 ℃ for 1 hour to completely remove the residual solvent or volatile components such as moistureThe positive electrode was obtained.

< production of negative electrode >

96.6 parts by mass of graphite ("Carbotron (registered trademark) P" manufactured by KUREHA, ltd.) as a negative electrode active material, and 3.4 parts by mass of polyvinylidene fluoride ("KF polymer (registered trademark) # 1120") as a binder in terms of solid content were added with an appropriate amount of NMP and mixed with stirring so that the total solid content was 50% by mass to obtain a slurry for a negative electrode.

On both sides of a copper foil having a thickness of 10 μm, an automatic coater was used to set the thickness to 70g/m²The prepared slurry for a negative electrode was applied one side by one side and pre-dried at 105 ℃ for 30 minutes. Then, the plate was pressed by a roll press at a line pressure of 0.1 to 3.0ton/cm to prepare a negative plate having a thickness of 90 μm on both sides. The negative electrode plate was then cut to a width of 54mm, and a strip-shaped negative electrode plate was produced. After a nickel collector sheet was ultrasonically welded to the end of the negative electrode plate, the negative electrode plate was dried at 105 ℃ for 1 hour to completely remove volatile components such as residual solvent and adsorbed moisture, thereby obtaining a negative electrode.

< production of lithium ion Secondary Battery >

The obtained positive electrode and negative electrode were combined, wound with a polyethylene microporous membrane separator having a thickness of 25 μm and a width of 60mm to prepare a spirally wound assembly, and then this assembly was inserted into a battery can. Next, LiPF was used as an electrolyte₆5ml of a nonaqueous electrolyte solution (ethylene carbonate/methylethyl carbonate mixed solution 30/70 (mass ratio)) dissolved at a concentration of 1mol/L was poured into a battery container, and then the inlet was closed to fabricate a cylindrical lithium secondary battery having a diameter of 18mm and a height of 65 mm. With respect to the produced lithium ion secondary batteries, the battery performance was evaluated by the following method.

(maintenance ratio of circulating Capacity after 50 cycles at 25 ℃ C.)

Constant-current constant-voltage charging at a charging voltage of 5.00 + -0.02V and 1ItA and constant-current discharging at 1ItA at a discharge end voltage of 3.00 + -0.02V were carried out at an ambient temperature of 25 ℃. The cycle of charge and discharge was repeated, and the discharge capacity ratio at the 50 th cycle with respect to the discharge capacity at the 1 st cycle was determined as the cycle capacity retention ratio after the 50 th cycle at 25 ℃.

(maintenance ratio of circulating Capacity after 50 cycles at 45 ℃ C.)

As a test at a high temperature, constant-current constant-voltage charging at a charging voltage of 5.00. + -. 0.02V and 1ItA and constant-current discharging at a discharging end voltage of 3.00. + -. 0.02V of 1ItA were carried out at an ambient temperature of 45 ℃. The cycle of charge and discharge was repeated, and the discharge capacity ratio at the 50 th cycle with respect to the discharge capacity at the 1 st cycle was determined as the cycle capacity retention ratio after the 50 th cycle at 45 ℃.

(rate of decrease in the circulating capacity maintenance rate due to temperature rise)

As an index of the cycle capacity maintenance rate at high temperature, a change in the "cycle capacity maintenance rate after 50 cycles at 45 ℃ with respect to the" cycle capacity maintenance rate after 50 cycles at 25 ℃ "was measured as a decrease rate of the cycle capacity maintenance rate due to a temperature increase. The fall rate is calculated from the following equation (5).

Rate of decrease in circulating capacity maintenance rate due to temperature rise (%)

(circulation capacity retention rate after 50 cycles at 45)/(circulation capacity retention rate after 50 cycles at 25)). times.100. cndot. (5)

[ examples 2 to 12& comparative examples 1 to 6]

The average polymerization degree, saponification degree and content of PVA shown in tables 1-2 were used; the type and content of the 1 st monomer unit; the kind and content of the 2 nd monomer unit were determined in the same manner as in example 1 to obtain compositions having the compositions and physical properties shown in tables 1 to 2.

The reference numerals used in the following tables and the like indicate the following compounds. The monomer units represent monomers derived from the monomer units.

AN: acrylonitrile

AA: acrylic acid

EEA: 2- (2-Ethoxyethoxy) acrylic acid ethyl ester

PPA: 2, 2, 3, 3, 3-Pentafluoroacrylic acid propyl ester

DA: acrylic acid dodecyl ester

MMA: methacrylic acid methyl ester

VA: vinyl acetate (VAA)

ST: styrene (meth) acrylic acid ester

PVA: polyvinyl alcohol

[ Table 1]

[ Table 2]