JPH076754A - Battery electrode and secondary battery using this electrode - Google Patents

Abstract

PURPOSE:To utilize the advantage of a carbon fiber and to increase charge/ discharge capacity by using the carbon fiber whose degree of orientation P is in a specified range. CONSTITUTION:A carbon fiber whose degree of orientation which is an index to show the degree of orientation of a carbon layer in the carbon fiber to the carbon axis direction is adequate is used as a high capacity carbon material. The carbon fiber whose degree of orientation P is in a range of 70%<=P<=85% is used. The carbon fiber is prepared in such a way that an acrylic fiber in which the degree of orientation of (400) of PAN(polyacrylonitrile) comprising 99.2 molar % acrylonitrile and 0.8 molar % methacrylic acid is 92% is heated at 200-250 deg.C in the air under no stretch as fire resistant treatment, and baked at 900-1500 deg.C for 5 minutes in the nitrogen atmosphere under no stretch. By using the carbon fiber, an electrode capable of charging/discharging at high rate and a secondary battery using the electrode can be provided.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electrode composed of carbon fiber having a low degree of orientation and a secondary battery using the electrode.

[0002]

2. Description of the Related Art In recent years, with the widespread use of portable devices such as video cameras and notebook computers, demand for small and high capacity secondary batteries has increased. Most of the secondary batteries currently used are nickel-cadmium batteries using an alkaline electrolyte, but the battery voltage is low at about 1.2 V, and it is difficult to improve the energy density. Therefore, high-energy secondary batteries have been studied using lithium metal, which is the base metal, as the negative electrode.

However, in a secondary battery in which lithium metal is used for the negative electrode, there is a risk that lithium will grow into dendrites due to repeated charging and discharging, causing a short circuit and igniting. In addition, since highly active metallic lithium is used, it is inherently dangerous, and there are many problems in using it for consumer use. In recent years, a lithium ion secondary battery using various carbonaceous materials has been devised as a device that solves such a safety problem and enables high energy peculiar to a lithium electrode. This method utilizes the fact that the carbonaceous material is doped with lithium ions during charging and has the same potential as that of metallic lithium, so that it can be used for the negative electrode instead of metallic lithium. In addition, during discharge, the doped lithium ions are dedoped from the negative electrode and return to the original carbonaceous material. When such a carbonaceous material doped with lithium ions is used as the negative electrode, there is no problem of dendrite generation, and since there is no metallic lithium, there is a feature that it is also excellent in safety, Currently, research and development are actively carried out.

As a secondary battery using an electrode using the above-mentioned carbonaceous material doped with lithium ions,
JP-A-57-208079, JP-A-58-931
76, JP-A-58-192266, JP-A-62-90863, JP-A-62-122066 and JP-A-3-66856 are known. Such a carbonaceous material is generally in the form of powder, and a polymer binder such as Teflon or vinylidene fluoride is required for molding the electrode. However, when carbon fiber or a carbon fiber structure is used as the carbonaceous material instead of powder, an electrode can be prepared without using a binder or in a small amount. Further, the carbon fiber or the carbon fiber structure is said to be excellent in terms of chemical stability with respect to the electrolyte, structural stability against volume expansion due to doping, and repeated charge / discharge characteristics. A secondary battery using such an electrode is disclosed in JP-A-60-
No. 36315, No. 60-54181, No. 62-103991, No. 62-1545.
64, JP-A-63-58763, JP-A-2
No. 82,466, etc. are known.

[0005]

However, an electrode using a carbonaceous material doped with ions of lithium or the like has a lower ion concentration per weight than that of lithium metal, so that the charge / discharge capacity of lithium metal is low. There is a problem that it is still lower than the case. In order to increase the charge / discharge capacity, it is necessary to use a carbonaceous material having an optimum structure. The present invention has advantages when using carbon fiber, and
It is an object of the present invention to provide an electrode having a sufficiently high charge / discharge capacity and a secondary battery using the electrode.

[0006]

The present invention has the following constitution in order to solve the above problems.

"(1) An electrode made of carbon fiber having a degree of orientation P of 70% ≤P≤85%.

(2) A secondary battery comprising the electrode described in (1) above. The intercalation (or doping) of carbon materials has been studied for a long time, and much knowledge has been accumulated. Conventionally, carbon materials that can be intercalated have a graphitization degree (crystallinity). It has been considered that it is limited to high quality items. However, in recent years, it has been found that intercalation is possible even for an amorphous carbon material such as a fired body of an organic material, and a high-performance secondary material utilizing intercalation into such a fired body of an organic material is used. Since the realization of batteries, there has been an increasing interest in amorphous carbon materials. Rechargeable batteries that utilize intercalation of lithium into a fired organic material can overcome the safety problems of lithium batteries and become high-capacity secondary batteries, which is a feature of lithium batteries. Is being watched as.

However, there are still many unclear points about the mechanism of intercalation into an amorphous carbon material, and a guideline for searching for a high-performance carbon material for a secondary battery has not been established yet. The current situation is that carbon materials are being developed with repeated mistakes. Currently, the search for high-performance carbon materials is directed toward amorphous carbon materials and toward crystalline carbon materials. As a result of diligent studies on the high-capacity carbon material, the present inventors have found that carbon fibers having an appropriate degree of orientation are excellent as the high-capacity carbon material.

Hereinafter, the carbon fibers constituting the battery electrode according to the present invention will be described in detail with reference to specific examples.

In the present invention, the orientation degree P is an index showing how much the carbon layer surface in the carbon fiber is oriented with respect to the fiber axis direction, and can be measured by the following method. When the carbon fiber axis is arranged on the fiber sample stand so as to be vertical and irradiated with X-rays (Cu, Kα) from a right angle direction, a strong diffraction line of (002) appears near the diffraction angle 2θ = 26 ° in the horizontal plane. . Next, while rotating the carbon fiber in a plane perpendicular to the incident X-ray, the rotation angle dependence of the diffraction intensity is measured at a position near the diffraction angle 2θ = 26 ° in the horizontal plane. The half width obtained from the angle dependence of this intensity is defined as the angle H,
The orientation degree P is calculated from the following formula.

P = {(180-H) / 180} × 100 (%) (1) The carbon fiber in the present invention is not particularly limited, and a carbonized organic material is generally used.
Specifically, PAN-based carbon fiber obtained from polyacrylonitrile (hereinafter abbreviated as PAN), pitch-based carbon fiber obtained from pitch of coal or petroleum, cellulose-based carbon fiber obtained from cellulose, gas of low molecular weight organic substance Examples thereof include vapor-grown carbon fibers, but other than these, carbon fibers obtained by firing polyvinyl alcohol, lignin, polyvinyl chloride, polyamide, polyimide, phenol resin, furfuryl alcohol, etc. may be used. Among these carbon fibers, it is preferable to appropriately select carbon fibers satisfying the characteristics according to the characteristics of the electrode and the battery in which the carbon fibers are used.

Among the above carbon fibers, when used as a negative electrode of a secondary battery using a non-aqueous electrolyte containing an alkali metal salt, PAN-based carbon fibers, pitch-based carbon fibers and vapor-grown carbon fibers are used. preferable. In particular, in terms of good doping with alkali metal ions, especially lithium ions, P
AN-based carbon fiber is preferably used.

Generally, the fibrous material has directionality because it is oriented in the fiber direction. Carbon fibers, which are fired bodies of such fibers, naturally have orientation, but in this case having orientation indicates that the carbon layer surfaces are aligned in a substantially constant direction. When carbon fiber is used as the active electrode material, if the orientation is too high, structural anisotropy is strong,
Since the intercalation is likely to occur depending on the direction, there is a drawback that the capacity does not increase. Further, if the orientation is too high, the crystallinity after firing is generally high, so that there is a problem that a high capacity cannot be obtained. Then, as a result of intensive studies on the high-capacity carbon material, the present inventors have found that carbon fibers having an appropriate degree of orientation are excellent as the high-capacity carbon material.

The method for producing PAN-based carbon fibers is described in JP-B-37-4405, JP-B-44-21175,
Japanese Patent Publication No. 47-24185, Japanese Patent Publication No. 51-6244, and many other known methods. However, most of them are methods of manufacturing carbon fiber as a reinforcing material, and therefore, a technique of increasing orientation in a yarn making and firing step is adopted. However, since it is important that the carbon fiber in the present invention is low as described above, a slight improvement is added to the known technique.
As a technique for controlling the orientation to be low, it is convenient and effective to perform the treatment under relaxation in the yarn making and firing steps.
That is, in a known method for producing a carbon fiber having a high orientation in order to achieve high strength, the raw yarn has a low draw ratio, and the firing step is performed under low tension and even under no tension. The improvements enable the production of carbon fibers with low orientation. As a method for producing a carbon fiber having a low orientation, for example, first, in the raw yarn, the main component is polyacrylonitrile, and co-treatment with a component such as methyl acrylate or methyl methacrylate, which tends to cause orientation relaxation by heating. A method using a polymerized polymer is preferably used. In the firing process,
As a specific example, in the method disclosed in JP-B-51-24603, the stretching ratio in the flameproofing step is 0.78 times, and the stretching ratio in the carbonization step is shrinking to 0.92 times. It is possible to adopt a method such as letting it occur. Furthermore, the degree of orientation is adjusted by setting the tension in the stabilization process, ie, the flameproofing process, disclosed in JP-A-62-117818 to 0.03 g / df, which is lower than 0.16 g / d in the comparative example. Is 80%. By reducing the draw ratio to 0.92 as the tension in the carbonization step, it becomes possible to manufacture carbon fibers having an orientation degree of 78%. Pitch-based carbon fibers, and also in carbon fibers starting from other organic polymers,
With the same technical idea, it is possible to manufacture low orientation carbon fibers.

When the carbon fiber in the present invention is used as an electrode, it may have any form, but it is preferably arranged in a uniaxial direction, or a fabric-like or felt-like structure. Become. Examples of the fabric-like or felt-like structure include woven fabrics, knitted fabrics, braids, laces, nets, felts, papers, non-woven fabrics, mats, and the like. Felt and the like are preferred.

The diameter of the carbon fibers used in the present invention should be determined so that each form can be easily adopted, but carbon fibers having a diameter of 1 to 1000 μm are preferably used, and 1 to 20 μm is more preferable. It is also preferable to use several kinds of carbon fibers having different diameters.

The electrode composed of the carbon fiber of the present invention is
It can be used as an active electrode of various batteries, and what kind of battery such as a primary battery or a secondary battery is used is not particularly limited. Among these, it is preferably used as a negative electrode of a secondary battery. Particularly preferable secondary batteries include secondary batteries using a non-aqueous electrolyte solution containing an alkali metal salt such as lithium perchlorate, lithium borofluoride, and phosphorus hexafluoride / lithium.

When the electrode of the present invention is used in a non-aqueous electrolyte secondary battery containing an alkali metal salt, the cation or anion doping of carbon fiber is utilized, and the cation-doped carbon fiber is used. Carbon fiber doped with anions will be used for the negative electrode.
These should be able to be used for the positive electrode or the negative electrode depending on various characteristics of the carbon fiber, but it is not always necessary to use both electrodes as the electrode of the present invention, and it may be used for either one. It is also a preferred embodiment that the constituted electrode is the negative electrode and the electrode containing no carbon fiber is the positive electrode. When using an electrode that does not contain carbon fibers for the positive electrode, in addition to carbonaceous materials other than fibers, artificial or natural graphite powder, fluorinated carbon, inorganic compounds such as metal or metal oxides, organic polymer compounds, etc. Can be used as the positive electrode. In this case, in the positive electrode made of an inorganic compound such as a metal or a metal oxide, charge / discharge reaction occurs by utilizing cation doping and dedoping. In the case of an organic polymer compound, a charge / discharge reaction occurs due to anion doping and undoping. As described above, various charging / discharging reaction modes are adopted depending on the substance, and these are appropriately selected according to the required positive electrode characteristics of the battery.

The positive electrode containing no carbon fiber includes inorganic compounds such as transition metal oxides and transition metal chalcogens containing alkali metal, polyacetylene, polyparaphenylene,
Examples of the positive electrode used in ordinary secondary batteries include conjugated polymers such as polyphenylene vinylene, polyaniline, polypyrrole, and polythiophene, crosslinked polymers having a disulfide bond, and thionyl chloride.
Among these, in the case of a secondary battery using a non-aqueous electrolyte containing a lithium salt, cobalt, manganese, molybdenum,
Transition metal oxides and transition metal chalcogens such as vanadium, chromium, iron, copper and titanium are preferably used.

The electrolytic solution of the secondary battery using the electrode of the present invention is not particularly limited, and a conventional electrolytic solution is used, and examples thereof include an acid or alkaline aqueous solution or a non-aqueous solvent. Among these, as the electrolytic solution of the secondary battery composed of the above-mentioned non-aqueous electrolytic solution containing an alkali metal salt, propylene carbonate, ethylene carbonate, γ-butyrolactone, N-methylpyrrolidone, acetonitrile, N, N-dimethylformamide, Dimethyl sulfoxide, tetrahydrofuran, 1,3-dioxolane,
Methyl formate, sulfolane, oxazolidone, thionyl chloride, 1,2-dimethoxyethane, diethylene carbonate, derivatives and mixtures of these are preferably used. The electrolyte contained in the electrolytic solution is an alkali metal,
Particularly, lithium halides, perchlorates, thiocyanates, borofluorides, phosphorous fluorides, arsenic fluorides, aluminum fluorides, trifluoromethylsulfates and the like are preferably used.

The secondary battery using the electrode of the present invention can be used as a video camera, a personal computer, a word processor, a radio-cassette, by utilizing the features of light weight, high capacity and high energy density.
It is widely applicable to portable small electronic devices such as mobile phones.

[0023]

EXAMPLES Specific embodiments of the present invention will be described below with reference to examples, but the present invention is not limited thereto.

Example 1 Acrylonitrile 99.2 mol%, methacrylic acid 0.
PAN (400) orientation ratio of 8 mol% is 92%
Acrylic fiber of 200 to 250 ℃ without tension in air
A carbon fiber was produced by making it flame resistant at 0 ° C. and subsequently firing it at 1100 ° C. for 5 minutes in nitrogen under no stress. The degree of orientation of the carbon fibers was measured by wide-angle X-ray diffraction (counter method). The degree of orientation of the carbon fiber obtained from the formula (1) was 76.7%.

Next, an electrode was prepared using the above carbon fiber,
Charge evaluation was performed. The electrolytic solution was evaluated by a three-electrode cell using propylene carbonate containing 1M lithium perchlorate, and a metal lithium foil for the counter electrode and the reference electrode. The current density per weight of carbon fiber is 0V (vs. Li
⁺ / Li). The discharge capacity of the carbon fiber electrode calculated from the amount of charge discharged after charging was 420 mAh / g.
The total shrinkage during processing was 30% of the original length.

Comparative Example 1 The same acrylic fiber as in Example 1 was subjected to constant length in air at 200 ° C.
To 250 ° C. to flame resistance, and then under nitrogen at a constant load of 26
Carbon fibers were produced by firing to 00 ° C for 7 minutes. The shrinkage ratio to the original length during graphitization was 6%.

The degree of orientation of the carbon fibers was measured by wide-angle X-ray diffraction (counter method). The degree of orientation of the carbon fiber obtained from the formula (1) was 90.4%.

Next, an electrode was prepared using the above carbon fiber,
Charge evaluation was performed. The electrolytic solution was evaluated by a three-electrode cell using propylene carbonate containing 1M lithium perchlorate, and a metal lithium foil for the counter electrode and the reference electrode. The current density per weight of carbon fiber is 0V (vs. Li
⁺ / Li). The discharge capacity of the carbon fiber electrode obtained from the amount of charge discharged after charging was 130 mAh / g.

Example 2 Acrylic fibers were subjected to flameproofing treatment in the same manner as in Example 1, and then fired at about 1200 ° C. for 5 minutes in a nitrogen atmosphere without tension. The total shrinkage during treatment was 31% of the original length. The degree of orientation of the carbon fibers was measured by wide-angle X-ray diffraction (counter method). The degree of orientation of the carbon fiber obtained from the formula (1) was 77.1%.

Next, an electrode was prepared using the above carbon fiber,
Charge evaluation was performed. The electrolytic solution was evaluated by a three-electrode cell using propylene carbonate containing 1M lithium perchlorate, and a metal lithium foil for the counter electrode and the reference electrode. The current density per weight of carbon fiber is 0V (vs. Li
⁺ / Li). The discharge capacity of the carbon fiber electrode obtained from the amount of charge discharged after charging was 380 mAh / g.

Example 3 Acrylic fibers were subjected to flameproofing treatment in the same manner as in Example 1, and then fired at about 1050 ° C. for 5 minutes under a nitrogen atmosphere in a tension state. The total shrinkage during processing was 30% of the original length. The degree of orientation of the carbon fibers was measured by wide-angle X-ray diffraction (counter method). The degree of orientation of the carbon fiber obtained from the formula (1) was 80.0%.

Next, an electrode was prepared using the above carbon fiber,
Charge evaluation was performed. The electrolytic solution was evaluated by a three-electrode cell using propylene carbonate containing 1M lithium perchlorate, and a metal lithium foil for the counter electrode and the reference electrode. The current density per weight of carbon fiber is 0V (vs. Li
⁺ / Li). The discharge capacity of the carbon fiber electrode obtained from the amount of charge discharged after charging was 400 mAh / g.

Example 4 Commercially available lithium carbonate (Li ₂ CO ₃ ) and basic cobalt carbonate
(2CoCO ₃ .3Co (OH) ₂ ) was weighed so that the molar ratio was Li / Co = 1/1, mixed with a ball mill, and heat-treated at 900 ° C. for 20 hours to obtain LiCoO ₂ . This was crushed with a ball mill, and artificial graphite was used as a conductive material, polyvinylidene fluoride (PVdF) was used as a binder, and N-methylpyrrolidone was used as a solvent. LiCoO ₂ / artificial graphite / PVdF = 80/15/5 To prepare a positive electrode slurry, which was applied on an aluminum foil, dried and pressed to obtain a positive electrode.

The carbon fiber electrode prepared in Example 1 was used as a negative electrode, and a porous polypropylene film (Celguard # 2) was used.
A secondary battery was manufactured by stacking the positive electrode prepared above through a separator 500 (manufactured by Daicel Chemical Industries, Ltd.). As the electrolytic solution, propylene carbonate containing 1M lithium perchlorate was used.

The charging evaluation of the secondary battery produced above was performed. The current density per carbon fiber weight was 40 mA / g at a constant current, and the battery was charged to 4.3 V. The discharge capacity of the secondary battery obtained from the amount of charge discharged after charging was 370 mAh / g based on the weight of the carbon fiber used in this battery.

Comparative Example 2 A secondary battery was manufactured in the same manner as in Example 4, using the carbon fiber electrode manufactured in Comparative Example 1 as the negative electrode. The weight of carbon fiber used in this battery was 110 mAh / g.

[0037]

According to the present invention, it is possible to provide a battery using a highly chargeable carbon fiber as an electrode.

Claims (4)

[Claims] 1. An electrode made of carbon fiber having an orientation degree P of 70% ≦ P ≦ 85%. 2. The carbon fiber comprises polyacrylonitrile 90
The electrode according to claim 1, which is obtained by firing at 0 ° C to 1500 ° C. 3. A secondary battery using the electrode according to claim 1. 4. A secondary battery using the electrode according to claim 2.