JP6149730B2 - Positive electrode for secondary battery, method for producing the same, slurry composition, and secondary battery - Google Patents

Description

  The present invention relates to a positive electrode for a secondary battery and a method for producing the same, a slurry composition for producing the positive electrode for the secondary battery, and a secondary battery provided with the positive electrode for the secondary battery.

  In recent years, portable terminals such as notebook computers, mobile phones, and PDAs (Personal Digital Assistants) have been widely used. As a secondary battery used as a power source of these portable terminals, for example, a nickel hydrogen secondary battery, a lithium ion secondary battery, or the like is used. Mobile terminals are required to have more comfortable portability, and are rapidly becoming smaller, thinner, lighter, and higher performance. As a result, mobile terminals are used in various places. In addition, secondary batteries are also required to be smaller, thinner, lighter, and have higher performance as with mobile terminals.

  A secondary battery usually includes an electrode, an electrolytic solution, and other battery members. In addition, the electrode usually includes a current collector and an electrode active material layer formed on the current collector. Further, the electrode active material layer includes a binder (binder) and an electrode active material. Conventionally, in order to improve the performance of a secondary battery, each component included in the electrode active material layer has been studied (see Patent Documents 1 to 4).

JP 2002-56896 A Japanese Patent No. 3601250 JP 2010-177079 A JP 2011-192644 A

  The electrode active material layer provided on the positive electrode is called a positive electrode active material layer. The positive electrode active material layer is obtained by, for example, mixing a conductive additive such as conductive carbon and a positive electrode active material in a liquid composition in which a polymer serving as a binder is dispersed or dissolved in a solvent such as water or an organic solvent. A slurry composition is obtained, and this slurry composition is applied to a current collector and dried.

  Conventionally, an organic solvent has often been used as the solvent. However, the use of an organic solvent may require a cost for recycling the organic solvent or may require safety by using the organic solvent. Therefore, in recent years, production of a positive electrode using water as a solvent has been studied (see Patent Documents 3 and 4).

  However, the conventional positive electrode produced using water as a solvent has a tendency to be inferior in the coating property of the slurry composition. In addition, the adhesion of the positive electrode active material layer to the current collector tends to be low. Furthermore, the pouring property of the positive electrode active material layer was low, and there was a tendency that the electrolytic solution did not soak. For this reason, the conventional positive electrode was inferior in the storage characteristic in a high temperature environment.

  The present invention was devised in view of the above-mentioned problems, and is excellent in balance in coating properties of the slurry composition, adhesion of the positive electrode active material layer to the current collector, and liquid injection property of the positive electrode active material layer, Secondary battery positive electrode for obtaining a secondary battery having high storage characteristics in a high temperature environment, a method for producing the secondary battery positive electrode, a slurry composition for producing the secondary battery positive electrode, and the secondary battery It aims at providing the secondary battery provided with the positive electrode.

As a result of intensive studies to solve the above problems, the present inventor has a tendency that the dispersibility of the positive electrode active material or the conductive additive tends to be low in the conventional positive electrode for a secondary battery. It has been found that some or all of coatability, adhesion of the positive electrode active material layer to the current collector, and liquid injection property of the positive electrode active material layer are low. And this inventor is a water-soluble polymer which has an acidic functional group containing monomer unit and a (meth) acrylic acid ester monomer unit in a predetermined ratio, a positive electrode active material, a conductive support agent, particles By forming a positive electrode including a combination with a binder, the coating property of the slurry composition, the adhesion of the positive electrode active material layer to the current collector, and the liquid injection property of the positive electrode active material layer can be improved in a balanced manner, The present inventors have found that a secondary battery having high storage characteristics in a high temperature environment can be realized, and the present invention has been completed.
That is, the present invention is as follows.

[1] A positive electrode for a secondary battery comprising a positive electrode active material layer comprising a positive electrode active material, a conductive additive, a particulate binder and a water-soluble polymer,
The water-soluble polymer includes a copolymer A having 15 to 60% by weight of acidic functional group-containing monomer units and 30 to 80% by weight of (meth) acrylic acid ester monomer units. , Positive electrode for secondary battery.
[2] The positive electrode for a secondary battery according to [1], wherein the copolymer A has a crosslinkable monomer unit.
[3] The positive electrode for a secondary battery according to [2], wherein the content ratio of the crosslinkable monomer unit in the copolymer A is 0.1 wt% to 2 wt%.
[4] The positive electrode for a secondary battery according to any one of [1] to [3], wherein the copolymer A has a reactive surfactant unit.
[5] The positive electrode for a secondary battery according to [4], wherein a content ratio of the reactive surfactant unit in the copolymer A is 0.1 wt% to 15 wt%.
[6] The positive electrode for a secondary battery according to any one of [1] to [5], wherein the copolymer A has a fluorine-containing (meth) acrylic acid ester monomer unit.
[7] The positive electrode for a secondary battery according to [6], wherein a content ratio of the fluorine-containing (meth) acrylic acid ester monomer unit in the copolymer A is 1% by weight to 15% by weight.
[8] The particulate binder includes a copolymer B having a (meth) acrylonitrile monomer unit and a (meth) acrylic acid ester monomer unit, according to any one of [1] to [7]. The positive electrode for secondary batteries as described.
[9] The weight ratio of the (meth) acrylonitrile monomer unit to the (meth) acrylic acid ester monomer unit in the copolymer B is “(meth) acrylonitrile monomer unit / (meth) acrylic acid ester”. [8] The secondary battery positive electrode according to [8], wherein the monomer unit is 1/99 to 30/70.
[10] The weight ratio of the particulate binder to the water-soluble polymer is 99.5 / 0.5 to 95/5 as “particulate binder / water-soluble polymer”, [1] to [9 ] The positive electrode for secondary batteries as described in any one of.
[11] A slurry composition for producing a positive electrode active material layer constituting a positive electrode for a secondary battery,
Including a positive electrode active material, a conductive additive, a particulate binder, a water-soluble polymer and water,
The water-soluble polymer contains a copolymer A containing 15% to 60% by weight of acidic functional group-containing monomer units and 30% to 80% by weight of (meth) acrylate monomer units. , Slurry composition.
[12] A method for producing a positive electrode for a secondary battery comprising a current collector and a positive electrode active material layer provided on the current collector,
[11] A method for producing a positive electrode for a secondary battery, comprising: applying the slurry composition according to [11] onto the current collector, and then drying the applied material to obtain the positive electrode active material layer.
[13] A positive electrode, a negative electrode, an electrolytic solution, and a separator are provided.
The secondary battery whose said positive electrode is a positive electrode for secondary batteries as described in any one of [1]-[10].

  According to the present invention, the secondary composition has excellent balance in coating properties of the slurry composition, adhesion of the positive electrode active material layer to the current collector, and liquid injection property of the positive electrode active material layer, and high storage characteristics in a high temperature environment. A positive electrode for a secondary battery from which a battery is obtained, a method for producing the positive electrode for the secondary battery, a slurry composition for producing the positive electrode for the secondary battery, and a secondary battery provided with the positive electrode for the secondary battery it can.

  Hereinafter, the present invention will be described in detail with reference to embodiments and examples. However, the present invention is not limited to the following embodiments and exemplifications, and may be arbitrarily modified and implemented without departing from the scope of the claims of the present invention and its equivalent scope.

  In this specification, “(meth) acryl” includes both “acryl” and “methacryl”. “(Meth) acrylate” includes both “acrylate” and “methacrylate”. Furthermore, “(meth) acrylonitrile” includes both “acrylonitrile” and “methacrylonitrile”. “(Meth) acryloyl” includes both “acryloyl” and “methacryloyl”. Further, “positive electrode active material” means an electrode active material for positive electrode, and “negative electrode active material” means an electrode active material for negative electrode. The “positive electrode active material layer” means an electrode active material layer provided on the positive electrode, and the “negative electrode active material layer” means an electrode active material layer provided on the negative electrode.

  Further, that a compound (including a polymer) is water-soluble means that an insoluble content is less than 0.5% by weight when 0.5 g of the compound is dissolved in 100 g of water at 25 ° C. . On the other hand, a compound being water-insoluble means that an insoluble content becomes 90% by weight or more when 0.5 g of the compound is dissolved in 100 g of water at 25 ° C.

[1. Positive electrode for secondary battery]
The positive electrode for a secondary battery of the present invention includes a positive electrode active material, a conductive additive, a particulate binder, and a water-soluble polymer. Usually, the positive electrode for secondary batteries of this invention is equipped with a collector and the positive electrode active material layer provided on the said collector. The positive electrode active material layer includes the positive electrode active material, a conductive additive, a particulate binder, and a water-soluble polymer.

[1.1. Cathode active material]
The positive electrode active material is an electrode active material used in the positive electrode, and is a material that transfers electrons in the positive electrode of the secondary battery. For example, when the secondary battery of the present invention is a lithium ion secondary battery, a material capable of inserting and extracting lithium ions is usually used as the positive electrode active material. Such positive electrode active materials are roughly classified into those made of inorganic compounds and those made of organic compounds.

  Examples of the positive electrode active material made of an inorganic compound include transition metal oxides, transition metal sulfides, lithium-containing composite metal oxides of lithium and transition metals, and the like. Examples of the transition metal include Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Mo.

Examples of the transition metal oxide include MnO, MnO ₂ , V ₂ O ₅ , V ₆ O ₁₃ , TiO ₂ , Cu ₂ V ₂ O ₃ , amorphous V ₂ O—P ₂ O ₅ , MoO _{3 and the} like. Among them, MnO, V ₂ O ₅ , V ₆ O ₁₃ , and TiO ₂ are preferable from the viewpoint of cycle stability and capacity of the secondary battery.

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

  Examples of the lithium-containing composite metal oxide include a lithium-containing composite metal oxide having a layered structure, a lithium-containing composite metal oxide having a spinel structure, and a lithium-containing composite metal oxide having an olivine structure.

Examples of the lithium-containing composite metal oxide having a layered structure include lithium-containing cobalt oxide (LiCoO ₂ ), lithium-containing nickel oxide (LiNiO ₂ ), lithium composite oxide of Co—Ni—Mn, Ni—Mn— Examples thereof include a lithium composite oxide of Al, a lithium composite oxide of Ni—Co—Al, and a solid solution of LiMaO ₂ and Li ₂ MbO ₃ . Examples of the solid solution of LiMaO ₂ and Li ₂ MbO ₃ include xLiMaO _2. (1-x) Li ₂ MbO ₃ . Here, x represents a number satisfying 0 <x <1, Ma represents one or more transition metals having an average oxidation state of 3+, and Mb represents one or more transition metals having an average oxidation state of 4+. Represent.

Among the lithium-containing composite metal oxides having a layered structure, it is preferable to use LiCoO ₂ from the viewpoint of improving the cycle characteristics of the secondary battery, and from the viewpoint of improving the energy density of the secondary battery, LiMaO. A solid solution of ₂ and Li ₂ MbO ₃ is preferred. Further, as a solid solution of LiMaO ₂ and Li ₂ MbO ₃ , in particular, xLiMaO ₂ · (1-x) Li ₂ MbO ₃ (x represents a number satisfying 0 <x <1, Ma represents Ni, Co, Mn Mb represents one or more selected from the group consisting of Mn, Zr and Ti). Among them, in particular, xLiMaO ₂ · (1-x) Li ₂ MnO ₃ (x represents a number satisfying 0 <x <1, Ma represents one or more selected from the group consisting of Ni, Co, Mn, Fe, and Ti. Are preferred).

Examples of the lithium-containing composite metal oxide having a spinel structure include a compound in which a part of Mn of lithium manganate (LiMn ₂ O ₄ ) is substituted with another transition metal. A specific example is Li _s [Mn _{2 -t} Md _t ] O ₄ . Here, Md represents one or more transition metals having an average oxidation state of 4+. Specific examples of Md include Ni, Co, Fe, Cu, and Cr. T represents a number satisfying 0 <t <1, and s represents a number satisfying 0 ≦ s ≦ 1.

Among _{them, Li s Fe t Mn 2-} t O 4-z where the Mn of the lithium manganate obtained by substituting Fe is preferred because the cost is inexpensive. Here, s represents a number satisfying 0 ≦ s ≦ 1, t represents a number satisfying 0 <t <1, and z represents a number satisfying 0 ≦ z ≦ 0.1.
Further, for example, LiNi _0.5 Mn _1.5 O _{4 in} which Mn of lithium manganate is substituted with Ni is also preferable. LiNi _0.5 Mn _1.5 O _{4 and the} like can replace all of Mn ³⁺ considered to be a factor of structural deterioration. Furthermore, since LiNi _0.5 Mn _1.5 O _{4 and the} like have an electrochemical reaction from Ni ²⁺ to Ni ⁴⁺ , a secondary battery having a high operating voltage and a high capacity can be realized.

Examples of the lithium-containing composite metal oxide having an olivine type structure include an olivine type lithium phosphate compound represented by Li _y McPO ₄ . Here, Mc represents one or more transition metals having an average oxidation state of 3+, and examples thereof include Mn and Co. Y represents a number satisfying 0 ≦ y ≦ 2. Furthermore, in the olivine-type lithium phosphate compound represented by Li _y McPO ₄ , Mn or Co may be partially substituted with another metal. Examples of the metal that can be substituted include Fe, Cu, Mg, Zn, V, Ca, Sr, Ba, Ti, Al, Si, B, and Mo.

Furthermore, examples of the positive electrode active material made of an inorganic compound include a positive electrode active material having a polyanion structure such as Li ₂ MeSiO ₄ , LiFeF ₃ having a perovskite structure, and Li ₂ Cu ₂ O ₄ having an orthorhombic structure. Can be mentioned. Here, Me represents Fe or Mn.

  Examples of the positive electrode active material made of an organic compound include conductive polymers such as polyacetylene and poly-p-phenylene.

  Alternatively, for example, a composite material covered with a carbon material may be produced by reducing and firing an iron-based oxide in the presence of a carbon source material, and the composite material may be used as a positive electrode active material. Iron-based oxides tend to have poor electrical conductivity, but can be used as a high-performance positive electrode active material by using a composite material as described above.

Among the above, the positive electrode active material is preferably a lithium-containing composite metal oxide because it has a high energy density. Many lithium-containing composite metal oxides have a hydrophilic group as a surface functional group. Therefore, by using a lithium-containing composite metal oxide, a slurry composition having high dispersion stability can be obtained, and the binding between the positive electrode active materials in the electrode can be kept strong.
Here, the surface state of the positive electrode active material can be determined by measuring the contact angle between the positive electrode active material and the solvent. For example, it can be confirmed by pressure-molding only the positive electrode active material to produce a pellet, and determining the contact angle of the pellet with respect to a polar solvent (for example, N-methylpyrrolidone). A lower contact angle indicates that the positive electrode active material is more hydrophilic.

  Moreover, when the secondary battery of this invention is a nickel hydride secondary battery, as a positive electrode active material, nickel hydroxide particle is mentioned, for example. For example, the nickel hydroxide particles may be solid-solved with cobalt, zinc, cadmium, or the like, or may be coated with a cobalt compound whose surface has been subjected to alkaline heat treatment.

  The positive electrode active material may be partially element-substituted. As the positive electrode active material, an inorganic compound and an organic compound may be used in combination. Furthermore, one type of positive electrode active material may be used alone, or two or more types may be used in combination at any ratio.

  The particle diameter of the positive electrode active material particles is usually selected as appropriate in consideration of the other constituent requirements of the secondary battery. The 50% volume cumulative diameter of the positive electrode active material particles is usually 0.1 μm or more, preferably 0.4 μm or more, more preferably 1 μm or more, from the viewpoint of improving battery characteristics such as load characteristics and cycle characteristics. It is 50 μm or less, preferably 30 μm or less, more preferably 20 μm or less. When the 50% volume cumulative diameter is within this range, a secondary battery having excellent output characteristics and a large charge / discharge capacity can be obtained. Moreover, the handling at the time of manufacturing the slurry composition for manufacturing a positive electrode active material layer and manufacturing a positive electrode is easy. The 50% volume cumulative diameter can be determined by measuring the particle size distribution by laser diffraction. That is, in the particle size distribution measured by the laser diffraction method, the particle diameter at which the cumulative volume calculated from the small diameter side becomes 50% is the 50% volume cumulative diameter.

[1.2. Conductive aid]
As a conductive support agent, the particle | grains which consist of an allotrope of carbon which has electroconductivity are mentioned, for example. By using a conductive additive, the electrical contact between the positive electrode active materials can be improved, and in particular when used in a lithium ion secondary battery, the discharge load characteristics can be improved.

  Specific examples of the conductive aid include conductive carbon such as acetylene black, ketjen black, carbon black, graphite, vapor grown carbon fiber, and carbon nanotube. Further, for example, carbon powder such as graphite, fibers and foils of various metals, and the like are also included. Here, one type of conductive assistant may be used alone, or two or more types may be used in combination at any ratio.

  Many conductive assistants exhibit surface hydrophobicity because there are many particles of carbon allotropes.

  The 50% volume cumulative diameter of the conductive additive is preferably smaller than the 50% volume cumulative diameter of the positive electrode active material. The specific range of the 50% volume cumulative diameter of the conductive assistant is usually 0.001 μm or more, preferably 0.05 μm or more, more preferably 0.01 μm or more, and usually 10 μm or less, preferably 5 μm or less, more preferably 1 μm or less. When the 50% volume cumulative diameter of the conductive assistant is within this range, high conductivity can be obtained with a smaller amount of use.

  The amount of the conductive assistant is usually 0.01 parts by weight or more, preferably 1 part by weight or more, and usually 20 parts by weight or less, preferably 10 parts by weight or less, with respect to 100 parts by weight of the positive electrode active material. When the amount of the conductive aid is within this range, the capacity of the secondary battery can be increased and high load characteristics can be exhibited.

[1.3. Particulate binder]
The particulate binder is usually contained in the positive electrode active material layer and has an effect of binding the positive electrode active material, the conductive additive and the current collector. By including the particulate binder, the positive electrode for the secondary battery can firmly hold the positive electrode active material and the conductive additive, so that the detachment of the positive electrode active material from the positive electrode for the secondary battery can be suppressed. The particulate binder can also bind particles other than the positive electrode active material and the conductive auxiliary agent usually contained in the positive electrode active material layer, and can also serve to maintain the strength of the positive electrode active material layer. In particular, since the particulate binder has a particulate shape, the binding property is particularly high, and deterioration due to capacity reduction and repeated charge / discharge can be remarkably suppressed.

  The compound that forms the particulate binder is not particularly limited as long as it is a compound that can bind the positive electrode active material and the conductive additive to each other. A suitable particulate binder is a dispersion type binder having a property of being dispersible in a solvent in the positive electrode slurry composition. Specific examples of the compound that forms the particulate binder include polymers such as diene polymers, acrylic polymers, fluoropolymers, and silicon polymers. Among these, a diene polymer and an acrylic polymer are preferable because they are excellent in binding property with the positive electrode active material and the strength and flexibility of the positive electrode to be obtained. Among them, an acrylic polymer is preferable from the viewpoint of high electrochemical stability.

  The diene polymer is a homopolymer of a conjugated diene or a copolymer obtained by polymerizing a monomer mixture containing a conjugated diene, or a hydrogenated product thereof. The ratio of the conjugated diene in the monomer mixture is usually 40% by weight or more, preferably 50% by weight or more, more preferably 60% by weight or more. Specific examples of the diene polymer include conjugated diene homopolymers such as polybutadiene and polyisoprene; aromatic vinyl / conjugated diene copolymers such as carboxy-modified styrene / butadiene copolymer (SBR); acrylonitrile -Vinyl cyanide * conjugated diene copolymers, such as a butadiene copolymer (NBR); Hydrogenated SBR, hydrogenated NBR, etc. are mentioned.

The acrylic polymer represents a polymer having a (meth) acrylic acid ester monomer unit. Moreover, a (meth) acrylic acid ester monomer unit represents a structural unit obtained by polymerizing a (meth) acrylic acid ester monomer.
The (meth) acrylic acid ester monomer, for example, of formula _(I): a compound represented by ^{CH 2} = CR 1 -COOR 2 and the like. In the formula (I), R ¹ represents a hydrogen atom or a methyl group, and R ² represents an alkyl group or a cycloalkyl group.

  Examples of (meth) acrylic acid ester monomers include methyl acrylate, ethyl acrylate, n-propyl acrylate, isopropyl acrylate, n-butyl acrylate, isobutyl acrylate, t-butyl acrylate, acrylic N-amyl acid, isoamyl acrylate, n-hexyl acrylate, 2-ethylhexyl acrylate, 2-methoxyethyl acrylate, 2-ethoxyethyl acrylate, hexyl acrylate, nonyl acrylate, lauryl acrylate, acrylic Acrylates such as stearyl acid and benzyl acrylate; methyl methacrylate, ethyl methacrylate, propyl methacrylate, isopropyl methacrylate, n-butyl methacrylate, isobutyl methacrylate, t-butyl methacrylate, n-amyl methacrylate, Acrylic acid isoamyl methacrylate n- hexyl, 2-ethylhexyl methacrylate, octyl methacrylate, isodecyl methacrylate, lauryl methacrylate, tridecyl methacrylate, stearyl methacrylate, and methacrylate and the like such as benzyl methacrylate. Among these, acrylate is preferable, and n-butyl acrylate and 2-ethylhexyl acrylate are particularly preferable in that the strength of the positive electrode for a secondary battery can be improved. Moreover, these monomers may be used individually by 1 type, and may be used combining two or more types by arbitrary ratios.

  Furthermore, among acrylic polymers, a copolymer having a (meth) acrylonitrile monomer unit and a (meth) acrylic acid ester monomer unit (hereinafter sometimes referred to as “copolymer B” as appropriate). preferable. Therefore, it is preferable that the particulate binder contains the copolymer B. This is because the copolymer B can increase the binding property and improve the strength of the positive electrode. Here, the (meth) acrylonitrile monomer unit represents a structural unit obtained by polymerizing a (meth) acrylonitrile monomer. As for the (meth) acrylonitrile monomer and the (meth) acrylonitrile monomer unit, one type may be used alone, or two or more types may be used in combination at any ratio.

  The weight ratio of the (meth) acrylonitrile monomer unit to the (meth) acrylic acid ester monomer unit is “(meth) acrylonitrile monomer unit / (meth) acrylic acid ester monomer unit”, usually 1 / 99 or more, preferably 5/95 or more, more preferably 10/90 or more, usually 30/70 or less, preferably 28/72 or less, more preferably 25/75 or less. By keeping the weight ratio of the (meth) acrylonitrile monomer unit and the (meth) acrylic acid ester monomer unit within the above range, the binding force of the particulate binder is increased and the strength of the positive electrode is remarkably improved. Can do.

  Moreover, the polymer which has a carboxylic acid group containing monomer unit can be used for an acrylic polymer. Therefore, for example, the copolymer B can have a carboxylic acid group-containing monomer unit. The carboxylic acid group-containing monomer unit represents a structural unit obtained by polymerizing a monomer containing a carboxylic acid group (—COOH; also referred to as a carboxyl group). Examples of the monomer containing a carboxylic acid group include an unsaturated carboxylic acid compound. Specific examples thereof include monomers containing monobasic acids such as acrylic acid and methacrylic acid; monomers containing dibasic acids such as maleic acid, fumaric acid and itaconic acid. Here, a carboxylic acid group containing monomer may be used individually by 1 type, and may be used combining two or more types by arbitrary ratios.

  The ratio of the carboxylic acid group-containing monomer unit in the copolymer B is preferably 0.1% by weight or more, more preferably 0.5% by weight or more, particularly preferably 1% by weight or more, preferably 50%. % By weight or less, more preferably 20% by weight or less, particularly preferably 10% by weight or less. When the proportion of the carboxylic acid group-containing monomer unit in the copolymer B is within this range, the binding property can be increased and the electrode strength can be improved.

  Furthermore, the acrylic polymer may have an arbitrary structural unit other than those described above as long as the effects of the present invention are not significantly impaired. Therefore, the copolymer B may have any structural unit other than those described above as long as the effects of the present invention are not significantly impaired. These arbitrary structural units are structural units obtained by polymerizing a monomer copolymerizable with the above-described monomer. Examples of monomers copolymerizable with the above-described monomers include carboxylic acid esters having two or more carbon-carbon double bonds such as ethylene glycol dimethacrylate, diethylene glycol dimethacrylate, and trimethylolpropane triacrylate. Styrene monomers such as styrene, chlorostyrene, vinyl toluene, t-butyl styrene, vinyl benzoic acid, methyl vinyl benzoate, vinyl naphthalene, chloromethyl styrene, hydroxymethyl styrene, α-methyl styrene, divinylbenzene; Amide monomers such as acrylamide, N-methylolacrylamide, and acrylamide-2-methylpropanesulfonic acid; Olefins such as ethylene and propylene; Diene monomers such as butadiene and isoprene; Vinyl chloride and vinylidene chloride Halogen atom-containing monomers such as vinyl acetate, vinyl propionate, vinyl butyrate, vinyl benzoate, etc .; vinyl ethers such as methyl vinyl ether, ethyl vinyl ether, butyl vinyl ether; methyl vinyl ketone, ethyl vinyl ketone, butyl And vinyl ketones such as vinyl ketone, hexyl vinyl ketone and isopropenyl vinyl ketone; heterocyclic ring-containing vinyl compounds such as N-vinyl pyrrolidone, vinyl pyridine and vinyl imidazole; One of these may be used alone, or two or more of these may be used in combination at any ratio.

  However, the amount of the arbitrary structural unit is such that the amount of the (meth) acrylic acid ester monomer unit in the acrylic polymer or the copolymer B is usually 50% by weight or more, preferably 70% by weight or more. It is desirable to keep it within the range.

  The polymer forming the particulate binder may have a crosslinked structure. Therefore, for example, the copolymer B may have a crosslinked structure. Examples of the method for introducing a crosslinked structure include a method of incorporating a crosslinkable group into a polymer and a method of using a combination of a polymer and a crosslinking agent. In this case, the polymer can be crosslinked by irradiation with heat or energy rays. The degree of crosslinking can be adjusted by the intensity of heating or irradiation with energy rays. Since the degree of swelling decreases as the degree of crosslinking increases, the degree of swelling of the particulate binder can be controlled by adjusting the degree of crosslinking.

  As described above, the particulate binder preferably includes the copolymer B. At this time, the amount of the copolymer B is preferably 70 parts by weight or more, more preferably 80 parts by weight or more, and preferably 100 parts by weight or less with respect to 100 parts by weight of the total amount of the particulate binder.

  The weight average molecular weight of the polymer forming the particulate binder is preferably 10,000 or more, more preferably 20,000 or more, preferably 1,000,000 or less, more preferably 500,000 or less. When the weight average molecular weight of the polymer forming the particulate binder is in the above range, the strength of the positive electrode for secondary batteries and the dispersibility of the positive electrode active material can be easily improved. The weight average molecular weight of the polymer forming the particulate binder can be determined by gel permeation chromatography (GPC) as a value in terms of polystyrene using tetrahydrofuran as a developing solvent.

  The glass transition temperature (Tg) of the particulate binder is preferably −50 ° C. or higher, more preferably −45 ° C. or higher, particularly preferably −40 ° C. or higher, preferably 25 ° C. or lower, more preferably 15 ° C. or lower. Especially preferably, it is 5 degrees C or less. When the glass transition temperature of the particulate binder is within the above range, a positive electrode for a secondary battery having excellent strength and flexibility and high output characteristics can be obtained. The glass transition temperature of the particulate binder can be adjusted by combining various monomers.

  Usually, the polymer forming the particulate binder is water-insoluble. Therefore, the particulate binder is usually in the form of particles in the slurry composition for producing the battery positive electrode, and is included in the secondary battery positive electrode while maintaining the particle shape.

  The number average particle size of the particulate binder is usually 0.0001 μm or more, preferably 0.001 μm or more, more preferably 0.01 μm or more, and usually 100 μm or less, preferably 10 μm or less, more preferably 1 μm or less. . When the number average particle diameter of the particulate binder is within this range, an excellent binding force can be expressed even with a small amount of use. Here, the number average particle diameter is a number average particle diameter calculated as an arithmetic average value obtained by measuring the diameters of 100 particulate binders randomly selected in a transmission electron micrograph. The shape of the particles may be either spherical or irregular.

  Further, as the particulate binder, one type of polymer may be used alone, or two or more types of polymers having different structures may be used in combination at any ratio.

  The amount of the particulate binder is usually 0.1 parts by weight or more, preferably 0.5 parts by weight or more, more preferably 0.8 parts by weight or more, and usually 50 parts by weight with respect to 100 parts by weight of the positive electrode active material. The amount is preferably 20 parts by weight or less, more preferably 10 parts by weight or less, and still more preferably 3 parts by weight or less. By setting the amount of the particulate binder within this range, sufficient adhesion can be secured, the capacity of the secondary battery can be increased, and the internal resistance of the positive electrode for the secondary battery can be decreased.

  The particulate binder can be produced, for example, by polymerizing a monomer composition containing the above-described monomer in an aqueous solvent to form polymer particles. The ratio of each monomer in the monomer composition is usually the same as the content ratio of the structural unit in the polymer forming the particulate binder.

  The aqueous solvent is not particularly limited as long as the particulate binder can be dispersed. Usually, an aqueous solvent having a boiling point at normal pressure of usually 80 ° C. or higher, preferably 100 ° C. or higher, and usually 350 ° C. or lower, preferably 300 ° C. or lower is used. Examples of the aqueous solvent will be given below. In the following examples, the number in parentheses after the solvent name is the boiling point (unit: ° C) at normal pressure, and the value after the decimal point is a value rounded off or rounded down.

Examples of aqueous solvents include water (100); ketones such as diacetone alcohol (169) and γ-butyrolactone (204); ethyl alcohol (78), isopropyl alcohol (82), normal propyl alcohol (97) and the like. Alcohols: propylene glycol monomethyl ether (120), methyl cellosolve (124), ethyl cellosolve (136), ethylene glycol tertiary butyl ether (152), butyl cellosolve (171), 3-methoxy-3-methyl-1-butanol (174) , Ethylene glycol monopropyl ether (150), diethylene glycol monobutyl pyrether (230), triethylene glycol monobutyl ether (271), dipropylene glycol monomethyl ether (188) Glycol ethers and the like; and 1,3-dioxolane (75), 1,4-dioxolane (101), ethers such as tetrahydrofuran (66) and the like. Among these, water is particularly preferable from the viewpoint that it is not flammable and easily obtains a particulate binder.
In addition, one type of aqueous solvent may be used alone, or two or more types may be used in combination at any ratio. For example, water may be used as the main solvent, and an aqueous solvent other than the above-described water may be mixed and used within a range where the dissolution of the particulate binder can be ensured.

  As a polymerization method, any method such as a suspension polymerization method or an emulsion polymerization method may be used. As a polymerization method, any method such as ionic polymerization, radical polymerization, and living radical polymerization may be used. Among them, it is easy to obtain a high molecular weight polymer, and since the polymer is obtained in the form of particles dispersed in water, a redispersion treatment is unnecessary, and a slurry for producing a positive electrode for a secondary battery as it is. From the viewpoint of production efficiency, such as being able to be used in a composition, an emulsion polymerization method is particularly preferred.

  The emulsion polymerization method is usually performed by a conventional method. For example, it can be performed by the method described in “Experimental Chemistry Course” Vol. 28, (Publisher: Maruzen Co., Ltd., edited by The Chemical Society of Japan). That is, water, an additive such as a dispersant, an emulsifier, a crosslinking agent, a polymerization initiator, and a monomer are added to a sealed container equipped with a stirrer and a heating device so as to have a predetermined composition. A method may be used in which the composition is stirred to emulsify monomers and the like in water, and the temperature is increased while stirring to initiate polymerization. Or after emulsifying the said composition, it can put into an airtight container, and the method of starting reaction similarly can be used.

  Examples of the polymerization initiator include organic compounds such as lauroyl peroxide, diisopropyl peroxydicarbonate, di-2-ethylhexyl peroxydicarbonate, t-butyl peroxypivalate, 3,3,5-trimethylhexanoyl peroxide. Peroxides; azo compounds such as α, α′-azobisisobutyronitrile; ammonium persulfate; potassium persulfate and the like. A polymerization initiator may be used individually by 1 type, and may be used combining two or more types by arbitrary ratios.

  As the dispersant, those used in usual synthesis may be used. Specific examples of the dispersant include benzenesulfonates such as sodium dodecylbenzenesulfonate and sodium dodecylphenylethersulfonate; alkyl sulfates such as sodium lauryl sulfate and sodium tetradodecylsulfate; sodium dioctylsulfosuccinate and sodium dihexylsulfosuccinate Sulfosuccinates such as: fatty acid salts such as sodium laurate; ethoxy sulfate salts such as polyoxyethylene lauryl ether sulfate sodium salt, polyoxyethylene nonylphenyl ether sulfate sodium salt; alkane sulfonate; alkyl ether phosphate sodium salt Polyoxyethylene nonylphenyl ether, polyoxyethylene sorbitan lauryl ester, polyoxyethylene- Nonionic emulsifiers such as reoxypropylene block copolymers; gelatin, maleic anhydride-styrene copolymers, polyvinyl pyrrolidone, sodium polyacrylate, water-soluble water such as polyvinyl alcohol having a polymerization degree of 700 or more and a saponification degree of 75% or more Functional polymers; and the like. Among these, benzenesulfonates such as sodium dodecylbenzenesulfonate and sodium dodecylphenylethersulfonate; alkyl sulfates such as sodium lauryl sulfate and sodium tetradodecylsulfate are preferable. More preferred are benzene sulfonates such as sodium dodecyl benzene sulfonate and sodium dodecyl phenyl ether sulfonate from the viewpoint of excellent oxidation resistance. In addition, these may be used individually by 1 type and may be used combining two or more types by arbitrary ratios. The amount of the dispersant is usually 0.01 part by weight to 10 parts by weight with respect to 100 parts by weight of the total amount of monomers.

  Further, in the polymerization, seed polymerization may be performed using seed particles.

In addition, an aqueous dispersion of the particulate binder obtained by the above-described method may be obtained by, for example, alkali metal (for example, Li, Na, K, Rb, Cs) hydroxide, ammonia, inorganic ammonium compound (for example, NH ₄ Cl). , It may be mixed with a basic aqueous solution containing an organic amine compound (eg, ethanolamine, diethylamine, etc.) and the pH may be adjusted to a range of usually 5 to 10, preferably 5 to 9. Of these, pH adjustment with an alkali metal hydroxide is preferable because it improves the binding between the current collector and the positive electrode active material.

  Further, the particulate binder may be a composite polymer particle composed of two or more kinds of polymers. The composite polymer particles are obtained, for example, by polymerizing at least one monomer component by a conventional method, then polymerizing at least one other monomer component and polymerizing by a conventional method (two-stage polymerization method). ) Or the like. In this way, by polymerizing the monomer stepwise, it is possible to obtain core-shell structured particles having a core layer present inside the particle and a shell layer covering the core layer.

[1.4. Water-soluble polymer)
The water-soluble polymer is a water-soluble copolymer having an acidic functional group-containing monomer unit and a (meth) acrylate monomer unit in a predetermined content ratio (hereinafter referred to as “copolymer A” as appropriate). May be included.) By using a water-soluble polymer containing such a copolymer A, the coating property of the slurry composition for producing a positive electrode for a secondary battery, the adhesion of the positive electrode active material layer to the current collector, and The liquid injection property of the positive electrode active material layer can be improved in a well-balanced manner, whereby a secondary battery having high storage characteristics in a high temperature environment can be realized. Here, the water-soluble polymer includes not only an embodiment containing only the copolymer A but also an embodiment containing the copolymer A and a polymer other than the copolymer A. Moreover, as the copolymer A contained in the water-soluble polymer, one type of polymer may be used alone, or two or more types of polymers having different structures may be used in combination at any ratio. . The water-soluble polymer does not include a water-soluble polymer compound (water-soluble natural polymer) derived from natural products such as carboxymethyl cellulose.

The reason why the water-soluble polymer according to the present invention contains the copolymer A is not as clear as described above, but according to the study of the present inventors, it is presumed as follows. The
Of the components contained in the positive electrode for secondary batteries, the positive electrode active material is generally hydrophilic, but the conductive additive is generally hydrophobic. For this reason, in the slurry composition containing the positive electrode active material and the conductive additive, it is difficult to disperse both the positive electrode active material and the conductive additive well. However, by using the water-soluble polymer according to the present invention, it is possible to favorably disperse both the positive electrode active material and the conductive additive. Thereby, when apply | coating a slurry composition, since it can apply | coat, suppressing aggregation of a positive electrode active material and a conductive support agent, applicability | paintability can be improved.

  Further, since the copolymer A contained in the water-soluble polymer has an acidic functional group, the copolymer A contained in the water-soluble polymer is dissolved in water by electrostatic interaction due to the acidic functional group. The viscosity can be increased. For this reason, since the viscosity of the slurry composition containing a water-soluble polymer increases, coatability can also be improved by this.

  Furthermore, since the dispersibility of the slurry composition is good, in the positive electrode active material layer, the bias and aggregation of the positive electrode active material, the conductive additive, and the particulate binder are suppressed. Therefore, since it is difficult for a portion having a small amount of particulate binder to be locally generated in the positive electrode active material layer, the adhesion strength of the positive electrode active material layer to the current collector does not locally increase or decrease. For this reason, the adhesiveness of the positive electrode active material layer with respect to a collector can be improved.

In addition, since the positive electrode active material and the conductive additive are well dispersed in the positive electrode active material layer, the constituent components are not easily biased in the positive electrode active material layer, and the structure uniformity of the positive electrode active material layer is improved. ing. For this reason, the distribution of the pores formed in the positive electrode active material layer is made uniform, and the electrolyte solution is easily soaked, so that the liquid injection property is improved.
Furthermore, since the uniformity of the composition of the positive electrode active material layer is also improved, the internal resistance of the positive electrode can be lowered.

  And the storage characteristic in the high temperature environment of the secondary battery of this invention can be improved according to the synergistic effect by the effect mentioned above. In addition, with the same mechanism, it is usually possible to improve the cycle characteristics and output characteristics of the secondary battery of the present invention.

  The reason why the copolymer A contained in the water-soluble polymer according to the present invention can improve the dispersibility of both the positive electrode active material and the conductive additive is that the copolymer contained in the water-soluble polymer Interaction between the acidic functional group in the polymer A and the polar group on the surface of the positive electrode active material, electrostatic repulsion by the acidic functional groups in the copolymer A contained in the water-soluble polymer, etc. Can be considered. That is, the copolymer A contained in the water-soluble polymer is appropriately adsorbed on the surface of the positive electrode active material by the interaction between the polar group on the surface of the positive electrode active material and the polar functional group of the copolymer A. The positive electrode active materials are less likely to aggregate due to the electrostatic repulsion effect of the adsorbed copolymer A. For this reason, it is considered that the conductive auxiliary agent can easily enter between the positive electrode active materials and the dispersibility is improved.

[1.4.1. Acidic functional group-containing monomer unit)
The acidic functional group-containing monomer unit represents a structural unit obtained by polymerizing a monomer containing an acidic functional group. Examples of the acidic functional group include a carboxylic acid group (—COOH), a sulfonic acid group (—SO ₃ H), and a phosphoric acid group (—PO ₃ H ₂ ). Among these, a carboxylic acid group is preferable. However, an acidic functional group may be used individually by 1 type, and may be used in combination of 2 or more types. Moreover, the number of acidic functional groups which the monomer containing an acidic functional group has may be one, and may be two or more.

  As the monomer containing a carboxylic acid group, a monomer having a carboxylic acid group and a polymerizable group is usually used. Examples of the monomer containing a carboxylic acid group include an unsaturated carboxylic acid monomer. An unsaturated carboxylic acid monomer is a monomer having a carbon-carbon unsaturated bond and having a carboxylic acid group.

  Examples of the unsaturated carboxylic acid monomer include unsaturated monocarboxylic acid and derivatives thereof; unsaturated dicarboxylic acid and acid anhydrides and derivatives thereof; and the like.

Examples of unsaturated monocarboxylic acids include ethylenically unsaturated monocarboxylic acids such as acrylic acid, methacrylic acid, and crotonic acid.
Examples of unsaturated monocarboxylic acid derivatives include 2-ethylacrylic acid, isocrotonic acid, α-acetoxyacrylic acid, β-trans-aryloxyacrylic acid, α-chloro-β-E-methoxyacrylic acid, and β -Derivatives of ethylenically unsaturated monocarboxylic acids, such as diaminoacrylic acid.

Examples of unsaturated dicarboxylic acids include ethylenically unsaturated dicarboxylic acids such as maleic acid, fumaric acid, and itaconic acid.
Examples of unsaturated dicarboxylic acid anhydrides include ethylenically unsaturated dicarboxylic acid anhydrides such as maleic anhydride, acrylic anhydride, methyl maleic anhydride, and dimethyl maleic anhydride.
Examples of derivatives of unsaturated dicarboxylic acids include methylmaleic acid, dimethylmaleic acid, phenylmaleic acid, chloromaleic acid, dichloromaleic acid, fluoromaleic acid, methylallyl maleate; and diphenyl maleate, nonyl maleate, maleate Examples thereof include maleate esters such as decyl acid, dodecyl maleate, octadecyl maleate, and fluoroalkyl maleate.

Among these, unsaturated monocarboxylic acids such as acrylic acid and methacrylic acid are preferable. It is because the dispersibility with respect to the water of the copolymer A can be improved more. Therefore, an unsaturated monocarboxylic acid monomer unit is preferable as the acidic functional group-containing monomer unit.
Moreover, the monomer containing an acidic functional group, and an acidic functional group containing monomer may be used individually by 1 type, and may be used combining two or more types by arbitrary ratios.

The content ratio of the acidic functional group-containing monomer unit in the copolymer A contained in the water-soluble polymer is usually 15% by weight or more, preferably 25% by weight or more, more preferably 30% by weight or more. It is 60 wt% or less, preferably 55 wt% or less, more preferably 40 wt% or less. By setting the content ratio of the acidic functional group-containing monomer unit to be equal to or higher than the lower limit of the above range, it is possible to exhibit electrostatic repulsion and obtain good dispersibility. On the other hand, by setting the content ratio of the acidic functional group-containing monomer unit to be equal to or less than the upper limit of the above range, excessive contact between the functional group and the electrolytic solution can be avoided, and durability can be improved. Moreover, since the adsorptivity of the copolymer A with respect to the positive electrode active material can be prevented from becoming excessively high and the positive electrode active material can be prevented from forming a pseudo-crosslinked structure, the positive electrode active material via the copolymer A can be prevented. Aggregation of substances can be prevented.
The content ratio of the acidic functional group-containing monomer unit in the copolymer A contained in the water-soluble polymer is usually a charging ratio of the monomer containing the acidic functional group used when the copolymer A is produced. Matches.

[1.4.2. (Meth) acrylic acid ester monomer unit]
A (meth) acrylic acid ester monomer unit represents a structural unit obtained by polymerizing a (meth) acrylic acid ester monomer. Examples of the (meth) acrylic acid ester monomer include compounds represented by the formula (I) as described in the section of the particulate binder.

  Examples of preferred (meth) acrylate monomers include methyl acrylate, ethyl acrylate, n-propyl acrylate, isopropyl acrylate, n-butyl acrylate, t-butyl acrylate, pentyl acrylate, hexyl acrylate, heptyl acrylate, octyl acrylate Acrylic acid alkyl esters such as 2-ethylhexyl acrylate, nonyl acrylate, decyl acrylate, lauryl acrylate, n-tetradecyl acrylate, stearyl acrylate; methyl methacrylate, ethyl methacrylate, n-propyl methacrylate, isopropyl methacrylate, n-butyl methacrylate, t -Butyl methacrylate, pentyl methacrylate, hexyl methacrylate, heptyl Methacrylic acid alkyl esters such as methacrylate, octyl methacrylate, 2-ethylhexyl methacrylate, nonyl methacrylate, decyl methacrylate, lauryl methacrylate, n-tetradecyl methacrylate, stearyl methacrylate; hydroxymethyl acrylate, 2-hydroxyethyl acrylate, hydroxypropyl acrylate, hydroxybutyl Acrylic acid (hydroxy) alkyl acrylates such as acrylates; and acrylic acid (hydroxy) alkyl metallates such as hydroxymethyl methacrylate, 2-hydroxyethyl methacrylate, hydroxypropyl methacrylate, and hydroxybutyl methacrylate.

  As for the (meth) acrylic acid ester monomer and the (meth) acrylic acid ester monomer unit, one type may be used alone, or two or more types may be used in combination at any ratio.

The content ratio of the (meth) acrylic acid ester monomer unit in the copolymer A contained in the water-soluble polymer is usually 30% by weight or more, preferably 35% by weight or more, more preferably 40% by weight or more. Also, it is usually 80% by weight or less, preferably 70% by weight or less. The flexibility of the positive electrode active material layer can be increased by setting the amount of the (meth) acrylic acid ester monomer unit to be not less than the lower limit value of the above range, and for secondary batteries by being not more than the upper limit value of the above range. The adhesion of the positive electrode can be improved.
The content ratio of the (meth) acrylate monomer unit in the copolymer A contained in the water-soluble polymer is usually that of the (meth) acrylate monomer used when the copolymer A is produced. It matches the preparation ratio.

[1.4.3. Crosslinkable monomer unit)
The water-soluble polymer preferably includes a polymer having a crosslinkable monomer unit. Accordingly, the copolymer A contained in the water-soluble polymer preferably has a crosslinkable monomer unit. By having a crosslinkable monomer unit, it is possible to increase the molecular weight of the water-soluble polymer within a range that does not impair the water-solubility of the water-soluble polymer, and to prevent the degree of swelling of the water-soluble polymer with respect to the electrolyte from becoming excessively high. . Here, the crosslinkable monomer unit represents a structural unit obtained by polymerizing the crosslinkable monomer. Moreover, a crosslinkable monomer represents the monomer which can form a crosslinked structure during superposition | polymerization or after superposition | polymerization by heating or irradiation of an energy ray. As an example of the crosslinkable monomer, a monomer having heat crosslinkability is usually mentioned. More specifically, for example, a monofunctional monomer having a thermally crosslinkable crosslinkable group and one olefinic double bond per molecule; a polyfunctional monomer having two or more olefinic double bonds per molecule; A functional monomer is mentioned.

  Examples of the thermally crosslinkable group include an epoxy group, an N-methylolamide group, an oxetanyl group, an oxazoline group, and a combination thereof. Among these, an epoxy group is more preferable in terms of easy adjustment of crosslinking and crosslinking density.

  Examples of the crosslinkable monomer having an epoxy group as a thermally crosslinkable group and having an olefinic double bond include vinyl glycidyl ether, allyl glycidyl ether, butenyl glycidyl ether, o-allylphenyl glycidyl. Unsaturated glycidyl ethers such as ether; butadiene monoepoxide, chloroprene monoepoxide, 4,5-epoxy-2-pentene, 3,4-epoxy-1-vinylcyclohexene, 1,2-epoxy-5,9-cyclododecadiene Monoepoxides of dienes or polyenes such as; alkenyl epoxides such as 3,4-epoxy-1-butene, 1,2-epoxy-5-hexene, 1,2-epoxy-9-decene; and glycidyl acrylate, glycidyl methacrylate, Glycidyl crotonate Unsaturated carboxylic acids such as sidyl-4-heptenoate, glycidyl sorbate, glycidyl linoleate, glycidyl-4-methyl-3-pentenoate, glycidyl ester of 3-cyclohexene carboxylic acid, glycidyl ester of 4-methyl-3-cyclohexene carboxylic acid Examples include glycidyl esters of acids.

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

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

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

  Examples of crosslinkable monomers having two or more olefinic double bonds per molecule include allyl (meth) acrylate, ethylene di (meth) acrylate, diethylene glycol di (meth) acrylate, and triethylene glycol di (meth). Acrylate, tetraethylene glycol di (meth) acrylate, trimethylolpropane-tri (meth) acrylate, dipropylene glycol diallyl ether, polyglycol diallyl ether, triethylene glycol divinyl ether, hydroquinone diallyl ether, tetraallyloxyethane, trimethylolpropane -Diallyl ether, allyl or vinyl ether of polyfunctional alcohols other than those mentioned above, triallylamine, methylenebisacrylamide, and divinylbenzene That.

Among these examples, ethylene dimethacrylate, allyl glycidyl ether, and glycidyl methacrylate are particularly preferable as the crosslinkable monomer.
Moreover, a crosslinking | crosslinked monomer and a crosslinking | crosslinked monomer unit may be used individually by 1 type, and may be used combining two or more types by arbitrary ratios.

The content ratio of the crosslinkable monomer unit in the copolymer A contained in the water-soluble polymer is usually 0.1% by weight or more, preferably 0.2% by weight or more, more preferably 0.5% by weight or more. It is usually 2% by weight or less, preferably 1.5% by weight or less, more preferably 1% by weight or less. By making the content ratio of the crosslinkable monomer unit more than the lower limit of the above range, the molecular weight of the copolymer A contained in the water-soluble polymer is increased, and the swelling of the water-soluble polymer due to the electrolytic solution is suppressed. The swelling of the positive electrode for a secondary battery can be suppressed. On the other hand, by setting the content ratio of the crosslinkable monomer unit to be equal to or less than the upper limit of the above range, the water solubility of the copolymer A contained in the water-soluble polymer can be increased and the dispersibility can be improved. it can. Therefore, by setting the content ratio of the crosslinkable monomer unit within the above range, both the degree of swelling and the dispersibility can be improved.
The content ratio of the crosslinkable monomer unit in the copolymer A contained in the water-soluble polymer usually coincides with the charge ratio of the crosslinkable monomer used when the copolymer A is produced.

[1.4.4. (Reactive surfactant unit)
The water-soluble polymer preferably includes a polymer having a reactive surfactant unit. Accordingly, the copolymer A contained in the water-soluble polymer preferably has a reactive surfactant unit. By having a reactive surfactant unit, the solubility of the water-soluble polymer in water and the dispersibility of the slurry composition can be enhanced. Here, the reactive surfactant unit represents a structural unit obtained by polymerizing a reactive surfactant monomer. The reactive surfactant monomer is a monomer having a polymerizable group that can be copolymerized with other monomers and having a surfactant group (that is, a hydrophilic group and a hydrophobic group). Represents a mer. The reactive surfactant unit obtained by polymerization of the reactive surfactant monomer constitutes a part of the molecule of a water-soluble polymer such as copolymer A, and can function as a surfactant.

  Usually, the reactive surfactant monomer has a polymerizable unsaturated group, and this polymerizable unsaturated group also acts as a hydrophobic group after polymerization. Examples of the polymerizable unsaturated group include a vinyl group, an allyl group, a vinylidene group, a propenyl group, an isopropenyl group, and an isobutylidene group. The type of the polymerizable unsaturated group may be one type or two or more types.

  Further, the reactive surfactant monomer usually has a hydrophilic group as a portion that exhibits hydrophilicity. Reactive surfactant monomers are classified into anionic, cationic and nonionic surfactants depending on the type of hydrophilic group.

Examples of the anionic hydrophilic group include —SO ₃ M, —COOM, and —PO (OH) ₂ . Here, M represents a hydrogen atom or a cation. Examples of cations include alkali metal ions such as lithium, sodium and potassium; alkaline earth metal ions such as calcium and magnesium; ammonium ions; ammonium ions of alkylamines such as monomethylamine, dimethylamine, monoethylamine and triethylamine; and Examples include ammonium ions of alkanolamines such as monoethanolamine, diethanolamine, and triethanolamine.
Examples of the cationic hydrophilic group include —Cl, —Br, —I, and —SO ₃ OR ^X. Here, R ^X represents an alkyl group. Examples of R ^X is methyl group, an ethyl group, a propyl group, and isopropyl group, and the like.
-OH is mentioned as an example of a nonionic hydrophilic group.

  Examples of suitable reactive surfactant monomers include compounds represented by the following formula (II).

In the formula (II), R represents a divalent linking group. Examples of R include -Si-O- group, methylene group, phenylene group and the like. In the formula (II), R ³ represents a hydrophilic group. An example of R ³ includes —SO ₃ NH ₄ . Further, in the formula (II), n represents an integer of 1 or more and 100 or less.

Another example of a suitable reactive surfactant monomer has a structural unit having a structure formed by polymerizing ethylene oxide and a structural unit having a structure formed by polymerizing butylene oxide, and Examples include compounds having an alkenyl group having a terminal double bond and —SO ₃ NH ₄ at the terminal (for example, trade names “Latemul PD-104” and “Latemul PD-105”, manufactured by Kao Corporation).
A reactive surfactant monomer may be used individually by 1 type, and may be used combining two or more types by arbitrary ratios.

The content of the reactive surfactant unit in the copolymer A contained in the water-soluble polymer is usually 0.1% by weight or more, preferably 0.2% by weight or more, more preferably 0.5% by weight or more. It is usually 15% by weight or less, preferably 10% by weight or less, more preferably 5% by weight or less. By making the content rate of a reactive surfactant unit more than the lower limit of the said range, the dispersibility of a slurry composition can be improved and a uniform electrode can be obtained. On the other hand, by setting the content ratio of the reactive surfactant unit to be equal to or less than the upper limit of the above range, the moisture content in the electrode plate can be kept low, and thus the durability of the positive electrode can be improved.
The content ratio of the cross-reactive surfactant unit in the copolymer A contained in the water-soluble polymer is usually the same as the charging ratio of the reactive surfactant monomer used when the copolymer A is produced. To do.

[1.4.5. Fluorine-containing (meth) acrylic acid ester monomer unit]
The water-soluble polymer preferably includes a polymer having a fluorine-containing (meth) acrylic acid ester monomer unit. Therefore, it is preferable that the copolymer A contained in the water-soluble polymer has a fluorine-containing (meth) acrylate monomer unit. By having a fluorine-containing (meth) acrylic acid ester monomer unit, the wettability of the water-soluble polymer with respect to the electrolyte solution is adjusted, and both the suppression of the swelling property of the positive electrode active material layer and the improvement of the liquid injection property are both achieved. Both can be realized. Here, the fluorine-containing (meth) acrylic acid ester monomer unit represents a structural unit obtained by polymerizing a fluorine-containing (meth) acrylic acid ester monomer. In the present specification, these fluorine-containing (meth) acrylic acid ester monomers and fluorine-containing (meth) acrylic acid ester monomer units containing fluorine are (meth) acrylic acid ester monomers not containing fluorine. And (meth) acrylic acid ester monomer units.

  Examples of the fluorine-containing (meth) acrylic acid ester monomer include a monomer represented by the following formula (III).

In the above formula (III), R ⁴ represents a hydrogen atom or a methyl group.
In the formula (III), R ⁵ represents a hydrocarbon group containing a fluorine atom. The carbon number of the hydrocarbon group is usually 1 or more and usually 18 or less, and the hydrocarbon group may be either linear or branched. In addition, the number of fluorine atoms contained in R ⁵ may be one or two or more.

  Examples of fluorine-containing (meth) acrylic acid ester monomers represented by the formula (III) include (meth) acrylic acid alkyl fluoride, (meth) acrylic acid fluoride aryl, and (meth) acrylic acid fluoride. Aralkyl is mentioned. Of these, alkyl fluoride (meth) acrylate is preferable.

  Specific examples of such monomers include 2,2,2-trifluoroethyl (meth) acrylate, β- (perfluorooctyl) ethyl (meth) acrylate, 2,2, (meth) acrylic acid. 3,3-tetrafluoropropyl, (meth) acrylic acid 2,2,3,4,4,4-hexafluorobutyl, (meth) acrylic acid 1H, 1H, 9H-perfluoro-1-nonyl, (meth) 1H, 1H, 11H-perfluoroundecyl acrylate, perfluorooctyl (meth) acrylate, 3 [4 [1-trifluoromethyl-2,2-bis [bis (trifluoromethyl) fluoro (meth) acrylate] And (meth) acrylic acid perfluoroalkyl esters such as methyl] ethynyloxy] benzooxy] 2-hydroxypropyl.

  One type of fluorine-containing (meth) acrylic acid ester monomer and fluorine-containing (meth) acrylic acid ester monomer unit may be used alone, or two or more types may be used in combination at any ratio. .

The content ratio of the fluorine-containing (meth) acrylate monomer unit in the copolymer A contained in the water-soluble polymer is usually 1% by weight or more, preferably 2% by weight or more, more preferably 5% by weight or more. It is usually 20% by weight or less, preferably 15% by weight or less, more preferably 10% by weight or less. By setting the content ratio of the fluorine-containing (meth) acrylic acid ester monomer unit to be equal to or higher than the lower limit of the above range, the copolymer A contained in the water-soluble polymer can be given repulsion to the electrolytic solution. The swelling property can be suppressed within an appropriate range. On the other hand, by making the content ratio of the fluorine-containing (meth) acrylic acid ester monomer unit below the upper limit of the above range, the copolymer A contained in the water-soluble polymer is given wettability to the electrolytic solution. And the low temperature output characteristics of the secondary battery can be improved.
The content ratio of the fluorine-containing (meth) acrylic acid ester monomer unit in the copolymer A contained in the water-soluble polymer is usually a fluorine-containing (meth) acrylic acid ester used when the copolymer A is produced. This is consistent with the monomer charge ratio.

[1.4.6. Other structural units)
The water-soluble polymer includes the above-mentioned acidic functional group-containing monomer unit, (meth) acrylate monomer unit, crosslinkable monomer unit, reactive surfactant unit, and fluorine-containing (meth) acrylate ester. In addition to the monomer unit, any structural unit may be included as long as the effects of the present invention are not significantly impaired. Therefore, the copolymer A may also have an arbitrary structural unit as long as the effects of the present invention are not significantly impaired.

  Examples of arbitrary structural units include structural units obtained by polymerizing the following arbitrary monomers. Examples of the optional monomer include styrene, chlorostyrene, vinyl toluene, t-butyl styrene, vinyl benzoic acid, methyl vinyl benzoate, vinyl naphthalene, chloromethyl styrene, hydroxymethyl styrene, α-methyl styrene, divinyl benzene. Styrene monomers such as acrylamide; amide monomers such as acrylamide and acrylamide-2-methylpropane sulfonic acid; α, β-unsaturated nitrile compound monomers such as acrylonitrile and methacrylonitrile; ethylene, propylene, etc. Olefin monomers; halogen atom-containing monomers such as vinyl chloride and vinylidene chloride; vinyl ester monomers such as vinyl acetate, vinyl propionate, vinyl butyrate and vinyl benzoate; methyl vinyl ether, ethyl vinyl ether, butyl vinyl ether Vinyl etc. Ether monomers; vinyl ketone monomers such as methyl vinyl ketone, ethyl vinyl ketone, butyl vinyl ketone, hexyl vinyl ketone, and isopropenyl vinyl ketone; and heterocyclic rings such as N-vinyl pyrrolidone, vinyl pyridine, and vinyl imidazole Vinyl compound monomers; and the like. One of these may be used alone, or two or more of these may be used in combination at any ratio.

  The content ratio of the arbitrary structural unit in the copolymer A contained in the water-soluble polymer is preferably 0% by weight to 10% by weight, more preferably 0% by weight to 5% by weight.

[1.4.7. Arbitrary polymer)
As the water-soluble polymer, one type of polymer may be used alone, or two or more types of polymers having different structures may be used in combination at any ratio. For example, one type of copolymer A may be used alone. Further, for example, two or more types of copolymers A having different structures may be used in combination at an arbitrary ratio. Furthermore, for example, the copolymer A and the copolymer A may be used in combination with any water-soluble polymer having a different structure.

  However, from the viewpoint of effectively exhibiting the advantages of using the copolymer A, the amount of the copolymer A is preferably 70 parts by weight or more, more preferably 100 parts by weight relative to the total amount of the water-soluble polymer. It is 80 parts by weight or more, preferably 100 parts by weight or less.

[1.4.8. Properties and amount of water-soluble polymer]
The weight average molecular weight of the copolymer A contained in the water-soluble polymer is usually smaller than the polymer that forms a particulate binder such as the copolymer B, for example, preferably 100 or more, more preferably 500 or more. Particularly preferably, it is 1,000 or more, preferably 500,000 or less, more preferably 250,000 or less, and particularly preferably 100,000 or less. A stable protective layer that covers the positive electrode active material by increasing the strength of the copolymer A by setting the weight average molecular weight of the copolymer A contained in the water-soluble polymer to be not less than the lower limit of the above range. Can be formed. For this reason, for example, the dispersibility of the positive electrode active material and the high-temperature storage characteristics of the secondary battery can be improved. On the other hand, the copolymer A can be softened by setting it to not more than the upper limit of the above range. For this reason, for example, suppression of the swelling of the positive electrode and improvement of the adhesion of the positive electrode active material layer to the current collector can be achieved.
Here, the weight average molecular weight of the copolymer A contained in the water-soluble polymer is polyethylene using a solution obtained by dissolving 0.85 g / ml sodium nitrate in a 10% by volume aqueous solution of acetonitrile by GPC. It can be obtained as a value in terms of oxide.

  The glass transition temperature of the water-soluble polymer is usually 0 ° C. or higher, preferably 5 ° C. or higher, and is usually 100 ° C. or lower, preferably 50 ° C. or lower. When the glass transition temperature of the water-soluble polymer is in the above range, both the adhesion and flexibility of the positive electrode can be achieved. From the same viewpoint, the glass transition temperature of the copolymer A is preferably within the above range. The glass transition temperature of the water-soluble polymer and copolymer A can be adjusted by combining various monomers.

The ionic conductivity of the copolymer A contained in the water-soluble polymer is usually 1 × 10 ⁻⁵ S / cm or more, preferably 2 × 10 ⁻⁵ S / cm or more, more preferably 5 × 10 ⁻⁵ S. / Cm or more, usually 1 × 10 ⁻³ S / cm or less, preferably 1 × 10 ⁻³ S / cm or less, more preferably 1 × 10 ⁻³ S / cm or less. By setting the ionic conductivity of the copolymer A contained in the water-soluble polymer to be not less than the lower limit of the above range, the low-temperature output characteristics of the secondary battery can be improved. Moreover, by making it into an upper limit value or less, the adhesiveness with respect to the electrical power collector of a positive electrode active material layer can be improved, and the durability of a positive electrode can be improved by extension.

Here, the “ionic conductivity of the copolymer A contained in the water-soluble polymer” refers to the ionic conductivity measured under the following predetermined conditions.
The aqueous solution of the copolymer A contained in the water-soluble polymer is poured into a silicon container so that the thickness after drying is 1 mm, and dried at room temperature for 72 hours to produce a 1 cm × 1 cm square film. This film is immersed in a 1.0 mol / L LiPF ₆ solution (solvent: a mixture of ethylene carbonate / diethyl carbonate in a 1/2 volume ratio) at 60 ° C. for 72 hours. The thickness d of the film after immersion is measured. Thereafter, the film is sandwiched between two copper foils, the resistance R is measured from the AC impedance at 0.001 Hz to 1000000 Hz, and ionic conductivity = R × 1 / d is calculated. This value is referred to as “ionic conductivity of the copolymer A contained in the water-soluble polymer”.

  The swelling ratio between the degree of swelling V1 of the water-soluble polymer measured under the predetermined conditions and the degree of swelling V0 of the particulate binder measured under the same conditions is preferably 1.0 to 2.0. More preferably, it is -1.5, and it is especially preferable that it is 1.0-1.2. Further, the swelling ratio between the degree of swelling of the copolymer A and the degree of swelling V0 of the particulate polymer, the degree of swelling V1 of the water-soluble polymer, and the degree of swelling of the copolymer B, measured under the predetermined conditions. It is preferable that the swelling ratio and the swelling ratio of the copolymer A and the swelling ratio of the copolymer B are also within the above ranges. When the swelling ratio is not less than the lower limit of the above range, the low-temperature output characteristics of the secondary battery can be improved. Moreover, by being below an upper limit, the distance between positive electrode active materials can be made into an appropriate narrow range, and favorable durability can be obtained.

Here, the swelling degree is a swelling degree with respect to a liquid having a solubility parameter of 8 to 13 (cal / cm ³ ) ^1/2 . A specific method for measuring the degree of swelling is as follows.
An aqueous dispersion of a particulate binder and an aqueous solution of a water-soluble polymer are each poured into a silicon container so that the thickness after drying is 1 mm, and dried at room temperature for 72 hours to form a 1 cm × 1 cm square film. Prepare and measure the weight M0. Thereafter, the film is immersed in a predetermined liquid at 60 ° C. for 72 hours, the weight M1 of the film after immersion is measured, and the degree of swelling is calculated from the formula (M1-M0) / M0. The ratio V1 / V0 is calculated from the swelling degree V0 of the particulate binder and the swelling degree V1 of the water-soluble polymer, and this value is defined as the swelling degree ratio. In the same manner, the swelling ratio between the swelling degree of the copolymer A and the swelling degree V0 of the particulate polymer, the swelling degree ratio between the swelling degree V1 of the water-soluble polymer and the swelling degree of the copolymer B, In addition, the ratio of the degree of swelling of the copolymer A and the degree of swelling of the copolymer B can also be measured.

Examples of the liquid having a predetermined solubility parameter for measuring the degree of swelling include a 1.0 mol / L LiPF ₆ solution (solvent: a mixture of ethylene carbonate / diethyl carbonate in a 1/2 volume ratio, solubility parameter 10. 8 (cal / cm ³ ) ^1/2 ). Moreover, when the electrolyte solution of the secondary battery of this invention has the solubility parameter of the said range, you may measure swelling degree using the said electrolyte solution.

  The values of the swelling degree V0 of the particulate binder and the swelling degree V1 of the water-soluble polymer are not particularly limited, but are preferably in the following ranges, respectively. That is, the degree of swelling V0 of the particulate binder is preferably 1.0 to 3.0 times, and more preferably 1.0 to 2.0 times. The degree of swelling V1 of the water-soluble polymer is preferably 1.0 times to 5.0 times, and more preferably 1.0 times to 4.0 times.

  The amount of the water-soluble polymer is such that the weight ratio of the particulate binder to the water-soluble polymer is usually within the range of 99.5 / 0.5 to 95/5 for the “particulate binder / water-soluble polymer”. To. Specifically, the weight ratio represented by “particulate binder / water-soluble polymer” is usually 95/5 or more, preferably 96/4 or more, more preferably 97/3 or more, and usually 99.5 / 0.5 or less, preferably 99/1 or less, more preferably 98.5 / 1.5 or less. By setting the weight ratio represented by “particulate binder / water-soluble polymer” to be not less than the lower limit of the above range, the adhesion of the positive electrode active material layer to the current collector can be enhanced. Moreover, durability can be improved by setting it as below an upper limit.

  Further, the weight ratio of the particulate binder to the copolymer A “particulate binder / copolymer A” is usually 95/5 or more, preferably 96/4 or more, more preferably 97/3 or more, and usually It is 99.5 / 0.5 or less, preferably 99/1 or less, more preferably 98.5 / 1.5 or less. By making this weight ratio more than the lower limit of the said range, the adhesiveness of the positive electrode active material layer with respect to a collector can be improved. Moreover, durability can be improved by setting it as below an upper limit.

[1.4.9. Production method of copolymer A]
The polymer A contained in the water-soluble polymer includes, for example, a monomer containing an acidic functional group and a (meth) acrylate monomer, and a crosslinkable monomer, a reactive interface as necessary. A monomer composition containing an activator monomer, a fluorine-containing (meth) acrylic acid ester monomer, and an optional monomer can be produced by polymerizing in an aqueous solvent. At this time, the ratio of each monomer in the monomer composition is usually the same as the content ratio of the structural unit in the copolymer A.

As the aqueous solvent, for example, the same solvent as in the production of the particulate binder may be used.
As the polymerization method, any method such as a solution polymerization method, a suspension polymerization method, a bulk polymerization method, and an emulsion polymerization method may be used. Furthermore, as a polymerization method, any method such as ionic polymerization, radical polymerization, and living radical polymerization may be used.
The polymerization temperature and the polymerization time can be arbitrarily selected depending on the polymerization method and the kind of the polymerization initiator. Usually, the polymerization temperature is about 30 ° C. or more, and the polymerization time is about 0.5 to 30 hours.
Further, for example, additives such as amines may be used as a polymerization aid.

  Thereby, an aqueous solution in which the copolymer A is usually dissolved in an aqueous solvent is obtained. Copolymer A may be taken out from the aqueous solution thus obtained. However, normally, a slurry composition for a positive electrode is produced using the copolymer A in a state dissolved in an aqueous solvent, and a positive electrode can be produced using the slurry composition.

  The aqueous solution containing the copolymer A in an aqueous solvent is usually acidic. Therefore, it may be alkalized to pH 7 to pH 13 as necessary. Thereby, the handleability of aqueous solution can be improved and the coating property of the slurry composition for manufacturing the positive electrode for secondary batteries can be improved. Examples of the method for alkalinizing to pH 7 to pH 13 include alkaline metal aqueous solutions such as lithium hydroxide aqueous solution, sodium hydroxide aqueous solution and potassium hydroxide aqueous solution; alkaline earth metal aqueous solutions such as calcium hydroxide aqueous solution and magnesium hydroxide aqueous solution; The method of mixing aqueous alkali solution, such as aqueous ammonia solution, is mentioned. One kind of the alkaline aqueous solution may be used alone, or two or more kinds may be used in combination at any ratio.

[1.5. (Optional ingredients)
The positive electrode for a secondary battery of the present invention may contain any component other than the above-described positive electrode active material, conductive additive, particulate binder and water-soluble polymer as long as the effects of the present invention are not significantly impaired. . For example, the positive electrode for a secondary battery of the present invention may contain additives such as a reinforcing material, a dispersant, a leveling agent, and an antioxidant. Usually, these optional components are contained in the positive electrode active material layer. Moreover, arbitrary components may be used individually by 1 type, and may be used combining two or more types by arbitrary ratios. The additive includes a water-soluble polymer compound derived from a natural product such as carboxymethylcellulose (CMC), and this CMC can be used as a binder, for example.

Examples of the reinforcing material include various inorganic and organic spherical, plate-like, rod-like, or fibrous fillers. By using the reinforcing material, a tough and flexible positive electrode can be obtained, and excellent long-term cycle characteristics can be exhibited in the secondary battery.
The amount of the reinforcing material used is usually 0.01 parts by weight or more, preferably 1 part by weight or more, and usually 20 parts by weight or less, preferably 10 parts by weight or less, with respect to 100 parts by weight of the positive electrode active material. By keeping the amount of the reinforcing material in the above range, a high capacity and a high load characteristic can be realized.

Examples of the dispersant include an anionic compound, a cationic compound, a nonionic compound, and a polymer compound. A specific dispersing agent is selected according to the positive electrode active material and conductive support agent to be used. By using the dispersant, the stability of the slurry composition for the positive electrode is improved and a smooth positive electrode is obtained, so that the battery capacity of the secondary battery can be increased.
The amount of the dispersant is usually 0.1 parts by weight or more, preferably 0.5 parts by weight or more, more preferably 0.8 parts by weight or more, and usually 10 parts by weight with respect to 100 parts by weight of the positive electrode active material. The amount is preferably 5 parts by weight or less, more preferably 2 parts by weight or less.

Examples of the leveling agent include surfactants such as alkyl surfactants, silicon surfactants, fluorine surfactants, and metal surfactants. By using the leveling agent, it is possible to prevent the repelling that occurs when the slurry composition is applied to the current collector, and to improve the smoothness of the positive electrode.
The amount of the leveling agent is preferably 0.01 to 10 parts by weight with respect to 100 parts by weight of the positive electrode active material. When the leveling agent is in the above range, the productivity, smoothness, and battery characteristics during the production of the positive electrode are excellent.

Examples of the antioxidant include a phenol compound, a hydroquinone compound, an organic phosphorus compound, a sulfur compound, a phenylenediamine compound, and a polymer type phenol compound. The polymer type phenol compound is a polymer having a phenol structure in the molecule, and a polymer type phenol compound having a weight average molecular weight of usually 200 or more, preferably 600 or more and usually 1000 or less, preferably 700 or less is used.
The amount of the antioxidant is usually 0.01 parts by weight or more, preferably 0.02 parts by weight or more, and usually 10 parts by weight or less, preferably 5 parts by weight or less with respect to 100 parts by weight of the positive electrode active material. .

[1.6. Current collector and electrode active material layer]
As described above, the positive electrode active material, the conductive additive, the particulate binder, the water-soluble polymer, and any components included as necessary are usually contained in the positive electrode active material layer. The positive electrode active material layer is usually provided on the surface of the current collector. Under the present circumstances, the positive electrode active material layer may be provided in the single side | surface of the electrical power collector, and may be provided in both surfaces.

  The current collector is not particularly limited as long as it is a material having electrical conductivity and electrochemical durability. From the viewpoint of heat resistance, the current collector is preferably made of metal, such as iron, copper, aluminum, nickel, stainless steel, titanium, tantalum, gold, or platinum. Among these, aluminum is particularly preferable for the positive electrode. One type of current collector material may be used alone, or two or more types may be used in combination at any ratio.

The shape of the current collector is not particularly limited, but a sheet shape having a thickness of about 0.001 mm to 0.5 mm is preferable.
In order to increase the adhesive strength of the positive electrode active material layer, the current collector is preferably used after being subjected to a roughening treatment on the surface. Examples of the roughening method include a mechanical polishing method, an electrolytic polishing method, and a chemical polishing method. In the mechanical polishing method, usually, for example, a polishing cloth with a fixed abrasive particle, a grindstone, an emery buff, a wire brush provided with a steel wire, or the like is used. Further, an intermediate layer may be formed on the surface of the current collector in order to increase the adhesive strength and conductivity of the positive electrode active material layer.

  The thickness of the positive electrode active material layer is usually 5 μm or more, preferably 10 μm or more, and usually 300 μm or less, preferably 250 μm or less. When the thickness of the positive electrode active material layer is in the above range, both load characteristics and energy density are high.

The water content in the positive electrode active material layer is preferably 1000 ppm or less, and more preferably 500 ppm or less. By setting the water content of the positive electrode active material layer within the above range, a positive electrode for a secondary battery having excellent durability can be realized. The water content can be measured by a known method such as the Karl Fischer method.
Such a low water content can be achieved, for example, by appropriately adjusting the composition of the structural unit in the water-soluble polymer such as the copolymer A. In particular, the water content can be reduced by including the fluorine-containing (meth) acrylic acid ester monomer unit in the water-soluble polymer in the above-described ratio.

[2. Method for producing positive electrode for secondary battery]
The positive electrode for a secondary battery of the present invention is prepared, for example, by preparing a slurry composition for producing a positive electrode active material layer constituting the positive electrode for a secondary battery and applying the slurry composition on a current collector. It can be manufactured by a manufacturing method including a step of drying the coated material to obtain the positive electrode active material layer.

  The slurry composition is a liquid composition containing a positive electrode active material, a conductive additive, a particulate binder, a water-soluble polymer and water, and optional components as necessary. The ratio of the positive electrode active material, conductive additive, particulate binder, water-soluble polymer, and optional components in the slurry composition is usually the same as the ratio of each component contained in the positive electrode active material layer.

  The slurry composition contains water as a solvent. Moreover, you may use the mixed solvent which combined water and the organic solvent as needed. In the slurry composition, usually, the positive electrode active material, the conductive additive and the particulate binder are dispersed in a solvent, and the water-soluble polymer is dissolved in the solvent.

  The amount of the solvent such as water is such that the content ratio of the positive electrode active material contained in the slurry composition is preferably 50% by weight or more, more preferably 60% by weight or more, and preferably 95% by weight or less, more preferably 90%. It is the range which becomes weight% or less. By setting the content ratio of the positive electrode active material in the slurry composition within the above range, a good slurry composition and positive electrode can be produced.

  The viscosity of the slurry composition is preferably 10 mPa · s or more, more preferably 100 mPa · s or more, preferably 100,000 mPa · s or less, more preferably from the viewpoint of the temporal stability and coating properties of the slurry composition. Is 50,000 mPa · s or less. The viscosity is a value measured using a B-type viscometer at 25 ° C. and a rotation speed of 60 rpm.

  The pH of the slurry composition is usually 7 or more, preferably 8 or more, and usually 12 or less, preferably 11.5 or less. By adjusting the pH of the slurry composition to the above range, the stability of the slurry composition can be increased, and the current collector can be prevented from being corroded.

  As a method of adjusting the pH of the slurry composition, for example, a method of adjusting the pH of the slurry composition by washing the positive electrode active material before preparing the slurry composition, bubbling carbon dioxide gas to the prepared slurry composition Examples thereof include a method for adjusting pH and a method for adjusting using a pH adjusting agent. Among these, it is preferable to use a pH adjuster.

Although the kind of pH adjuster is not specifically limited, It is preferable that it is a water-soluble substance which shows acidity. Either a strong acid or a weak acid may be used.
Examples of water-soluble substances that exhibit weak acidity include organic compounds having acid groups such as carboxylic acid groups, phosphoric acid groups, and sulfonic acid groups. Among these, an organic compound having a carboxylic acid group is particularly preferably used. Specific examples of the compound having a carboxylic acid group include succinic acid, phthalic acid, maleic acid, succinic anhydride, phthalic anhydride, maleic anhydride and the like. These compounds can be made into acid anhydrides having little influence in the secondary battery by drying.
Examples of water-soluble substances that exhibit strong acidity include hydrochloric acid, nitric acid, sulfuric acid, and acetic acid.

Among the pH adjusting agents, it is preferable that the agent is decomposed or volatilized in the drying step of the slurry composition. In this case, no pH adjuster remains in the obtained positive electrode. Examples of such a pH adjuster include acetic acid and hydrochloric acid.
Moreover, a pH adjuster may be used individually by 1 type, and may be used combining two or more types by arbitrary ratios.

  The amount of the pH adjusting agent is preferably 0.1 parts by weight or more and preferably 0.5 parts by weight or less with respect to 100 parts by weight of the positive electrode mixture. Here, the positive electrode mixture is the total amount of materials constituting the positive electrode active material layer including the positive electrode active material, the conductive additive, the particulate binder, the water-soluble polymer, and optional components. By setting the amount of the pH adjusting agent to be not less than the lower limit of the above range, the pH of the slurry composition can be stably improved. Here, the improvement in pH means that the acidity of the pH becomes stronger when an acid is used as the pH adjuster. Moreover, the pH adjuster is sufficient below the upper limit of the above range.

  The slurry composition is obtained by mixing a positive electrode active material, a conductive additive, a particulate binder, a water-soluble polymer and water, and optional components used as necessary. At this time, the mixing method and the mixing order are not limited. Since the slurry composition of the present invention uses a water-soluble polymer, it is possible to highly disperse the positive electrode active material, the conductive auxiliary agent and the particulate binder in any mixing method and mixing order. is there.

  As the mixing apparatus, for example, a bead mill, a ball mill, a roll mill, a sand mill, a pigment disperser, a crusher, an ultrasonic disperser, a homogenizer, a planetary mixer, a fill mix, or the like may be used. Among these, it is particularly preferable to use a ball mill, a roll mill, a pigment disperser, a crusher, or a planetary mixer because dispersion at a high concentration is possible.

  The pH of the slurry composition may be adjusted at any time as long as the slurry composition is being produced. Especially, after adjusting a slurry composition to desired solid content concentration, it is preferable to adjust pH with a pH adjuster. By adjusting the pH after adjusting the slurry composition to a predetermined solid content concentration, it is possible to easily adjust the pH while preventing dissolution of the positive electrode active material.

  After preparing the slurry composition, the slurry composition is applied onto the current collector. At this time, the slurry composition may be applied to only one side of the current collector or may be applied to both sides. Since the slurry composition of the present invention is excellent in dispersibility, uniform application is easy.

  There is no restriction | limiting in the coating method, For example, methods, such as a doctor blade method, a zip method, a reverse roll method, a direct roll method, a gravure method, an extrusion method, a brush coating method, are mentioned. By applying the slurry composition, a film of the slurry composition is formed on the surface of the current collector. At this time, the thickness of the slurry composition film can be appropriately set according to the target thickness of the positive electrode active material layer.

  Thereafter, a liquid such as water is removed from the film of the slurry composition by drying. Thereby, the positive electrode active material layer containing a positive electrode active material, a conductive support agent, a particulate-form binder, and a water-soluble polymer is formed in the surface of an electrical power collector, and the positive electrode for secondary batteries of this invention is obtained.

  The drying temperature and drying time are not particularly limited. For example, you may heat-process at 120 degreeC or more for 1 hour or more. Examples of the drying method include drying with warm air, hot air, and low-humidity air; vacuum drying; drying by irradiation with infrared rays, far infrared rays, electron beams, and the like.

  After forming the positive electrode active material layer on the surface of the current collector, it is preferable to apply pressure treatment to the positive electrode active material layer using a die press or a roll press. By the pressure treatment, the porosity of the positive electrode can be lowered. The porosity is preferably 5% or more, more preferably 7% or more, preferably 15% or less, more preferably 13% or less. By setting the porosity to be equal to or higher than the lower limit value of the above range, a high volume capacity can be easily obtained, and the positive electrode active material layer can be hardly separated from the current collector. Moreover, high charging efficiency and discharge efficiency are obtained by setting it as an upper limit or less.

  Furthermore, when the positive electrode active material layer includes a curable polymer, the polymer may be cured after the positive electrode active material layer is formed.

  Moreover, a powder molding method is mentioned as an example of another manufacturing method of the positive electrode of this invention. The powder molding method is the following method. That is, the slurry composition for manufacturing the positive electrode for secondary batteries is prepared. Thereafter, composite particles containing a positive electrode active material, a conductive additive, a particulate binder, and a water-soluble polymer are prepared from the slurry composition. Furthermore, the composite particles are supplied onto a current collector, and if desired, are further roll-pressed and formed to form a positive electrode active material layer to obtain a battery positive electrode. At this time, as the slurry composition, the same slurry composition as described above may be used.

[3. Secondary battery]
The secondary battery of the present invention includes a positive electrode, a negative electrode, an electrolytic solution, and a separator. Moreover, in the secondary battery of this invention, a positive electrode is a positive electrode for secondary batteries of this invention. Since the secondary battery of the present invention uses the positive electrode containing the water-soluble polymer according to the present invention, the secondary battery is excellent in storage characteristics in a high temperature environment, and is usually excellent in output characteristics and cycle characteristics in a high temperature environment.

  The secondary battery of the present invention may be any of a lithium ion secondary battery, a nickel hydride secondary battery, and the like. Among these, lithium ion secondary batteries are preferable because performance improvement effects such as improvement of long-term cycle characteristics and output characteristics are particularly remarkable. Hereinafter, the case where the secondary battery of the present invention is a lithium ion secondary battery will be described.

[3.1. Electrolyte)
As an electrolytic solution for a lithium ion secondary battery, for example, a nonaqueous electrolytic solution in which a supporting electrolyte is dissolved in a nonaqueous solvent is used. As the supporting electrolyte, a lithium salt is usually used. Examples of the lithium salt include LiPF ₆ , LiAsF ₆ , LiBF ₄ , LiSbF ₆ , LiAlCl ₄ , LiClO ₄ , CF ₃ SO ₃ Li, C ₄ F ₉ SO ₃ Li, CF ₃ COOLi, (CF ₃ CO) ₂ NLi , (CF ₃ SO ₂ ) ₂ NLi, (C ₂ F ₅ SO ₂ ) NLi, and the like. Among these, LiPF ₆ , LiClO ₄ , and CF ₃ SO ₃ Li that are easily soluble in a solvent and exhibit a high degree of dissociation are preferable. One of these may be used alone, or two or more of these may be used in combination at any ratio. Since the lithium ion conductivity increases as the supporting electrolyte having a higher degree of dissociation is used, the lithium ion conductivity can be adjusted depending on the type of the supporting electrolyte.

  The concentration of the supporting electrolyte in the electrolytic solution is usually 1% by weight or more, preferably 5% by weight or more, and usually 30% by weight or less, preferably 20% by weight or less. Moreover, according to the kind of supporting electrolyte, you may use normally by the density | concentration of 0.5 mol / L-2.5 mol / L. If the concentration of the supporting electrolyte is too low or too high, the ionic conductivity may decrease.

  The non-aqueous solvent is not particularly limited as long as it can dissolve the supporting electrolyte. Examples of non-aqueous solvents include carbonates such as dimethyl carbonate (DMC), ethylene carbonate (EC), diethyl carbonate (DEC), propylene carbonate (PC), butylene carbonate (BC), methyl ethyl carbonate (MEC); and esters such as γ-butyrolactone and methyl formate; ethers such as 1,2-dimethoxyethane and tetrahydrofuran; sulfur-containing compounds such as sulfolane and dimethyl sulfoxide; Among these, carbonates are preferable because they have a high dielectric constant and a wide stable potential region. A non-aqueous solvent may be used individually by 1 type, and may be used combining two or more types by arbitrary ratios. The lower the viscosity of the non-aqueous solvent, the higher the lithium ion conductivity. Therefore, the lithium ion conductivity can be adjusted depending on the type of solvent.

  Moreover, the electrolyte solution may contain an additive. Examples of the additive include carbonate compounds such as vinylene carbonate (VC). An additive may be used individually by 1 type and may be used combining two or more types by arbitrary ratios.

Moreover, as electrolytes other than the above, for example, polymer electrolytes such as polyethylene oxide and polyacrylonitrile; gel polymer electrolytes in which the polymer electrolyte is impregnated with an electrolyte; inorganic solid electrolytes such as LiI and Li ₃ N; Also good.

[3.2. Negative electrode)
As the negative electrode, one having a current collector and a negative electrode active material layer formed on the surface of the current collector is usually used.

  As the negative electrode current collector, for example, the same as the positive electrode current collector may be used. Among these, copper is preferable as the current collector for the negative electrode.

The negative electrode active material layer is a layer containing a negative electrode active material and a binder.
Examples of the negative electrode active material include carbonaceous materials such as amorphous carbon, graphite, natural graphite, mesocarbon microbeads, and pitch-based carbon fibers; conductive polymers such as polyacene; silicon, tin, zinc, manganese, iron, nickel Metals or alloys thereof; oxides or sulfates of the metals or alloys; metal lithium; lithium alloys such as Li—Al, Li—Bi—Cd, Li—Sn—Cd; lithium transition metal nitrides; silicon, etc. Is mentioned. Further, as the negative electrode active material, a material obtained by attaching a conductive additive to the surface of the negative electrode active material particles by, for example, a mechanical modification method may be used. Moreover, a negative electrode active material may be used individually by 1 type, and may be used combining two or more types by arbitrary ratios.

  The particle size of the particles of the negative electrode active material is usually selected as appropriate in consideration of the other components of the secondary battery of the present invention. Among these, from the viewpoint of improving battery characteristics such as initial efficiency, load characteristics, and cycle characteristics, the 50% volume cumulative diameter of the negative electrode active material particles is preferably 1 μm or more, more preferably 15 μm or more, and preferably 50 μm or less. More preferably, it is 30 μm or less.

  The content ratio of the negative electrode active material in the negative electrode active material layer is preferably 90% by weight or more, more preferably 95% by weight or more, preferably 99.9% by weight or less, more preferably 99% by weight or less. By setting the content of the negative electrode active material in the above range, the capacity of the secondary battery of the present invention can be increased, and the flexibility of the negative electrode and the binding property between the current collector and the negative electrode active material layer are improved. Can be made.

  As the binder used in the negative electrode active material layer, for example, the same binder as the particulate binder used in the positive electrode active material layer may be used. In addition, for example, polymers such as polyethylene, polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), polyacrylic acid derivatives, polyacrylonitrile derivatives; acrylics A soft polymer such as a soft polymer, a diene-based soft polymer, an olefin-based soft polymer, or a vinyl-based soft polymer may be used. Moreover, these may be used individually by 1 type and may be used combining two or more types by arbitrary ratios.

  In addition, the negative electrode active material layer may contain components other than the negative electrode active material and the binder as necessary. When the example is given, the arbitrary components etc. which may be contained in the positive electrode active material layer of the positive electrode for secondary batteries of this invention are mentioned. Moreover, these may be used individually by 1 type and may be used combining two or more types by arbitrary ratios.

  The total thickness of the current collector and the negative electrode active material layer is usually 5 μm or more, preferably 10 μm or more, and usually 300 μm or less, preferably 250 μm or less. When the thickness of the negative electrode is in the above range, both load characteristics and energy density can be improved.

  The negative electrode, for example, similarly to the positive electrode for secondary batteries of the present invention, prepare a slurry composition for the negative electrode containing a negative electrode active material, a binder and water, and form a layer of the slurry composition on a current collector, The layer may be produced by drying.

[3.3. separator〕
As the separator, for example, a polyolefin resin such as polyethylene or polypropylene, or a microporous film or nonwoven fabric containing an aromatic polyamide resin; a porous resin coat containing an inorganic ceramic powder; Specific examples include microporous membranes made of polyolefin resins (polyethylene, polypropylene, polybutene, polyvinyl chloride), and resins such as mixtures or copolymers thereof; polyethylene terephthalate, polycycloolefin, polyether sulfone, polyamide, Examples thereof include a microporous film made of a resin such as polyimide, polyimide amide, polyaramid, nylon, and polytetrafluoroethylene; a polyolefin fiber woven or non-woven fabric thereof; and an aggregate of insulating substance particles. Among these, a microporous film made of a polyolefin-based resin is preferable because the entire separator can be thinned to increase the active material ratio in the secondary battery and increase the capacity per volume.

  The thickness of the separator is usually 0.5 μm or more, preferably 1 μm or more, and is usually 40 μm or less, preferably 30 μm or less, more preferably 10 μm or less. Within this range, the resistance due to the separator in the secondary battery becomes small, and the workability when manufacturing the secondary battery is excellent.

[3.4. Secondary battery manufacturing method)
As a specific method for manufacturing a secondary battery, for example, a positive electrode and a negative electrode are overlapped via a separator, and this is wound into a battery container according to the shape of the battery. The method of injecting and sealing is mentioned. Further, if necessary, an expanded metal; an overcurrent prevention element such as a fuse or a PTC element; a lead plate or the like may be inserted to prevent an increase in pressure inside the battery or overcharge / discharge. The shape of the secondary battery may be any of a coin shape, a button shape, a sheet shape, a cylindrical shape, a square shape, a flat shape, and the like.

Hereinafter, the present invention will be specifically described with reference to examples. However, the present invention is not limited to the following examples, and may be arbitrarily modified within the scope of the claims of the present invention and its equivalents.
In the following description, “%” and “part” representing amounts are based on weight unless otherwise specified. In addition, the operations described below were performed under normal temperature and normal pressure conditions unless otherwise specified.

〔Evaluation method〕
1. Adhesive strength The positive electrodes for lithium ion secondary batteries produced in the examples and comparative examples were cut into rectangles having a length of 100 mm and a width of 10 mm to obtain test pieces. Cellophane tape was affixed on the surface of the positive electrode active material layer of this test piece with the surface of the positive electrode active material layer facing down. At this time, a cellophane tape defined in JIS Z1522 was used. Moreover, the cellophane tape was fixed to the test bench. Then, the stress when one end of the current collector was pulled vertically upward at a pulling speed of 50 mm / min and peeled was measured. This measurement was performed 3 times, the average value was calculated | required, and the said average value was made into peel strength. The higher the peel strength, the greater the binding force of the positive electrode active material layer to the current collector, that is, the higher the adhesion strength.

2. Coatability The positive electrode slurry composition produced in Examples and Comparative Examples was applied on a 20 μm thick aluminum foil as a current collector so that the film thickness after drying was about 200 μm, and then dried. I let you. This drying was performed by conveying the aluminum foil in an oven at 60 ° C. at a speed of 0.5 m / min for 2 minutes. Thereafter, heat treatment was performed at 120 ° C. for 2 minutes to obtain a positive electrode. The obtained positive electrode was cut out with a size of 10 cm × 10 cm, and the number of pinholes having a diameter of 0.1 mm or more was visually measured. The smaller the number of pinholes, the better the coatability.

3. Moisture content of the electrode plate The moisture content in the positive electrode was measured by the Karl Fischer coulometric titration method for the positive electrode for lithium ion secondary batteries produced in Examples and Comparative Examples. The measurement was performed with a moisture measuring device (manufactured by Kyoto Electronics Co., Ltd., anolyte: Aquamicron AX, catholyte: Aquamicron CXU). Thereby, the moisture content of the electrode plate (weight per unit weight of the electrode active material layer, unit “ppm”) was measured.

4). Electrolyte injection property The positive electrode for lithium ion secondary batteries produced in Examples and Comparative Examples was punched out to a diameter of 12 mm and immersed in 10 ml of an electrolyte solvent (propylene carbonate) in a dry room for 5 minutes. The liquid absorption was calculated from the change in weight.
Weight before immersion = A (mg)
Weight after immersion = B (mg)
Liquid absorption = BA

5). High-temperature storage characteristics After the lithium-ion secondary battery of the laminate type cell is allowed to stand for 24 hours, the charge / discharge operation is carried out for 2 cycles at a charge / discharge rate of 3V to 4.2V, 0.1C. The capacity was set as the initial discharge capacity C0. Further, the battery was charged at 4.2 V, stored at 60 ° C. for 4 weeks, discharged to 3 V, and the discharge capacity C1 after storage at high temperature was measured. The high-temperature storage characteristics were evaluated by a capacity change rate ΔCS represented by ΔCS = C1 / C0 × 100 (%). It shows that it is excellent in a high temperature storage characteristic, so that the value of this capacity | capacitance change rate (DELTA) CS is high.

[Example 1]
(1-1. Production of water-soluble polymer)
In a 5 MPa pressure vessel with a stirrer, 31 parts of methacrylic acid (acidic functional group-containing monomer), 0.8 part of ethylene dimethacrylate (crosslinkable monomer), 2,2,2-trifluoroethyl methacrylate (fluorine-containing ( (Meth) acrylic acid ester monomer) 7.5 parts, butyl acrylate ((meth) acrylic acid ester monomer) 59.2 parts, polyoxyalkylene alkenyl ether ammonium sulfate (reactive surfactant monomer, manufactured by Kao) , Trade name "Latemul PD-104") 1.5 parts, ion-exchanged water 150 parts, and potassium persulfate (polymerization initiator) 0.5 part, after sufficiently stirring, warm to 60 ℃ Polymerization was started. When the polymerization conversion rate reached 96%, the reaction was stopped by cooling to obtain a mixture containing a water-soluble polymer (copolymer A). 10% aqueous ammonia was added to the mixture containing the water-soluble polymer to adjust the pH to 8 to obtain an aqueous solution containing the desired water-soluble polymer.

(1-2. Production of binder composition for positive electrode)
To polymerization can A, 10.65 parts of 2-ethylhexyl acrylate, 1.15 parts of acrylonitrile, 0.12 part of sodium lauryl sulfate, and 79 parts of ion-exchanged water were added. To this polymerization can A, 0.2 parts of ammonium persulfate and 10 parts of ion-exchanged water were further added as a polymerization initiator, heated to 60 ° C., and stirred for 90 minutes.

  In another polymerization vessel B, 67 parts of 2-ethylhexyl acrylate, 19 parts of acrylonitrile, 2.0 parts of methacrylic acid, 0.2 part of allyl methacrylate, 0.7 part of sodium lauryl sulfate and 46 parts of ion-exchanged water were added. Stir to make an emulsion. This emulsion was sequentially added from polymerization vessel B to polymerization vessel A over about 180 minutes. Thereafter, the mixture was stirred for about 120 minutes, and when the monomer consumption reached 95%, the reaction was terminated by cooling. Then, pH was adjusted with 4% NaOH aqueous solution, and the composition containing the particulate binder A (copolymer B) was obtained.

The obtained particulate binder A had a glass transition temperature of −32 ° C. and a number average particle size of 0.15 μm. The content ratio of the (meth) acrylic acid ester monomer unit in the particulate binder A is 77.6%, the structural unit of the vinyl monomer having an acid component is 2.0%, and the (meth) acrylonitrile monomer The content ratio of the units was 20.2%, and the content ratio of the structural units of allyl methacrylate was 0.2%.
To the composition containing the particulate binder A, a 5% aqueous sodium hydroxide solution was added to adjust the pH to 8. Then, the unreacted monomer was removed by heating under reduced pressure. Then, it cooled to 30 degrees C or less, and obtained the water dispersion containing a desired particulate binder.

  The aqueous solution containing the water-soluble polymer obtained in (1-1. Production of water-soluble polymer) was diluted with ion-exchanged water to adjust the concentration to 5%. This was mixed with the aqueous dispersion containing the particulate binder obtained above so that the particulate binder / water-soluble polymer = 95/5 (weight ratio) corresponding to the solid content. A composition was obtained.

(1-3. Production of positive electrode)
100 parts of LiCoO ₂ having a volume average particle diameter of 20 μm and a layered structure as a positive electrode active material, 2 parts of acetylene black as a conductive assistant, and a 1% aqueous solution of carboxymethyl cellulose as a dispersant (manufactured by Daiichi Kogyo Seiyaku “ BSH-6 ") 1 part by solid content, and the binder composition obtained in the above (1-2. Production of binder composition for positive electrode) as a particulate binder and water-soluble polymer. 1 part and ion exchange water were mixed. Here, the solid content of the binder composition means a particulate binder and a water-soluble polymer. These were mixed by a planetary mixer to prepare a slurry composition for a positive electrode. At this time, the amount of ion-exchanged water was such that the total solid concentration of the slurry composition was 40%.

  The slurry composition for positive electrode was applied on a 20 μm-thick aluminum foil as a current collector by a comma coater so that the film thickness after drying was about 200 μm and dried. This drying was performed by conveying the aluminum foil in an oven at 60 ° C. at a speed of 0.5 m / min for 2 minutes. Thereafter, heat treatment was performed at 120 ° C. for 2 minutes to obtain a positive electrode having a positive electrode active material. With respect to the positive electrode thus obtained, adhesion strength, coatability, moisture content of the electrode plate and electrolyte solution pouring property were measured.

(1-4. Production of slurry composition for negative electrode)
In a planetary mixer with a disper, 100 parts of artificial graphite (average particle size: 24.5 μm) having a specific surface area of 4 m ² / g as a negative electrode active material, and a 1% aqueous solution of carboxymethyl cellulose as a dispersant (Daiichi Kogyo Seiyaku Co., Ltd.) 1 part of “BSH-12” produced in the same manner as solid content was added, and the solid content concentration was adjusted to 55% with ion-exchanged water, and then mixed at 25 ° C. for 60 minutes. Next, the solid content concentration was adjusted to 52% with ion-exchanged water. Thereafter, the mixture was further mixed at 25 ° C. for 15 minutes to obtain a mixed solution.

  A 40% aqueous dispersion containing a styrene-butadiene copolymer (with a glass transition temperature of −15 ° C.) in an amount of 1.0 part in terms of solid content and ion-exchanged water are added to the mixed solution, and the final solid content concentration is The mixture was adjusted to 50% and further mixed for 10 minutes. This was defoamed under reduced pressure to obtain a slurry composition for a negative electrode having good fluidity.

(1-5. Production of negative electrode)
A film after drying the slurry composition for a negative electrode obtained in the above (1-4. Production of slurry composition for negative electrode) on a copper foil having a thickness of 20 μm as a current collector with a comma coater It was applied to a thickness of about 150 μm and dried. This drying was performed by conveying the copper foil in an oven at 60 ° C. at a speed of 0.5 m / min for 2 minutes. Then, the negative electrode original fabric was obtained by heat-processing at 120 degreeC for 2 minute (s). This negative electrode original fabric was rolled with a roll press to obtain a negative electrode having a negative electrode active material layer having a thickness of 80 μm.

(1-6. Preparation of separator)
A single-layer polypropylene separator (width 65 mm, length 500 mm, thickness 25 μm, manufactured by dry method, porosity 55%) was cut into a square of 5 × 5 cm ² .

(1-7. Lithium ion secondary battery)
An aluminum packaging exterior was prepared as the battery exterior. The positive electrode obtained in the above (1-3. Production of positive electrode) was cut into a square of 4 × 4 cm ² and arranged so that the surface on the current collector side was in contact with the aluminum packaging exterior. On the surface of the positive electrode active material layer of the positive electrode, the square separator obtained in the above (1-6. Preparation of separator) was disposed. Furthermore, the negative electrode obtained in the above (1-5. Production of negative electrode) was cut into a square of 4.2 × 4.2 cm ² so that the surface on the negative electrode active material layer side faces the separator on the separator. Arranged. Furthermore, a 1.0 μM LiPF ₆ solution containing 1.5% vinylene carbonate (VC) was charged. The solvent of this LiPF ₆ solution is a mixed solvent (EC / EMC = 3/7 (volume ratio)) of ethylene carbonate (EC) and ethyl methyl carbonate (EMC). Furthermore, in order to seal the opening of the aluminum packaging material, heat sealing at 150 ° C. was performed to close the aluminum exterior, and a lithium ion secondary battery was manufactured.
About the obtained lithium ion secondary battery, the high temperature storage characteristic was evaluated.

[Example 2]
In the same manner as in Example 1 except that the amount of methacrylic acid was changed to 20 parts and the amount of butyl acrylate was changed to 70.2 parts in (1-1. Production of water-soluble polymer). A lithium ion secondary battery was manufactured and evaluated.

Example 3
In the same manner as in Example 1 except that the amount of methacrylic acid was changed to 25 parts and the amount of butyl acrylate was changed to 65.2 parts in (1-1. Production of water-soluble polymer). A lithium ion secondary battery was manufactured and evaluated.

Example 4
In the same manner as in Example 1 except that the amount of methacrylic acid was changed to 40 parts and the amount of butyl acrylate was changed to 50.2 parts in (1-1. Production of water-soluble polymer). A lithium ion secondary battery was manufactured and evaluated.

Example 5
In the same manner as in Example 1 except that the amount of methacrylic acid was changed to 45 parts and the amount of butyl acrylate was changed to 45.2 parts in (1-1. Production of water-soluble polymer). A lithium ion secondary battery was manufactured and evaluated.

Example 6
The same as in Example 1 except that in (1-1. Production of water-soluble polymer), the amount of ethylene dimethacrylate was changed to 0.1 part and the amount of butyl acrylate was changed to 59.9 parts. A positive electrode and a lithium ion secondary battery were manufactured and evaluated.

Example 7
The same as in Example 1 except that the amount of ethylene dimethacrylate was changed to 1.2 parts and the amount of butyl acrylate was changed to 58.8 parts in (1-1. Production of water-soluble polymer). A positive electrode and a lithium ion secondary battery were manufactured and evaluated.

Example 8
The same as in Example 1 except that in (1-1. Production of water-soluble polymer), the amount of ethylene dimethacrylate was changed to 1.8 parts and the amount of butyl acrylate was changed to 58.2 parts. A positive electrode and a lithium ion secondary battery were manufactured and evaluated.

Example 9
In the above (1-1. Production of water-soluble polymer), a positive electrode and a lithium ion secondary battery were produced and evaluated in the same manner as in Example 1 except that glycidyl methacrylate was used instead of ethylene dimethacrylate. .

Example 10
In the above (1-1. Production of water-soluble polymer), a positive electrode and a lithium ion secondary battery were produced and evaluated in the same manner as in Example 1 except that allyl glycidyl ether was used instead of ethylene dimethacrylate. did.

Example 11
Example 1 except that the amount of ammonium polyoxyalkylene alkenyl ether sulfate was changed to 0.4 parts and the amount of butyl acrylate was changed to 60.3 parts in (1-1. Production of water-soluble polymer). In the same manner as above, a positive electrode and a lithium ion secondary battery were produced and evaluated.

Example 12
Example 1 except that in (1-1. Production of water-soluble polymer), the amount of ammonium polyoxyalkylene alkenyl ether sulfate was changed to 2.5 parts and the amount of butyl acrylate was changed to 58.2 parts. In the same manner as above, a positive electrode and a lithium ion secondary battery were produced and evaluated.

Example 13
A positive electrode and a lithium ion secondary battery in the same manner as in Example 1 except that in (1-1. Production of water-soluble polymer), sodium dodecylbenzenesulfonate was used instead of ammonium polyoxyalkylene alkenyl ether sulfate. Were manufactured and evaluated.

Example 14
In Example (1-1. Production of water-soluble polymer), except that 2,2,2-trifluoroethyl methacrylate was not used, and the amount of butyl acrylate was changed to 66.7 parts. In the same manner as in Example 1, a positive electrode and a lithium ion secondary battery were manufactured and evaluated.

Example 15
In the above (1-1. Production of water-soluble polymer), except that the amount of 2,2,2-trifluoroethyl methacrylate was changed to 17 parts and the amount of butyl acrylate was changed to 49.7 parts. In the same manner as in Example 1, positive electrodes and lithium ion secondary batteries were produced and evaluated.

Example 16
In the above (1-1. Production of water-soluble polymer), positive electrode and lithium ion were prepared in the same manner as in Example 1 except that trifluoromethyl methacrylate was used instead of 2,2,2-trifluoroethyl methacrylate. Secondary batteries were manufactured and evaluated.

Example 17
In the above (1-2. Production of positive electrode binder composition), the weight ratio of the water-soluble polymer to the particulate binder in the positive electrode binder composition is particulate binder / water-soluble polymer = 99.5 /. A positive electrode and a lithium ion secondary battery were produced and evaluated in the same manner as in Example 1 except that the value was 0.5.

Example 18
In the above (1-2. Production of positive electrode binder composition), the weight ratio of the water-soluble polymer to the particulate binder in the positive electrode binder composition is particulate binder / water-soluble polymer = 90.0 / A positive electrode and a lithium ion secondary battery were produced and evaluated in the same manner as in Example 1 except that the value was 10.0.

Example 19
To polymerization can A, 11 parts of 2-ethylhexyl acrylate, 1 part of acrylonitrile, 0.12 part of sodium lauryl sulfate and 79 parts of ion-exchanged water were added. To this polymerization can A, 0.2 parts of ammonium persulfate and 10 parts of ion-exchanged water were further added as a polymerization initiator, heated to 60 ° C., and stirred for 90 minutes.

  In another polymerization vessel B, 67 parts of 2-ethylhexyl acrylate, 19 parts of acrylonitrile, 2.0 parts of itaconic acid, 0.7 parts of sodium lauryl sulfate and 46 parts of ion-exchanged water were added and stirred to prepare an emulsion. did. This emulsion was sequentially added from polymerization vessel B to polymerization vessel A over about 180 minutes. Thereafter, the mixture was stirred for about 120 minutes, and when the monomer consumption reached 95%, the reaction was terminated by cooling. Then, pH was adjusted with 4% NaOH aqueous solution, and the composition containing the particulate binder B (copolymer B) was obtained.

The obtained particulate binder B had a glass transition temperature of −32 ° C. and a number average particle size of 0.18 μm. The content ratio of the (meth) acrylic acid ester monomer unit in the particulate binder B is 78%, the structural unit of the monomer having an acid component is 2.0%, and the content of the (meth) acrylonitrile monomer unit The percentage was 20%.
A 5% aqueous sodium hydroxide solution was added to the composition containing the particulate binder B to adjust the pH to 8. Then, the unreacted monomer was removed by heating under reduced pressure. Then, it cooled to 30 degrees C or less, and obtained the water dispersion containing a desired particulate binder.

  The aqueous dispersion containing the particulate binder thus produced in Example 19 was used in place of the aqueous dispersion containing the particulate binder produced in Example 1-2 (1-2. Production of binder composition for positive electrode). A positive electrode and a lithium ion secondary battery were produced and evaluated in the same manner as in Example 1 except for that.

Example 20
To polymerization can A, 10 parts of 2-ethylhexyl acrylate, 1 part of acrylonitrile, 0.12 part of sodium lauryl sulfate, and 79 parts of ion-exchanged water were added. To this polymerization can A, 0.2 parts of ammonium persulfate and 10 parts of ion-exchanged water were further added as a polymerization initiator, heated to 60 ° C., and stirred for 90 minutes.

  In another polymerization vessel B, 67 parts of 2-ethylhexyl acrylate, 18 parts of acrylonitrile, 2.0 parts of itaconic acid, 2 parts of hydroxyethyl acrylate, 0.7 part of sodium lauryl sulfate and 46 parts of ion-exchanged water were added and stirred. Thus, an emulsion was prepared. This emulsion was sequentially added from polymerization vessel B to polymerization vessel A over about 180 minutes. Thereafter, the mixture was stirred for about 120 minutes, and when the monomer consumption reached 95%, the reaction was terminated by cooling. Then, pH was adjusted with 4% NaOH aqueous solution, and the composition containing the particulate binder C (copolymer B) was obtained.

The obtained particulate binder C had a glass transition temperature of −33 ° C. and a number average particle diameter of 0.18 μm. The content ratio of the (meth) acrylic acid ester monomer unit in the particulate binder C is 77%, the structural unit of the monomer having an acid component is 2%, and the content ratio of the (meth) acrylonitrile monomer unit is The content rate of the structural unit of 19% and hydroxyalkyl acrylate was 2%.
To the composition containing the particulate binder C, a 5% aqueous sodium hydroxide solution was added to adjust the pH to 8. Then, the unreacted monomer was removed by heating under reduced pressure. Then, it cooled to 30 degrees C or less, and obtained the aqueous dispersion containing a desired particulate binder.

  Thus, the aqueous dispersion containing the particulate binder produced in Example 20 was used in place of the aqueous dispersion containing the particulate binder produced in Example 1-2 (1-2. Production of binder composition for positive electrode). A positive electrode and a lithium ion secondary battery were produced and evaluated in the same manner as in Example 1 except for that.

[Comparative Example 1]
In (1-2. Production of binder composition for positive electrode), an aqueous dispersion containing an aqueous solution containing the water-soluble polymer obtained in (1-1. Production of water-soluble polymer) and a particulate binder. The aqueous dispersion containing the particulate binder was used as it was as a positive electrode binder composition. Thereafter, except that the binder composition containing no water-soluble polymer was used as the binder composition for the positive electrode, (1-3. Production of positive electrode) to (1-7. Lithium ion secondary battery) and Similarly, a positive electrode and a lithium ion secondary battery were manufactured and evaluated.

[Comparative Example 2]
Example (1-2. For positive electrode) of Example 1 except that an aqueous polyacrylic acid solution was used instead of the aqueous solution containing the water-soluble polymer obtained in (1-1. Production of water-soluble polymer). Production of Binder Composition) to (1-7. Lithium Ion Secondary Battery) were produced and evaluated for positive electrodes and lithium ion secondary batteries.

[Comparative Example 3]
In a 5 MPa pressure vessel equipped with a stirrer, 80 parts of methacrylic acid, 1.5 parts of sodium dodecylbenzenesulfonate, 20.0 parts of butyl acrylate, 150 parts of ion-exchanged water, and 0.5 part of potassium persulfate were placed and stirred sufficiently. Thereafter, the polymerization was started by heating to 60 ° C. When the polymerization conversion reached 96%, the reaction was stopped by cooling to obtain a mixture containing a water-soluble polymer. 10% aqueous ammonia was added to the mixture containing the water-soluble polymer to adjust the pH to 8 to obtain an aqueous solution containing the desired water-soluble polymer.

  The above (1) of Example 1 except that the aqueous solution containing the water-soluble polymer thus obtained was used instead of the aqueous solution containing the water-soluble polymer obtained in (1-1. Production of water-soluble polymer). -2. Production of positive electrode binder composition)-(1-7. Lithium ion secondary battery) In the same manner, a positive electrode and a lithium ion secondary battery were produced and evaluated.

[Comparative Example 4]
In a 5 MPa pressure vessel equipped with a stirrer, 10 parts of methacrylic acid, 1.5 parts of sodium dodecylbenzenesulfonate, 2.5 parts of trifluoromethyl methacrylate, 87.5 parts of butyl acrylate, 150 parts of ion-exchanged water, and potassium persulfate After 5 parts were added and sufficiently stirred, the polymerization was started by heating to 60 ° C. When the polymerization conversion reached 96%, the reaction was stopped by cooling to obtain a mixture containing a water-soluble polymer. 10% aqueous ammonia was added to the mixture containing the water-soluble polymer to adjust the pH to 8 to obtain an aqueous solution containing the desired water-soluble polymer.

  The above (1) of Example 1 except that the aqueous solution containing the water-soluble polymer thus obtained was used instead of the aqueous solution containing the water-soluble polymer obtained in (1-1. Production of water-soluble polymer). -2. Production of positive electrode binder composition)-(1-7. Lithium ion secondary battery) In the same manner, a positive electrode and a lithium ion secondary battery were produced and evaluated.

〔result〕
The results of Examples and Comparative Examples are shown in Tables 1 to 6 below.
Here, the abbreviations in the table represent the following.
2EHA: 2-ethylhexyl acrylate AN: acrylonitrile MAA: methacrylic acid EDMA: ethylene dimethacrylate POAAE: polyoxyalkylene alkenyl ether ammonium sulfate TFEMA: 2,2,2-trifluoroethyl methacrylate BA: butyl acrylate SDBS: sodium dodecylbenzenesulfonate TFMMA : Trifluoromethyl methacrylate GMA: glycidyl methacrylate AGE: allyl glycidyl ether AMA: allyl methacrylate β-HEA: hydroxyethyl acrylate

〔Consideration〕
In the examples, the number of pinholes generated tends to be smaller than in the comparative example. From this, in the Example, since the slurry composition for positive electrodes is excellent in a dispersibility, it turns out that coating property is favorable. Further, from this, it is considered that the obtained positive electrode active material layer has little coating unevenness and the pores are uniformly distributed.

  Moreover, in an Example, there exists a tendency for peel strength to be high compared with a comparative example. From this, in the Example, since the slurry composition for positive electrodes is excellent in a dispersibility, it turns out that there is no bias | inclination of a positive electrode active material and a conductive support agent, and it has high adhesive strength.

  Furthermore, in the examples, there is a tendency that the amount of electrolyte absorbed is larger than in the comparative example. From this, in an Example, it is thought that the positive electrode has the outstanding liquid injectability, and, for this reason, the internal resistance in a positive electrode is small.

  When each of the coating property, adhesion strength, and liquid pouring property is observed, there are some in which the comparative example gives better results than some examples. However, in any of the examples, good results with a better balance than those of the comparative examples are obtained as a whole in terms of coating property, adhesion strength, and liquid injection property. Thus, since the coating property, the adhesion strength, and the liquid injection property are well balanced, it is considered that excellent high-temperature storage characteristics are exhibited in any of the examples. In general, when the coating property, the adhesion strength and the liquid injection property are good, not only the high-temperature storage characteristics but also the output characteristics and the high-temperature cycle characteristics are obtained, so the secondary of the present invention. It is presumed that the battery also has excellent output characteristics and high temperature cycle characteristics.

Claims (9)

A positive electrode for a secondary battery comprising a positive electrode active material layer comprising a positive electrode active material, a conductive additive, a particulate binder and a water-soluble polymer,
The water-soluble polymer, an acidic functional group-containing monomer units 15 wt% to 60 wt%, cross-linking monomer units 0.1% to 2 wt%, (meth) acrylic acid ester monomer units 30% by weight to 80% by weight , a reactive surfactant unit, and a copolymer A having a fluorine-containing (meth) acrylate monomer unit ,
The particulate binder is a positive electrode for a secondary battery including 0.1 wt% to 10 wt% of a carboxylic acid group-containing monomer unit. The content of the reactive surfactant unit in the copolymer A is 0.1 wt% to 15 wt%, a positive electrode for a secondary battery according to claim 1, wherein. The positive electrode for a secondary battery according to claim 1 or 2, wherein a content ratio of the fluorine-containing (meth) acrylic acid ester monomer unit in the copolymer A is 1 wt% to 15 wt%. The secondary battery according to any one of claims 1 to 3 , wherein the particulate binder includes a copolymer B having a (meth) acrylonitrile monomer unit and a (meth) acrylic acid ester monomer unit. Positive electrode. The weight ratio of the (meth) acrylonitrile monomer unit to the (meth) acrylic acid ester monomer unit in the copolymer B is “(meth) acrylonitrile monomer unit / (meth) acrylic acid ester monomer”. The positive electrode for a secondary battery according to claim 4 , wherein the unit is 1/99 to 30/70. The weight ratio of the water-soluble polymer and the particulate binder is 99.5 / 0.5 to 95/5 "particulate binder / water-soluble polymer", any one of claims 1 to 5 The positive electrode for secondary batteries as described in the paragraph. A slurry composition for producing a positive electrode active material layer constituting a positive electrode for a secondary battery,
Including a positive electrode active material, a conductive additive, a particulate binder, a water-soluble polymer and water,
The water-soluble polymer comprises 15% to 60% by weight of acidic functional group-containing monomer units, 0.1% to 2% by weight of crosslinkable monomer units , ( meth) acrylic acid ester monomer units 30 Weight% to 80% by weight , a reactive surfactant unit, and a copolymer A containing a fluorine-containing (meth) acrylate monomer unit ,
The slurry composition in which the particulate binder contains 0.1 wt% to 10 wt% of carboxylic acid group-containing monomer units. A method for producing a positive electrode for a secondary battery comprising a current collector and a positive electrode active material layer provided on the current collector,
The manufacturing method of the positive electrode for secondary batteries including the process of apply | coating the slurry composition of Claim 7 on the said electrical power collector, and drying this coating material and obtaining the said positive electrode active material layer. A positive electrode, a negative electrode, an electrolyte and a separator are provided.
The secondary battery whose said positive electrode is a positive electrode for secondary batteries as described in any one of Claims 1-6 .