JPH11329440A - Non-aqueous electrolyte secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a non-aqueous electrolyte secondary battery which is excellent in characteristics of both an initial capacity and a capacity after the passage of cycles, by enabling adhesion to be secured to a collector or the like without damaging conductivity of a lithium ion. SOLUTION: A core/shell combined fine grain, having both a core part made of fluorocarbon polymer which is insoluble in organic solvent, and a shell part made of a polymer which is soluble or swells in organic solvent, and polyimide, for example, polysulfone polyimide which dissolves in common organic solvent with the shell part of the core/shell combined fine grain, are used as binders of a positive electrode 2 and a negative electrode 4. A mean grain size of the core/shell combined fine grain is 0.05-1 μm, and the weight ratio of the core part to the shell part is 98:2 to 50:50.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a non-aqueous electrolyte secondary battery, and more particularly to an improvement in a binder for a positive electrode or a negative electrode.

[0002]

2. Description of the Related Art In recent years, with the advance of electronic technology, high performance, portable and cordless electronic devices have been rapidly advanced. Accordingly, batteries used as a power supply for these portable electronic devices are also required to be smaller, lighter, and have higher energy density.

Under such circumstances, non-aqueous electrolyte secondary batteries, especially lithium ion secondary batteries, have high voltage, high energy density, low self-discharge, and no memory effect. It has features such as,
Since the battery has high safety, it has been actively studied in various fields.

As a positive electrode active material for producing these lithium ion secondary batteries, many lithium transition metal composite oxides have been studied, and good performance has been obtained.

[0005]

When the above-described lithium ion secondary battery is manufactured, it is necessary to process the positive electrode active material and the negative electrode active material into a predetermined shape, for example, a sheet shape. Is essential.

For example, in order to bind lithium transition metal composite oxides to each other and to adhere them to aluminum or the like used as a current collector, Japanese Patent Laid-Open Publication No.
As described in US Pat.
An N-methylpyrrolidone solution of polyvinylidene fluoride (PVdF) is mixed with each of a positive electrode made of graphite and a lithium transition metal composite oxide as in ₂ , and a negative electrode made of a carbonaceous material, and processed into a sheet. In this case, polyvinylidene fluoride (PVdF) is the binder.

[0007] International Publication No. WO 96/12764
The publication discloses a battery comprising, as a binder, core-shell composite fine particles having an average particle size of 0.05 to 1 μm having a fibril-forming polytetrafluoroethylene as a core and a non-fibril-forming polymer as a shell. There is an example.
Japanese Patent No. 106897 describes an example in which polytetrafluoroethylene (PTFE) is used as a first binder and polyvinylidene fluoride (PVdF) is used as a second binder.

However, polyvinylidene fluoride (P
When VdF) is used alone as a binder, since it is used in a solution, if it is used in an amount sufficient for adhesion, it may cover the surface of the active material and hinder the conductivity of lithium ions and the like. It becomes a problem.

Further, since the core-shell composite fine particles themselves have a weak bonding force, if they are not used in a large amount or in the form of fibers, sufficient adhesion to the current collector cannot be ensured.

Further, PTFE is used as the first binder.
In the example of using PVdF as the second binder,
The stability of PTFE in solvents is not sufficient, rendering the mixture unstable and impairing the properties of the final processed electrode.

[0011] The above problems cause the detachment of the positive electrode mixture and the like, and reduce the discharge capacity of the lithium ion secondary battery, leaving room for improvement.

The present invention has been proposed in view of such a conventional situation, and it is possible to secure the adhesiveness to a current collector or the like without impairing the conductivity of lithium ions, and to improve the initial capacity. It is another object of the present invention to provide a non-aqueous electrolyte secondary battery having excellent capacity characteristics after a cycle.

[0013]

In order to achieve the above-mentioned object, the present invention comprises a core made of a fluorocarbon polymer which is insoluble in an organic solvent and a shell made of a polymer soluble or swellable in an organic solvent. Core-shell composite fine particles and polyimide dissolved in an organic solvent common to the shell of the core-shell composite fine particles are contained as a binder for the positive electrode and / or the negative electrode.

That is, according to the present invention, the core-shell composite fine particles are used as a first binder, and a polyimide soluble in a common organic solvent with the shell polymer is used in combination as a second binder. Is the gist.

In the electrode mixture mix, the core-shell composite fine particles and a polyimide, for example, a polysulfone polyimide are used in combination, and the mixture is applied to a metal foil as a current collector by the action of the polyimide as a surfactant. When the solvent is dried, each material forms an electrode mixture layer without agglomeration, and the adhesive force of the polymer in the shell of the core-shell composite fine particles is efficiently expressed, so that the active material inherently has It becomes possible to take out without losing the capacity.

Furthermore, since the core-shell composite fine particles are dispersed in the electrode mixture, the stress in the electrode mixture based on the high cohesive force of the polyimide having a large adhesive force is efficiently absorbed, A decrease in capacity due to the electrode mixture being detached from the current collector (metal foil) during the charge / discharge cycle can be avoided.

[0017]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, each battery material and components in the non-aqueous electrolyte secondary battery of the present invention will be described in detail.

First, a fluorocarbon polymer which is insoluble in an organic solvent constituting the core of the core-shell composite fine particles is a monomer such as tetrafluoroethylene, hexafluoroethylene, fluoroalkylethylene, or fluoroalkylfluorovinyl ether. It is the same as microparticles having an average particle size of 0.05 to 1 μm produced by emulsion polymerization of a polymer or a group of monomers centered on these monomers, and coagulates a commercially available emulsion polymer. The same fine particles as those constituting a fluorocarbon-based fine powder obtained by drying and a fluorocarbon-based aqueous dispersion (dispersion) obtained by concentrating and stabilizing an emulsion polymer can be used.

Specific production methods are described in JP-B-37-4643, JP-B-46-14466, and JP-B-56-2.
No. 6242, and the like.

Further, when a polymer of a monomer having tetrafluoroethylene as a main component (PTFE and modified PTFE) is used, its fibril-forming ability is problematic.
Final performance can be affected by the presence or absence of fibril formation, but there is no direct relationship between this and the components of the present invention.

The polymer which is soluble or swells in the organic solvent constituting the shell of the core-shell composite fine particles is that the shell of the core is easily reacted after the synthesis of the fluorocarbon polymer. A polymer containing vinylidene fluoride as a monomer component is preferably used,
Polymers of α, β-ethylenically unsaturated compound monomers such as methyl acrylate, methyl methacrylate, ethyl acrylate, styrene, acrylonitrile, vinylpyrrolidone, and alkyl vinyl ether, or a group of monomers centered on these monomers May also be used.

The above-mentioned core-shell composite fine particles have a shell portion of vinylidene fluoride-based resin and have an international publication number WO94 / WO94 /.
Japanese Patent Application Laid-Open No. 1475/1988 discloses a case where the shell is a polymer of an α, β-ethylenically unsaturated carboxylic acid ester monomer.
It can be produced according to the methods described in JP-A-312836.

The polyimide used as the second binder polymer is not particularly limited as long as it is soluble in an organic solvent. For example, it is necessary to ensure the adhesion to the positive electrode active material and the current collector aluminum foil, A polyimide having a sulfone group in the molecule and having no amino group and no carboxyl group in the molecule is suitable from the viewpoint of avoiding oxidation at the potential. Specific examples include equimolar amounts of 3,3'-
Polycondensation polymer of 4,4'-diphenylsulfonetetracarboxylic dianhydride and bis [4- (4-aminophenoxy) phenyl] sulfone (for example, manufactured by Sony Chemical
Trade name NIV1001).

The core-shell composite fine particles and the second binder polymer are used as a binder for a positive electrode or a negative electrode.

The lithium composite oxide capable of removing and inserting lithium used for the positive electrode is not particularly limited, but may be an oxide or a salt of lithium, cobalt, nickel, or manganese as a starting material. The raw materials are mixed according to the composition, and the mixture is heated at 600 ° C. to 10
It is obtained by firing in a temperature range of 00 ° C., and is made of LiCoO ₂ , Li _x Ni _y M _{1 -y} O ₂ (where M
Is at least one or more metal elements selected from transition metal elements or Al, preferably Co, Fe, Mn,
Represents at least one or more metal elements selected from Ti, Cr, V, and Al; 0.05 ≦ x ≦ 1.10, 0.5
≤ y ≤ 1.0. ), LiNiO ₂ , LiMnO ₂ , LiMn ₂ O _{4 and the} like. The starting material is not limited to oxides or salts, but can be synthesized from a hydroxide or the like.

The conductive agent used in the positive electrode mixture is not particularly limited, but, for example, metal powder, carbon powder and the like are used. Particularly, in the case of carbon powder, pyrolytic carbon such as carbon black and its graphitized product, artificial and natural flaky graphite powder, carbon fiber and its graphitized product, and the like are preferable. Also, a mixture of these carbon powders can be used.

The negative electrode is not particularly limited as long as it is a material usually used in this type of battery. However, any negative electrode may be used as long as it can absorb and release lithium metal, a lithium alloy or lithium. , An alloy with indium or the like, a carbon material capable of inserting and extracting lithium, or a polymer such as polyacetylene and polypyrrole. Among them, a carbon material or the like capable of inserting and extracting lithium is preferably used because a nonaqueous electrolyte battery having excellent cycle life can be obtained.

The carbon material for the negative electrode of the non-aqueous electrolyte secondary battery is not particularly limited, but any material capable of doping and undoping lithium can be used, and pyrolytic carbons, cokes ( Pitch coke, needle coke,
Petroleum coke, etc.), graphites, glassy carbons,
Organic polymer compound fired bodies (phenol resins and furan resins fired at an appropriate temperature and carbonized), carbon fibers, activated carbon, and the like can be used.

As the non-aqueous electrolyte, for example, an electrolyte obtained by using a lithium salt as an electrolyte and dissolving it in an organic solvent is used. Here, as the organic solvent, for example, propylene carbonate, ethylene carbonate, 1,2-dimethoxyethane, γ-butyrolactone, diethyl ether,
Tetrahydrofuran, 2-methyl-tetrahydrofuran, 1,3-dioxolan, sulfolane, acetonitrile, dimethyl carbonate, diethyl carbonate, dipropyl carbonate, methyl ethyl carbonate, methyl propyl carbonate, and the like, and these may be used alone or in combination of two or more. It is possible to use.

As the electrolyte, LiClO ₄ , LiAs
F ₆ , LiPF ₆ , LiBF ₄ , LiB (C ₆ H ₅ ) ₄ , Li
Cl, LiBr, LiSO ₃ CH ₃ , LiSO ₃ CF ₃ ,
LiN (SO ₂ CF ₃ ) ₂ , LiC (SO ₂ CF ₃ ) _{3 and the} like can be used.

The separator is not particularly limited, but examples thereof include a woven fabric, a nonwoven fabric, and a microporous synthetic resin membrane. Particularly, a synthetic resin microporous membrane is preferably used, and among them, a polyolefin-based microporous membrane is preferable in terms of thickness, film strength, and film resistance. Specifically, it is a microporous film made of polyethylene and polypropylene, or a microporous film obtained by combining these.

The shape of the current collector used for the positive electrode and the negative electrode is not particularly limited, but may be a foil, a mesh, or a mesh such as expanded metal.

For example, in the case of a positive electrode current collector, examples of the current collector used include an aluminum foil, a stainless steel foil, and a nickel foil. As its thickness, 10
Those having a size of 40 μm are preferred.

In the case of the negative electrode current collector, examples of the current collector used include a copper foil, a stainless steel foil, and a nickel foil. In this case, the thickness is 5 to 20 μm
Are preferred.

Although the structure of the non-aqueous electrolyte secondary battery is arbitrary, a more secure sealed non-aqueous electrolyte battery can be obtained by detecting an increase in the internal pressure of the battery when an abnormality such as overcharge occurs. It is desirable to have a means for shutting off.

[0036]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Specific embodiments of the present invention will be described below with reference to experimental results and the like.

Example 1 An example of a coin cell using core-shell composite fine particles whose core is made of polytetrafluoroethylene and whose shell is made of polyvinylidene fluoride will be described.

First, tetrafluoroethylene (T
EF) and vinylidene fluoride (VdF) in the shell are used in a weight ratio of 95: 5, and the production method described in International Publication No. WO94 / 01475, that is, the initiator is used in an autoclave. Was performed in two steps using ammonium perfluorooctanoate to obtain a fine particle aqueous dispersion, which was then evaporated with a spray drying apparatus to prepare core-shell composite fine particles.

Next, fully dried core-shell composite fine particles 3
70 parts by weight of N-methylpyrrolidone as a solvent was added to the parts by weight and stirred to obtain a uniform dispersion.

Further, 2 parts by weight of a polysulfone polyimide (manufactured by Sony Chemical Co., trade name: NIV1001) as a second binder polymer, 5 parts by weight of acetylene black as a conductive agent, and LiCoO ₂ as a cathode active material
Was added and stirred to obtain a uniform slurry.

The slurry was applied to one side of the aluminum foil using a doctor blade method, dried with hot air at 80 ° C., and pressed with a roller press to obtain a positive electrode. At this time, the positive electrode mixture was firmly fixed to the aluminum foil.

Next, the non-graphitizable carbon was used as the negative electrode active material, the non-aqueous solution of 1M propylene carbonate / 1,2-dimethyl carbonate mixed using LiPF ₆ as the electrolyte was used as the electrolyte, and the coin cells were respectively formed using the above-mentioned positive electrode. Ten were produced.

The manufactured coin cell has a structure as shown in FIG. 1, in which a cathode 2 is accommodated in a cathode can 1, and a negative electrode 4 is mounted thereon via a separator 3 and a gasket 5 is provided thereon. This is covered with an anode cap 6 and caulked with the cathode can 1 so as to be sealed.

Example 2 An example of a coin cell using core-shell composite fine particles whose core is made of polytetrafluoroethylene and whose shell is made of polymethyl methacrylate will be described.

First, tetrafluoroethylene (T
FE) and the shell methyl methacrylate in a weight ratio of 9
5: 5 and disclosed in JP-A-63-312836.
No. 4,078,098, i.e., after obtaining an aqueous dispersion using polytetrafluoroethylene powder and an anionic surfactant, post-polymerizing methyl methacrylate using potassium persulfate as an initiator. After obtaining a fine particle aqueous dispersion in this way, the core-shell composite fine particles were prepared by evaporating water with a spray drying apparatus.

To 3 parts by weight of sufficiently dried core-shell composite fine particles, 70 parts by weight of dimethyl sulfoxide as a solvent was added and stirred to obtain a uniform dispersion.

Thereafter, ten coin cells were produced in the same manner as in Example 1.

Comparative Example 1 Core-shell composite fine particles were obtained in the same manner as in Example 1. To 8 parts by weight of sufficiently dried core-shell composite fine particles, N
-Methylpyrrolidone was added, and a coin cell was prepared in the same manner as in Example 1 except that polysulfone polyimide as the second binder polymer was not used.
0 were produced.

Comparative Example 2 Example 1 was repeated except that 4 parts by weight of polyvinylidene fluoride was used in place of the core-shell composite fine particles, and that the polysulfone polyimide as the second binder polymer was not used. Ten coin cells were produced in the same manner.

Comparative Example 3 The same method as in Example 1 was used except that the core-shell composite fine particles were not used, and the amount of the polysulfone polyimide as the second binder polymer was changed to 4 parts by weight. Thus, ten coin cells were produced.

Evaluation of Charge / Discharge Characteristics For each of the coin cells of Examples and Comparative Examples, the upper limit voltage at the time of charge was 4.2 V, the cut-off voltage at the time of discharge was 3.0 V,
Charge / discharge was performed at a constant current of 0.5 mA / cm ² .

At this time, the average discharge capacity in the first cycle and the average discharge capacity in the 200th cycle were measured, and the average discharge capacity retention ratio was calculated. Table 1 shows the results.

[0053]

[Table 1]

As is clear from Table 1, in the example to which the present invention was applied, the average discharge capacity in the first cycle was higher than that in the comparative example, and the average discharge capacity retention was 90% or more. On the other hand, in the comparative example batteries, the average discharge capacity retention ratio was lower than 90%.

[0055]

As is clear from the above description, according to the present invention, since the core-shell composite fine particles and the second polymer binder (polyimide) are used as a binder for an electrode, lithium Adhesiveness to current collectors etc. can be secured without impairing ion conductivity, and a non-aqueous electrolyte secondary battery with excellent initial capacity and capacity characteristics after cycling can be obtained. It is.

[Brief description of the drawings]

FIG. 1 is a schematic sectional view showing a structure of a coin cell manufactured in an example and a comparative example.

[Explanation of symbols]

1 cathode can, 2 cathode, 3 separator, 4 anode, 5 gasket, 6 anode cap

Claims (9)

[Claims] 1. Core-shell composite fine particles having a core of a fluorocarbon polymer insoluble in an organic solvent and a shell soluble or swellable in an organic solvent, and a shell of the core-shell composite fine particles. A non-aqueous electrolyte secondary battery comprising a polyimide dissolved in a common organic solvent as a binder for a positive electrode and / or a negative electrode. 2. The core-shell composite fine particles having an average particle size of 0.1.
And the weight ratio of the core to the shell is 98:
The non-aqueous electrolyte secondary battery according to claim 1, wherein the ratio is 2 to 50:50. 3. The non-aqueous electrolyte secondary battery according to claim 1, wherein the fluorocarbon polymer insoluble in the organic solvent is polytetrafluoroethylene. 4. The non-aqueous electrolyte secondary battery according to claim 1, wherein the polymer soluble or swellable in the organic solvent is a polymer of a hydrocarbon α, β-ethylenically unsaturated compound. 5. The non-aqueous electrolyte secondary battery according to claim 4, wherein the polymer of the hydrocarbon α, β-ethylenically unsaturated compound is polyvinylidene fluoride. 6. The non-aqueous solution according to claim 1, wherein the polyimide dissolved in an organic solvent common to the shell of the core-shell composite fine particles is a polyimide having no amino group or carboxyl group in the molecule. Electrolyte secondary battery. 7. The non-aqueous electrolyte secondary battery according to claim 1, wherein the polyimide dissolved in an organic solvent common to the shell of the core-shell composite fine particles is a polysulfone polyimide. 8. The non-aqueous electrolyte secondary battery according to claim 1, wherein the core-shell composite fine particles are contained in a positive electrode together with an active material and a conductive material. 9. The non-aqueous electrolyte secondary battery according to claim 1, wherein the positive electrode active material contains a lithium compound and the negative electrode active material contains any of lithium metal, a lithium alloy, and a material capable of inserting and extracting lithium. .