Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Separator – retainer – spacer or materials for use therewith
Reexamination Certificate
1999-12-14
2002-10-08
Chaney, Carol (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Separator, retainer, spacer or materials for use therewith
C429S245000, C429S246000, C429S249000, C429S253000, C429S254000
Reexamination Certificate
active
06461772
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a battery diaphragm suitable as a diaphragm (separator) of a secondary battery such as an alkaline battery and a redox flow battery. More specifically, the present invention relates to a battery diaphragm having superior chemical resistance, mechanical strength and ion selective permeability as well as low resistance, formed of a composite film including a complex of a porous base substrate with a crosslinked polymer functioning as ion exchange resin.
2. Description of the Background Art
In a battery, a diaphragm is used to prevent short-circuit between positive and negative electrodes. Generally, a battery diaphragm is required to have electrical insulation and resistance to electrolyte (chemical resistance). Additionally, it is required that the diaphragm is poromeric allowing good permeation of the electrolyte, and that the electrolyte contained in the diaphragm has low electrical resistance. Further, the battery diaphragm is also required to have heat resistance and mechanical strength high enough to withstand physical variation of active material associated with charging/discharging operations.
Recently, an ion exchange membrane has come to be used as a diaphragm for separating positive electrolyte and negative electrolyte from each other in a secondary battery such as an alkaline battery or a redox flow battery. Conventional ion exchange membrane applied to the battery diaphragm includes, for example, an ion exchange membrane having an anion group or a cation group introduced to divinylbenzene-styrene copolymer, an ion exchange membrane of divinylbenzene-vinylpyridine copolymer adapted to have quarternary pyridinium group, and an anion-exchange membrane having chloromethylated aromatic polysulfone polymer crosslinked by polyamine (Japanese Patent Laying-Open No. 2-68146).
Though such an ion exchange membrane has superior chemical resistance, heat resistance and ion selective permeability, the membrane does not have sufficient mechanical strength by itself. Insufficient mechanical strength of the ion exchange membrane hinders reduction in thickness of the membrane, and hence, it is difficult to lower film resistance. When the membrane is reinforced by other material, it also becomes difficult to lower film resistance.
Japanese Patent Laying-Open No. 6-271688 proposes an anion exchange membrane having a multilayered structure of polyolefin or polyfluorolefin porous film subjected to hydrophilization and an anion exchange membrane of chloromethylated aromatic polysulfonate type polymer aminated by monoamine and crosslinked by polyamine. It is described that the multilayered anion exchange membrane has superior corrosion resistance, high ion permeability and mechanical strength as a thin film of ion exchange membrane is provided on a porous support body having high mechanical strength.
The multilayered anion exchange membrane, however, unavoidably suffers from the problem of peeling of the layers at the interface of adhesion, as it has two-layered structure having the ion-exchange membrane and the porous film adhered with each other. A diaphragm for a secondary battery such as a redox flow battery or an alkaline battery employing an alkali electrolyte or a liquid active material must have high chemical resistance and mechanical strength. When the ion exchange membrane having the multilayered structure is used as the diaphragm of a secondary battery, the chemical resistance and the mechanical strength at the adhering portion are not sufficient, and therefore the membrane is prone to layer peeling.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a battery diaphragm having superior chemical resistance, low resistance, superior ion selective permeability and high mechanical strength.
As a result of intensive study to overcome the problems of the prior art described above, the inventors of the present invention have attained a battery diaphragm formed of a composite film including a complex of a crosslinked polymer containing repeated units of vinyl heterocyclic compound having two or more hydrophilic groups. Preferably, the battery diaphragm in accordance with the present invention can be obtained by impregnating a porous structure of a porous base substrate with a monomer mixture containing crosslinking monomer and vinyl heterocyclic compound with two or more hydrophilic groups and causing polymerization to generate crosslinked polymer in the porous structure.
As a monomer mixture containing vinyl heterocyclic compound having two or more hydrophilic groups such as 1-vinyl imidazole, a crosslinking monomer such as divinyl benzene and a monomer which allows co-polymerization as needed is used, it becomes possible to increase degree of crosslinking without reducing hydrophilic groups, whereby highly balanced ion permeability and low resistance can be attained. Further, since the crosslinked polymer as the ion exchange resin is firmly bonded within the porous structure of the porous base substrate, the problem of layer peeling or the like can be prevented. When polytetrafluoroethylene porous film is used as the porous base substrate, a battery diaphragm having yet higher heat resistance, chemical resistance, ion selectivity, low resistance and high mechanical strength can be obtained.
As a result of intensive study to overcome the problems of the prior art described above, the inventors of the present invention have attained a battery diaphragm formed of a composite film including a complex of a porous base substrate with a crosslinked polymer containing repeated units of vinyl compound each having a functional group and thereafter decomposing part of the functional groups. Preferably, the battery diaphragm in accordance with the present invention can be obtained by impregnating a porous structure of a porous base substrate with a monomer mixture containing a vinyl compound having functional groups and crosslinking monomer, causing polymerization to generate crosslinked polymer in the porous structure, and thereafter decomposing part of the functional groups.
As a vinyl compound having an amon-exchange group such as 4-vinylpyridine, a vinyl compound having a hydrolyzable functional group such as vinyl acetate, vinyl propionate or vinyl-n-butyrate, and a crosslinking monomer such as divinyl benzene are used as the monomer mixture, first, degree of crosslinking can be increased by co-polymerization and, secondary, by decomposition of the functional group such as the acetate group, propionate group and n-butyrate group, hydrophilicity can be improved and spaces are formed in the crosslinked polymer to increase the amount of water molecules existing therein. Accordingly, highly balanced ion permeability and low resistance can be attained. Further, since crosslinked polymer as the ion exchange resin is firmly bonded within the porous structure of the porous base substrate, the problem of layer peeling or the like can be prevented.
Further, the inventors of the present invention have attained a composite film obtained by impregnating a porous base substrate with a monomer mixture containing a vinyl compound having a functional group and a crosslinking monomer and thereafter causing photopolymelization.
The present invention has been completed based on these findings.
The present invention provides a battery diaphragm formed of a composite film including a complex of a porous base substrate (A) with a crosslinked polymer (B
1
) containing repeated units of vinyl heterocyclic compound having two or more hydrophilic groups.
Further, the present invention provides a battery diaphragm including a complex of a porous base substrate (A) with a crosslinked polymer (B
2
) containing repeated units of vinyl compound each having a functional group, and thereafter decomposing part of the functional groups.
Further, the present invention provides a battery diaphragm formed of a composite film including a complex of a porous base substrate (A) with a crosslinked polymer (B
3
) containing r
Miyake Shinichi
Tokuda Nobuyuki
Chaney Carol
McDermott & Will & Emery
Sumitomo Electric Industries Ltd.
Yuan Dah Wei
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