Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Separator – retainer – spacer or materials for use therewith
Reexamination Certificate
2000-12-27
2003-05-13
Weiner, Laura (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Separator, retainer, spacer or materials for use therewith
C429S129000, C429S249000
Reexamination Certificate
active
06562519
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a microporous film. More particularly, the invention relates to a microporous film having excellent heat resistance, a separator for a nonaqueous-electrolyte battery which comprises the microporous film, and a nonaqueous-electrolyte battery employing the separator.
DESCRIPTION OF THE RELATED ART
Nonaqueous-electrolyte batteries employing a light metal such as lithium as an electrode have a high energy density and are reduced in self discharge, and the range of applications thereof has hence widened considerably in the trend toward performance advancement and size reduction in electronic apparatus, etc. As the electrodes of such nonaqueous-electrolyte batteries is used a spirally wound structure which is formed by superposing a positive electrode, a negative electrode, and a separator each in a strip form and coiling them together. Because of this constitution, a wide effective electrode area is ensured. The separator basically functions to prevent short-circuit between the electrodes and to allow ions to pass therethrough due to its microporous structure and thereby enable cell (battery) reactions. However, from the standpoint of safety improvement, separators having the so-called shutdown (SD) mechanism are used in which if an abnormal current flows due to erroneous connection, etc., the resin thermally deforms and the micropores are closed with increasing temperature in the cell to thereby stop the cell reactions.
Known as separators having such an SD function are, for example, microporous polyethylene films and microporous films having a multilayer structure comprising a polyethylene layer and a polypropylene layer
However, as a result of recent progress in lithium ion secondary batteries and the like, the separators have come to be required to not only have the shutdown function but satisfy a requirement concerning heat resistance. Specifically, the separators are desired to cope with higher temperatures because there may be cases where a further increase in temperature after shutdown causes the separator itself to suffer meltdown or to be plasticated and break. In particular, since the number of factors contributing to heat generation increases with increasing capacity and reducing internal resistance in batteries, heat resistance becomes increasing important.
In view of the problem described above, JP-A-63-308866 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”) discloses a method for obtaining a microporous film having high strength and excellent high-temperature properties by superposing a film of low-melting polyethylene and a film of high-melting polypropylene. However, this separator has increased internal resistance due to the layered structure and is hence unsuitable for use as a separator for high-performance batteries such as those for use in high-output applications. In JP-A-10-298325 is disclosed a method for obtaining a microporous film comprising a high-molecular polyethylene composition containing low-molecular polyethylene and polypropylene. However, this microporous film is still insufficient in that there may be cases where heat resistance higher than that of polypropylene is required for an abrupt temperature increase, and that there is a possibility that even a separator which itself has excellent heat resistance might break when its mechanical strength is low, because it receives a high pressure upon thermal abrupt gasification of the internal electrolyte. For example, the low-boiling electrolytes in frequent use include dimethoxyethane (boiling point, 83° C.), dimethyl carbonate (boiling point, 90° C.), methyl ethyl carbonate (boiling point, 107° C.), and diethyl carbonate (boiling point, 126° C.). When such a electrolyte is used, a gas pressure is applied to the separator at 150° C., which is a temperature used for battery evaluation in a safety test.
SUMMARY OF THE INVENTION
An object of the invention is to provide, in view of the problems described above, a microporous film which is excellent in permeability and mechanical strength and has breakage resistance at high temperatures, a separator for a nonaqueous-electrolyte battery which comprises the microporous film, and a nonaqueous-electrolyte battery employing the separator.
The present inventors made intensive investigations in order to eliminate the problems described above. As a result, they have found that a microporous film having a gel content of from 20 to 90% as determined through 3-hour immersion in boiling xylene has not only heat resistance but excellent mechanical strength and is hence suitable for use as a separator for a nonaqueous-electrolyte battery. The invention has been achieved based on this finding.
The invention provides:
[1] A microporous film having a gel content of from 20 to 90% as determined through 3-hour immersion in boiling xylene;
[2] A separator for a nonaqueous-electrolyte battery which comprises the microporous film described in [1] above; and
[3] A nonaqueous-electrolyte battery employing the separator described in [2] above.
DETAILED DESCRIPTION OF THE INVENTION
The microporous film of the invention has a gel content of from 20 to 90% as determined through 3-hour immersion in boiling xylene. Because the film has a gel content within that range, it produces an effect that film breakage resistance is obtained in which the heat resistance and excellent mechanical strength attributable to the crosslinked structure are maintained even at high temperatures and high pressures. The gel content of the film is preferably from 25 to 85%, more preferably from 30 to 85%, most preferably from 35 to 80%. The lower limit of the gel content thereof is 20% from the standpoint of obtaining a separator having sufficient heat resistance. The upper limit thereof is 90% from the standpoint of attaining sufficient SD. The values of gel content herein are obtained by the method of measurement described in the Examples which will be given later.
The microporous film of the invention preferably comprises resin having a crosslinked structure. The resin desirably comprises a blend of two or more polymers from the standpoint that the microporous film made of a polymer blend can be efficiently and stably made to have properties such as excellent permeability, high mechanical strength, and excellent high-temperature breakage resistance. Examples of polymers usable for the microporous film include crosslinkable ingredients which impart a crosslinked structure to the microporous film, and further include other resin ingredients. Desirable as the crosslinkable ingredients imparting a crosslinked structure to the microporous film are unsaturated compounds. Especially preferred of these is a polymer formed by the ring-opening polymerization of either an unsaturated condensed alicyclic compound or a derivative thereof, because the backbone of this polymer has aliphatic rings and double bonds both derived from the monomer and, hence, the efficient formation of a crosslinked structure and an improvement in the heat resistance of the microporous film to be obtained can be expected. The polymer formed by ring-opening polymerization may be one in which the double bonds have been partly hydrogenated.
Examples of the unsaturated condensed alicyclic compound are roughly divided into the following three groups. The first group includes unsaturated compounds which are classified as condensed alicyclic compounds in a narrow sense and have, in one of the rings, a double bond to be incorporated into the backbone to be formed therefrom through ring-opening polymerization. Also usable as the unsaturated condensed alicyclic compounds are derivatives formed by replacing some of the hydrogen atoms of those unsaturated compounds with substituents. Examples thereof include bicyclo[3.2.0]hept-6-ene, bicyclo[4.2.0]oct-7-ene, and derivatives thereof.
The second group includes unsaturated compounds which are classified as bridge-ring compounds and have,
Emori Hideyuki
Noumi Shunsuke
Uetani Yoshihiro
Yamaguchi Mutsuko
Yamamoto Kazushige
Nitto Denko Corporation
Weiner Laura
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