JPH0722019A - Battery electrode and secondary battery using the same - Google Patents

Abstract

(57) [Summary] [Structure] (1) A battery electrode made of an amorphous carbon film. (2) A secondary battery using the electrode described in 1 above. [Effect] According to the present invention, it is possible to provide a secondary battery in which the step of forming a sheet such as powder and fibers is omitted by forming a carbon film.

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 film 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-2-66856 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. If carbon fiber or a carbon fiber structure is used as the carbonaceous material instead of powder as an improvement technique, it is possible to prepare an electrode 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. Secondary batteries using such electrodes include JP-A-60-36315, JP-A-60-54181, JP-A-62-103991, and JP-A-62-
154564, JP-A-63-58763,
Japanese Unexamined Patent Publication No. 2-82466 and the like are known.

[0005]

However, in the electrode using the conventional carbonaceous material doped with ions such as lithium, the ion concentration per weight is lower than that in the case of lithium metal. There is a problem that it is still lower than that of metal. In order to increase the charge / discharge capacity, it is essential to obtain a carbonaceous material having an optimum structure. In addition, it goes without saying that in order to make a practical high-performance battery, each component is assembled into a battery and a completed product has a high capacity per unit weight or unit volume.

That is, in the present invention, it is an object of the present invention to eliminate the step of forming a sheet such as powder or fiber by forming a carbon film, and to further improve the performance.

[0007]

The present invention has the following constitution in order to achieve the above object.

[(1) A battery electrode made of an amorphous carbon film having a low degree of orientation.

(2) A secondary battery using 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. At present, as a direction of searching for high-performance carbon materials, there are a direction aiming at an amorphous carbon material in terms of material and a direction aiming at a crystalline carbon material.
Other shapes are being considered. As a result of diligent studies on the high capacity carbon material, the present inventors have found that the carbon film is excellent as the high capacity carbon material.

Hereinafter, the carbon film constituting the battery electrode according to the present invention will be described in detail with reference to specific examples. The carbon film in the present invention is not particularly limited, and a film obtained by firing an organic material is generally used. Specific examples thereof include polyacrylonitrile (PAN), cellulose, polybutadiene, and the like. In addition, pitches such as coal or petroleum, polyvinyl alcohol, lignin, polyvinyl chloride, polyamide, polyimide, phenol resin, furfuryl alcohol, etc. A carbon film obtained by firing is also acceptable. Among these carbon films, it is necessary to appropriately select a carbon film satisfying the characteristics according to the characteristics of the electrode and the battery in which the carbon film is used. Among the above carbon films, PAN, cellulose, and polybutadiene are preferable when used in the negative electrode of a secondary battery using a non-aqueous electrolyte containing an alkali metal salt. In particular, PAN is preferably used because the doping of alkali metal ions, particularly lithium ions is good.

In general, a film material is uniaxially or biaxially stretched in order to give it thickness and strength according to its use. For this reason, the carbon film, which is a fired body of such a film, naturally has orientation, but in this case having orientation indicates that the carbon layer surface is aligned substantially parallel to the film surface. . When a carbon film is used as the active electrode material, if the orientation is too high, structural anisotropy is strong and intercalation easily occurs depending on the direction, so that the capacity tends not to increase. On the other hand, if the orientation is too high, the crystallinity after firing is generally high, so that a high capacity may not be obtained. Therefore, in the present invention, it is preferable to use an amorphous carbon film having a low orientation degree, for example, an amorphous film having an orientation degree of 80% or less. Further, an amorphous carbon film having a large specific surface area is preferable as the high capacity carbon material.

As a method for producing a carbon film, Japanese Patent Laid-Open No. 60-179102 and Japanese Examined Patent Publication No. 1-49642 are available.
Known methods such as Japanese Patent Publication can be used. However, since it is important that the carbon film of the present invention has a low degree of orientation as described above, it is necessary to improve the known technique to some extent. As a technique for controlling the orientation to be low, a wet method is preferable for film formation, and the coagulation conditions are such that the processability of the subsequent stretching step is not extremely deteriorated, and the condition that the structure is as coarse as possible is selected. Is preferred. The rough structure can reduce the carbon layer surface parallel to the film surface.

The carbon film of the present invention is preferably used in the form of a roll or a laminate in order to improve the performance of the battery.

The thickness of the carbon film used in the present invention is preferably made as thin as possible in order to increase the specific surface area, and is determined by the restrictions on processability during manufacturing and handleability during battery assembly. However, a carbon film having a thickness of 10 μm or less is preferably used, and 1 to 7 μm is more preferable. Further, since it is amorphous, the carbonization temperature is suitably in the range of 900 to 1500 ° C.

The electrode composed of the carbon film of the present invention 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. is not. Among these, it is preferably used as a negative electrode of a secondary battery. As a particularly preferable secondary battery, a secondary battery using a non-aqueous electrolytic solution containing an alkali metal salt such as lithium perchlorate, lithium borofluoride, and phosphorus hexafluoride / lithium can be mentioned.

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 in the carbon film is utilized, and the cation-doped carbon film is used. On the negative electrode,
A carbon film doped with anions will be used for the positive electrode. These are those which should be able to be used for the positive electrode or the negative electrode depending on various characteristics of the carbon film, but it is not always necessary to use both electrodes as the electrodes of the present invention, and an electrode composed of the carbon film of the present invention for the negative electrode, It is also a preferred embodiment to use an electrode not containing a carbon film as a positive electrode.

When an electrode containing no carbon film is used for the positive electrode, in addition to carbonaceous materials other than the film, artificial or natural graphite powder, carbon fiber, fluorinated carbon, inorganic compounds such as metal or metal oxides. An organic polymer compound or the like 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 film includes inorganic compounds such as transition metal oxides and transition metal chalcogens containing alkali metals, conjugated polymers such as polyacetylene, polyparaphenylene, polyphenylenevinylene, polyaniline, polypyrrole, and polythiophene. Examples of the positive electrode used in ordinary secondary batteries include 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, transition metal oxides and transition metal chalcogens such as cobalt, manganese, molybdenum, vanadium, chromium, iron, copper and titanium are It is 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 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.

[0022]

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.
A polyacrylonitrile film having a thickness of 8 μm and having a thickness of 6 μm is heated to 200 ° C. under tension while suppressing free shrinkage
To flameproof at 250 ° C. The area retention of the flameproofed film with respect to the raw material film was 61%. Then, a carbon film was produced by firing at 1100 ° C. in nitrogen without tension.

Next, an electrode was prepared using the above carbon film, 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 weight of carbon fiber is 40V / g at a constant current of 0V (v
s.Li ⁺ / Li). The discharge capacity of the carbon film electrode, which was obtained from the amount of charge discharged after charging, was measured. The results are shown in Table 1.

Example 2 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 in a ball mill, and then 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. The weight ratio of 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 film electrode prepared in Example 1 was used as a negative electrode, and the positive electrode prepared as described above was superposed through a separator made of a porous polypropylene film (Celguard # 2500, manufactured by Daicel Chemical Industries) to form an AA size. A secondary battery of a size was produced. 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 weight of carbon film was 40 mA / g, and the battery was charged to 4.3 V at a constant current. As the discharge capacity of the secondary battery obtained from the amount of charge discharged after charging, the discharge capacity per weight of the carbon film used in this battery and per unit weight of the battery was measured. The results are shown in Table 1. Comparative Example 1 Novolak resin was obtained by carbonizing at 1100 ° C. and crushing.
90 parts by weight of carbon powder having a size of 00 mesh or less and 10 parts by weight of polyvinylidene fluoride as a binder were made into a paste using dimethylamide, applied on an aluminum foil and pressure-bonded, and after drying, an electrode was evaluated in the same manner as in Example 1. . The results are shown in Table 1.
Shown in.

Comparative Example 2 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 in a ball mill, and then 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. The weight ratio of 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 powder molded sheet electrode prepared in Comparative Example 1 was used as a negative electrode, and the positive electrode prepared as described above was superposed with a porous polypropylene film (Celguard # 2500, manufactured by Daicel Chemical Industries) separator interposed therebetween. AA size secondary battery was prepared. As the electrolytic solution, propylene carbonate containing 1M lithium perchlorate was used.

The charging evaluation of the secondary battery manufactured as described above was performed. Current density per weight of carbon powder molded sheet is 40mA
It was charged to 4.3 V with a constant current of / g. Regarding the discharge capacity of the secondary battery obtained from the amount of charge discharged after charging, the discharge capacity per weight of the carbon powder molded sheet used in this battery and per unit weight of the battery was measured. The results are shown in Table 1.

[0031]

[Table 1]

[0032]

According to the present invention, a secondary battery can be provided by forming a carbon film and omitting the step of forming a sheet such as powder and fibers.

Claims (4)

[Claims] 1. A battery electrode comprising an amorphous carbon film. 2. The battery electrode according to claim 1, wherein the carbon film is obtained by firing polyacrylonitrile at 900 ° C. to 1500 ° C. 3. A secondary battery using the electrode according to claim 1. 4. The secondary battery according to claim 3, wherein the carbon film is obtained by firing polyacrylonitrile at 900 ° C. to 1500 ° C.