Chemistry: electrical current producing apparatus – product – and – Fluid active material or two-fluid electrolyte combination... – Active material in molten state
Patent
1990-06-08
1992-06-02
Kalafut, Stephen
Chemistry: electrical current producing apparatus, product, and
Fluid active material or two-fluid electrolyte combination...
Active material in molten state
296232, H01M 1039
Patent
active
051185741
DESCRIPTION:
BRIEF SUMMARY
This invention relates to alkali metal energy conversion devices, such as for example, alkali metal cells and particularly sodium sulphur cells and methods of constructing such devices. Such cells typically employ a solid electrolyte element separating cathodic and anodic reactants which are liquid at the cell operating temperature.
A known construction of such a device comprises an external casing, a solid electrolyte element dividing the interior of the casing into two electrode regions, an electrically insulating element joined to the electrolyte element, and at least one metal member sealed to the insulating element. This structure typically forms part of the sealing arrangement for the device, sealing off the two electrode regions both from each other and from the ambient environment. For example, the external casing of the device may be of metal, so that any sealing of an electrode region requires a seal to be made between the metal of the casing and the electrolyte element. However the metal of the casing must be electrically insulated from the electrolyte element and the insulation is provided by the intervening electrically insulating element.
An example of such an arrangement applied to a sodium sulphur cell is shown in GB-A-2102622 which has an alpha alumina lid closing a tubular electrolyte element. A centrally located current collector is mounted in an aperture through the alpha alumina lid and insulated by the lid from the electrolyte element. The outer electrode region, on the outside of the electrolyte element, is sealed by means of a thin metal closure member welded about its periphery to a metal casing for the cell, and sealed about an inner periphery to the alpha alumina lid. This latter seal between the thin metal closure member and the alpha alumina lid has sometimes been made by compression bonding using an intermediate layer, between the thin metal closure element and the ceramic lid, of a soft material, e.g. aluminium, to provide the necessary bonding.
Such bonding is carried out after the insulating element, i.e. the alpha alumina ceramic lid, as attached to the electrolyte element since this attachment is effected by glazing at elevated temperatures which would destroy seals manufactured by thermocompression bonding using an intermediate layer between the thin metal closure element and the ceramic lid.
Alternatively, EP-A-0166605 discloses the techniques of directly thermocompression bonding the thin metal member to the insulating ceramic lid to provide a seal between the two which is not adversely effected by subsequent temperature cycling such as may be employed if the insulating element is subsequently joined to the electrolyte element by glazing. The ability to form such a seal between the thin metal member and the insulating element before the latter is joined to the electrolyte element greatly facilitates the making of this seal. For instance, a stack of metal members and insulating elements may be simultaneously sealed to one another to provide a plurality of sealed pairs.
It will be appreciated that the sealing of sodium sulphur cells and other alkali metal energy conversion devices is of critical importance in the manufacture of the cell to ensure good performance and safety and is a particularly difficult problem because of the high operating temperatures of these cells, typically 350.degree. C. Bonding techniques using cements have not proved practical.
In particular, the seal to the outer electrode region effected by the welding of the thin metal closure member about its periphery to the metal casing is dependent upon the integrity of a thin film, generally aluminium oxide with the materials commonly used, that is formed on the thin metal member during thermocompression. This thin film protects the metal member from attack by corrosive substances in the outer, i.e. sulphur electrode, region, these in particular being polysulphides formed therein.
Hitherto, the thin metal member has been directly welded to the metal casing after thermocompression sealing of the metal me
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Ball Roger A.
Bindin Peter J.
McLachlan Stuart
O'Neil Bell Christopher
Sands Gilbert
Chloride Silent Power Limited
Kalafut Stephen
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