Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Separator – retainer or spacer insulating structure
Reexamination Certificate
1999-03-29
2001-10-02
Weiner, Laura (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Separator, retainer or spacer insulating structure
C429S144000, C429S249000, C429S251000
Reexamination Certificate
active
06296969
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a separator for a battery and a method for producing the same, more particularly to a separator for a battery having a function for avoiding dangers caused by overheat of a high-performance battery such as a lithium battery, etc., and having excellent film shape maintaining properties even at a high temperature.
2. Description of the Related Arts
In recent years, a compact battery of high performance has been required as a portable compact device such as a lap top computer, a mobile telephone, a VTR camera, and a liquid crystal television, is developed. In accordance with this, a lithium battery comes to public notice and is practically used since it has a high electromotive force, a high energy, a small self discharge, and the like.
The battery generally has a separator for preventing a short circuit between positive and negative electrodes. A polyolefin-based organic porous film is used as a separator in the lithium battery in practical use at present (Japanese Laid-Open Patent Publication Nos. 5-25305 and 8-138643).
The polyolefin-based organic porous film is used to restrain a battery reaction when the battery performs unusually. Namely, when a large electric current flows through the lithium battery caused by an external short circuit, a misconnection of the positive and negative electrodes, etc., the lithium battery is heated and an internal temperature of this battery is excessively increased. At this time, this organic porous film is melted and its micropores are clogged, thereby preventing migration of ions in an electrolyte. Thus, the battery reaction is stopped and further heating is restrained.
Such a function is called a shutdown function and is indispensable to the separator of a lithium battery. The lithium battery is designed such that the shutdown function is fulfilled at a temperature close to 130° C. when using polyethylene and a temperature close to 160° C. when using polypropylene as the separator.
However, this shutdown state can be continuously maintained only when micropores of the separator are clogged by a melted substance of polyolefin as a material of the separator. When a large heating state is continued, the separator is completely melted, so that the electrodes come in contact with each other and a short circuit current flows through the inside of the battery. Thus, there is a danger that the heating state is further continued and the battery is further overheated.
As mentioned above, the state in which the separator is completely melted is called meltdown.
As disclosed in Japanese Laid-Open Patent Publication No. 8-111214, a separator for a battery comprising a laminated structure of a glass porous body and a polyolefin porous body is proposed so as to prevent this meltdown.
However, in this case, when the temperature of a resin layer (polyolef in) attains to a complete melting temperature, an internal pressure of the battery is increased and force for expanding the glass porous body is applied thereto. Since the glass porous body used here is a nonwoven fabric of glass fibers, the glass fibers are simply entwined with each other. Therefore, the clearances between the fibers are increased and cracks generate, resulting in a flowing of a short circuit electric current through the battery and also overheat of the battery.
SUMMARY OF THE INVENTION
In consideration of such conventional problems, the present invention provides a separator for a battery with high safety and a method for producing the same in which the separator has a shutdown function and capability of preventing meltdown, complete melting and cracks when the battery performs unusually.
The present invention is a separator for a battery comprising a porous ceramic sheet having ceramic fibers bonded to each other at their intersection points and polyolefin and comprising micropores for impregnation of an electrolyte.
In the separator for the battery of the present invention, when the battery is heated by an external short circuit, etc., a porous ceramic sheet is clogged by melting polyolefin at a temperature close to a melting point of polyolefin, and thus an electric current is shut down.
In addition, the ceramic fibers have heat resistance and are bonded to each other at their intersection points, even when the battery temperature further rises and polyolefin attains to a complete melting state. Thus, the separator for the battery is maintained in a sheet shape as it is and does not swell as in a case using the nonwoven fabric of glass fibers shown in the conventional arts described above. Accordingly, no cracks generate in the separator for the battery. Namely, an original film shape is maintained and the shutdown function of the separator for the battery is continued even when the above meltdown occurs. Therefore, the separator has excellent safety.
Thus, in accordance with the present invention, it is possible to provide a separator for a battery with high safety and a method for producing the same in which the separator has the shutdown function and capability of preventing meltdown, complete melting and cracks when the battery performs unusually.
REFERENCES:
patent: 5102745 (1992-04-01), Tatarchuk et al.
patent: 5989750 (1999-11-01), Ohba et al.
patent: 56-59456 (1981-05-01), None
patent: 2-204373 (1990-08-01), None
patent: 5-25305 (1993-02-01), None
patent: 5-55977 (1993-08-01), None
patent: 5-55975 (1993-08-01), None
patent: 7-183021 (1995-07-01), None
patent: 8-111214 (1996-04-01), None
patent: 8-138643 (1996-05-01), None
English Abstract of JP 56-59456, May 22, 1981, Tetsuo Kawai, Hitachi Maxell, Ltd.
English Abstract of JP 5-55975, Aug. 18, 1993, Yoshiteru Miwa, et al., Nippon Sheet Glass Co., Ltd.
English Abstract of JP 5-55977, Aug. 18, 1993, Yoshiteru Miwa, et al., Nippon Sheet Glass Co., Ltd.
Iwata Misao
Matsunaga Hirokazu
Shibata Yukio
Tanimizu Yoshio
Yano Kenji
Noritake Co. Ltd.
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Weiner Laura
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