Chemistry: electrical current producing apparatus – product – and – Fluid active material or two-fluid electrolyte combination... – Active material in molten state
Patent
1987-03-24
1988-03-22
Skapars, Anthony
Chemistry: electrical current producing apparatus, product, and
Fluid active material or two-fluid electrolyte combination...
Active material in molten state
429 49, H01M 620
Patent
active
047328240
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to an alkali metal electro-chemical storage cell, for example a sodium/sulphur cell.
Typically, such cells are formed with a metal casing, a solid electrolyte element dividing the interior of the casing into anode and cathode regions and current collectors in the anode and cathode regions associated with external terminals for making connections to the cell. The anode region contains alkali metal, for example sodium, which is in liquid form at the operating temperature of the cell, and the cathode region contains a cathode reactant material, such as sulphur. The solid electrolyte material is formed of an electronically insulating material which is conductive to cations of the alkali metal. In a fully charged condition, the cathode region of a sodium/sulphur cell contains substantially pure sulphur and the anode region contains liquid sodium up to a maximum level. As the cell discharges, the level of sodium in the anode region declines and sodium polysulphides are formed in the cathode region.
A problem with such cells is in the fabrication of an appropriate electrolyte element. In sodium sulphur cells, the electrolyte element is normally made of beta alumina and may be formed as a tube fitting within the outer metal casing of the cell, so that the inside of the tube forms one of the anode and cathode regions and the annular space between the tube and the outer casing forms and other region. It is difficult to make tubes of beta alumina which have high strength, especially in view of the high temperatures, in the order of 350.degree. C. for sodium/sulphur cells, at which the cell has to operate, and the stresses resulting from temperature cycling in use. Accordingly, a problem with such cells is the possibility of the solid electrolyte element failing in use so that there can be direct reaction between the anode and cathode reactants.
A solid electrolyte element can fail by a small crack developing which may permit sulphur from the cathode region to penetrate into the anode region and react directly with the sodium therein. As a result, the anode region becomes progressively more contaminated with sodium polysulphides and the cell eventually loses its open circuit potential. In practice, alkali metal cells are configured in batteries comprising a plurality of cells connected in parallel and in series parallel configurations. Series connection of the cells is necessary so that the battery has a desired output voltage in excess of the potential of a single cell. Parallel connection is desirable so that the battery has the necessary storage capacity.
When the solid electrolyte element of a cell is breached as mentioned above, and the cell loses its output potential due to direct reaction between the cathode and anode reactants, the damaged cell has a finite resistance between the anode and cathode terminals. As a result, a damaged cell in a parallel connected set of cells in a battery can result in the other cells connected in parallel therewith discharging by current flowing in the reverse direction through the damaged cell.
The present invention sets out to provide an alkali metal electro chemical storage cell which becomes open circuit in the event of a breach in the solid electrolyte element of the cell. It can be seen that if the damaged cell becomes open circuit. other parallel connected cells in the battery will not become discharged through the damaged cell and can still provide current to an external load.
In accordance with the present invention, an alkali metal electro-chemical storage cell comprises anode and cathode regions containing alkali metal and cathodic reactant respectively, a cationically conductive solid electrolyte member separating the anode and cathode regions, and respective anode and cathode current collectors and associated connection terminals, the anode current collector comprising a further cationically conductive solid electrolyte member extending into alkali metal in said anode region and separating a contact breaker region from alkali metal in sa
REFERENCES:
patent: 4124739 (1978-11-01), Sudworth
patent: 4252869 (1981-02-01), Heitz et al.
patent: 4638555 (1987-01-01), MacLachlan et al.
patent: 4657830 (1987-04-01), Kagawa
Chloride Silent Power Limited
Skapars Anthony
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