1. Field of the Invention
This invention relates to electrode structures for electro-chemical cells of the kind having an electrochemical reactant contained in a porous electronically-conducting matrix.
2. Prior Art
Such an electrode structure is used, for example, as a cathode electrode in cells having a molten alkali metal as the anode separated by a solid ionically-conductive membrane from the cathode. A typical example of such a cell is a sodium-sulphur cell in which the anode comprises sodium, which is molten at the operating temperature of the cell and in which the cathode comprises an electronically-conducting matrix, typically of carbon or graphite fibrous material impregnated with sulphur/polysulphides, the anode and cathode being separated by a solid electrolyte permitting the passage of sodium ions, for example, a beta-alumina. Because the sulphur/polysulphides has a poor electrical conductivity, it is necessary to provide an electronically-conductive matrix in the cathodic region, this matrix permitting the movement of the cathodic reactant, which is liquid at the operating temperature of the cell into the region adjacent the electrolyte surface where the electro-chemical reaction takes place, the conductive matrix providing an electronic path between this region and a current collector. Typically, the matrix is formed of carbon felt or carbon fibres or foamed carbon. See for example U.S. Pat. Nos. 3,982,957; 3,980,496; 3,985,575; 3,993,503; 4,052,535 and 4,123,596.
In a tubular sodium-sulphur cell, in which the solid electrolyte is in the form of an open-ended tube, it is possible to locate the cathodic reactant within the tube and the sodium outside or vice-versa (see for example U.S. Pat. No. 3,922,176). In either case, the matrix is held in contact with an electronically-conducting current collector to enable an external circuit to be connected to the cell. In the former case, this current collector is typically a solid cylindrical rod coaxial with the electrolyte tube. In the latter case, the current collector is typically a cylindrical metal sheath forming part of the outer casing of the cell. In both cases the cathodic reactant and the porous matrix are contained within a cylindrical annulus between the electrolyte tube and the current collector.
Within the sodium electrode it is preferable to provide a capillary means to maintain an adequate supply of liquid sodium over the entire surface area of one side of the solid electrolyte. In order to ensure efficient operation of this capillary means, it is desirable to initially fill the cell with sodium in liquid form. For safety reasons this operation is preferably performed before sulphur has been introduced into the cell. Because the melting point of sulphur (113.degree. C.) is quite close to that of sodium (98.degree. C.), stringent temperature control would be necessary during liquid sodium filling of a cell already containing sulphur to prevent local remelting of sulphur, which could constitute a fire hazard should the cell be subjected to accidental rough handling resulting in fracture of the ceramic. For the same reason, it is also undesirable to fill with liquid sulphur a cell that has been previously filled with liquid sodium.
There is a further disadvantage of filling a cell with liquid sulphur, which is that, on cooling of the cell to room temperature, the sulphur volume will diminish due to a large increase in density at the solidification temperature, and thermal contraction of the solid. The thermal expansion coefficient of beta-alumina electrolyte (6.0.times.10.sup.-6 K.sup.-1) is lower than that of sulphur (6.4.times.10.sup.-5 K.sup.-1) so that subsequent rewarming of the cell can result in the electrolyte being subjected to stresses that may exceed the ceramic strength. This effect will be particularly troublesome when the sulphur electrode is located within the electrolyte tube, as hoop stresses will be set up within the electrolyte, which calculations show may well exceed the electrolyte strength.
Yet another disadvantage of liquid filling is that the distribution of sulphur within the porous matrix cannot easily be made uniform. Because the volume of the cathodic reactant increases considerably (30%) during discharge to a composition of approximately Na.sub.2 S.sub.3, it is necessary to provide within the sulphur electrode expansion space to accommodate this increase in volume. It is desirable that the expansion space and cathodic reactant be uniformly distributed throughout the volume of the sulphur electrode. This is not possible if the cell is filled with liquid sulphur, as gravity will ensure that this will accumulate in the lower regions of the cell.
It has been proposed in U.S. Pat. No. 4,118,545, to pack fibre material between layers of cloth to facilitate their handling and more specifically it has been proposed to utilise the cloth to join a plurality of elongate elements, along their length so that the assembly can be formed into an annular unit to fit within a cell. This assembly has to be impregnated with sulphur after it has been formed.
The problem of manufacture of the sulphur electrodes has been considered by D. Chatterji "Development of sodium-sulphur batteries for utility application" Electric Power Research Institute report EM--266 Dec. 1976, which describes a method in which a cylindrical solid plug of carbon felt plus sulphur is cast and an axial hole is drilled through it conforming to the electrolyte dimensions. An earlier EPRI report 127-2 of Dec. 1975 "Development Program for solid electrolyte Batteries" discloses a preformed cylindrical sulphur electrode manufactured by forming a composite of carbon felt and sulphur and then machining the composite to the precise dimensions required.