Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Separator – retainer – spacer or materials for use therewith
Reexamination Certificate
1999-08-13
2001-04-17
Zitomer, Fred (Department: 1713)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Separator, retainer, spacer or materials for use therewith
C524S500000, C524S567000, C526S253000, C526S254000, C526S255000
Reexamination Certificate
active
06218051
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to separators to be used in secondary lithium batteries.
2. Description of the Prior Art
A secondary lithium battery fundamentally consists of an anode, a cathode and a separator located between these electrodes. This separator contains an electrolyte solution in pores of a separator material, which enables ions liberated from the electrode materials to migrate between the electrodes during the charge/discharge cycle.
Known examples of separator materials for secondary lithium batteries include glass fiber fabrics, microporous polyolefin membranes and non-woven fabrics, and a microporous material obtained by extracting with a solvent a mixture of a vinylidene fluoride-hexafluoropropylene copolymer (hereinafter sometimes referred to as (VdF-HFP) copolymer resin) and a plasticizer so as to eliminate the plasticizer from the mixture and then making the residue microporous as disclosed in National Publication No. 8-509100 of the Japanese Translation of International Publication WO95/06332 (PCT/US94/08772) and National Publication No. 9-500485 of the Japanese Translation of International Publication WO95/15589 (PCT/US94/12641).
Regarding secondary lithium batteries using separator materials of the latter type, such a separator comprises a (VdF-HFP) copolymer resin employed as the base material, silica employed as a filler and dibutyl phthalate employed as a plasticizer. This separator material is inserted between a fluorinated polymer-containing cathode membrane and an anode membrane to give a three-layered laminate. Next, the above plasticizer is extracted with a solvent to thereby form pores having electrolyte sustained therein. Owing to this constitution, the battery can achieve a remarkably elevated ion conductivity compared with conventional secondary lithium batteries of the polymer (solid electrolyte) type. Namely, the battery of this type can give an ion conductivity comparable to those of liquid-type secondary lithium batteries.
When the above-mentioned separator material having a (VdF-HFP) copolymer resin as a base resin is laminated onto the a electrodes, however, there arises a problem of short-circuiting. The present inventor studied factors causative of this short-circuiting and, as a result, estimated that short-circuiting would be induced as follows. In the step of the lamination, the electrode membranes and separator are softened by heating. In practice, not only the interfaces thereof but also the whole electrode membranes and separator are softened. When pressure for adhering the interfaces is applied in this state, active material particles having electrically conductive carbon in the electrode membranes adhering thereto penetrate into the softened separator membrane, thus causing short-circuiting.
SUMMARY OF THE INVENTION
The present invention aims at providing separator materials which are free from short-circuiting even in the process of fabricating batteries, in particular, in the step of the lamination onto electrode membranes.
As the result of intensive studies, the present inventors have found that when polymer particles having a higher softening point than a (VdF-HFP) copolymer resin constituting a separator are dispersed in the separator, these polymer particles serve as a reinforcement to thereby prevent the occurrence of short-circuiting caused by the penetration of active materials contained in electrodes, even when the (VdF-HFP) copolymer is softened. The present invention has been completed on the basis of this finding.
Accordingly, the present invention relates to:
(1) a separator material for secondary lithium batteries comprising as the base material, a vinylidene fluoride-hexafluoropropylene (VdF-HFP) copolymer resin, wherein polymer particles having a higher softening point than the copolymer are blended and dispersed in the separator;
(2) a separator material for secondary lithium batteries as set forth in the above item (1) wherein the polymer particles are fluorinated polymer resin particles; and
(3) a separator material for secondary lithium batteries as set forth in the above item (1) wherein the polymer particles are particles of one or more polymers selected from the group consisting of tetrafluoro- ethylene-perfluoroalkyl vinyl ether copolymer resins (PFA), tetrafluoroethylene-hexafluoropropylene copolymer resins (FEP), tetrafluoroethylene-ethylene copolymer resins (ETFE), trifluorochloroethylene-ethylene polymer resins (ECTFE), tetrafluoroethylene polymer resins (PTFE), vinylidene fluoride-hexafluoropropylene copolymer resins (VdF-HFP) and vinylidene fluoride polymer resins (PVdF).
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The (VdF-HFP) copolymer resin to be used in the present invention is a copolymer resin of vinylidene fluoride containing 8 to 25% by weight of hexafluoropropylene (HFP) as disclosed in National Publication No. 8-509100 of the Japanese Translation of WO95/06332 and National Publication No. 9-500485 of the Japanese Translation of W095/15589. The polymer particles to be dispersed in this base material should have a higher softening point than the base resin so that the polymer can remain as particles dispersed in the base resin.
These polymer particles are employed so that the base resin of the separator material is reinforced and prevented from the penetration of active material particles having electrically conductive substances adhering thereto, which exist on the electrode surface in the step of the heat lamination of the separator onto the electrodes, thus preventing short-circuiting.
As these polymer particles, use may be made of those having a higher softening point than the (VdF-HFP) copolymer resin employed as the base resin so that the polymer can remain as particles dispersed in the base resin in the process of producing the separator material and in the step of the lamination for the fabrication of battery units. It is still preferable to use polymer particles having a good adhesiveness to the base resin.
Preferable examples of the material of the polymer particles include fluorinated resins such as vinylidene fluoride polymer resins (PVdF), vinylidene fluoride-hexafluoropropylene copolymer resins (PVdF-HFP), tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer resins (PFA), tetrafluoroethylene-hexafluoropropylene copolymer resins (FEP), tetrafluoroethylene-ethylene copolymer resins (ETFE), trifluorochloroethylene-ethylene copolymer resins (ECTFE) and tetrafluoroethylene polymer resins (PTFE). However, the polymer particles to be used in the present invention are not restricted to fluorinated resins but use can be made therefor of any polymer, so long as it has a higher softening point (for example, not being softened at 120 to 130°°C.), has a good adhesiveness to the base resin and remains stable within the range of the lithium battery operation voltage (4.5 to 2.5 V vs Li/Li
+
).
The particle diameter of the polymer particles in the base resin ranges from 1 to 100 &mgr;m, preferably from 1 to 50 &mgr;m and still preferably from 5 to 10 &mgr;m, though it varies depending on the membrane thickness of the separator material. Although it is preferable that the polymer particles are superior in solvent resistance to the base resin, regulation may be made by, for example, controlling the time of contact with plasticizers (solvents) in the kneading step of the process for producing the separator material.
In the separator material according to the present invention, it is an essential point that the (VdF-HFP) copolymer base resin contains polymer particles having a higher softening point than the base resin dispersed therein and other related matters are not particularly restricted. That is to say, the separator material may further contain an inorganic filler and a plasticizing solvent (plasticizer).
Examples of the inorganic filler usable in the separator material of the present invention include silica and alumina. As the plasticizer, use may be made of ethylene carbonate, propylene carbonate, dimethoxyethane, dieth
Flynn ,Thiel, Boutell & Tanis, P.C.
NGK Spark Plug Co. Ltd.
Zitomer Fred
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