KR20150063958A - Lithium ion secondary battery - Google Patents

Abstract

[PROBLEMS] To provide a high-capacity lithium ion secondary battery which is flexible and has no electrode layer cracking at the time of bending and excellent in high-potential cycle characteristics.
[MEANS FOR SOLVING PROBLEMS] A lithium ion secondary battery according to the present invention comprises a negative electrode, a positive electrode and a nonaqueous electrolyte, wherein the negative electrode contains an alloy active material and the positive electrode contains a positive electrode active material, a positive electrode binder and a conductive material , The nitrile group-containing acrylic polymer and the nitrile group-containing acrylic polymer have a swelling degree of not more than 3 times and a THF insoluble matter content of not more than 30 mass% And the particle diameter of the ash is 5 to 40 nm.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a lithium ion secondary battery,

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a lithium ion secondary battery, and more particularly, to a lithium ion secondary battery capable of high capacity.

2. Description of the Related Art In recent years, portable terminals such as a notebook computer, a mobile phone, and a PDA (Personal Digital Assistant) have become popular. BACKGROUND ART A nickel-hydrogen secondary battery, a lithium ion secondary battery, and the like are widely used in secondary batteries used for power supply of these portable terminals. Portable terminals are demanded to be more comfortable to carry, and miniaturization, thinness, light weight, and high performance have been rapidly progressed, and as a result, portable terminals have been used in various places.

In addition, as for the battery, the miniaturization, thinness, weight reduction, and high performance of the portable terminal have been demanded. In order to increase the amount of the active material in the active material layer in order 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 ensuring 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 is increased, the voltage at the time of charging and discharging is also increasing. An electrolyte made of ethylene carbonate, propylene carbonate or the like can not withstand a high voltage and may be decomposed. Therefore, a fluorine electrolyte additive is also used in combination.

On the other hand, in the positive electrode, a fluorine-containing polymer such as polyvinylidene fluoride (PVdF) has been used as a binder for forming an electrode layer. Since the fluorine-containing polymer is not dissolved in the electrolytic solution, stable binding property is expected, but the fluorine-containing polymer such as PVdF is hard and does not bend well. Therefore, depending on the shape and size of the battery, in the case of using only the fluorine-containing polymer when forming the electrode into a predetermined shape by crushing after pressing the electrode, a crack may occur in the electrode layer.

In addition, in order to achieve high capacity, it has been investigated that carbon black or the like, which is a conductive material, is made fine in the positive electrode. However, when the fluorine-containing polymer is used singly as the binder, the finely divided conductive material aggregates, the dispersibility becomes insufficient, conductivity is not increased, and the capacity is not improved.

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

When a fluorine-containing polymer is used as the binder in the positive electrode in order to achieve high capacity for the lithium ion secondary battery, there is a problem that a crack is generated in the electrode layer. It has also been studied to use a fluorine-containing polymer and a nitrile rubber in combination as a binder used for a positive electrode, and to use a fluorine-containing polymer in combination with a crosslinked acrylate-based polymer.

For example, in Patent Document 2, a fluorine-containing polymer and nitrile rubber are used in combination as a binder for forming an electrode layer of a lithium ion secondary battery. However, as the amount of the binder increases, the active material density is relatively lowered, so that sufficient battery capacity may not be obtained. Further, since the amount of the binder is increased, the degree of swelling of the electrode layer with respect to the electrolytic solution is increased, and in particular, the peel strength at the time of high potential cycle is lowered, and the cycle characteristics are sometimes deteriorated.

In Patent Document 3, a fluorine-containing polymer and a crosslinked acrylate polymer are used in combination as a binder. However, since the crosslinked acrylate polymer is used, swelling of the electrode layer with respect to the electrolytic solution is suppressed, but the dispersibility is insufficient because the crosslinked acrylate polymer exists in the form of particles, and as a result, the high-potential cycle characteristic is sometimes lowered .

Accordingly, an object of the present invention is to provide a high-capacity lithium ion secondary battery which is flexible, has no electrode layer cracks at the time of bending, and has excellent high-voltage cycle characteristics.

The present inventors have intensively studied in order to solve the above problems and found that by using a nitrile group-containing acrylic polymer as a binder for a positive electrode and by adjusting the swelling degree and the THF insoluble content of a nitrile group-containing acrylic polymer to a non- , The nitrile group-containing acrylic polymer is dissolved in the dispersion medium of the slurry composition for the positive electrode, but is not dissolved in the electrolyte solution having a near dissolution parameter (SP value), so that it can be swelled in an appropriate range. By using the alloy active material as the negative electrode active material and the conductive material powder finely impregnated in the positive electrode, it is possible to achieve high capacity, and by using the nitrile group-containing acrylic polymer in combination with the fluorine- It was found that a lithium ion secondary battery having a high capacity and excellent in output characteristics and high potential cycle characteristics can be obtained without cracking of the electrode layer at the time of bending and an electrode with high active material density. Based on these findings, the present invention has been completed.

The gist of the present invention is as follows.

(1) A lithium ion secondary battery comprising a negative electrode, a positive electrode, and a nonaqueous electrolyte,

Wherein the negative electrode contains an alloy-based active material,

Wherein the positive electrode contains a positive electrode active material, a positive electrode binder and a conductive material,

Wherein the binder for a positive electrode contains a nitrile group-containing acrylic polymer and a fluorine-containing polymer,

The swelling degree of the nitrile group-containing acrylic polymer with respect to the nonaqueous electrolyte is 3 times or less, the THF insoluble content is 30 mass% or less,

Wherein the conductive material has a particle diameter of 5 to 40 nm.

(2) The lithium ion secondary battery according to (1), wherein the conductive material is contained in 1 to 3 mass parts and the positive electrode binder is contained in 0.5 to 2 mass parts with respect to 100 mass parts of the positive electrode active material.

(3) The positive electrode binder according to (1) or (2), wherein the content of the nitrile group-containing acrylic polymer is 50 to 5 mass% and the content of the fluorine-containing polymer is 50 to 95 mass% Ion secondary battery.

(4) The lithium ion secondary battery described in any one of (1) to (3), wherein the fluorine-containing polymer is polyvinylidene fluoride.

(5) The lithium ion secondary battery according to any one of (1) to (4), wherein the nitrile group-containing acrylic polymer contains an ethylenic unsaturated acid monomer unit.

(6) The lithium ion secondary battery according to (5), wherein the content of the ethylenically unsaturated acid monomer unit in the nitrile group-containing acrylic polymer is 10 to 30 mass%.

(7) The lithium ion secondary battery according to any one of (1) to (6), which is a wound pouch cell.

According to the present invention, by using the alloy-based active material for the negative electrode and the finely divided conductive material powder as the positive electrode and by using the fluorine-containing polymer and the nitrile group-containing acrylic polymer together as the binder, An electrode having a high active material density can be obtained in addition to the absence of cracks in the electrode layer. In addition to excellent output characteristics and high potential cycle characteristics, a lithium ion secondary battery having a high initial capacity can be provided.

Hereinafter, the present invention will be described in more detail. A lithium ion secondary battery according to the present invention comprises a negative electrode, a positive electrode and a nonaqueous electrolyte. The negative electrode contains an alloy-based active material, and the positive electrode contains a positive electrode active material, a positive electrode binder and a conductive material. The positive electrode binder contains a nitrile group-containing acrylic polymer and a fluorine-containing polymer. The degree of swelling of the nitrile group-containing acrylic polymer with respect to the non-aqueous electrolyte is 3 times or less, and the amount of THF insoluble matter is 30 mass% or less. The particle diameter of the positive electrode conductive material is 5 to 40 nm. Hereinafter, each of the negative electrode, the positive electrode and the non-aqueous electrolyte will be described more specifically.

(Negative electrode)

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 contains the alloy active material (a1) as the negative electrode active material (a) and contains other carbonaceous active material (a2) if necessary, and usually contains the binder for the negative electrode (b) And the like.

(a) Negative electrode active material

The negative electrode active material is a material that receives electrons (lithium ions) in the negative electrode. As the negative electrode active material, the alloy-based active material (a1) is used, and if necessary, the 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. By using the alloy-based active material and the carbon-based active material in combination, it is possible to obtain a battery having a capacity larger than that of the negative electrode obtained using only the alloy- , Degradation of cycle characteristics, and the like can be solved.

(a1) alloy-based active material

The alloy-based active material includes an element capable of inserting lithium in its structure, and has a theoretical capacity per unit weight of 500 mAh / g or more when lithium is inserted (the upper limit of the theoretical capacity is not particularly limited, And may be, for example, 5000 mAh / g or less), specifically, a single metal or alloy thereof forming a lithium alloy, and an oxide, a sulfide, a nitride, a silicide, a carbide, or a phosphide thereof.

Examples of the metal and the alloy that form the lithium alloy include a metal such as Ag, Al, Ba, Bi, Cu, Ga, Ge, In, Ni, P, Pb, Sb, Si, Sn, . ≪ / RTI > Among them, a single metal such as silicon (Si), tin (Sn) or lead (Pb), an alloy containing these atoms, or a compound of these metals is preferable. Among them, a single metal of Si capable of intercalating lithium at a low electric potential is more preferable.

The alloy-based active material may further contain one or more non-metallic elements. Specifically, 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 = less than _{0.01 2), SnO x (0} <x ≤ 2), LiSiO, there may be mentioned LiSnO, etc., insertion desorption of lithium are possible on that particular low potential SiOC, SiO _x, and SiC is preferred, and SiOC, SiO _x is more preferable. For example, SiOC can be obtained by firing a polymeric material containing silicon. Among SiOC, a range of 0.8? X? 3 and 2? Y? 4 is preferably used in balance of capacity and cycle characteristics.

Examples of the oxides, sulfides, nitrides, silicides, carbides and phosphides of the single metals and their alloys forming the lithium alloy include oxides, sulfides, nitrides, silicides, carbides and phosphides of elements capable of intercalating lithium, Of these, oxides are particularly preferable. Specifically, a lithium-containing metal complex oxide containing a metal element selected from the group consisting of tin oxide, manganese oxide, titanium oxide, niobium oxide, oxide such as vanadium oxide, Si, Sn, Pb and Ti atoms is preferable.

As the lithium-containing metal composite oxide, a lithium-titanium composite oxide represented by Li _x Ti _y M _z O ₄ (0.7 ≤ x ≤ 1.5, 1.5 ≤ y ≤ 2.3, 0 ≤ z ≤ 1.6, and M is Na, K, Co, Al Li ₄ Ti ₅ O ₃ , Li ₁ Ti ₂ O ₄ , Li _4/5 Ti _{11/5, and the like.} O ₄ is preferred.

Among these alloy-based active materials, an active material containing silicon is preferable. By using the silicon-containing active material, the electric capacity of the secondary battery can be increased. Among the silicon-containing active materials, SiO _x C _y , SiO _x , and SiC are more preferable. In the active material containing silicon and carbon in combination, it is presumed that Li intercalates into Si (silicon) on the high electric potential and C (carbon) on the low electric potential, and expansion and contraction are suppressed more than other alloy active materials , The effect of the present invention is more likely to be obtained.

It is preferable that the alloy-based active material is formed into a particle shape. If the shape of the particles is spherical, a higher density electrode can be formed at the time of electrode formation. When the alloy-based active material is particles, the volume average particle diameter thereof is preferably 0.1 to 50 μm, more preferably 0.5 to 20 μm, particularly preferably 1 to 10 μm. When the volume average particle diameter of the alloy-based active material is within this range, it is easy to manufacture the slurry composition used for producing the negative electrode. The volume average particle diameter in the present invention can be obtained by measuring the particle diameter distribution by laser diffraction.

The tap density of the alloy-based active material is not particularly limited, but 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, and particularly preferably 4.0 to 10.0 m 2 / g. Since the specific surface area of the alloy-based active material is in the above range, the active points of the surface of the alloy-based active material are increased, so that the output characteristics of the lithium ion secondary battery are excellent. In the present invention, the "BET specific surface area" refers to the BET specific surface area measured by the nitrogen adsorption method and is a value measured according to ASTM D 3037-81.

(a2) a carbon-based active material

The carbon-based active material refers to an active material having carbon as a main skeleton capable of intercalating lithium, specifically, a carbonaceous material and a graphite material. The carbonaceous material is generally a carbon material having a low degree of graphitization (that is, low crystallinity) obtained by carbonizing a carbon precursor at a temperature of 2000 占 폚 or less. The lower limit of the heat treatment temperature is not particularly limited, but may be 500 ° C or higher, for example. The graphite material is a graphite material having a high crystallinity close to graphite obtained by heat-treating graphitizing carbon at 2000 占 폚 or higher. The upper limit of the treatment temperature is not particularly limited, but may be set at 5000 占 폚 or lower, for example.

Examples of the carbonaceous material include graphite carbon which easily changes the structure of carbon by a heat treatment temperature, and non-graphitizable carbon which has a structure similar to an amorphous structure typified by glassy carbon.

As the graphitizing carbon, for example, a carbon material obtained by using tar pitch obtained from petroleum or coal as a raw material can be mentioned. Specific examples thereof include coke, mesocarbon microbeads (MCMB), mesophase pitch-based carbon fibers, pyrolysis-phase grown carbon fibers, and the like. MCMB is a carbon fine particle obtained by separating and extracting mesophase spherules produced in the course of heating the pitch at about 400 ° C. The mesophase pitch-based carbon fiber is a carbon fiber whose mesophase pitch is obtained by the growth and coalescence of the mesophase spherules as a raw material. The pyrolysis vapor-grown carbon fiber refers to a method of pyrolyzing acrylic polymer fibers and the like, (2) pyrolysis by spinning the pitch, (3) catalytic vapor phase growth (pyrolysis of hydrocarbons using nanoparticles such as iron) (Catalytic CVD) method.

Examples of the non-graphitizable carbon include a phenol resin fired body, a polyacrylonitrile carbon fiber, a pseudo-isotropic carbon, a furfuryl alcohol resin fired body (PFA), hard carbon and the like.

Examples of the graphite material include natural graphite and artificial graphite. Artificial graphite includes artificial graphite heat-treated at 2800 ° C or higher, graphitized MCMB heat-treated at MCMB at 2000 ° C or higher, graphitized mesophase pitch carbon fibers heat-treated at a temperature of 2000 ° C or above, mesophase pitch- .

Among the 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 2.2 g / cm 3 or less) Can be easily manufactured. If the negative electrode has a negative electrode active material layer having a density of the negative electrode active material layer in the above range, the effect of the present invention is remarkable.

It is preferable that the carbon-based active material is formed into a particle shape. When the shape of the particles is spherical, a higher density electrode can be formed at the time of electrode formation. When the carbonaceous active material is a particle, the volume average particle diameter of the carbonaceous active material is preferably 0.1 to 100 占 퐉, more preferably 0.5 to 50 占 퐉, and particularly preferably 1 to 30 占 퐉. When the volume average particle diameter of the carbon-based active material is within this range, it is easy to prepare the slurry composition used for producing the negative electrode.

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

The specific surface area of the carbonaceous 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. Since the specific surface area of the carbonaceous active material is in the above range, the active sites of the surface of the carbonaceous active material are increased, so that the output characteristics of the lithium ion secondary battery are excellent. The specific surface area can be measured by, for example, the BET method.

The negative electrode active material may be a single alloy type active material, or two or more types may be used in combination at an arbitrary ratio. As a preferable embodiment of the negative electrode active material, there can be mentioned an active material in which an alloy type active material and a carbon type active material are combined. When the alloy active material (a1) and the carbonaceous active material (a2) are used in combination as the negative electrode active material (a), the mixing method is not particularly limited and includes conventionally known dry mixing and wet mixing.

When the alloy-based active material (a1) and the carbon-based active material (a2) are used together in the negative electrode active material (a), the alloyed active material (a1) is contained in an amount of 1 to 50 parts by mass relative to 100 parts by mass of the carbon- . By mixing the alloy-based active material and the carbon-based active material within the above range, it is possible to obtain a battery having a capacity larger than that of the negative electrode obtained using only the conventional carbon-based active material, and to prevent the adhesion strength of the negative electrode and the cycle characteristics from being lowered. The effect of the present invention is remarkable when the negative electrode has a negative electrode active material layer in which the alloy-based active material (a1) and the carbonaceous active material (a2) are used together in the above range.

(b) Binder for negative electrode

The binder for negative electrode is preferably a component which binds the electrode active material to the surface of the current collector in the negative electrode and which has excellent performance of holding the negative electrode active material and has high adhesion to the current collector. Usually, a polymer is used as the material of the binder. The polymer as the binder material may be a homopolymer or a copolymer. The polymer of the binder for negative electrode is not particularly limited, and examples thereof include polymer compounds such as fluoropolymer, diene polymer, acrylate polymer, polyimide, polyamide and polyurethane. Among them, A polymer or an acrylate polymer is preferable, and a diene polymer or an acrylate polymer is more preferable because it can increase a withstanding voltage and can increase an energy density of an electrochemical device. In order to improve the strength of an electrode, Is particularly preferable.

The diene polymer is a polymer containing a structural unit (hereinafter may be referred to as &quot; conjugated diene monomer unit &quot;) formed by polymerization of a conjugated diene monomer, specifically, a homopolymer of a conjugated diene; Copolymers of different types of conjugated dienes; A copolymer obtained by polymerizing a monomer mixture containing a conjugated diene, and hydrogenated products thereof. Examples of the conjugated diene include 1,3-butadiene, 2-methyl-1,3-butadiene, 2,3-dimethyl-1,3-butadiene, 1,3-butadiene, 1,3-pentadiene, 1,3-hexadiene, and 2,4-hexadiene. Of these, 1,3-butadiene and 2-methyl-1,3-butadiene are preferred. The conjugated dienes may be used singly or two or more kinds may be used in combination at any ratio. 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 more preferably 30% by mass or more and 55% by mass or less.

The diene polymer may contain, in addition to the conjugated diene, a structural unit (hereinafter sometimes referred to as a "nitrile group-containing monomer unit") formed by polymerizing a nitrile group-containing monomer. Specific examples of the nitrile group-containing monomer include an?,? - unsaturated nitrile compound such as acrylonitrile, methacrylonitrile,? -Chloroacrylonitrile,? -Ethyl acrylonitrile and the like, Nitrile is preferred. The proportion of the nitrile group-containing monomer units in the diene polymer is preferably in the range of 5 to 40% by mass, and more preferably 5 to 30% by mass. By setting the amount of the nitrile group-containing monomer unit within the above range, the obtained electrode strength is further improved. The nitrile group-containing monomers may be used singly or two or more kinds may be used in combination at an arbitrary ratio.

The diene polymer may contain a structural unit formed by polymerizing a monomer other than the monomer unit. Other monomers 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, vinylbenzoate, vinylnaphthalene, chloromethylstyrene, hydroxymethylstyrene,? -Methylstyrene, divinylbenzene; Olefins such as ethylene and propylene; Vinyl esters such as vinyl acetate, vinyl propionate, vinyl butyrate and vinyl benzoate; Amide 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; Vinylpyrrolidone, vinylpyridine, vinylimidazole, and other heterocyclic-containing vinyl compounds. Each of the other monomers may be used alone, or two or more different monomers may be used in combination at an arbitrary ratio.

Acrylate polymer is obtained by polymerizing a monomer derived from a compound represented by the general formula (1): CH ₂ ═CR ¹ -COOR ² (wherein R ¹ represents a hydrogen atom or a methyl group, and R ² represents an alkyl group or a cycloalkyl group) (Hereinafter may be referred to as a &quot; (meth) acrylic acid ester monomer unit &quot;). Specific examples of the monomer 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, Acrylic acid esters such as n-hexyl acrylate, 2-ethylhexyl acrylate, lauryl acrylate and stearyl acrylate; Propyl methacrylate, isopropyl methacrylate, n-butyl methacrylate, isobutyl methacrylate, t-butyl methacrylate, n-amyl methacrylate, N-hexyl acrylate, 2-ethylhexyl methacrylate, lauryl methacrylate, stearyl methacrylate and the like. Of these, acrylic acid esters are preferable, and it is particularly preferable that the strength of an electrode obtained from n-butyl acrylate and 2-ethylhexyl acrylate can be improved. The proportion of the (meth) acrylic acid ester monomer units in the acrylate polymer is usually 50% by mass or more, and preferably 70% by mass or more. The use of an acrylate polymer having a ratio of the (meth) acrylic acid ester monomer unit within the above range can provide a high heat resistance and can reduce the internal resistance of the resulting electrode.

It is preferable that the acrylate polymer contains a monomer unit containing a nitrile group in addition to a (meth) acrylic acid ester monomer unit. Examples of the nitrile group-containing monomer include acrylonitrile and methacrylonitrile. Of these, acrylonitrile is preferable because the binder strength of the current collector and the electrode mixture layer can be improved to improve the electrode strength. The proportion of the nitrile group-containing monomer units in the acrylate polymer is preferably in the range of 5 to 35 mass%, and more preferably 10 to 30 mass%. By setting the amount of the nitrile group-containing monomer unit within the above range, the obtained electrode strength is further improved.

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

The acrylate polymer may contain, in addition to the monomer, a structural unit obtained by polymerizing other copolymerizable monomer. Examples of the other monomer include carboxylic acid esters having two or more carbon-carbon double bonds such as ethylene glycol dimethacrylate, diethylene glycol dimethacrylate, and trimethylolpropane triacrylate; Unsaturated esters containing fluorine in the side chains such as perfluorooctyl ethyl acrylate and perfluorooctyl ethyl methacrylate; Styrene monomers such as styrene, chlorostyrene, vinyltoluene, t-butylstyrene, vinylbenzoic acid, vinylbenzoate, vinylnaphthalene, chloromethylstyrene, hydroxymethylstyrene,? -Methylstyrene, divinylbenzene; Amide monomers such as acrylamide, N-methylol acrylamide and acrylamide-2-methylpropanesulfonic acid; Olefins such as ethylene and propylene; Diene monomers such as butadiene and isoprene; Halogen-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-containing vinyl compounds such as N-vinylpyrrolidone, vinylpyridine and vinylimidazole; Glycidyl ethers such as allyl glycidyl ether; And glycidyl esters such as glycidyl acrylate and glycidyl methacrylate. The content of these copolymerizable other monomer units in the acrylate polymer may be suitably adjusted according to the intended use.

In addition to the above, examples of the binder for negative electrode include a binder such as polyethylene, polypropylene, polyisobutylene, polyvinyl chloride, polyvinylidene chloride, polyvinylidene fluoride, polytetrafluoroethylene, polyvinyl acetate, polyvinyl alcohol, Vinyl polymers such as butyl ether, polyacrylonitrile, polymethacrylonitrile, polymethyl methacrylate, methyl polyacrylate, ethyl polymethacrylate, allyl polyacetate, and polystyrene; Ether polymers containing hetero atoms in the main chain such as polyoxymethylene, polyoxyethylene, poly-cyclic thioether, polydimethylsiloxane and the like; Condensation 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 polypyromellitimide, and thickeners described later.

The shape of the binder for negative electrode is not particularly limited, but it is preferable that the shape of the binder for the negative electrode is in particulate form because the adhesion to the current collector is good and deterioration due to capacity reduction or charge / discharge repetition of the produced electrode can be suppressed. The particulate binder may be one that retains its particle shape in the state of being dispersed in a dispersion medium. It is preferable that the particulate binding agent can exist in a state in which the particle shape is maintained even in the negative electrode active material layer. In the present invention, the &quot; state in which the particle state is maintained &quot; is not necessarily the state in which the particle shape is completely maintained, but the state of the particle shape may be 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 obtained in the form of a powder obtained by drying such a dispersion.

The glass transition temperature (Tg) of the binder for negative electrode is preferably 50 占 폚 or less, and more preferably -40 to 0 占 폚. When the glass transition temperature (Tg) of the binder is within this range, the adhesion is excellent with a small amount of use, the strength of the electrode is high, and the flexibility is abundant, so that the electrode density can be easily increased by the pressing step have.

When the binder for negative electrode is a particulate binder, the number average particle diameter is not particularly limited, but is usually 0.01 to 1 mu m, preferably 0.03 to 0.8 mu m, more preferably 0.05 to 0.5 mu m. When the number average particle diameter of the binder is in this range, an excellent adhesion can be imparted to the negative electrode active material layer even by using a small amount. Here, the number average particle diameter is the number average particle diameter calculated as the arithmetic mean value by measuring the diameter of 100 randomly selected binder particles in a transmission electron microscope photograph. The shape of the particles may be spherical, deformed, or any shape. These binders may be used alone or in combination of two or more.

The amount of the binder for negative electrode is usually in the range of 0.1 to 50 parts by mass, preferably 0.5 to 20 parts by mass, and more preferably 1 to 10 parts by mass based on 100 parts by mass of the negative electrode active material. When the amount of the binder is within this range, the adhesion between the obtained negative electrode active material layer and the current collector can be sufficiently secured, and the capacity of the electrochemical device can be increased and the internal resistance can be lowered.

(c)

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

Other negative electrode components

The negative electrode active material layer may further contain optional components such as a reinforcing material, a leveling agent, an electrolyte additive having a function of inhibiting decomposition of an electrolyte, etc. In addition, a thickener or the like contained in the slurry to be adjusted during negative electrode production may remain .

As the reinforcing material, various inorganic and organic spherical, plate, rod-like or fibrous fillers can be used. By using the reinforcing material, it is possible to obtain a strong and flexible negative electrode, and to exhibit excellent long-term cycle characteristics. 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 negative electrode active material. By including the reinforcing material in the above range, high capacity and high load characteristics can be exhibited.

Examples of the leveling agent include surfactants such as alkyl surfactants, silicone surfactants, fluorochemical surfactants, and metal surfactants. By mixing the leveling agent, it is possible to prevent cratering that occurs during coating or to improve the smoothness of the negative electrode active material layer. The content of the leveling agent is preferably 0.01 to 10 parts by mass based on 100 parts by mass of the total amount of the negative electrode active material. When the leveling agent is contained in the above range, productivity, smoothness and battery characteristics at the time of negative electrode production are excellent.

As the electrolyte additive, vinylene carbonate or the like used for an electrolytic solution can be used. The content of the electrolyte additive is preferably 0.01 to 10 parts by mass based on 100 parts by mass of the total amount of the negative electrode active material. When the content of the electrolyte additive is within the above range, the obtained secondary battery has excellent cycle characteristics and high temperature characteristics. Other additives include nanoparticles such as fumed silica and fumed alumina. It is possible to control the tin firing of the slurry composition to be adjusted when the negative electrode is produced by mixing nanoparticles and to improve the leveling property of the negative electrode active material layer obtained thereby. The content of the nano-particles is preferably 0.01 to 10 parts by mass based on 100 parts by mass of the total amount of the negative electrode active material. When the nanoparticles are used in the negative electrode slurry composition in the above proportions, the slurry stability and productivity are excellent and high battery characteristics are exhibited.

Examples of the thickener include cellulose such as carboxymethylcellulose, carboxyethylcellulose, hydroxyethylcellulose, ethylcellulose, hydroxymethylcellulose, hydroxyethylcellulose, hydroxypropylcellulose, hydroxyethylmethylcellulose and the like (Including salts such as ammonium salts and alkali metal salts thereof), starches of oxidized starch, starch of phosphoric acid, casein, various modified starches, polyethylene oxide, polyethylene glycol, polyvinyl alcohol, polyvinyl pyrrolidone, Polycarboxylic acid, acrylic acid or methacrylic acid copolymer (including salts such as ammonium salts and alkali metal salts thereof (sodium salt and lithium salt)), and the like. These may be used in combination of two or more.

Among them, a cellulose compound (including salts such as ammonium salts and alkali metal salts thereof), a polysulfonic acid, a polycarboxylic acid, an acrylic acid or the like is preferable from the viewpoint of excellent slurry stability in the production of the negative electrode and from the viewpoint of suppressing swelling of the obtained negative electrode. Methacrylic acid copolymers (including salts such as ammonium salts and alkali metal salts thereof) are preferable. At this time, the acrylic acid or methacrylic acid copolymer (including salts such as ammonium salts and alkali metal salts thereof) may be copolymerized with acrylic acid or a copolymerizable component other than methacrylic acid, for example, methyl acrylate or methyl methacrylate And it is preferable from the viewpoint of controlling various properties, and furthermore, when the copolymer and the cellulose compound are used in combination, the viscosity stability of the negative electrode slurry composition can also be improved.

The content of the thickener is preferably from 0.05 to 10 parts by mass, more preferably from 0.08 to 3 parts by mass, based on 100 parts by mass of the negative electrode active material.

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-mentioned negative electrode active material (a), negative electrode binder (b), conductive material (c) and other optional components in a dispersion medium. As the dispersion medium, any of water and 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, methylcyclohexanone and ethylcyclohexanone; Chlorinated aliphatic hydrocarbons such as methylene chloride, chloroform, and carbon tetrachloride; Esters such as ethyl acetate, butyl acetate,? -Butyrolactone and? -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; And amides such as N-methylpyrrolidone and N, N-dimethylformamide.

These dispersion media may be used alone, or two or more of them may be mixed and used as a mixed solvent. Among these, a dispersant having excellent dispersibility of each component, low boiling point and high volatility is preferable because it can be removed at a low temperature in a short time. Specifically, acetone, toluene, cyclohexanone, cyclopentane, tetrahydrofuran, cyclohexane, xylene, water, 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 stirring, shaking, rotating, and the like. In addition, a method using a dispersion kneader such as a homogenizer, a ball mill, a sand mill, a roll mill, and a planetary kneader may be used.

Lithium ion secondary battery anode

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 the same.

The manufacturing method of the lithium ion secondary battery negative electrode includes a step of forming the negative electrode active material layer by applying the negative electrode slurry composition to one surface or both surfaces of the current collector and drying the same.

The method of applying the negative electrode slurry composition onto the current collector is not particularly limited. For example, methods such as a doctor blade method, a dipping method, a reverse roll method, a direct roll method, a gravure method, an extrusion method, and a brush coating method can be used.

Examples of the drying method include a drying method by hot air, hot air, low-humidity air, vacuum drying, and irradiation with (circle) infrared rays or electron beams. The drying time is usually 5 to 30 minutes, and the drying temperature is usually 40 to 180 ° C.

It is preferable to have a step of coating the negative electrode slurry composition on the current collector and drying the negative electrode active material layer and then lowering the porosity of the negative electrode active material layer by pressure treatment using a die press or a roll press when manufacturing the lithium ion secondary battery negative electrode Do. 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 excessively high, the charging efficiency and the discharge efficiency may deteriorate. If the porosity is too low, it is difficult to obtain a high volume capacity, and the negative electrode active material layer tends to peel off from the current collector, so that defects may easily occur. When a curable polymer is used as the binder, curing is preferred.

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

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

The density of the negative electrode active material layer is preferably 1.6 to 1.9 g / cm3, more preferably 1.65 to 1.85 g / cm3. When the density of the negative electrode active material layer is in the above range, a high capacity secondary battery can be obtained.

The current collector is not particularly limited as long as it is an electrically conductive and electrochemically durable material. However, the current collector is preferably a metal material because it has heat resistance, and examples thereof include iron, copper, aluminum, nickel, stainless steel, titanium, , Platinum, and the like. Among them, copper is particularly preferable as a current collector used for a lithium ion secondary battery negative electrode.

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

The collector may be subjected to surface roughening in advance in order to increase the bonding strength with the negative electrode active material layer. Examples of roughening methods include mechanical polishing, electrolytic polishing, and chemical polishing. In the mechanical polishing, a wire brush having polishing abrasive particles fixed to abrasive particles, a grinding stone, an emery buff, a steel wire, or the like is used. A primer layer or the like may be formed on the surface of the collector in order to increase the adhesive strength and conductivity of the negative electrode active material layer.

(Positive electrode)

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 the positive electrode active material (A), the positive electrode binder (B) and the conductive material (C), and contains other components as necessary.

(A) positive electrode active material

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

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

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

Examples of transition metal oxides include 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 Among them, MnO, V ₂ O ₅ , V ₆ O ₁₃ and TiO ₂ are preferable from the cycle characteristics and the capacity.

Transition metal sulfides include TiS ₂ , TiS ₃ , amorphous MoS ₂ , and FeS.

The lithium-containing composite metal oxide includes, for example, a lithium-containing composite metal oxide having a layered structure, a lithium-containing composite metal oxide having a spinel structure, and a lithium-containing composite metal oxide having an olivine structure.

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

Examples of the lithium-containing composite metal oxide having a spinel structure include lithium manganese oxide (LiMn ₂ O ₄ ) and Li [Mn _3/2 M _1/2 ] O _{4 in} which a part of Mn is substituted with another transition metal M is Cr, Fe, Co, Ni, Cu, etc.).

As the lithium-containing composite metal oxide having olivine structure, for example, Li _X MPO ₄ (where M is Mn, Fe, Co, Ni, Cu, Mg, Zn, V, Ca, Sr, Ba, , At least one kind selected from Si, B and Mo, 0? X? 2).

Among them, lithium-containing cobalt oxide (LiCoO ₂ ), lithium-containing nickel oxide (LiNiO ₂ ), lithium composite oxide of Co-Ni-Mn (Li (Co Mn Ni) O ₂ ) Li-excess layer compound _{(Li [Ni 0.17 Li 0.2 Co} 0.07 Mn 0.56] O 2), lithium-containing composite metal oxide having a spinel structure (LiNi _0.5 Mn _1.5 O ₄₎ are preferable, and lithium-containing cobalt oxide (LiCoO _2), A lithium excess layered compound (Li [Ni _0.17 Li _0.2 Co _0.07 Mn _0.56 ] O ₂ ) is more preferable.

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

The iron-based oxide lacking electrical conductivity may be used as an electrode active material covered with a carbon material by causing a carbon source material to exist at the time of reduction calcination.

These compounds may be partially substituted by an element.

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

The volume average particle diameter of the positive electrode active material is usually 1 to 50 占 퐉, preferably 2 to 30 占 퐉. When the 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 the deterioration of the battery capacity can be suppressed. In order to form the positive electrode active material layer, a slurry containing a positive electrode active material and a binder for positive electrode (hereinafter sometimes referred to as &quot; positive electrode slurry composition &quot;) is usually prepared, It is easy to formulate it with an appropriate viscosity, and a uniform positive electrode active material layer can be obtained.

The content 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%. By setting the content of the positive electrode active material in the positive electrode active material layer within the above range, flexibility and adhesion can be exhibited while exhibiting high capacity.

(B) Binder for positive electrode

The positive-electrode binder (B) contains a nitrile group-containing acrylic polymer (B1) and a fluorine-containing polymer (B2).

(B1) a 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 nitrile group-containing monomer, 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 (B1) contains nitrile group-containing monomer units, preferably contains a (meth) acrylic acid ester monomer unit, and optionally contains ethylenic unsaturated acid monomer units and other Containing monomer units derived from monomers. These monomer units are structural units formed by polymerizing the monomers. Here, the content of each monomer is ordinarily the same as the content of each monomer unit in the nitrile group-containing acrylic polymer.

Specific examples of the nitrile group-containing monomer include acrylonitrile and methacrylonitrile. Among them, acrylonitrile is preferable because it enhances the adhesion with the current collector and improves the strength of the electrode.

The content of the nitrile group-containing monomer units in the nitrile group-containing acrylic polymer (B1) is preferably 5 to 35 mass%, more preferably 10 to 30 mass%, and particularly preferably 15 to 25 mass% to be. When the amount 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.

(Meth) acrylic acid ester monomer unit is a monomer derived from a compound represented by the general formula (1): CH ₂ ═CR ¹ -COOR ² (wherein R ¹ represents a hydrogen atom or a methyl group and R ² represents an alkyl group or a cycloalkyl group) Is a structural unit formed by polymerization.

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, Hexyl acrylate, 2-ethylhexyl acrylate, lauryl acrylate, and stearyl acrylate; Propyl methacrylate, n-butyl methacrylate, t-butyl methacrylate, n-amyl methacrylate, isoamethacrylate, naphthyl methacrylate, n-hexyl methacrylate, methacrylic acid 2-ethylhexyl methacrylate, lauryl methacrylate, and stearyl methacrylate. Of these, acrylate is preferable, and n-butyl acrylate and 2-ethylhexyl acrylate are particularly preferable because they can improve the strength of the electrode.

(Meth) acrylic acid ester monomers may be used singly or two or more kinds thereof may be combined at an arbitrary ratio. Therefore, the nitrile group-containing acrylic polymer (B1) may contain only one kind of (meth) acrylic acid ester monomer, or may contain two or more kinds in combination at an arbitrary ratio.

The content of the (meth) acrylic acid ester monomer unit in the nitrile group-containing acrylic polymer (B1) is preferably 35 to 85% by mass, more preferably 45 to 75% by mass, and particularly preferably 50 to 70% by mass. When the nitrile group-containing acrylic polymer (B1) having a content of the (meth) acrylic acid ester monomer unit in the above range is used, the flexibility of the electrode active material is high and the swelling property of the electrolytic solution is suppressed. In addition, the heat resistance is high, and the internal resistance of the resulting electrode for an electrochemical device can be reduced.

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

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

Concrete examples of the ethylenically unsaturated carboxylic acid monomer include an ethylenically unsaturated monocarboxylic acid and a derivative thereof, an ethylenically unsaturated dicarboxylic acid and an acid anhydride thereof and derivatives thereof.

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

Examples of the derivative of the ethylenically unsaturated monocarboxylic acid include 2-ethyl acrylic acid, isocrotonic acid,? -Acetoxy acrylic acid,? -Trans-aryloxy acrylic acid,? -Chloro-?-E- methoxy acrylic acid, And diaminoacrylic acid.

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

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

Examples of derivatives of ethylenically unsaturated dicarboxylic acids 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 maleic acid diphenyl, maleic anonil, maleic decyl, maleic acid dodecyl, maleic acid octadecyl, and maleic acid fluoroalkyl.

Specific examples of the ethylenically unsaturated sulfonic acid monomer include vinylsulfonic acid, methylvinylsulfonic acid, styrenesulfonic acid, (meth) acrylsulfonic acid, (meth) acrylic acid-2-sulfonic acid, 2- Amide-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-hydroxypropane phosphate.

The alkali metal salt or ammonium salt of the ethylenic unsaturated acid monomer may also be used.

The ethylenic unsaturated acid monomer may be used singly or in combination of two or more. Therefore, the nitrile group-containing acrylic polymer (B1) may contain only one kind of the ethylenic unsaturated acid monomer, or two or more kinds of the ethylenic unsaturated acid monomer may be combined in an arbitrary ratio.

Among these, from the viewpoint of improving the dispersibility of the nitrile group-containing acrylic polymer (B1), as the ethylenic unsaturated acid monomer, an ethylenic unsaturated carboxylic acid monomer or an ethylenic unsaturated sulfonic acid monomer may be used singly or an ethylenic unsaturated carboxylic acid The combination of the unsaturated carboxylic acid monomer and the ethylenically unsaturated sulfonic acid monomer is preferable, and the combination of the ethylenic unsaturated carboxylic acid monomer and the ethylenically unsaturated sulfonic acid monomer is more preferable.

Among the ethylenically unsaturated carboxylic acid monomers, ethylenically unsaturated monocarboxylic acid is preferable, and acrylic acid or methacrylic acid is more preferable in view of expressing good dispersibility in the nitrile group-containing acrylic polymer (B1) It is preferably methacrylic acid.

Among the ethylenically unsaturated sulfonic acid monomers, 2-acrylamide-2-hydroxypropane sulfonic acid, 2-acrylamide-2-methylpropane (BEA) Sulfonic acid, and more preferably 2-acrylamide-2-methylpropanesulfonic acid.

The content of the ethylenically unsaturated acid monomer unit in the nitrile group-containing acrylic polymer (B1) is preferably 10 to 30% by mass, more preferably 12 to 28% by mass, and particularly preferably 14 to 26% by mass Range. When the ethylenically unsaturated carboxylic acid monomer and the ethylenically unsaturated sulfonic acid monomer are used in combination as the ethylenic unsaturated acid monomer, the content of the ethylenically unsaturated carboxylic acid monomer in the nitrile group-containing acrylic polymer (B1) is preferably 10 To 30% by mass, more preferably 12 to 28% by mass, and the content of the ethylenically unsaturated sulfonic acid monomer is preferably 0.1 to 10% by mass.

By setting the content ratio of the ethylenically unsaturated acid monomer unit within the above range, the dispersibility of the nitrile group-containing acrylic polymer (B1) upon slurrying can be high and a highly uniform positive electrode active material layer can be formed, The resistance can be reduced.

The nitrile group-containing acrylic polymer (B1) may contain a conjugated diene monomer unit in addition to the monomer unit having a nitrile group and the (meth) acrylic acid ester monomer unit. The conjugated diene monomer unit is a structural unit formed by polymerizing a structural unit and / or a conjugated diene monomer formed by polymerizing a conjugated diene monomer and hydrogenating it.

Specific examples of the conjugated diene monomer include 1,3-butadiene, isoprene, 2,3-dimethyl-1,3-butadiene, 1,3-pentadiene, chloroprene, And 1,3-butadiene is more preferable. The conjugated diene monomers may be used singly or in combination of two or more. Therefore, the nitrile group-containing acrylic polymer (B1) may contain only one type of conjugated diene monomer unit, or two or more kinds of the conjugated diene monomer units may be contained in combination in an arbitrary ratio.

The proportion of the conjugated diene monomer unit (content ratio containing hydrogenated monomer units) in the nitrile group-containing acrylic polymer (B1) is preferably 20 to 98% by mass, more preferably 20 to 80% by mass, And preferably 20 to 70% by mass.

The nitrile group-containing acrylic polymer (B1) may further contain, in addition to each monomer unit, a crosslinkable monomer unit within a range that does not affect the THF insoluble content of the nitrile group-containing acrylic polymer (B1). The crosslinkable monomer unit is a structural unit capable of forming a crosslinked structure during polymerization or after polymerization by heating or energy irradiation of the crosslinkable monomer. Examples of the cross-linkable monomer include monomers having thermal crosslinkability. More specifically, there can be mentioned a monofunctional monomer having a thermally crosslinkable crosslinkable group and one olefinic double bond per molecule, and a polyfunctional monomer having at least two olefinic double bonds per molecule.

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 a combination thereof. Of these, the epoxy group is more preferable because the crosslinking and the crosslinking density can be easily controlled.

Examples of the crosslinkable monomer having an epoxy group as the thermally crosslinkable crosslinkable group and having an olefinic double bond include vinyl glycidyl ether, allyl glycidyl ether, butenyl glycidyl ether, o-allylphenyl glycidyl ether Unsaturated glycidyl ether; Butadiene monoepoxide, chloroprene monoepoxide, 4,5-epoxy-2-pentene, 3,4-epoxy-1-vinylcyclohexene and 1,2-epoxy-5,9-cyclododecadien Monoepoxide of diene or polyene; Alkenyl epoxide such as 3,4-epoxy-1-butene, 1,2-epoxy-5-hexene and 1,2-epoxy-9-decene; And glycidyl methacrylate, glycidyl methacrylate, glycidyl crotonate, glycidyl-4-heptenoate, glycidyl sorbate, glycidyl linoleate, glycidyl-4-methyl Glycidyl esters of unsaturated carboxylic acids such as glycidyl esters of 3-cyclohexenecarboxylic acid and glycidyl esters of 4-methyl-3-cyclohexenecarboxylic acid, .

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

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

Vinyl-2-oxazoline, 2-vinyl-4-methyl-2-oxazoline, 2-vinyl-2-oxazoline, and the like can be given as examples of the crosslinkable monomer having an oxazoline group as the thermally crosslinkable crosslinkable group and having an olefinic double bond. Methyl-2-oxazoline, 2-isopropenyl-2-oxazoline, 2-isopropenyl- And 2-isopropenyl-5-ethyl-2-oxazoline.

Examples of the polyfunctional monomer having two or more olefinic double bonds include allyl (meth) acrylate, ethylenedi (meth) acrylate, diethyleneglycol di (meth) acrylate, triethyleneglycol di , Tetraethylene glycol di (meth) acrylate, trimethylolpropane tri (meth) acrylate, dipropylene glycol diallyl ether, polyglycol diallyl ether, triethylene glycol divinyl ether, hydroquinone diaryl ether, Trimethylolpropane-diallyl ether, allyl or vinyl ether of polyfunctional alcohols other than the above, triallylamine, methylene bisacrylamide, and divinylbenzene.

As the crosslinkable monomer, allyl (meth) acrylate, ethylenedi (meth) acrylate, allyl glycidyl ether, and glycidyl methacrylate can be preferably used.

The crosslinking monomer may be used singly or in combination of two or more. Therefore, the nitrile group-containing acrylic polymer (B1) may contain only one type of crosslinkable unsaturated acid monomer, or two or more kinds thereof may be combined in an arbitrary ratio.

When the crosslinkable monomer unit is contained in the nitrile group-containing acrylic polymer (B1), the content thereof is preferably 0.1% by mass or more, more preferably 0.2% by mass or more, particularly preferably 0.5% by mass or more, Is not more than 5% by mass, more preferably not more than 4% by mass, particularly preferably not more than 2% by mass. By setting the content ratio of the crosslinkable monomer unit to the lower limit value or more of the above range, the weight average molecular weight of the nitrile group-containing acrylic polymer (B1) can be increased to prevent the degree of swelling from excessively increasing. On the other hand, when the ratio of the crosslinkable monomer units is not more than the upper limit of the above range, the dispersibility of the nitrile group-containing acrylic polymer (B1) can be improved. Therefore, by making the content ratio of the crosslinkable monomer unit fall within the above range, both swelling degree and dispersibility can be improved.

The nitrile group-containing acrylic polymer (B1) may further contain an aromatic vinyl monomer unit, an ethylenically unsaturated carboxylic acid amide monomer unit and the like in addition to the above.

Examples of the aromatic vinyl monomer include styrene,? -Methylstyrene, vinyltoluene, chlorostyrene, and hydroxymethylstyrene.

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

By containing these monomer units, the dispersibility of the nitrile group-containing acrylic polymer (B1) when slurried is high, and the active material layer having high uniformity can be formed, and the resistance of the positive electrode active material layer can be reduced. These monomer units may be contained in a proportion of 10 mass% or less.

Here, the content of the respective monomers is usually such that the monomer units (e.g., (meth) acrylic acid ester monomer unit, ethylenic unsaturated acid monomer unit, conjugated diene monomer unit, and crosslinkable monomer unit) in the nitrile group- Is equal to the content ratio.

Next, the degree of swelling of the nitrile group-containing acrylic polymer (B1) with respect to the non-aqueous electrolyte and the amount of the tetrahydrofuran (THF) insoluble portion of the nitrile group-containing acrylic polymer (B1) will be described.

The degree of swelling of the nitrile group-containing acrylic polymer (B1) with respect to the non-aqueous electrolyte is preferably from 1.0 times to 3 times, more preferably from 1.0 time to 2.8 times, more preferably from 1.0 times to 2.8 times to avoid a significant change in the volume of such polymer in the electrolyte Is 1.0 or more and 2.6 times or less. Here, the non-aqueous electrolyte is an electrolyte constituting the lithium ion secondary battery of the present invention. When the degree of swelling of the nitrile group-containing acrylic polymer (B1) relative to the non-aqueous electrolyte is in the above range, the adhesion of the positive electrode active material layer to the current collector is maintained even after the charge / discharge cycle is repeated, and the cycle characteristics are improved. The degree of swelling with respect to the non-aqueous electrolyte can be controlled, for example, by 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. Also, it decreases when the content of the ethylenically unsaturated monomer unit is increased.

The tetrahydrofuran (THF) insoluble content of the nitrile group-containing acrylic polymer (B1) is 30% by mass or less, preferably 25% by mass or less, more preferably 30% by mass or less in order to appropriately dissolve such a polymer in the slurry dispersion medium 20% by mass or less. The THF-insoluble matter is an indicator of the amount of gel. When the amount of the THF-insoluble matter is large, the possibility that the organic solvent such as N-methylpyrrolidone (hereinafter sometimes referred to as NMP) The dispersibility in the solution may be impaired. The THF insoluble content can be controlled by the polymerization reaction temperature, the addition time of the monomers, the amount of the polymerization initiator and the like as described later. Specifically, the amount of the insoluble fraction is reduced by a method such as a polymerization initiator for increasing the polymerization reaction temperature and a large amount of a chain transfer agent.

Since the electrolyte solution and the organic solvent as the slurry dispersion agent are close to each other in the solubility parameter (SP value), if the degree of swelling of the polymer used as the binder in the electrolyte falls within an appropriate range, the organic solvent, which is a slurry dispersion agent, The swelling degree of the polymer with respect to the electrolytic solution may be out of an appropriate range. On the other hand, in the present invention, these swelling degrees and THF insoluble contents All are within reasonable limits.

The production method of the nitrile group-containing acrylic polymer (B1) is not particularly limited, but the monomer mixture containing the monomer constituting the nitrile group-containing acrylic polymer (B1) is subjected to emulsion polymerization and, if necessary, hydrogenation Can be obtained. The emulsion polymerization method is not particularly limited, and conventionally known emulsion polymerization methods 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, shaking type, and rotary type. 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 may be used. The hydrogenation method is not particularly limited, and a known method may be employed.

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; butyl peroxide, t-butyl peroxide, cumene hydroperoxide, p-menthol hydroperoxide, di-t-butyl peroxide, t-butyl cumyl peroxide, acetyl peroxide, isobutyryl peroxide, octanoyl peroxide, , Organic peroxides such as 3,5,5-trimethylhexanoyl peroxide and t-butyl peroxyisobutyrate; Azo compounds such as azobisisobutyronitrile, azobis-2,4-dimethylvaleronitrile, azobiscyclohexanecarbonitrile, and methyl azobisisobutyrate.

Of these, inorganic peroxides can be preferably used. These polymerization initiators may be used alone or in combination of two or more. The peroxide initiator may be used as a redox polymerization initiator in combination with a reducing agent such as sodium bisulfite.

The amount of the polymerization initiator to be 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 in the polymerization. By using the polymerization initiator in the above range, the THF-insoluble content of the obtained nitrile group-containing acrylic polymer can be appropriately controlled.

In order to control the THF-insoluble content of the resulting copolymer, it is preferable to use a chain transfer agent in emulsion polymerization. Examples of the chain transfer agent include an alkylmercury such as n-hexylmercaptan, n-octylmercaptan, t-octylmercaptan, n-dodecylmercaptan, t-dodecylmercaptan and n-stearylmercaptan &Lt; / RTI &gt; Xanthene compounds such as dimethylxanthogen disulfide and diisopropylxanthogen disulfide; Thiuram-based compounds such as terpinolene, tetramethylthiuram disulfide, tetraethylthiuram disulfide and tetramethylthiuram monosulfide; 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-ethylhexyl thioglycolate, diphenylethylene and? -Methylstyrene dimer.

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

The amount of the chain transfer agent to be 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.

A surfactant may be used in emulsion polymerization. The surfactant may be an anionic surfactant, a nonionic surfactant, a cationic surfactant, or an amphoteric surfactant. 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, Sulfuric acid ester salts of higher alcohols such as phthalate, sodium octadecylsulfate; Alkyl benzene sulfonic acid salts such as sodium dodecylbenzenesulfonate, sodium laurylbenzenesulfonate and sodium hexadecylbenzenesulfonate; Aliphatic sulfonic acid salts such as sodium laurylsulfonate, sodium dodecylsulfonate and sodium tetradecylsulfonate, and the like.

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

In emulsion polymerization, pH adjusting agents such as sodium hydroxide and ammonia; Various additives such as a dispersant, a chelating agent, an oxygen scavenger, a builder, and a seed latex for controlling the particle diameter can be suitably used. In particular, emulsion polymerization using seed latex is preferred. The seed latex refers to a dispersion of fine particles which becomes the nucleus of the reaction during emulsion polymerization. Many fine particles have a particle diameter of 100 nm or less. The fine particles are not particularly limited, and a general-purpose polymer such as a diene polymer is used. According to the seed polymerization method, copolymer particles having relatively uniform particle diameters can be obtained.

The polymerization temperature at the time of carrying out the polymerization reaction is not particularly limited, but is usually 0 to 100 占 폚, preferably 40 to 80 占 폚. The polymerization reaction is terminated by emulsion polymerization at such a temperature range, adding a polymerization terminator at a predetermined polymerization conversion rate, cooling the polymerization system or the like. The polymerization conversion rate for stopping the polymerization reaction is preferably 93% by mass or more, and more preferably 95% by mass or more. By setting the polymerization temperature to the above range, the THF insoluble content of the resulting copolymer can be appropriately controlled.

The nitrile group-containing acrylic polymer (B1) is formed into a form (latex) in which the copolymer is dispersed in the dispersion medium by removing the unreacted monomers as desired and adjusting the pH or solid content concentration after stopping the polymerization reaction and the hydrogenation reaction, Is obtained. Thereafter, the dispersion medium may be replaced as necessary, or the dispersion medium may be evaporated to obtain a particulate copolymer in powder form.

To the dispersion of the nitrile group-containing acrylic polymer (B1), known dispersants, thickeners, antioxidants, antifoaming agents, antiseptics, antibacterial agents, blister preventives, pH adjusting agents and the like may be added as required.

(B2) Fluorine-containing polymer

In the positive electrode binder, the fluorine-containing polymer (B2) is used in addition to the nitrile group-containing acrylic polymer (B1). The stability of the slurry is improved by containing the fluorine-containing polymer as the binder for the positive electrode, and the swelling of the binder to the electrolyte is suppressed and the cycle characteristics are improved. In addition, since the positive electrode binder contains a nitrile group-containing acrylic polymer in addition to the fluorine-containing polymer, the cyclic characteristics at high electric potential are further improved.

The fluorine-containing polymer (B2) 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 specifically includes 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- And copolymers of other fluorine-containing monomers and monomers copolymerizable therewith.

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 units in the fluorine-containing polymer is usually 70% by mass or more, and preferably 80% by mass or more.

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,? -Methylstyrene, p-t-butylstyrene, vinyltoluene and chlorostyrene; Unsaturated nitrile compounds such as (meth) acrylonitrile (the abbreviation of acrylonitrile and methacrylonitrile, hereinafter the same); (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; Unsaturated compounds containing a sulfuric acid group such as 3-allyloxy-2-hydroxypropane sulfuric acid; Propyl (meth) acrylate-3-chloro-2-phosphate, and 3-allyloxy-2-hydroxypropanephosphoric acid.

The proportion of the monomer units copolymerizable with the fluorine-containing monomer in the fluorine-containing polymer (B2) is usually 30% by mass or less, preferably 20% by mass or less.

Of the fluorine-containing polymer (B2), a polymer containing vinylidene fluoride as the fluorine-containing monomer, specifically, a homopolymer of vinylidene fluoride, a copolymer of vinylidene fluoride and another fluorine-containing monomer copolymerizable therewith, vinylidene fluoride Copolymers of other fluorine-containing monomers copolymerizable therewith and monomers copolymerizable therewith are preferred.

Among such 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 (B2) may be used singly or in combination of two or more. Particularly, it is preferable to use a combination of a low molecular weight material and a high molecular weight material. Concretely, the fluorine-containing polymer having a melt viscosity of less than 35 kpoise measured at ASTM D 3835/232 캜 of 100 sec ^-1 has a low molecular weight and a molecular weight of not less than 35 kpoise is preferably a high molecular weight.

Examples of the high molecular weight polyvinylidene fluoride include KYNAR HSV900 manufactured by Arcemasia, Solef6020 manufactured by Solvay, Solef6010, Solef1015, Solef 5130, and KF7208 manufactured by Kureha Corporation. Examples of the low molecular weight polyvinylidene fluoride include KYNAR 710 720 740 760 760A manufactured by Arcemasia, Solef 6008 manufactured by Solvay, and KF1120 manufactured by Kureha Corporation.

When the high molecular weight compound and the low molecular weight compound are used in combination as the fluorine-containing polymer (B2), the weight ratio (low molecular weight / high molecular weight) of the low molecular weight compound and the high molecular weight compound of the fluorine-containing polymer is preferably 30/70 to 70/30.

When the low-molecular weight compound and the high-molecular weight compound are used in combination in such a range, the binding properties of the positive electrode active materials, the binding property of the current collector and the active material, and the uniformity of the slurry can be maintained more effectively.

The weight average molecular weight of the fluorine-containing polymer (B2) in terms of polystyrene determined by gel permeation chromatography 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 (B2) within the above-mentioned range, desorption (falling off of the powder) of the positive electrode active material and the conductive material in the positive electrode active material layer is suppressed and viscosity adjustment of the positive electrode slurry becomes easy.

The glass transition temperature (Tg) of the fluorine-containing polymer (B2) is preferably 0 占 폚 or lower, more preferably -20 占 폚 or lower, particularly preferably -30 占 폚 or lower. The lower limit of the Tg of the fluorine-containing polymer (B2) is not particularly limited, but is preferably -50 ° C or higher, and more preferably -40 ° C or higher. When the Tg of the fluorine-containing polymer (B2) is in the above range, desorption (dropping of the powder) of the positive electrode active material and the conductive material in the positive electrode active material layer can be suppressed. The Tg of the fluorine-containing polymer (B2) can be adjusted by combining various monomers. Further, Tg was measured using a differential scanning calorimeter according to JIS K 7121; 1987. &lt; / RTI &gt;

The melting point (Tm) of the fluorine-containing polymer (B2) is preferably 190 占 폚 or lower, more preferably 150 to 180 占 폚, and still more preferably 160 to 170 占 폚. When the Tm of the fluorine-containing polymer (B2) is in the above range, an electrode excellent in flexibility and adhesion strength can be obtained. The Tm of the fluorine-containing polymer (B2) can be adjusted by combining various monomers or by controlling the polymerization temperature. Further, Tm is measured using a differential scanning calorimeter according to JIS K 7121; 1987. &lt; / RTI &gt;

The method for producing the fluorine-containing polymer (B2) is not particularly limited, and any method such as solution polymerization, suspension polymerization, bulk polymerization or emulsion polymerization can be used. Of these, the suspension polymerization method and the emulsion polymerization method are preferable, and the emulsion polymerization method is more preferable. The productivity of the fluorine-containing polymer (B2) can be improved by producing the fluorine-containing polymer (B2) by the emulsion polymerization method, and the fluorine-containing polymer (B2) having the 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. Examples of the polymerization initiator to be used in the polymerization include, for example, lauroyl peroxide, diisopropyl peroxydicarbonate, di-2-ethylhexyl peroxydicarbonate, t-butyl peroxypivalate, 3,3,5-trimethylhexa Organic peroxides such as nonyl peroxide, and azo compounds such as?,? '- azobisisobutyronitrile, and ammonium persulfate and potassium persulfate.

The fluorine-containing polymer (B2) is used in the form of a dispersion or a 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 (B2), 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, methylcyclohexanone and ethylcyclohexanone; Chlorinated aliphatic hydrocarbons such as methylene chloride, chloroform, and carbon tetrachloride; Esters such as ethyl acetate, butyl acetate,? -Butyrolactone and? -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; And amides such as N-methylpyrrolidone and N, N-dimethylformamide.

These dispersion media may be used alone, or two or more of them may be mixed and used as a mixed solvent. Particularly, in view of being industrially used at the time of preparing the electrode slurry, not being well volatilized in the production, and as a result of suppressing the volatilization of the electrode slurry and improving the smoothness of the resultant positive electrode, water or N-methylpyrrolidone , Cyclohexanone, and toluene are preferable.

In the case where the fluorine-containing polymer (B2) is dispersed in a particulate form in the dispersion medium, the solid content concentration of the dispersion containing the fluorine-containing polymer is usually 1 to 25 mass%, preferably 3 to 20 mass% More preferably 5 to 15% by mass.

When the fluorine-containing polymer (B2) is dissolved in N-methylpyrrolidone (hereinafter sometimes referred to as "NMP") so as to be an 8% solution, the viscosity is preferably 10 to 5000 mPa · s, More preferably 100 to 2000 mPa · s. By adjusting the viscosity of the 8% NMP solution of the fluorine-containing polymer (B2) within the above-mentioned range, it is easy to adjust the viscosity of the positive electrode slurry composition to a viscosity that is easy to apply during the production of the positive electrode slurry composition. The viscosity of the 8% NMP solution of the fluorine-containing polymer (B2) was determined by dissolving the fluorine-containing polymer (B2) in NMP so as to be an 8% solution and measuring the viscosity at 25 ° C and 60 rpm using a B-type viscometer ) Was used to prepare a resin composition according to JIS K 7117-1; 1999. &lt; / RTI &gt;

The positive electrode binder contains the nitrile group-containing acrylic polymer (B1) and the fluorine-containing polymer (B2). The positive electrode binder contains the nitrile group-containing acrylic polymer (B1) and the fluorine-containing polymer (B2), so that the positive electrode having excellent bending property of the winding body and the lithium secondary ion battery having excellent initial capacity, Is obtained. The proportion of the nitrile group-containing acrylic polymer (B1) relative to 100% by mass of the total amount of the positive electrode binder is preferably 5 to 50% by mass, more preferably 5 to 40% by mass, particularly preferably 5 to 30% % to be. By containing the nitrile group-containing acrylic polymer in such a ratio, it is possible to obtain a lithium ion secondary battery having a high capacity and excellent in high-voltage cycle characteristics without causing an increase in internal resistance or a decrease in initial capacity.

The proportion of the fluorine-containing polymer (B2) relative to 100% by mass of the total amount of the positive-electrode binder is 50 to 95% by mass, preferably 60 to 90% by mass, and more preferably 70 to 85% by mass.

The positive electrode binder may contain, in addition to the nitrile group-containing acrylic polymer (B1) and the fluorine-containing polymer (B2), other polymers usable as binders, if necessary. Other polymers that may be used in combination include, for example, resins such as polyacrylic acid derivatives and polyacrylonitrile derivatives, and soft polymers such as acrylate soft polymer, diene soft polymer, olefin soft polymer and vinyl soft polymer. These may be used alone, or two or more of them may be used in combination. The other polymer may be contained in an amount of 30 mass% or less, more preferably 0.1 to 20 mass%, particularly 0.2 to 10 mass%, based on 100 mass% of the total amount of the positive electrode binder.

The amount of the positive electrode binder is preferably 0.5 to 2 parts by mass, more preferably 1 to 2 parts by mass, and particularly preferably 1.5 to 2 parts by mass based on 100 parts by mass of the positive electrode active material. When the amount of the positive electrode binder is in this range, the adhesion between the positive electrode active material layer and the current collector can be sufficiently secured, and the capacity of the lithium secondary battery can be increased and the internal resistance can be lowered.

(C) Conductive material

The positive electrode contains a conductive material. The particle diameter of the conductive material contained in the positive electrode is 5 to 40 nm, preferably 10 to 38 nm, more preferably 15 to 36 nm in terms of number average particle diameter. If the particle size of the conductive material in the positive electrode is too small, it tends to be aggregated and becomes difficult to disperse uniformly. As a result, the internal resistance of the positive electrode active material layer tends to increase, making it difficult to improve the capacity. However, by using the binding agent for positive electrode described above, the atomized conductive material can be uniformly dispersed, and the capacity can be improved. When the particle diameter of the conductive material is too large, it is difficult to exist between the positive electrode active materials, and the internal resistance of the positive electrode active material layer increases, making it difficult to improve the capacity. The number average particle diameter of the conductive material is measured by a dynamic light scattering type particle diameter / particle size distribution measuring apparatus (for example, Nanotrac Wave-size measuring apparatus, manufactured by Nikkiso Co., Ltd.) after ultrasonic dispersion of the conductive material at 0.01 mass% EX150). &Lt; / RTI &gt;

The specific surface area (BET formula) of the conductive material in the positive electrode is preferably 400 m 2 / g or less, more preferably 300 m 2 / g or less, and particularly preferably 200 m 2 / g or less. If the specific surface area of the conductive material is too large, it tends to aggregate and becomes difficult to disperse uniformly. As a result, the internal resistance of the positive electrode active material layer increases and it becomes difficult to improve the capacity. As the conductive material, one type of conductive material having the above-described specific surface area may be used alone, or two or more kinds of conductive materials having different specific surface areas may be used as the conductive material after the BET specific surface area of the conductive material after the mixing is in the above- May be used in combination.

As the conductive material, conductive carbon such as acetylene black, ketjen black, carbon black, graphite, vapor grown carbon fiber, and carbon nanotube can be used in the same manner as the negative electrode. By containing the conductive material, the stability in the production of the positive electrode slurry can be improved, and the electrical contact between the positive electrode active materials in the positive electrode active material layer can be improved, thereby increasing the capacity. The content of the conductive material is preferably 1 to 3 parts by mass, more preferably 1.2 to 2.8 parts by mass, particularly preferably 1.5 to 2.5 parts by mass based on 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 at the positive electrode may increase, making it difficult to increase the capacity. If the content of the conductive material is too large, it is difficult to increase the density of the electrode, and the initial capacity may be lowered.

Other components of the positive electrode

The positive electrode may further contain, as optional components, an electrolyte additive having functions such as a reinforcing material, a leveling agent, an electrolyte decomposition inhibiting function, and the like in the same manner as the above negative electrode. In addition, May remain.

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

The lithium ion secondary battery positive electrode slurry composition is obtained by mixing the above-mentioned positive electrode active material (A), positive electrode binder (B), conductive material (C) and other additives in a dispersion medium. As the dispersion medium, any of water and 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, methylcyclohexanone and ethylcyclohexanone; Chlorinated aliphatic hydrocarbons such as methylene chloride, chloroform, and carbon tetrachloride; Esters such as ethyl acetate, butyl acetate,? -Butyrolactone and? -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; And amides such as N-methylpyrrolidone and N, N-dimethylformamide.

These dispersion media may be used alone, or two or more of them may be mixed and used as a mixed solvent. Among these, a dispersant having a high non-conductive particle dispersibility and a low boiling point and a high volatility is preferable because it can be removed at a low temperature in a short time. Specifically, acetone, toluene, cyclohexanone, cyclopentane, tetrahydrofuran, cyclohexane, xylene, water, or N-methylpyrrolidone or a mixed solvent thereof is preferable.

Although the mixing method is not particularly limited, for example, there can be mentioned a method using a mixing device such as a stirring type, shaking type, and rotary type. In addition, a method using a dispersion kneader such as a homogenizer, a ball mill, a sand mill, a roll mill, and a planetary kneader may be used.

Lithium ion secondary battery positive

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

The manufacturing method of the positive electrode of the lithium ion secondary battery includes a step of forming the positive electrode active material layer by applying the positive electrode slurry composition to one surface or both surfaces of the current collector and drying the same.

The method of applying the positive electrode slurry composition onto the current collector is not particularly limited. For example, methods such as a doctor blade method, a dipping method, a reverse roll method, a direct roll method, a gravure method, an extrusion method, and a brush coating method can be used.

Examples of the drying method include a drying method by hot air, hot air, low-humidity air, vacuum drying, and irradiation with (circle) infrared rays or electron beams. The drying time is usually 5 to 30 minutes, and the drying temperature is usually 40 to 180 ° C.

It is preferable to have a step of coating the positive electrode slurry composition on the current collector and then drying and then lowering the porosity of the positive electrode active material layer by pressure treatment using a die press or a roll press when manufacturing the positive electrode of the lithium ion secondary battery Do. 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, the charging efficiency and the discharge efficiency may deteriorate. On the other hand, if the porosity of the positive electrode active material layer is too low, a high volume capacity can not be obtained well, and the positive electrode active material layer tends to peel off from the current collector, so that defects tend to occur easily. When a curable polymer is used as the positive electrode binder, it is preferable to cure the positive electrode.

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

The content of the positive electrode active material in the positive electrode active material layer is preferably 85 to 99% by mass, and 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 in the above range, it is possible to obtain a secondary battery exhibiting flexibility and binding property while exhibiting high capacity.

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 in the above range, a high capacity secondary battery can be obtained.

The current collector is not particularly limited as long as it is an electrically conductive and electrochemically durable material. However, a metallic material is preferable because it has heat resistance. For example, iron, copper, aluminum, nickel, stainless steel, titanium, tantalum, Gold, platinum, and the like. Among them, aluminum is particularly preferable as the current collector used for the positive electrode of the lithium ion secondary battery. The shape of the current collector is not particularly limited, but a sheet-like sheet having a thickness of about 0.001 to 0.5 mm is preferable. The collector may be subjected to surface roughening in advance in order to increase the bonding strength with the positive electrode active material layer. Examples of roughening methods include mechanical polishing, electrolytic polishing, and chemical polishing. In the mechanical polishing, a wire brush having polishing abrasive particles fixed to abrasive particles, a grinding stone, an emery buff, a steel wire, or the like is used. 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.

(Lithium ion secondary battery)

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

Non-aqueous electrolyte

The non-aqueous electrolyte is not particularly limited, and a non-aqueous solvent in which a lithium salt is dissolved as a supporting electrolyte can be used. The lithium salt is, 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 NLi, (CF ₃ SO ₂₎ can be cited lithium salts such as _{2 NLi, (C 2 F 5} SO 2) NLi. _{LiPF 6, LiClO 4, CF 3} SO 3 Li , particularly exhibiting a high degree of dissociation easily soluble in the solvent is preferably used. These may be used alone or in combination of two or more. The amount of the supporting electrolyte is usually not less than 1% by mass, preferably not less than 5% by mass, and usually not more than 30% by mass, preferably not more than 20% by mass with respect to the electrolytic solution. If the amount of the supporting electrolyte is excessively large or excessively large, the ionic conductivity decreases and the charging and discharging characteristics of the battery deteriorate.

The solvent used for the electrolytic solution is not particularly limited as long as it dissolves the supporting electrolyte. The solvent is usually selected from the group consisting of dimethyl carbonate (DMC), ethylene carbonate (EC), diethyl carbonate (DEC), propylene carbonate (PC), butylene carbonate ), And methyl ethyl carbonate (MEC); ethers such as tetrahydrofuran and 1,2-dimethoxyethane; Sulfolane, and dimethyl sulfoxide are used. Particularly, dimethyl carbonate, ethylene carbonate, propylene carbonate, diethyl carbonate, and methyl ethyl carbonate are preferable because it is easy to obtain high ionic conductivity and has a wide use temperature range. These may be used alone or in combination of two or more.

Examples of the electrolytic solution other than the above include gelated polymer electrolytes in which a polymer electrolyte such as polyethylene oxide or polyacrylonitrile is impregnated with an electrolytic solution, and inorganic solid electrolytes such as lithium sulfide, LiI, Li ₃ N and the like.

It is also possible to add an additive to the electrolytic solution. As additives, in addition to carbonate compounds such as vinylene carbonate (VC), fluorine carbonates such as fluoroethylene carbonate and ethyl methyl sulfone are preferable. Among them, fluorine-based electrolytic solution additives such as fluorine carbonates have a high withstanding voltage. The electrolytic solution composed of ethylene carbonate or propylene carbonate can not withstand a high voltage and may be decomposed. Therefore, it is preferable to add the fluorine-based electrolytic solution additive to the electrolytic solution.

Separator

The separator is a porous substrate having pores. Examples of usable separators include (a) a porous separator having air holes, (b) a porous separator having a polymer coating layer formed on one surface or both surfaces thereof, or (c) a porous A porous separator having a resin coating layer formed thereon. Nonlimiting examples of these include polypropylene-based, polyethylene-based, polyolefin-based or aramid-based 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 coating layer, or a separator coated with a porous film layer made of an inorganic filler or a dispersant for an inorganic filler.

Manufacturing Method of Lithium Ion Secondary Battery

The method for producing the lithium ion secondary battery of the present invention is not particularly limited. For example, the negative electrode and the positive electrode described above are superimposed with a separator interposed therebetween, and they are wound or folded in accordance with the shape of the battery, put into a battery container, and an electrolyte is injected into the battery container and pinched. If necessary, it is also possible to prevent the rise of the internal pressure of the battery and the overcharge discharge by inserting an over-current prevention element such as X-fund metal, fuse, PTC element, or a lead plate. The shape of the battery may be any of a laminate cell type, a coin type, a button type, a sheet type, a cylindrical type, a square type, a flat type, and a wound type pouch cell. Particularly, according to the present invention, since the electrode layer is flexible and cracks occur in the electrode layer at the time of bending, it can be preferably applied to the production of the wound pouch cell.

Example

Hereinafter, the present invention will be described by way of examples, but the present invention is not limited thereto. In the present embodiment, parts and% are based on mass unless otherwise specified. In Examples and Comparative Examples, various physical properties were evaluated as follows.

(Measurement of non-aqueous electrolytic solution swelling degree)

A 8% N-methylpyrrolidone (NMP) solution of the nitrile group-containing acrylic polymer was poured into a Teflon chalet so that the thickness after drying was 100 占 퐉 to prepare a polymer film. The obtained film was punched out with a diameter of 16 mm and its weight was measured (its weight was referred to as &quot; A &quot;). A mixture of ethylene carbonate and ethyl methyl carbonate at a weight ratio of 3 to 7 was mixed with 5% of fluoroethylene carbonate, and lithium hexafluorophosphate (LiPF ₆ ) was dissolved to a concentration of 1 mol / l to prepare a non-aqueous electrolyte. 20 g of the nonaqueous electrolytic solution was dipped in a 16 mmφ film and completely swelled at 60 ° C for 72 hours. Thereafter, the swollen film was taken out, the non-aqueous electrolyte on the surface was lightly wiped off, and its weight was measured (its weight was referred to as "B"). The non-aqueous electrolyte swelling degree (= B / A) was determined from these values. The larger the non-aqueous electrolyte swelling degree, the greater the deformation in the non-aqueous electrolyte.

(Measurement of THF Insoluble Content)

An 8% NMP solution of the nitrile group-containing acrylic polymer was poured into a Teflon chalet so as to have a thickness of 100 mu m after drying to prepare a polymer film. The obtained film was punched out with a diameter of 16 mm and its weight was measured (its weight was referred to as &quot; C &quot;). 20 g of tetrahydrofuran was immersed in a film having a thickness of 16 mmφ, and the soluble matter was completely dissolved at 25 ° C for 24 hours. Thereafter, the insoluble residual solid matter was taken out, the tetrahydrofuran was completely volatilized with an infrared drier, and the weight was measured (the weight was referred to as "D"). From these values, the THF insoluble content (= D / C x 100) was obtained. The smaller the THF insoluble content, the less crosslinking between polymer molecules.

(Bending property of winding)

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

A ... No crack

B ... Micro crack

C ... Peeling from electrode

(Initial capacity)

The obtained nonaqueous electrolyte battery was charged at a constant current of 140 mA until the battery voltage reached 4.2 V under the environment of 25 캜, and charged at 4.2 V until charging current reached 14 mA. Subsequently, constant current discharge was carried out until the battery voltage reached 3 V at 140 mA to obtain the initial capacity. The initial capacity at this time was evaluated according to the following evaluation criteria.

A ... 700 mAh or more

B ... 697 ㎃ or more and less than 700 ㎃

C ... 694 ㎃ or more and less than 697 ㎃

D ... Less than 690 ㎃ and less than 694 ㎃

E ... Less than 690 mAh

(Output characteristics)

The nonaqueous electrolyte battery in which the initial capacity was measured was charged at a constant current of 140 mA at 25 캜 until the battery voltage reached 4.2 V and charged at 4.2 V until charging current reached 14 mA. Subsequently, a constant current discharge was carried out until the battery voltage reached 3 V at 1400 mA to obtain a 2 C capacity. (2 C capacity) / (initial capacity) x 100 as output characteristics, the evaluation was carried out according to the following evaluation criteria.

A ... over 90

B ... 87% to less than 90%

C ... 84% or more and less than 87%

D ... 80% or more and less than 84%

E ... Less than 80%

(High potential cycle characteristics)

The nonaqueous electrolyte battery in which the output characteristics were evaluated was charged 100 times under a 25 캜 environment until the battery voltage reached 4.4 V and was discharged 100 times until the battery voltage reached 3 V at 600 mA . Then, the ratio of the 100th discharge capacity to the first discharge capacity was determined and evaluated according to the following criteria.

A ... More than 80%

B ... 77% or more and less than 80%

C ... 74% or more and less than 77%

D ... 70% or more and less than 74%

E ... Less than 70%

The positive electrode active material used for the positive electrode, the positive electrode active material used for the positive electrode, the binder for the positive electrode, and the conductive material are as follows. Herein, the particle diameter of the active material described below means the volume average particle diameter, and the particle diameter of the conductive material means the number average particle diameter.

(Negative electrode active material a)

Gr / SiOx: A mixture of 90 parts of spherical artificial graphite (particle diameter: 12 占 퐉) and 10 parts of alloy-based active material SiOx (particle diameter: 10 占 퐉)

Gr / SiOC: A mixture of 90 parts of spherical artificial graphite (particle diameter: 12 占 퐉) and 10 parts of an alloy active material SiOC (volume average particle diameter: 10 占 퐉)

Gr: spherical artificial graphite (particle diameter: 12 占 퐉)

(Positive electrode active material A)

LCO: lithium cobalt oxide (LiCoO ₂ ) (particle diameter: 12 탆)

LNM: Li [Ni _0.17 Li _0.2 Co _0.07 Mn _0.56 ] O ₂ (particle diameter: 15 탆)

(Positive electrode conductive material C)

AB23: Acetylene black (Denka black powder product, manufactured by Denki Kagaku Kogyo Co., Ltd., particle size: 23 nm, specific surface area: 133 m 2 / g)

AB35: acetylene black (trade name: DENKA BLACK POWDER, manufactured by Denki Kagaku Kogyo Co., Ltd., particle diameter 35 nm, specific surface area 68 m2 / g)

AB48: acetylene black (trade name: DENKA BLACK POWDER, manufactured by Denki Kagaku Kogyo Co., Ltd., particle size: 48 nm, specific surface area: 39 m 2 / g)

, 1.8 parts of AB23 + HiPCO: acetylene black (trade name: DENKA BLACK POWDER, manufactured by Denki Kagaku Kogyo K.K., particle diameter 23 nm, specific surface area 133 m 2 / g) and HiPCO (carbon nanotube manufactured by Unidym Corp., particle size: 26 nm, specific surface area 700 m 2 / g) (specific surface area after mixing: 190 m &lt; 2 &gt; / g)

(Binder for positive electrode B)

The nitrile group-containing acrylic polymers (B1-1) to (B1-11) were prepared as follows.

(Preparation example 1)

Preparation of nitrile group-containing acrylic polymer (B1-1)

Into an autoclave equipped with a stirrer, 164 parts of ion exchanged water, 59.5 parts of 2-ethylhexyl acrylate (2EHA), 20 parts of methacrylic acid (MAA), 20 parts of acrylonitrile (AN) 0.5 parts of methyl propanesulfonic acid (AMPS), 0.3 parts of potassium persulfate as a polymerization initiator, and 1.6 parts of sodium laurylsulfate as an emulsifier were placed. After sufficiently stirring, polymerization was carried out by heating at 70 DEG C for 3 hours and 80 DEG C for 2 hours, To obtain an aqueous dispersion of the group-containing acrylic polymer (B1-1). The polymerization conversion rate, which was determined from the solid content concentration, was 96%. Further, 500 parts of N-methylpyrrolidone was added to 100 parts of this aqueous dispersion, and water and remaining monomers were all evaporated under reduced pressure. Then, 81 parts of N-methylpyrrolidone was evaporated to obtain 8% by mass of NMP solution was obtained. The obtained polymer (B1-1) had a non-aqueous electrolyte swelling degree of 1.7 times and a THF insoluble content of 10% or less.

(Preparation Examples 2 to 11)

Preparation of nitrile group-containing acrylic polymers (B1-2) to (B1-11)

The same procedure as in Preparation Example 1 was carried out except that the injection amount and type of the monomers were changed as shown in Table 1. In Table 1, AN denotes acrylonitrile, 2EHA denotes 2-ethylhexyl acrylate, MAA denotes methacrylic acid, AA denotes acrylic acid, AMPS denotes 2-acrylamide-2-methylpropane sulfonic acid, St denotes styrene, AMA denotes Refers to allyl methacrylate. Table 1 shows the non-aqueous electrolyte swelling degree and the THF insoluble content of the obtained polymers (B1-1) to (B1-11).

(Preparation Example 12)

Preparation of nitrile group-containing acrylic polymer (B1-12)

240 parts of ion-exchanged water, 2.5 parts of sodium alkylbenzenesulfonate, 20 parts of acrylonitrile, and 35 parts of butyl acrylate (BA) were charged in this order to the autoclave equipped with a stirrer, the inside of the bottle was replaced with nitrogen, , And 45 parts of 3-butadiene (BD) were charged, and 0.25 part of ammonium persulfate was added to carry out a polymerization reaction at a reaction temperature of 40 ° C to obtain a polymer containing a nitrile group-containing monomer unit, an acrylic acid ester monomer unit and a conjugated diene monomer unit . The polymerization conversion rate was 85% and the iodine value was 280 mg / 100 mg.

A solution of 400 ml (total solid content: 48 g) in which the total solids concentration of the polymer was adjusted to 12 mass% by using water was added to a 1 liter autoclave equipped with a stirrer, and nitrogen gas was flowed for 10 minutes, After removal of oxygen, 75 mg of palladium acetate as a hydrogenation catalyst was dissolved in 180 ml of water to which 4 moles of nitric acid was added relative to Pd. The contents of the autoclave were heated to 50 占 폚 under pressure with hydrogen gas to 3 MPa after the inside of the system was substituted twice with hydrogen gas, and the hydrogenation reaction for 6 hours (referred to as "hydrogenation at the first step" ). At this time, the iodine value of the polymer was 35 mg / 100 mg.

Subsequently, the autoclave was returned to atmospheric pressure, and 25 mg of palladium acetate as a hydrogenation catalyst was dissolved in 60 ml of water to which 4-fold molar amount of nitric acid relative to Pd was added. The contents of the autoclave were heated to 50 占 폚 in a state where the inside of the autoclave was displaced twice with hydrogen gas and pressurized with hydrogen gas to 3 MPa, and the hydrogenation reaction for 6 hours (referred to as "hydrogenation in the second step" ).

Thereafter, the contents were returned to room temperature, the inside of the system was brought to a nitrogen atmosphere, and the mixture was concentrated using an effervescent solution until the solid concentration reached 40%, thereby obtaining an aqueous dispersion of the nitrile group-containing acrylic polymer (B1-12) . Further, 320 parts of N-methylpyrrolidone was added to 100 parts of this aqueous dispersion, and water and residual monomers were all evaporated under reduced pressure. N-methylpyrrolidone was then added to obtain an aqueous 8% by mass solution of the polymer (B1-12) NMP solution was obtained. The obtained polymer (B1-12) had a non-aqueous electrolyte swelling degree of 2.9 times and a THF insoluble content of 10% or less. The iodine value of the nitrile group-containing acrylic polymer (B1-12) was 10 mg / 100 mg.

As the fluorine-containing polymer (B2), the following were used.

Mixed PVdF: Polyvinylidene fluoride (1: 1 (weight ratio) mixture of KYNAR HSV 900 manufactured by Arcemasia and KYNAR 720 manufactured by Kabushiki Kaisha)

In addition, the melt viscosity measured by ASTM D3835 / 232 ° C of 100 sec ^-1 of KYNAR HSV 900 is 50 kpoise, and the melt viscosity of KYNAR 720 is 9 kpoise.

High molecular weight PVdF: KYNAR HSV900

Low molecular weight PVdF: KYNAR720

(Example 1)

[Preparation of slurry composition for positive electrode and positive electrode]

100 parts of lithium cobaltate (LiCoO ₂ ) (particle diameter: 12 μm) as a positive electrode active material, and 100 parts of acetylene black (AB35, Denka Black Powder Co., product of Denka Kagaku Kogyo; particle diameter 35 nm, specific surface area 68 m 2 / , 1.6 parts of a mixed polyvinylidene fluoride (a 1: 1 mixture of KYNAR HSV900 and KYNAR720 manufactured by Arkema) as a fluorine-containing polymer for a positive electrode binder, and 1.6 parts of a polymer B1-1 as a nitrile group- , And an appropriate amount of NMP were stirred with a planetary mixer to prepare a positive electrode slurry composition.

An aluminum foil having a thickness of 15 mu m was prepared as a current collector. The slurry composition for positive electrode was applied on both sides of an aluminum foil so that the coating amount after drying was 25 mg / cm 2, dried at 60 ° C for 20 minutes, at 120 ° C for 20 minutes, and then at 150 ° C for 2 hours to obtain a positive electrode disk . The original plate of the positive electrode was rolled by a roll press to produce a sheet-like positive electrode made of a positive electrode active material layer having a density of 3.9 g / cm &lt; 3 &gt; and an aluminum foil. This was cut to a width of 4.8 mm and a length of 50 cm, and an aluminum lead was connected.

[Preparation of negative electrode slurry composition and negative electrode]

90 parts of spherical artificial graphite (particle diameter: 12 占 퐉), 10 parts of SiOx (particle diameter: 10 占 퐉) and 1 part of a styrene butadiene rubber (particle diameter: 180 nm, glass transition temperature: , 1 part of carboxymethylcellulose as a thickener and an appropriate amount of water were stirred with a planetary mixer to prepare a negative electrode slurry composition.

A copper foil having a thickness of 15 탆 was prepared as a current collector. The negative electrode slurry composition was coated on both surfaces of the copper foil so that the coating amount after drying was 10 mg / cm 2, dried at 60 ° C for 20 minutes, at 120 ° C for 20 minutes, and then heat treated at 150 ° C for 2 hours to obtain negative electrode disks. This original negative electrode was rolled by a roll press to produce a sheet-like negative electrode made of a negative active material layer having a density of 1.8 g / cm &lt; 3 &gt; and a copper foil. This was cut into 5.0 mm in width and 52 cm in length, and nickel lead was connected.

The obtained sheet-like positive electrode and sheet-negative electrode were wound using a core of 20 mm in diameter via a separator to obtain a rolled body. As the separator, a microporous membrane made of polypropylene having a thickness of 20 mu m was used. The winding was compressed in one direction until it reached a thickness of 4.5 mm at a speed of 10 mm / sec. The ratio of the long 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 ethylene carbonate and ethyl methyl carbonate at a ratio of 3 to 7 (weight ratio), and lithium hexafluorophosphate (LiPF ₆ ) was dissolved so as to have a concentration of 1 mol / 2% by volume of vinylene carbonate was added to prepare a non-aqueous electrolyte.

The electrode plate group was housed in a predetermined aluminum laminate case together with 3.2 g of a non-aqueous electrolyte. Then, after the negative electrode lead and the positive electrode lead were connected to a predetermined point, the opening of the case was sealed with heat to complete the nonaqueous electrolyte battery. This battery is a pouch type having a width of 35 mm, a height of 48 mm and a thickness of 5 mm, and a nominal capacity of the battery is 700 mAh. Table 2 shows the initial capacity, output characteristics, and high-potential cycle characteristics of the obtained battery.

(Example 2)

Except that the negative electrode active material was changed to Gr / SiOC (a mixture of 90 parts of spherical artificial graphite (particle diameter: 12 μm) and 10 parts of an alloy active material SiOC (volume average particle diameter: 10 μm)) . The results are shown in Table 2.

(Example 3)

The procedure of Example 1 was repeated except that the positive electrode conductive material was changed to acetylene black (AB23, acetylene black (trade name: Denka Black powder, manufactured by Denki Kagaku Kogyo Co., Ltd., particle size: 23 nm, specific surface area: 133 m 2 / g) 2.

(Example 4)

Except that the positive electrode active material was changed to LNM (Li [Ni _0.17 Li _0.2 Co _0.07 Mn _0.56 ] O ₂ (particle diameter: 15 μm)). The results are shown in Table 2.

(Examples 5 to 12)

Except that the nitrile group-containing acrylic polymers (B1-2) to (B1-9) were used in place of the nitrile group-containing acrylic polymer (B1-1) of the positive electrode binder and the amount thereof was changed as shown in Table 2 , The same procedure as in Example 1 was carried out. The results are shown in Table 2.

(Example 13)

The procedure of Example 1 was repeated except that the high molecular weight polyvinylidene fluoride was used instead of the mixed polyvinylidene fluoride of the positive electrode binder. The results are shown in Table 2.

(Example 14)

The procedure of Example 1 was repeated except that low molecular weight polyvinylidene fluoride was used in place of the mixed polyvinylidene fluoride of the positive electrode binder. The results are shown in Table 2.

(Example 15)

1.8 parts of acetylene black (DENKA BLACK POWDER, manufactured by Denki Kagaku Kogyo, particle size: 23 nm, specific surface area: 133 m 2 / g), and 1.8 parts of HiPCO (carbon nanotubes manufactured by Unidym Corp., particle size: 26 nm, specific surface area: 700 M &lt; 2 &gt; / g). The results are shown in Table 2.

(Example 16)

The same procedure as in Example 1 was carried out except that the nitrile group-containing acrylic polymer (B1-12) was replaced with the nitrile group-containing acrylic polymer (B1-12). The results are shown in Table 2.

(Example 17)

[Preparation of slurry composition for negative electrode]

Aqueous solution of polyacrylic acid lithium salt (manufactured by Aldrich Co., Ltd., viscosity average molecular weight: 12.5 million) and an appropriate amount of water were added to the disperser so as to be a 10% aqueous solution and then dissolved by adding lithium hydroxide thereto. &Lt; / RTI &gt;

90 parts of spherical artificial graphite (particle diameter: 12 占 퐉), 10 parts of SiOx (particle diameter: 10 占 퐉) as an anode active material, and an amount of water equivalent to 1 part of the aqueous solution of the lithium salt of polyacrylic acid as a solid component, To prepare a negative electrode slurry composition.

The procedure of Example 1 was repeated except that the slurry composition for negative electrode was changed to the slurry composition for negative electrode. The results are shown in Table 2.

(Comparative Example 1)

The same procedure as in Example 1 was carried out except that the negative electrode active material was changed to only spherical artificial graphite (particle diameter: 12 μm). The results are shown in Table 2.

(Comparative Example 2)

The same procedure as in Example 1 was carried out except that the positive electrode conductive material was changed to acetylene black (AB48, acetylene black (trade name: Denka Black Powder manufactured by Denki Kagaku Kogyo Co., Ltd., particle size: 48 nm, specific surface area: 39 m 2 / g) 2.

(Comparative Example 3)

The same procedure as in Example 1 was carried out except that the fluorine-containing polymer of the positive-electrode binder was not used and 2 parts of the nitrile group-containing acrylic polymer (B1-1) was used. The results are shown in Table 2.

(Comparative Example 4)

The same procedure as in Example 1 was carried out except that the nitrile group-containing acrylic polymer of the positive electrode binder was not used and 2 parts of mixed polyvinylidene fluoride was used. The results are shown in Table 2.

(Comparative Examples 5 and 6)

Except that the nitrile group-containing acrylic polymer (B1-10) or (B1-11) was used in place of the nitrile group-containing acrylic polymer (B1-1) of the positive electrode binder. The results are shown in Table 2.

(Comparative Example 7)

Except that a modified acrylic rubber polymer (trade name: BM500B, swelling degree: 2.7 times, insoluble content: 80%) was used in place of the nitrile group-containing acrylic polymer (B1-1) 1. The results are shown in Table 2.

From Tables 1 and 2, good results with excellent balance were obtained with respect to all the evaluation items that satisfied the requirements of the present invention. On the other hand, in Comparative Example 1 in which the alloy active material was not used for the negative electrode active material, the initial capacity was markedly inferior. In Comparative Example 2 in which the number average particle diameter of the positive electrode conductive material exceeded the specific range and the large conductive material was used, The results were inferior to the evaluation items, and especially the initial capacity and output characteristics were markedly inferior. In Comparative Examples 3 and 4 in which the nitrile group-containing acrylic polymer and the fluorine-containing polymer were not used in combination, the results were inferior in almost all the evaluation items, and the initial capacity and the output characteristics were particularly inferior. In Comparative Examples 5 and 6 in which the degree of swelling and the THF insoluble amount did not satisfy the requirements of the present invention, the results were inferior in almost all of the evaluation items, and in particular, the high-potential cycle characteristics were inferior. In Comparative Example 7 in which modified acryl rubber microparticles were used in place of the nitrile group-containing acrylic polymer, the high-potential cycle characteristics were very inferior.

Claims (7)

A lithium ion secondary battery comprising a negative electrode, a positive electrode, and a nonaqueous electrolyte,
Wherein the negative electrode contains an alloy-based active material,
Wherein the positive electrode contains a positive electrode active material, a positive electrode binder and a conductive material,
Wherein the binder for a positive electrode contains a nitrile group-containing acrylic polymer and a fluorine-containing polymer,
The swelling degree of the nitrile group-containing acrylic polymer with respect to the non-aqueous electrolyte is 3 times or less, the THF insoluble content is 30 mass% or less,
Wherein the conductive material has a particle diameter of 5 to 40 nm. The method according to claim 1,
Wherein 1 to 3 parts by mass of the conductive material and 0.5 to 2 parts by mass of the binder for the positive electrode are contained in 100 parts by mass of the positive electrode active material. 3. The method according to claim 1 or 2,
Wherein the content of the nitrile group-containing acrylic polymer in the positive electrode binder is 50 to 5 mass% and the content of the fluorine-containing polymer is 50 to 95 mass%. 4. The method according to any one of claims 1 to 3,
Wherein the fluorine-containing polymer is polyvinylidene fluoride. 5. The method according to any one of claims 1 to 4,
Wherein the nitrile group-containing acrylic polymer contains an ethylenic unsaturated acid monomer unit. 6. The method of claim 5,
And the content of the ethylenically unsaturated acid monomer unit in the nitrile group-containing acrylic polymer is 10 to 30 mass%. 7. The method according to any one of claims 1 to 6,
A lithium ion secondary battery which is a wound pouch cell.