GB2042243A

GB2042243A - Sodium sulphur cells

Info

Publication number: GB2042243A
Application number: GB7904967A
Authority: GB
Original assignee: Chloride Silent Power Ltd
Current assignee: Chloride Silent Power Ltd
Priority date: 1979-02-13
Filing date: 1979-02-13
Publication date: 1980-09-17
Also published as: GB2042243B

Abstract

In a sodium sulphur cell having a cathode electrode comprising a porous carbon material between a solid electrolyte 12 and a cathode current collector 14, to prevent or reduce corrosion of the current collector, the porous carbon material comprises a main body of first material 19 with a bulk electronic resistivity not exceeding 20 ohm cm measured in the direction across the cathodic region between the electrolyte 12 and the current collector 14 together with a second material of at least twice the conductivity of the first material and extending not more than 25% of the distance from the current collector to the electrolyte. <IMAGE>

Description

SPECIFICATION Sodium-sulphur cells This invention relates to sodium-sulphur cells.

In a sodium-sulphur cell, a membrane of solid electrolyte material, such as beta alumina, separates sodium, which is liquid at the operating temperature of the cell and which forms the anode, from a cathodic reactant comprising sulphur and sodium polysulphides. On discharge of the cell, the sodium ions pass through the electrolyte and, in the cathodic region, give up an electron and combine with sulphur to form a sodim polysulphide or combine with a higher polysulphide to form a lower polysulphide. The converse action takes place on recharging the cell. The liquid sulphur and polysulphides have a low electronic conductivity and, in order to enable the electrons to pass to an external circuit and in order to disperse the electro-chemical reaction through the cathodic region, it is the common practice to use a porous carbon, e.g.

graphite felt, in the cathodic region between the electrolyte and a current collector which is electrically connected to an external circuit.

In a sodium-sulphur cell of the central sodium type, the electrolyte is a tube with the sodium inside the tube and, in this case, a cylindrical casing for the cell commonly constitutes the cathode current collector. In a cell of the central sulphur type, the sodium is around the outside of the tube and the cathode current collector is formed by a rod or tube axially located within the electrolyte tube.

Corrosion of the cathode current collector has long been recognised as a problem in sodium-sulphur cells.

Carbon will withstand the corrosive condition in the cathodic region but, particularly bearing in mind the very high current which can be drawn from a sodium-sulphur cell and which is one of the major advantages of this type of cell, the electrical resistance of carbon is a drawback. Many proposals have been made for using other materials and, particularly, coated and composite structures. Nevertheless, corrosion of the cathode current collector still presents a problem and it is one of the factors limiting the life of this type of cell.

According to this invention, in a sodium-sulphur cell having a cathode electrode comprising a porous carbon material impregnated with the cathodic reactant in a cathode region between a solid electrolyte and a cathode current collector, the cathode electrode comprises a main body of a first porous carbon material having a bulk electronic resistivity not exceeding 20 ohm cm measured in the direction across the cathodic region between the electrolyte and the current collector and having a second porous carbon material having a bulk electronic conductivity at least twice that of the first porous carbon material, the second material being adjacent the current collector and extending not more than 25% of the distance from the current collector to the electrolyte.

The porous carbon material may be a fibrous material, e.g. a graphite felt or cloth but non-fibrous material such as reticulated vitreous carbon or powdered graphite may be used, more particularly for the second material. If a fibrous material is employed it may be anisotropic in bulk properties, for example, having a higher conductivity in the direction between the electrolyte and current collector than in orthogonal directions; the bulk resistivity of the first and second materials in the direction between the electrolyte and current collector is the relevant resistivity in considering the present invention.

By providing the second porous carbon material having the higher bulk conductivity in the region near the current collector, the electro-chemical reaction is reduced in this region compared with the main body of the cathodic region where the porous carbon material is of lower conductivity. It will be particularly noted however that in this main body, the bulk resistivity does not exceed 20 ohm cm. Typically it may be substantially less than this, for example in the range of 5 to 10 ohm cm. The second material typically has a bulk resistivity, in the direction between the electrolyte and current collector, not exceeding 3 ohm cm.

It has been proposed in the past to degrade the conductivity of the porous material adjacent the electrolyte and even to use non-conductive fibrous material in this region. Such prior proposals have been with the object of inhibiting the electro-chemical reaction in the immediate neighbourhood of the electrolyte.

Because the ionic conductivity of the cathodic reactant material is less than the electronic conductivity of the carbon fibre, there is a tendency for the reaction to occur adjacent the electrolyte surface with the result that elemental sulphur collects adjacent the electrolyte. Since the sulphur is non-conductive, charge transfer is inhibited and the charging process is hindered or terminated. Such prior proposals, as examplified for example in U.S. Specification No. 3,993,503, are concerned therefore with removing the electro-chemical reaction away from the electrolyte during the charge cycle. This may be done by degrading the conductivity of the fibres adjacent the electrolyte.The present invention however is concerned with a completely different problem, namely the prevention or reduction of corrosion of the cathode current collector and, to achieve this objective, a porous material is used, in the neighbourhood of the cathode current collector, which material is of higher conductivity than the main body of porous material in the cathodic region where the reaction is to occur. To protect the cathode current collector, the second porous material preferably extends at least 5% of the distance from the current collector to the electrolyte.As stated above, the second material does not extend more than 25% of this distance from the current collector and preferably it does not extend more than 20% of the distance from the current collector to the electrolyte since it is undesirable to prevent the electro-chemical reaction ocurring in too great a volume of the cathodic region.

In a flat plate type of cell, the first and second porous carbon materials may comprise sheets or blocks of material between a current collector plate and the electrolyte plate; preferably the two materials together completely fill the region between these two plates. In a tubular cell, that is to say a cell having a tubular electrolyte separating a cathodic region from an anodic region, the cathodic region may be inside or outside the electrolyte, the current collector being an axial rod or tube in the first case or being a surrounding cylinder, typically the housing, in the second case. In either case, the cathodic region is annular and preferably each of the first and second porous materials forms an annular body. For means of construction, this body may be sub-divided into segments but preferably the cathodic region is completely filled with porous material.

The following is a description of one embodiment of the invention, reference being made to the accompanying drawing which is a diagrammatic transverse cross-section through a tubular sodium sulphur cell.

Referring to the drawing there is shown, in diagrammatic cross-section, a sodium sulphur cell of the central sulphur type having a cathodic region 10 inside a beta alumina tube 11 which separates this cathodic region from liquid sodium in an outer region 12 around the electrolyte tube but within an outer housing 13. A current collector rod 14 is arranged axially within the electrolyte tube so that the cathodic region is of annular form. This current collector rod 14 may for example be formed of a nickel-chromium alloy and may have a core of a more highly conductive metal such as aluminium.

The present invention is concerned more particularly with the cathodic region 10. As previously explained, it is the known practice to provide, in this region, a porous electronically conductive body impregnated with the sulphur/sodium polysulphides constituting the cathodic reactant. In the arrangement of the present invention, the part of the cathodic region 10 adjacent the current collector is formed of a porous carbon material 18 which has a higher bulk conductivity than the porous carbon material 19 filling the remainder of the annular space between the current collector and the electrolyte tube. The material adjacent the current collector has a bulk electronic conductivity which is at least twice that of the material 19. The material 19 however has adequate conductivity to enable the electro-chemical reaction to take place throughout the body of that material.

The following is a list of typical porous carbon materials such as might be used in the cathodic region of a sodium sulphur cell, showing their conductivity: Thornel VMA 7 ohm cm Sigrathern KFB2 3.3 ohm cm Kureha KGF 205 2.31 ohm cm Reticulated Vitreous Carbon 0.6- 1.0 ohm cm FMI FiberForm 0.88 ohm cm Le Carbone RVC 4000 0.8 ohm cm Sigrathern KFA 10 0.5 ohm cm Sigrathern KGA 10 0.3 ohm cm Powdered graphite 0.1 The conductivity figures quoted in the above list are obtained from measurements of material in situ in a sodium sulphur cell and may differ from manufactures' quoted values of electronic conductivity as measured under other conditions. The materials in this list, with the exception of the reticulated vitreous carbon and the powdered graphite, are all fibre materials.Fibrous materials may be used in the form of a woven fabric or as a felt or as a packing of loose fibres. Carbonised polymeric materials, such as polyacrylonitrile based fibre, are conveniently employed to make a woven cloth of relatively high bulk conductivity. Pitch based fibre mats are more economical but have higher resistivity than the carbonised polymers.

It will be seen that it is readily possible to choose two different materials of which one has a bulk conductivity at least twice as great as the other. The higher conductivity of the material 18 tends to make the electro-chemical reaction take place away from the immediate neighbourhood of the current collector and it is found that, by doing so, corrosion of the current collector can be substantially reduced or inhibited. The thickness of the annular layer of higher conductivity material is preferably at least 5% of the spacing between the current collector and electrolyte tube so as to give a substantial reduction of electro-chemical reaction immediately adjacent the current collector. The material however does not extend beyond 25% of the distance across to the electrolyte in order to leave adequate volume in the cathodic region for the electro-chemical reaction to take place.

Claims

1. A sodium-sulphur cell having a cathode electrode comprising a porous carbon material impregnated with the cathodic reactant in a cathode region between a solid electrolyte and a cathode current collector, wherein the cathode electrode comprises a main body of a first porous carbon material having a bulk electronic resistivity not exceeding 20 ohm cm measured in the direction across the cathodic region between the electrolyte and the current collector and having a second porous material having a bulk electronic conductivity at least twice that of the first porous carbon material, the second material being adjacent the current collector and extending not more than 25% of the distance from the current collector to the electrolyte.

2. A sodium-sulphur cell as claimed in claim 1 wherein said first material has a bulk resistivity less than 10 ohm cm.

3. A sodium sulphur cell as claimed in either claim 1 or claim 2 wherein the second material has a bulk electronic resistivity, in the direction between the electrolyte and the current collector, not exceeding 3 ohm cm.

4. A sodium sulphur cell as claimed in any of the preceding claims wherein the second material extends between 5% and 25% of the distance from the current collector to the electrolyte.

5. A sodium sulphur cell as claimed in any of claims 1 to 3 wherein the second material extends between 5% and 20% of the distance from the current collector to the electrolyte.

6. A sodium sulphur cell as claimed in any of the preceding claims wherein the first and second materials completely fill the cathodic region.

7. A sodium sulphur cell as claimed in any of the preceding claims and of the tubular type with the cathodic region of annular form either inside or outside an electrolyte tube, wherein the first and second materials are in the form of concentric annuli.

8. A sodium sulphur cell as claimed in any of the preceding claims wherein the first material is a pitch-based fibrous material.

9. A sodium sulphur cell as claimed in any of the preceding claims wherein the second material is a carbonised polymeric material.

10. A sodium sulphur cell as claimed in claim 9 wherein the second material is a woven cloth.

11. A sodium sulphur cell substantially as hereinbefore described with reference to the accompanying drawing.