KR102184050B1 - Lithium ion secondary battery - Google Patents

Abstract

To provide a high-capacity lithium-ion secondary battery that is flexible, does not generate cracks in an active material layer during bending, and has excellent high potential cycle characteristics. The lithium ion secondary battery according to the present invention includes a positive electrode, a negative electrode, and a non-aqueous electrolyte, the positive electrode contains a positive electrode active material, a binder for a positive electrode, and a conductive material, and the positive electrode active material has an SP value of 9 to 11 ( cal/cm 3) ^1/2 of a nitrile group-containing acrylic polymer, and the binder for positive electrodes contains a fluorine-containing polymer.

Description

Lithium ion secondary battery {LITHIUM ION SECONDARY BATTERY}

The present invention relates to a lithium ion secondary battery, and more particularly, to a lithium ion secondary battery capable of increasing capacity.

In recent years, the spread of portable terminals such as notebook computers, mobile phones, and personal digital assistants (PDAs) is remarkable. As secondary batteries used for power supplies of these portable terminals, nickel hydride secondary batteries, lithium ion secondary batteries, and the like are widely used. Portable terminals are required to have more comfortable portability, so that miniaturization, thinness, weight reduction, and high performance are rapidly progressing, and as a result, portable terminals are being used in various places.

In addition, for batteries, similar to those for portable terminals, miniaturization, thinning, weight reduction, and high performance are required. Further, in order to increase the amount of the active material in the active material layer to increase the capacity of the battery, it is required to reduce materials such as a binder for fixing the active material on the current collector and a conductive material for securing conductivity.

For the purpose of increasing the capacity of a lithium ion secondary battery, a negative electrode for a lithium ion secondary battery using an alloy-based active material containing Si or the like has been developed (for example, Patent Document 1). As the capacity increases, the voltage at the time of charging and discharging is also increasing, and since the electrolytic solution made of ethylene carbonate or propylene carbonate may not withstand high voltage and may be decomposed, a combination of a fluorine-based electrolytic solution additive is also used.

On the other hand, in the positive electrode, as a binder for forming the active material layer, a fluorine-containing polymer such as polyvinylidene fluoride (PVdF) has been used. Since the fluorine-containing polymer does not dissolve in the electrolytic solution, stable binding properties are expected, but fluorine-containing polymers such as PVdF are hard and difficult to bend. For this reason, depending on the shape and size of the battery, cracks may occur in the active material layer when only the fluorine-containing polymer is used when the electrode is wound and then crushed to form a predetermined shape.

In addition, in order to achieve a high capacity, it has been studied to fine-grain carbon black, which is a conductive material, particularly in the positive electrode. However, when the fluorine-containing polymer is used alone as a binder, the fine-grained conductive material aggregates, the dispersibility becomes insufficient, the conductivity does not increase, and the capacity cannot be improved in some cases.

Japanese Patent No. 4025995 Japanese Patent No. 3598153 Japanese Patent No. 4929573

For lithium ion secondary batteries, when a fluorine-containing polymer is used as a binder in the positive electrode in order to achieve a high capacity, there is a problem that cracks are generated in the active material layer. In addition, as a binder used for the positive electrode, it is being studied to use a fluorine-containing polymer and a nitrile rubber in combination, or to use a fluorine-containing polymer and a crosslinked acrylate polymer in combination.

For example, in Patent Document 2, a fluorine-containing polymer and nitrile rubber are used in combination as a binder for forming an active material layer of a lithium ion secondary battery. However, as the amount of the binder increases, the density of the active material relatively decreases, and thus sufficient battery capacity may not be obtained. In addition, since the amount of the binder increases, the degree of swelling of the active material layer with respect to the electrolyte solution also increases, and in particular, the peel strength in the high potential cycle decreases, and the cycle characteristics may decrease.

In addition, in Patent Document 3, a fluorine-containing polymer and a crosslinked acrylate polymer are used together as a binder. However, since the crosslinked acrylate polymer is used, swelling of the active material layer to the electrolyte is suppressed, but since the crosslinked acrylate polymer exists in the form of particles, the dispersibility becomes insufficient, and as a result, the high potential cycle characteristics decrease. There are cases.

Accordingly, it is an object of the present invention to provide a high-capacity lithium ion secondary battery that is flexible, does not cause cracks in the active material layer during bending, and has excellent high potential cycle characteristics.

In order to solve the above problems, the inventors of the present invention have studied intensively in order to solve the above problems. As a result of coating the positive electrode active material with a nitrile group-containing acrylic polymer having specific physical properties, the nitrile group-containing acrylic polymer is dissolved in the dispersion medium of the slurry composition for the positive electrode, while the dissolution parameter It was found out that (SP value) could be swelled in an appropriate range without being dissolved in an electrolytic solution close to. In addition, by using a conductive material powder for the positive electrode, while trying to increase the capacity, by using a fluorine-containing polymer as a binder for the positive electrode, it is flexible, there is no crack in the active material layer during bending, and an electrode having a high active material density is obtained. In addition, it was found that a high-capacity lithium ion secondary battery having excellent high potential cycle characteristics can be obtained. Based on these findings, the present invention has been completed.

The summary of the present invention is as follows.

[1] As a lithium ion secondary battery comprising a positive electrode, a negative electrode and a non-aqueous electrolyte,

The positive electrode contains a positive electrode active material, a binder for a positive electrode, and a conductive material,

The positive electrode active material is coated with a nitrile group-containing acrylic polymer having an SP value of 9 to 11 (cal/cm 3) ^1/2 ,

The lithium ion secondary battery, wherein the binder for positive electrodes contains a fluorine-containing polymer.

[2] The lithium ion secondary battery according to [1], wherein the positive electrode active material is an excess lithium layered compound containing Li, Mn, Co, and Ni.

[3] The lithium ion secondary battery according to [1] or [2], wherein the negative electrode contains an alloy-based active material.

[4] The lithium ion secondary battery according to any one of [1] to [3], wherein the swelling degree of the nitrile group-containing acrylic polymer in the nonaqueous electrolyte solution is 3 times or less, and the amount of THF insoluble content is 30 mass% or less.

[5] The lithium ion secondary battery according to any one of [1] to [4], wherein the conductive material has a particle diameter of 5 to 40 nm.

[6] The lithium ion secondary battery according to any one of [1] to [5], wherein 1 to 3 parts by mass of a conductive material and 0.4 to 2 parts by mass of a binder for a positive electrode are contained with respect to 100 parts by mass of the positive electrode active material.

[7] The lithium ion secondary battery according to any one of [1] to [6], wherein the fluorine-containing polymer is polyvinylidene fluoride.

[8] The lithium ion secondary battery according to any one of [1] to [7], wherein the nitrile group-containing acrylic polymer contains an ethylenically unsaturated acid monomer unit.

[9] The lithium ion secondary battery according to [8], wherein the content ratio of the ethylenically unsaturated acid monomer unit in the nitrile group-containing acrylic polymer is 10 to 30 mass%.

[10] The lithium ion secondary battery according to any one of [1] to [9], which is a wound pouch cell.

According to the present invention, by using a conductive material powder for the positive electrode, coating the positive electrode active material with a nitrile group-containing acrylic polymer having specific physical properties, and using a fluorine-containing polymer as a binder, it is flexible, and the active material layer is formed during bending. In addition to having no cracking, an electrode having a high active material density is obtained. In addition, it is possible to provide a lithium ion secondary battery having excellent high potential cycle characteristics.

Hereinafter, the present invention will be described in more detail. The lithium ion secondary battery according to the present invention (hereinafter, simply referred to as "secondary battery" in some cases) includes a positive electrode, a negative electrode, and a non-aqueous electrolyte. The positive electrode contains a positive electrode active material, a binder for positive electrodes, and a conductive material. The positive electrode active material is coated with a nitrile group-containing acrylic polymer having an SP value of 9 to 11 (cal/cm 3) ^1/2 . The binder for the positive electrode is a fluorine-containing polymer. Hereinafter, each of the positive electrode, the negative electrode, and the non-aqueous electrolyte will be described in more detail.

(Positive)

The positive electrode comprises a current collector and a positive electrode active material layer laminated on the current collector. The positive electrode active material layer contains a positive electrode active material (A), a binder for positive electrodes (B), and a conductive material (C), and contains other components as necessary. Further, the positive electrode active material (A) is a positive electrode active material coated with a nitrile group-containing acrylic polymer having an SP value of 9 to 11 (cal/cm 3) ^1/2 .

(A) positive electrode active material

As the positive electrode active material, an active material capable of intercalating and desorbing lithium ions is used, and such a positive electrode active material is roughly classified into an inorganic compound and an organic compound.

Examples of the positive electrode active material made of the inorganic compound include transition metal oxides, transition metal sulfides, and lithium-containing composite metal oxides of lithium and transition metals.

As the transition metal, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Mo, and the like are used.

As transition metal oxides, MnO, MnO ₂ , V ₂ O ₅ , V ₆ O ₁₃ , TiO ₂ , Cu ₂ V ₂ O ₃ , amorphous V ₂ OP ₂ O ₅ , MoO ₃ , V ₂ O ₅ , V ₆ O ₁₃ And the like, among them, MnO, V ₂ O ₅ , V ₆ O ₁₃ , and TiO ₂ are preferable in terms of cycle characteristics and capacity.

Examples of the transition metal sulfide include TiS ₂ , TiS ₃ , amorphous MoS ₂ , and FeS.

Examples of the lithium-containing composite metal oxide 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-type structure.

As a lithium-containing composite metal oxide having a layered structure, for example, a lithium-containing cobalt oxide (LiCoO ₂ ), a lithium-containing nickel oxide (LiNiO ₂ ), and a lithium composite oxide of Co-Ni-Mn (Li(CoMnNi)O ₂ ) , Li, Mn, Co and Ni containing lithium excess layered compound (Li[Ni _0.17 Li _0.2 Co _0.07 Mn _0.56 ]O ₂ ), Ni-Mn-Al lithium composite oxide, Ni-Co-Al lithium composite oxide And the like.

As the lithium-containing composite metal oxide having a spinel structure, for example, lithium manganate (LiMn ₂ O ₄ ) or Li[Mn _3/2 M _1/2 ]O ₄ in which a part of Mn is substituted with another transition metal (here M is Cr, Fe, Co, Ni, Cu, etc.), etc. are mentioned.

As a lithium-containing composite metal oxide having an olivine structure, for example, Li _X MPO ₄ (in the formula, M is, Mn, Fe, Co, Ni, Cu, Mg, Zn, V, Ca, Sr, Ba, Ti , At least one selected from Al, Si, B and Mo, and an olivine-type lithium phosphate compound represented by 0≦X≦2).

Among these, in terms of excellent cycle characteristics and initial capacity, lithium-containing cobalt oxide (LiCoO ₂ ), lithium-containing nickel oxide (LiNiO ₂ ), lithium composite oxide of Co-Ni-Mn (Li(CoMnNi)O ₂ ), Li , Mn, Co and a lithium-excessive layered compound containing Ni (Li[Ni _0.17 Li _0.2 Co _0.07 Mn _0.56 ]O ₂ ), a lithium-containing composite metal oxide having a spinel structure (LiNi _0.5 Mn _1.5 O ₄ ) is preferred, A lithium-containing cobalt oxide (LiCoO ₂ ), a lithium-excessive layered compound containing Li, Mn, Co and Ni (Li[Ni _0.17 Li _0.2 Co _0.07 Mn _0.56 ]O ₂ ) is more preferable.

As a positive electrode active material made of an organic compound, for example, a conductive polymer such as polyacetylene and poly-p-phenylene may be used.

Further, the iron-based oxide, which is poor in electrical conductivity, may be used as an electrode active material covered with a carbon material by making a carbon source material present during reduction firing.

Moreover, you may use what element substituted these compounds partially.

The positive electrode active material for a lithium ion secondary battery may be a mixture of the above inorganic compound and organic compound.

The volume average particle diameter of the positive electrode active material is usually 1 to 50 µm, preferably 2 to 30 µm. When the volume average particle diameter of the positive electrode active material is in the above range, the amount of the positive electrode binder in the positive electrode active material layer can be reduced, and a decrease in battery capacity can be suppressed. In addition, in order to form a positive electrode active material layer, a slurry containing a positive electrode active material and a positive electrode binder (hereinafter, sometimes referred to as “a positive electrode slurry composition”) is usually prepared, but this slurry composition for a positive electrode is prepared. It becomes easy to prepare with a viscosity suitable for coating, and a uniform positive electrode active material layer can be obtained.

The content ratio of the positive electrode active material in the positive electrode active material layer is preferably 90 to 99.9 mass%, more preferably 95 to 99 mass%. When the content of the positive electrode active material in the positive electrode active material layer is in the above range, flexibility and binding properties can be exhibited while exhibiting high capacity.

The positive electrode active material (A) in the present invention is coated with a nitrile group-containing acrylic polymer. By using the positive electrode active material (A) coated with a nitrile group-containing acrylic polymer, the cycle characteristics at a high potential of a lithium ion secondary battery are improved. Hereinafter, a nitrile group-containing acrylic polymer will be described.

Nitrile group-containing acrylic polymer

The nitrile group-containing acrylic polymer is a polymer containing a nitrile group-containing monomer unit and a (meth)acrylic acid ester monomer unit. The nitrile group-containing monomer unit refers to a structural unit formed by polymerizing a monomer having a nitrile group, and the (meth)acrylic acid ester monomer unit refers to a structural unit formed by polymerizing a (meth)acrylic acid ester monomer. The nitrile group-containing acrylic polymer contains a nitrile group-containing monomer unit and a (meth)acrylic acid ester monomer unit, and further, if necessary, a monomer derived from other monomers such as ethylenic unsaturated acid monomer units and crosslinkable monomers. Contains units. These monomer units are structural units formed by polymerizing the monomer. Here, the content ratio of each monomer (injection ratio of the monomer) generally matches the content ratio of each monomer unit in the nitrile group-containing acrylic polymer.

Specific examples of the nitrile group-containing monomer include acrylonitrile, methacrylonitrile, and the like, and among them, acrylonitrile is preferable from the viewpoint of increasing the adhesion to the current collector and improving electrode strength.

The content ratio of the nitrile group-containing monomer unit in the nitrile group-containing acrylic polymer is preferably 5 to 35% by mass, more preferably 10 to 30% by mass, and particularly preferably 15 to 25% by mass. When the content ratio of the nitrile group-containing monomer unit is within this range, the adhesion to the current collector is excellent, and the strength of the obtained electrode is improved.

The (meth)acrylic acid ester monomer unit is a compound represented by the general formula (1): CH ₂ =CR ¹ -COOR ² (in the formula, R ¹ represents a hydrogen atom or a methyl group, and R ² represents an alkyl group or a cycloalkyl group.) It is a structural unit formed by polymerizing the derived monomer.

Specific examples of the compound represented by the general formula (1) include ethyl acrylate, propyl acrylate, isopropyl acrylate, n-butyl acrylate, isobutyl acrylate, t-butyl acrylate, n-amyl acrylate, isoamyl acrylate, and n-hexyl acrylate. Acrylates such as 2-ethylhexyl acrylate, lauryl acrylate, and stearyl acrylate; Ethyl methacrylate, propyl methacrylate, isopropyl methacrylate, n-butyl methacrylate, isobutyl methacrylate, t-butyl methacrylate, n-amyl methacrylate, isoamyl methacrylate, meth And methacrylates such as n-hexyl acrylate, 2-ethylhexyl methacrylate, lauryl methacrylate, and stearyl methacrylate. Among these, acrylate is preferable, and n-butyl acrylate and 2-ethylhexyl acrylate are particularly preferable from the viewpoint of improving the strength of the obtained electrode.

One type of (meth)acrylic acid ester monomer may be used alone, or two or more types may be combined in an arbitrary ratio. Accordingly, the nitrile group-containing acrylic polymer may contain only one type of (meth)acrylic acid ester monomer, or may contain two or more types in combination in an arbitrary ratio.

The content ratio of the (meth)acrylic acid ester monomer unit in the nitrile group-containing acrylic polymer is preferably 35 to 85% by mass, more preferably 45 to 80% by mass, still more preferably 45 to 75% by mass, particularly preferably Is 50 to 70 mass%. When a nitrile group-containing acrylic polymer in which the content ratio of the (meth)acrylic acid ester monomer unit is in the above range is used, the flexibility of the electrode is high and the swelling property to the non-aqueous electrolyte is suppressed. Moreover, heat resistance is high, and the internal resistance of the electrode obtained can be made small.

The nitrile group-containing acrylic polymer may contain an ethylenically unsaturated acid monomer unit in addition to the nitrile group-containing monomer unit and the (meth)acrylic acid ester monomer unit.

The ethylenically unsaturated acid monomer unit is a structural unit formed by polymerizing an ethylenically unsaturated acid monomer. The ethylenically unsaturated acid monomer is an ethylenically unsaturated monomer having an acid group such as a carboxyl group, a sulfonic acid group, and a phosphinyl group, and is not limited to a specific monomer. Specific examples of the ethylenically unsaturated acid monomer include an ethylenically unsaturated carboxylic acid monomer, an ethylenically unsaturated sulfonic acid monomer, and an ethylenically unsaturated phosphoric acid monomer.

Specific examples of the ethylenically unsaturated carboxylic acid monomer include ethylenically unsaturated monocarboxylic acids and derivatives thereof, ethylenically unsaturated dicarboxylic acids and acid anhydrides thereof, and derivatives thereof.

Examples of ethylenically unsaturated monocarboxylic acids include acrylic acid, methacrylic acid, and crotonic acid.

Examples of derivatives of ethylenically unsaturated monocarboxylic acids include 2-ethylacrylic acid, isocrotonic acid, α-acetoxyacrylic acid, β-trans-aryloxyacrylic acid, α-chloro-β-E-methoxyacrylic acid, and β- Diaminoacrylic acid may be mentioned.

Examples of ethylenically unsaturated dicarboxylic acids include maleic acid, fumaric acid, and itaconic acid.

Examples of the acid anhydride of the ethylenically unsaturated dicarboxylic acid include maleic anhydride, acrylic anhydride, methyl maleic anhydride, and dimethyl maleic anhydride.

Examples of derivatives of ethylenically unsaturated dicarboxylic acid include methyl allyl maleate such as methyl maleic acid, dimethyl maleic acid, phenyl maleic acid, chloromaleic acid, dichloromaleic acid, and fluoromaleic acid; And maleic acid esters, such as diphenyl maleate, nonyl maleate, decyl maleate, dodecyl maleate, octadecyl maleate, and fluoroalkyl maleate, are mentioned.

Specific examples of ethylenically unsaturated sulfonic acid monomers are vinylsulfonic acid, methylvinylsulfonic acid, styrenesulfonic acid, (meth)acrylic sulfonic acid, (meth)acrylic acid-2-ethyl sulfonic acid, 2-acrylamide-2-hydroxypropanesulfonic acid, 2- Acrylamide-2-methylpropanesulfonic acid and the like.

Specific examples of the ethylenically unsaturated phosphoric acid monomer include propyl (meth)acrylate-3-chloro-2-phosphate, ethyl (meth)acrylate-2-phosphate, and 3-allyloxy-2-hydroxypropanephosphate.

Further, an alkali metal salt or an ammonium salt of the ethylenically unsaturated acid monomer can also be used.

One type of the ethylenically unsaturated acid monomer may be used alone, or two or more types may be combined in an arbitrary ratio. Accordingly, the nitrile group-containing acrylic polymer may contain only one type of ethylenically unsaturated acid monomer, or may contain two or more types in combination in an arbitrary ratio.

Among these, from the viewpoint of improving the dispersibility of the nitrile group-containing acrylic polymer, as the ethylenically unsaturated acid monomer, an ethylenically unsaturated carboxylic acid monomer or an ethylenically unsaturated sulfonic acid monomer is used alone, or an ethylenically unsaturated carboxylic acid monomer And ethylenically unsaturated sulfonic acid monomers are preferably used in combination.

Among ethylenically unsaturated carboxylic acid monomers, from the viewpoint of expressing good dispersibility in the nitrile group-containing acrylic polymer, it is preferably an ethylenically unsaturated monocarboxylic acid, more preferably acrylic acid or methacrylic acid, and particularly preferably Is methacrylic acid.

In addition, among ethylenically unsaturated sulfonic acid monomers, 2-acrylamide-2-hydroxypropanesulfonic acid and 2-acrylamide-2-methylpropanesulfonic acid are preferred from the viewpoint of exhibiting good dispersibility in the nitrile group-containing acrylic polymer. , More preferably 2-acrylamide-2-methylpropanesulfonic acid.

The content ratio of the ethylenically unsaturated acid monomer unit in the nitrile group-containing acrylic polymer is preferably 3 to 30% by mass, more preferably 10 to 30% by mass, still more preferably 12 to 28% by mass, particularly preferably It is in the range of 14 to 26 mass%. When an ethylenically unsaturated carboxylic acid monomer and an ethylenically unsaturated sulfonic acid monomer are used in combination as the ethylenically unsaturated acid monomer, the content ratio of the ethylenically unsaturated carboxylic acid monomer in the nitrile group-containing acrylic polymer is preferably 3 to It is 30 mass %, more preferably 10 to 30 mass %, particularly preferably 12 to 28 mass %, and the content ratio of the ethylenically unsaturated sulfonic acid monomer is preferably 0.1 to 10 mass %.

By setting the content ratio of the ethylenically unsaturated acid monomer unit in the above range, the dispersibility of the nitrile group-containing acrylic polymer when slurry is improved, the positive electrode active material can be satisfactorily coated, and as a result, the positive electrode having high uniformity An active material layer can be formed, and the resistance of the positive electrode can be reduced.

The nitrile group-containing acrylic polymer may further contain a crosslinkable monomer unit in a range that does not affect the amount of insoluble THF of the nitrile group-containing acrylic polymer in addition to the respective monomer units. The crosslinkable monomer unit is a structural unit capable of forming a crosslinked structure during or after polymerization by heating or irradiating the crosslinkable monomer with energy. As an example of a crosslinkable monomer, a monomer which has thermal crosslinkability is mentioned normally. More specifically, a monofunctional monomer having a thermally crosslinkable crosslinkable group and one olefinic double bond per molecule, and a polyfunctional monomer having two or more olefinic double bonds per molecule may be mentioned.

Examples of the thermally crosslinkable crosslinkable group contained in the monofunctional monomer include an epoxy group, an N-methylolamide group, an oxetanyl group, an oxazoline group, and combinations thereof. Among these, an epoxy group is more preferable because it is easy to control crosslinking and crosslinking density.

Examples of the crosslinkable monomer having an epoxy group as a thermally crosslinkable crosslinkable group and an olefinic double bond include vinyl glycidyl ether, allyl glycidyl ether, butenyl glycidyl ether, o-allylphenyl glycidyl ether Unsaturated glycidyl ethers such as; Dienes such as butadiene monoepoxide, chloroprene monoepoxide, 4,5-epoxy-2-pentene, 3,4-epoxy-1-vinylcyclohexene, 1,2-epoxy-5,9-cyclododecadiene, or Monoepoxide of polyene; Alkenyl epoxides such as 3,4-epoxy-1-butene, 1,2-epoxy-5-hexene, and 1,2-epoxy-9-decene; And glycidyl acrylate, glycidyl methacrylate, glycidyl crotonate, glycidyl-4-heptenoate, glycidyl sorbate, glycidylinoleate, glycidyl-4-methyl Glycidyl esters of unsaturated carboxylic acids such as glycidyl esters of -3-pentenoate, 3-cyclohexenecarboxylic acid, and glycidyl esters of 4-methyl-3-cyclohexenecarboxylic acid. I can.

Examples of the crosslinkable monomer having an N-methylolamide group as a thermally crosslinkable crosslinkable group and an olefinic double bond include (meth)acrylamides having a methylol group such as N-methylol (meth)acrylamide. I can.

Examples of the crosslinkable monomer having an oxetanyl group as a thermally crosslinkable crosslinkable group and an olefinic double bond include 3-((meth)acryloyloxymethyl)oxetane, 3-((meth)acryloyloxy Methyl)-2-trifluoromethyloxetane, 3-((meth)acryloyloxymethyl)-2-phenyloxetane, 2-((meth)acryloyloxymethyl)oxetane, and 2-( (Meth)acryloyloxymethyl)-4-trifluoromethyloxetane.

Examples of a crosslinkable monomer having an oxazoline group as a thermally crosslinkable crosslinkable group and an olefinic double bond include 2-vinyl-2-oxazoline, 2-vinyl-4-methyl-2-oxazoline, 2-vinyl- 5-methyl-2-oxazoline, 2-isopropenyl-2-oxazoline, 2-isopropenyl-4-methyl-2-oxazoline, 2-isopropenyl-5-methyl-2-oxazoline, And 2-isopropenyl-5-ethyl-2-oxazoline.

Examples of the polyfunctional monomer having two or more olefinic double bonds include allyl (meth) acrylate, ethylene di (meth) acrylate, diethylene glycol di (meth) acrylate, triethylene glycol di (meth) acrylate, Tetraethylene glycol di(meth)acrylate, trimethylolpropane-tri(meth)acrylate, dipropylene glycol diallyl ether, polyglycol diallyl ether, triethylene glycol divinyl ether, hydroquinone diallyl ether, tetraallyloxy Ethane, trimethylolpropane-diallyl ether, allyl or vinyl ether of polyfunctional alcohols other than the above, triallylamine, methylenebisacrylamide, and divinylbenzene.

As the crosslinkable monomer, in particular, allyl (meth)acrylate, ethylene di (meth)acrylate, allyl glycidyl ether, and glycidyl methacrylate can be preferably used.

The said crosslinkable monomer may be used individually by 1 type, and may combine 2 or more types by arbitrary ratios. Accordingly, the nitrile group-containing acrylic polymer may contain only one type of crosslinkable monomer, or may contain two or more types in combination in an arbitrary ratio.

When the crosslinkable monomer unit is contained in the nitrile group-containing acrylic polymer, the content ratio is preferably 0.1% by mass or more, more preferably 0.2% by mass or more, particularly preferably 0.5% by mass or more, and preferably It is 5% by mass or less, more preferably 4% by mass or less, and particularly preferably 2% by mass or less. By making the content ratio of the crosslinkable monomer unit more than the lower limit of the above range, the degree of crosslinking of the nitrile group-containing acrylic polymer can be increased, and the degree of swelling can be prevented from excessively increasing. On the other hand, by making the ratio of the crosslinkable monomer unit below the upper limit of the above range, the dispersibility of the nitrile group-containing acrylic polymer can be improved. Therefore, both the swelling degree and the dispersibility can be made favorable by making the content ratio of a crosslinkable monomer unit within the said range.

Further, in addition to the above, the nitrile group-containing acrylic polymer may contain an aromatic vinyl monomer unit, an ethylenically unsaturated carboxylic acid amide monomer unit, or the like.

Examples of the aromatic vinyl monomer include styrene, α-methylstyrene, vinyltoluene, chlorostyrene, and hydroxymethylstyrene.

Examples of ethylenically unsaturated carboxylic acid amide monomers include (meth)acrylamide and N-methoxymethyl (meth)acrylamide.

By containing these monomer units, the dispersibility of the nitrile group-containing acrylic polymer when it is made into a slurry is improved, and the positive electrode active material can be satisfactorily coated. These monomer units may be contained in a ratio of 10% by mass or less.

Here, the content ratio of each monomer (injection ratio of the monomer) is usually each monomer unit in the nitrile group-containing acrylic polymer (for example, a (meth)acrylic acid ester monomer unit, an ethylenically unsaturated acid monomer unit, and It coincides with the content ratio of the crosslinkable monomer unit).

Next, the SP value of the nitrile group-containing acrylic polymer, the degree of swelling in the non-aqueous electrolyte, and the amount of tetrahydrofuran (THF) insoluble in the nitrile group-containing acrylic polymer will be described.

The SP value of the nitrile group-containing acrylic polymer is 9 to 11 (cal/cm 3) ^1/2 , preferably 9 to 10.5 (cal/cm 3) ^1/2 , more preferably 9.5 to 10 (cal/cm 3) ) ^1/2 . By setting the SP value of the nitrile group-containing acrylic polymer in the above range, while maintaining the solubility in the dispersion medium (hereinafter sometimes referred to as “slurry dispersion medium”) used when preparing the slurry composition for a positive electrode described later. It can have an appropriate swelling property for a non-aqueous electrolyte. Thereby, the uniformity of the obtained electrode is further improved, and the cycle characteristic of the secondary battery using it can be improved.

The SP value means a solubility parameter. This SP value can be obtained according to the method described in J. Brandrup and E. H. Immergut edition "Polymer Handbook" VII Solubility Parameter Values, pp 519-559 (John Wiley & Sons, 3rd edition, published in 1989).

In addition, the SP value of a polymer that is not described in the above document "Polymer Handbook" can be obtained according to the "molecular attraction constant method" proposed by Small. This method is a method of determining the SP value (δ) according to the following equation from the characteristic value of the functional group (atomic group) constituting the compound molecule, that is, the statistics of the molecular attraction constant (G) and the molecular weight.

In the above formula, δ represents the SP value, ΣG represents the total number of molecular attraction constants G, V represents the cost, M represents the molecular weight, and d represents the specific gravity.

In addition, when two or more types of nitrile group-containing acrylic polymers are combined to cover the surface of the positive electrode active material, the SP value of the entire nitrile group-containing acrylic polymer is calculated from the SP value and the mixing molar ratio of the respective nitrile group-containing acrylic polymers. You can get it. Specifically, the SP value of the entire nitrile group-containing acrylic polymer is calculated as a weighted average weighted by a molar ratio with respect to the SP value of each nitrile group-containing acrylic polymer.

When the nitrile group-containing acrylic polymer has the above SP value, it means that the nitrile group-containing acrylic polymer is dissolved in the slurry dispersion medium, but swells in the non-aqueous electrolyte solution. Therefore, in the lithium ion secondary battery, since the portion coated with the nitrile group-containing acrylic polymer of the positive electrode active material does not contact the non-aqueous electrolyte, deterioration of the positive electrode active material is suppressed, and the cycle characteristics decrease even if the charge/discharge cycle is repeated. Can be prevented.

In addition, the fact that the nitrile group-containing acrylic polymer has the above SP value means that the nitrile group-containing acrylic polymer can swell in the non-aqueous electrolyte solution of the lithium ion secondary battery. Therefore, in the lithium ion secondary battery of the present invention, since the nitrile group-containing acrylic polymer does not hinder the movement of ions, the internal resistance in the positive electrode of the lithium ion secondary battery can be suppressed to a small extent. In addition, since water is not usually contained in a lithium ion secondary battery, or even if it is contained in a small amount, even if the nitrile group-containing acrylic polymer is swollen, corrosion of the current collector due to elution of the positive electrode active material is difficult to proceed.

The degree of swelling of the nitrile group-containing acrylic polymer in the nonaqueous electrolyte solution is preferably 1.0 times or more and 3 times or less, and more preferably 1.0 times or more and 2.8 times or less, in order to avoid remarkable changes in the volume of these polymers in the nonaqueous electrolyte. , More preferably 1.0 times or more and 2.6 times or less. Here, the non-aqueous electrolytic solution is an electrolytic solution constituting the lithium ion secondary battery of the present invention. By making the swelling degree of the nitrile group-containing acrylic polymer with respect to the non-aqueous electrolyte solution in the above range, even if the charge/discharge cycle is repeated, the adhesion of the positive electrode active material layer to the current collector is maintained, and the cycle characteristics are improved. The degree of swelling to the non-aqueous electrolytic solution can be controlled by, for example, the content ratio of each monomer unit described above. Specifically, it increases when the content ratio of the nitrile group-containing monomer unit is increased. Further, it decreases when the content ratio of the ethylenically unsaturated acid monomer unit is increased.

In addition, the amount of tetrahydrofuran (THF) insoluble in the nitrile group-containing acrylic polymer is preferably 30 mass% or less, more preferably 25 mass% or less, in order to properly dissolve such a polymer in the slurry dispersion medium. Preferably, it is in the range of 20 mass% or less. THF insoluble matter is an index of the amount of gel, and if the amount of THF insoluble matter is large, the possibility of present in the form of particles increases in a slurry using an organic solvent such as N-methylpyrrolidone (hereinafter, it may be described as NMP). , The dispersibility in the slurry may be impaired. The amount of THF insoluble can be controlled by the polymerization reaction temperature, the addition time of the monomer, the amount of the polymerization initiator, and the like, as described later. Specifically, the amount of THF insoluble content decreases by increasing the polymerization reaction temperature or increasing the polymerization initiator or chain transfer agent. In addition, the lower limit of the amount of THF insoluble content is not particularly limited, but is preferably 0% by mass or more, more preferably more than 0% by mass, and particularly preferably 5% by mass or more.

Since the dissolution parameter (SP value) of the nonaqueous electrolyte and the organic solvent as the slurry dispersion medium is close, if the swelling degree of the polymer to the nonaqueous electrolyte is within an appropriate range, such a polymer is not dissolved in the organic solvent as the slurry dispersion medium (THF insoluble content) In some cases, conversely, if such a polymer is easily dissolved in an organic solvent, the swelling degree of the polymer in the non-aqueous electrolyte solution may be out of the appropriate range, but in the present invention, the swelling degree and THF insoluble All of them are within an appropriate range.

The method for producing the nitrile group-containing acrylic polymer is not particularly limited, but as described above, a monomer mixture containing a monomer constituting the nitrile group-containing acrylic polymer can be obtained by emulsion polymerization. The method of emulsion polymerization is not particularly limited, and a conventionally known emulsion polymerization method may be employed. The mixing method is not particularly limited, and examples thereof include a method using a mixing device such as a stirring type, a shaking type, and a rotary type. Further, a method using a dispersion kneading apparatus such as a homogenizer, a ball mill, a sand mill, a roll mill, a planetary mixer, and a planetary kneader can be mentioned.

Examples of the polymerization initiator used in the emulsion polymerization include inorganic peroxides such as sodium persulfate, potassium persulfate, ammonium persulfate, potassium perphosphate, and hydrogen peroxide; t-butyl peroxide, cumene hydroperoxide, p-mentane hydroperoxide, di-t-butyl peroxide, t-butyl cumyl peroxide, acetyl peroxide, isobutyryl peroxide, octanoyl peroxide, benzoyl peroxide Organic peroxides such as 3,5,5-trimethylhexanoyl peroxide and t-butylperoxyisobutyrate; Azo compounds, such as azobisisobutyronitrile, azobis-2,4-dimethylvaleronitrile, azobiscyclohexanecarbonitrile, and methyl azobisisobutyrate, etc. are mentioned.

Among these, inorganic peroxides can be preferably used. These polymerization initiators can be used individually or in combination of two or more, respectively. Moreover, the peroxide initiator can also be used as a redox-based polymerization initiator in combination with a reducing agent such as sodium bisulfite.

The amount of the polymerization initiator used is preferably 0.05 to 5 parts by mass, more preferably 0.1 to 2 parts by mass, based on 100 parts by mass of the total amount of the monomer mixture used for polymerization. Within the above range, by using a polymerization initiator, the amount of THF insoluble content of the obtained nitrile group-containing acrylic polymer can be appropriately adjusted.

In order to control the amount of THF insoluble content of the obtained nitrile group-containing acrylic polymer, a chain transfer agent may be used during emulsion polymerization. Examples of the chain transfer agent include alkyl mers such as n-hexyl mercaptan, n-octyl mercaptan, t-octyl mercaptan, n-dodecyl mercaptan, t-dodecyl mercaptan, and n-stearyl mercaptan. Captan; Xanthogen compounds, such as dimethyl xanthogen disulfide and diisopropyl xanthogen disulfide; Thiuram compounds, such as tapinorene, tetramethyl thiuram disulfide, tetraethyl thiuram disulfide, and tetramethyl thiuram monosulfide; Phenolic compounds such as 2,6-di-t-butyl-4-methylphenol and styrenated phenol; Allyl compounds such as allyl alcohol; Halogenated hydrocarbon compounds such as dichloromethane, dibromomethane, and carbon tetrabromide; Thioglycolic acid, thiomalic acid, 2-ethylhexylthioglycolate, diphenylethylene, α-methylstyrene dimer, etc. are mentioned.

Among these, alkyl mercaptan is preferable, and t-dodecyl mercaptan can be more preferably used. These chain transfer agents can be used alone or in combination of two or more.

The amount of the chain transfer agent used is preferably 0.05 to 2 parts by mass, more preferably 0.1 to 1 part by mass, based on 100 parts by mass of the monomer mixture.

In the case of emulsion polymerization, a surfactant may be used. The surfactant may be any of anionic surfactants, nonionic surfactants, cationic surfactants, and amphoteric surfactants. Specific examples of the anionic surfactant include sodium lauryl sulfate, ammonium lauryl sulfate, sodium dodecyl sulfate, ammonium dodecyl sulfate, sodium octyl sulfate, sodium decyl sulfate, sodium tetradecyl sulfate, sodium hexadecyl sulfate. Sulfates of higher alcohols such as pate and sodium octadecyl sulfate; Alkylbenzene sulfonates, such as sodium dodecylbenzenesulfonate, sodium laurylbenzenesulfonate, and sodium hexadecylbenzenesulfonate; Aliphatic sulfonic acid salts such as sodium lauryl sulfonate, sodium dodecyl sulfonate, and sodium tetradecyl sulfonate; And the like.

The surfactant is used in an amount of preferably 0.5 to 10 parts by mass, and more preferably 1 to 5 parts by mass, based on 100 parts by mass of the monomer mixture.

Moreover, at the time of emulsion polymerization, pH adjusters, such as sodium hydroxide and ammonia; Various additives, such as a dispersing agent, a chelating agent, an oxygen scavenger, a builder, and sidratex, for particle diameter control can be used suitably. Cydratex refers to a dispersion of fine particles that become the nucleus of a reaction during emulsion polymerization. The fine particles often have a particle diameter of 100 nm or less. The microparticles are not particularly limited, and general-purpose polymers such as diene polymers and acrylic polymers are used. According to the emulsion polymerization method using Sidratex, polymer particles having a relatively even particle size are obtained.

The polymerization temperature at the time of performing the polymerization reaction is not particularly limited, but is usually 0 to 100°C, preferably 40 to 80°C. Emulsion polymerization is carried out in such a temperature range, and a polymerization terminator is added or the polymerization system is cooled to stop the polymerization reaction at a predetermined polymerization conversion rate. The polymerization conversion rate at which the polymerization reaction is stopped is preferably 93% by mass or more, and more preferably 95% by mass or more. Moreover, by making the polymerization temperature into the said range, the amount of THF insoluble content of the obtained nitrile group-containing acrylic polymer can be adjusted suitably.

After stopping the polymerization reaction, if desired, unreacted monomers are removed, and the pH or solid content concentration is adjusted to obtain an acrylic polymer containing a nitrile group in a form in which the polymer is dispersed in a dispersion medium (latex). Thereafter, if necessary, the dispersion medium may be substituted, or the dispersion medium may be evaporated to obtain a particulate nitrile group-containing acrylic polymer in a powder form.

To the dispersion of the nitrile group-containing acrylic polymer, a known dispersing agent, a thickener, an anti-aging agent, an antifoaming agent, an antiseptic agent, an antibacterial agent, an anti-blister agent, a pH adjuster, and the like may be added as necessary.

The method of coating a positive electrode active material using the nitrile group-containing acrylic polymer is not particularly limited, and for example, when preparing a slurry composition for a positive electrode, it can be obtained by mixing and stirring the positive electrode active material and a nitrile group-containing acrylic polymer. . The method of mixing and stirring is not particularly limited. Further, the amount of the nitrile group-containing acrylic polymer is preferably 0.05 to 1.95 parts by mass, more preferably 0.1 to 1.0 parts by mass, and particularly preferably 0.2 to 0.6 parts by mass per 100 parts by mass of the positive electrode active material. When the amount of the nitrile group-containing acrylic polymer is in the above range, the positive electrode active material can be satisfactorily coated. In addition, in the present invention, the nitrile group-containing acrylic polymer may have not only a function of covering a positive electrode active material, but also a function as a binder.

(B) Binder for positive electrode

The binder for positive electrodes (B) contains a fluorine-containing polymer.

Fluorine-containing polymer

As the binder for positive electrodes, a fluorine-containing polymer is used. When the binder for positive electrodes contains a fluorine-containing polymer, the stability of the slurry is improved, and swelling of the binder to the non-aqueous electrolyte is suppressed, and cycle characteristics are improved.

The fluorine-containing polymer is a polymer containing a fluorine-containing monomer unit. The fluorine-containing monomer unit is a structural unit formed by polymerizing a fluorine-containing monomer. The fluorine-containing polymer is specifically a homopolymer of a fluorine-containing monomer, a copolymer of a fluorine-containing monomer and another fluorine-containing monomer copolymerizable therewith, a copolymer of a fluorine-containing monomer and a monomer copolymerizable therewith, a fluorine-containing monomer and this And copolymers of other fluorine-containing monomers copolymerizable with and a monomer copolymerizable with these.

Examples of the fluorine-containing monomer include vinylidene fluoride, tetrafluoroethylene, hexafluoropropylene, vinyl trifluoride, vinyl fluoride, and perfluoroalkyl vinyl ether, but vinylidene fluoride is preferable.

The proportion of the fluorine-containing monomer unit in the fluorine-containing polymer is usually 70% by mass or more, and preferably 80% by mass or more. In addition, the upper limit of the ratio of the fluorine-containing monomer unit in the fluorine-containing polymer is 100% by mass.

Examples of the monomer copolymerizable with the fluorine-containing monomer include 1-olefins such as ethylene, propylene, and 1-butene; Aromatic vinyl compounds such as styrene, α-methylstyrene, p-t-butylstyrene, vinyltoluene, and chlorostyrene; Unsaturated nitrile compounds, such as (meth)acrylonitrile (abbreviation of acrylonitrile and methacrylonitrile. The same hereinafter); (Meth)acrylic acid ester compounds such as methyl (meth)acrylate, butyl (meth)acrylate, and 2-ethylhexyl (meth)acrylate; (Meth)acrylamide compounds such as (meth)acrylamide, N-methylol (meth)acrylamide, and N-butoxymethyl (meth)acrylamide; Carboxyl group-containing vinyl compounds, such as (meth)acrylic acid, itaconic acid, fumaric acid, crotonic acid, and maleic acid; Epoxy group-containing unsaturated compounds such as allyl glycidyl ether and glycidyl (meth)acrylate; Amino group-containing unsaturated compounds such as dimethylaminoethyl (meth)acrylate and diethylaminoethyl (meth)acrylate; Sulfonic acid group-containing unsaturated compounds such as styrenesulfonic acid, vinylsulfonic acid, and (meth)allylsulfonic acid; Sulfuric acid group-containing unsaturated compounds such as 3-allyloxy-2-hydroxypropane sulfuric acid; Phosphoric acid group-containing unsaturated compounds, such as propyl (meth)acrylic acid-3-chloro-2-phosphate and 3-allyloxy-2-hydroxypropane phosphoric acid, etc. are mentioned.

The ratio of the fluorine-containing monomer and the monomer unit copolymerizable with the fluorine-containing polymer is usually 30% by mass or less, and preferably 20% by mass or less.

Among the fluorine-containing polymers, a polymer containing vinylidene fluoride as a fluorine-containing monomer, specifically, a homopolymer of vinylidene fluoride, a copolymer of vinylidene fluoride and other fluorine-containing monomers copolymerizable therewith, and vinylidene fluoride and this A copolymer of another copolymerizable fluorine-containing monomer and a monomer copolymerizable therewith, and a copolymer of vinylidene fluoride and a monomer copolymerized therewith are preferable.

Among the above-described fluorine-containing polymers, a homopolymer of vinylidene fluoride (polyvinylidene fluoride), a vinylidene fluoride-hexafluoropropylene copolymer, and polyvinyl fluoride are preferable, and polyvinylidene fluoride is more preferable.

The fluorine-containing polymer may be used alone or in combination of two or more. When using two or more types together, it is preferable to use a low molecular weight body and a high molecular weight body together. Specifically, the melt viscosity of the fluorine-containing polymer measured in ASTM D3835/232° C. 100 sec ^-1 is lower than 35 kpoise as low molecular weight, and 35 kpoise or more as high molecular weight, and both are preferably used in combination.

Examples of the high molecular weight polyvinylidene fluoride include KYNAR HSV900 manufactured by Arkema, Solef6020, Solef6010, Solef1015, Solef5130, KF7208 manufactured by Kureha. Further, examples of the low molecular weight polyvinylidene fluoride include Arkema's KYNAR710 720 740 760 760A, Solvay's Solef6008, and Kureha's KF1120.

When a high molecular weight and a low molecular weight are used in combination as the fluorine-containing polymer, the weight ratio (low molecular weight/high molecular weight) of the low molecular weight and high molecular weight of the fluorine-containing polymer is preferably 30/70 to 70/30.

By using a low molecular weight body and a high molecular weight body together in such a range, the binding property between the positive electrode active materials, the binding property between the current collector and the positive electrode active material, and the uniformity of the slurry can be more effectively maintained.

The weight average molecular weight of the polystyrene conversion value by gel permeation chromatography of the fluorine-containing polymer is preferably 100,000 to 2,000,000, more preferably 200,000 to 1,500,000, and particularly preferably 400,000 to 1,000,000.

By setting the weight average molecular weight of the fluorine-containing polymer in the above range, the removal (pouring) of the positive electrode active material (A) and the conductive material (C) in the positive electrode active material layer is suppressed, and the viscosity adjustment of the slurry composition for positive electrodes is It becomes easier.

The glass transition temperature (Tg) of the fluorine-containing polymer is preferably 0°C or less, more preferably -20°C or less, and particularly preferably -30°C or less. The lower limit of Tg of the fluorine-containing polymer is not particularly limited, but is preferably -50°C or higher, and more preferably -40°C or higher. When Tg of the fluorine-containing polymer is in the above range, it is possible to suppress detachment (powder dropping) of the positive electrode active material (A) and the conductive material (C) in the positive electrode active material layer. Moreover, Tg of a fluorine-containing polymer can be adjusted by combining various monomers. In addition, Tg uses a differential scanning calorimeter, and is JIS K 7121; It can be measured based on 1987.

The melting point (Tm) of the fluorine-containing polymer is preferably 190°C or less, more preferably 150 to 180°C, and still more preferably 160 to 170°C. When Tm of the fluorine-containing polymer is in the above range, a positive electrode excellent in flexibility and adhesion strength can be obtained. In addition, Tm of the fluorine-containing polymer can be adjusted by combining various monomers or controlling the polymerization temperature. In addition, Tm uses a differential scanning calorimeter, and is JIS K 7121; It can be measured based on 1987.

The method for producing the fluorine-containing polymer 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. Among these, a suspension polymerization method and an emulsion polymerization method are preferable. By producing the fluorine-containing polymer by the emulsion polymerization method, the productivity of the fluorine-containing polymer can be improved, and a fluorine-containing polymer having a desired average particle diameter can be obtained. As the polymerization reaction, any reaction such as ionic polymerization, radical polymerization, and living radical polymerization can be used. As a polymerization initiator used for polymerization, for example, lauroyl peroxide, diisopropylperoxydicarbonate, di-2-ethylhexylperoxydicarbonate, t-butylperoxypivalate, 3,3,5-trimethylhexanoyl Organic peroxides such as peroxide, azo compounds such as α,α'-azobisisobutyronitrile, or ammonium persulfate, potassium persulfate, and the like.

The fluorine-containing polymer is used in the form of a dispersion or dissolved solution dispersed in a dispersion medium. The dispersion medium is not particularly limited as long as it can uniformly disperse or dissolve the fluorine-containing polymer, and water or an organic solvent can be used. Examples of the organic solvent include cyclic aliphatic hydrocarbons such as cyclopentane and cyclohexane; Aromatic hydrocarbons such as toluene, xylene, and ethylbenzene; Ketones such as acetone, ethyl methyl ketone, diisopropyl ketone, cyclohexanone, methyl cyclohexanone, and ethyl cyclohexanone; Chlorine aliphatic hydrocarbons such as methylene chloride, chloroform, and carbon tetrachloride; Esters such as ethyl acetate, butyl acetate, γ-butyrolactone, and ε-caprolactone; Alkyl nitriles such as acetonitrile and propionitrile; Ethers such as tetrahydrofuran and ethylene glycol diethyl ether; Alcohols such as methanol, ethanol, isopropanol, ethylene glycol, and ethylene glycol monomethyl ether; Amides, such as N-methylpyrrolidone and N,N-dimethylformamide, are mentioned.

These dispersion media may be used alone, or two or more of them may be mixed and used as a mixed solvent. Among these, in particular, since it is industrially used in the production of a slurry composition for a positive electrode, it is difficult to volatilize during production, and as a result, the volatilization of the slurry composition for a positive electrode is suppressed, and the smoothness of the resulting positive electrode is improved, water, or N-methylpyrrolidone, cyclohexanone, toluene, and the like are preferred.

In the case where the fluorine-containing polymer is dispersed in the form of particles in the dispersion medium, the solid content concentration of the dispersion liquid containing the fluorine-containing polymer is usually 1 to 25% by mass, preferably 3 to 20% by mass, from the viewpoint of handleability, 5 to 15 mass% is more preferable.

In addition, the viscosity when dissolved in N-methylpyrrolidone (hereinafter, referred to as "NMP" may be described as an 8% solution) is preferably 10 to 5000 mPa·s, more Preferably it is 100 to 2000 mPa·s. By setting the viscosity of the 8% NMP solution of the fluorine-containing polymer into the above range, it is easy to adjust the viscosity of the positive electrode slurry composition to a viscosity that is easy to coat when preparing the positive electrode slurry composition. The viscosity of the 8% NMP solution of the fluorine-containing polymer is dissolved in NMP so that the fluorine-containing polymer becomes an 8% solution, and for this, at 25°C and 60 rpm, using a B-type viscometer (RB-80L manufactured by Toki Industries), JIS K 7117-1; It can be measured based on 1999.

The binder for positive electrodes contains the above-described fluorine-containing polymer. In addition to the positive electrode active material coated with a specific nitrile group-containing acrylic polymer, the positive electrode binder contains a fluorine-containing polymer, so that the positive electrode has excellent bendability of the wound body, and the initial capacity, output characteristics, and high potential cycle characteristics This excellent lithium ion secondary battery is obtained.

The proportion of the fluorine-containing polymer is preferably 50 to 100% by mass, more preferably 60 to 90% by mass, and still more preferably 70 to 85% by mass with respect to 100% by mass of the total amount of the binder for positive electrodes.

The binder for positive electrodes may contain, in addition to the above-described fluorine-containing polymer, other polymers usable as a binder, if necessary. Examples of other polymers that may be used in combination include resins such as polyacrylic acid derivatives and polyacrylonitrile derivatives, and soft polymers such as acrylate soft polymers, diene soft polymers, olefin soft polymers, and vinyl soft polymers. . These may be used alone, or two or more of them may be used in combination. The other polymers described above may be contained in a proportion of 30 mass% or less, further 0.1 to 20 mass%, particularly 0.2 to 10 mass% with respect to 100 mass% of the total amount of the binder for positive electrodes. In addition, the amount of the nitrile group-containing acrylic polymer having an SP value of 9 to 11 (cal/cm 3) ^1/2 is not included in the amount of the binder for positive electrodes herein.

The amount of the binder for the positive electrode is in the range of preferably 0.4 to 2 parts by mass, more preferably 1 to 2 parts by mass, and particularly preferably 1.5 to 2 parts by mass per 100 parts by mass of the positive electrode active material. When the amount of the positive electrode binder is in such a range, the adhesion between the obtained positive electrode active material layer and the current collector can be sufficiently ensured, and the capacity of the lithium ion secondary battery can be increased and the internal resistance can be reduced.

(C) Conductive material

The positive electrode contains a conductive material. The particle diameter of the conductive material contained in the positive electrode is a number average particle diameter, preferably 5 to 40 nm, more preferably 10 to 38 nm, and still more preferably 15 to 36 nm. If the particle diameter of the conductive material in the positive electrode is too small, aggregation tends to occur and uniform dispersion becomes difficult. As a result, the internal resistance of the electrode increases, and improvement of the capacity tends to be difficult. However, by using the binder for a positive electrode described above, it becomes possible to uniformly disperse the finely divided conductive material, and the capacity is improved. In addition, when the particle diameter of the conductive material is too large, it becomes difficult to exist between the positive electrode active materials, the internal resistance of the positive electrode active material layer increases, and it becomes difficult to improve the capacity. The number average particle diameter of the conductive material is, after ultrasonically dispersing the conductive material at 0.01% by mass in water, a dynamic light scattering type particle size/particle size distribution measuring device (e.g., Nikkiso Corporation, a particle size distribution measuring device Nanotrac Wave-EX150) is used. It can be calculated|required by using and measuring.

Further, the specific surface area (BET formula) of the conductive material in the positive electrode is preferably 1500 m 2 /g or less, more preferably 1000 m 2 /g or less, and particularly preferably 400 m 2 /g or less. If the specific surface area of the conductive material is too large, it is likely to be aggregated and uniform dispersion becomes difficult. As a result, the internal resistance of the positive electrode active material layer increases, making it difficult to improve the capacity. In addition, as the conductive material, one type of conductive material having the above-described specific surface area may be used alone, or two or more types of conductive material having different specific surface areas are mixed, and the BET specific surface area of the conductive material is the range described above. You may use it in combination so that it may become the inner size.

As the conductive material, similarly to the negative electrode, conductive carbon such as acetylene black, Ketjen black, carbon black, graphite, vapor-grown carbon fiber, and carbon nanotubes can be used. By containing a conductive material, stability at the time of manufacturing the slurry composition for positive electrodes is improved, and the electrical contact between positive electrode active materials in a positive electrode active material layer can be improved, and high capacity|capacitance is aimed at. The content of the conductive material is preferably 1 to 3 parts by mass, more preferably 1.2 to 2.8 parts by mass, and particularly preferably 1.5 to 2.5 parts by mass with respect to 100 parts by mass of the total amount of the positive electrode active material. If the content of the conductive material is too small, the internal resistance in the positive electrode active material layer increases, and it may be difficult to increase the capacity. Moreover, when the content of the conductive material is too large, it becomes difficult to increase the density of the positive electrode, and the initial capacity may decrease.

Other positive electrode components

In addition, as an optional component, the positive electrode may further contain a reinforcing material, a leveling agent, and an electrolyte solution additive having a function of inhibiting electrolyte decomposition, and the like, as an optional component, and contained in a slurry prepared at the time of manufacturing the positive electrode. The thickener to be used may remain.

As the reinforcing material, various inorganic and organic spherical, plate, rod, or fibrous fillers can be used. By using a reinforcing material, a strong and flexible positive electrode can be obtained, and excellent long-term cycle characteristics can be exhibited. The content of the reinforcing material is usually 0.01 to 20 parts by mass, preferably 1 to 10 parts by mass based on 100 parts by mass of the total amount of the positive electrode active material. By including the reinforcing material in the above range, it may exhibit high capacity and high load characteristics.

As a leveling agent, surfactants, such as an alkyl type surfactant, a silicone type surfactant, a fluorine type surfactant, and a metal type surfactant, are mentioned. By mixing the leveling agent, it is possible to prevent cratering that occurs during coating or to improve the smoothness of the positive electrode. The content of the leveling agent is preferably 0.01 to 10 parts by mass with respect to 100 parts by mass of the total amount of the positive electrode active material. When the leveling agent is included in the above range, productivity, smoothness, and battery characteristics at the time of manufacturing a positive electrode are excellent.

As the electrolyte solution additive, vinylene carbonate or the like used in the electrolyte solution can be used. The content of the electrolyte solution additive is preferably 0.01 to 10 parts by mass with respect to 100 parts by mass of the total amount of the positive electrode active material. When the content of the electrolyte solution additive is within the above range, the resulting lithium ion secondary battery has excellent cycle characteristics and high temperature characteristics. Other than that, nanoparticles, such as fumed silica and fumed alumina, are mentioned. By mixing the nanoparticles, the thixotropy of the slurry composition to be adjusted when producing the positive electrode can be controlled, and the leveling property of the positive electrode obtained thereby can be improved. The content of the nanoparticles is preferably 0.01 to 10 parts by mass with respect to 100 parts by mass of the total amount of the positive electrode active material. When nanoparticles are used in the above ratio in the slurry composition for a positive electrode, slurry stability and productivity are excellent, and high battery characteristics are exhibited.

Method for producing a slurry composition for a positive electrode of a lithium ion secondary battery

The slurry composition for a lithium ion secondary battery positive electrode is obtained by mixing the above-described positive electrode active material, a nitrile group-containing acrylic polymer, a binder for positive electrodes (B), a conductive material (C), and other additives in a dispersion medium. By mixing the positive electrode active material and the nitrile group-containing acrylic polymer in a dispersion medium, cyano groups contained in the nitrile group-containing acrylic polymer, or acid groups contained in the nitrile group-containing acrylic polymer in the case of containing an ethylenically unsaturated acid monomer unit, etc. By interacting with functional groups such as hydroxyl groups on the surface of the active material, the nitrile group-containing acrylic polymer is preferentially adsorbed to the positive electrode active material. Then, the obtained slurry composition for a lithium ion secondary battery positive electrode is applied to a collector described later and dried to form a positive electrode active material coated with a nitrile group-containing acrylic polymer, that is, a positive electrode active material (A) for use in the present invention. I can.

In addition, in the slurry composition obtained as described above, since the cyano groups, acid groups, etc. are adsorbed by interacting with functional groups such as hydroxyl groups on the surface of the current collector to be described later, the adhesion between the positive electrode active material layer and the current collector is improved. I can make it.

As the dispersion medium, an organic solvent can be used. Examples of the organic solvent include cyclic aliphatic hydrocarbons such as cyclopentane and cyclohexane; Aromatic hydrocarbons such as toluene, xylene, and ethylbenzene; Ketones such as acetone, ethyl methyl ketone, diisopropyl ketone, cyclohexanone, methylcyclohexane, and ethylcyclohexane; Chlorine-based aliphatic hydrocarbons such as methylene chloride, chloroform, and carbon tetrachloride; Esters such as ethyl acetate, butyl acetate, γ-butyrolactone, and ε-caprolactone; Alkyl nitriles such as acetonitrile and propionitrile; Ethers such as tetrahydrofuran and ethylene glycol diethyl ether; Alcohols such as methanol, ethanol, isopropanol, ethylene glycol, and ethylene glycol monomethyl ether; Amides, such as N-methylpyrrolidone and N,N-dimethylformamide, are mentioned.

These dispersion media may be used alone, or two or more of them may be mixed and used as a mixed solvent. Among these, particularly, a dispersion medium having excellent dispersibility of the positive electrode active material, low boiling point and high volatility is preferable because it can be removed in a short time and at a low temperature. Specifically, acetone, toluene, cyclohexanone, cyclopentane, tetrahydrofuran, cyclohexane, xylene, or N-methylpyrrolidone, or a mixed solvent thereof is preferable.

The mixing method is not particularly limited, and examples thereof include a method using a mixing device such as a stirring type, a shaking type, and a rotary type. Further, a method using a dispersion kneading device such as a homogenizer, a ball mill, a sand mill, a roll mill, and a planetary kneader can be mentioned.

Lithium ion secondary battery positive electrode

The lithium ion secondary battery positive electrode is formed by applying the above-described slurry composition for a lithium ion secondary battery positive electrode to a current collector and drying it.

A method for producing a positive electrode for a lithium ion secondary battery includes a step of applying and drying the slurry composition for a positive electrode on one or both surfaces of a current collector to form a positive electrode active material layer.

The method of applying the slurry composition for positive electrodes on the current collector is not particularly limited. For example, a method such as a doctor blade method, a dip method, a reverse roll method, a direct roll method, a gravure method, an extrusion method, and a brush coating method may be mentioned.

As a drying method, a drying method by irradiation, such as warm air, hot air, drying by low humidity air, vacuum drying, (far) infrared rays, an electron beam, etc. is mentioned, for example. The drying time is usually 5 to 30 minutes, and the drying temperature is usually 40 to 180°C.

In manufacturing a lithium ion secondary battery positive electrode, a step of lowering the porosity of the positive electrode active material layer by applying and drying the slurry composition for a positive electrode on a current collector, and then using a mold press or a roll press, etc. It is preferable to have. The porosity of the positive electrode active material layer is preferably 5 to 30%, more preferably 7 to 20%. If the porosity of the positive electrode active material layer is too high, charging efficiency or discharging efficiency may deteriorate. On the other hand, when the porosity of the positive electrode active material layer is too low, it is difficult to obtain a high volume capacity, and the positive electrode active material layer is easily peeled from the current collector, and defects are likely to occur in some cases. Moreover, when using a curable polymer as the binder (B) for positive electrodes, it is preferable to cure.

The thickness of the positive electrode active material layer in the lithium ion secondary battery positive electrode is usually 5 to 300 µm, and preferably 30 to 250 µm. When the thickness of the positive electrode active material layer is in the above range, a secondary battery having high load characteristics and cycle characteristics can be obtained.

The content ratio of the positive electrode active material in the positive electrode active material layer is preferably 85 to 99% by mass, more preferably 88 to 97% by mass. When the content ratio of the positive electrode active material in the positive electrode active material layer is within the above range, it is possible to obtain a secondary battery exhibiting high capacity and flexibility and binding properties.

The density of the positive electrode active material layer is preferably 3.0 to 4.0 g/cm 3, more preferably 3.4 to 4.0 g/cm 3. When the density of the positive electrode active material layer is within the above range, a high-capacity secondary battery can be obtained.

The current collector is not particularly limited as long as it is a material that has electrical conductivity and is electrochemically durable, but a metal material is preferable to have heat resistance. For example, iron, copper, aluminum, nickel, stainless steel, titanium, tantalum , Gold, platinum, and the like. Among them, aluminum is particularly preferred as a current collector used for the positive electrode of a lithium ion secondary battery. The shape of the current collector is not particularly limited, but it is preferably a sheet shape having a thickness of about 0.001 to 0.5 mm. The current collector may be used after roughening treatment in advance in order to increase the adhesive strength with the positive electrode active material layer. Examples of the roughening method include a mechanical polishing method, an electrolytic polishing method, and a chemical polishing method. In the mechanical polishing method, a wire brush provided with an abrasive cloth, grindstone, emery buff, steel wire, etc. to which abrasive particles are fixed are used. Further, in order to increase the adhesive strength and conductivity of the positive electrode active material layer, a primer layer or the like may be formed on the surface of the current collector.

(Negative)

The negative electrode comprises a current collector and a negative electrode active material layer laminated on the current collector. The negative electrode active material layer, as a negative electrode active material (a), preferably contains an alloy-based active material (a1), contains other carbon-based active material (a2) as necessary, and usually contains a binder for negative electrodes (b ), a conductive material (c), and the like.

(a) negative electrode active material

The negative electrode active material is a substance that transfers electrons (lithium ions) within the negative electrode. As the negative electrode active material, an alloy-based active material (a1) is used, and if necessary, a carbon-based active material (a2) can be used. The negative electrode active material preferably contains an alloy-based active material and a carbon-based active material, and by using the alloy-based active material and the carbon-based active material together, a secondary battery having a larger capacity than the negative electrode obtained using only the alloy-based active material can be obtained. Problems such as a decrease in adhesion strength of the negative electrode and a decrease in cycle characteristics can also be solved.

(a1) alloy-based active material

The alloy-based active material includes an element capable of inserting lithium in its structure, and the theoretical electric capacity per weight when lithium is inserted is 500 mAh/g or more (the upper limit of the theoretical electric capacity is not particularly limited, but, for example, It can be 5000 mAh/g or less.) Phosphorus active material. Specifically, a single metal forming a lithium alloy and its alloy, and oxides, sulfides, nitrides, silicides, carbides, phosphides, and the like are used.

As simple metals and alloys that form lithium alloys, metals such as Ag, Al, Ba, Bi, Cu, Ga, Ge, In, Ni, P, Pb, Sb, Si, Sn, Sr, Zn, and other metals are contained. The compound to be mentioned is mentioned. Among them, a single metal of silicon (Si), tin (Sn) or lead (Pb), an alloy containing these atoms, or a compound of these metals is preferable. In addition, among these, a simple Si metal capable of intercalating and deintercalating lithium at a low potential is more preferable.

The alloy-based active material may further contain one or more non-metal elements. Specifically, for example, SiC, SiO _x C _y (hereinafter referred to as "SiOC") (0 <x ≤ 3, 0 <y ≤ 5), Si ₃ N ₄ , Si ₂ N ₂ O, SiO _x ( x = 0.01 or more and less than 2), SnO _x (0 <x ≤ 2), LiSiO, LiSnO, etc., among others, SiOC, SiO _x , and SiC capable of intercalation and desorption of lithium at a low potential are preferred, SiOC, SiO _x is more preferred.

For example, SiOC can be obtained by firing a high molecular material containing silicon. Among SiOCs, in the balance between capacity and cycle characteristics, a range of 0.8≦x≦3 and 2≦y≦4 is preferably used.

Examples of the single metal forming a lithium alloy and oxides, sulfides, nitrides, silicides, carbides, and phosphides of the alloys include oxides, sulfides, nitrides, silicides, carbides, and phosphides of elements that can be inserted into lithium. Oxides are particularly preferred. Specifically, a lithium-containing metal composite oxide containing an oxide such as tin oxide, manganese oxide, titanium oxide, niobium oxide, and vanadium oxide, and a metal element selected from the group consisting of Si, Sn, Pb, and Ti atoms is preferable.

As the lithium-containing metal composite oxide, a lithium titanium composite oxide further represented by Li _x Ti _y M _z O ₄ (0.7 ≤ x ≤ 1.5, 1.5 ≤ y ≤ 2.3, 0 ≤ z ≤ 1.6, M silver, Na, K, Co, Al, Fe, Ti, Mg, Cr, Ga, Cu, Zn, and Nb), among others, Li _4/3 Ti _5/3 O ₄ , Li ₁ Ti ₂ O ₄ , Li _4/5 Ti _{11/ 5} O ₄ is preferred.

Among these alloy-based active materials, an active material containing silicon is preferred. By using an active material containing silicon, it becomes possible to increase the electric capacity of the secondary battery. Moreover, among the active materials containing silicon, SiO _x C _y , SiO _x , and SiC are more preferable. In an active material containing a combination of silicon and carbon, it is assumed that insertion and removal of Li into Si (silicon) at a high potential and C (carbon) at a low potential occurs, and expansion and contraction are suppressed compared to other alloy-based active materials. , The effect of the present invention is more easily obtained.

It is preferable that the alloy-based active material is sieved in a particulate form. When the shape of the particles is spherical, a higher density electrode can be formed during electrode molding. When the alloy-based active material is a particle, the volume average particle diameter is preferably 0.1 to 50 µm, more preferably 0.5 to 20 µm, and particularly preferably 1 to 10 µm. When the volume average particle diameter of the alloy-based active material is within this range, the production of the slurry composition used to produce the negative electrode becomes easy. In addition, the volume average particle diameter in this invention can be calculated|required by measuring a particle diameter distribution by laser diffraction.

The tap density of the alloy-based active material is not particularly limited, but it is preferably 0.6 g/cm 3 or more.

The specific surface area (BET formula) of the alloy-based active material is preferably 3.0 to 20.0 m 2 /g, more preferably 3.5 to 15.0 m 2 /g, particularly preferably 4.0 to 10.0 m 2 /g. When the specific surface area of the alloy-based active material is in the above range, the active point on the surface of the alloy-based active material increases, and thus the output characteristics of the lithium ion secondary battery are excellent. In addition, in this invention, "BET specific surface area" means a BET specific surface area by a nitrogen adsorption method, and is a value measured according to ASTM D3037-81.

(a2) carbon-based active material

The carbon-based active material refers to an active material having as a main skeleton carbon into which lithium can be inserted, and specifically, a carbonaceous material and a graphite material are exemplified. The carbonaceous material is generally a carbon material with a low degree of graphitization (ie, low crystallinity) obtained by heat-treating a carbon precursor at 2000° C. or lower to carbonize it. The lower limit of the heat treatment temperature is not particularly limited, but may be, for example, 500°C or higher. The graphite material is a graphite material having high crystallinity close to graphite obtained by heat-treating easily graphitic carbon at 2000°C or higher. The upper limit of the treatment temperature is not particularly limited, but may be, for example, 5000° C. or less.

Examples of the carbonaceous material include graphitic carbon that easily changes the structure of carbon by the heat treatment temperature, and non-graphitic carbon having a structure close to an amorphous structure typified by glassy carbon.

Examples of the graphitic carbon include a carbon material made of tar pitch obtained from petroleum or coal as a raw material. Specific examples thereof include coke, mesocarbon microbeads (MCMB), mesophase pitch-based carbon fibers, and pyrolytic vapor-grown carbon fibers. MCMB is carbon fine particles obtained by separating and extracting mesophase globules generated in the process of heating pitches at around 400°C. The mesophase pitch-based carbon fiber is a carbon fiber having a mesophase pitch obtained by growing and combining the mesophase globules as a raw material. Pyrolysis vapor-grown carbon fiber means (1) a method of pyrolyzing acrylic polymer fibers, (2) a method of pyrolyzing by spinning pitch, (3) a catalytic gas phase pyrolysis of hydrocarbons using nanoparticles such as iron as a catalyst. It is a carbon fiber obtained by a growth (catalytic CVD) method.

Examples of the non-graphitizing carbon include phenol resin fired body, polyacrylonitrile carbon fiber, pseudoisotropic carbon, furfuryl alcohol resin fired body (PFA), and hard carbon.

As a graphite material, natural graphite and artificial graphite are mentioned, for example. Examples of artificial graphite include artificial graphite heat-treated at 2800°C or higher, graphitized MCMB heat-treated at 2000°C or higher for MCMB, and graphitized mesophase pitch-based carbon fiber heat-treated at 2000°C or higher for mesophase pitch-based carbon fibers. I can.

Among the above carbon-based active materials, graphite materials are preferred. By using a graphite material, it becomes easy to increase the density of the negative electrode active material layer, and the density of the negative electrode active material layer is 1.6 g/cm 3 or more (the upper limit of the density is not particularly limited, but it can be 2.2 g/cm 3 or less. ) It becomes easy to manufacture negative electrode. If the density of the negative electrode active material layer is a negative electrode having a negative electrode active material layer in the above range, the effect of the present invention is remarkably exhibited.

It is preferable that the carbon-based active material is sized in a particulate form. When the shape of the particles is spherical, a higher density electrode can be formed during electrode molding. When the carbon-based active material is a particle, the volume average particle diameter of the carbon-based active material is preferably 0.1 to 100 µm, more preferably 0.5 to 50 µm, and particularly preferably 1 to 30 µm. When the volume average particle diameter of the carbon-based active material is within this range, production of the slurry composition used to produce the negative electrode becomes easy.

The tap density of the carbon-based active material is not particularly limited, but it is preferably 0.6 g/cm 3 or more.

The specific surface area of the carbon-based active material is preferably 3.0 to 20.0 m 2 /g, more preferably 3.5 to 15.0 m 2 /g, and particularly preferably 4.0 to 10.0 m 2 /g. When the specific surface area of the carbon-based active material is in the above range, the active point on the surface of the carbon-based active material increases, and thus the output characteristics of the lithium ion secondary battery are excellent. The specific surface area can be measured, for example, by the BET method.

As for the negative electrode active material, one type of alloy-based active material may be used alone, or two or more types may be used in combination in an arbitrary ratio. Moreover, as a preferable aspect of a negative electrode active material, an active material in which an alloy type active material and a carbon type active material are combined is mentioned. When the alloy-based active material (a1) and the carbon-based active material (a2) are used in combination as the negative electrode active material (a), the mixing method is not particularly limited, and conventionally known dry mixing and wet mixing are mentioned.

In the negative electrode active material (a), when the alloy-based active material (a1) and the carbon-based active material (a2) are used in combination, 1 to 50 parts by mass of the alloy-based active material (a1) per 100 parts by mass of the carbon-based active material (a2) It is preferable to contain. By mixing the alloy-based active material and the carbon-based active material in the above range, a battery having a larger capacity than the negative electrode obtained by using only the conventional carbon-based active material can be obtained, and a decrease in adhesion strength of the negative electrode and a decrease in cycle characteristics can be prevented. have. If it is a negative electrode having a negative electrode active material layer in which an alloy-based active material (a1) and a carbon-based active material (a2) are used in combination within the above range, the effect of the present invention is remarkably exhibited.

(b) Binder for negative electrode

The binder for the negative electrode is a component that binds the negative electrode active material to the surface of the current collector in the negative electrode, and it is preferable to use a material having excellent performance for holding the negative electrode active material and high adhesion to the current collector. Usually, a polymer is used as the material of the binder. The polymer as the material of the binder may be a homopolymer or a copolymer. Although it does not specifically limit as a polymer of the binder for negative electrodes, For example, polymer compounds, such as a fluoropolymer, a diene polymer, an acrylate polymer, a polyimide, a polyamide, a polyurethane, are mentioned, Among them, a fluoropolymer , A diene polymer or an acrylate polymer is preferable, and a diene polymer or an acrylate polymer is more preferable in that the withstand voltage can be increased and the energy density of the secondary battery can be increased, and the strength of the electrode is improved. A diene polymer is particularly preferred.

The diene polymer is a polymer containing a structural unit formed by polymerizing a conjugated diene monomer (hereinafter, it may be described as a "conjugated diene monomer unit"), and specifically, a homopolymer of a conjugated diene; Copolymers of different kinds of conjugated dienes; A copolymer obtained by polymerizing a monomer mixture containing a conjugated diene, or a hydrogenated product thereof. Examples of the conjugated diene include 1,3-butadiene, 2-methyl-1,3-butadiene, 2,3-dimethyl-1,3-butadiene, 2-ethyl-1,3-butadiene, and 2-chlor -1,3-butadiene, 1,3-pentadiene, 1,3-hexadiene, and 2,4-hexadiene. Among these, 1,3-butadiene and 2-methyl-1,3-butadiene are preferable. Moreover, conjugated diene 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 diene polymer is preferably 20% by mass or more and 60% by mass or less, and preferably 30% by mass or more and 55% by mass or less.

The diene polymer may contain a nitrile group-containing monomer unit other than the conjugated diene monomer unit. Specific examples of the nitrile group-containing monomer constituting the nitrile group-containing monomer unit include α,β-unsaturated nitrile compounds such as acrylonitrile, methacrylonitrile, α-chloroacrylonitrile, and α-ethylacrylonitrile. And acrylonitrile is preferable among them. The ratio of the nitrile group-containing monomer unit in the diene polymer is preferably 5 to 40% by mass, more preferably 5 to 30% by mass. By making the ratio of the nitrile group-containing monomer unit into the above range, the obtained electrode strength is further improved. Moreover, a nitrile group-containing monomer may be used individually by 1 type, and may be used combining two or more types by arbitrary ratios.

Further, the diene polymer may contain a structural unit formed by polymerizing another monomer in addition to the monomer unit. Examples of other monomers constituting the other monomer units include unsaturated carboxylic acids such as acrylic acid, methacrylic acid, itaconic acid and fumaric acid; Styrene monomers such as styrene, chlorostyrene, vinyltoluene, t-butylstyrene, vinylbenzoic acid, methyl vinylbenzoate, vinylnaphthalene, chloromethylstyrene, hydroxymethylstyrene, α-methylstyrene, and divinylbenzene; Olefins such as ethylene and propylene; Vinyl esters such as vinyl acetate, vinyl propionate, vinyl butyrate, and vinyl benzoate; Amide-based monomers such as acrylamide, N-methylol acrylamide, and acrylamide-2-methylpropanesulfonic acid; Vinyl ethers such as methyl vinyl ether, ethyl vinyl ether, and butyl vinyl ether; Vinyl ketones such as methyl vinyl ketone, ethyl vinyl ketone, butyl vinyl ketone, hexyl vinyl ketone, and isopropenyl vinyl ketone; And heterocycle-containing vinyl compounds such as N-vinylpyrrolidone, vinylpyridine, and vinylimidazole. Moreover, the said other monomer may be used individually by 1 type, respectively, and may be used combining two or more types by arbitrary ratios.

The acrylate polymer is a monomer derived from a compound represented by the general formula (1): CH ₂ =CR ¹ -COOR ² (in the formula, R ¹ represents a hydrogen atom or a methyl group, and R ² represents an alkyl group or a cycloalkyl group.) It is a polymer containing a monomer unit ((meth)acrylic acid ester monomer unit) formed by polymerization. Specific examples of the monomers constituting the (meth)acrylic acid ester monomer unit include ethyl acrylate, propyl acrylate, isopropyl acrylate, n-butyl acrylate, isobutyl acrylate, t-butyl acrylate, n-amyl acrylate, isoamyl acrylate, and acrylic acid. acrylic acid esters such as n-hexyl, 2-ethylhexyl acrylate, lauryl acrylate, and stearyl acrylate; Ethyl methacrylate, propyl methacrylate, isopropyl methacrylate, n-butyl methacrylate, isobutyl methacrylate, t-butyl methacrylate, n-amyl methacrylate, isoamyl methacrylate, meth And methacrylic acid esters such as n-hexyl acrylate, 2-ethylhexyl methacrylate, lauryl methacrylate, and stearyl methacrylate. Among these, acrylic acid esters are preferred, and n-butyl acrylate and 2-ethylhexyl acrylate are particularly preferred from the viewpoint of improving the strength of the obtained electrode. The ratio of the (meth)acrylic acid ester monomer unit in the acrylate polymer is usually 50% by mass or more, preferably 70% by mass or more. When an acrylate polymer in which the ratio of the (meth)acrylic acid ester monomer unit is in the above range is used, heat resistance is high and the internal resistance of the obtained electrode can be reduced.

It is preferable that the said acrylate polymer contains a nitrile group-containing monomer unit in addition to a (meth)acrylic acid ester monomer unit. Examples of the nitrile group-containing monomer include acrylonitrile and methacrylonitrile, and among them, acrylonitrile is preferable in that the binding property between the current collector and the negative electrode active material layer increases, and the electrode strength can be improved. . The ratio of the nitrile group-containing monomer unit in the acrylate polymer is preferably 5 to 35% by mass, more preferably 10 to 30% by mass. By setting the amount of the nitrile group-containing monomer unit in the above range, the obtained electrode strength is further improved.

In addition to the monomer units, a structural unit formed by polymerizing a copolymerizable carboxylic acid group-containing monomer (hereinafter, referred to as a "carboxylic acid group-containing monomer unit" may be used) as the acrylate polymer. Specific examples of the carboxylic acid group-containing monomer include monobasic acid-containing monomers such as acrylic acid and methacrylic acid; Dibasic acid-containing monomers, such as maleic acid, fumaric acid, and itaconic acid, are mentioned. Among them, dibasic acid-containing monomers are preferred, and itaconic acid is particularly preferred from the viewpoint of enhancing the binding property with the current collector and improving electrode strength. These monobasic acid-containing monomers and dibasic acid-containing monomers can be used alone or in combination of two or more. The proportion of the carboxylic acid group-containing monomer unit in the acrylate polymer is preferably 1 to 50% by mass, more preferably 1 to 20% by mass, and particularly preferably 1 to 10% by mass. By setting the amount of the carboxylic acid group-containing monomer unit in the above range, the strength of the obtained electrode is further improved.

Further, the acrylate polymer may contain, in addition to the monomer unit, a structural unit obtained by polymerizing a copolymerizable monomer, and specific examples of the other monomers include ethylene glycol dimethacrylate, diethylene glycol dimethacrylate, Carboxylic acid esters having two or more carbon-carbon double bonds such as trimethylolpropane triacrylate; Unsaturated esters containing fluorine in side chains such as perfluorooctylethyl acrylate and perfluorooctylethyl methacrylate; Styrene monomers such as styrene, chlorostyrene, vinyltoluene, t-butylstyrene, vinylbenzoic acid, methyl vinylbenzoate, vinylnaphthalene, chloromethylstyrene, hydroxymethylstyrene, α-methylstyrene, and divinylbenzene; Amide monomers such as acrylamide, N-methylolacrylamide, and acrylamide-2-methylpropanesulfonic acid; Olefins such as ethylene and propylene; Diene monomers such as butadiene and isoprene; Halogen atom-containing monomers such as vinyl chloride and vinylidene chloride; Vinyl esters such as vinyl acetate, vinyl propionate, vinyl butyrate, and vinyl benzoate; Vinyl ethers such as methyl vinyl ether, ethyl vinyl ether, and butyl vinyl ether; Vinyl ketones such as methyl vinyl ketone, ethyl vinyl ketone, butyl vinyl ketone, hexyl vinyl ketone, and isopropenyl vinyl ketone; Heterocyclic vinyl compounds, such as N-vinylpyrrolidone, vinylpyridine, and vinylimidazole; Glycidyl ethers such as allyl glycidyl ether; Glycidyl esters, such as glycidyl acrylate and glycidyl methacrylate, etc. are mentioned. The ratio of the other copolymerizable monomer units in the acrylate polymer may be appropriately adjusted according to the purpose of use.

In addition to the above, as binders for negative electrodes, polyethylene, polypropylene, polyisobutylene, polyvinyl chloride, polyvinylidene chloride, polyvinylidene fluoride, polytetrafluoroethylene, polyvinyl acetate, polyvinyl alcohol, polyvinyl iso Vinyl polymers such as butyl ether, polyacrylonitrile, polymethacrylonitrile, polymethyl methacrylate, methyl polyacrylate, polyethyl methacrylate, allyl polyacetate, and polystyrene; Ether polymers containing a hetero atom in the main chain such as polyoxymethylene, polyoxyethylene, polycyclic thioether, and polydimethylsiloxane; Condensed ester polymers such as polylactone, polycyclic anhydride, polyethylene terephthalate, and polycarbonate; Condensed amide polymers, such as nylon 6, nylon 66, poly-m-phenylene isophthalamide, poly-p-phenylene terephthalamide, and polypyromelitimide, and a thickener described below.

The shape of the binder for negative electrodes is not particularly limited, but it is preferable to be in particulate form in order to have good adhesion to the current collector and to suppress a decrease in the capacity of the manufactured electrode or deterioration due to repeated charge/discharge. . The particulate binder may be one that maintains and exists a particle shape in a state dispersed in a dispersion medium, but it is preferable that the particulate binder can exist in a state that maintains the particle shape also in the negative electrode active material layer. In the present invention, the "state in which the particle state is maintained" does not need to be a state in which the particle shape is completely maintained, but may be a state in which the particle shape is maintained to some extent. Examples of the particulate binder include those in which particles of a binder such as latex are dispersed in water, and those in the form of powder obtained by drying such a dispersion liquid.

The glass transition temperature (Tg) of the negative electrode binder is preferably 50°C or less, more preferably -40 to 0°C. When the glass transition temperature (Tg) of the binder is in this range, the adhesion is excellent with a small amount of use, the electrode strength is strong, and the flexibility is abundant, and the electrode density can be easily increased by the pressing process during electrode formation. .

When the binder for negative electrodes is a particulate binder, the number average particle diameter is not particularly limited, but is usually 0.01 to 1 µm, preferably 0.03 to 0.8 µm, more preferably 0.05 to 0.5 µm. When the number average particle diameter of the binder is within this range, excellent adhesion can be imparted to the negative electrode active material layer even with a small amount of use. Here, the number average particle diameter is a number average particle diameter calculated as an arithmetic average value by measuring the diameter of 100 binder particles randomly selected from a transmission electron micrograph. The shape of the particles may be spherical or deformed.

These binders can be used alone or in combination of two or more.

The amount of the binder for the negative electrode is usually 0.1 to 50 parts by mass, preferably 0.5 to 20 parts by mass, and more preferably 1 to 10 parts by mass per 100 parts by mass of the negative electrode active material. When the amount of the binder is in this range, the adhesion between the obtained negative electrode active material layer and the current collector can be sufficiently ensured, and the capacity of the secondary battery can be increased and the internal resistance can be reduced.

(c) conductive material

The negative electrode active material layer may contain a conductive material. The particle diameter of the conductive material contained in the negative electrode is a number average particle diameter of 5 to 40 nm, preferably 10 to 38 nm, and more preferably 15 to 36 nm. As the conductive material, conductive carbon such as acetylene black, Ketjen black, carbon black, graphite, vapor-grown carbon fiber, and carbon nanotubes can be used. By containing a conductive material, electrical contact between negative electrode active materials can be improved, and when used for a lithium ion secondary battery, discharge rate characteristics can be improved. The content of the conductive material is preferably 1 to 20 parts by mass, more preferably 1 to 10 parts by mass with respect to 100 parts by mass of the total amount of the negative electrode active material.

Other negative electrode ingredients

In addition, the negative electrode may further contain, as an optional component, a reinforcing material, a leveling agent, an electrolyte solution additive having a function of inhibiting electrolyte decomposition, and the like, as in the above-described positive electrode. The contained thickener or the like may remain.

Method for producing slurry composition for negative electrode of lithium ion secondary battery

The slurry composition for a lithium ion secondary battery negative electrode is obtained by mixing the above-described negative electrode active material (a), a negative electrode binder (b), a conductive material (c), and other additives in a dispersion medium. As the dispersion medium, both water and organic solvents can be used. Examples of the organic solvent include cyclic aliphatic hydrocarbons such as cyclopentane and cyclohexane; Aromatic hydrocarbons such as toluene, xylene, and ethylbenzene; Ketones such as acetone, ethyl methyl ketone, diisopropyl ketone, cyclohexanone, methyl cyclohexanone, and ethyl cyclohexanone; Chlorine-based aliphatic hydrocarbons such as methylene chloride, chloroform, and carbon tetrachloride; Esters such as ethyl acetate, butyl acetate, γ-butyrolactone, and ε-caprolactone; Acylonitriles such as acetonitrile and propionitrile; Ethers such as tetrahydrofuran and ethylene glycol diethyl ether; Alcohols such as methanol, ethanol, isopropanol, ethylene glycol, and ethylene glycol monomethyl ether; Amides, such as N-methylpyrrolidone and N,N-dimethylformamide, are mentioned.

These dispersion mediums may be used alone or as a mixed solvent by mixing two or more. Among these, in particular, a dispersion medium having excellent dispersibility of each component, a low boiling point and high volatility, is preferable because it can be removed in a short time and at a low temperature. Specifically, acetone, toluene, cyclohexanone, cyclopentane, tetrahydrofuran, cyclohexane, xylene, water, N-methylpyrrolidone, or a mixed solvent thereof is preferable.

The mixing method is not particularly limited, and examples thereof include a method using a mixing device such as a stirring type, a shaking type, and a rotary type. Further, a method using a dispersion kneading device such as a homogenizer, a ball mill, a sand mill, a roll mill, and a planetary kneader can be mentioned.

Lithium ion secondary battery negative electrode

The lithium ion secondary battery negative electrode is formed by applying the above-described slurry composition for a lithium ion secondary battery negative electrode to a current collector and drying it.

A method for producing a negative electrode for a lithium ion secondary battery includes a step of applying and drying the slurry composition for negative electrodes on one or both surfaces of a current collector to form a negative electrode active material layer.

The method of applying the slurry composition for negative electrodes on the current collector is not particularly limited. For example, a method such as a doctor blade method, a dip method, a reverse roll method, a direct roll method, a gravure method, an extrusion method, and a brush coating method may be mentioned.

As a drying method, a drying method by irradiation, such as warm air, hot air, drying by low humidity air, vacuum drying, (far) infrared rays, an electron beam, etc. is mentioned, for example. The drying time is usually 5 to 30 minutes, and the drying temperature is usually 40 to 180°C.

In manufacturing a lithium ion secondary battery negative electrode, a step of lowering the porosity of the negative electrode active material layer by applying a pressure treatment to the negative electrode active material layer after coating and drying the negative electrode slurry composition on a current collector, using a mold press or a roll press. It is desirable to have. The porosity of the negative electrode active material layer is preferably 5 to 30%, more preferably 7 to 20%. If the porosity of the negative electrode active material layer is too high, charging efficiency or discharging efficiency may deteriorate. When the porosity is too low, it is difficult to obtain a high volume capacity, and the negative electrode active material layer is easily peeled from the current collector, and defects are likely to occur in some cases. Moreover, when using a curable polymer as a binder for negative electrodes, it is preferable to cure.

The thickness of the negative electrode active material layer in the negative electrode of the lithium ion secondary battery is usually 5 to 300 µm, and preferably 30 to 250 µm. When the thickness of the negative electrode active material layer is within the above range, it is possible to obtain a secondary battery exhibiting high characteristics in both the load characteristics and the cycle characteristics.

The content ratio of the negative electrode active material in the negative electrode active material layer is preferably 85 to 99% by mass, more preferably 88 to 97% by mass. When the content ratio of the negative electrode active material in the negative electrode active material layer is within the above range, a secondary battery exhibiting high capacity and flexibility and binding properties can be obtained.

The density of the negative electrode active material layer is preferably 1.6 to 1.9 g/cm 3, more preferably 1.65 to 1.85 g/cm 3. When the density of the negative electrode active material layer is within the above range, a high-capacity secondary battery can be obtained.

The current collector is not particularly limited as long as it is a material that has electrical conductivity and is electrochemically durable, but a metal material is preferable to have heat resistance. For example, iron, copper, aluminum, nickel, stainless steel, titanium, tantalum , Gold, platinum, and the like. Among them, copper is particularly preferable as a current collector used for the negative electrode of a lithium ion secondary battery.

The shape of the current collector is not particularly limited, but it is preferably a sheet shape having a thickness of about 0.001 to 0.5 mm.

In order to increase the adhesive strength with the negative electrode active material layer, the current collector may be previously roughened and used. Examples of the roughening method include a mechanical polishing method, an electrolytic polishing method, and a chemical polishing method. In the mechanical polishing method, a wire brush provided with an abrasive cloth, grindstone, emery buff, steel wire, etc. to which abrasive particles are fixed are used. Further, in order to increase the adhesive strength and conductivity of the negative electrode active material layer, a primer layer or the like may be formed on the surface of the current collector.

(Lithium ion secondary battery)

The lithium ion secondary battery according to the present invention includes the positive electrode and the negative electrode described above, has a non-aqueous electrolyte solution, and usually includes a separator.

Nonaqueous electrolyte

The non-aqueous electrolyte solution is not particularly limited, and a lithium salt dissolved in a non-aqueous solvent as a supporting electrolyte can be used. As a lithium salt, for example, LiPF ₆ , LiAsF ₆ , LiBF ₄ , LiSbF ₆ , LiAlCl ₄ , LiClO ₄ , CF ₃ SO ₃ Li, C ₄ F ₉ SO ₃ Li, CF ₃ COOLi, (CF ₃ CO) ₂ Lithium salts, such as NLi, (CF ₃ SO ₂ ) ₂ NLi, and (C ₂ F ₅ SO ₂ )NLi, are mentioned. In particular, LiPF ₆ , LiClO ₄ , and CF ₃ SO ₃ Li which are easily soluble in a solvent and exhibit a high degree of dissociation are preferably used. These can be used alone or in combination of two or more. The amount of the supporting electrolyte is usually 1% by mass or more, preferably 5% by mass or more, and usually 30% by mass or less, preferably 20% by mass or less with respect to the non-aqueous electrolyte. Even if the amount of the supporting electrolyte is too small or too large, the ionic conductivity is lowered, and the charging and discharging characteristics of the battery are lowered.

The solvent used for the non-aqueous electrolyte is not particularly limited as long as it dissolves the supporting electrolyte, but typically, dimethyl carbonate (DMC), ethylene carbonate (EC), diethyl carbonate (DEC), propylene carbonate (PC), butylene carbonate Alkyl carbonates such as (BC) and methyl ethyl carbonate (MEC); esters such as γ-butyrolactone and methyl formate, ethers such as 1,2-dimethoxyethane, and tetrahydrofuran; Sulfur-containing compounds such as sulfolane and dimethyl sulfoxide; Is used. In particular, dimethyl carbonate, ethylene carbonate, propylene carbonate, diethyl carbonate, and methyl ethyl carbonate are preferable because high ionic conductivity is easily obtained and the operating temperature range is wide. These can be used alone or in combination of two or more.

Examples of the non-aqueous electrolyte solution other than the above include a gel polymer electrolyte in which a polymer electrolyte such as polyethylene oxide and polyacrylonitrile is impregnated with a non-aqueous electrolyte solution, and inorganic solid electrolytes such as lithium sulfide, LiI, and Li ₃ N.

Moreover, it is also possible to contain an additive in a non-aqueous electrolyte solution and use it. As the additive, in addition to carbonate-based compounds such as vinylene carbonate (VC), fluorine-containing carbonates such as fluoroethylene carbonate, and ethylmethylsulfone are preferable. Among these, a fluorine-based electrolyte solution additive material such as fluorinated carbonate has a high withstand voltage. As the capacity increases, the voltage at the time of charging and discharging is also increasing, and since an electrolyte made of ethylene carbonate or propylene carbonate may not withstand high voltage and may decompose, it is preferable to mix the above fluorine-based electrolyte additive into a non-aqueous electrolyte. .

Separator

The separator is a porous substrate having pores, and the available separators include (a) a porous separator having pores, (b) a porous separator having a polymer coat layer formed on one or both sides, or (c) inorganic ceramic powder. And a porous separator in which a porous resin coat layer containing conductive particles and a binder was formed. Non-limiting examples thereof include polypropylene, polyethylene, polyolefin, or aramid porous separators, polyvinylidene fluoride, polyethylene oxide, polyacrylonitrile, or polyvinylidene fluoride hexafluoropropylene copolymer. A polymer film for a solid polymer electrolyte or a gel polymer electrolyte, a separator coated with a gelled polymer coat layer, or a separator coated with a porous membrane layer made of an inorganic filler or a dispersant for inorganic fillers.

Manufacturing method of lithium ion secondary battery

The manufacturing method of the lithium ion secondary battery of this invention is not specifically limited. For example, the above-described positive electrode and negative electrode are superimposed through a separator, and this is wound or bent according to the shape of the battery to be put into a battery container, and an electrolyte solution is injected into the battery container to seal. In addition, if necessary, an expandable metal, an overcurrent prevention element such as a fuse or a PTC element, a lead plate, etc. may be inserted to prevent an increase in pressure inside the battery and overcharge and discharge. The shape of the battery may be any of a laminate cell type, a coin type, a button type, a sheet type, a cylinder type, a square type, a flat type, a wound pouch cell, or the like. In particular, according to the present invention, since the active material layer is flexible and there is no cracking in the active material layer during bending, it can be preferably applied to the manufacture of a wound pouch cell.

Example

Hereinafter, although an Example is given and this invention is demonstrated, this invention is not limited to this. In addition, parts and% in this example are based on mass unless otherwise specified. In Examples and Comparative Examples, various physical properties were evaluated as follows.

(SP value)

The SP value means a solubility parameter. This SP value was determined by the "molecular attraction constant method" proposed by Small. This method is a method of determining according to the following formula from the statistics and molecular weight of the molecular attraction constant (G), which is the characteristic value of the functional group (atomic group) constituting the compound molecule.

In the above formula, δ represents the SP value, ΣG represents the total number of molecular attraction constants G, V represents the cost, M represents the molecular weight, and d represents the specific gravity.

In the case of combining two or more types of polymers, the SP value of the entire polymer was calculated from the SP value and the mixing molar ratio of the individual polymers, and calculated using the above calculation formula.

(Confirmation of coating of positive electrode active material)

A slurry was prepared by setting the mixing ratio of the positive electrode active material, the positive electrode binder, and the nitrile group-containing acrylic polymer to 100:1.6:0.4, and applied and dried on a current collector (aluminum foil) to obtain an electrode.

The electrode surface was observed with a scanning electron microscope (S-3400N manufactured by Hitachi Corporation). Observation conditions performed image observation of a 100 µm x 100 µm square with a magnification of 2000 times and an acceleration voltage of 15 kV. In addition, elemental mapping of a nickel atom, a nitrogen atom, and a carbon atom was performed with an energy dispersive X-ray analyzer (Quantax manufactured by Bruker) attached to the scanning electron microscope, and each mapping image was produced. This operation was performed 5 times by randomly selecting 5 points on the electrode.

On the image, 10 positive electrode active material particles having a long side and a short side of 10 µm or more and which the surface of the positive electrode active material particles do not overlap with other particles and can be observed at 90% or more were randomly selected.

In the above selected element-mapped positive electrode active material particle image, the area of the portion in which the presence of a nitrogen element was confirmed was judged to be the active material surface coated with the nitrile group-containing acrylic polymer. In addition, when the coverage ratio was 60% or more, it was evaluated as "possible", assuming that the nitrile group-containing acrylic polymer was covering the surface of the positive electrode active material particles.

(Measurement of swelling degree of non-aqueous electrolyte solution)

An 8% N-methylpyrrolidone (NMP) solution of the nitrile group-containing acrylic polymer was poured into a Teflon (registered trademark) dish so that the thickness after drying became 100 µm, to prepare a polymer film. The obtained film was punched out with 16 mmφ and the weight was measured (the weight is referred to as "A"). 5% of fluoroethylene carbonate was mixed in a mixture of 3 to 7 weight ratio of ethylene carbonate and ethyl methyl carbonate, and lithium hexafluoride phosphate (LiPF ₆ ) was dissolved so that the concentration of 1 mol/liter was prepared as a non-aqueous electrolyte. . The film punched at 16 mm? was immersed in 20 g of the non-aqueous electrolytic solution, followed by immersion at 60°C for 72 hours. Thereafter, the immersed film was taken out, the non-aqueous electrolyte solution on the surface was lightly wiped off, and the weight was measured (the weight is referred to as "B"). The non-aqueous electrolyte swelling degree (=B/A) was calculated from these values. The higher the degree of swelling of the nonaqueous electrolyte solution, the greater the deformation in the nonaqueous electrolyte solution.

(Measurement of THF insoluble content)

An 8% NMP solution of a nitrile group-containing acrylic polymer was poured into a Teflon (registered trademark) dish so that the thickness after drying became 100 µm, to prepare a polymer film. The resulting film was measured for punching weight by 16 mmφ (the weight is referred to as "C"). The film punched at 16 mm? was immersed in 20 g of tetrahydrofuran, and the soluble component was completely dissolved over 24 hours at 25°C. After that, the residual solid which is an insoluble matter was taken out, and the tetrahydrofuran was completely volatilized with an infrared dryer, and then the weight was measured (the weight is referred to as "D"). From these values, the amount of THF insoluble content (= D/C × 100) was calculated. The smaller the amount of THF insoluble content, the less crosslinking between polymer molecules is.

(Bending characteristics of winding body)

The sheet-shaped positive electrode and the sheet-shaped negative electrode were wound using a core having a diameter of 20 mm through a separator to obtain a wound body. As the separator, a microporous membrane made of polypropylene having a thickness of 20 µm was used. The wound body was compressed in one direction at a speed of 10 mm/sec until it became 4.5 mm in thickness. After compression, the wound body was disassembled, the positive electrode was observed, and evaluation was performed according to the following evaluation criteria.

A… No crack

B… Smile crack

C… Peeling from the electrode

(High potential cycle characteristics)

With respect to the nonaqueous electrolyte battery, the operation of charging the nonaqueous electrolyte battery under a 25°C environment until the battery voltage reached 4.8 V at 600 mA, and discharging it at 600 mA until the battery voltage reached 3 V was repeated 100 times.

Then, the ratio of the discharge capacity at the 100th time to the discharge capacity at the first time was calculated.

(Nitrile group-containing acrylic polymer)

The nitrile group-containing acrylic polymers (1) to (6) were adjusted as follows.

[Preparation Example 1]

Preparation of nitrile group-containing acrylic polymer (1)

In an autoclave equipped with a stirrer, 164 parts of ion-exchanged water, 67.5 parts of 2-ethylhexylacrylate (2EHA), 17 parts of methacrylic acid (MAA), 15 parts of acrylonitrile (AN), 2-acrylamide-2 -0.5 parts of methylpropanesulfonic acid (AMPS), 0.3 parts of potassium persulfate as a polymerization initiator, and 1.6 parts of sodium lauryl sulfate as a surfactant were added, sufficiently mixed, and then heated at 70° C. for 3 hours and again at 80° C. for 2 hours for polymerization. And obtained the aqueous dispersion of the nitrile group-containing acrylic polymer (1). In addition, the polymerization conversion rate determined from the solid content concentration was 96%. Further, 500 parts of N-methylpyrrolidone was added to 100 parts of the aqueous dispersion, water and residual monomer were evaporated under reduced pressure, and 81 parts of N-methylpyrrolidone were evaporated to obtain a nitrile group-containing acrylic polymer (1). An 8 mass% of NMP solution was obtained. At this time, the degree of swelling of the non-aqueous electrolyte solution was twice, and the amount of THF insoluble content was 10% or less. Moreover, the SP value of the nitrile group-containing acrylic polymer was 9.9 (cal/cm 3) ^1/2 .

[Preparation Examples 2 to 6]

Preparation of nitrile group-containing acrylic polymers (2) to (6)

It was the same as in Preparation Example 1 except that the injection amount and type of the monomers were changed as shown in Table 1. In addition, in Table 1, AN is acrylonitrile, 2EHA is 2-ethylhexyl acrylate, EA is ethyl acrylate, BA is butyl acrylate, MAA is methacrylic acid, AMPS is 2-acrylamide-2-methyl Propanesulfonic acid and GMA refer to glycidyl methacrylate. Table 1 shows the degree of swelling of the non-aqueous electrolyte, the amount of THF insoluble, and the SP value.

(Example 1)

(Preparation of slurry composition for positive electrode and positive electrode)

100 parts of Li-excessive layered compound (Li[Ni _0.17 Li _0.2 Co _0.07 Mn _0.56 ]O ₂ ) as a positive electrode active material, and acetylene black (AB35, Denki Chemical Co., Ltd.) as a positive electrode conductive material in powder form: number particle diameter 35 nm, Specific surface area 68 m 2 /g) 2.0 parts and 1.6 parts of mixed polyvinylidene fluoride (a 1:1 mixture of KYNAR HSV900 and KYNAR720 manufactured by Arkema) as a fluorine-containing polymer of the binder for the positive electrode, and the nitrile group-containing acrylic polymer described above. The nitrile group-containing acrylic polymer (1) was stirred in an amount equivalent to a solid content of 0.4 parts and an appropriate amount of NMP with a planetary mixer to prepare a slurry composition for a positive electrode. In addition, the melt viscosity of KYNAR HSV900 measured at ASTM D3835/232° C. 100 sec ^-1 is 50 kpoise, and the melt viscosity of KYNAR720 is 9 kpoise.

An aluminum foil having a thickness of 15 µm was prepared as a current collector. The positive electrode slurry composition was applied on both sides of an aluminum foil so that the amount of applied after drying was 25 mg/cm 2, dried at 60° C. for 20 minutes and 120° C. for 20 minutes, and then heated at 150° C. for 2 hours to form a positive electrode fabric ( I got a cause. By these operations, the surface of the current collector that was not coated with the positive electrode active material and the positive electrode active material was coated with the nitrile group-containing acrylic polymer (1). This positive electrode original fabric was rolled by a roll press to prepare a sheet-like positive electrode comprising a positive electrode active material layer having a density of 3.9 g/cm 3 and an aluminum foil. This was cut into a width of 4.8 mm and a length of 50 cm, and an aluminum lead was connected. Moreover, about the obtained positive electrode, the bending characteristic of a wound body was evaluated. Table 2 shows the results.

(Preparation of slurry composition for negative electrode and negative electrode)

As a negative electrode active material, 90 parts of spherical artificial graphite (particle size: 12 µm) and 10 parts of SiOx (particle size: 10 µm), styrene butadiene rubber (particle size: 180 nm, glass transition temperature: -40°C) 1 part as a binder, as a thickener 1 part of carboxymethylcellulose and an appropriate amount of water were stirred with a planetary mixer to prepare a slurry composition for negative electrodes.

A copper foil having a thickness of 15 µm was prepared as a current collector. The negative electrode slurry composition was applied on both sides of the copper foil so that the applied amount after drying was 10 mg/cm 2, dried at 60° C. for 20 minutes and 120° C. for 20 minutes, and then heated at 150° C. for 2 hours to obtain a negative electrode fabric. . This negative electrode original fabric was rolled by a roll press to produce a sheet-like negative electrode composed of a negative electrode active material layer having a density of 1.8 g/cm 3 and copper foil. This was cut into a width of 5.0 mm and a length of 52 cm, and a nickel lead was connected.

[Manufacture of lithium ion secondary battery]

The obtained sheet-like positive electrode and sheet-like negative electrode were wound using a core having a diameter of 20 mm through a separator to obtain a wound body. As the separator, a microporous membrane made of polypropylene having a thickness of 20 µm was used. The wound body was compressed in one direction at a speed of 10 mm/sec until it became 4.5 mm in thickness. The ratio of the major diameter to the short diameter of the approximately ellipse is 7.7.

Further, 5% by mass of fluoroethylene carbonate was mixed with a mixture of 3 to 7 weights of ethylene carbonate and ethyl methyl carbonate, lithium hexafluoride phosphate was dissolved so as to have a concentration of 1 mol/liter, and 2 vol% of vinylene carbonate Was added to prepare a non-aqueous electrolytic solution.

The said wound body was accommodated together with 3.2 g of non-aqueous electrolyte solution in a predetermined aluminum laminated case. Then, after connecting the negative electrode lead and the positive electrode lead to predetermined points, the opening of the case was sealed with heat, thereby completing a lithium ion secondary battery as a wound pouch cell. This battery is a pouch type with a width of 35 mm, a height of 48 mm, and a thickness of 5 mm, and the nominal capacity of the battery is 700 mAh. Table 2 shows the high potential cycle characteristics of the obtained secondary battery.

(Example 2)

In place of the nitrile group-containing acrylic polymer (1), it was carried out similarly to Example 1 except having used the said polymer (2). Table 2 shows the results.

(Example 3)

In place of the nitrile group-containing acrylic polymer (1), it was carried out similarly to Example 1 except having used the said polymer (3). Table 2 shows the results.

(Example 4)

In place of the nitrile group-containing acrylic polymer (1), it was carried out similarly to Example 1 except having used the said polymer (4). Table 2 shows the results.

(Example 5)

In place of the nitrile group-containing acrylic polymer (1), it was carried out in the same manner as in Example 1 except that the polymer (5) was used. Table 2 shows the results.

(Comparative Example 1)

It was carried out in the same manner as in Example 1 except that the nitrile group-containing acrylic polymer (1) was not used. Table 2 shows the results.

(Comparative Example 2)

In place of the nitrile group-containing acrylic polymer (1), except having used polypropylene (Polypropylene manufactured by Wako Junyaku), it was carried out in the same manner as in Example 1. The results are shown in Table 2. In addition, the SP value of polypropylene was 8.35 (cal/cm 3) ^1/2 , and the positive electrode active material could not be coated with polypropylene. Moreover, the swelling degree of the non-aqueous electrolyte solution of polypropylene was 1.1 times, and the amount of THF insoluble content was 0 mass %.

(Comparative Example 3)

In place of the nitrile group-containing acrylic polymer (1), it was carried out in the same manner as in Example 1 except that the polymer (6) was used. Table 2 shows the results.

In Tables 1 and 2, about the examples satisfying the requirements of the present invention, good results were obtained in a good balance for all evaluation items. On the other hand, in Comparative Examples 1 and 2 in which the nitrile group-containing acrylic polymer was not used, and in Comparative Example 3 in which the nitrile group-containing acrylic polymer was used outside the specific range, the results were inferior to all evaluation items.

Claims (10)

A lithium ion secondary battery comprising a positive electrode, a negative electrode and a non-aqueous electrolyte,
The positive electrode contains a positive electrode active material, a binder for a positive electrode, and a conductive material,
The positive electrode active material is coated with a nitrile group-containing acrylic polymer having an SP value of 9 to 11 (cal/cm 3) ^1/2 ,
The nitrile group-containing acrylic polymer contains 50 to 70 mass% of a (meth)acrylic acid ester monomer unit,
The degree of swelling of the nitrile group-containing acrylic polymer in the non-aqueous electrolyte solution is 3 times or less, and the amount of THF insoluble content is 30% by mass or less,
The lithium ion secondary battery, wherein the binder for positive electrodes contains a fluorine-containing polymer. The method of claim 1,
The lithium ion secondary battery, wherein the positive electrode active material is an excess lithium layered compound containing Li, Mn, Co and Ni. The method of claim 1,
The lithium ion secondary battery, wherein the negative electrode contains an alloy-based active material. The method of claim 1,
A lithium ion secondary battery, wherein the conductive material has a particle diameter of 5 to 40 nm. The method of claim 1,
A lithium ion secondary battery, wherein 1 to 3 parts by mass of a conductive material and 0.4 to 2 parts by mass of a binder for a positive electrode are contained with respect to 100 parts by mass of the positive electrode active material. The method of claim 1,
The lithium ion secondary battery, wherein the fluorine-containing polymer is polyvinylidene fluoride. The method of claim 1,
The lithium ion secondary battery, wherein the nitrile group-containing acrylic polymer contains an ethylenically unsaturated acid monomer unit. The method of claim 7,
The lithium ion secondary battery, wherein the content ratio of the ethylenically unsaturated acid monomer unit in the nitrile group-containing acrylic polymer is 10 to 30% by mass. The method according to any one of claims 1 to 8,
Lithium-ion secondary battery, which is a wound pouch cell. delete