JPH07220725A - Electrically conductive composition and electrode using same - Google Patents

Abstract

PURPOSE:To prevent carbon particles from separating during preservation and application so as to provide an excellent adhesion property and stable electrical conductivity by allowing an organic resin and the carbon particles to meet respective predetermined requirements, and setting the amount of the organic resin to a predetermined wt.%. CONSTITUTION:A copolymer, wherein a unit capable of absorbing stresses caused by the expansion and contraction of electrodes due to a charge and discharge cycle and a heat cycle and a unit having a group that provides an adhesive property to the metal of a current collector are combined within molecules, is used as an organic resin binder, the amount of the organic binder being 1 to 10wt.%. Carbon particles having a specific pH range and covered at their surfaces with a water-soluble high polymer are used as conductive particles. A variety of batteries that use electrodes made of a conductive composition containing such an organic resin and carbon particles can keep a large discharge capacity without causing peeling of the electrodes from the current collector due to the charge and discharge cycle and the heat cycle.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a conductive composition suitable for electrodes such as lithium batteries. The present invention also relates to an electrode obtained with such a conductive composition.

[0002]

2. Description of the Related Art It is well known to use a carbon material for the negative electrode of a lithium battery (see JP-A-63-24555 and JP-A-64-7258). This type of battery is summarized in Osaka Industrial Laboratory Report, Volume 42, pages 150-159 (1992). A lithium battery is assembled into a battery by, for example, sandwiching a separator made of porous polyolefin between a positive electrode and a negative electrode, winding both electrodes in a spiral shape. At this time, as a binding agent for adhering the electrode and the current collector, polytetrafluoroethylene, polyvinylidene fluoride, etc. are mixed with the carbon particles and used for the electrode.

These fluororesins have a weak adhesion to metal, and the outermost periphery of the electrode is peeled off from the current collector during repeated charging and discharging, resulting in a decrease in battery capacity.

[0004]

The object of the present invention is to prevent peeling between the electrode and the aggregate when used as an electrode mixture and to separate carbon particles during coating. It is to provide a conductive composition that is unlikely to occur. Another object of the present invention is to provide an electrode using such a conductive composition as a mixture.

[0005]

Means for Solving the Problems As a result of repeated studies to solve this problem, the present inventors have found that as an organic resin,
An organic resin is a copolymer obtained by combining a unit capable of absorbing stress due to expansion and contraction of an electrode due to charge / discharge cycle or heat cycle, and a unit having a group which imparts adhesiveness to a metal of a current collector in a molecule. By using as a binder, and having a specific pH range, by using carbon particles whose surface is coated with a water-soluble polymer as conductive particles, it was found that the above problems can be achieved, and the present invention is completed. Came to.

That is, the present invention is a conductive composition containing an organic resin and carbon particles, wherein the organic resin satisfies the requirement (A) below and the carbon particles satisfy the requirement (B) below. , An electrically conductive composition in which the amount of the organic resin is 1 to 10% by weight. (A) (1) one or more kinds of aliphatic hydrocarbon monomers having an ethylenically unsaturated double bond or a conjugated double bond, 70 to 99.99 mol%; and (2) 1 in a molecule.
Containing two or more carboxyl groups or an acid anhydride group to be subsequently hydrolyzed, or having a hydroxyl group bonded to a carbon atom where an aliphatic carbon-carbon unsaturated bond does not exist, containing an ethylenically unsaturated double bond Monomers 0.01-2
It is a copolymer obtained by copolymerizing 0 mol%. (B) (a) The carbon particles are mixed with ethanol-water in a weight ratio of 1:
The liquid obtained by dispersing 10% by weight in the mixed liquid of 1 has a pH of 6 to 9; and (b) the surface is coated with a water-soluble polymer.

The organic resin used as a binder in the conductive composition of the present invention is a mixture of the aliphatic hydrocarbon monomer (1) and the monomer (2) having a carboxyl group, an acid anhydride group or a hydroxyl group. It is a polymer. The organic resin may contain units derived from other monomers, if necessary, and can be directly obtained by copolymerization of (1) and (2), as well as hydrolysis, hydrogenation and / or ammonium salt. Alternatively, it may be synthesized through a reaction for forming an amine salt. By using the copolymer having such a constitution, the unit derived from the monomer (1) can absorb the expansion and contraction stress of the electrode due to the charge / discharge cycle or the heat cycle, and the monomer (2) By the unit derived from, a conductive composition having excellent adhesion to the metal of the current collector and suitable as an electrode mixture can be obtained.

As the aliphatic hydrocarbon monomer (1), an aliphatic hydrocarbon having an ethylenically unsaturated double bond or a conjugated double bond is used. Specifically, ethylene, propylene, butene-1, isobutene, pentene-
1,1-methylpentene-1, butadiene and isoprene may be mentioned. When butadiene or isoprene is used, it may be saturated by hydrogenation after the copolymerization.
These may be used alone or in combination of two or more, and ethylene, propylene and butadiene (saturated after copolymerization) are preferable.

The amount of this monomer (1) is 70 to 99.99 mol%, preferably 75 to 99.9 mol%, and more preferably 77 to 99.99% of the monomers involved in the copolymerization.
It is 5 mol%. When (1) is less than 70 mol%, the copolymer lacks the ability to absorb the stress due to volume expansion and contraction of the electrode, and peeling occurs during use due to charge / discharge cycle or heat cycle of the electrode. If it exceeds 99.9 mol%, the adhesion to a metal such as a current collector will be poor.

Examples of the monomer (2) include carboxyl group-containing monomers such as acrylic acid, methacrylic acid, crotonic acid, maleic acid, itaconic acid and 4-carboxystyrene. Examples of the monomer having an acid anhydride group, which later yields two carboxyl groups by hydrolysis, include maleic anhydride, itaconic anhydride, Δ ⁴ tetrahydrophthalic anhydride, and the like; and Diels of cyclopentadiene and itaconic anhydride. -Alder reaction adducts are exemplified. 2 as a hydroxyl group-containing monomer
-Hydroxyethyl acrylate, 2-hydroxyethyl methacrylate, 3-hydroxypropyl acrylate, 3-hydroxypropyl methacrylate, 4-hydroxystyrene, and other carbon atoms without an aliphatic carbon-carbon unsaturated bond, that is, a polymer, An example is a monomer in which a hydroxyl group is bonded to the carbon atom located in the side chain after formation. These may be used alone or in combination of two or more, and acrylic acid, maleic anhydride and 4-hydroxystyrene are preferable. When a monomer having an acid anhydride group is used as (2), it is converted into two carboxyl groups by adding a hydrolysis step.

This monomer (2) is 0.01 to 20 mol% of the monomer that participates in the copolymerization. In the case of a carboxyl group-containing monomer and an acid anhydride-containing monomer,
0.05 to 10 mol% is preferable, and 0.1 to 5 mol% is more preferable. In the case of a hydroxyl group-containing monomer, 0.1 to
15 mol% is preferable, and 1-10 mol% is more preferable. If it is less than 0.01 mol%, the adhesiveness to a metal such as a current collector is poor, and if it exceeds 20 mol%, the stress due to volume expansion and contraction of the electrode cannot be absorbed.

Further, if necessary, the copolymer may further contain another monomer having an ethylenically unsaturated double bond.
It can be copolymerized at a mol% or less. As a monomer for introducing such a unit, styrene, acrylonitrile, and a (meth) acrylic acid ester having no hydroxyl group in the side chain, for example, methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, butyl acrylate, butyl methacrylate, etc. Is exemplified. When the content of these exceeds 30 mol%, the copolymer swells or dissolves in the electrolytic solution and fails to function as a binder. Another example of another monomer is vinylidene fluoride. If this content exceeds 30 mol%, the adhesive strength of the copolymer will decrease and peeling from a metal such as a current collector will occur.

The organic resin, which is a copolymer satisfying the requirement (A), is obtained by copolymerizing the monomer (1), the monomer (2) and, if necessary, other monomers by a conventional method. Obtainable. Furthermore, (1)
When a conjugated diene such as butadiene or isoprene is used as a part or all of the above monomer, the monomer (2) having an acid anhydride group such as maleic anhydride is added to the double bond remaining in the polymer chain. It is then hydrolyzed and converted to maleic acid units. Also, the double bond of the polymer chain similarly obtained by using the conjugated diene may be saturated by hydrogenation. Furthermore, when the obtained copolymer contains a carboxyl group-containing unit, it may be converted into an ammonium salt or an amine salt by using an alkali metal ammonium salt or an organic amine.

The carbon particles used as the conductive particles in the conductive composition of the present invention have a pH of 6 to 6 when dispersed in a mixture of ethanol and water at a weight ratio of 1: 1 by 10% by weight. It is a neutral to weakly alkaline one, which is adjusted to 9, preferably 6.5 to 8.8. When the pH is less than 6, the carbon particles are likely to settle and separate, and particularly when a conductive paste having an apparent viscosity of 20 Pa · s or less is prepared, the carbon particles agglomerate and the carbon particles are separated from the organic resin and the solvent during standing. On the other hand, even when the pH exceeds 9, carbon particles aggregate and settle. The shape of the carbon particles may be spherical, flaky or fibrous. Its size, that is, the spherical one is the diameter,
The flaky shape has an average of the major axis and the minor axis of the flat surface, and the fibrous shape has a length of preferably 0.1 to 50 μm, more preferably 2 to 30 μm. If it is less than 0.1 μm, it is difficult to apply the conductive composition uniformly,
Also, the binding strength, coating strength, and especially bending strength are poor. 30μ
When it exceeds m, the surface of the formed composition loses smoothness,
The packing density decreases, and the discharge capacity per unit volume decreases. Alternatively, the size of each carbon particle is 0.02.
Particles having a size of .about.0.1 .mu.m may be aggregated in a chain shape or a lump shape to form secondary particles having the above-mentioned size.

It is necessary to coat the surface of such carbon particles with a water-soluble polymer. As a result, dispersibility in the organic resin that is a binder can be improved and sedimentation can be prevented.

As the water-soluble polymer used for coating the surface of the carbon particles, polyvinyl alcohol, polyethylene oxide, polypropylene oxide, ethylene oxide / propylene oxide copolymer, polyvinylpyrrolidone, styrene / maleic anhydride copolymer are hydrolyzed. Or a water-soluble salt thereof, a hydrolyzate of a vinyl methyl ether / maleic anhydride copolymer or a water-soluble salt thereof, poly (meth) acrylic acid or a water-soluble salt thereof, methyl cellulose, carboxymethyl cellulose or a water-soluble salt thereof, etc. It is exemplified, and one kind or a combination of two or more kinds may be used. As the above water-soluble salt, lithium salt,
Examples are alkali metal salts such as sodium salts, ammonium salts or amine salts, with lithium salts being preferred.

The coating of the surface of the carbon particles is carried out, for example, by immersing the carbon particles in an aqueous solution of a water-soluble polymer, stirring, filtering and drying. The amount of the water-soluble polymer used for coating is preferably 0.005 to 1% by weight, more preferably 0.01 to 0.5% by weight, based on the carbon particles.
If it is less than 0.005% by weight, sedimentation or separation of carbon particles will occur, and if it exceeds 5% by weight, the battery capacity will decrease.

The conductive composition of the present invention contains an organic resin and carbon particles which satisfy the above-mentioned conditions. The content of the organic resin in the composition is 1 to 10% by weight, preferably 2 to
6% by weight. If it is less than 1% by weight, the binding force of the carbon particles is weak, and the carbon particles are peeled and desorbed by charge / discharge cycles and heat cycles, resulting in a decrease in battery capacity. On the other hand, if it exceeds 10% by weight, the filling amount of carbon particles in the electrode is small and the battery capacity is also small.

To dissolve the organic resin of the conductive composition of the present invention and disperse the carbon particles, a solvent may be added to prepare a conductive paste having an apparent viscosity suitable for the treatment. Examples of the solvent include aromatic hydrocarbons such as toluene, xylene, ethylbenzene, and aromatic components of petroleum fraction; and aliphatic hydrocarbons such as hexane, heptane, octane, cyclohexane, mineral spirits, and kerosene. It If there are many carboxyl groups and hydroxyl groups in the organic resin, to help dissolve the organic resin,
You may use together 5-20 weight% of propanol or butanol.

The apparent viscosity of the conductive paste varies depending on the purpose of use, but when used for forming the electrode, it is preferably in the range of 5 to 20 Pa · s so that the electrode is formed by one coating. If it is less than 5 Pa · s, the coating thickness becomes insufficient, and if it exceeds 20 Pa · s, the coating becomes uneven.

The conductive composition of the present invention may be blended with nitrile rubber, isoprene rubber, ethylene propylene rubber, etc., if necessary, as long as the purpose is not violated.

To prepare the electrically conductive composition and electrically conductive paste of the present invention, an organic resin, carbon particles and, if necessary, other components are mixed with a propeller stirrer, a desolver, a kneader crusher, a triple roll, a ball mill. It is possible by mixing and dispersing with a mixing and dispersing machine such as.

The coating may be carried out by any method capable of coating in a thin film form, such as coating with a doctor blade or an applicator; screen printing or stencil printing. The thickness of the applied layer is preferably 7 to 200 μm, and 8
0 to 150 μm is more preferable. If it is less than 70 μm, the battery capacity per volume is low, and if it exceeds 200 μm, the bending strength of the coating film is poor and cracks occur.

By using the conductive composition of the present invention as an electrode mixture, various electrodes such as a negative electrode and a positive electrode of a lithium battery can be prepared. The other elements of these batteries can be used as known. For example, an electrolytic solution used in a lithium battery includes non-aqueous solvents such as ethers, ketones, lactones, nitriles, esters,
Carbonates or sulfolanes are used, especially cyclic carbonates. Examples of such a non-aqueous solvent include ethylene carbonate, propylene carbonate, γ-butyrolactone, tetrahydrofuran, 2-methyltetrahydrofuran, sulfolane, dimethoxyethane and the like. Further, as the electrolyte to be dissolved therein, LiClO ₄ , LiBF ₄ , LiPF ₆ , Li
Such as AsF _6, CF ₃ SO ₃ Li and the like. Support of lithium as an active material may be carried out by a conventional method.

[0025]

Industrial Applicability According to the present invention, a conductive composition having excellent adhesiveness and stable conductivity, in which carbon particles are less likely to be separated during storage and during coating, can be obtained. Various batteries using the electrode produced using the conductive composition of the present invention can maintain a high discharge capacity without peeling between the electrode and the current collector due to charge / discharge cycle or heat cycle. Therefore, the conductive composition of the present invention is useful as an electrode of various batteries including a negative electrode of a lithium battery.

[0026]

The present invention will be described in detail below with reference to Examples and Comparative Examples. In these examples, parts represent parts by weight. The invention is not limited by these examples.

In the following examples, the pH of carbon particles was measured as follows. That is, 10% by weight of carbon particles were dispersed in a mixed solution of ethanol and water at a weight ratio of 1: 1 and left for 5 minutes. Then, the carbon particles were removed by centrifugation, and the pH of the mixed solution was measured with a pH meter.

Reference Example 1: Surface treatment of carbon material Polyvinyl alcohol 0.1 having an average molecular weight of 30,000
50 μl of an aqueous solution containing 20 parts of water, a size of 20 μm and a pH of 7.
After 100 parts of spherical carbon particles of No. 6 were mixed, stirred and dispersed, the carbon particles were filtered off and dried at 150 ° C. for 30 minutes to obtain surface-treated carbon particles C1. Similarly, carbon particles having the size, shape and pH shown in Table 1 were surface-treated with various water-soluble polymers shown in Table 1 to obtain surface-treated carbon particles C2 to C6. For these surface-treated carbon particles, the weight loss of the particles up to 400 ° C. by TG-DTA was determined and used as the surface treatment amount. However, the water-soluble polymer is L
In the case of the i salt, the particles were dispersed in water, boiled, the amount of the metal salt attached was determined by an ion chromatogram, and the value added to the above weight loss was used as the surface treatment amount. In addition, in Table 1, the untreated carbon particles C7 used for comparison are also described.

[0029]

[Table 1]

Reference Example 2: Synthesis of Organic Resin Copolymers B1 to B7 having the monomer compositions shown in Table 2 were described in "Experimental Method of Polymer Synthesis" by Takayuki Otsu, Masayoshi Kinoshita, Kagaku Dojin). According to the method, using an autoclave, under high pressure, in the presence of azobisisobutyronitrile,
Radical polymerization was carried out in benzene. The synthesized monomer was purified by throwing it into anhydrous diethyl ether to precipitate the polymer, and dried. B7 is
It is an ethylene propylene rubber for comparison, which does not contain a carboxyl group or a hydroxyl group.

[0031]

[Table 2]

Examples 1 to 9 and Comparative Examples 1 to 5: Negative Electrodes of Lithium Batteries As shown in Table 3, the organic resins B1 to 1 synthesized in Reference Example 2
B6, and ethylene propylene rubber B for comparison
7 and polyvinylidene fluoride B8 were each dissolved in xylene containing 10% by weight of n-butyl alcohol. In addition, carbon particles C1 to C6 surface-treated with a water-soluble polymer in Reference Example 1 and carbon particles C7 for comparison were used.
Was added to each of the above components, and the mixture was stirred with a raiser and mixed until the carbon particles were uniformly dispersed in the system, thereby preparing a conductive paste. Use this conductive paste at room temperature for 24 hours
After standing for a time, the degree of separation of carbon particles was observed. The results are shown in Table 3.

This conductive paste was applied to a copper foil having a thickness of 12 μm using an applicator so that the coating film thickness after drying would be 100 μm, and dried at 50 ° C. for 10 minutes,
A negative electrode was formed. The following evaluation was performed on this negative electrode.

Volume resistivity: 1 cm area on the carbon layer of the negative electrode
Another copper foil of No. ² was placed, and the resistance value between the copper foil of the negative electrode and the copper foil of the negative electrode was measured by the four-terminal method. The thickness of the carbon layer was measured with a micrometer, and the volume resistivity was calculated.

Bending strength: Using a bending tester (manufactured by Yasuda Seiki Seisakusho) having a diameter of 2 mm, the negative electrode was bent with the copper foil facing down and the carbon layer facing up, and the state of cracks and peeling was observed under a 20 × microscope. did.

2 cm in size formed as described above
Using a 1 cm negative electrode, the negative electrode; manganese dioxide 85% by weight, pH 7.5, 10% by weight of carbon particles having a size of 760Å, and 5% by weight of the organic resin B1 synthesized in Reference Example 2 were applied to a copper foil. A lithium battery of a three-electrode method (see SANYO TECHNICAL REVIEW Vol. 20, No. 3, p. 76 (November 1988)) equipped with a positive electrode obtained by drying; and a lithium metal was assembled. As the electrolyte, 1,000 ml of a mixed solution of propylene carbonate and dimethoxyethane with a molar ratio of 1: 1 was added to L
IClO ₄ was used after 1 mole dissolved. For the battery thus produced, the initial value of the discharge capacity and the value after 50 cycles of heat cycle with the temperature of −20 ° C. for 30 minutes and the temperature of 90 ° C. for 30 minutes as one cycle were measured. The state of the negative electrode after the heat cycle test was observed.

The results are shown in Examples 1 to 9 using the conductive composition of the present invention, Comparative Example 1 containing a large amount of an organic resin as a binder, Comparative Example 2 using ethylene propylene rubber, and surface treatment. In comparison with Comparative Examples 3 and 4 using carbon particles not subjected to the above and Comparative Example 5 using a combination of polyvinylidene fluoride and carbon particles not subjected to the surface treatment,
It shows in Table 3.

[0038]

[Table 3]

Claims (2)

[Claims] 1. A conductive composition containing an organic resin and carbon particles, wherein the organic resin satisfies the requirement (A) below, and the carbon particles satisfy the requirement (B) below. Of 1 to 10% by weight. (A) (1) one or more kinds of aliphatic hydrocarbon monomers having an ethylenically unsaturated double bond or a conjugated double bond, 70 to 99.99 mol%; and (2) 1 in a molecule.
Ethylenic unsaturated double bond having one or two carboxyl groups or an acid anhydride group to be hydrolyzed later, or having a hydroxyl group bonded to a carbon atom having no aliphatic carbon-carbon unsaturated bond One or two of the contained monomers
It is a copolymer or a salt thereof obtained by copolymerizing 0.01 to 20 mol% of one or more species. (B) (a) The pH of the liquid in which 10% by weight of the carbon particles are dispersed in a mixture of ethanol and water at a weight ratio of 1: 1 is 6 to 9; and (b) the surface is a water-soluble polymer. It is covered. 2. An electrode using the conductive composition according to claim 1 as a mixture.