SODIUM/SULPHUR CELL

Abstract: A sodium/sulphur cell has a cathodic reaction region including a member of aluminium/silicon alloy.

Description: SODIUM/SULPHUR CELL This invention relates to a sodium/sulphur cell. Many forms of electrical batteries are known, and can be divided into two main types, mainly primary batteries which have a relatively short life and which are discarded when exhausted and secondary batteries which are rechargeable when exhausted and thus have a relatively long life. Both primary and secondary batteries of dry alkaline type are known, as are secondary batteries of lead acid type. Recently secondary batteries of sodium/sulphur type have become known, such batteries having the advantages of light weight, high storage capacity and relatively quick recharging time. Further, such batteries use sodium and sulphur both of which are cheap and abundant materials. Unlike conventional lead acid batteries in which a liquid electrolyte - dilute sulphuric acid - separates two solid electrodes, in a sodium/sulphur cell a solid electrolyte - generally beta alumina - separates two liquid electrodes, namely liquid sulphur and liquid sodium electrodes. Such a sodium/sulphur cell is shown in the drawing which is a perspective view of the cell with part broken away. As shown the cell comprises a case 1 of, for example steel, in the form of a right circular cylinder and containing a solid electrolyte cup 2 of beta alumina, the cup 2 containing a sodium electrode 3, while a space between the case 1 and the cup 2 contains a sulphur electrode 4. For use, the cell is maintained at a temperature of between 300°C and 400°C such that the sodium and sulphur electrodes 3 and 4 are in liquid form. The open end of the cup 2 is closed by an insulating disc 5 of alpha alumina, while the case 1 is closed by an annular steel disc 6. The case 1 serves as a terminal for the sulphur electrode 4, while the sodium electrode 3 contains an elongate metal current collector 8 which extends axially of the case 1 out through the disc 5 where it is connected to a centre terminal disc 7 mounted on the disc 5, the necessary connections being made by welding. As sulphur is essentially electronically non-conducting a means of providing electronic conduction to all parts of the sulphur electrode from the case 1 has to be provided, and this is generally achieved by forming the sulphur electrode 4 as a carbon fibre mat impregnated with the sulphur. It will be appreciated that with such a cell the sodium and sulphur electrodes 3 and 4 can have their locations reversed. The atomic structure of beta alumina is such that it acts as a selective ion filter. As the cell is discharged sodium ions are transported through the beta alumina electrolyte cup 2 from the sodium electrode 3, to the sulphur electrode 4, and there combine with the sulphur to form some species of sodium polysulphide. In order for discharge to continue, the sodium polysulphide has to be transported away from the surface of the cup 2 and replaced with fresh sulphur. During recharging of the cell it has to be ensured that a continuous supply of sodium polysulphide is maintained at the surface of the cup 2. If sodium ions are removed from the sodium polysulphide at such a rate that fresh polysulphides cannot be taken to the surface of the cup 2 , a layer of sulphur will be formed at the surface, and this will cause the cell to suffer an increase in internal resistance, and its performance will be impaired. To overcome this problem some form of preferential wetting at, and to, the surface of the cup 2 by sodium polysulphide has to be provided. According to US-A-4767684 the problem discussed above, which is known as polarisation, can be overcome by including in the cathodic reaction region of the cell a device which provides a continuous release of an additive into the cathodic reaction. In particular this document proposes the use of a device comprising an additive material which is a metal from Groups, I, II and III of the Periodic Table of Elements; a Transition Series Metal; antimony, lead, tin or bismuth, or an alloy, salt, oxide, phosphide, arsenide, antimonide, carbide or nitride of any of the proposed metals. or a mixture of any of these, contained in a coating of an inert material such as porcelain, enamel, or relatively inert molybdenum metal. The inert coating does not completely cover the additive material, an area of the surface thereof being left uncovered in dependence upon the required rate of release of the additive material into the cathodic reaction of the cell. According to this invention a sodium/sulphur cell has a cathodic reaction region including a member of aluminium/silicon alloy. It is believed that the inclusion of the aluminium/silicon alloy member in the cathodic reaction region of the cell results in corrosion products from the member being taken into solution and then redeposited on the material of the cathode so promoting preferential wetting thereof whereby the polarisation problem discussed above is overcome. A particular advantage of the cell of the invention is that the aluminium/silicon alloy member does not require an inert coating to control the rate of corrosion thereof, since the use of silicon in the alloy reduces the rate of corrosion of the aluminium to an acceptable level due to the formation of a stable, tenacious, reaction product on the surface of the aluminium of the member. A preferred composition for the aluminium/silicon member is:- Aluminium 90 - 99.5% Silicon 0.2 - 10% Magnesium 0 - 4% Manganese 0 - 2% Other Materials 0 - 2% The aluminium/silicon alloy member can be provided in the form of a wire and will serve as an inactive electrical current carrying member in the cathodic reaction region of the cell, and may be connected to a current carrying member of the cell. Otherwise the aluminium/silicon alloy member can be provided as a coating on a wall or walls of the cathodic reaction region of the cell. Tests have shown that cells in accordance with the invention show virtually no charge to discharge asymmetry, no polarisation problems, and a very low discharge resistance rise..

Claims: CLAIMS 1. A sodium/sulphur cell having a cathodic reaction region including a member of aluminium/silicon alloy. 2. A cell as claimed in Claim 1, in which said member is provided in the cathodic reaction region in the form of a wire. 3. A cell as claimed in Claim 1, in which said member is provided as a coating on a wall or walls of the cathodic reaction region. 4. A cell as claimed in any preceding claim, in which said member is composed of:- Aluminium 90 - 99.5% Silicon 0.2 - 10% Magnesium 0 - 4% Manganese 0 - 2% Other Materials 0 - 2% 5. A cell as claimed in any preceding claim, substantially as hereinbefore described with reference to the drawing.

Inventors: O'NEIL-BELL CHRISTOPHER, O'NEIL-BELL, CHRISTOPHER
Applicants: CHLORIDE SILENT POWER LTD, CHLORIDE SILENT POWER LIMITED
Application year: 1991
Main classification: H01M10
IPC codes: C22C21, H01M4, H01M10