Chemistry: electrical current producing apparatus – product – and – Fluid active material or two-fluid electrolyte combination... – Active material in molten state
Patent
1992-09-14
1994-03-22
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
052963160
DESCRIPTION:
BRIEF SUMMARY
This invention relates to alkali metal energy conversion devices, such as for example alkali metal cells and particularly sodium sulphur cells. 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 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 of sealing components 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.
It will be appreciated that there are a number of options for the design of the external casing, in particular the choice of material to be used.
For example, reference may be made to U.S. Pat. No. 4,546,056 which shows an alkali metal energy conversion device having a particular arrangement of external casing, in this case comprising a cup shaped inner housing made of aluminium separated from a steel outer housing element by a layer of electrical insulation. The aluminium inner housing element serves as an electrical conductor whilst the steel outer housing element effectively provides support for the inner housing element. Aluminium has a number of properties which make it attractive for use in such alkali metal devices, for example low density, high electrical conductivity and resistance to corrosion from, for example, species such as polysulphides. Corrosion resistance against such species is due to the formation of a layer of aluminium sulphide, which is, however, electrically insulating. In some applications, it is know to arc, plasma or flame spray the surface of an aluminium can with a material which forms an electrically conductive layer in such an environment. Nichrome is often used for this.
It will be appreciated that an aluminium can cannot be used on its own for the external casing since the substantial overpressures generated would necessitate a can of substantial thickness and therefore cost. U.S. Pat. No. 4,546,056 addresses this by providing a thin aluminium can supported by an outer steel housing.
It is well known to hot dip aluminise steel to provide a composite material which is not only relatively economic to produce but also highly resistant to corrosion and sufficiently strong. Hitherto, such materials have not been employed in alkali metal energy conversion devices because they posess a number of severe drawbacks. The drawbacks associated with hot dipped aluminised material stem from there being an inter-metallic structure which is inevitably formed between the steel and the aluminium during coating. This inter-metalic structure is essentially columnar and comparatively brittle. Consequently, it is not possible to diffusion bond to a hot dip aluminised steel material since, at the temperatures involved, the inter-metallic alloy layer is weak and the material is unable to sustain the stresses placed on it. As a result, the material simply breaks away if diffusion bonding is attempted. A further problem is that hod dipped aluminised materials can become porous and thus allow corrosion.
Typically, the aluminium used in the hot dip process has a silicon content of approximately 10-14%. This has the effect of reducing the processing temperature required to coat the steel substrate by up to 60.degree. C. The silicon is also used to aid in the formation of a more uniform intermetallic layer between the aluminium and steel. However, the use of such high levels of sili
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Candidate Materials For The Sulfur Electrode Current Collectors--II. Aluminum and Its Alloys, R. P. Tischer et al., Corrosion Science, 26(5):371-375, 1986.
The Corrosion of Metals and Alloys by Sodium Polysulfide Melts at 350.degree. C., Brown et al., Proceedings of the Symposium on Sodium-Sulfur Batteries, 87(5): 237-245 (1986).
Silent Power GmbH fur Energiespeichertechnik
Skapars Anthony
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