Chemistry: electrical current producing apparatus – product – and – Fluid active material or two-fluid electrolyte combination... – Active material in molten state
Patent
1990-06-28
1992-11-17
Skapars, Anthony
Chemistry: electrical current producing apparatus, product, and
Fluid active material or two-fluid electrolyte combination...
Active material in molten state
H01M 1039
Patent
active
051642727
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention is concerned with alkali metal cells of the kind comprising an outer container with a solid electrolyte member dividing the interior of the container into anodic and cathodic regions. The anodic region will contain a molten alkali metal, typically sodium, and the cathodic region will contain a suitable cathodic reactant also in liquid state, typically sulphur and sodium polysulphides.
DESCRIPTION OF THE PRIOR ART
It is known to form such alkali metal cells with the solid electrolyte member shaped as a cup located inside the outer container, and with the alkali metal forming the anode contained within the electrolyte cup and the cathodic reactant contained in the outer region between the cup and the outer container. The outer container is typically made of metal and forms a current collector for the cathode. Current may be collected from the alkali metal anode within the electrolyte cup by an electronically conductive current collecting member located within the cup so as normally to be in contact with any alkali metal remaining in the cup, and extending out through a closure member for the electrolyte cup so as to provide a current lead-out.
An example of such an alkali metal cell, taking the form of a sodium/sulphur cell is illustrated in GB-A-2161017.
It is also know to improve the performance of alkali metal cells by providing within the electrolyte cup or tube containing the alkali metal anode, an additional metal foil tubular form immediately adjacent the inner surface of the electrolyte tube. This foil tubular form can be made from a sheet of spring foil rolled into the tube before insertion into the electrolyte cup, whereupon the resilience of the foil tends to press the foil against the cylindrical inner face of the electrolyte.
The purpose of such an inner foil tubular form is to encourage the molten alkali metal to wet a maximum area of the internal cylindrical surface of the electrolyte. It may be appreciated that as the cell discharges, the quantity of alkali metal, typically sodium, within the electrolyte cup or tube declines which can result in a reducing surface area of the electrolyte being in direct contact with the alkali metal. This has hitherto resulted in the internal resistance of cells increasing as they become discharged. However, the metal foil tubular form located within the electrolyte cup or tube causes liquid alkali metal to be drawn up by capillary action in the narrow gap between the foil and the inner cylindrical face of the tube, thereby increasing the surface area of the electrolyte which is wetted and maintaining internal cell resistance at a low level. An example of the above arrangement is disclosed in GB-A-1461071.
The solid electrolyte employed in alkali metal cells is typically made of a ceramic material which though electronically insulating is conductive to cations of the alkali metal. Beta alumina is the usual material for sodium sulphur cells. An additional problem which arises with such cells employing ceramic electrolytes is the possibility of the electrolyte failing, e.g. cracking, and so allowing direct contact and mixing of the anodic and cathodic materials. Any direct mixing results in a highly exothermic reaction which, if not checked, can result in the temperature of the cell rising so high that the outer cell container is breached. This in turn can result in the highly corrosive reaction products leaking into the battery cavity containing the cells.
Whilst substantial effort is directed to the preparation of electrolyte members which have a very high integrity, thereby reducing the risk of a damaging failure of the cell to a very low level, some risk may still remain and efforts have been directed at substantially eliminating this remaining risk of catastrophic failure.
As one example, GB-A-1517287 describes a sodium sulphur cell having a molybdenum tube located inside a tubular solid electrolyte to act as a sodium reservoir. The molybdenum tube defines with the electrolyte a shallow wicking space through
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Patent Abstracts of Japan, vol. 8, No. 181 (E-261) (1618), Aug. 21, 1984, corresponding to Japanese Patent No. JP-A-5973863 (Yuasa Denchi K.K.), Apr. 26, 1984.
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Lewis Ian M.
Lowton Ronald P.
Stackpool Michael F.
Sykes Alan
Chloride Silent Power Limited
Skapars Anthony
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