JP2002033106A - Slurry for lithium ion secondary battery electrode and its utilization - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium ion secondary battery having high capacity and a reduced charging time. SOLUTION: This slurry for a lithium ion secondary battery electrode contains a cellulose ether compound, rubber particles, water and an active material. The electrode is obtained by using the slurry for the lithium ion secondary battery electrode characterized in that the average polymerization degree of the cellulose ether compound is 150 to 300 and the active material is a carbonaceous material having a graphitization degree of 0.80 or more, and the lithium ion secondary battery is manufactured by using it.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a slurry for an electrode of a lithium ion secondary battery and its use. More specifically, the present invention relates to a slurry for a lithium ion secondary battery electrode containing a cellulose ether compound, rubber particles, water and an active material, and its use.

[0002]

2. Description of the Related Art In recent years, notebook personal computers, mobile phones, P
The spread of mobile terminals such as DA is remarkable. As a secondary battery used as a power source of these portable terminals, a lithium ion secondary battery (hereinafter, sometimes simply referred to as “battery”) is frequently used. As for mobile terminals, smaller, thinner, lighter, and higher-performance mobile phones have been demanded for more comfortable portability. As a result, mobile terminals have been used in various places. As the range of use has increased, batteries have also been required to be smaller, thinner, lighter, and have higher performance, as in portable terminals.

In order to improve the performance of such batteries, electrodes,
Improvements in electrolytes and other battery members are being studied. Regarding the electrode, in addition to the study of the active material and the current collector itself, studies have been made on a binder for holding the active material on the current collector. Usually, a binder made of an arbitrary polymer holds an active material on a current collector to form an electrode. Such an electrode is obtained by mixing a binder and a liquid medium, adding an optional additive to the mixture as necessary to obtain a binder composition, adding an active material to the slurry, and forming a slurry for an electrode. And dried. Conventionally, polyvinylidene fluoride (PVD) has been used as the binder.
Use of a fluorine-based polymer such as F) or a non-fluorine-based polymer such as a diene rubber has been proposed. These polymers are used for electrode production as a slurry obtained by adding an active material to a binder composition dissolved in an organic solvent or dispersed in water.

JP-A-11-67213 discloses that a binder composition comprising a polymer latex has an average degree of polymerization of 300.
It has been proposed to add carboxymethylcellulose having a degree of etherification of 0.5 to 1,800 and a degree of etherification of 0.5 to 1. Although it is described that this binder composition provides an electrode having excellent discharge characteristics, high capacity, charge-discharge cycle characteristics, and stability, it is not clear how much the battery capacity is. Recently, it has been desired to extend the use time of the portable terminal and the charging time, and it is urgently required to increase the capacity and the charging speed of the battery, that is, to improve the rate characteristics. However, even with the use of the previously known binder composition, it has been difficult to achieve both high battery capacity and improved rate characteristics.

[0005]

SUMMARY OF THE INVENTION An object of the present invention is to provide a slurry for an electrode capable of providing a lithium ion secondary battery having a high capacity and excellent rate characteristics in view of the above state of the art. Is to do.

[0006]

Means for Solving the Problems The inventors have used a cellulose ether compound having a low degree of polymerization in a slurry for a lithium ion secondary battery electrode containing a cellulose ether compound, rubber particles, water, and an active material. It has been surprisingly found that, when a negative electrode is manufactured using a combination of a carbonaceous material having a high degree of graphitization as a substance, both high capacity and improved rate characteristics of a lithium ion secondary battery can be achieved.

Thus, according to the present invention, as a first invention, there is provided a slurry for a lithium ion secondary battery electrode comprising a cellulose ether compound, rubber particles, water and an active material, wherein the average degree of polymerization of the cellulose ether compound is Is 15
0 or more and less than 300, and the active material has a degree of graphitization of 0.8
A slurry for a lithium ion secondary battery electrode characterized by being the above carbonaceous material is provided.

Further, as a second invention, there is provided a negative electrode for a lithium ion secondary battery manufactured by using the slurry according to the first invention, and as a third invention, a negative electrode according to the second invention is provided. Provided is a lithium ion secondary battery comprising the same.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION The slurry for an electrode of a lithium ion secondary battery The slurry for an electrode of the present invention is a slurry containing a cellulose ether compound having a low degree of polymerization, rubber particles, water, and a carbonaceous material having a high degree of graphitization as an active material. The cellulose ether compound contained in the slurry is characterized by having an average degree of polymerization of 150 or more and less than 300. When the average degree of polymerization of the cellulose ether compound is within this range, the solubility of the cellulose ether compound in water can be ensured, and an electrode having a smooth surface can be obtained. Can be compatible. If the average degree of polymerization is too small, the rate characteristics will be good, but the battery capacity will be low. Conversely, if the degree of polymerization exceeds the above range, it will be difficult to secure a high battery capacity at a high rate (a large amount of current during charging). Becomes The degree of etherification of the cellulose ether compound is not particularly limited, but is usually 0.5 to
1.0 is preferably used.

Specific examples of the cellulose ether compound include carboxymethylcellulose, carboxyethylcellulose and hydroxyethylcellulose, and alkali metal salts such as lithium salt, sodium salt and potassium salt, and ammonium salts thereof. Of these, alkali metal salts of carboxymethyl cellulose are preferred. The content of the cellulose ether compound in the slurry of the present invention is usually 0.1 to 10% by weight, preferably 0.2 to 5% by weight, based on the weight of the slurry, and the rubber content (in terms of solid content). Is usually 0.1 to
It is 60% by weight, preferably 0.2 to 40% by weight.

The rubber particles and water in the slurry may be usually prepared in the form of an aqueous rubber dispersion. Examples of the aqueous dispersion of such rubber particles include, for example, styrene / conjugated diene copolymer (SB) rubber latex such as styrene / butadiene copolymer rubber (SBR), and nitrile such as nitrile / butadiene copolymer rubber (NBR).・ Conjugated diene copolymer rubber latex, silicone rubber latex such as polyorganosiloxane, polymerization of alkyl acrylate, or alkyl acrylate and ethylenically unsaturated carboxylic acid and / or other ethylenically unsaturated monomer And fluororubber latex such as vinylidene fluoride copolymer rubber.

The method for producing the rubber aqueous dispersion is not particularly limited, but it can be usually produced by subjecting each of the predetermined monomers to emulsion polymerization or suspension polymerization according to a conventional method. For example, the method described in “Experimental Chemistry Course” Vol. 28 (edited by The Chemical Society of Japan, published by Maruzen Co., Ltd.), ie, water, a dispersant, an emulsifier, a crosslinking agent, etc. The additives, initiators and monomers are added so as to have a predetermined composition, and the monomers are dispersed or emulsified in water by stirring, and the polymerization is started by a method such as raising the temperature while stirring. By a method or the like, a dispersion in which rubber particles are dispersed in water can be obtained.

In the slurry for a lithium ion secondary battery electrode of the present invention, a carbonaceous material having a graphitization degree of 0.8 or more is used as an active material. The degree of graphitization is preferably 0.85 or more. The degree of graphitization is measured from a diffraction pattern such as a 002 plane of X-ray diffraction. If the degree of graphitization is less than 0.8, the time required for charging the battery increases, and the rate characteristics deteriorate. Specific examples of the carbonaceous material having a degree of graphitization of 0.8 or more include artificial graphite, graphite, natural graphite, MCMB, and pitch-based carbon fibers fired at 2,000 ° C. or more. Natural graphite is preferred.

The amount of the active material in the battery electrode slurry of the present invention is not particularly limited, but is usually 10 to 1,000 times, preferably 30 to 500 times, the weight of rubber particles. If the amount of the active material is too small, the active material layer formed on the current collector has many inactive portions, the function as an electrode becomes insufficient, and it becomes difficult to improve the rate characteristics of the battery. On the other hand, if the amount of the active material is too large, the active material is not sufficiently fixed to the current collector, and easily falls off. The water in the slurry of the present invention is necessary to adjust the concentration to be easily applied to the current collector, and the amount thereof is 20 to 1, based on 100 parts by weight of the total of the cellulose ether compound, the rubber particles, and the active material. 0
00 parts by weight, preferably 50 to 500 parts by weight.

If necessary, the slurry of the present invention may contain additives such as a viscosity modifier and a fluidizing agent for improving the paint properties. Specific examples thereof include polyacrylates such as sodium polyacrylate, polyvinyl alcohol, polyethylene oxide, polyvinylpyrrolidone, copolymers of acrylic acid or acrylates and vinyl alcohol, maleic anhydride or maleic acid or fumaric acid. Water-soluble polymers such as fully or partially saponified copolymers of vinyl acetate, modified polyvinyl alcohol, modified polyacrylic acid, polyethylene glycol, polycarboxylic acid, ethylene-vinyl alcohol copolymer, and vinyl acetate polymer. . The usage ratio of these additives can be freely selected as needed. In addition, a conductive material such as carbon such as activated carbon or metal powder can be added as long as the object of the present invention is not hindered.

2. The negative electrode for a lithium ion secondary battery The negative electrode for a lithium ion secondary battery of the present invention is obtained by applying the slurry of the present invention to a current collector such as a metal foil, drying the slurry, and fixing the active material on the current collector surface. Manufactured in. The current collector is not particularly limited as long as the current collector is made of a conductive material. Usually, a metal collector such as iron, copper, aluminum, nickel, and stainless steel is used. Although the shape is not particularly limited, a sheet having a thickness of about 0.001 to 0.5 mm is usually used.

The method of applying the slurry to the current collector is not particularly limited. For example, doctor blade method, dip method,
Reverse roll method, direct roll method, gravure method,
It is applied by an extrusion method, immersion, brush painting, or the like. The amount to be applied is not particularly limited, but the thickness of the active material layer formed after drying the slurry is usually 0.0.
The amount is about 0.05 to 5 mm, preferably about 0.01 to 2 mm. The drying method is not particularly limited, for example, warm air,
Drying by hot air, low-humidity air, vacuum drying, drying by irradiation with (far) infrared rays, electron beams, or the like can be given. Drying conditions are
Usually, stress concentration occurs and cracks occur in the active material layer,
Adjustment is made so that the active material layer can be dried as quickly as possible within a speed range in which the active material layer does not peel off from the current collector. Further, the electrode may be stabilized by pressing the dried current collector. As a pressing method, a method such as a die press or a roll press is used.

3. Lithium-ion secondary battery The lithium-ion secondary battery of the present invention includes an electrolytic solution and the above-described negative electrode for a lithium-ion secondary battery, and further includes a positive electrode, and is manufactured according to a conventional method using components such as a separator as necessary. Things. The method for producing the lithium ion secondary battery may be a commonly used method. For example, it is manufactured by stacking a positive electrode and a negative electrode with a separator interposed therebetween, winding or folding according to the shape of the battery, placing the battery in a battery container, injecting an electrolyte, and sealing the battery. The shape of the battery may be any of a coin type, a button type, a sheet type, a cylindrical type, a square type, a flat type and the like.

The positive electrode is made of LiCoO ₂ , L as an active material.
It is manufactured using a lithium-containing composite metal oxide such as iNiO ₂ , LiMnO ₂ , and LiMn ₂ O ₄ . The electrolytic solution may be any one as long as it is generally used for a lithium ion secondary battery, and an electrolyte that exhibits a function as a battery may be selected according to the types of the negative electrode active material and the positive electrode active material. As the electrolyte, for example, any of conventionally known lithium salts can be used, and LiClO ₄ , LiBF ₆ , Li
Such as PF _6, and the like.

Solvents for dissolving these electrolytes (electrolyte solvents) are not particularly limited as long as they are commonly used, but carbonates such as propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate and the like. Γ-
Lactones such as butyl lactone; trimethoxymethane, 1,2-dimethoxyethane, diethyl ether, 2
Ethers such as ethoxyethane, tetrahydrofuran and 2-methyltetrahydrofuran; sulfoxides such as dimethylsulfoxide; 1,3-dioxolane;
Oxolanes such as -methyl-1,3-dioxolane;
Nitrogen-containing compounds such as acetonitrile and nitromethane;
Organic acid esters such as methyl formate, methyl acetate, ethyl acetate, butyl acetate, methyl propionate and ethyl propionate; diglymes; triglymes; sulfolanes; oxazolidinones such as 3-methyl-2-oxazolidinone; Sultones such as propane sultone, 1,4-butane sultone and naphtha sultone; or a mixture of two or more solvents.

[0021]

EXAMPLES The present invention will be described in detail below with reference to examples, but the present invention is not limited to these examples. In the examples, parts and% are based on weight unless otherwise specified. In Examples and Comparative Examples, battery characteristics were measured by the following methods. Discharge capacity: 0 to 1.2 V in a 25 ° C. atmosphere using a coin-type battery manufactured by the following method.
The discharge capacity at the 30th cycle (unit: mAh / g (per active material)) was determined by a constant current method of 01C. The larger the value, the smaller the capacity reduction, which is a good result.

Charge / discharge rate characteristics: Except that the measurement conditions were changed to a test temperature of 40 ° C. and a constant current amount of 0.1 C, 0.5 C, and 1 C, a method similar to the above-described discharge capacity measurement was used. The discharge capacity at the 30th cycle (unit: mAh / g (per active material)) at each constant current amount was measured. 3 measured by a constant current method at 25 ° C. and 0.01 C.
The discharge capacity at the 30th cycle measured by each constant current method at 40 ° C. with respect to the discharge capacity at the 0th cycle (unit: mA)
h / g (per active material)) was calculated in percentage. The higher the value, the faster the charging is possible.

Production of Coin-Type Battery The slurry for the negative electrode of the lithium ion secondary battery prepared in Examples 1 and 2 and Comparative Examples 1 and 2 was made of copper foil (18 μm thick)
Is uniformly applied by a doctor blade method.
After drying with a dryer at 15 ° C. for 15 minutes, further drying under reduced pressure at 5 mmHg and 120 ° C. for 2 hours with a vacuum dryer, the active material density was 1.5 g / cm ³ by a biaxial roll press.
To obtain a negative electrode. The electrode thus obtained was cut into a circle having a diameter of 15 mm. As the positive electrode, metallic lithium having a diameter of 15 mm was used.

With a separator made of a circular polypropylene porous membrane having a diameter of 18 mm and a thickness of 25 μm interposed,
A stainless steel coin with the active material superimposed on the opposite lithium metal, placed so that the positive electrode contacts the bottom of the outer container, expanded metal placed on the negative electrode, and polypropylene packing installed It was housed in a mold outer container (diameter 20 mm, height 1.8 mm, stainless steel thickness 0.25 mm).

An electrolyte is injected so that no air remains in the container, and a 0.2 mm-thick stainless steel cap is fixed on the outer container via a polypropylene packing, and the battery can is sealed. And diameter 20mm, thickness about 2mm
Was manufactured. The electrolyte was propylene carbonate / ethylene carbonate / diethyl carbonate / dimethyl carbonate / methyl ethyl carbonate =
A solution in which LiPF ₆ was dissolved at a concentration of 1 mol / liter in 20/20/20/20/20 (volume ratio at 20 ° C.) was used.

Examples 1 and 2, Comparative Examples 1 and 2 3 parts of sodium carboxymethylcellulose having a polymerization degree and an etherification degree shown in Table 1 and a styrene / butadiene copolymer rubber latex (styrene content 35%, solid content concentration) 40%) to obtain a binder composition, which was mixed with natural graphite (graphitization degree 0.9).
9) 94 parts were added, and 110 parts of water were further added and mixed well to obtain a slurry for a battery electrode. A negative electrode was manufactured using the obtained slurry, and a battery was further manufactured. The discharge capacity and charge / discharge rate characteristics were evaluated. Table 1 shows the results.

[0027]

[Table 1]

[0028]

According to the present invention, a negative electrode prepared from the slurry of the present invention containing a cellulose ether compound having a low degree of polymerization, rubber particles, water and a carbonaceous material having a high degree of graphitization is used.
A lithium ion secondary battery having a high capacity and an improved charging rate can be obtained.

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Claims (3)

[Claims] 1. A slurry for a lithium ion secondary battery electrode comprising a cellulose ether compound, rubber particles, water and an active material, wherein the cellulose ether compound has an average degree of polymerization of 150 to less than 300, and the active material is A slurry for a lithium ion secondary battery electrode, which is a carbonaceous material having a degree of graphitization of 0.8 or more. 2. A negative electrode for a lithium ion secondary battery produced by using the slurry according to claim 1. 3. A lithium ion secondary battery comprising the negative electrode according to claim 2.