Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Separator – retainer or spacer insulating structure
Reexamination Certificate
2000-12-11
2003-03-04
Ryan, Patrick (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Separator, retainer or spacer insulating structure
C429S252000, C429S254000
Reexamination Certificate
active
06528205
ABSTRACT:
TECHNICAL FIELD
This invention relates to a separator which prevents a sealed cell from suffering short-circuiting between electrode plates and holds an electrolyte in the cell.
BACKGROUND ART
A sealed cell (hereinafter simply referred to as “cell” ) has electrode plates inside and a sheet-form separator interposed therebetween for preventing short-circuiting. This separator usually comprises a non-woven fabric, which is produced by a method comprising subjecting a slurry solution of thin short fibers (about 50% thereof have a diameter of 4 &mgr;m or smaller; length, 25 mm or shorter) to sheet formation by a papermaking process. Size and thickness of the separator are suitably regulated according to the intended use thereof.
A cell functions when charge transfer occurs between the electrode plates and the electrolyte, and this electrolyte is present in the state of being held by the separator. The electrolyte generally is an aqueous solution, and there are cases where an additive is added to the aqueous solution to make it have a higher viscosity or be in a gel state so as to be more readily held by the separator.
Cell performances improve with decreasing internal resistance and as the electrolyte becomes difficult to form a layer (this layer formation by an electrolyte is hereinafter referred to simply as “layer formation”). The internal resistance of a cell (hereinafter referred to simply as “internal resistance”) is considerably influenced by charge mobility in the electrolyte and chars transferability between the electrode plates and the electrolyte.
Although the separator itself is a factor which elevates internal resistance because it inhibits the transport of ions, it is indispensable to prevent short-circuiting between electrode plates and layer formation. A separator comprising a non-woven fabric has innumerable pores therein; the more and larger the pores, the higher the ion transportability and the lower the internal resistance. However, a separator in which the degree thereof is too high shows impaired electrolyte retention and is apt to cause layer formation.
Charge transfer between the electrode plates and the electrolyte occurs by oxidation-reduction reactions between the electrode plates and ions. It is important for the separator to be in intimate contact with the electrode plates because charge transfer between the electrode plates and the electrolyte occurs only when the ions come into contact with the electrode plates. Namely, the degree of intimate contact between the separator and the electrode plates is an important factor in internal resistance. In order to increase the degree of intimate contact, a separator is compressed and inserted between electrode plates. As a result of the compression, the separator comes into intimate contact with the electrode plates due to its repulsive force and restoring force.
It is, however, known that upon injection of an electrolyte, the separator decreases its repulsive force and restoring force. It is generally thought that this decrease in separator restoring force is attributable to the surface tension of the electrolyte which has come into the spaces among the constituent fibers of the separator. Namely, the injection of an electrolyte reduces the degree of intimate contact between the separator and the electrode plates.
Furthermore, since the electrode plates expand/contract upon every charge/discharge, the separator repeatedly receives an external compressive/relaxation force. Because of this, separators having low restoring force suffer the so-called “fatigue phenomenon” in an early stage, resulting in an abrupt decrease in the degree of intimate contact.
Improvement in separators has enthusiastically been made to increase the degree of intimate contact between a separator and electrode plates. Examples thereof include an invention in which inorganic particles which gel upon contact with an electrolyte are caused to be present in a separator beforehand (Unexamined Published Japanese Patent Application No. 4-32158) and an invention in which fine polyethylene particles are mixed into a separator to impart elasticity to the separator (Unexamined Published Japanese Patent Application No. 5-67463).
However, the conventional inventions have had the following problems.
In the invention in which inorganic particles are caused to be present in a separator beforehand, the inorganic particles in the separator did not completely gel upon impregnation with an electrolyte. Consequently, since the restoring force of this separator does not considerably differ from that in the case where no inorganic particles are contained, the degree of intimate contact between electrodes and the separator was not sufficiently improved.
Furthermore, when the amount of the inorganic particles present in the separator is large, inorganic particles and the inorganic particles which have gelled occupy the pores of inner parts of the separator and this makes the transport of ions difficult. There have been further problems, for example, that since the separator itself has a reduced compressibility, operation for inserting the separator between electrode plates becomes difficult. Before being inserted between electrode plates, a separator is compressed by mechanically applying a high pressure thereto until the thickness thereof decreases to about a half. If this compression becomes difficult, an apparatus for this use only becomes separately necessary.
On the other hand, the invention in which fine polyethylene particles are caused to be present in a separator aims at weakening the adhesion between fibers to thereby make the separator elastic. Namely, this technique is intended to enable the degree of intimate contact between the separator and electrode plates to be uniform throughout. Consequently, even when fine polyethylene particles are caused to be present, the degree of intimate contact between the separator and electrode plates cannot be improved.
The present invention has been achieved in view of the problems accompanying such conventional techniques. An object thereof is to provide a separator for sealed cells which attains a high degree of intimate contact with electrode plates, i.e., attains reduced internal resistance, and which is less apt to suffer the fatigue phenomenon even in repetitions of use of the cell and improves the life of the cell.
DISCLOSURE OF THE INVENTION
In order to achieve the object shown above, the separator for sealed cells of the invention comprises a non-woven fabric having fine rubber particles adhered thereto.
In one aspect of the invention, the fine rubber particles are a nitrile rubber, a chloroprene rubber, or a chlorosulfonated polyethylene.
In another aspect of the invention, the fine rubber particles have a particle diameter of 10 &mgr;m or smaller.
In another aspect of the invention, the content of the adhered fine rubber particles is from 0.1 to 10% by weight.
In yet another aspect of the invention, the non-woven fabric comprises glass fibers.
BEST MODE FOR CARRYING OUT THE INVENTION
Practical modes of this invention will be explained in detail below.
This invention relates to a separator which comprises a non-woven fabric having fine rubber particles adhered thereto and is to be interposed between the electrode plates of a sealed cell. This separator comprises a non-woven fabric formed by randomly superposing fibers and has pores in inner parts thereof due to its structure.
The shape of the separator is not particularly limited, but is preferably in a sheet form so as to fit to the electrode plates of a sealed cell.
The fine rubber particles are adhered to the surface of the non-woven fabric, i.e., to the surface of the constituent fibers (hereinafter simply referred to as “fibers”) of the non-woven fabric and are present among the fibers. In particular, the particles are present at intersections of fibers and serve as an adhesive. These fine rubber particles function as elastic bodies to absorb relative positional changes of the fibers and restore them. Namely, due to the presence of the fine rubber
Kameda Tsuyoshi
Kishimoto Kenjiro
Kitami Toshio
Kobayashi Makoto
Sasaki Masaaki
Ryan Patrick
Tsang-Foster Susy
Yuasa Corporation
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