WO2010092977A1

WO2010092977A1 - Electrode mixture slurry for lithium secondary batteries, and electrode and lithium secondary battery that use said slurry

Info

Publication number: WO2010092977A1
Application number: PCT/JP2010/051955
Authority: WO
Inventors: 明天高; 博之有馬; 英郎坂田; 瞳中澤
Original assignee: ダイキン工業株式会社
Priority date: 2009-02-12
Filing date: 2010-02-10
Publication date: 2010-08-19
Also published as: KR20110111481A; CN102318108A; JP5625917B2; JPWO2010092977A1

Abstract

Disclosed is an electrode mixture slurry that is stable with no gelation, and additionally disclosed are an electrode with good adhesion between the binder and collector and with ample flexibility, and a lithium secondary battery with excellent battery characteristics. The electrode mixture slurry for lithium secondary batteries comprises an electrode active material, a binder, and an organic solvent. The binder is a fluorine-containing polymer represented by the composition formula: (VDF)_m(TFE)_n(HFP)_l (In the formula, VDF represents structural unit derived from vinylidene fluoride; TFE represents a structural unit derived from tetrafluoroethylene; HFP represents a structural unit derived from hexafluoropropylene; 0.45 ≦ m ≦ 1; 0.05 ≦ n ≦ 0.5; 0 ≦ 1 ≦ 0.1; where m + n + l = 1.), and includes a solvent-soluble thermoplastic resin that is not a fluorine-containing polymer.

Description

Slurry for electrode mixture of lithium secondary battery, electrode using the slurry, and lithium secondary battery

The present invention relates to a slurry for an electrode mixture of a lithium secondary battery excellent in stability, a flexible electrode using the slurry, and a lithium secondary battery having improved battery characteristics.

Lithium secondary batteries are widely used as power sources for various portable electric and electronic devices or as batteries for electric vehicles.

A lithium secondary battery is equipped with a positive electrode, a negative electrode, a non-aqueous electrolyte, and usually a separator, and development and improvement of each member is actively performed.

Of these, the positive electrode is usually prepared by, for example, dispersing a positive electrode active material in a binder, and if necessary, a conductive material together with an organic solvent to prepare a slurry for a positive electrode mixture. After applying to a positive electrode current collector, the solvent is removed by drying. It is produced by rolling.

As a binder for a positive electrode of a lithium secondary battery, polyvinylidene fluoride (PVdF) has been often used conventionally. For example, in Patent Document 1, a positive electrode mixture prepared by mixing lithium-containing oxide such as LiCoO ₂ as a positive electrode active material and graphite as a conductive agent with PVdF is dispersed in N-methylpyrrolidone to form a slurry. Is applied to a positive electrode current collector made of aluminum foil, and a negative electrode mixture prepared by mixing a carbonaceous material as a negative electrode active material and PVdF is dispersed in N-methylpyrrolidone to form a slurry. The technique which apply | coats on the copper foil which is a body, and after each drying is compression-molded with a roller press machine and processed into an electrode sheet is disclosed. However, PVdF easily swells with respect to the organic solvent of the non-aqueous electrolyte such as propylene carbonate, ethylene carbonate, diethyl carbonate, or a mixture thereof used in the lithium ion secondary battery. For this reason, the adhesiveness with the metal foil as the current collector becomes worse as charging and discharging are repeated, resulting in a problem that the battery internal resistance increases and the battery performance deteriorates. Furthermore, the electrode sheet using PVdF as a binder is poor in flexibility, and the electrode sheet used in the production of the square battery is folded at 180 degrees, or the electrode sheet used in the production of the cylindrical battery is used in the process of rounding the electrode sheet small. The problem that the electrode mixture is peeled off from the sheet is likely to occur, and the production yield becomes difficult.

Patent Document 2 discloses vinylidene fluoride (VdF) -hexafluoropropylene (HFP) for the purpose of imparting binding properties to the expansion and contraction of the positive electrode active material during charge and discharge in a non-aqueous electrolyte secondary battery. A material having rubber elasticity, which is mainly composed of a fluorine-based binary copolymer such as a copolymer, a VdF-3 fluoroethylene chloride (CTFE) copolymer, is described as a binder. However, such a copolymer has poor crystallinity as compared with PVdF, and therefore, it is more likely to swell with respect to the organic solvent of the non-aqueous electrolyte than PVdF, and depending on the type of the electrolyte, it is eluted and used as a binder. No longer play a role.

As a similar binder, Patent Document 3 describes that a fluorine-based polymer copolymer mainly composed of VdF, tetrafluoroethylene (TFE) and HFP is used as a binder instead of PVdF. Has been. The composition range of the copolymer described in the claims is, in terms of molar fraction, VdF of 0.3 to 0.9, HFP of 0.03 to 0.5, and TFE of 0 to 0.5. The sum of the molar fractions of these three monomers is 0.80 to 1.

In addition, Patent Document 4 describes a binder that is soluble in a general-purpose solvent but hardly swells in an organic solvent of an electrolytic solution. The binder disclosed in Patent Document 4 includes VdF 50 to 80 mol% and TFE 20 to 50 mol% binary fluorine-containing copolymer, VdF 50 to 80 mol%, TFE 17 to 50 mol%, and other copolymerization monomers. It is a ternary fluorine-containing copolymer of less than 3 mol%, and VdF / TFE copolymer and VdF / TFE / HFP copolymer are described as VdF / TFE copolymer used in the examples. Yes. In order to improve the adhesion to the current collector, the content of resin such as polymethacrylate, polymethylmethacrylate, polyacrylonitrile, polyimide, polyamide, polyamideimide, polycarbonate, etc. in the binder is about 20% by volume. It is described that it may be included below.

Patent Document 5 proposes to use a polyimide on the positive electrode side and an aromatic polyamide on the negative electrode side in addition to PVdF as a binder in order to improve the cycle characteristics at high temperature. .

Furthermore, Patent Document 6 proposes a method of treating the surface of the current collector with an acrylic polymer in order to improve the adhesion between the current collector and the binder. Further, it is described that a mixture of 50 to 95% by weight of PVdF and a copolymer of VdF and another polymer (for example, TFE, HFP, CTFE, etc.) can be used.

On the other hand, various positive electrode active materials have been developed from the viewpoints of battery characteristics, safety, and resource (rare metal) depletion, and recently, positive electrode active materials that contain Ni and Mn and that contain less Co, which is a rare metal, have emerged. is doing. However, these positive electrode materials containing Ni and Mn have high basicity, so that the slurry is easily gelled. Further, as for the negative electrode active material, an active material made of a basic material has appeared in addition to the carbon-based material that has been used conventionally.

Japanese Unexamined Patent Publication No. 04-249859 Japanese Patent Laid-Open No. 04-095363 Japanese Patent Publication No. 08-004007 Japanese Patent Laid-Open No. 10-233217 Japanese Patent Laid-Open No. 11-031513 JP 09-199133 A

By the way, lithium-containing composite oxides including LiCoO ₂ , LiNiO ₂ , LiMn ₂ O ₄ are basically basic, and the reason for this is not confirmed, but they were coexisted with PVdF and many VdF-based copolymers. The positive electrode mixture slurry has a problem that gelation occurs and the stability of the slurry is impaired. In the negative electrode, when a basic material is used as the negative electrode active material, there are similar problems. Further, since the basic negative electrode is swelled severely, the conventional PVdF tends to cause the negative electrode active material to fall off. Although the use of polyimide as a binder has been studied, there is a problem that the obtained electrode is very hard and easily cracked.

Further, the VdF / TFE copolymer is rich in flexibility, but has a slight difficulty in adhesion to the current collector, and its improvement is also required.

An object of the present invention is to provide a stable slurry for an electrode mixture that does not cause gelation and a method for producing the same, and thus an electrode having high flexibility due to improved adhesion between the mixture and the current collector, and further battery characteristics. It is in providing the lithium secondary battery excellent in the.

As a result of further investigations by the present inventors, a VdF / TFE copolymer obtained by copolymerizing a specific amount of TFE with VdF is surprisingly a basic electrode active material. The electrode mixture slurry prepared by mixing is found to be homogeneous and stable, and the electrode formed using this electrode mixture slurry has excellent flexibility. It was found that the electrode mixture and the current collector were not peeled off, and the battery characteristics of the lithium secondary battery were improved.

Such excellent base resistance is specifically seen in VdF / TFE copolymers not found in other VdF copolymers such as VdF / HFP copolymers and VdF / CTFE copolymers. Is a characteristic.

That is, the present invention is a slurry for electrode mixture of a lithium secondary battery containing an electrode active material (A), a binder (B) and an organic solvent (C), wherein the binder (B)
(B1) Composition formula (B1):
(VDF) _m (TFE) _n (HFP) _l
(In the formula, VDF is a structural unit derived from vinylidene fluoride; TFE is a structural unit derived from tetrafluoroethylene; HFP is a structural unit derived from hexafluoropropylene; 0.45 ≦ m ≦ 1; 0.05 ≦ n ≦ 0. 5; 0 ≦ l ≦ 0.1, where m + n + l = 1) and (B2) a solvent-soluble thermoplastic resin other than the fluorine-containing polymer (B1). The present invention relates to a slurry for an electrode mixture of a lithium secondary battery.

The electrode active material (A) has the formula (A1):
Li _x M ¹ _y M ² _1-y O ₂
(Wherein 0.4 ≦ x ≦ 1; 0.3 ≦ y ≦ 1; M ¹ is at least one selected from the group consisting of Ni and Mn; M ² is selected from the group consisting of Co, Al and Fe) Particularly suitable when the positive electrode active material (A1) includes a lithium-containing composite metal oxide represented by at least one kind, and the electrode active material (A) is a basic material containing Si and / or Sn. It is particularly suitable when it is a negative electrode active material (A2) containing.

The present invention also relates to an electrode of a lithium secondary battery obtained by applying the slurry for electrode mixture of the present invention to a current collector and drying it.

Furthermore, the present invention also relates to a lithium secondary battery including the non-aqueous electrolyte using the electrode of the present invention as a positive electrode and / or a negative electrode.

According to the present invention, a homogeneous and stable slurry for an electrode mixture can be provided, and an electrode having excellent adhesiveness with a current collector formed using the slurry for an electrode mixture and rich in flexibility. A lithium secondary battery excellent in battery characteristics can be provided using an electrode mixture.

The electrode mixture slurry of the lithium secondary battery of the present invention includes an electrode active material (A), a binder (B), and an organic solvent (C). Hereinafter, each component will be described.

(A) Electrode active material In this invention, a positive electrode active material (A1) or a negative electrode active material (A2) may be sufficient.

(A1) Cathode Active Material As the cathode active material (A1), the formula (A1):
Li _x M ¹ _y M ² _1-y O ₂
(Wherein 0.4 ≦ x ≦ 1; 0.3 ≦ y ≦ 1; M ¹ is at least one selected from the group consisting of Ni and Mn; M ² is selected from the group consisting of Co, Al and Fe) A lithium-containing composite metal oxide represented by at least one).

In particular,
Formula (A1-1):
LiNi _x Co _y Al _z O ₂
(Wherein 0.7 ≦ x ≦ 1; 0 ≦ y ≦ 0.3; 0 ≦ z ≦ 0.03; 0.9 ≦ x + y + z ≦ 1.1),
Formula (A1-2):
LiNi _x Co _y Mn _z O ₂
(Wherein 0.3 ≦ x ≦ 0.6; 0 ≦ y ≦ 0.4; 0.3 ≦ z ≦ 0.6; 0.9 ≦ x + y + z ≦ 1.1),
Formula (A1-3):
Li _x Mn _z O ₂
(Wherein 0.4 ≦ x ≦ 0.6; 0.9 ≦ z ≦ 1), or formula (A1-4):
LiFe _x Co _y Mn _z O ₂
(Wherein 0.3 ≦ x ≦ 0.6; 0.1 ≦ y ≦ 0.4; 0.3 ≦ z ≦ 0.6; 0.9 ≦ x + y + z ≦ 1.1)
The lithium-containing composite metal oxide represented by

Specific examples of the lithium-containing composite metal oxide represented by the formula (A1-1) include, for example, LiNi _0.8 Co _0.2 O ₂ , LiNi _0.7 Co _0.3 O ₂ , LiNi _0.82 Co _0.15 Al _0.03 O ₂ , LiNi _0.7 Co _0.2 Al Examples thereof include _0.1 O ₂ and LiNi _0.85 Co _0.1 Al _0.5 O _2. Among them, LiNi _0.82 Co _0.15 Al _0.03 O ₂ (NCA) is preferable.

Specific examples of the lithium-containing composite metal oxide represented by the formula (A1-2) include, for example, LiNi _0.5 Mn _0.5 O ₂ , LiNi _0.75 Mn _0.25 O ₂ , LiNi _0.25 Mn _0.75 O ₂ , LiNi _1/3 Co _{1 / 3} Mn _1/3 O ₂ , LiNi _0.4 Co _0.2 Mn _0.4 O ₂ , LiNi _0.3 Co _0.5 Mn _0.2 O _2, etc., among which LiNi _1/3 Co _1/3 Mn _1/3 O ₂ (NCM) preferable.

Specific examples of the lithium-containing composite metal oxide represented by the formula (A1-3) include Li _0.5 MnO ₂ (spinel manganese), LiMnO _{2 and the} like.

Specific examples of the lithium-containing composite metal oxide represented by the formula (A1-4) include, for example, LiFe _1/3 Co _1/3 Mn _1/3 O ₂ , Li _0.5 Fe _1/3 Co _1/3 Mn _{1 / 3} O ₂ , LiFe _0.4 Co _0.3 Mn _0.3 O ₂ , Li _0.5 Fe _0.4 Co _0.3 Mn _0.3 O _{2 and the} like can be mentioned.

In addition, LiCoO ₂ , LiNiO ₂ , LiMn ₂ O _{4 and the} like can also be used.

(A2) Negative electrode active material Examples of the negative electrode active material (A2) include known basic materials such as a basic material containing Si and / or Sn. Specifically, metal compounds capable of inserting lithium ions, such as metal oxides and metal nitrides, Si, SiCuAl, SiNiAg, and CoSn ₂ can also be used. Examples of the metal oxide include metal oxides containing Si and Sn, and examples of the metal nitride include Li _2.6 Co _0.4 N.

(B) Binder In the present invention, as the binder, a VdF / TFE-based fluoropolymer (B1) represented by the composition formula (B1) and a solvent-soluble thermoplastic resin (B2) (however, 2 types of polymers are used.

(B1) VdF / TFE fluorine-containing polymer The binder (B1) has a composition formula (B1):
(VDF) _m (TFE) _n (HFP) _l
(In the formula, VDF is a structural unit derived from vinylidene fluoride; TFE is a structural unit derived from tetrafluoroethylene; HFP is a structural unit derived from hexafluoropropylene; 0.45 ≦ m ≦ 1; 0.05 ≦ n ≦ 0. 5; 0 ≦ l ≦ 0.1, where m + n + l = 1). The structural units VDF, TFE, and HFP can be connected in an arbitrary order, and may exist at random.

Among these, as the fluoropolymer, in the formula (B1), 0.50 ≦ m ≦ 0.90, 0.10 ≦ n ≦ 0.50 and 0 ≦ l ≦ 0.08 (where m + n + 1 = 1) VdF / TFE fluorine-containing copolymer is preferable from the viewpoint of good flexibility and alkali resistance.

Among them, in the formula (B1), a VdF / TFE binary fluorine-containing copolymer satisfying 0.50 ≦ m ≦ 0.90 and 0.10 ≦ n ≦ 0.50 (where m + n = 1) is obtained. From the viewpoint of good flexibility and alkali resistance. Furthermore, n (TFE) is preferably from 0.10 to 0.40, particularly preferably from 0.15 to 0.40 because of good base resistance and flexibility.

In addition, VdF / TFE / HFP ternary including 0.50 ≦ m ≦ 0.90, 0.09 ≦ n ≦ 0.49 and 0.01 ≦ l ≦ 0.04 (where m + n + l = 1) is included. A fluorine copolymer is preferred from the viewpoint of good flexibility and alkali resistance. Furthermore, since the base resistance and flexibility are good, a copolymer of 0.60 ≦ m ≦ 0.90 and 0.09 ≦ n ≦ 0.45 and 0.01 ≦ l ≦ 0.04 is obtained. Further, a copolymer satisfying 0.60 ≦ m ≦ 0.70, 0.30 ≦ n ≦ 0.40 and 0.02 ≦ l ≦ 0.04 is preferable.

Regardless of binary or ternary system, if the content of TFE is too much above the above range, it will be difficult to dissolve in an organic solvent, while if it is too low, the base resistance will be low and the flexibility will be low. The effects of the present invention may not be fully achieved.

The molecular weight of the VdF / TFE copolymer preferably has a number average molecular weight of 10,000 to 500,000 in terms of polystyrene as measured by GPC (gel permeation chromatography). If it is less than 10,000, the molecular weight is too low to form a film, and if it exceeds 500,000, the thixotropy of the electrode mixture becomes very large and it tends to be difficult to apply to the electrode current collector. . In order to improve the cycle characteristics, it is preferable that the molecular weight is relatively high. From this point, for example, in the case of a terpolymer, 150,000 to 500,000 are preferable.

The VdF / TFE copolymer used as the binder (B1) in the present invention can be polymerized by a known polymerization method, and among them, the radical copolymerization method is mainly preferred. That is, the polymerization method is not limited as long as it proceeds radically, but is initiated by, for example, an organic or inorganic radical polymerization initiator, heat, light, ionizing radiation, or the like. As the polymerization mode, solution polymerization, bulk polymerization, suspension polymerization, emulsion polymerization and the like can be used.

This VdF / TFE copolymer has excellent base resistance and is generally well used, not to mention nitrogen-containing organic solvents such as N-methylpyrrolidone, dimethylformamide, and dimethylacetamide, which are used as solvents for PVdF. It is also soluble in low-boiling general-purpose organic solvents that are used, does not cause gelation even when mixed with an electrode active material, can impart flexibility to the electrode, and has low swellability with respect to non-aqueous electrolytes.

(B2) Solvent-soluble thermoplastic resin In the present invention, the binder (B2) is a solvent-soluble thermoplastic resin that functions to improve the adhesion to the current collector. In the present invention, the “solvent-soluble thermoplastic resin” is a thermoplastic resin that dissolves 5% by mass or more in an organic solvent (C) at 25 ° C. to form a uniform solution. Polyvinylidene fluoride (PVdF ), At least one selected from the group consisting of polyacrylic acid polymers, polymethacrylic acid polymers, polyimides, polyamides and polyamideimides.

As PVdF, those conventionally used as binders for lithium secondary batteries can be used as they are. When PVdF is used as the binder (B2), it is 10 to 90% by mass of the total amount of the binders (B1) and (B2), and more preferably 50 to 90% by mass. However, it is preferable from the viewpoint of good adhesion. That is, since the binder (B1) has a role of imparting flexibility and the binder (B2) has a role of imparting adhesiveness, the composition may be arbitrarily balanced according to the purpose.

Examples of polyacrylic acid polymers include polyacrylic acid, ammonium salts and sodium salts thereof; polyacrylic acid alkyl esters; polyacrylic acid amides; alkoxysilyl-modified polyacrylic acid esters.

Examples of the polymethacrylic acid polymer include polymethacrylic acid, ammonium salts and sodium salts thereof; polymethacrylic acid alkyl esters; polymethacrylic acid amides; alkoxysilyl-modified polymethacrylic acid esters.

When at least one selected from the group consisting of an acrylic acid polymer, a polymethacrylic acid polymer, polyimide, polyamide and polyamideimide is used as the binder (B2), the binders (B1) and (B2) are used. 1) to 20% by mass of the total amount of ()) is preferred from the standpoint of maintaining flexibility and good adhesion.

When using as a negative electrode binder, when graphite is used as the negative electrode active material, the above composition is preferable, but when using an active material with high swellability such as silica, metal, and alloy, polyimide, polyamide, or polyamideimide is used as a binder ( The binder (B1) is preferably used in an amount of 1 to 40% by mass of the total amount of (B1) and (B2). In this case, the binder (B2) plays a role of suppressing swelling, and the binder (B1) plays a role of imparting flexibility.

(C) Organic solvent The electrode mixture slurry of the present invention can be obtained by mixing and dispersing an electrode active material (A), a binder (B), and an electrode material such as a conductive material described later in an organic solvent. .

Examples of the organic solvent (C) used for preparing the electrode mixture slurry of the present invention include nitrogen-containing organic solvents such as N-methylpyrrolidone, dimethylformamide, dimethylacetamide, acetone, methyl ethyl ketone, cyclohexanone, methyl isobutyl ketone, and the like. Ketone solvents; ester solvents such as ethyl acetate and butyl acetate; ether solvents such as tetrahydrofuran and dioxane; and general-purpose organic solvents having a low boiling point such as mixed solvents thereof. Of these, N-methylpyrrolidone is particularly preferred because of its excellent slurry stability and coating properties.

Furthermore, in order to produce a stable slurry for electrode mixture, the water content of the organic solvent (C) is important. That is, when the water content is 100 ppm or less, further 30 ppm or less, the basic expression due to the basic electrode active material is small, and gelation can be suppressed.

(D) Other electrode material In this invention, in the range which does not impair the effect of this invention, another electrode material can be mix | blended as needed.

As another electrode material, for example, a conductive material is exemplified. Examples of the conductive material include carbon blacks such as acetylene black and ketjen black, and carbon materials such as graphite.

As a method for preparing the slurry for electrode mixture of the present invention, the electrode active material (A), the conductive material (D) and the like are dispersed and mixed in a solution obtained by dissolving the binder (B) in the organic solvent (C). The method is common. In addition, for example, the powder of the binder (B), the electrode active material (A), and the conductive material (D) may be mixed first, and then the organic solvent (C) may be added to prepare a slurry.

In the slurry for electrode mixture of the present invention, the blending ratio of the binder (B) (the sum of (B1) and (B2)) is a solid content (electrode active material ( A), binder (B), conductive material (D), etc.) is 0.1 to 20% by mass, preferably 1 to 10% by mass. The compounding amount of the electrode active material (A) is 80 to 98% by mass, preferably 90 to 97% by mass in the solid content. When blending the conductive material (D), the blending amount of the conductive material (D) is 1 to 20% by mass, preferably 2 to 10% by mass in the solid content. The solid content concentration of the slurry is preferably 40 to 70% by mass from the viewpoint of good workability, coating property, and slurry stability.

The slurry for electrode mixture of the present invention is a stable and homogeneous fluid that does not gel, and can be applied to a current collector, dried, rolled, and cut into a predetermined size to produce an electrode. Conventional methods and conditions can be adopted as the method and conditions for producing the positive electrode and the negative electrode.

Examples of the current collector on which the electrode mixture slurry is applied include aluminum foil, etched aluminum foil, and aluminum foil coated with a conductive paste.

The electrode of the present invention uses a VdF / TFE copolymer that is flexible and does not cause gelation as the binder (B1), and improves the adhesion to the current collector with the binder (B2). As a result, the electrode mixture and the current collector have good adhesion, and even when processed into a spiral or fold type electrode, the electrode mixture layer does not crack or peel off. Since it is difficult to swell with respect to electrolyte solution, even if charging / discharging is repeated, a battery characteristic does not fall large.

The present invention also relates to a lithium secondary battery in which the electrode of the present invention is used as a positive electrode and / or a negative electrode and provided with a non-aqueous electrolyte.

When the electrode of the present invention is used as a positive electrode, the negative electrode may include an electrode of the present invention containing a negative electrode active material made of a basic material such as an alloy, or a negative electrode using a known carbon material as a negative electrode active material. It may be. A negative electrode using a carbon material is prepared using a negative electrode active material and a negative electrode binder by a known material and method, and is applied or adhered to a negative electrode current collector such as a copper foil. Can be produced. As the negative electrode active material of the carbon material, a carbonaceous material that can be doped / undoped with lithium or the like is used. For example, a conductive polymer such as polyacene or polypyrrole, coke, polymer charcoal, carbon fiber, or the like per unit volume. Because of its high energy density, pyrolytic carbons, cokes (petroleum coke, pitch coke, coal coke, etc.), carbon black (acetylene black, etc.), glassy carbon, organic polymer material fired bodies (organic polymer materials Preferred are those fired in an inert gas stream or in vacuum at a temperature of 500 ° C. or higher.

As the non-aqueous electrolyte, a solution obtained by dissolving a known electrolyte salt in a known electrolyte salt dissolving organic solvent can be used.

The organic solvent for dissolving the electrolyte salt is not particularly limited, but propylene carbonate, ethylene carbonate, butylene carbonate, γ-butyrolactone, 1,2-dimethoxyethane, 1,2-diethoxyethane, dimethyl carbonate, diethyl Known hydrocarbon solvents such as carbonate; one or more of fluorine solvents such as fluoroethylene carbonate, fluoroether and fluorinated carbonate can be used.

Examples of the electrolyte salt include LiClO ₄ , LiAsF ₆ , LiBF ₄ , LiPF ₆ , LiN (SO ₂ CF ₃ ) ₂ , and LiN (SO ₂ C ₂ F ₅ ) _2. LiPF ₆ , LiBF ₄ , LiN (SO ₂ CF ₃ ) ₂ , LiN (SO ₂ C ₂ F ₅ ) ₂ or combinations thereof are preferred.

The concentration of the electrolyte salt is required to be 0.8 mol / liter or more, and further 1.0 mol / liter or more. Although the upper limit depends on the organic solvent for dissolving the electrolyte salt, it is usually 1.5 mol / liter.

The lithium secondary battery of the present invention can be produced by enclosing these members in a battery case and sealing them. A separator may be interposed between the positive electrode and the negative electrode.

Next, the present invention will be described more specifically based on examples, but the present invention is not limited to these examples.

Example 1
(Preparation of slurry for positive electrode mixture)
The ratio of each electrode material shown in Table 1 is measured so that the positive electrode active material (A1): binder (B1) + (B2): conductive material (D) has a mass ratio of 95: 5: 5. did. The binder (B1) + (B2) is dissolved in N-methylpyrrolidone (NMP) so as to have a concentration of 10% by mass, and then a predetermined amount of the positive electrode active material (A1) is added to the NMP solution of the binder. And a conductive material (D) were added and mixed thoroughly with a stirrer. While stirring, NMP was sequentially added so that the solid content concentration was 50% by mass to prepare a slurry for positive electrode mixture.

(Preparation of positive electrode)
The prepared slurry for positive electrode mixture was filtered through a Ni mesh (200 mesh) sieve to make the particle size of the solid content uniform. Subsequently, the positive electrode mixture slurry after filtration was subjected to vacuum defoaming treatment. After the defoaming of the positive electrode mixture slurry is completed, the positive electrode mixture slurry is applied onto an Al foil having a thickness of 22 μm, which is a current collector plate, with an applicator (the amount by which the dry weight of the positive electrode coating film is 18 mg / cm ² ). ) After the application, NMP was completely volatilized while drying at 100 to 120 ° C. using a blast dryer or a hot plate to produce a strip-shaped positive electrode.

Each component for preparing the positive electrode mixture slurry was as follows.

Positive electrode mixture slurry (A1)
(A1-1): LiNi _0.82 Co _0.15 Al _0.03 O ₂ (manufactured by Toda Kogyo Co., Ltd.)
(A1-2): LiNi _1/3 Co _1/3 Mn _1/3 O ₂ (manufactured by Nippon Chemical Industry Co., Ltd.)
Binder (B1)
(B1-1): VdF / TFE copolymer (VdF / TFE = 80/20 mol% ratio)
(B1-2): VdF / TFE / HFP copolymer (VdF / TFE / HFP = 65 / 32.5 / 2.5 mol% ratio)
Binder (B2)
(B2-1): PVdF (KF1120 manufactured by Kureha Chemical Co., Ltd.)
Organic solvent (C)
(C-1): N-methylpyrrolidone (water content 30 ppm)
The density of the produced positive electrode was measured in the following manner. The results are shown in Table 1.

(Density measurement)
Pass the positive electrode twice through a roll press with a gap of 75 μm at 70 ° C., and further change the gap to 35 μm and pass twice. Then, measure the area / film thickness / weight of the positive electrode to determine the density (g / cm ³ ). calculate.

(With or without cracks)
The produced positive electrode was cut into a length of 3 cm and a width of 6 cm, then folded by 180 ° and then expanded, and the presence or absence of cracks in the positive electrode was visually confirmed. The results are shown in Table 1.

From the results in Table 1, it can be seen that when compared with PVdF, the copolymer obtained by copolymerizing TFE has higher flexibility as a binder, and therefore the density is likely to be improved. In addition, it can be seen that the copolymer having higher alkali resistance than PVdF and copolymerized with flexible TFE can suppress cracking of the positive electrode.

Example 2
A positive electrode was produced in the same manner as in Example 1 except that the resin shown in Table 2 was used as the binder (B2) in the ratio shown in Table 2, and the density and crack presence were examined. The results are shown in Table 2.

Binder (B2)
(B2-2): Polymethyl methacrylate (PMMA) (manufactured by Aldrich)
(B2-3): Methyl methacrylate (MMA) / methacrylic acid (MA) (MMA / MA = 1: 0.016 molar ratio) (manufactured by Aldrich)
(B2-4): Polyamideimide (PAI) (HPC7200 manufactured by Hitachi Chemical Co., Ltd.)
(B2-5): Polyimide (PI) (HCI-7000 manufactured by Hitachi Chemical Co., Ltd.)

From the results of Table 2, it can be seen that the binders (B1) and (B2) used in combination have high flexibility and are difficult to break.

Example 3
A positive electrode was prepared in the same manner as in Example 2 except that the binders (B1) and (B2) were used in the types and proportions shown in Table 3, and examined for cracks. The results are shown in Table 3.

From the results in Table 3, it can be seen that even if the amount of the binder (B2) is reduced, the flexibility is high and the cracking is difficult.

Example 4
A positive electrode was produced in the same manner as in Example 2 except that the binders (B1) and (B2) were used in the types and proportions shown in Table 4, and an adhesive tape (PR51 manufactured by Ace Global Co., Ltd.) was applied to the produced positive electrode. ) Was peeled off and the state of the positive electrode mixture layer was visually observed. The results are shown in Table 4.

From the results in Table 4, it can be seen that the adhesiveness of the binders (B1) and (B2) used to the current collector is good.

Example 5
Using the positive electrode shown in Table 5, a lithium secondary battery (laminate cell) was produced by the following method. For these lithium secondary batteries, rate characteristics and cycle characteristics were examined as follows. The results are shown in Table 5.

(Production of lithium secondary battery (laminate cell))
To artificial graphite powder (manufactured by Hitachi Chemical Co., Ltd., trade name: MAG-D), styrene-butadiene rubber dispersed with distilled water is added to a solid content of 6% by mass and mixed with a disperser to form a slurry. Was uniformly coated on a negative electrode current collector (copper foil having a thickness of 10 μm) and dried to form a negative electrode mixture layer, then compression-molded with a roller press, cut, and then dried. The lead body was welded to produce a strip-shaped negative electrode.

The strip-shaped positive electrode was cut to 40 mm × 72 mm (with a positive electrode terminal of 10 mm × 10 mm), the strip-shaped negative electrode was cut to 42 mm × 74 mm (with a negative electrode terminal of 10 mm × 10 mm), and a lead body was welded to each terminal. Further, a microporous polyethylene film having a thickness of 20 μm was cut into a size of 78 mm × 46 mm to form a separator, and a positive electrode and a negative electrode were set so as to sandwich the separator, and these were put in an aluminum laminate packaging material. Next, 2 ml of electrolyte solution (LiPF ₆ dissolved at a concentration of 1 mol / liter in a solvent having a volume ratio of 3/7 of ethylene carbonate (EC) and ethyl methyl carbonate (EMC)) was put into the packaging material 2 ml at a time and sealed. A laminate cell having a capacity of 72 mAh was produced.

(Rate characteristics)
As for charging, charging is performed until the charging current becomes 1/10 C at 4.2 V at 1.0 C, and discharging is performed to 3.0 V at a current equivalent to 0.2 C, and the discharge capacity is obtained. Subsequently, the battery is charged at 1.0 C at 4.2 V until the charging current becomes 1/10 C, discharged at a current equivalent to 2 C until 3.0 V, and the discharge capacity is obtained. From the ratio of the discharge capacity at 2C and the discharge capacity at 0.2C, the rate characteristic is obtained by substituting into the following calculation formula.
Rate characteristic (%) = 2C discharge capacity (mAh) /0.2C discharge capacity (mAh) × 100

(Cycle characteristics)
As for the cycle characteristics, a charge / discharge cycle performed under the above-described charge / discharge conditions (charging at 1.0 V until the charging current becomes 1/10 C at 4.2 V and discharging to 3.0 V at a current equivalent to 1 C) is 1 The discharge capacity after the first cycle and the discharge capacity after 100 cycles are measured. For the cycle characteristics, the value obtained by the following formula is used as the capacity retention rate.
Capacity retention rate (%) = 100 cycle discharge capacity (mAh) / 1 cycle discharge capacity (mAh) × 100

From the results of Table 5, it is understood that the battery characteristics are maintained even when the positive electrode using the binders (B1) and (B2) is used.

Example 6
Si (negative electrode active material; manufactured by Fuji Silysia Chemical Co., Ltd.), acetylene black (Denka Black manufactured by Denki Kagaku Kogyo Co., Ltd.), and binders (B) ((B1) and (B2)) shown in Table 6 Was mixed in a disperser using NMP as a solvent at a mass ratio of 45:45:10 to prepare a slurry for negative electrode mixture. This slurry was uniformly applied onto a negative electrode current collector (copper foil having a thickness of 10 μm) and dried to form a negative electrode mixture layer, then compression-molded with a roller press, cut, and then dried. The lead body was welded to produce a strip-shaped negative electrode.

The positive electrode uses A1-2 (LiNi _1/3 Co _1/3 Mn _1/3 O ₂ ) as the positive electrode active material, and the binder B1-1 and the binder B2-1 are 50/50 (mass ratio). It was produced in the same manner as in Example 1 except that it was used in combination.

Using this negative electrode and positive electrode, a lithium secondary battery (laminate cell) was produced in the same manner as in Example 5, and the cycle characteristics were measured in the same manner as in Example 5. The results are shown in Table 6.

From the results in Table 6, it can be seen that the cycle characteristics are maintained even when a basic material is used as the negative electrode active material.

Example 7
In Example 6, the negative electrode active material was changed to SiO ₂ (manufactured by Aldrich) or Sn particles (manufactured by Aldrich), and the negative electrode binder was blended in the types and proportions shown in Table 7, and Similarly, lithium secondary batteries were produced, and the cycle characteristic test of Example 5 and the bending test of Example 3 were performed on them. The results are shown in Table 7.

From the results of Table 7, even when SiO ₂ or Sn particles are used as the negative electrode active material, there is no flexibility when polyamideimide (B2-4) or polyimide (B2-5) is used alone. It can be seen that by using the VdF / TFE / HFP copolymer (B1-2) in combination, flexibility can be imparted while maintaining the capacity.

Claims

A slurry for electrode mixture of a lithium secondary battery containing an electrode active material (A), a binder (B), and an organic solvent (C), wherein the binder (B) is:
(B1) Composition formula (B1):
(VDF) m (TFE) n (HFP) l
(In the formula, VDF is a structural unit derived from vinylidene fluoride; TFE is a structural unit derived from tetrafluoroethylene; HFP is a structural unit derived from hexafluoropropylene; 0.45 ≦ m ≦ 1; 0.05 ≦ n ≦ 0. 5; 0 ≦ l ≦ 0.1, where m + n + l = 1) and (B2) a solvent-soluble thermoplastic resin other than the fluorine-containing polymer (B1). A slurry for an electrode mixture of a lithium secondary battery.
The electrode active material (A) is the positive electrode active material (A1), and the formula (A1):
Li x M 1 y M 2 1-y O 2
(Wherein 0.4 ≦ x ≦ 1; 0.3 ≦ y ≦ 1; M 1 is at least one selected from the group consisting of Ni and Mn; M 2 is selected from the group consisting of Co, Al and Fe) The slurry for electrode mixture of Claim 1 containing the lithium containing complex metal oxide shown by at least 1 sort (s).
The positive electrode active material (A1) is
Formula (A1-1):
LiNi x Co y Al z O 2
(Wherein 0.7 ≦ x ≦ 1; 0 ≦ y ≦ 0.3; 0 ≦ z ≦ 0.03; 0.9 ≦ x + y + z ≦ 1.1),
Formula (A1-2):
LiNi x Co y Mn z O 2
(Wherein 0.3 ≦ x ≦ 0.6; 0 ≦ y ≦ 0.4; 0.3 ≦ z ≦ 0.6; 0.9 ≦ x + y + z ≦ 1.1),
Formula (A1-3):
Li x Mn z O 2
(Wherein 0.4 ≦ x ≦ 0.6; 0.9 ≦ z ≦ 1), or formula (A1-4):
LiFe x Co y Mn z O 2
(Wherein 0.3 ≦ x ≦ 0.6; 0.1 ≦ y ≦ 0.4; 0.3 ≦ z ≦ 0.6; 0.9 ≦ x + y + z ≦ 1.1)
The slurry for an electrode mixture according to claim 2.
The slurry for electrode mixture according to claim 1, wherein the electrode active material (A) is a negative electrode active material (A2) containing a basic material containing Si and / or Sn.
The binder (B2) is at least one selected from the group consisting of polyvinylidene fluoride, polyacrylic acid polymer, polymethacrylic acid polymer, polyimide, polyamide and polyamideimide. A slurry for electrode mixture according to claim 1.
The binder (B1) is 0.50 ≦ m ≦ 0.90, 0.10 ≦ n ≦ 0.50, and 0 ≦ l ≦ 0.08 (where m + n + 1 = 1) in the formula (B1). The slurry for an electrode mixture according to any one of claims 1 to 5, comprising a fluorine-containing copolymer.
A binary fluorine-containing copolymer in which the binder (B1) is 0.50 ≦ m ≦ 0.90 and 0.10 ≦ n ≦ 0.50 (where m + n = 1) in the formula (B1) The slurry for electrode mixture according to any one of claims 1 to 6.
In the formula (B1), the binder (B1) is 0.50 ≦ m ≦ 0.90, 0.09 ≦ n ≦ 0.49, and 0.01 ≦ l ≦ 0.04 (where m + n + 1 = 1). The slurry for an electrode mixture according to any one of claims 1 to 6, comprising a fluorine-containing copolymer which is:
The slurry for electrode mixture according to any one of claims 1 to 8, wherein the water content of the organic solvent (C) is 30 ppm or less.
An electrode of a lithium secondary battery obtained by applying the slurry for electrode mixture according to any one of claims 1 to 9 to a current collector and drying it.
A lithium secondary battery comprising the electrode according to claim 10 as a positive electrode and / or a negative electrode and comprising a non-aqueous electrolyte.
In the method of producing a slurry for an electrode mixture of a lithium secondary battery by dispersing the electrode active material (A) and the binder (B) in an organic solvent (C), the binder (B) is:
(B1) Composition formula (B1):
(VDF) m (TFE) n (HFP) l
(In the formula, VDF is a structural unit derived from vinylidene fluoride; TFE is a structural unit derived from tetrafluoroethylene; HFP is a structural unit derived from hexafluoropropylene; 0.45 ≦ m ≦ 1; 0.05 ≦ n ≦ 0. 5; 0 ≦ l ≦ 0.1, where m + n + l = 1), and (B2) a solvent-soluble thermoplastic resin other than the fluorine-containing polymer (B1), and A method for producing a slurry for an electrode mixture of a lithium secondary battery, wherein the water content of the organic solvent (C) is 100 ppm or less.
The production method according to claim 12, wherein the water content of the organic solvent (C) is 30 ppm or less.
The process according to claim 12 or 13, wherein the organic solvent (C) is N-methylpyrrolidone.
A slurry for an electrode mixture of a lithium secondary battery obtained by the production method according to any one of claims 12 to 14.