Metal working – Method of mechanical manufacture – Electrical device making
Reexamination Certificate
1999-08-23
2001-05-22
Weiner, Laura (Department: 1745)
Metal working
Method of mechanical manufacture
Electrical device making
C029S623400
Reexamination Certificate
active
06235066
ABSTRACT:
TECHNICAL FIELD
This invention relates to a process for producing a lithium ion secondary battery. More particularly, it relates to a process for producing a lithium ion secondary battery having an arbitrary shape with a reduced thickness and a reduced weight.
BACKGROUND OF THE INVENTION
In order to meet the demand for reduction in size and weight of portable electronic equipment, increase of capacity of the battery used therefor is deemed the most important subject for improving battery performance. Development and improvement of a variety of batteries have been proceeding along this line. Of the batteries developed to date, lithium ion secondary batteries are expected to achieve the highest capacity and have been and will be given intense improvement.
A lithium ion secondary battery mainly comprises a positive electrode, a negative electrode, and an ion conducting layer interposed between the electrodes. The ion conducting layer used in a lithium ion secondary battery now available for practical use is a separator made of a porous film of polypropylene, etc. filled with an electrolytic solution.
In the lithium ion secondary batteries now available for practical use, electrical connections among the positive electrode, the ion conducting layer, and the negative electrode are maintained by pressure application by using a firm battery case made of stainless steel, etc. However, such a case increases the weight of a lithium ion secondary battery, making it difficult to realize size and weight reduction. Moreover, the rigidness of the case narrows the freedom of shape design.
In order to realize size and weight reduction and freedom of shape design of a lithium ion secondary battery, it is necessary to join an ion conducting layer to a positive and a negative electrode and to maintain the joined state without applying pressure from the outside.
In this connection, U.S. Pat. No. 5,437,692 discloses a structure in which a lithium ion-conducting polymer is used as an ion conducting layer, and a positive electrode and a negative electrode are joined to the ion-conducting layer with an adhesive layer containing a lithium compound. WO95/15589 discloses a structure having a plastic ion-conducting layer to which a positive and a negative electrode are joined.
According to the method disclosed in U.S. Pat. No. 5,437,692 supra, however, the joint strength attained is not enough, the battery cannot be made sufficiently thin, and the ion conduction resistance between the positive and the negative electrodes through the ion-conducting layer is high so that the battery characteristics such as charge and discharge characteristics are insufficient for practical use. According to WO95/15589 supra, the ion-conducting layer, being plastic, cannot secure sufficient joint strength, and the thickness of the battery cannot be reduced sufficiently.
The present invention has been made in order to solve these problems. It provides a process for producing a lithium ion secondary battery in which a positive and a negative electrode are brought into intimate contact with an ion-conducting layer (a separator) with an adhesive resin to secure sufficient joint strength among the electrodes and the separator while suppressing ion conduction resistance among them on the same level as in a conventional battery put in a case.
DISCLOSURE OF THE INVENTION
A first process for producing a lithium ion secondary battery according to the present invention comprises the steps of forming a positive electrode active material -layer and a negative electrode active material layer on a positive electrode current collector or a negative electrode current collector, respectively, to prepare a positive electrode and a negative electrode, respectively, applying to a separator a binder resin solution having a fluorocarbon resin or polyvinyl alcohol as a main component dissolved in a solvent, forming a plurality of laminates in which the positive electrode and the negative electrode alternate with the separator therebetween, drying the plurality of laminates while applying pressure to evaporate the solvent to form a tabular laminated battery body, and impregnating the tabular laminated battery body with an electrolytic solution.
A second process for producing a lithium ion secondary battery of the invention is the above-described first process, wherein the plurality of laminates are formed via a cut sheet of the separator.
A third process for producing a lithium ion secondary battery of the invention is the above-described first process, wherein the plurality of laminates are formed via a rolled separator.
A fourth process for producing a lithium ion secondary battery of the invention is the above-described first process, wherein the plurality of laminates are formed via a folded separator.
According to the first to fourth processes for producing a lithium ion secondary battery, separation between a separator and each electrode composed of a positive or negative electrode active material and a positive or negative electrode current collector joined to the respective active material is prevented, and the battery structure can be maintained without a rigid case. This makes it feasible to reduce the weight and thickness of a battery. The binder resin solution applied to the separators brings about improved charge and discharge characteristics. While having a plurality of laminates, a compact lithium ion secondary battery with stable characteristics can be obtained. In case some outer force that would deform the battery or some internal thermal stress is imposed, the laminate is destroyed not at the interface between the separator and the electrode but at the interface between the active material layer and the current collector, which is effective for safety security.
A fifth process for producing a lithium ion secondary battery of the invention is the above-described first process, wherein the binder resin solution is a solution containing a fluorocarbon resin or polyvinyl alcohol in dimethylformamide.
A sixth process for producing a lithium ion secondary battery of the invention is the above-described fifth process, wherein the binder resin solution is a solution containing 3 to 25 parts by weight, preferably 5 to 15 parts by weight, of the fluorocarbon resin or polyvinyl alcohol in dimethylformamide. According to this embodiment, the time required for the step of evaporating the solvent is shortened, and a lithium ion secondary battery having excellent charge and discharge characteristics can be obtained.
A seventh process for producing a lithium ion secondary battery of the invention is the above-described first process, wherein the step of drying is carried out in a flow of air at 80° C. or lower, whereby the time required for drying can be shortened.
An eighth process for producing a lithium ion secondary battery of the invention is the above-described first process, wherein the surface of the separators is subjected to a plasm treatment before the binder resin solution is applied thereto, whereby the adhesion can be improved further.
REFERENCES:
patent: 5456000 (1995-10-01), Gozdz et al.
patent: 5571634 (1996-11-01), Gozdz et al.
patent: 62-8471 (1987-01-01), None
patent: 9-500485 (1997-01-01), None
patent: 9-293518 (1997-11-01), None
Aihara Shigeru
Hamano Kouji
Inuzuka Takayuki
Murai Michio
Shiota Hisashi
Mitsubishi Denki & Kabushiki Kaisha
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Weiner Laura
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