Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Cell enclosure structure – e.g. – housing – casing – container,...
Reexamination Certificate
1998-09-16
2001-03-27
Chaney, Carol (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Cell enclosure structure, e.g., housing, casing, container,...
C429S175000, C429S165000, C429S185000
Reexamination Certificate
active
06207321
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a sodium secondary battery which is sealed in an improved manner, and in particular to a rechargeable secondary battery applied to batteries for storage of electric power for road grading and to electric vehicles.
2. Background Art
FIG. 9
(PRIOR ART) is a schematic illustration of a conventional sodium secondary battery.
As shown in
FIG. 9
, in the conventional sodium secondary battery, a negative electrode chamber is formed by placing sodium
3
into a bottom-closed, hollow cylindrical solid electrolyte
2
provided inside an outer case
1
; and between the outer case
1
and the solid electrolyte
2
is disposed a porous electrode
4
impregnated with sulfur
5
serving as a positive electrode active substance, to thereby form a positive electrode chamber. An outer case metal fitting
6
having an L-shaped cross section is welded to the opening portion of the outer case
1
. For a cover
12
, a metal fitting (hereinafter referred to as a cover metal fitting
8
) is provided. An electric insulator
7
, attached along the outer periphery of the solid electrolyte
2
in the vicinity of its opening, is sandwiched between the outer case metal fitting
6
and the cover metal fitting
8
via an aluminum alloy
9
serving as a brazing material, and undergoes hot-pressing to thereby provide sealing of the structure.
With the above structure, in the discharge process the sodium
3
contained in the negative electrode chamber dissociates into sodium ions and electrons. The sodium ions pass through the solid electrolyte
2
to migrate into the positive electrode chamber outside the solid electrolyte and are combined with the sulfur
5
and electrons circulating outside the cell to thereby form sodium polysulfide.
Meanwhile, in the charge process, sodium polysulfide present in the positive electrode chamber dissociates into sodium ions, electrons, and sulfur. The formed sodium ions pass through the solid electrolyte
2
to migrate into the negative electrode chamber defined by the inside of the tubular solid electrolyte and are combined with electrons circulating outside the cell to thereby form sodium
3
.
The process for manufacturing the above cell will next be described.
(1) The insulator
7
is bonded to the upper portion of the bottom-closed, tubular solid electrolyte
2
by use of a glass solder
10
.
(2) The upper face of the insulator
7
is bonded to the cover metal fitting
8
, and the lower part of the insulator
7
is bonded to the outer case metal fitting
6
, both by hot-press bonding by the mediation of aluminum alloy
9
serving as a brazing material. As used herein, the term “hot-press bonding” refers to bonding between heterogeneous materials by the application of pressure in an atmosphere of about 600° C., which is close to the melting point of aluminum alloy
9
.
(3) The positive electrode
4
impregnated with the sulfur
5
serving as a positive electrode active substance is placed in the outer case
1
, and then the outer case metal fitting
6
is welded to the outer case
1
.
(4) A wick
11
, which also serves as a safety tube and has a sodium discharge outlet
11
a
, is secured onto the cover
12
, which is then welded with the cover metal fitting
8
.
(5) The sodium
3
, in the form of liquid, is injected from a sodium-injection-hole, and the hole is sealed with a sealing member
13
.
Problems that arise in relation to hot-press bonding for manufacturing the sodium secondary battery will next be described.
(1) A high temperature is required for melting the aluminum alloy
9
serving as a brazing material; thus, a heating apparatus, such as an electric furnace achieving a temperature as high as approximately 600° C., is required. A vacuum condition may also be required, depending on the bonding method employed.
(2) Under the aforementioned conditions (i.e., high temperature and in vacuo), pressurization must be performed, which raises disadvantages associated with scaling up of the apparatus employed and an increase in the number of manufacturing steps, such as cooling from high temperatures and raising pressure from the vacuum condition to atmospheric pressure.
(3) There may be a case in which &bgr;-alumina, serving as an insulator, breaks due to high temperature. The breakage induces reaction between sodium and sulfur to suddenly cause a high temperature condition. When the temperature is higher than the melting point of aluminum, the cell is broken.
(4) Conventional planar-type sodium secondary batteries suffer a problem of poor sealing caused by a large proportion of hot-press-welded parts since flanges located at the periphery of a positive electrode container and a negative electrode container being opposite to each other are hot-press welded.
SUMMARY OF THE INVENTION
The present invention was accomplished in order to solve the problems remaining in the conventional techniques, and an object of the present invention is to provide a rechargeable secondary battery applied to batteries for storage of electric power for road-grading and to electric vehicles.
To solve the above-described problems, in a first aspect of the present invention, there is provided a sodium secondary battery in which a negative electrode chamber is formed inside a bottom-closed, hollow cylindrical, solid electrolyte, which is accommodated in an outer case, and a positive electrode chamber is formed outside the solid electrolyte, wherein a cover which closes the opening of the outer case is fastened to the outer case by bolts and the interposition of an insulator.
According to the first aspect of the invention, the following four advantages are obtained, since the cover, which closes the opening of the outer case, is sealed by the bolts and the interposition of an insulator.
i) A secondary battery can be manufactured without thermal stress being generated and with remarkably increased yield;
ii) An electric furnace is eliminated from the manufacturing facility, whereby the time for heating in the electric furnace is saved, a cooling step may be omitted, manufacture is simplified, and the facility cost is reduced remarkably.
iii) Since the solid electrolyte
2
will not be broken, possibility of disintegration of the brazed members is reduced even when the temperature of the brazed portion becomes higher than the melting point of aluminum as a result of reaction between sodium and sulfur; and
iv) The battery is easily disassembled by simple loosening of the bolts, to thereby facilitate recycling the battery, which is more difficult in the case of batteries fabricated through melt bonding.
In the above-described first aspect of the invention, preferably, the cover and the opening portion of the outer case of the sodium secondary battery are formed of a common material and the linear expansion coefficient thereof is greater than that of the insulator. This is advantageous in that more secure fastening with bolts is achieved to thereby provide improved sealing.
Preferably, a space is provided, at the opening portion of the outer case, to allow expansion of the bolts when the cell temperature rises to the operation temperature of the battery. This structure is advantageous in that need for washers is eliminated.
Preferably, the insulator also serves as a cover. With the employment of an insulator which also functions as a cover, the number of members is reduced to thereby simplify the structure of the casing. Preferably, the flanges are formed such that their bolt-receiving portions have an increased thickness to accommodate the expansion of the bolts when the cell temperature rises to the operation temperature of the battery. With this structure, use of washers is advantageously eliminated.
In a second aspect of the present invention, there is provided a sodium secondary battery in which a positive electrode chamber is formed inside a bottom-closed, hollow cylindrical, solid electrolyte, which is accommodated in an outer case, and a negative electrode chamber is formed outside the solid electrolyte, wh
Fukagawa Masayuki
Iwamoto Keiichi
Kawasetsu Nozomu
Sawata Akihiro
Sudou Katsuzou
Chaney Carol
Dove Tracy
Mitsubishi Heavy Industries Ltd.
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