JP2013008485A - Positive electrode for secondary battery and secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide: a positive electrode having high dispersion stability and high potential stability; and a secondary battery including the positive electrode and having excellent room temperature cycle characteristics and high temperature cyclic characteristics.SOLUTION: A positive electrode for a secondary battery includes a collector and a positive electrode active material layer stacked on the collector and containing a positive electrode active material and a binder. In the positive electrode for a secondary battery, an average charge voltage of the positive electrode active material relative to lithium metal is 3.9 V or greater, the binder includes a polymerization unit having a nitrile group and a structural unit of a straight chain alkylene having 4 or more carbon atoms, a containing ratio of the polymerization unit having a nitrile group in the binder is 2-50 mass%, and an iodine value of the binder is 3-60 mg/100 mg.

Description

  The present invention relates to a positive electrode for a secondary battery, and more particularly to a positive electrode for a secondary battery having excellent peel strength and cycle characteristics used for a lithium ion secondary battery and the like. The present invention also relates to a secondary battery having such a positive electrode for a secondary battery.

  Lithium ion secondary batteries exhibit the highest energy density among batteries in practical use, and are widely used especially for small electronics. In addition, development for automobile applications is also expected, and lithium ion secondary batteries exhibiting excellent characteristics such as high energy and long life even in a wide temperature range are demanded particularly in electric vehicle (EV) applications.

  In order to increase the energy of the lithium ion secondary battery, it is most effective to increase the operating voltage of the positive electrode active material and increase the electric capacity. Various positive electrode active materials have been studied as positive electrode active materials that may meet this demand.

In Patent Document 1 (Japanese Patent No. 3539518), a solid solution comprising an electrochemically inactive layered Li ₂ MnO ₃ and an electrochemically active layered LiMO ₂ (transition metals such as M = Co and Ni). A positive electrode active material has been studied. Patent Document 1 describes that when these materials are used, the charging average voltage is 4.0 V, and the capacity shows a large electric capacity exceeding 200 mAh / g. However, when the charge / discharge potential is increased and the charge / discharge cycle is repeated, there is a problem that the capacity is quickly deteriorated due to the structural change of the positive electrode active material itself.

In Patent Document 2 (Japanese Patent Laid-Open No. 2008-270201), the structure is stabilized by performing oxidation treatment on a positive electrode active material made of xLiMO _2. (1-x) Li ₂ NO ₃ (M and N are transition metals). And improving the cycle characteristics of the battery.

Further, studies have been made to replace a part of Mn of LiMn ₂ O ₄ having a spinel structure with Ni, Fe, Co, Cu, Cr, etc., to increase the charge / discharge potential and increase the energy density. These are materials that can constitute a LIB having a 5V class electromotive force, and are also called 5V class materials.

For example, Patent Document 3 (Japanese Patent No. 4189457) and Patent Document 4 (Japanese Patent Laid-Open No. 2010-97845) disclose LiFe _x Mn _2-x O ₄ (0 <x <1) and the like. These documents describe that when these materials are used, the charging average voltage is 4.2 V or more and the capacity is 90 to 110 mAh / g.

  Moreover, the positive electrode used for a lithium ion secondary battery usually has a structure in which a positive electrode active material layer is laminated on a current collector, and the positive electrode active material layer includes a positive electrode active material in addition to the positive electrode active material. A binder is used to bind each other and the positive electrode active material and the current collector.

  As a binder generally used for a positive electrode of a lithium ion secondary battery, a synthetic rubber polymer such as a fluorine resin (PVDF) or a styrene-butadiene rubber (SBR) has been proposed. In particular, when a positive electrode active material having a high charge / discharge potential is used, it is necessary to design a material suitable for the positive electrode active material.

Japanese Patent No. 3539518 JP 2008-270201 A Japanese Patent No. 4189457 JP 2010-97845 A

Patent Documents 1, 3, and 4 exemplify PVDF as a binder, and Patent Document 2 exemplifies PVDF and SBR.
According to the study by the present inventors, the binder has a large number of double bonds in the structure of the binder, or the bias of the charge occurs, so that the binder is within the operating voltage range of the high potential positive electrode active material. However, it was found that the structure of the positive electrode was broken and the cycle characteristics of the battery deteriorated by repeating the charge / discharge cycle without being able to maintain the structure stably.
Further, when a positive electrode is formed using PVDF or SBR as a binder, the positive electrode active material and the conductivity-imparting agent may not be uniformly dispersed in the positive electrode and may be unevenly distributed. As a result, a uniform positive electrode could not be obtained, the resistance increased, and the long-term cycle characteristics tended to deteriorate. In order to improve the long-term cycle characteristics, it is considered important to obtain a uniform positive electrode and further to maintain the binding state of the binder for a long time inside the battery.

  In view of the above circumstances, an object of the present invention is to provide a positive electrode having high dispersion stability and a secondary battery having excellent room temperature cycle characteristics and high temperature cycle characteristics using the positive electrode.

The gist of the present invention aimed at solving such problems is as follows.
(1) A current collector and a positive electrode active material layer laminated on the current collector and containing a positive electrode active material and a binder,
The charge average voltage of the positive electrode active material with respect to lithium metal is 3.9 V or more,
The binder comprises a polymer unit having a nitrile group and a linear alkylene structural unit having 4 or more carbon atoms,
The content ratio of the polymer unit having the nitrile group in the binder is 2 to 50% by mass, and
The positive electrode for secondary batteries whose iodine value of the said binder is 3-60 mg / 100 mg.

(2) The positive electrode for secondary batteries as described in (1) in which the binder contains 5 to 30% by mass of the polymer unit having the nitrile group.

(3) The positive electrode for a secondary battery according to (1) or (2), wherein the binder further comprises a polymer unit having a hydrophilic group.

(4) The positive electrode for a secondary battery according to any one of (1) to (3), wherein the positive electrode active material is represented by any one of the following general formulas (I) to (III);
xLiMaO ₂ · (1-x) Li ₂ MbO ₃ (I)
LiyMcPO ₄ (II)
LiMd _0.5 Mn _1.5 O ₄ (III)
(Where x is a number that satisfies 0 <x <1, y is a number that satisfies 0 ≦ y ≦ 2, Ma and Mc are one or more transition metals having an average oxidation state of 3+, and Mb and Md is one or more transition metals with an average oxidation state of 4+).

(5) A secondary battery having a positive electrode, a negative electrode, a separator and an electrolyte solution,
The secondary battery whose said positive electrode is a positive electrode for secondary batteries in any one of (1)-(4).

  In the positive electrode for a secondary battery of the present invention, the positive electrode active material is uniformly dispersed in the positive electrode by using a specific positive electrode active material and a specific binder. Moreover, the positive electrode for secondary batteries of this invention has the outstanding peel strength. As a result, the secondary battery using the positive electrode exhibits excellent cycle characteristics over a wide temperature range (for example, 20 to 60 ° C.) even when the charge / discharge potential is increased.

Positive electrode for secondary battery The positive electrode for a secondary battery of the present invention comprises a current collector and a positive electrode active material layer laminated on the current collector. The positive electrode active material layer contains a positive electrode active material and a binder, and may contain other components added as necessary.

(Positive electrode active material)
As the positive electrode active material used for the positive electrode for a secondary battery, a lithium-containing composite metal oxide having a charge average voltage of 3.9 V or more with respect to lithium metal and capable of occluding and releasing lithium ions is used. By setting the average charge voltage to 3.9 V or more, the energy density of the secondary battery obtained by scanning the potential to a high potential is excellent. In the present invention, the charging average voltage refers to a potential (plateau) at which the secondary battery is charged to the upper limit voltage by the constant current method and lithium is desorbed at that time. The upper limit voltage is a voltage that exceeds the voltage and may cause expansion and heat generation of the battery, which is the limit of ensuring safety.

  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 an olivine structure, and a lithium-containing composite metal oxide having a spinel structure.

The lithium-containing composite metal oxide having a layered structure is a solid solution of LiMaO ₂ and Li ₂ MbO ₃ , xLiMaO _2. (1-x) Li ₂ MbO ₃ (0 <x <1, Ma has an average oxidation state) One or more transition metals that are 3+, and Mb is one or more transition metals that have an average oxidation state of 4+). In particular, xLiMaO ₂ · (1-x) Li ₂ MbO ₃ (0 <x <1, Ma = Ni, Co, Mn, Fe, Ti, etc., Mb = Mn, Zr, Ti, etc.) is preferable, and xLiMaO ₂ · (1-x) Li ₂ MnO ₃ (0 <x <1, Ma = Ni, Co, Mn, Fe, Ti, etc.) is preferable. When charging / discharging is performed in a region where the potential of the positive electrode active material with respect to lithium metal is 3 to 5.2 V, the charge average voltage of these materials is about 4.0 to 4.5 V, and the theoretical capacity is 200 to 300 mAh / g. Indicates.

As the lithium-containing composite metal oxide having an olivine structure, LiyMcPO ₄ (wherein Mc is one or more transition metals having an average oxidation state of 3+, Mc = Mn, Co, etc., 0 ≦ y ≦ 2) The olivine type lithium phosphate compound represented is mentioned. Mn or Co may be partially substituted with other metals, and examples of metals that can be substituted include Fe, Cu, Mg, Zn, V, Ca, Sr, Ba, Ti, Al, Si, B, and Mo. Can be mentioned. When charging / discharging was performed in a region where the potential of the positive electrode active material with respect to lithium metal was 3 to 5.2 V, the charge average voltage of these materials was about 4.0 to 4.8 V, and the theoretical capacity was 165 to 170 mAh / g. Indicates.

As the lithium-containing composite metal oxide having a spinel structure, Li _a [Mn _2−x Md _x ] O _{4 in} which a part of Mn of lithium manganate (LiMn ₂ O ₄ ) is substituted with another transition metal (here, Md includes one or more transition metals having an average oxidation state of 4+, Md = Ni, Co, Fe, Cu, Cr, etc., 0 <x <1, 0 ≦ a ≦ 1), and the like. Among them, _Li a was replaced Mn with _{Fe Fe x Mn 2-x O} 4-z (0 ≦ a ≦ 1,0 <x <1,0 ≦ z ≦ 0.1) , since the cost is inexpensive Preferably, LiNi _0.5 Mn _1.5 O ₄ or the like in which Mn is replaced with Ni can replace all of Mn ³⁺ which is considered to be a structural deterioration factor, and perform an electrochemical reaction from Ni ²⁺ to Ni ⁴⁺ . Therefore, a high operating voltage and a high capacity can be obtained, which is preferable. When charging / discharging was performed in the region where the potential of the positive electrode active material with respect to lithium metal was 3 to 5.2 V, the charge average voltage of these materials was about 4.6 to 4.8 V, and the theoretical capacity was 110 to 140 mAh / g. Indicates.

In addition, high theories such as a positive electrode active material having a polyanion structure such as Li ₂ MeSiO ₄ (where Me is Fe, Mn), LiFeF ₃ having a perovskite structure, Li ₂ Cu ₂ O ₄ having an orthorhombic structure, etc. Although it is expected as a material having a capacity, each material has a wide operating voltage range of around 1 to 5 V, and there are significant problems with the material itself, such as the inability to maintain the structure when Li is desorbed.

Among the active materials, a solid solution of LiMaO ₂ and Li ₂ MnO ₃ which is a lithium-containing composite metal oxide having a layered structure, spinel-structured LiNi _0.5 Mn _1.5 O ₄ , olivine-type LiCoPO ₄ and LiMnPO ₄ It is preferable to use an active material such as ₄ .

  The particle diameter of the positive electrode active material used in the present invention is appropriately selected in consideration of other constituent elements of the battery. From the viewpoint of improving battery characteristics such as load characteristics and cycle characteristics, the 50% volume cumulative diameter is Usually, it is 0.1-50 micrometers, Preferably it is 0.4-30 micrometers, More preferably, it is 1-20 micrometers. 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, and a positive electrode slurry and a positive electrode for forming a positive electrode active material layer can be produced. Is easy to handle. The 50% volume cumulative diameter can be determined by measuring the particle size distribution by laser diffraction.

  The amount of the positive electrode active material contained in the positive electrode active material layer of the positive electrode for secondary battery of the present invention is 50 to 99% by mass, more preferably 70 to 99% by mass, and the most preferable range is 80 It is -99 mass%. By setting it as the said range, a high capacity | capacitance lithium ion secondary battery can be produced.

(binder)
The binder comprises a polymer unit having a nitrile group and a linear alkylene structural unit having 4 or more carbon atoms.

  By including a polymer unit having a nitrile group in the polymer constituting the binder, the dispersibility of the positive electrode active material in the positive electrode slurry for forming the positive electrode active material layer is improved, and the slurry is stored in a stable state for a long period of time. can do. As a result, a uniform positive electrode active material layer can be easily manufactured. Moreover, since the lithium ion conductivity is good, the internal resistance in the battery can be reduced, and the output characteristics of the battery can be improved.

  The content ratio of the polymer unit having the nitrile group in the binder is 2 to 50% by mass, preferably 5 to 30% by mass, and more preferably 10 to 25% by mass. When the content ratio of the polymer unit having a nitrile group in the binder is too large, the solubility in the electrolytic solution is increased, and the cycle characteristics of the battery are inferior particularly in the high temperature cycle test. Moreover, when there is too little content rate of the polymerization unit which has a nitrile group, it is inferior to the dispersibility of a positive electrode active material, and a favorable positive electrode slurry cannot be obtained, but the uniformity of the positive electrode obtained falls. As a result, the resistance of the positive electrode increases, and further, the cycle characteristics of a battery using the positive electrode are inferior.

  In the polymer constituting the binder, the linear alkylene structural unit has 4 or more carbon atoms, preferably 4 to 16, more preferably 4 to 12.

  Dispersibility of the conductivity-imparting agent added as necessary to the slurry for forming the positive electrode (positive electrode slurry) by introducing a non-polar linear alkylene structural unit into the polymer constituting the binder. And the production of a uniform positive electrode is facilitated. In addition, since the conductivity imparting agent is uniformly dispersed in the electrode, it is easy to form a conductive network, thereby reducing internal resistance, and as a result, cycle characteristics and output characteristics of a battery using this electrode are improved. Further, by introducing a linear alkylene structural unit having a chain length of a predetermined length or more, the swellability of the positive electrode with respect to the electrolytic solution is optimized, and the battery characteristics are improved.

  The binder preferably contains 50 to 98% by mass, more preferably 50 to 80% by mass, and particularly preferably 50 to 70% by mass of a linear alkylene structural unit having 4 or more carbon atoms.

  The binder used in the present invention preferably further contains a polymer unit having a hydrophilic group. The hydrophilic group in the present invention refers to a functional group that liberates protons in an aqueous solvent, and a salt in which the proton is substituted with a cation, and specifically includes a carboxylic acid group, a sulfonic acid group, and a phosphoric acid group. , Hydroxyl groups and salts thereof.

  By introducing a hydrophilic group into the polymer constituting the binder, the adhesion between the positive electrode active materials and between the positive electrode active material layer and the current collector described later is improved, and the positive electrode active material layer in the positive electrode production process Part peeling (powder falling) can be reduced.

  The binder preferably contains 0.05 to 10% by mass, more preferably 0.1 to 8% by mass, and particularly preferably 1 to 6% by mass of the polymer unit having the hydrophilic group.

  The iodine value of the binder is 3 to 60 mg / 100 mg, preferably 3 to 30 mg / 100 mg, more preferably 8 to 10 mg / 100 mg. When the iodine value of the binder exceeds 60 mg / 100 mg, the stability at the oxidation potential is low due to the unsaturated bond contained in the binder, and the high temperature cycle characteristics of the battery are inferior. Conversely, when the iodine value of the binder is less than 3 mg / 100 mg, the flexibility of the binder is lowered. As a result, part of the positive electrode active material can be detached due to chipping of the electrode, powder falling, or the like when the electrode is wound or pressed. When powder falling or the like occurs, the detached lump (positive electrode active material) causes damage to the separator, a short circuit between the positive electrode and the negative electrode, and is inferior in safety and long-term characteristics. When the iodine value of the binder is in the above range, the binder is chemically structurally stable with respect to a high potential, the electrode structure can be maintained even in a long-term cycle, and the high-temperature cycle characteristics are excellent. The iodine value is determined according to JIS K6235;

  The weight average molecular weight in terms of polystyrene by gel permeation chromatography of the binder used in the present invention is preferably 10,000 to 700,000, more preferably 50,000 to 500,000, particularly preferably 100,000 to. 300,000. By making the weight average molecular weight of a binder into the said range, a positive electrode can be made flexible and it is easy to adjust to the viscosity which is easy to apply at the time of manufacture of a positive electrode slurry.

  As described above, the binder preferably contains a polymer unit having a nitrile group and a linear alkylene structural unit having 4 or more carbon atoms, and further contains a polymer unit having a hydrophilic group. Such a binder is obtained by polymerizing a monomer that leads to a polymer unit having a nitrile group, a monomer that leads to a linear alkylene structural unit, and a monomer that leads to a polymer unit having a hydrophilic group. The linear alkylene structural unit can be formed by obtaining a polymer having a structural unit having an unsaturated bond and then hydrogenating it.

Hereinafter, the manufacturing method of the binder used for this invention is demonstrated.
As a monomer for deriving a polymerization unit having a nitrile group, an α, β-ethylenically unsaturated nitrile monomer may be mentioned. The α, β-ethylenically unsaturated nitrile monomer is not particularly limited as long as it is an α, β-ethylenically unsaturated compound having a nitrile group. For example, acrylonitrile; α-chloroacrylonitrile, α-bromoacrylonitrile, etc. [Alpha] -halogenoacrylonitrile; [alpha] -alkylacrylonitrile such as methacrylonitrile; and the like. Among these, acrylonitrile and methacrylonitrile are preferable. These can be used individually by 1 type or in combination of multiple types.

  The method for introducing the linear alkylene structural unit into the binder is not particularly limited, but a method of hydrogenating the conjugated diene monomer unit after introduction is simple and preferable.

  The conjugated diene monomer is preferably a conjugated diene having 4 or more carbon atoms, and examples thereof include 1,3-butadiene, isoprene, 2,3-dimethyl-1,3-butadiene, 1,3-pentadiene and the like. Of these, 1,3-butadiene is preferred. These can be used individually by 1 type or in combination of multiple types.

  Introduction of the hydrophilic group into the binder is carried out by polymerizing monomers having a carboxylic acid group, a sulfonic acid group, a phosphoric acid group, a hydroxyl group, and a salt thereof.

Examples of the monomer having a carboxylic acid group include monocarboxylic acids and derivatives thereof, dicarboxylic acids, and derivatives thereof.
Examples of monocarboxylic acids include acrylic acid, methacrylic acid, and crotonic acid.
Examples of monocarboxylic acid derivatives include 2-ethylacrylic acid, isocrotonic acid, α-acetoxyacrylic acid, β-trans-aryloxyacrylic acid, α-chloro-β-E-methoxyacrylic acid, β-diaminoacrylic acid, and the like. Can be mentioned.
Examples of the dicarboxylic acid include maleic acid, fumaric acid, itaconic acid and the like.
Dicarboxylic acid derivatives include methyl maleic acid, dimethyl maleic acid, phenyl maleic acid, chloromaleic acid, dichloromaleic acid, fluoromaleic acid and the like methyl allyl maleate, diphenyl maleate, nonyl maleate, decyl maleate, dodecyl maleate, And maleate esters such as octadecyl maleate and fluoroalkyl maleate.
Moreover, the acid anhydride which produces | generates a carboxyl group by hydrolysis can also be used.
Examples of the acid anhydride of dicarboxylic acid include maleic anhydride, acrylic anhydride, methyl maleic anhydride, and dimethyl maleic anhydride.
In addition, monoethyl maleate, diethyl maleate, monobutyl maleate, dibutyl maleate, monoethyl fumarate, diethyl fumarate, monobutyl fumarate, dibutyl fumarate, monocyclohexyl fumarate, dicyclohexyl fumarate, monoethyl itaconate, diethyl itaconate Also included are monoesters and diesters of α, β-ethylenically unsaturated polyvalent carboxylic acids such as monobutyl itaconate and dibutyl itaconate.

  Examples of the monomer having a sulfonic acid group include vinyl sulfonic acid, methyl vinyl sulfonic acid, (meth) allyl sulfonic acid, styrene sulfonic acid, (meth) acrylic acid-2-ethyl sulfonate, 2-acrylamido-2-methyl. Examples thereof include propanesulfonic acid and 3-allyloxy-2-hydroxypropanesulfonic acid.

  Examples of the monomer having a phosphate group include phosphoric acid-2- (meth) acryloyloxyethyl phosphate, methyl-2- (meth) acryloyloxyethyl phosphate, and ethyl phosphate- (meth) acryloyloxyethyl phosphate. .

Examples of the monomer having a hydroxyl group include ethylenically unsaturated alcohols such as (meth) allyl alcohol, 3-buten-1-ol, 5-hexen-1-ol; 2-hydroxyethyl acrylate, acrylic acid-2 -Ethylene such as hydroxypropyl, 2-hydroxyethyl methacrylate, 2-hydroxypropyl methacrylate, di-2-hydroxyethyl maleate, di-4-hydroxybutyl maleate, di-2-hydroxypropyl itaconate alkanol esters of unsaturated carboxylic acids; formula ^{_{CH 2 = CR 1 -COO- (C}} n H 2n O) m -H (m is 2 to 9 integer, n represents 2 to 4 integer, ^{R 1} is hydrogen Or an ester of polyalkylene glycol represented by (meth) acrylic acid represented by 2-hydro; Mono (meth) acrylic acid esters of dihydroxy esters of dicarboxylic acids such as cyethyl-2 ′-(meth) acryloyloxyphthalate, 2-hydroxyethyl-2 ′-(meth) acryloyloxysuccinate; 2-hydroxyethyl vinyl ether; Vinyl ethers such as 2-hydroxypropyl vinyl ether; (meth) allyl-2-hydroxyethyl ether, (meth) allyl-2-hydroxypropyl ether, (meth) allyl-3-hydroxypropyl ether, (meth) allyl-2- Mono (meth) allyl ethers of alkylene glycols such as hydroxybutyl ether, (meth) allyl-3-hydroxybutyl ether, (meth) allyl-4-hydroxybutyl ether, (meth) allyl-6-hydroxyhexyl ether Polyoxyalkylene glycol (meth) monoallyl ethers such as diethylene glycol mono (meth) allyl ether and dipropylene glycol mono (meth) allyl ether; glycerin mono (meth) allyl ether, (meth) allyl-2-chloro Mono (meth) allyl ethers of halogen and hydroxy-substituted products of (poly) alkylene glycols such as -3-hydroxypropyl ether, (meth) allyl-2-hydroxy-3-chloropropyl ether; eugenol, isoeugenol Mono (meth) allyl ether of monohydric phenol and its halogen-substituted product; (meth) allyl of alkylene glycol such as (meth) allyl-2-hydroxyethylthioether and (meth) allyl-2-hydroxypropylthioether Thioether compounds; and the like.

  Among these, the hydrophilic group is preferably a carboxylic acid group or a sulfonic acid group because it is excellent in the adhesion between the positive electrode active materials and the adhesion between the positive electrode active material layer and the current collector described later. A carboxylic acid group is preferable because it efficiently captures transition metal ions that may be eluted from the positive electrode active material.

  Moreover, the binder used for this invention may contain the other monomer unit copolymerizable with the monomer which forms these monomer units other than the said monomer unit. The content ratio of such other monomer units is preferably 30% by mass or less, more preferably 20% by mass or less, and still more preferably 10% by mass or less with respect to the total monomer units.

  Examples of such other copolymerizable monomers include, for example, methyl acrylate, ethyl acrylate, n-propyl acrylate, isopropyl acrylate, n-butyl acrylate, t, which constitute the polymer unit of the (meth) acrylate monomer. -Acrylic acid alkyl esters such as butyl acrylate, pentyl acrylate, hexyl acrylate, heptyl acrylate, octyl acrylate, 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, Nyl methacrylate, hexyl methacrylate, heptyl methacrylate, octyl methacrylate, 2-ethylhexyl methacrylate, nonyl methacrylate, decyl methacrylate, lauryl methacrylate, n-tetradecyl methacrylate, methacrylic acid alkyl esters such as stearyl methacrylate; styrene, α-methylstyrene, vinyl Aromatic vinyl compounds such as toluene; fluorine-containing vinyl compounds such as fluoroethyl vinyl ether, fluoropropyl vinyl ether, o-trifluoromethylstyrene, vinyl pentafluorobenzoate, difluoroethylene, tetrafluoroethylene; 1,4-pentadiene, 1, Non-conjugated diene compounds such as 4-hexadiene, vinylnorbornene and dicyclopentadiene; Α-olefin compounds such as tylene, propylene, 1-butene, 4-methyl-1-pentene, 1-hexene, 1-octene; methoxyethyl (meth) acrylate, methoxypropyl (meth) acrylate, (meth) acrylic Alkoxyalkyl esters of α, β-ethylenically unsaturated carboxylic acids such as butoxyethyl acid; divinyl compounds such as divinylbenzene; dialkyl such as ethylene di (meth) acrylate, diethylene glycol di (meth) acrylate, and ethylene glycol di (meth) acrylate In addition to polyfunctional ethylenically unsaturated monomers such as (meth) acrylic acid esters; trimethacrylic acid esters such as trimethylolpropane tri (meth) acrylate; N-methylol (meth) acrylamide, N, N ′ -Dimethylol (meth) a Self-crosslinking compound such as Riruamido; and the like. Among them, it shows solubility in NMP used as a solvent for the positive electrode slurry without eluting into the electrolytic solution, improves the flexibility of the positive electrode, and suppresses peeling of the positive electrode when a wound cell is produced. In addition, since it is excellent in characteristics (cycle characteristics, etc.) of the secondary battery using the positive electrode, an alkyl acrylate is preferable, and an alkyl acrylate having 2 to 12 carbon atoms of an alkyl group bonded to a non-carbonyl oxygen atom. Esters are more preferable, and among them, ethyl acrylate, n-butyl acrylate, 2-ethylhexyl acrylate, nonyl acrylate, and lauryl acrylate are particularly preferable. In addition, when N-methylpyrrolidone (NMP) is used as a solvent for the positive electrode slurry without eluting into the electrolytic solution, it exhibits solubility in NMP, and in addition, the positive electrode active material has excellent dispersibility and a uniform positive electrode. Since it is obtained, aromatic vinyl compounds such as styrene and α-methylstyrene are preferred.

  Furthermore, the binder used for this invention may contain the monomer copolymerizable with these other than the monomer component mentioned above. Monomers copolymerizable with these include vinyl esters such as vinyl acetate, vinyl propionate and vinyl butyrate; vinyl ethers such as methyl vinyl ether, ethyl vinyl ether and butyl biether; methyl vinyl ketone, ethyl vinyl ketone and butyl And vinyl ketones such as vinyl ketone, hexyl vinyl ketone and isopropenyl vinyl ketone; and heterocyclic ring-containing vinyl compounds such as N-vinyl pyrrolidone, vinyl pyridine and vinyl imidazole. By graft-copolymerizing these monomers by an appropriate technique, the binder having the above-described structure can be obtained.

The binder used in the present invention is used in the state of a dispersion in which the binder is dispersed in a dispersion medium (water or an organic solvent) or a dissolved solution (hereinafter, these are collectively referred to as “binder dispersion”). To do.) The dispersion medium is not particularly limited as long as it can uniformly disperse or dissolve the binder. In the present invention, it is preferable to use water as a dispersion medium from the viewpoints of excellent environmental viewpoint and high drying speed. Examples of the organic solvent include cycloaliphatic hydrocarbons such as cyclopentane and cyclohexane; aromatic hydrocarbons such as toluene, xylene and ethylbenzene; acetone, ethyl methyl ketone, diisopropyl ketone, cyclohexanone, methylcyclohexane, ethylcyclohexane and the like. Ketones; chlorinated aliphatic hydrocarbons such as methylene chloride, chloroform, carbon tetrachloride; esters such as ethyl acetate, butyl acetate, γ-butyrolactone, and ε-caprolactone; acylonitriles such as acetonitrile and propionitrile; tetrahydrofuran , Ethers such as ethylene glycol diethyl ether: alcohols such as methanol, ethanol, isopropanol, ethylene glycol, ethylene glycol monomethyl ether; N- Amides such as methylpyrrolidone and N, N-dimethylformamide can be mentioned.
These dispersion media may be used alone, or two or more of these dispersion media may be mixed and used as a mixed solvent. Among these, since it is used industrially at the time of positive electrode slurry preparation described later, it is difficult to volatilize in production, and as a result, volatilization of the slurry can be suppressed, and the smoothness of the obtained positive electrode is improved. Or N-methylpyrrolidone is preferable.

  When the binder is dispersed in the dispersion medium in the form of particles, the average particle diameter (dispersed particle diameter) of the binder dispersed in the form of particles is preferably 50 to 500 nm, more preferably 70 to 400 nm, and particularly preferably. 100-250 nm. When the average particle size of the binder is within this range, the strength and flexibility of the positive electrode obtained are good.

  When the binder is dispersed in the dispersion medium in the form of particles, the solid content concentration of the binder dispersion is usually 15 to 70% by mass, preferably 20 to 65% by mass, and more preferably 30 to 60% by mass. When the solid content concentration is within this range, workability in producing a positive electrode slurry described later is good.

  The glass transition temperature (Tg) of the binder used in the present invention is preferably −50 to 25 ° C., more preferably −45 to 15 ° C., and particularly preferably −40 to 5 ° C. When the Tg of the binder is in the above range, the positive electrode for a secondary battery of the present invention has excellent strength and flexibility, so that the output characteristics of a secondary battery using the positive electrode can be improved. The glass transition temperature of the binder can be adjusted by combining various monomers.

  The method for producing the binder used in the present invention is not particularly limited, and any method such as a solution polymerization method, a suspension polymerization method, a bulk polymerization method, and an emulsion polymerization method can be used. As the polymerization reaction, any reaction such as ionic polymerization, radical polymerization, and living radical polymerization can be used. Examples of the polymerization initiator used for polymerization include lauroyl peroxide, diisopropyl peroxydicarbonate, di-2-ethylhexyl peroxydicarbonate, t-butyl peroxypivalate, 3,3,5-trimethylhexanoyl peroxide, and the like. Organic peroxides, azo compounds such as α, α′-azobisisobutyronitrile, ammonium persulfate, potassium persulfate, and the like.

  When the linear alkylene structural unit is formed by hydrogenation after introduction of the conjugated diene monomer unit, the method of hydrogenation is not particularly limited. Carbon derived from a conjugated diene monomer unit in an unsaturated polymer (a polymer comprising a nitrile group-containing polymer unit and a conjugated diene monomer unit) obtained by the above-described polymerization method by hydrogenation reaction Only the carbon unsaturated bond can be selectively hydrogenated to obtain the binder used in the present invention. Moreover, the iodine value of the binder used for this invention can be made into the range mentioned above by hydrogenation reaction. The binder used in the present invention is preferably a hydrogenated acrylonitrile-butadiene copolymer having a hydrophilic group (hereinafter sometimes referred to as “hydrogenated NBR”).

  As a selective hydrogenation method for selectively hydrogenating only the carbon-carbon unsaturated bond derived from the conjugated diene monomer unit in the unsaturated polymer, a known method may be used, such as an oil layer hydrogenation method, Any of the aqueous layer hydrogenation methods can be used, but the aqueous layer hydrogenation method is preferable because the content of impurities (for example, a coagulant or metal described later) is small in the obtained binder.

  When the binder used in the present invention is produced by the oil layer hydrogenation method, the following method is preferred. That is, first, a dispersion of an unsaturated polymer prepared by emulsion polymerization is coagulated by salting out, dissolved in an organic solvent through filtration and drying. Subsequently, the unsaturated polymer dissolved in the organic solvent is subjected to a hydrogenation reaction (oil layer hydrogenation method) to obtain a hydride, and the obtained hydride solution is coagulated, filtered and dried. A binder to be used is obtained.

  When an alkali metal caprate is used as an emulsifier, the caprate in the binder finally obtained in each step of coagulation, filtration and drying by salting out the dispersion of the unsaturated polymer is used. It is preferable to prepare such that the amount is 0.01 to 0.4% by mass. For example, a known coagulant such as magnesium sulfate, sodium chloride, calcium chloride, or aluminum sulfate can be used in coagulation by salting out of the dispersion, but preferably an alkali such as magnesium sulfate, magnesium chloride, or magnesium nitrate. By using an earth metal salt; or a Group 13 metal salt such as aluminum sulfate; the amount of caprate contained in the unsaturated polymer can be reduced. Therefore, it is preferable to use an alkaline earth metal salt or a Group 13 metal salt as the coagulant, more preferably an alkaline earth metal salt, and finally by controlling the amount of use and the solidification temperature, The amount of caprate in the obtained binder can be within the above range. The amount of the coagulant used is preferably 1 to 100 parts by mass, more preferably 5 to 50 parts by mass, and particularly preferably 10 to 50 parts by mass, when the amount of unsaturated polymer to be hydrogenated is 100 parts by mass. It is. The coagulation temperature is preferably 10 to 80 ° C.

  The solvent for the oil layer hydrogenation method is not particularly limited as long as it is a liquid organic compound that dissolves the unsaturated polymer, but benzene, toluene, xylene, hexane, cyclohexane, tetrahydrofuran, methyl ethyl ketone, ethyl acetate, cyclohexanone, and acetone are preferable. used.

  As a catalyst for the oil layer hydrogenation method, any known selective hydrogenation catalyst can be used without limitation, and a palladium-based catalyst and a rhodium-based catalyst are preferable, and a palladium-based catalyst (such as palladium acetate, palladium chloride, and palladium hydroxide) is used. More preferred. Two or more of these may be used in combination, but when a rhodium catalyst and a palladium catalyst are used in combination, it is preferable to use a palladium catalyst as the main active ingredient. These catalysts are usually used by being supported on a carrier. Examples of the carrier include silica, silica-alumina, alumina, diatomaceous earth, activated carbon and the like. The amount of the catalyst used is preferably 10 to 5000 ppm, more preferably 100 to 3000 ppm, in terms of the amount of metal of the hydrogenation catalyst, relative to the amount of the unsaturated polymer to be hydrogenated.

  The hydrogenation reaction temperature in the oil layer hydrogenation method is preferably 0 to 200 ° C., more preferably 10 to 100 ° C., and the hydrogen pressure is preferably 0.1 to 30 MPa, more preferably 0.2 to 20 MPa. The reaction time is preferably 1 to 50 hours, more preferably 2 to 25 hours.

Alternatively, when the binder used in the present invention is produced by the aqueous layer hydrogenation method, the dispersion of the unsaturated polymer prepared by emulsion polymerization is diluted with water as necessary to carry out the hydrogenation reaction. Preferably it is done.
Here, in the aqueous layer hydrogenation method, hydrogen is supplied to a reaction system in the presence of a hydrogenation catalyst to hydrogenate (I) an aqueous layer direct hydrogenation method, and in the presence of an oxidizing agent, a reducing agent and an activator. There are (II) water layer indirect hydrogenation methods in which hydrogenation is carried out by reduction.

(I) In the aqueous layer direct hydrogenation method, the concentration of the unsaturated polymer in the aqueous layer (concentration in the dispersion state) is preferably 40% by mass or less in order to prevent aggregation.
The hydrogenation catalyst used is not particularly limited as long as it is a compound that is difficult to decompose with water. As specific examples of hydrogenation catalysts, palladium catalysts include palladium salts of carboxylic acids such as formic acid, propionic acid, lauric acid, succinic acid, oleic acid, and phthalic acid; palladium chloride, dichloro (cyclooctadiene) palladium, dichloro (norbornadiene) ) Palladium chloride such as palladium and ammonium hexachloropalladium (IV); Iodide such as palladium iodide; Palladium sulfate dihydrate and the like. Of these, palladium salts of carboxylic acids, dichloro (norbornadiene) palladium and ammonium hexachloropalladium (IV) are particularly preferred. The amount of the hydrogenation catalyst used may be determined as appropriate, but is preferably 5 to 6000 ppm, more preferably 10 to 4000 ppm in terms of the amount of metal of the hydrogenation catalyst with respect to the amount of the unsaturated polymer to be hydrogenated. is there.

  The reaction temperature in the aqueous layer direct hydrogenation method is preferably 0 to 300 ° C, more preferably 20 to 150 ° C, and particularly preferably 30 to 100 ° C. If the reaction temperature is too low, the reaction rate may decrease. Conversely, if the reaction temperature is too high, side reactions such as hydrogenation of nitrile groups may occur. The hydrogen pressure is preferably 0.1 to 30 MPa, more preferably 0.5 to 20 MPa. The reaction time is selected in consideration of the reaction temperature, hydrogen pressure, target hydrogenation rate, and the like.

  On the other hand, in the (II) aqueous layer indirect hydrogenation method, the concentration of the unsaturated polymer in the aqueous layer (concentration in the dispersion state) is preferably 1 to 50% by mass, more preferably 1 to 40% by mass. .

  Examples of the oxidizing agent used in the water layer indirect hydrogenation method include oxygen, air, and hydrogen peroxide. The amount of these oxidizing agents used is a molar ratio to the carbon-carbon double bond (oxidant: carbon-carbon double bond), preferably 0.1: 1 to 100: 1, more preferably 0.8: 1. It is in the range of -5: 1.

  As the reducing agent used in the aqueous layer indirect hydrogenation method, hydrazines such as hydrazine, hydrazine hydrate, hydrazine acetate, hydrazine sulfate, and hydrazine hydrochloride, or compounds that liberate hydrazine are used. The amount of these reducing agents used is a molar ratio to the carbon-carbon double bond (reducing agent: carbon-carbon double bond), preferably 0.1: 1 to 100: 1, more preferably 0.8: It is in the range of 1-5: 1.

  As the activator used in the water layer indirect hydrogenation method, ions of metals such as copper, iron, cobalt, lead, nickel, iron, tin and the like are used. These activators are used in a molar ratio to the carbon-carbon double bond (activator: carbon-carbon double bond), preferably 1: 1000 to 10: 1, more preferably 1:50 to 1: 2.

  The reaction in the aqueous layer indirect hydrogenation method is carried out by heating within the range from 0 ° C. to the reflux temperature, whereby the hydrogenation reaction is carried out. The heating range at this time is preferably 0 to 250 ° C, more preferably 20 to 100 ° C, and particularly preferably 40 to 80 ° C.

  In both the direct hydrogenation method and the indirect hydrogenation method in the aqueous layer, it is preferable to carry out solidification by salting out, filtration and drying after the hydrogenation. In order to control the amount of caprate in the binder after hydrogenation, the salting out is carried out in the same manner as the salting out of the dispersion of the unsaturated polymer in the oil layer hydrogenation method. It is preferable to use a Group 13 metal salt, and it is particularly preferable to use an alkaline earth metal salt. Further, the filtration and drying steps subsequent to coagulation can be performed by known methods.

  In addition, the method for producing the binder used in the present invention is particularly preferably a method in which hydrogenation is carried out in two or more stages. Even when the same amount of hydrogenation catalyst is used, the hydrogenation efficiency can be increased by carrying out the hydrogenation in two or more stages. That is, when the conjugated diene monomer unit is converted into a linear alkylene structural unit, the iodine value of the binder can be further reduced.

  In addition, when hydrogenation is performed in two or more stages, it is preferable to achieve hydrogenation of 50% or more, more preferably 70% or more at the hydrogenation rate (hydrogenation rate) (%) in the first stage. . That is, when the numerical value obtained by the following formula is the hydrogenation rate (%), this numerical value is preferably 50% or more, and more preferably 70% or more.

Hydrogenation rate (hydrogenation rate) (%)
= 100 × (carbon-carbon double bond amount before hydrogenation-carbon-carbon double bond amount after hydrogenation) / (carbon-carbon double bond amount before hydrogenation)
In addition, the amount of carbon-carbon double bonds can be analyzed using NMR.

  After completion of the hydrogenation reaction, the hydrogenation catalyst in the dispersion is removed. As the method, for example, an adsorbent such as activated carbon or ion exchange resin can be added to adsorb the hydrogenation catalyst with stirring, and then the dispersion can be filtered or centrifuged. It is also possible to leave the hydrogenation catalyst in the dispersion without removing it.

  Moreover, it is preferable that the binder used for this invention contains a hydrophilic group. The method for introducing a hydrophilic group into the binder is not particularly limited. In the above-described binder production process, a method for introducing a hydrophilic group into the polymer constituting the binder (a monomer having a hydrophilic group is used together). And a method of mixing a binder with an ethylenically unsaturated carboxylic acid or an anhydride thereof (method of acid-modifying the binder) after completion of the hydrogenation reaction. Among these, a method of copolymerizing a monomer having a hydrophilic group is preferable because it is simple in the process. When the binder includes a hydrophilic group, the positive electrode active material is excellent in dispersibility and a uniform positive electrode can be obtained. Moreover, the resistance in a positive electrode is reduced, As a result, the secondary battery which shows the outstanding cycling characteristics can be obtained. Furthermore, the binding property with the current collector becomes good, the positive electrode structure can be maintained even after repeated charge and discharge, and the high temperature cycle characteristics are excellent.

  In the following, a binder containing a hydrophilic group by mixing an ethylenically unsaturated carboxylic acid or anhydride thereof with the binder after completion of the hydrogenation reaction (hereinafter, sometimes referred to as “acid-modified binder”). The method for producing the will be described in detail.

  The ethylenically unsaturated carboxylic acid or anhydride thereof used for producing the acid-modified binder is not particularly limited, but the ethylenically unsaturated dicarboxylic acid or anhydride thereof having 4 to 10 carbon atoms, particularly anhydrous Maleic acid is preferred.

Examples of the ethylenically unsaturated carboxylic acid include ethylenically unsaturated monocarboxylic acids such as acrylic acid and methacrylic acid:
Ethylenically unsaturated dicarboxylic acids such as maleic acid, fumaric acid, itaconic acid, citraconic acid:
Ethylenically unsaturated dicarboxylic acid anhydrides such as maleic anhydride, itaconic anhydride and citraconic anhydride:
Monomethyl maleate, monoethyl maleate, monopropyl maleate, mono-n-butyl maleate, monoisobutyl maleate, mono-n-pentyl maleate, mono-n-hexyl maleate, mono-2-ethylhexyl maleate, Monomethyl fumarate, monoethyl fumarate, monopropyl fumarate, mono-n-butyl fumarate, monoisobutyl fumarate, mono-n-pentyl fumarate, mono-n-hexyl fumarate, mono-2-ethylhexyl fumarate, Monomethyl itaconate, monoethyl itaconate, monopropyl itaconate, mono-n-butyl itaconate, monoisobutyl itaconate, mono-n-pentyl itaconate, mono-n-hexyl itaconate, mono-2-ethylhexyl itaconate, Citraconate monomethyl, cyto Conethyl monoethyl, citraconic acid monopropyl, citraconic acid mono-n-butyl, citraconic acid monoisobutyl, citraconic acid mono-n-pentyl, citraconic acid mono-n-hexyl, citraconic acid mono-2-ethylhexyl, mesaconic acid monomethyl, Mesaconic acid monoethyl, mesaconic acid monopropyl, mesaconic acid mono-n-butyl, mesaconic acid monoisobutyl, mesaconic acid mono-n-pentyl, mesaconic acid mono-n-hexyl, mesaconic acid mono-2-ethylhexyl, glutaconic acid monomethyl, Monoethyl glutaconate, monopropyl glutaconate, mono-n-butyl glutaconate, monoisobutyl glutaconate, monoisobutyl glutaconate, mono-n-pentyl glutaconate, mono-n-hexyl glutaconate, mono-glutaconic acid mono- -Ethylhexyl, monomethyl allylmalonate, monoethyl allylmalonate, monopropyl allylmalonate, mono-n-butyl allylmalonate, monoisobutyl allylmalonate, mono-n-pentyl allylmalonate, mono-n-hexyl allylmalonate, mono-2 allylmalonate -Ethylhexyl, monomethyl teraconic acid, monoethyl teraconic acid, monopropyl teraconic acid, mono-n-butyl teraconic acid, monoisobutyl teraconic acid, mono-n-pentyl teraconic acid, mono-n-hexyl teraconic acid, mono-2 teraconic acid -Unsaturated dicarboxylic acid monoalkyl ester such as ethylhexyl.

  The acid-modified binder is obtained, for example, by subjecting the binder used in the present invention to an ethylenically unsaturated carboxylic acid or an anhydride thereof to an ene type addition reaction.

  The ene-type addition reaction usually occurs by kneading the binder used in the present invention with an ethylenically unsaturated carboxylic acid or its anhydride at a high temperature without using a radical generator. The use of a radical generator increases the generation of gels and increases the Mooney viscosity of the binder, and also causes the radical type addition reaction between the ethylenically unsaturated carboxylic acid or its anhydride and the binder. Can not be.

  The amount of the ethylenically unsaturated carboxylic acid or anhydride thereof is not particularly limited, but is usually 0.05 to 10 parts by mass of the ethylenically unsaturated carboxylic acid or anhydride thereof, preferably 100 parts by mass of the binder, 0.2 to 6 parts by mass.

  In an ene type addition reaction, for example, when an open type kneader such as a roll type kneader is used, molten ethylenically unsaturated carboxylic acid such as maleic anhydride or its anhydride is scattered, In some cases, it is impossible to carry out an additional reaction. In addition, when a continuous kneader such as a single-screw extruder, a same-direction twin-screw extruder, a different-direction rotary twin-screw extruder, etc. is used, the binder remaining at the outlet of the extruder is gelled, so that the die head The addition reaction cannot be performed efficiently because clogging occurs. Further, a large amount of unreacted ethylenically unsaturated carboxylic acid or its anhydride remains in the binder.

  In the ene type addition reaction, it is preferable to use a heated closed kneader. The heat-sealed kneader can be arbitrarily selected from batch-type heat-sealed kneaders such as a pressure kneader, Banbury mixer, Brabender, etc. Among them, a pressure kneader is preferable.

  In the above production method, first, before adding the ethylenically unsaturated carboxylic acid or its anhydride to the binder used in the present invention by the ene type addition reaction, at a temperature at which the ene type addition reaction does not substantially occur, Specifically, at 60 to 170 ° C., preferably at 100 to 150 ° C., the ethylenically unsaturated carboxylic acid or its anhydride and the binder are pre-kneaded, and the ethylenically unsaturated carboxylic acid or its anhydride is put into the binder. Disperse uniformly. If the pre-kneading temperature is excessively low, the binder may slip in the kneader, and the ethylenically unsaturated carboxylic acid or its anhydride and the binder may not be sufficiently mixed. On the other hand, if the pre-kneading temperature is excessively high, the ethylenically unsaturated carboxylic acid or anhydride thereof thrown into the kneader may be scattered in a large amount, and the ene type addition reaction rate may be lowered.

  Next, in order to carry out an ene type addition reaction, the temperature of the mixture of the binder being kneaded and the ethylenically unsaturated carboxylic acid or anhydride thereof is usually kept at 200 to 280 ° C, preferably 220 to 260 ° C. The method for maintaining the temperature is not particularly limited, but is usually achieved by flowing warm water or steam through the jacket of the kneader, or using shear heat generation.

When flowing warm water or steam through the jacket of the heat-sealed kneader, the jacket temperature is usually maintained at 70 to 250 ° C, preferably 130 to 200 ° C. Moreover, when utilizing shear heat_generation | fever, it is preferable to continue kneading | mixing with a kneading machine with the shear rate of 30-1000S < ^-1 >, Preferably it is 300-700S- ¹ . In particular, the use of shear heat generation is preferable because the temperature of the mixture can be easily controlled. The kneading time in the heat-sealed kneader is not particularly limited, but is usually 120 seconds to 120 minutes, preferably 180 seconds to 60 minutes.

  If the temperature of the mixture during kneading is excessively low, the ene type addition reaction may not proceed sufficiently. Moreover, when too high, generation | occurrence | production of gelation or a burnt thing etc. will occur, and as a result, gel may mix in a product. In addition, if the shear rate is excessively high, it is difficult to control the temperature of the mixture by shearing heat generation, the temperature of the mixture becomes too high, and generation of gels and burned products occurs, which is not preferable as an industrial production method. . On the other hand, if the shear rate is excessively low, the temperature of the mixture becomes too low, so that a sufficient ene-type addition reaction cannot be expected.

  In the ene-type addition reaction, the addition of an anti-aging agent during kneading can prevent the gelation of the binder and the increase in Mooney viscosity. Although the kind of anti-aging agent is not particularly limited, amine type, amine ketone type, phenol type, benzimidazole type and other anti-aging agents for binders can be used.

  Examples of amine-based antioxidants include phenyl-1-naphthylamine, alkylated diphenylamine, octylated diphenylamine, 4,4-bis (α, α-dimethylbenzyl) diphenylamine, p- (p-toluenesulfonylamide) diphenylamine, N, N-di-2-naphthyl-p-phenylenediamine, N, N-diphenyl-p-phenylenediamine, N-phenyl-N-isopropyl-p-phenylenediamine, N-phenyl-N- (1,3- And dimethylbutyl) -p-phenylenediamine and N-phenyl-N- (3-methacryloyloxy-2-hydroxypropyl) -p-phenylenediamine.

  Examples of amine ketone antioxidants include 2,2,4-trimethyl-1,2-dihydroquinoline, 6-ethoxy-1,2-dihydro-2,2,4-trimethylquinoline, and the like.

  Examples of phenolic antioxidants include 2,6-di-tert-butyl-4-methylphenol, 2,6-di-tert-butyl-4-ethylphenol, 2,2-methylenebis (4-ethyl- 6-tert-butylphenol), 2,2-methylenebis (4-methyl-6-tert-butylphenol), 4,4-butylidenebis (3-methyl-6-tert-butylphenol), 4,4-thiobis (3-methyl) -6-tert-butylphenol), 2,5-di-tert-butylhydroquinone, 2,5-di-tert-amylhydroquinone, and the like.

  Examples of the benzimidazole anti-aging agent include 2-mercaptobenzimidazole, 2-mercaptomethylbenzimidazole, metal salt of 2-mercaptomethylbenzimidazole, and the like.

  The usage-amount of these anti-aging agents is 0.01-5 mass parts normally with respect to 100 mass parts of binders, Preferably it is 0.1-2 mass parts.

  According to the production method described above, 80% or more of the charged amount of the ethylenically unsaturated carboxylic acid or anhydride used for the ene type addition reaction can be added to the binder, and it remains in the binder. Unreacted ethylenically unsaturated carboxylic acid or anhydride thereof can be reduced to 5% or less of the charged amount. Therefore, this method is extremely useful for industrially stable production.

  The binder used in the present invention is preferably obtained through a particulate metal removal step of removing particulate metal contained in the binder dispersion in the binder production step. When the content of the particulate metal component contained in the binder is 10 ppm or less, it is possible to prevent metal ion crosslinking over time between the polymers in the positive electrode slurry, which will be described later, and to prevent an increase in viscosity. Furthermore, there is little concern about self-discharge increase due to internal short circuit of the secondary battery or dissolution / precipitation during charging, and the cycle characteristics and safety of the battery are improved.

  The method for removing the particulate metal component from the binder dispersion in the particulate metal removal step is not particularly limited. For example, the removal method by filtration using a filtration filter, the removal method by a vibrating screen, the removal method by centrifugation. And a method of removing by magnetic force. Especially, since the removal object is a metal component, the method of removing by magnetic force is preferable. The method for removing by magnetic force is not particularly limited as long as it is a method capable of removing a metal component, but in consideration of productivity and removal efficiency, it is preferably performed by arranging a magnetic filter in the production line of the binder.

  In the production process of the binder used in the present invention, the dispersant used in the above polymerization method may be one used in ordinary synthesis, and specific examples include sodium dodecylbenzenesulfonate, sodium dodecylphenylethersulfonate, and the like. Benzene sulfonates; alkyl sulfates such as sodium lauryl sulfate and sodium tetradodecyl sulfate; sulfosuccinates such as sodium dioctyl sulfosuccinate and sodium dihexyl sulfosuccinate; fatty acid salts such as sodium laurate; polyoxyethylene lauryl ether sulfate sodium Salts, ethoxy sulfate salts such as polyoxyethylene nonyl phenyl ether sulfate sodium salt; alkane sulfonate salts; alkyl ether phosphate sodium salts; Nonionic emulsifiers such as xylethylene nonylphenyl ether, polyoxyethylene sorbitan lauryl ester, polyoxyethylene-polyoxypropylene block copolymer; gelatin, maleic anhydride-styrene copolymer, polyvinyl pyrrolidone, sodium polyacrylate, Examples thereof include water-soluble polymers such as polyvinyl alcohol having a polymerization degree of 700 or more and a saponification degree of 75% or more, and these may be used alone or in combination of two or more. Among these, benzenesulfonates such as sodium dodecylbenzenesulfonate and sodium dodecylphenylethersulfonate; alkyl sulfates such as sodium lauryl sulfate and sodium tetradodecylsulfate are preferable, and oxidation resistance is more preferable. From this point, it is a benzenesulfonate such as sodium dodecylbenzenesulfonate and sodium dodecylphenylethersulfonate. The addition amount of a dispersing agent can be set arbitrarily, and is about 0.01-10 mass parts normally with respect to 100 mass parts of monomer total amounts.

  The pH when the binder used in the present invention is dispersed in the dispersion medium is preferably 5 to 13, more preferably 5 to 12, and most preferably 10 to 12. When the pH of the binder is in the above range, the storage stability of the binder is improved, and further, the mechanical stability is improved.

  PH adjusting agents for adjusting the pH of the binder include alkali metal hydroxides such as lithium hydroxide, sodium hydroxide and potassium hydroxide, alkaline earth metal oxides such as calcium hydroxide, magnesium hydroxide and barium hydroxide, Hydroxides such as hydroxides of metals belonging to Group IIIA in a long periodic table such as aluminum hydroxide; carbonates such as alkali metal carbonates such as sodium carbonate and potassium carbonate, alkaline earth metal carbonates such as magnesium carbonate Examples of organic amines include alkylamines such as ethylamine, diethylamine and propylamine; alcohol amines such as monomethanolamine, monoethanolamine and monopropanolamine; ammonia such as ammonia water; Can be mentioned. Among these, alkali metal hydroxides are preferable from the viewpoints of binding properties and operability, and sodium hydroxide, potassium hydroxide, and lithium hydroxide are particularly preferable.

  The content ratio of the binder in the positive electrode for a secondary battery is preferably 0.5 to 15% by mass, more preferably 5 to 15% by mass, and particularly preferably 8 to 15% by mass. When the content ratio of the binder in the positive electrode is in the above range, a part of the positive electrode active material layer described later is prevented from falling off (pouring off) from the positive electrode for the secondary battery of the present invention, and the flexibility of the positive electrode is improved. Since it can improve, the cycling characteristics of the secondary battery using this positive electrode can be improved.

  The binder may further contain other binder components in addition to the polymer having the nitrile group and the polymer each having a linear alkylene structural unit. As other binder components, various resin components can be used in combination. For example, polyethylene, polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), polyacrylic acid, polyacrylonitrile, polyacrylate, polymethacrylate, or the like may be used. it can. A copolymer containing 50% or more of the above resin component can also be used. For example, polyacrylic acid derivatives such as acrylic acid-styrene copolymer and acrylic acid-acrylate copolymer; acrylonitrile-styrene copolymer, acrylonitrile A polyacrylonitrile derivative such as an acrylate copolymer can also be used. Among these, it is preferable to use PVDF or a polyacrylonitrile derivative because the strength of the positive electrode and the resistance to electrolytic solution are excellent.

Furthermore, the soft polymers exemplified below can also be used as other binders.
Acrylic acid such as polybutyl acrylate, polybutyl methacrylate, polyhydroxyethyl methacrylate, polyacrylamide, polyacrylonitrile, butyl acrylate / styrene copolymer, butyl acrylate / acrylonitrile copolymer, butyl acrylate / acrylonitrile / glycidyl methacrylate copolymer Or an acrylic soft polymer which is a homopolymer of a methacrylic acid derivative or a copolymer with a monomer copolymerizable therewith;
Silicon-containing soft polymers such as dimethylpolysiloxane, diphenylpolysiloxane, dihydroxypolysiloxane;
Liquid polyethylene, polypropylene, poly-1-butene, ethylene / α-olefin copolymer, propylene / α-olefin copolymer, ethylene / propylene / diene copolymer (EPDM), ethylene / propylene / styrene copolymer, etc. Olefinic soft polymers of
Vinyl-based soft polymers such as polyvinyl alcohol, polyvinyl acetate, polyvinyl stearate, vinyl acetate / styrene copolymer;
Epoxy-based soft polymers such as polyethylene oxide, polypropylene oxide, epichlorohydrin rubber;
Fluorine-containing soft polymers such as vinylidene fluoride rubber and tetrafluoroethylene-propylene rubber;
And other soft polymers such as natural rubber, polypeptide, protein, polyester-based thermoplastic elastomer, vinyl chloride-based thermoplastic elastomer, and polyamide-based thermoplastic elastomer.
These soft polymers may have a cross-linked structure or may have a functional group introduced by modification. These may be used alone or in combination of two or more. Among these, a polyacrylonitrile derivative is preferable in order to improve the dispersibility of the positive electrode active material.

  The content of other binders is preferably 5 to 80% by mass, more preferably 10 to 70% by mass, based on 100% by mass of the total amount of binder (total amount of binder and other binders). %, Particularly preferably 20 to 60% by mass. When the content ratio of the other binder is within the above range, the internal resistance of the battery is not increased and high cycle characteristics can be exhibited.

  The content of the total binder (binder and other binders) in the positive electrode active material layer is preferably 0.1 to 10 parts by mass, more preferably 0.5 to 100 parts by mass of the positive electrode active material. -5 parts by mass. The content of the total binder in the positive electrode active material layer in the positive electrode for secondary battery of the present invention is in the above range, so that the positive electrode active materials and the positive electrode active material and the current collector are excellent in binding properties, Furthermore, while maintaining the flexibility of the positive electrode, the resistance does not increase without inhibiting the movement of Li.

(Other ingredients)
In addition to the above components (positive electrode active material and binder), the positive electrode active material layer has functions such as conductivity-imparting materials, reinforcing materials, dispersants, leveling agents, antioxidants, thickeners, and electrolyte decomposition inhibition. Other components such as an electrolytic solution additive having other properties and other binders may be contained, and may be contained in a positive electrode slurry described later. These are not particularly limited as long as they do not affect the battery reaction.

  As the conductivity-imparting material, conductive carbon such as acetylene black, ketjen black, carbon black, graphite, vapor-grown carbon fiber, and carbon nanotube can be used. Examples thereof include carbon powders such as graphite, and fibers and foils of various metals. By using the conductivity imparting material, the electrical contact between the positive electrode active materials can be improved, and the discharge load characteristic can be improved particularly when used for a lithium ion secondary battery.

  As the reinforcing material, various inorganic and organic spherical, plate-like, rod-like or fibrous fillers can be used. By using a reinforcing material, a tough and flexible positive electrode can be obtained, and excellent long-term cycle characteristics can be exhibited. The usage-amount of an electroconductivity imparting material or a reinforcing material is 0.01-20 mass parts normally with respect to 100 mass parts of positive electrode active materials, Preferably it is 1-10 mass parts. By being included in the said range, a high capacity | capacitance and a high load characteristic can be shown.

  Examples of the dispersant include an anionic compound, a cationic compound, a nonionic compound, and a polymer compound. A dispersing agent is selected according to the positive electrode active material and electroconductivity imparting material to be used. The content ratio of the dispersant in the positive electrode active material layer is preferably 0.01 to 10% by mass. When the amount of the dispersant is within the above range, a positive electrode slurry described later is excellent in stability, a smooth electrode can be obtained, and a high battery capacity can be exhibited.

  Examples of the leveling agent include surfactants such as alkyl surfactants, silicone surfactants, fluorine surfactants, and metal surfactants. By mixing the surfactant, it is possible to prevent the repelling that occurs during coating or to improve the smoothness of the positive electrode. The content ratio of the leveling agent in the positive electrode active material layer is preferably 0.01 to 10% by mass. 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 200 to 1000, preferably 600 to 700 is preferably used. The content ratio of the antioxidant in the positive electrode active material layer is preferably 0.01 to 10% by mass, more preferably 0.05 to 5% by mass. When the antioxidant is in the above range, the stability of the positive electrode slurry, the battery capacity, and the cycle characteristics described later are excellent.

  Examples of thickeners include cellulosic polymers such as carboxymethylcellulose, methylcellulose, hydroxypropylcellulose, and ammonium salts and alkali metal salts thereof; (modified) poly (meth) acrylic acid and ammonium salts and alkali metal salts thereof; ) Polyvinyl alcohols such as polyvinyl alcohol, copolymers of acrylic acid or acrylate and vinyl alcohol, maleic anhydride or copolymers of maleic acid or fumaric acid and vinyl alcohol; polyethylene glycol, polyethylene oxide, polyvinyl pyrrolidone, modified Examples include polyacrylic acid, oxidized starch, phosphate starch, casein, and various modified starches. When the amount of the thickener used is within this range, the coating property and the adhesion between the positive electrode active material layer and the current collector are good. In the present invention, “(modified) poly” means “unmodified poly” or “modified poly”, and “(meth) acryl” means “acryl” or “methacryl”. The content ratio of the thickener in the positive electrode active material layer is preferably 0.01 to 10% by mass. When the thickener is in the above range, it is possible to obtain a smooth positive electrode having excellent dispersibility of a positive electrode active material in a positive electrode slurry described later, and exhibit excellent load characteristics and cycle characteristics.

  As the electrolytic solution additive, vinylene carbonate used in the electrolytic solution described later can be used. The content ratio of the electrolyte solution additive in the positive electrode active material layer is preferably 0.01 to 10% by mass. When the electrolytic solution additive is in the above range, the cycle characteristics and the high temperature characteristics are excellent. Other examples include nanoparticles such as fumed silica and fumed alumina. By mixing the nanoparticles, the thixotropy of the positive electrode slurry can be controlled, and the leveling property of the positive electrode obtained thereby can be improved. The content ratio of nanoparticles and the like in the positive electrode active material layer is preferably 0.01 to 10% by mass. When the nanoparticles are in the above range, the slurry stability and productivity are excellent, and high battery characteristics are exhibited.

(Current collector)
The current collector used in the present invention is not particularly limited as long as it has electrical conductivity and is electrochemically durable, but from the viewpoint of heat resistance, for example, iron, copper, etc. Metal materials such as aluminum, nickel, stainless steel, titanium, tantalum, gold, and platinum are preferable. Among these, aluminum is particularly preferable for the positive electrode of the lithium ion secondary battery. The shape of the current collector is not particularly limited, but a sheet shape having a thickness of about 0.001 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 roughening in advance. Examples of the roughening method include a mechanical polishing method, an electrolytic polishing method, and a chemical polishing method. In the mechanical polishing method, an abrasive cloth paper 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 current collector surface in order to increase the adhesive strength and conductivity of the positive electrode active material layer.

(Method for producing positive electrode for secondary battery)
As a method for producing the positive electrode for a secondary battery of the present invention, any method may be used as long as the positive electrode active material layer is bound in layers on at least one surface, preferably both surfaces of the current collector. For example, a positive electrode slurry for a secondary battery, which will be described later, is applied to a current collector and dried, and then heated at 120 ° C. or higher for 1 hour or longer to form a positive electrode. The method for applying the positive electrode slurry to the current collector is not particularly limited. Examples thereof include a doctor blade method, a zip method, a reverse roll method, a direct roll method, a gravure method, an extrusion method, and a brush coating method. Examples of the drying method include drying by warm air, hot air, low-humidity air, vacuum drying, and drying by irradiation with (far) infrared rays or electron beams.

  Next, it is preferable to lower the porosity of the positive electrode active material layer by pressure treatment using a mold press or a roll press. The preferable range of the porosity is 5 to 15%, more preferably 7 to 13%. If the porosity is too high, charging efficiency and discharging efficiency are deteriorated. When the porosity is too low, there are problems that it is difficult to obtain a high volume capacity, or that the positive electrode active material layer is easily peeled off and a defect is likely to occur. Further, when a curable binder is used, it is preferably cured.

  The thickness of the positive electrode for secondary batteries of the present invention is usually 5 to 300 μm, preferably 10 to 250 μm. When the thickness of the positive electrode is in the above range, both load characteristics and energy density are high.

(Positive electrode slurry for secondary batteries)
The positive electrode slurry for secondary battery used when manufacturing the positive electrode for secondary battery of the present invention contains a positive electrode active material, a binder and a solvent. As the positive electrode active material and the binder, those described above are used.

(solvent)
The solvent is not particularly limited as long as it can uniformly dissolve or disperse the binder used in the present invention. As the solvent used for the positive electrode slurry, either water or an organic solvent can be used. Examples of organic solvents include cycloaliphatic hydrocarbons such as cyclopentane and cyclohexane; aromatic hydrocarbons such as toluene, xylene, and ethylbenzene; ketones such as acetone, ethylmethylketone, diisopropylketone, cyclohexanone, methylcyclohexane, and ethylcyclohexane. Chlorinated aliphatic hydrocarbons such as methylene chloride, chloroform and carbon tetrachloride; esters such as ethyl acetate, butyl acetate, γ-butyrolactone and ε-caprolactone; acylonitriles such as acetonitrile and propionitrile; tetrahydrofuran; Ethers such as ethylene glycol diethyl ether; alcohols such as methanol, ethanol, isopropanol, ethylene glycol, ethylene glycol monomethyl ether; N-methyl Amides such as lupyrrolidone and N, N-dimethylformamide may be mentioned.

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

  The solid content concentration of the positive electrode slurry used in the present invention is not particularly limited as long as it can be applied and immersed and has a fluid viscosity, but is generally about 10 to 80% by mass.

  In addition to the above-described positive electrode active material, binder and solvent, the positive electrode slurry further includes a dispersant used in the above-described positive electrode for a secondary battery, an electrolytic solution additive having a function of suppressing electrolytic decomposition, etc. Other components may be included. These are not particularly limited as long as they do not affect the battery reaction.

(Method for producing positive electrode slurry for secondary battery)
In the present invention, the method for producing the positive electrode slurry for the secondary battery is not particularly limited, and can be obtained by mixing the above-described positive electrode active material, binder, solvent, and other components added as necessary.

  In the present invention, by using the above components, a positive electrode slurry in which the components are highly dispersed can be obtained regardless of the mixing method and the mixing order. The mixing device is not particularly limited as long as it can uniformly mix the above-mentioned components. Bead mill, ball mill, roll mill, sand mill, pigment disperser, crusher, ultrasonic disperser, homogenizer, planetary mixer, fill mix, etc. Among them, 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 viscosity of the positive electrode slurry is preferably 10 to 100,000 mPa · s, more preferably 100 to 50,000 mPa · s, from the viewpoints of uniform coatability and slurry aging stability. The viscosity is a value measured using a B-type viscometer at 25 ° C. and a rotation speed of 60 rpm.

Secondary battery The secondary battery of the present invention comprises a positive electrode, a negative electrode, a separator and an electrolytic solution, and the positive electrode is the positive electrode for the secondary battery.

  Examples of the secondary battery of the present invention include a lithium ion secondary battery and a nickel hydride secondary battery. Among these, a lithium ion secondary battery is preferable as an application because long-term cycle characteristics and output characteristics are most improved. Hereinafter, a lithium ion secondary battery will be described.

(Electrolyte for lithium ion secondary battery)
As the electrolytic solution for the lithium ion secondary battery, an organic electrolytic solution in which a supporting electrolyte is dissolved in an organic solvent is used. A lithium salt is used as the supporting electrolyte. The lithium salt is not particularly _{limited, 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. Two or more of these may be used in combination. 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 organic solvent used in the electrolyte for the lithium ion secondary battery is not particularly limited as long as it can dissolve the supporting electrolyte, but dimethyl carbonate (DMC), ethylene carbonate (EC), diethyl carbonate (DEC), propylene Carbonates such as carbonate (PC), butylene carbonate (BC), methyl ethyl carbonate (MEC); esters such as γ-butyrolactone and methyl formate; ethers such as 1,2-dimethoxyethane and tetrahydrofuran; sulfolane, dimethyl sulfoxide Sulfur-containing compounds such as are preferably used. Moreover, you may use the liquid mixture of these solvents. Among these, carbonates are preferable because they have a high dielectric constant and a wide stable potential region. Since the lithium ion conductivity increases as the viscosity of the solvent used decreases, the lithium ion conductivity can be adjusted depending on the type of the solvent.

  Moreover, it is also possible to use an electrolyte containing an additive. Examples of the additive include carbonate compounds such as vinylene carbonate (VC).

  The density | concentration of the supporting electrolyte in the electrolyte solution for lithium ion secondary batteries is 1-30 mass% normally, Preferably it is 5-20 mass%. Further, it is usually used at a concentration of 0.5 to 2.5 mol / L depending on the type of the supporting electrolyte. If the concentration of the supporting electrolyte is too low or too high, the ionic conductivity tends to decrease.

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

(Separator for lithium ion secondary battery)
As a separator for a lithium ion secondary battery, a known one such as a microporous film or non-woven fabric containing a polyolefin resin such as polyethylene or polypropylene or an aromatic polyamide resin; a porous resin coat containing an inorganic ceramic powder; Can do. For example, a polyolefin film (polyethylene, polypropylene, polybutene, polyvinyl chloride), a microporous film made of a resin such as a mixture or copolymer thereof, polyethylene terephthalate, polycycloolefin, polyether sulfone, polyamide, polyimide, polyimide amide, Examples thereof include a microporous membrane made of a resin such as polyaramid, polycycloolefin, nylon, and polytetrafluoroethylene, or a woven fabric of polyolefin fibers, a nonwoven fabric thereof, an aggregate of insulating substance particles, or the like. Among these, a microporous film made of a polyolefin-based resin is preferable because the thickness of the entire separator can be reduced and the active material ratio in the secondary battery can be increased to increase the capacity per volume.

  The thickness of a separator is 0.5-40 micrometers normally, Preferably it is 1-30 micrometers, More preferably, it is 1-10 micrometers. Within this range, the resistance due to the separator in the battery is reduced, and the workability during battery production is excellent.

(Anode for lithium ion secondary battery)
The negative electrode for a lithium ion secondary battery is formed by laminating a negative electrode active material layer including a negative electrode active material and a negative electrode binder on a current collector. Examples of the current collector include the same materials as those described for the positive electrode for the secondary battery, and are not particularly limited as long as the materials have electrical conductivity and are electrochemically durable. Copper is particularly preferable for the negative electrode of the secondary battery.

(Anode active material for lithium ion secondary battery)
Examples of the negative electrode active material for a lithium ion secondary battery negative electrode include amorphous carbon, graphite, natural graphite, mesocarbon microbeads, carbonaceous materials such as pitch-based carbon fibers, and conductive polymers such as polyacene. . In addition, as the negative electrode active material, metals such as silicon, tin, zinc, manganese, iron, nickel, alloys thereof, oxides or sulfates of the metals or alloys are used. In addition, lithium alloys such as lithium metal, Li—Al, Li—Bi—Cd, and Li—Sn—Cd, lithium transition metal nitride, silicone, and the like can be used. As the negative electrode active material, a material obtained by attaching a conductivity imparting material to the surface by a mechanical modification method can also be used. The particle diameter of the negative electrode active material is appropriately selected in consideration of other constituent elements of the battery. From the viewpoint of improving battery characteristics such as initial efficiency, load characteristics, and cycle characteristics, a 50% volume cumulative diameter is usually The thickness is 1 to 50 μm, preferably 15 to 30 μm.

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

  In addition to the above components, the negative electrode for a lithium ion secondary battery further includes other components such as a dispersant used in the above-described positive electrode for a secondary battery and an electrolyte additive having a function of inhibiting decomposition of the electrolyte. May be included. These are not particularly limited as long as they do not affect the battery reaction.

(Negative electrode binder for lithium ion secondary battery)
The negative electrode binder for the lithium ion secondary battery is not particularly limited, and known materials can be used. For example, polyethylene, polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), polyacrylic acid used for the above-described positive electrode for lithium ion secondary batteries Resins such as derivatives and polyacrylonitrile derivatives, and soft polymers such as acrylic soft polymers, diene soft polymers, olefin soft polymers, and vinyl soft polymers can be used. These may be used alone or in combination of two or more.

  The thickness of the negative electrode for a lithium ion secondary battery is usually 5 to 300 μm, preferably 10 to 250 μm. When the negative electrode thickness is in the above range, both load characteristics and energy density are high.

  The negative electrode for lithium ion secondary batteries can be manufactured by the same method as the positive electrode for lithium ion secondary batteries described above.

  As a specific method for producing a lithium ion secondary battery, 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. If necessary, an expanded metal, an overcurrent prevention element such as a fuse or a PTC element, a lead plate, or the like can be inserted to prevent an increase in pressure inside the battery and overcharge / discharge. The shape of the 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 described with reference to examples, but the present invention is not limited thereto. In addition, unless otherwise indicated, the part and% in a present Example are a mass reference | standard. In the examples and comparative examples, various physical properties are evaluated as follows.

<Measurement of charging average voltage>
The battery was charged to the upper limit voltage in the evaluation of the room temperature cycle characteristics and the high temperature cycle characteristics of each Example / Comparative Example by the constant current method of 0.2C, and the potential (plateau) at which lithium desorption occurred at that time The charge average voltage was used. The upper limit voltage is a voltage that exceeds the voltage and may cause expansion and heat generation of the battery, which is the limit of ensuring safety.

<Measurement of iodine value>
100 g of the NMP solution of the binder was coagulated with 1 liter of methanol and then vacuum dried at 60 ° C. overnight. The iodine value of the dried binder was measured according to JIS K6235;

<Electrode characteristics: peel strength>
The positive electrode on which the positive electrode active material layer is formed is cut into a rectangle having a width of 1.0 cm and a length of 10 cm to form a test piece, and fixed with the positive electrode active material layer surface facing up. After applying the cellophane tape to the surface of the positive electrode active material layer of the test piece, the stress when the cellophane tape was peeled off from one end of the test piece at a rate of 50 mm / min in the 180 ° direction was measured. The measurement was performed 10 times, the average value was obtained, and this was taken as the peel strength (N / m) and evaluated according to the following criteria. The higher the peel strength, the better the binding property of the positive electrode active material layer.
A: 15 N / m or more B: 10 N / m or more and less than 15 N / m C: 5.0 N / m or more and less than 10 N / m D: Less than 5.0 N / m

<Electrode characteristics: electrode resistance>
The positive electrode on which the positive electrode active material layer is formed is cut into a rectangle having a width of 1.0 cm and a length of 10 cm to obtain a test piece. Using a resistivity meter Loresta GP (Mitsubishi Chemical Corporation), the four-terminal resistance of the positive electrode active material layer was measured in accordance with JIS K7194; It shows that it is excellent in dispersion stability, so that resistance value is small.
A: Less than 1000Ω B: 1000Ω or more and less than 5000Ω C: 5000Ω or more and less than 10,000Ω D: 10,000Ω or more and less than 50000Ω E: 50000Ω or more

<Battery characteristics: Room temperature cycle characteristics>
A 10-cell half-cell coin-type lithium ion secondary battery was charged to 4.3 to 5 V by a constant current method of 0.2 C in a 20 ° C. atmosphere, and charged and discharged to 3.0 V was repeated 50 cycles. Charge / discharge capacity retention represented by the ratio (%) of the electric capacity at the end of 50 cycles and the electric capacity at the end of 5 cycles is obtained, and this is used as an evaluation standard for room temperature cycle characteristics, and evaluated according to the following criteria. It shows that it is excellent in room temperature cycling characteristics, so that this value is large.
A: 80% or more B: 70% or more and less than 80% C: 50% or more and less than 70% D: 30% or more and less than 50% E: Less than 30%

<Battery characteristics: High-temperature cycle characteristics>
A 10-cell half-cell coin-type lithium ion secondary battery was charged to 4.3 to 5 V by a constant current method of 0.2 C in an atmosphere of 60 ° C., and charging / discharging to 3.0 V was repeated 50 cycles. The charge / discharge capacity retention ratio represented by the ratio (%) of the electric capacity at the end of 50 cycles and the electric capacity at the end of 5 cycles is obtained, and this is used as an evaluation standard for high-temperature cycle characteristics, and evaluated according to the following criteria. It shows that it is excellent in high temperature cycling characteristics, so that this value is large.
A: 60% or more B: 50% or more and less than 60% C: 40% or more and less than 50% D: 30% or more and less than 40% E: 20% or more and less than 30% F: Less than 20%

Example 1
(Manufacture of binder)
In an autoclave equipped with a stirrer, 240 parts of ion exchange water, 2.5 parts of sodium alkylbenzene sulfonate, 20 parts of acrylonitrile, 30 parts of butyl acrylate and 4.5 parts of methacrylic acid were put in this order, and the inside of the bottle was replaced with nitrogen. Thereafter, 45.5 parts of butadiene is injected, 0.25 part of ammonium persulfate is added, and a polymerization reaction is performed at a reaction temperature of 40 ° C. to obtain a polymer comprising a polymer unit having a nitrile group and a conjugated diene monomer unit. Obtained. The polymerization conversion was 85% and the iodine value was 280.

  400 ml of the above polymer having a total solid content concentration adjusted to 12% by weight (48 g of total solid content) is put into a 1 liter autoclave equipped with a stirrer, and nitrogen gas is allowed to flow for 10 minutes to dissolve dissolved oxygen in the polymer. After the removal, 75 mg of palladium acetate as a hydrogenation catalyst was dissolved in 180 ml of water to which 4 times moles of nitric acid had been added relative to Pd and added. After the inside of the system was replaced twice with hydrogen gas, the autoclave contents were heated to 50 ° C. while being pressurized with hydrogen gas up to 3 MPa, and the hydrogenation reaction (referred to as “first-stage hydrogenation reaction”) for 6 hours. ) At this time, the iodine value of the polymer was 35.

  Next, the autoclave was returned to atmospheric pressure, and 25 mg of palladium acetate was dissolved in 60 ml of water to which 4 times moles of nitric acid had been added to Pd as a hydrogenation catalyst and added. After the inside of the system was replaced twice with hydrogen gas, the contents of the autoclave were heated to 50 ° C. while being pressurized with hydrogen gas up to 3 MPa, and the hydrogenation reaction (referred to as “second stage hydrogenation reaction”) was performed for 6 hours. )

  Thereafter, the contents were returned to room temperature, the inside of the system was made into a nitrogen atmosphere, and then concentrated using an evaporator until the solid concentration was 40% to obtain a binder aqueous dispersion. The binder had an iodine value of 10. Further, when a dried product was obtained from the binder aqueous dispersion in the same manner as described in the above “Measurement of iodine value” and analyzed by NMR, the binder was a polymer unit having a nitrile group with respect to the total amount of the polymer. 20% by mass of (acrylonitrile monomer unit), 45.5% by mass of linear alkylene structural unit having 4 or more carbon atoms (butadiene monomer unit), and polymerized unit (methacrylic group) having a hydrophilic group (carboxylic acid group) Acid monomer unit) was contained in an amount of 4.5% by mass, and another monomer unit (butyl acrylate monomer unit) was contained in an amount of 30% by mass.

  To 100 parts of this binder aqueous dispersion, 320 parts of N-methylpyrrolidone (hereinafter referred to as “NMP”) was added, and water was evaporated under reduced pressure to obtain an NMP solution of a positive electrode binder.

[Production of positive electrode slurry and positive electrode]
100 parts of a solid solution Li [Ni _0.15 Li _0.2 Co _0.1 Mn _0.55 ] O _{2 of} LiNiO ₂ and Li ₂ MnO ₃ which is a lithium-containing composite metal oxide having a layered structure as a positive electrode active material, , 2.0 parts of acetylene black (HS-100: Denki Kagaku Kogyo), 1.0 part of NMP solution of the binder (solid content, solid content concentration 8.0%), and an appropriate amount of NMP in a planetary mixer And stirred to prepare a positive electrode slurry. Moreover, the charge average voltage was measured about the positive electrode active material. The results are shown in Table 1.

An aluminum foil with a thickness of 20 μm was prepared as a current collector. The positive electrode slurry was applied on an aluminum foil with a comma coater so that the film thickness after drying was about 65 μm, dried at 60 ° C. for 20 minutes, 120 ° C. for 20 minutes, and then heat-treated at 150 ° C. for 2 hours to produce a positive electrode raw material. Got anti. This positive electrode original fabric was rolled by a roll press to produce a positive electrode comprising a positive electrode active material layer having a density of 2.5 g / cm ³ and an aluminum foil. The positive electrode had a thickness of 70 μm. Peel strength measurement was performed using the produced positive electrode. The results are shown in Table 1.

[Production of battery]
The positive electrode is cut into a disk shape with a diameter of 16 mm, and a separator made of a disk-shaped polypropylene porous film with a diameter of 18 mm and a thickness of 25 μm, a metallic lithium used as the negative electrode, and an expanded metal are sequentially laminated on the surface of the positive electrode active material layer. This was stored in a stainless steel coin-type outer container (diameter 20 mm, height 1.8 mm, stainless steel thickness 0.25 mm) provided with polypropylene packing. The electrolyte is poured into the container so that no air remains, and the outer container is fixed with a 0.2 mm thick stainless steel cap through a polypropylene packing, and the battery can is sealed, and the diameter is A lithium ion coin battery (half cell) having a thickness of 20 mm and a thickness of about 2 mm was produced. As the electrolyte, ethylene carbonate (EC) and ethyl methyl carbonate (EMC) EC: EMC = 3 : 7 to LiPF ₆ 1 mixed in a mixed solvent comprising at (20 ° C. volume ratio in) mol / A solution dissolved at a concentration of 1 liter was used. Using this battery, room temperature cycle characteristics and high temperature cycle characteristics were evaluated. The upper limit voltage in the evaluation of the room temperature cycle characteristics and the high temperature cycle characteristics was 4.5V. The results are shown in Table 1.

(Example 2)
LiNi _0.5 Mn _1.5 O ₄ (particle diameter: 16 μm), which is a lithium-containing composite metal oxide having a spinel structure, was used as the positive electrode active material instead of the solid solution of LiNiO ₂ and Li ₂ MnO _3. Except for the above, the same operation as in Example 1 was performed to obtain a positive electrode slurry and a positive electrode, and a battery was produced. The results of each evaluation are shown in Table 1. The upper limit voltage in the evaluation of the room temperature cycle characteristics and the high temperature cycle characteristics was 5.0V.

(Example 3)
As a binder, except having used the following binder, operation similar to Example 2 was performed, the positive electrode slurry and the positive electrode were obtained, and the battery was produced. The results of each evaluation are shown in Table 1. The upper limit voltage in the evaluation of the room temperature cycle characteristics and the high temperature cycle characteristics was 5.0V.

(Manufacture of binder)
In an autoclave equipped with a stirrer, 240 parts of ion-exchanged water, 2.5 parts of sodium alkylbenzenesulfonate, 20 parts of acrylonitrile and 30 parts of butyl acrylate are placed in this order, and the inside of the bottle is replaced with nitrogen, and then 50 parts of butadiene is injected. Then, 0.25 part of ammonium persulfate was added and polymerized at a reaction temperature of 40 ° C. to obtain a polymer comprising a polymer unit having a nitrile group and a conjugated diene monomer unit. The polymerization conversion was 85% and the iodine value was 280.

  400 ml of the above polymer having a total solid content concentration adjusted to 12% by weight (48 g of total solid content) is put into a 1 liter autoclave equipped with a stirrer, and nitrogen gas is allowed to flow for 10 minutes to dissolve dissolved oxygen in the polymer. After the removal, 75 mg of palladium acetate as a hydrogenation catalyst was dissolved in 180 ml of water to which 4 times moles of nitric acid had been added relative to Pd and added. After the inside of the system was replaced twice with hydrogen gas, the autoclave contents were heated to 50 ° C. while being pressurized with hydrogen gas up to 3 MPa, and the hydrogenation reaction (referred to as “first-stage hydrogenation reaction”) for 6 hours. ) At this time, the iodine value of the polymer was 35.

  Next, the autoclave was returned to atmospheric pressure, and 25 mg of palladium acetate was dissolved in 60 ml of water to which 4 times moles of nitric acid had been added to Pd as a hydrogenation catalyst and added. After the inside of the system was replaced twice with hydrogen gas, the contents of the autoclave were heated to 50 ° C. while being pressurized with hydrogen gas up to 3 MPa, and the hydrogenation reaction (referred to as “second stage hydrogenation reaction”) was performed for 6 hours. )

  Thereafter, the contents were returned to room temperature, the inside of the system was made into a nitrogen atmosphere, and then concentrated using an evaporator until the solid concentration was 40% to obtain a binder aqueous dispersion. The binder had an iodine value of 10. Further, when a dried product was obtained from the binder aqueous dispersion in the same manner as described in the above “Measurement of iodine value” and analyzed by NMR, the binder was a polymer unit having a nitrile group with respect to the total amount of the polymer. 20% by mass (acrylonitrile monomer units), 50% by mass of linear alkylene structural units having 4 or more carbon atoms (butadiene monomer units), and 30% other monomer units (butyl acrylate monomer units). It contained mass%.

  To 100 parts of this binder aqueous dispersion, 320 parts of N-methylpyrrolidone (hereinafter referred to as “NMP”) was added, and water was evaporated under reduced pressure to obtain an NMP solution of a positive electrode binder.

Example 4
As a binder, except having used the following binder, operation similar to Example 1 was performed, the positive electrode slurry and the positive electrode were obtained, and the battery was produced. The results of each evaluation are shown in Table 1. The upper limit voltage in the evaluation of the room temperature cycle characteristics and the high temperature cycle characteristics was 4.5V.

(Manufacture of binder)
In a metal bottle, 0.2 parts of sodium carbonate is dissolved in 200 parts of ion-exchanged water, and 2.5 parts of potassium caprate is added thereto to prepare a soap solution, and formaldehyde naphthalene sulfonate is used as a dispersant. 1.0 part of polycondensate was added. Next, 36.2 parts of acrylonitrile and 0.5 part of t-dodecyl mercaptan (molecular weight modifier) were charged in this order in the soap solution, and the internal gas was replaced with nitrogen three times, and then 63.8 parts of 1,3-butadiene. Was charged. The metal bottle was kept at 5 ° C., 0.1 part of cumene hydroperoxide (polymerization initiator), an appropriate amount of a reducing agent and a chelating agent were charged, and the polymerization reaction was carried out for 16 hours while keeping the temperature at 5 ° C. Thereafter, 0.1 part of a 10 wt% aqueous hydroquinone (polymerization terminator) aqueous solution was added to stop the polymerization reaction, and the residual monomer was removed using a rotary evaporator at a water temperature of 60 ° C. to give a polymer unit having a nitrile group. And an aqueous dispersion (solid content concentration: 25% by weight) of a polymer comprising conjugated diene monomer units. The polymerization conversion was 85% and the iodine value was 300.

  Then, the aqueous dispersion of the above polymer is added to an aqueous solution of magnesium sulfate in an amount of 12 parts when the solid content in the aqueous dispersion is 100 parts, and the aqueous dispersion is solidified by stirring. Then, after washing with water and filtering, a polymer was obtained by vacuum drying at 60 ° C. for 12 hours.

Next, the obtained polymer was dissolved in acetone so as to have a concentration of 12% by mass, put in an autoclave, and a palladium / silica catalyst having an amount of Pd metal of 1000 ppm with respect to the amount of the polymer was added. The hydrogenation reaction was performed at 0.0 MPa. After completion of the hydrogenation reaction, the mixture was poured into a large amount of water to coagulate, filtered and dried to obtain a binder. The iodine value of the binder was 7. Further, when the binder was analyzed by H ¹ -NMR, the binder was a polymer unit having a nitrile group (acrylonitrile monomer unit) of 36.2% by mass and a straight chain having 4 or more carbon atoms, based on the total amount of the polymer. It contained 63.8% by mass of a chain alkylene structural unit (butadiene monomer unit).

  This binder was dissolved in NMP so that the solid content concentration was 8% by mass to obtain an NMP solution of a positive electrode binder.

(Comparative Example 1)
Except that LiCoO ₂ (particle diameter: 19 μm) was used instead of the solid solution of LiNiO ₂ and Li ₂ MnO ₃ as the positive electrode active material, the same operation as in Example 4 was performed to adjust the positive electrode slurry, and the density Produced a positive electrode comprising a positive electrode active material layer of 3.7 g / cm ³ and an aluminum foil. The positive electrode had a thickness of 70 μm.

  Using the positive electrode, the same operation as in Example 4 was performed to produce a battery. The results of each evaluation are shown in Table 1. The room temperature cycle characteristics and the high temperature cycle characteristics of the battery of Comparative Example 1 could not be evaluated because the battery expanded during charging and discharging. The upper limit voltage in the evaluation of the room temperature cycle characteristics and the high temperature cycle characteristics was 4.5V.

(Comparative Example 2)
The following binders were used as binders. Further, LiNi _0.5 Mn _1.5 O ₄ (particle diameter: 16 μm), which is a lithium-containing composite metal oxide having a spinel structure, was used as the positive electrode active material.

100 parts of LiNi _0.5 Mn _1.5 O ₄ (particle diameter: 16 μm), 2.0 parts of acetylene black (HS-100: Electrochemical Industry), 2.5 parts of an aqueous dispersion of the following binder (solid content) Concentration 40%), 40 parts of a carboxymethyl cellulose aqueous solution (solid content concentration 2%) having a degree of etherification of 0.8 as a thickener, and an appropriate amount of water are stirred with a planetary mixer to prepare a positive electrode slurry. Prepared.

The positive electrode slurry is coated on a 20 μm thick aluminum foil with a comma coater so that the film thickness after drying is about 70 μm, dried at 60 ° C. for 20 minutes, and then heat treated at 150 ° C. for 2 hours to form a positive electrode raw material. Got. This positive electrode original fabric was rolled with a roll press to produce a positive electrode comprising a positive electrode active material layer having a density of 2.1 g / cm ³ and an aluminum foil. The positive electrode had a thickness of 65 μm. Peel strength measurement was performed using the produced positive electrode. The results are shown in Table 1.

  A battery was fabricated in the same manner as in Example 1 except that the positive electrode obtained above was used as the positive electrode. The results of each evaluation are shown in Table 1. The upper limit voltage in the evaluation of the room temperature cycle characteristics and the high temperature cycle characteristics was 5.0V.

(Manufacture of binder)
In a 5 MPa pressure vessel with a stirrer, 46 parts of styrene, 49 parts of 1,3-butadiene, 5 parts of methacrylic acid, 5 parts of sodium dodecylbenzenesulfonate, 150 parts of ion-exchanged water, 1 part of potassium persulfate as a polymerization initiator, After sufficiently stirring, the polymerization was started by heating to 50 ° C. When the monomer consumption reached 95.0%, the reaction was stopped by cooling, and a binder aqueous dispersion having a solid concentration of 40% (number average particle diameter of polymer particles: 100 nm, glass transition temperature of polymer particles: − 15 ° C.). The binder did not contain a polymer unit having a nitrile group or a linear alkylene structural unit having 4 or more carbon atoms.

(Comparative Example 3)
As a binder, except having used the following binder, operation similar to the comparative example 2 was performed, the positive electrode slurry and the positive electrode were obtained, and the battery was produced. The results of each evaluation are shown in Table 1. The upper limit voltage in the evaluation of the room temperature cycle characteristics and the high temperature cycle characteristics was 5.0V.

(Manufacture of binder)
To polymerization can A, 0.5 parts of sodium lauryl sulfate and 15 parts of ion-exchanged water were added, 0.25 part of ammonium persulfate and 10 parts of ion-exchanged water were added as polymerization initiators, and the mixture was heated to 70 ° C. The emulsion prepared by adding 35 parts of butyl acrylate, 60 parts of ethyl acrylate, 5 parts of methacrylic acid, 0.7 part of sodium lauryl sulfate and 60 parts of ion-exchanged water to B is polymerized from the polymerization vessel B over about 180 minutes. After sequentially adding to Can A, the mixture was stirred for about 120 minutes, and when the monomer consumption reached 95%, the reaction was terminated by cooling, and then the pH was adjusted with a 4% NaOH aqueous solution to obtain a binder aqueous dispersion.

  To 100 parts of this binder aqueous dispersion, 320 parts of NMP was added, and water was evaporated under reduced pressure to obtain a binder NMP solution.

From the results of Table 1, the positive electrodes of Examples 1 to 4 are excellent in peel strength and dispersion stability. Moreover, the secondary batteries of Examples 1 to 4 are excellent in room temperature cycle characteristics and high temperature cycle characteristics. On the other hand, a positive electrode and a secondary battery (Comparative Example 1) using a positive electrode active material having an average charge voltage of less than 3.9 V, and a positive electrode and a secondary battery (Comparative Example) using a binder not containing a polymer unit having a nitrile group. 2, 3) is inferior to the balance of peel strength, dispersion stability, room temperature cycle characteristics and high temperature cycle characteristics as compared with Examples 1 to 4.

Claims (5)

A current collector and a positive electrode active material layer laminated on the current collector and containing a positive electrode active material and a binder;
The charge average voltage of the positive electrode active material with respect to lithium metal is 3.9 V or more,
The binder comprises a polymer unit having a nitrile group and a linear alkylene structural unit having 4 or more carbon atoms,
The content ratio of the polymer unit having the nitrile group in the binder is 2 to 50% by mass, and
The positive electrode for secondary batteries whose iodine value of the said binder is 3-60 mg / 100 mg.   The positive electrode for a secondary battery according to claim 1, wherein the binder contains 5 to 30% by mass of the polymer unit having the nitrile group.   The positive electrode for a secondary battery according to claim 1, wherein the binder further comprises a polymer unit having a hydrophilic group. The positive electrode for a secondary battery according to any one of claims 1 to 3, wherein the positive electrode active material is represented by any one of the following general formulas (I) to (III):
xLiMaO ₂ · (1-x) Li ₂ MbO ₃ (I)
LiyMcPO ₄ (II)
LiMd _0.5 Mn _1.5 O ₄ (III)
(Where x is a number that satisfies 0 <x <1, y is a number that satisfies 0 ≦ y ≦ 2, Ma and Mc are one or more transition metals having an average oxidation state of 3+, and Mb and Md is one or more transition metals with an average oxidation state of 4+). A secondary battery having a positive electrode, a negative electrode, a separator and an electrolyte solution,
The secondary battery whose said positive electrode is a positive electrode for secondary batteries in any one of Claims 1-4.