METHOD OF MANUFACTURING A CATHODE ELECTRODE MATRIX FOR A SODIUM/SULPHURE CELL

Abstract: In a method of manufacturing a cathode electrode matrix for a sodium/sulphur cell, a block of fibrous material is impregnated with a cathodic reactant material before being cut into parts which are then fused together to form the matrix.

Description: METHOD OF MANUFACTURING A CATHODE ELECTRODE MATRIX FOR A SODIUM/SULPHUR CELL This invention relates to a method of manufacturing a cathode electrice matrix for a sodium/sulphur cell. In a sodium/sulphur cell, a solid electrolyte material separates molten sodium, forming the anode, from a sulphur/ polysulphide cathodic reactant. The solid electrolyte is a material, such as beta-alumina, which conducts sodium ions. On discharge of the cell, the sodium gives up electrons at the anodic interface of the solid electrolyte. Sodium ions pass through the electrolyte into the cathode region adjacent the opposite face of the electrolyte. The electrons pass through the sodium to the anode current collector and thence around an external circuit to a cathode current collector e.g. a rod or tube formed of or coated with a material chemically inert to the cathode reactant. The electrons must pass from this cathode current collector to the region of the cathode adjacent the surface of the solid electrolyte where they react with the sulphur to form sulphide ions. Sulphide ions and sodium ions form a polysulphide. The electronic conductivity of molten sulphur is low and hence it is the practice to pack the cathodic region with a fibrous carbon or graphite material to provide the required electronic conductivity, the fibrous material forming a matrix through which the cathodic reactant can move . Sodium sulphur cells are commonly of tubular form. They may be of the kind known as a central sodium cell in which the sodium is inside an electrolyte cup and the cathodic region lies between the outer surface of the electrolyte cup and a tubular current collector which might constitute or form part of the cell housing. , Such a sodium/sulphur cell is shown in Figure 13 of the drawings, which is a perspective view of the cell with part broken away. As shown the cell comprises a case 50 of, for example steel, in the form of a right-circular cylinder and containing a solid electrolyte cup 51 of beta alumina, the cup 51 containing a sodium electrode 52, while a space between the case 50 and the cup 51 contains a sulphur electrode matrix 53. For use, the cell is maintained at a temperature of between 300°C and 400°C such that the sodium and sulphur of the electrodes 52 and 53 are in liquid form. The open end- of the cup 51 is closed by an insulating disc 54 of alpha alumina, while the case 50 is closed by an annular steel disc 55. The case 50 serves as a terminal for the sulphur electrode matrix 53, while the sodium electrode 52 contains an elongate metal current collector 56 which extends axially of the case 50 out through the disc 54 where it is connected to a centre terminal disc 57 mounted on the disc 54, the necessary connections being made by welding. As sulphur is essentially non-conducting a means of making an electrical connection between the case 50 and the cup 51 has to be provided, and this is generally achieved as discussed above by forming the sulphur electrode matrix 53 as a conductive fibre matrix impregnated with sulphur. Alternatively the cell may be of the type known as a central sulphur cell in which the sodium is outside the electrolyte cup and the cathodic reactant is in an annular region between the inner surface of the electrolyte cup and a central current collector rod or tube. In each of these constructions, the cathodic region is of annular form. The common practice has been to use carbon fibre felt as the electronically-conductive matrix of the cathode electrode. Such felt may be formed into annular elements which may be packed axially into the cathodic region, the felt subsequently being impregnated with sulphur. The matrix material in the cathodic region has to be porous to permit of free access of the cathodic reactant material to the neighbourhood of the electrolyte cup. Electrically however this conductor forms the path to transfer electrons from the reaction zone to the cathode current collector when charging the cell and provides the path between the current collector and the regions near the surface of the electrolyte cup where the sulphide ions have to be formed on discharge of the cell. One of the problems in sodium sulphur cells is to obtain sufficient overall conductance in the cathode electrode. Thi is of particular importance in sodium sulphur cells becaus these cells can pass very large currents on discharge. Th conductance of the fibre matrix material in the cathodic regio constitutes one of the limitations on the performance of suc cells. It is possible to increase the bulk conductance o carbon fibre felt by packing the felt more tightly. Thi however impedes the free movement of the cathodic reactan material which must have access to the neighbourhood of th electrolyte cup. Other techniques have therefore been proposed to improv the bulk conductance in the cathodic region. For example, i GB-A-1513682 there is described a cathode electrode matrix fo a sodium sulphur cell formed of a plurality of discret elements with electronically-conductive material such a graphite foil between the elements and extending across th region between the current collector and the electrolyte cup t increase the conductivity across that region. It is also known to employ loose fibres instead of fel as the matrix in the cathodic region of a sodium sulphur cell as described in GB-A-1528672 which describes the use of mixture of graphite or carbon fibres with fibres of anothe material, for example an oxide material such as alumina o zirconia, which is preferentially wetted by the sulphides i the cathodic reactant to improve the physical transfer of th cathodic reactant material. In GB-A-2042299 there is described a method of manufacturing a cathode electrode matrix for a sodium/sulphur cell comprising the steps of forming a block of electronically conductive fibrous material which is chemically resistant to hot cathodic reactant, the fibres in the block predominantly extending parallel to one plane, and cutting the block in a plurality of parallel planes normal to said one plane to form slices in which the fibres predominantly have a component of direction normal to the plane of the slice. In this known method the slices thus formed are then further treated by compression in a series of parallel regions to form segments of trapezoidal section between the compressed regions with the fibres having a component of direction normal to the parallel surfaces of the trapezoids, the slice either before compressing or after having been compressed being impregnated with the cathodic reactant at a temperature at which the reactant is liquid. A slice thus produced can be folded to form an annular member to serve as the required cathode electrode matrix. However, as discussed above, it is also known to build up a cathode electrode matrix from a plurality of disc and annular members cut from a sheet of fibrous material rather than use a one-piece structure formed from a slice as described above, and this can present difficulties in view of the lack of bonding in a sheet produced as set out above. This is a particular problem if a sheet is used in which the fibres lie predominantly in one direction in the plane i which they lie, use of such a sheet being advantageous sinc the bulk conductance in the radial direction for any disc o annular member cut from the sheet is enhanced compared wit that which would be obtained if the fibres were randoml oriented, as in a felt. According to this invention in a method as set out abov the block is impregnated with a cathodic reactant at temperature at which the reactant is liquid and then allowed t cool, prior to being cut to form said slices, cathode electrod matrix parts then being formed from the slices. With the method of this invention the impregnate reactant serves to bind the fibres of the block togethe whereby the block can be readily cut to obtain the slices fro which the parts, for example discs or annular members necessary to produce a .cathode electrode matrix are then formed The block can, in known manner, be formed from plurality of layers of conductive fibre material of, fo example carbon or graphite, separated by layers of alumin fibre material, for example material as sold under the trad mark "Saffil". The cathodic reactant can be sulphur or a sodiu sulphide. If sulphur is used, the melting point of which i 113°C, it is injected at approximately 150° at a moul temperature of between 1 - 12 C. This invention will now be described by way of example with reference to the drawings in which Figures 1 to 12 illustrate various steps in the manufacture of a cathode electrode matrix for a sodium/sulphur cell by a method according to the invention. Referring to the drawings, as shown in Figure 1 a plurality of rectangles 1 each 275 mm by 175 mm are cut from a roll 2 of carbon fibre felt material, the fibre direction being random in each rectangle 1. The rectangles 1 are then stacked to form a block 3 about 20 to 25 mm high. As shown in Figure 2, rectangles 4 each again 275 mm by 175 mm, are cut from a roll 5 of alumina fibre material, and the rectangles 4 then separated by peeling to provide rectangular layers 6 each about 0.25 mm thick. Next, as shown in Figure 3, a composite block 7 is then formed by sandwiching an alumina fibre material layer 7 between two of the carbon fibre material blocks 3. To produce a complete cathode electrode matrix various parts are required, and in particular side wall parts and base parts. As shown in Figure 4, side wall parts are produced from a plurality of composite blocks 8 arranged in a compressed stack 9 35 mm high, the composite blocks 8 each being 100 mm by 47 mm and being cut from a composite block 7 as shown in Figure 3. Base parts are similarly produced as shown in Figure 5 from a plurality of composite blocks 10 each 47 mm by 35 mm, and again cut from a composite block 7 as shown in Figure 3. The composite blocks 10 are arranged in a compressed horizontal stack 11 about 100 mm long. The side part blocks 9 of Figure 4 and the base part blocks 11 of Figure 5 are then separately introduced into a mould 100 as shown in Figure 6 into which molten sulphur is injected through a nozzle 101 and gate 102, thereby to impregnate the blocks 9 and 11 with the sulphur as required. Referring now to Figure 7, after impregnation with sulphur the side part block 9 of Figure 4 is sliced lengthwise by means of a circular saw 200 to provide slices 12 each about 6.7- 0.2 mm thick. The base part block 11 of Figure 5 is similarly sliced lengthwise as shown in Figure 8 to provide slices 13 again about 6.7^ 0.2 mm thick. It will be appreciated that in the slices 12 cut from a side part block 9 the layers in the slice run lengthwise of the slice, while in the slices 13 cut from a base part block 11 the layers in the slice run widthwise of the slice, this as clearly shown in Figures 7 and 8. To form the -base of the cathode electrode a disc and a ring are required, and these are produced as shown in Figures 9 and 10 by cutting discs 14 about 41.75 mm in diameter from a slice 13 as shown in Figure 8 using a circular saw blade 300, and then cutting a centre portion about 29 mm in diameter from one such disc using a further, smaller circular saw blade 400 to form a ring 15, as shown in Figure 10. Referring now to Figure 11, the base of the cathode electrode is finally formed by placing a disc 14 of Figure 9 and a ring 15 of Figure 10 together with a disc 16 of alumina fibre material in a two part mould 500, 501. The mould 500, 501 is partially closed and heated until the sulphur in the disc 14 and ring 15 is molten, and then the mould 500, 501 is fully closed, as shown, to compress the disc 14, ring 15 and disc 16 into a composite structure 17 in the form of a dish having a base, an upstanding peripheral wall, and an internal lining of alumina fibre material, the dish 17 being removed from the mould 500, 501 after cooling. An alternative method of producing the base is to cut the disc 14 of greater thickness than indicated, and then remove, as by machining, a central portion to leave a base of the required dish shape. Referring now to Figures 12 and 13, the side wall parts of the cathode electrode are finally formed by placing a slice 12 of Figure 7 together with a sheet 18 of alumina fibre material in a two part mould 600, 601 and then carrying out a moulding operation as for the dish 17 of Figure 11 to form a side wall part 19 in the form of part of a right-circular cylinder with the alumina fibre material 18 lining its inner surface. After such moulding the side wall part 19 is cut to the required length for the cathode electrode matrix to be produced. Finally, a base part 17 of Figure 11 and a plurality of side wall parts 19 of Figure 12 are assembled together to form a cathode electrode matrix having a base formed by the part 17 and a side wall formed by the parts 19, the inner surface of the matrix being lined with alumina fibre material and the base and side wall being impregnated with sulphur. This final assembly can be carried out in a cell, with the parts 17 and 19 becoming fused together when the cell is brought up to its operating temperature. Otherwise the parts 17 and 19 can be assembled outside of ' cell and remoulded to fuse them together to form an integral matrix structure for introduction into a cell.

Claims: CLAIMS 1. A method of manufacturing a cathode electrode matrix for a sodium/sulphur cell, comprising the steps of forming a block of electronically conductive fibrous material which is chemically resistant to hot cathodic reactant, the fibres in the block predominantly extending parallel to one plane, and cutting the block in a plurality of parallel planes normal to said one plane to form slices in which the fibres predominantly have a component of direction normal to the plane of the slice, in which the block is impregnated with a cathodic reactant at a temperature at which the reactant is liquid and then allowed to cool, prior to being cut to form said slices, cathode electrode matrix parts then being formed from the slices. 2. A method as claimed in Claim 1, in which the block is formed from a plurality of layers of conductive fibre material separated by layers of alumina fibre material. 3. A method as claimed in Claim 2, in which base parts for the cathode electrode matrix are formed from slices in which the layers extend widthwise of the slice. 4. A method as claimed in Claim 3, in which the base of the cathode electrode matrix is formed from disc and ring base parts which are moulded together to form a composite base structure in the form of a dish having a base and an upstanding peripheral wall. 5. A method as claimed in Claim 4, in which an alumina fibre material layer is moulded onto the inner surface of said composite base structure. 6. A method as claimed in any one of Claims 2 to 5, in which side wall parts for the cathode electrode matrix are formed from slices in which the layers extend lengthwise of the slice. 7. A method as claimed in Claim 6, in which side wall parts of the cathode electrode matrix are moulded from said slices, each part being in the form of part of a right-circular cylinder with the layers extending lengthwise thereof. 8. A method as claimed in Claim 7, in which an alumina fibre material layer is moulded onto the inner surface of each side wall part. 9. A method as claimed in any one of Claims 3 to 8, in which the cathode electrode matrix is formed from one of said composite base structure parts and a plurality of said sid wall parts fused together. 10. A method as claimed in Claim 9, in which said parts ar fused together in situ in a cell. 11. A method as claimed in Claim 9, in which said parts ar fused together prior to being inserted into a cell.

Inventors: GREEN RALPH, JACKSON STEPHEN, GREEN, RALPH, JACKSON, STEPHEN
Applicants: CHLORIDE SILENT POWER LTD, CHLORIDE SILENT POWER LIMITED
Application year: 1989
Main classification: H01M4
IPC codes: H01M4, H01M10