JPH1125954A - Secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To increase a charging/discharging capacity, to facilitate fabrication and to prevent the occurrence of short-circuiting by including a carbon fiber as a carbon material and not including a carbon fiber having a specified length or higher substantially. SOLUTION: A carbon fiber is used in a short fiber form, no inclusion of a carbon fiber of 30 μm or more in a carbon body is necessary, and no inclusion of a carbon fiber of 20 μm or more in the carbon body is preferable. This is because short-circuiting caused by a long fiber is prevented by a short fiber. As a method for not including a carbon fiber of 30 μm or more, for example, a method using a classifier is available. The diameter of the carbon fiber is set to facilitate forming each shape, but 1 to 20 μm is preferred. An electrode using a carbon material as an electrode material is used for either one of positive and negative electrodes or both, but since a charging/discharging potential is low, the electrode is preferably used as the negative electrode.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a secondary battery using carbon fibers having a reduced fiber length as an electrode material.

[0002]

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

However, in a secondary battery using lithium metal for the negative electrode, there is a danger that lithium may grow dendritic by repeated charge and discharge, causing a short circuit and causing ignition. In addition, since highly active metallic lithium is used, the risk is inherently high, 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 solution to such a problem of safety and attaining high energy peculiar to a lithium electrode. This method utilizes the fact that during charging, the carbonaceous material is doped with lithium ions and has the same potential as metal lithium, so that it can be used as a negative electrode instead of metal lithium. At the time of discharging, the doped lithium ions are undoped from the negative electrode, and return to the original carbonaceous material. When such a carbonaceous material doped with lithium ions is used as a negative electrode, there is no problem of dendrite generation, and since there is no metallic lithium, there is a feature that it is excellent in safety, Currently, research and development are being actively conducted.

A secondary battery using an electrode utilizing the above-described doping of a carbonaceous material with lithium ions is disclosed in JP-A-57-20807 and JP-A-58-9317.
No. 6, JP-A-58-192266, JP-A-58-192266
JP-A-2-90863, JP-A-62-212066, JP-A-2-66656 and the like are known. Such a carbonaceous material is generally in the form of a powder, and is used for an electrode as a sheet-like molded body to which a polymer binder such as Teflon or vinylidene fluoride is added for electrode molding. Secondary batteries using carbon fibers or carbon fiber structures as electrodes as carbonaceous materials are disclosed in JP-A-60-36315, JP-A-60-54181, and JP-A-62-103991. JP-A-62-2
154564, JP-A-63-58763,
JP-A-2-82466 is known.

[0005]

However, an electrode using carbon fibers doped with ions such as lithium has a problem in electrode processing when long fibers are used, and when short fibers are used. However, the conventional battery described above has a problem that a micro short circuit easily occurs in the battery.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a secondary battery which solves the above-mentioned disadvantages, has a high charge / discharge capacity, is easy to mold, and does not easily cause a short circuit.

[0007]

SUMMARY OF THE INVENTION The present invention has the following arrangement to solve the above-mentioned problems.

[0008] In a secondary battery using a carbonaceous material as an electrode material in at least one of the electrodes, it is required that the carbonaceous material contains carbon fibers and is substantially free of carbon fibers having a length of 30 µm or more. A rechargeable battery that features it. "

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION As a result of intensive studies on high-capacity carbon materials, the present inventors have found that carbon fibers are excellent as high-capacity carbon materials. With respect to the problem of a minute short circuit between electrodes, it has been found that it is important to reduce carbon fibers of a certain length or more contained in a carbonaceous material.

In the present invention, it is necessary that a carbonaceous material is used in at least one of the electrodes and that the carbonaceous material contains carbon fibers.

The carbon fiber in the present invention is not particularly limited, and is generally obtained by firing an organic substance,
Specifically, PAN-based carbon fiber obtained from polyacrylonitrile (hereinafter abbreviated as PAN), pitch-based carbon fiber obtained from pitch such as coal or petroleum, cellulosic carbon fiber obtained from cellulose, and gas of low molecular weight organic matter The obtained vapor-grown carbon fiber and the like can be mentioned. In addition, a carbon fiber obtained by firing polyvinyl alcohol, lignin, polyvinyl chloride, polyamide, polyimide, phenol resin, furfuryl alcohol and the like may be used. Among these carbon fibers, it is preferable that a carbon fiber satisfying the characteristics is appropriately selected according to the characteristics of the electrode and the battery in which the carbon fibers are used.

In the present invention, these carbon fibers are used after being shortened. As a method of cutting or pulverizing the carbon fiber into short fibers, various pulverizers and the like can be used. In the present invention, it is necessary that carbon fibers of 30 μm or more are not substantially contained in the carbon body,
More preferably, carbon fibers having a size of 20 μm or more are not substantially contained in the carbon body. The term "substantially not included" means that there is no short circuit between the electrodes due to the long fiber. However, from a morphological observation, at least three positions on the electrode by a scanning electron microscope are longer than a predetermined length. Is 1/5 of the number of carbon bodies seen in the field of view.
0 or less.

In the present invention, a method for substantially preventing the carbon fibers having a size of 30 μm or more from being contained is not particularly limited, but, for example, a method using a classifier or the like is mentioned as a preferable means.

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

In the present invention, in terms of improvement in bulk density,
It is preferable to use carbon powder as an active material together with carbon fibers. As the carbon powder, graphite powder is preferably used. It is also preferable to add acetylene black, carbon black, or the like as a conductive material.

In the present invention, it is also preferable to use a binder. Examples of the binder include various epoxy resins, polyimides, and polyamides. In particular, fluoride polymers such as polyvinylidene fluoride and polytetrafluoroethylene are preferable from the viewpoint of stability. The content of the binder is preferably less than 20% by weight based on the carbonaceous material from the viewpoint of battery capacity.

In the present invention, an electrode using the carbonaceous material as an electrode material is provided on one of a positive electrode and a negative electrode,
Although it is possible to use it for both electrodes, it is preferable to use it as a negative electrode because the charge and discharge potential is low.

Examples of the positive electrode active material include 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, and disulfides. Examples of the positive electrode used in ordinary secondary batteries, such as a crosslinked polymer having a bond, can be given. Among these, in the case of a secondary battery using a non-aqueous electrolyte containing a lithium salt, cobalt, manganese, nickel, molybdenum, vanadium,
Transition metal oxides such as chromium, iron, copper, and titanium, and transition metal chalcogens are preferably used. More specifically,
For example, LiCoO ₂ , LiNiO ₂ , LiMn ₂ O ₄ , or a mixture thereof, to which a different atom such as boron, aluminum, strontium, or silicon is added, is used.

As the electrolyte for the secondary battery of the present invention, a conventional electrolyte is used without any particular limitation, and examples thereof include an aqueous acid or alkali solution and a non-aqueous solvent. Among them, propylene carbonate, ethylene carbonate, γ-butyrolactone, N-methylpyrrolidone, acetonitrile, N, N-
Dimethylformamide, dimethylsulfoxide, tetrahydrofuran, 1,3-dioxolan, methyl formate,
Sulfolane, oxazolidone, thionyl chloride, 1,2-
Dimethoxyethane, diethylene carbonate, their derivatives and mixtures are preferably used. Examples of the electrolyte contained in the electrolytic solution include alkali metal, especially lithium halide, perchlorate, thiocyanate, borofluoride, phosphorus fluoride, arsenic fluoride, aluminum fluoride, trifluoromethyl sulfate, and the like. Is preferably used.

The method of manufacturing a secondary battery according to the present invention includes forming a sheet from a slurry containing a carbonaceous material containing carbon fibers, a binder, carbon powder, and the like, if necessary, and a solvent. I do. As the current collector, copper foil, stainless steel foil and the like are used without any particular limitation, and a carbonaceous material and a slurry formed of a binder or the like are provided on both or one side of the current collector to form a sheet. Can be molded.

The secondary battery of the present invention can be used as various secondary batteries. Particularly preferred secondary batteries include alkali metal salts such as lithium perchlorate, lithium borofluoride, and lithium lithium hexafluoride. And a secondary battery using a non-aqueous electrolytic solution containing:

The secondary battery of the present invention is widely used in portable small electronic devices such as a video camera, a personal computer, a word processor, a radio-cassette, a cellular phone, etc. by utilizing the features of light weight, high capacity, and high energy density. It is possible.

[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 PAN-based carbon fiber (manufactured by Toray Industries, Inc.) was roughly pulverized by a hammer mill and then pulverized by a roll mill to obtain milled fiber. The average length of the carbon fibers at this time was about 30 μm. Thereafter, the classification of the long fibers was deleted using a classification device. To this was added 10 parts by weight of polyvinylidene fluoride as a binder, and a slurry was prepared using 1-methyl-2-pyrrolidone as a solvent. The slurry was applied to a copper foil, dried and pressed to produce a negative electrode. When the carbon fibers were observed at three places on the negative electrode by a scanning electron microscope, no carbon fibers having a fiber length of 30 μm or more were observed in any case.

As the positive electrode material, commercially available lithium carbonate (Li ₂
CO ₃ ) and basic cobalt carbonate (2CoCO ₃・ 3Co (OH) ₂ ) were weighed so that the molar ratio Li / Co = 1/1, mixed in a ball mill, and heat-treated at 900 ° C. for 20 hours. LiCoO ₂ was obtained. 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 ₂ / artificial graphite / PVdF at a weight ratio
= 80/15/5 to prepare a positive electrode slurry,
This slurry was applied on an aluminum foil, dried and pressed to obtain a positive electrode.

The positive electrode prepared above was overlapped with a porous polypropylene film (Celgard # 2500, manufactured by Daicel Chemical Co., Ltd.) via a separator to produce a secondary battery. As the electrolyte, propylene carbonate containing 1 M LiBF ₄ was used.

[0027] Twenty of the secondary batteries prepared as described above were prepared and their performance was evaluated. The current density per carbonaceous material weight is
The battery was charged to 4.2 V at a constant current of 40 mA / g. The discharge capacity of the secondary battery determined from the amount of charge discharged after charging is
340 mAh per weight of carbonaceous material used in this battery
/ g. The short-circuit occurrence rate of this battery was 0 out of 20 batteries.

Comparative Example 1 Except that the classification and removal of the long fibers of the carbon fibers were not performed,
A battery was manufactured in the same manner as in Example 1. When the carbon fibers of the negative electrode produced here were observed at three places by a scanning electron microscope, carbon fibers having a fiber length of 30 μm or more were found at a ratio of 3/50 in all cases.
The short-circuit occurrence rate of this battery was 4 out of 20 batteries.

Example 2 Short fibrous carbon fibers produced in the same manner as in Example 1
A battery was manufactured in the same manner as in Example 1, except that a mixture of graphite powders at a weight ratio of 1: 3 was used as the negative electrode active material. When the carbon fibers were observed by a scanning electron microscope at three places of the negative electrode, 30
No carbon fiber having a fiber length of at least μm was found.

At this time, the battery capacity was 360 mAh / g per weight of the carbonaceous material used, and the short circuit occurrence rate of the batteries was 0 out of 20 batteries.

Example 3 Short fibrous carbon fibers produced in the same manner as in Example 1
A battery was fabricated in the same manner as in Example 1, except that a mixture of polyvinylidene fluoride as a binder and acetylene black as a conductive material in a ratio of 80: 15: 5 was used as a negative electrode active material. . When the carbon fibers were observed at three places on the negative electrode by a scanning electron microscope, no carbon fibers having a fiber length of 30 μm or more were observed in any case.

At this time, the battery capacity was 330 mAh / g per weight of the carbonaceous material used, and the short-circuit occurrence rate of the batteries was 0 out of 20 batteries.

Example 4 The milled fiber of the PAN-based carbon fiber before classification prepared in Example 1 was further pulverized by a roll mill, and then classified by using a classification device under the conditions except for short fibers having a length of 20 μm or more. . 15 parts by weight of polyvinylidene fluoride as a binder and 1 part by weight of acetylene black as a conductive material were added to 84 parts by weight of a mixed carbon body obtained by mixing the obtained short fibers and graphite powder at a weight ratio of 1: 3. A slurry was prepared using 1-methyl-2-pyrrolidone as a solvent. The slurry was applied to a copper foil, dried and pressed to produce a negative electrode. As a result of observation by a scanning electron microscope at three places of the negative electrode, no carbon fiber having a length of 20 μm or more was found in any case.

Using the above-mentioned negative electrode, 20 batteries were produced and charged and discharged in the same manner as in Example 1. The discharge capacity of the obtained secondary battery was 360 mAh / g per weight of the mixed carbon material, and the short-circuit occurrence rate of the batteries was 0 out of 20 batteries.

[0035]

According to the present invention, it is possible to manufacture a battery having a short circuit in the battery and to provide a battery having a high production yield.

Claims (7)

[Claims] 1. A secondary battery using a carbonaceous material as an electrode material in at least one electrode, wherein the carbonaceous material contains carbon fibers and does not substantially contain carbon fibers having a length of 30 μm or more. A secondary battery characterized by the above-mentioned. 2. The secondary battery according to claim 1, further comprising a carbon powder as an electrode material together with said carbon fiber. 3. The battery according to claim 2, wherein said carbon powder is made of graphite powder. 4. The secondary battery according to claim 1, further comprising a binder in an amount of less than 20% by weight based on the carbonaceous material. 5. The secondary battery according to claim 1, wherein at least one electrode is an electrode having an electrode material containing a carbon body, a carbon powder and a binder on both surfaces or one surface of a metal foil. 6. The secondary battery according to claim 1, wherein an electrode using a carbonaceous material as an electrode material is used as a negative electrode, and a transition metal is used as a positive electrode material. 7. The secondary battery according to claim 6, wherein the positive electrode material is selected from LiCoO ₂ , LiNiO ₂ and Li Mn ₂ O ₄ .