Coating processes – Electrical product produced – Carbon coating
Reexamination Certificate
1999-11-24
2002-02-05
Maples, John S. (Department: 1745)
Coating processes
Electrical product produced
Carbon coating
C427S077000, C427S189000, C427S201000
Reexamination Certificate
active
06344235
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a method for producing an electrode for a non-aqueous electrolytic cell, and a method for producing an electrode for a non-aqueous electrolytic cell, in which a collector is coated with an electrode active material layer comprising an active material, flake graphite and a binder.
BACKGROUND ART
Of non-aqueous electrolytic cells high in discharge potential and discharge capacity, lithium ion secondary cells occluding and releasing lithium have recently come in practice. Electrodes of these cells are produced by preparing paints for active material layers using active materials and binders, applying the paints onto collectors, and drying them.
The active materials used in the electrodes are poor in electric conductivity, except for some materials, so that conductive materials are used.
In the electrodes, the role of the conductive materials is important. If the conductive materials do not act effectively, the problem arises that the capacity of the cells is decreased, or that the cycle life is reduced.
In the lithium ion secondary cells, for example, when carbon is used as the active materials in negative electrodes, the introduction of lithium into the active materials expands them, and the release of lithium therefrom contracts them. In the lithium ion secondary cells, charge and discharge are repeated, which causes the active materials to repeat expansion and contraction. When the active materials are contracted, the contact of the active materials with the conductive materials becomes poor. The cells are therefore gradually deteriorated.
The non-aqueous electrolytic secondary cells include cells using carbon black such as acetylene black as the conductive materials, for example, “a non-aqueous electrolytic secondary cell having a negative electrode in which an alkali metal is used as an active material, non-aqueous electrolyte and a positive electrode, wherein a conductive material of said positive electrode comprises a large amount of metallic Ti powder and a small amount of carbon black” (Japanese Patent Unexamined Publication No. 62-15761), and cells using LiMn
2
O
4
as the active materials and graphite as the conductive materials, for example, “a non-aqueous electrolytic secondary cell having a positive electrode mainly comprising LiMn
2
O
4
and graphite, a negative electrode and non-aqueous electrolyte, wherein the ratio of graphite to the total amount of said LiMn
2
O
4
and graphite is 8% to 22% by weight” (Japanese Patent Unexamined Publication No. 1-105459).
Further, methods for producing related electrodes for cells include, for example, “a method for producing a cell electrode comprising an insoluble, infusible substrate, a heat-treated product of a phenol resin, having a polyacene skeleton structure with a hydrogen/carbon atom number ratio of 0.5 to 0.05, and having a specific surface area value according to a BET method of at least 600 m
2
/g, wherein a powder of said insoluble, infusible substrate are mixed with a conductive material and a binder, and the resulting mixture is applied to press molding or coating or adhered under pressure onto a support” (Japanese Patent Unexamined Publication No. 63-301460).
When carbon black such as acetylene black is used as the conductive materials, as described in Japanese Patent Unexamined Publication No. 62-15761 mentioned above, the problems are encountered that the adhesion of a coating film to a collector is poor, resulting in easy separation, and that a coating film becomes too hard to obtain good flexibility, resulting in easy cracking of the electrode, because of large surface area of acetylene black.
When the graphite is used as the conductive material, for example, as described in Japanese Patent Unexamined Publication No. 1-105459 mentioned above, the flexibility of a coating film is good, but the graphite having a small central particle size reduces the cycle life. On the other hand, the graphite having a large size is difficult to exhibit an effect as the conductive material, if it is not added in large amounts.
In the invention of Japanese Patent Unexamined Publication No. 63-301460, the particle size of the conductive material is decreased. However, the use of the graphite having a small particle size results in reduced cycle life.
In view of the problems as described above, the present invention provides an electrode for a non-aqueous electrolytic cell having good charge and discharge characteristics such as discharge capacity and charge and discharge cycle life, and improved in physical characteristics.
DISCLOSURE OF INVENTION
In order to solve the above-mentioned problems, the present inventors have conducted intensive investigation. As a result, the present inventors have discovered that the above-mentioned problems can be solved to achieve an object of the present invention by producing an electrode for a non-aqueous electrolytic cell using an active material and flake graphite wherein a pulverization process of said graphite is carried out twice or more, followed by mixing and pulverization of said graphite using said active material, thus completing the present invention.
That is to say, the present invention provides (1) a method for producing an electrode for a non-aqueous electrolytic cell, in which a collector is coated with an electrode active material layer comprising an active material, flake graphite and a binder, which is characterized by that said graphite is subjected to a pulverization process twice or more, and thereafter, mixed and pulverized using said active material; (2) the method described in the above (1), wherein the central particle size of said graphite is larger than that of said active material; (3) the method described in the above (1) or (2), wherein the amount of the conductive material contained in a coating film is from 0.1% to 15% by weight; (4) the method described in the above (1) or (2), wherein the active material is a negative electrode active material; and (5) the method described in the above (1) or (2), wherein the active material is a positive electrode active material.
The present invention will be described below in detail.
An object of the present invention is to provide a method for producing an electrode for a non-aqueous electrolytic cell, the electrode being improved in charge and discharge characteristics such as discharge capacity and charge and discharge cycle life, and in physical characteristics of a coating film.
In the electrodes for non-aqueous electrolytic cells, the active materials are poor in electric conductivity, except for some materials, so that conductive materials are used. In general, when the conductive materials are contained in larger amounts, the activity of the active materials is brought out more easily. However, the addition of the conductive materials in large amounts decreases the amounts of the active materials in the volumes of the electrodes, resulting in the decreased capacity of the cells. Efforts have therefore been made to decrease the amounts of the conductive materials while bringing out the activities of the active materials.
Further, the electrodes for non-aqueous electrolytic secondary cells are decreased in capacity to allow deterioration to occur each time they are used. As one of the causes of deterioration of the electrodes for non-aqueous electrolytic secondary cells, it is considered that the contact of the active materials with the conductive materials in the electrodes becomes poor to cause the failure to bring out electricity outside, resulting in deterioration as the cells. Usually, carbon black such as acetylene black or graphite is used as the conductive materials. Acetylene black has a large specific surface area. However, when the active materials are carbon, it has been conceivable that the contact of acetylene black with carbon is not so good.
In addition, the coating films using acetylene black is poor in adhesion thereof to the collectors, so that the problem is liable to occur that the coating films are easily separated, or that the coating films o
Iijima Tadayoshi
Kurose Shigeo
Takahashi Tetsuya
Maples John S.
TDK Corporation
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