Chemistry: electrical current producing apparatus – product – and – Fluid active material or two-fluid electrolyte combination...
Reexamination Certificate
1998-07-06
2001-06-12
Weiner, Laura (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Fluid active material or two-fluid electrolyte combination...
C429S235000, C429S238000
Reexamination Certificate
active
06245455
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a sodium-sulfur secondary battery, and more particularly, to the structure of a cathode chamber of a sodium-sulfur battery having superior charge-discharge cycle characteristics and a simple structure, and a method of manufacturing the same.
The sodium-sulfur secondary battery is one of the sealed type secondary batteries which uses a solid electrolyte tube having a sodium ion conductivity comprising at least either one of &bgr;-alumina and &bgr;″-alumina as an electrolyte, and which separates sodium as an anode active material from at least either one of sulfur and sodium polysulfide as a cathode active material, at least one of which is contained in a battery vessel. Such a battery is capable of being operated at a temperature in the range of 300° C.-350° C. In the present application, and as can be seen in the foregoing, a cathode is defined as an electrode filled with sulfur and/or sodium polysulfide, and an anode is defined as an electrode filled with sodium.
Regarding a sodium-sulfur battery, a technique to place a conductive felt-like carbon mat (rug) material, i.e. an electronic conductor, between the solid electrolyte tube and the cathode vessel, in order to collect electricity of the battery and to maintain the sulfur and sodium polysulfide, i.e. cathode active materials, has been disclosed, for example, in JP-A-6-283201 (1994), and JP-A-7-122294 (1995).
The carbon mat material used in the above mentioned battery was manufactured by perforating needle punches to a web made of flame resistant carbon fiber and calcining the web. However, this technique had problems in that the number of manufacturing steps was large, and the production cost of the mat itself was high, because plural carbon mat materials which had been fabricated in the shape of circular arc needed to be contained in the cathode chamber.
The electronic conductor made of carbon mat material operated as an electric collector between the solid electrolyte tube and the cathode container. Therefore, the contact resistance of the carbon mat material with the solid electrolyte tube was decreased by installing the mat into the cathode chamber in a compressed condition. However, if the compressing force applied to the mat was not uniform, local tensile stresses were generated on the surface of the solid electrolyte tube, and a danger of possible breakage of the solid electrolyte tube could be anticipated after a long period of operation of the battery. Furthermore, when the mat was compressed, a fluctuation in the fiber density was readily generated, and consequently, another problem arose in the form of a fluctuation in the internal resistance of the battery.
In accordance with JP-A-54-109134 (1979), a technique to provide a cathode collector by piling up short carbon fibers and bonding them with a carbide was disclosed. However, the technique had a problem in that the number of steps of the manufacturing process could not be decreased, because a step for piling up short fibers and a step for carbonizing the binder were necessary. Furthermore, fluctuation in electronic conductivity of the binder occurred depending on the carbonizing condition of the binder, and consequently, a problem in which a incrementing of the internal resistance was generated.
JP-A-57-27572 (1982) disclosed a technique to use swelled graphite of 2-50% by weight as a cathode collector. The swelled graphite is graphite which has swelled in a direction of the c-axis among crystalline axes of the graphite. However, in the case of the cathode electric collector disclosed in the above publication, the electricity collecting resistance became higher in comparison with a case when carbon fiber was used as the collector, and consequently, the charge-discharge efficiency was decreased. Several electric collectors have been disclosed in JP-A-61-156640 (1986) and U.S. Pat. No. 4,169,120, in addition to the above references.
SUMMARY OF THE INVENTION
One of the objects of the present invention is to provide a sodium-sulfur secondary battery having superior charge-discharge cycle characteristics, a cathode chamber having a structure which is not difficult to manufacture, and a method of manufacturing same.
One of the other objects of the present invention is to solve the above problems, and to provide a sodium-sulfur secondary battery having superior charge-discharge resistance characteristics and which is not difficult to manufacture.
One of the features of the present invention relates to a sodium-sulfur battery which has such a structure in which sulfur, and/or sodium polysulfide, and an electronic conductor can be arranged in a cathode chamber between a battery vessel, operating concurrently as a cathode electric collector, and a solid electrolyte tube, and wherein a material having a low electronic conductivity and superior corrosion resistance against sulfur and sodium polysulfide is arranged between the solid electrolyte tube and the electronic conductor; and this feature involves composing the electronic conductor of an assembly of a plural number of balls made of an electronic conducting substance having an elasticity and a porosity equal to or more than 80%.
By using the above assembly of a plural number of balls made of an electronic conductor (primary assembly balls) having an elasticity and a porosity equal to or more than 80%, uniform packing of the electronic conductor into the cathode chamber becomes possible.
Consequently, preferable charge-discharge cycle characteristics can be obtained, because a diffusion of the active material in the cathode chamber is not disturbed when the charge-discharge reaction progresses.
Furthermore, the danger of a significant breakage of the battery resulting on account of breakage of the solid electrolyte tube can be decreased, because a restriction force generated by coagulation of the cathode active material is reduced when the temperature of the battery is decreased, and any tensile stress generated at the surface of the electrolyte tube is small. By reducing the stress in the solid electrolyte tube, a reduction of the wall thickness of the electrolyte tube becomes possible, and the internal resistance of the battery can be reduced. Accordingly, a battery adequate for assembling a module, which is formed by connecting plural batteries in series, can be provided, because heat generation with a high current density operation is reduced. If the porosity of the electronic conductor ball is reduced to less than 80%, the packing ratio of the cathode active material is decreased, and the capacity of the battery is reduced. The porosity of the electronic conductor ball can be controlled by mixing powder of either the same material as the electronic conductor ball or an electronic conducting substance having a superior corrosion resistance against sulfur and/or sodium polysulfide. An uniform packing of the assembly of the balls into the cathode chamber is facilitated by mixing and using at least two kinds of balls having typical outer diameters different from each other.
Another one of features of the present invention relates to a sodium-sulfur secondary battery having a cathode active material and a cathode electric collector in the cathode chamber using a mixed material of conductive fiber balls having a porosity equal to or more than 80% and conductive grains as the cathode electric collector.
In this case, the grains size of the conductive grain in the range from 10 &mgr;m to 5000 &mgr;m can be used, and the conductive grains can be composed of a single material or a mixture of at least two kinds of materials, selected from the group consisting of carbon made from polyacrylonitrile, carbon made from coal-tar pitch, non-crystalline carbon, natural graphite, artificial graphite, acetylene black, kitchen black, Cr—Co base alloys, and Al—Si alloys.
Still another one of features of the present invention relates to a sodium-sulfur secondary battery having a cathode active material and a cathode electric collector in the cathode chamber using a m
Doi Toshiya
Kadoshima Masaru
Kamo Tomoichi
Kohno Kazushige
Koike Seizi
Antonelli Terry Stout & Kraus LLP
Hitachi , Ltd.
Weiner Laura
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