JPH08171913A - Electrode and battery employing the electrode - Google Patents

Abstract

PURPOSE: To provide the electrode and a battery employing the electrode where the electrode is high in charging/discharging capacity, and can be easily molded by using carbonaceous material whose average length in the direction of orientation is less than a specified value, which can be obtained out of carbon substances having a specified degree of orientation. CONSTITUTION: The electrode is formed out of carbonaceous material whose average length in the direction of orientation is less than 5mm, which can be obtained out of carbon substances whose degree P of orientation is 70%<=P<=85%. For example, acrylo fibers of polyacrylonitride composed of 99.2% acrylonitride by mole and 0.8% metha-acrylic acid by mole where the degree of orientation of acrylo fibers is 92%, is made fire resistant in the air at the temperature of 200 to 250 deg.C with no attention strained, and is subsequently sintered in the nitrogen at 1100 deg.C for 5 minutes with no attention strained so as to be formed into carbon fibers. The aforesaid carbon fibers are smashed into middle fibers whose average length is about 30μm, and binding and solvent are added thereto so as to be formed into slurry. The slurry is then applied onto a copper foil, dried and pressed so as to be formed into the electrode.

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 a carbon body 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.

Secondary batteries using an electrode utilizing the above-mentioned carbonaceous material doped with lithium ions are disclosed in JP-A-57-208079 and JP-A-58-9317.
6, JP-A-58-192266, JP-A-6-
No. 2-90863, Japanese Patent Laid-Open No. 62-122066, Japanese Patent Laid-Open No. 2-66856 and the like are known. Such a carbonaceous material is generally in the form of powder, and is used for an electrode as a sheet-shaped molded body to which a polymer binder such as Teflon or vinylidene fluoride is added for electrode molding. On the other hand, as a secondary battery using a carbon fiber or a carbon fiber structure as an electrode instead of a powder as a carbonaceous material, JP-A-60-36315 and JP-A-60-6315 are available.
-54181, JP-A-62-103991, JP-A-62-154564, JP-A-63-5.
Japanese Laid-Open Patent Publication No. 8763 and Japanese Laid-Open Patent Publication No. 2-82466 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.

It is an object of the present invention to solve the above-mentioned drawbacks and to provide an electrode using a carbon body having an orientation and having a high charge / discharge capacity and being easily molded and a secondary battery using the electrode. And

[0007]

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

"(1) An electrode made of a carbonaceous material having an orientation degree P of 70% ≤P≤85% and having an average length of 5 mm or less in the orientation direction obtained from a carbon body.

(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 a carbon body having an appropriate degree of orientation is excellent as the high-capacity carbon material.

The carbon body used in the battery electrode of the present invention will be described in detail below by using carbon fiber as a typical example.

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. Assuming that the full width at half maximum obtained from the angle dependence of this intensity is 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 generally fired organic material is 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 preferable. In particular, in terms of good doping with alkali metal ions, especially lithium ions, P
AN-based carbon fiber is preferably used.

Generally, a 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.

As a method for producing a PAN-based carbon fiber, Japanese Patent Publication No. 37-4405 and Japanese Patent Publication No. 44-211.
No. 75, Japanese Patent Publication No. 47-24185, Japanese Patent Publication No. 5
No. 1-6244, 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 the co-precipitation with a component such as methyl acrylate or methyl methacrylate that easily causes orientation relaxation by heating. A method using a polymerized polymer is preferably used. In the firing process, a specific example is
In the method disclosed in Japanese Patent No. 51-24603, it is possible to adopt a method in which the stretching ratio in the flameproofing step is 0.78 times, and the stretching ratio in the carbonization step is also contracted to 0.92 times. it can. Furthermore, the tension in the stabilization process, that is, the flameproofing process disclosed in JP-A-62-117818 is 0.03 g, which is lower than 0.16 g / d of the comparative example.
By setting / df, the degree of orientation becomes 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, by the same technical idea,
It is possible to manufacture low orientation carbon fibers.

In the present invention, a carbonaceous material having an average length in the orientation direction of 5 mm or less, preferably 1 mm or less, and more preferably 100 μm or less, which is obtained from the carbon fiber having the appropriate degree of orientation, is used. The average length is, for example, S
It is determined by measuring the length of 20 or more carbonaceous materials in the orientation direction by observing with a microscope such as EM. The above carbonaceous material is used as an electrode by forming a sheet from a slurry containing a binder and a solvent, but if the length exceeds 5 mm, the coatability deteriorates. As a method of cutting or crushing the carbon fiber to a length of 5 mm or less, various fine crushers can be used.

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.

Although carbon fibers have been described in detail as the carbon body having an appropriate degree of orientation of the present invention, the present invention can be applied to any carbon body having an orientation other than carbon fibers. The electrode composed of a carbonaceous material having a length of 5 mm or less, which is obtained from the carbon body of the present invention, can be used as an active electrode of various batteries, and is used for any battery such as a primary battery or a secondary battery. It 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 to the 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 are those which should 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 is sufficient to use either one of them for the carbon fiber of the present invention. 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 may be used, and examples thereof include an acid or alkaline aqueous solution or a non-aqueous solvent. Among them, 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.

[0024]

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, the above-mentioned carbon fiber is crushed to obtain an average length of about 3
It was a 0 μm milled fiber. The average length was obtained by measuring the lengths of 20 particles in the orientation direction by SEM observation and determining the average value. To this, 10 parts by weight of polyvinylidene fluoride was added as a binder, and 1-methyl-2-pyrrolidone was used as a solvent to prepare a slurry. After applying this slurry to a copper foil, it was dried and pressed to form an electrode, and the charge was evaluated. The electrolytic solution was evaluated by a three-electrode cell using propylene carbonate containing 1 M LiBF ₄ and a metallic lithium foil for the counter electrode and the reference electrode. The current density per weight of carbonaceous material was 40 mA / g constant current, and the battery was charged to 0 V (vs. Li ⁺ / Li). The discharge capacity of the carbonaceous material electrode determined from the amount of charge discharged after charging was 390 mAh / g.

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 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, using the above carbon fiber, an electrode was prepared in the same manner as in Example 1 and the charge was evaluated. 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 carbonaceous material was a constant current of 40 mA / g, and it was charged to 0 V (vs. Li ⁺ / Li). The discharge capacity of the carbonaceous material electrode obtained from the amount of charge discharged after charging is
It 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 10 minutes in a nitrogen atmosphere without tension. Total shrinkage during processing is 31% of original length
Met. 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 78.0%. Next, an electrode was prepared in the same manner as in Example 1 using the above carbon fiber, and the charge was evaluated. The electrolytic solution was evaluated by a three-electrode cell using propylene carbonate containing 1 M LiBF ₄ and a metallic lithium foil for the counter electrode and the reference electrode. The current density per weight of carbonaceous material was 40 mA / g constant current, and the battery was charged to 0 V (vs. Li ⁺ / Li). The discharge capacity of the carbonaceous material electrode determined from the amount of charge discharged after charging was 375 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 10 minutes under a nitrogen atmosphere in a tension state. 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 79.8%.

Next, using the above carbon fiber, an electrode was prepared in the same manner as in Example 1 and the charge was evaluated. The electrolyte is 1ML
Evaluation was carried out in a three-electrode cell using propylene carbonate containing iBF ₄ , and metallic lithium foil for the counter electrode and the reference electrode. The current density per weight of carbonaceous material was 40 mA / g constant current, and the battery was charged to 0 V (vs. Li ⁺ / Li). The discharge capacity of the carbonaceous material electrode calculated from the amount of charge discharged after charging is 41
It was 0 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 pulverized in a ball mill, artificial graphite as a conductive material, polyvinylidene fluoride as a binder (PVdF), using N- methylpyrrolidone as a solvent, LiCoO at a weight ratio of 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 carbonaceous material electrode prepared in Example 1 was used as a negative electrode, and a porous polypropylene film (Celgard #
A secondary battery was manufactured by stacking the positive electrode prepared above through a separator 2500 (manufactured by Daicel Chemical Industries, Ltd.). As the electrolyte, propylene carbonate containing 1M LiBF ₄ was used.

The charging evaluation of the secondary battery manufactured as described above was performed. The current density per weight of carbonaceous material was 40 mA / g, and the battery was charged to 4.3 V at a constant current. The discharge capacity of the secondary battery obtained from the amount of charge discharged after charging was 390 mAh / g based on the weight of the carbonaceous material used in the battery.

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

[0036]

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

Claims (5)

[Claims] 1. An electrode made of a carbonaceous material having a degree of orientation P of 70% ≦ P ≦ 85% and having an average length in the orientation direction of 5 mm or less, which is obtained from a carbon body. 2. The electrode according to claim 1, wherein the carbon body is carbon fiber. 3. The carbon fiber comprises polyacrylonitrile 90
The electrode according to claim 2, which is obtained by firing at 0 ° C to 1500 ° C. 4. A secondary battery using the electrode according to claim 1. 5. A secondary battery using the electrode according to claim 2.