WO1991017582A1

WO1991017582A1 - Cathode electrode structures for sodium sulphur cells and their manufacture

Info

Publication number: WO1991017582A1
Application number: PCT/GB1991/000746
Authority: WO
Inventors: David Joseph Riley; Andrew Charles Stribley
Original assignee: Chloride Silent Power Limited
Priority date: 1990-05-10
Filing date: 1991-05-10
Publication date: 1991-11-14
Also published as: EP0527860A1; GB9221476D0; GB9010501D0; GB2259602A; AU7862491A; CA2081669A1; JPH05507172A

Abstract

A method of forming a cathode structure for a sodium sulphur cell uses sheets of electrically conductive fibrous material which have fibres predominantly extending parallel to one plane. A sheet of the fibrous material is cut into a plurality of strips which are then assembled to form at least one sheet-like structure in which the fibres predominantly have a component of direction normal to the plane of the sheet-like structure. The structure is shaped and compressed to form at least one cathode component, which component comprises at least a section of the required shape of the cathode structure. The compressed component is impregnated with a cathodic reactant.

Description

CATHODE ELECTRODE STRUCTURES FOR SODIUM SULPHUR CELLS AND THEIR MANUFACTURE

This invention relates to cathode electrode structures for sodium sulphur cells.

In a sodium sulphur cell, a solid electrolyte material, typically beta alumina, separates molten sodium forming the anode from a sulphur/sodium polysulphide cathodic reactant. On discharge of the cell, the sodium gives up electrons at the anodic interface of the solid electrolyte and sodium ions pass through the electrolyte into the cathode 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 in the cathodic region of the cell. The electrons must pass from this cathode current collector to the region of the cathode adjacent the surface of the solid electrolyte; here 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 and they may be of the kind known as a central sodium cell in which the s r.ium is inside the electrolyte tube and the cirnodic region lies between the outer surface of the electrolyte tube and a tubular current collector, which might constitute or form part of the cell housing. Alternatively the cell may be of the type known as a central sulphur cell in which the sodium is outside the electrolyte tube and the cathodic reactant is in the annular region between the inner surface of the electrolyte tube and the central current collector rod or tube. In each of these constructions, the cathodic region is of annular form. It has been a common practice to use graphite felt as the electronically conductive packing material in the cathodic region. Such felt may be formed for example into annular elements which may be packed axially into the cathodic region, the felt subsequently being impregnated with sulphur. It is also known for example from UK Patents GB-A-1472975 and GB-A-1513682, to fabricate a cathodic matrix from preformed elements which have been shaped by compression whilst impregnated with liquid sulphur, the elements being cooled whilst being pressed so that they can be fitted into the annular region of the cell. When the cell is raised to the operating temperature, the sulphur melts and the compressed fibrous material expands to provide good contact with both the current collector and the electrolyte surface.

In US Patent Specification No. 4,076,902, there is described a sodium sulphur cell in which graphite fibres are arranged in an annular region between a beta alumina electrolyte tube and a surrounding cell housing with the fibres extending in a direction normal to the cathodic current collector constituted by the cell housing and thus normal to the electrolyte tube. It has long been appreciated that radial orientation of the fibres would be desirable to improve the radial conductivity but the above-mentioned US patent specification does not disclose any method by which the graphite fibres can be so oriented. One method of radially orienting fibres is disclosed in UK Patent GB-B-2042244. In this specification there is described a method in which a compressible block of electronically conductive fibrous materials is formed with the fibres extending predominantly in parallel planes; the block is then cut in a plurality of parallel planes normal to said one plane to form at least one sheet in which all the fibres have a component of direction normal to the plane of the sheet, the sheet is then compressed along a series of parallel regions which extend across the sheet in its plane to form segments of trapezoidal section between the compressed regions with the fibres having a component of direction normal to the parallel surface of the trapezoids. The fibrous sheet is impregnated either before compression or whilst compressed, with the cathodic reactant at a temperature such that the cathodic reactant is liquid and the compressed impregnated sheet is cooled to solidify the reactant. The segments of trapezoidal form thus a plurality of segments which may be assembled together, each extending at least part of the length of the cell, to form an annular cathode electrode structure.

In UK Patent GB-B-2095026 there is described a method of forming a suitable cathode structure by needling a suitable fibrous sheet having fibres lying in planes parallel to the planes of the major surfaces of the sheet, cutting the sheet into longitudinal strips, turning each strip through substantially 90 degrees at its longitudinal axis and assembling the strips into a sheet-like structure and compressing the sheet structure in a series of parallel regions to form a series of parallel groove sloping sides defining segments of trapezoidal section such that the fibres lie normal to the parallel surfaces of the trapezoid. The assembly is finally impregnated with liquid sulphur or sodium polysulphide which when cooled and solidified provides a mat of trapezoids connected together by threads which can be used as hinges to form an annular cathodic element for insertion into a cell.

It is one of the objects of the present invention to provide an improved method of forming a cathode structure for a cylindrical sodium sulphur cell with fibres extending in a radial direction between the electrolyte surface and the current collector.

It is a further object of the present invention to provide improved apparatus for manufacturing a cathode structure.

According to one aspect of the present invention there is provided a method of forming a cathode structure for a sodium sulphur cell comprising the steps of cutting into a plurality of strips a sheet of electrically conductive fibrous material which has fibres predominantly extending parallel to one plane, assembling the strips to form at least one sheet-like structure in which the fibres predominantly have a component of direction normal to the plane of the sheet-like structure, shaping and compressing the structure to form at least one cathode component, which component comprises at least a section of the required shape of the cathode structure and impregnating the compressed component with a cathodic reactant.

Preferably the shaping is effected by compressing said sheet-like structure in a mould. The mould is preferably shaped so that the required number of components for at least one single cathode structure are formed simultaneously.

The sheet of fibrous material may be compressed during the cutting step to between 0.05 and 0.4 of its uncompressed thickness and preferably to between 0.1 and 0.3 of its uncompressed thickness and more preferably to 0.2 of its uncompressed thickness.

In the preferred embodiment of the invention the cut strips of the sheet are successively pushed into a holding device transversely to the plane of the sheet to assemble the sheet like structure, which holding device holds the sheet like structure in a compressed state during further handling.

Preferably the strips are tied together by passing at least one thread through the strips to join them together, the thread being parallel to one of the larger faces of the sheet-like structure the thread being preferably a plied yarn of twisted graphite.

Preferably the sheet is predominantly formed of carbon fibre material and is preferably combined with a layer of fibrous alumina material before the cutting step.

According to the invention there is also provided a method of forming a cathode structure for a sodium sulphur cell comprising the steps of combining a sheet of electrically conductive fibrous material which has fibres predominantly parallel to one plane with a layer of fibrous alumina material, cutting the combined sheet into a plurality of strips extending in a direction normal to said one plane, placing the strips into a holding device which maintain the strips under compression, threading at least one thread through the strips to join them together, transferring the joined strips under compression from the holding device to shaping means which cuts and shapes the combined material into side and base components, said components being sections of the required shape of the cathode structure, impregnating the shaped components with liquid sulphur or sulphide and maintaining compression until the sulphur or sulphide has solidified. According to the invention there is further provided apparatus for forming a cathode structure for a sodium sulphur cell comprising cutting means for cutting a sheet of electrically conductive fibrous material into strips, tying means for passing a thread through said strips to join them together injection moulding means for shaping the joined strips to form at least one component for a cathode structure and a holding device for compressing and transferring the strips from the cutting means to the tying means and from the tying means to the moulding means.

Preferably the apparatus further comprises feeding means for feeding the sheet of fibrous material to the cutting means in an indexing manner.

Preferably the feeding means further comprise means for combining the fibrous alumina material with said sheet of fibrous material before cutting and feeding it to the cutting means in an indexing manner. The feeding means preferably comprise at least one conveyor driven in an indexing manner.

A preferred embodiment of the invention the cutting means comprise a vertically movable cutting blade and the compression block also movable in a vertical direction, to enable the sheet of fibrous material to be compressed during cutting.

Preferably the cutting means further comprise an adjustable stop for determining the length of cut strips of fibrous materials and a fed blade for putting the cut strips into the holding device transversely to the plane of the sheet.

The tying means may comprise at least one hollow needle carrying a continous thread operable to pass through the strips held compressed in the holding device and to withdraw therefrom leaving the thread in position. The holding device preferably has a pair of cutting blades operable to cut the thread at either end of a row of strips held therein after the needle has been withdrawn.

Conveniently the holding device further comprises a shutter operable to open the holding device and pusher means operable to eject the sheet-like structure held therein.

The moulding means preferably comprise a cavity mould having a plurality of lands and valleys, the shape of which is such that on closure the mould cuts and shapes the joined strips into the required shape. The moulding means preferably comprise means for injecting liquid sulphur or sulphide into the closed mould. A preferred embodiment of the invention the apparatus further comprises means for moving the holding device between the cutting, tying and moulding means and means for locating the device relative thereto. A preferred embodiment of the invention will now be described with reference to the accompanying drawings in which:-

Fig. 1 is a typical sodium sulphur cell having a cathode structure; Fig. 2 is a perspective diagram of a sheet of carbon fibre material to be formed into a cathode structure for a sodium sulphur cell according to the invention;

Fig. 3 is a front elevation of a carbon-alumina composite which is an intermediate product made from strips of the carbon fibre material of Fig. 2 during the forming of the cathode structure;

Fig. 4 is an end elevation of the carbon- alumina composite of Fig. 3; Fig. 5 and 6 are block diagrams of processing apparatus for the forming of the cathode structure for a sodium sulphur cell;

Fig. 7 is a side elevation of a part of the feeding and cutting station of the processing apparatus of Figs. 5 and 6 (with parts omitted for clarity) ;

Fig. 8 is a side elevation of a further part of the feeding and cutting station of Fig. 7 (with parts omitted for clarity) ;

Fig. 9 is the same view of the apparatus of Fig. 8 at an alternative stage of processing showing the relative location of a transfer tool for moving material from one station to another in the processing apparatus of Figs. 5 and 6;

Fig. 10 is a sectional side view of the transfer tool;

Fig. 10a is an underneath plan view of the transfer tool of Figure 10 with the base shutter removed;

Fig. 11 is an end sectional view of the transfer tool of figure 10 on line XI-XI;

Figs. 12 a, b, c and d are side elevations of the transfer tool of Fig. 9 at a tying station (of which parts have been omitted for clarity) .

Fig. 13 is a plan view of a lower cavity mould of a mould cavity filling station;

Fig. 14 is a partial section of the lower cavity mould of Fig. 13 on XIV-XIV;

Fig. 15 is a part sectional elevation of the lower cavity mould of Fig. 13 on XV-XV; Fig. 16 is another partial sectional view of the lower cavity mould of Fig. 13 on XVI-XVI;

Figs. 17 a, b, c, d and e are side elevations of the location of the transfer tool of Fig. 10 relative to a section of a mould cavity filling station;

Figs 18a, b and c are different views of the side portions of the cathode structure;

Figs 19a and 19b are various views of the base portion the cathode structure.

Fig. 1 illustrates a typical sodium sulphur cell 5 having a sodium electrode 6, a beta alumina electrolyte 7 and a sulphur electrode 8 within an outer case 9. The sulphur electrode 8 is a composite structure of sulphur, carbon fibre, alumina fibre and graphite yarn and each electrode 8 is built from three side elements 70 and one base element 71 (as shown in Figs. 18a, b, c and 19a, b) by the manufacturing process described below.

Referring to Fig. 2 there is shown a sheet 10 of fibrous electrically conductive material having fibres 12 lying in planes parallel to the plane of the sheet shown in this embodiment parallel to one another lying generally parallel to line A-A. Fibrous material of this nature, formed of carbon fibre material, is commerically available in sheet form in which substantially all the fibres have at least a component of direction extending in a predetermined direction. In a preferred embodiment the material is of a non-woven structure which is built up in the layers to a thickness of approximately 30 mm. This sheet 10 is the starting material for forming the cathode structure 8 for a sodium sulphur cell 5.

The sheet of carbon material 10 is used to form a carbon-alumina composite 14 (see Figs. 3 and 4) which consists of an oriented carbon mat interleaved with layers of fibrous alumina material 15 with the layers 17 being tied by graphite yarn 16. Strips 11 are cut from the sheet 10 along lines 13 and are placed vertically on top of each other so that the fibres all run in the same direction across the width of the mat i.e. thus "orienting" the mat. The direction of the fibres 12 is generally indicated by arrow B. The alumina material 15 or saffil (Trade Mark) is similar to glass filter paper and is beneficial in improving the distribution of the cathodic reactant because alumina is more easily wetted by polysulphides than the conductive carbon material. The graphite yarn 16 is preferably a plied yarn of twisted sliver e.g. 0.8-1 gm"¹ and has good tensile properties e.g. breaking load of 6kg min. The purpose of this material within the fibre composite structure 14 is to constrain the layers of compressed carbon mat 11 by acting as a tie. The overall composite is compressed in direction of arrows C. The handling of these materials is difficult.

The carbon mat 10 has a loose structure and requires constraining whilst being handled. Furthermore the fibres 12 can be physically abrasive and this must be taken into consideration during handling. To achieve the required electrical performance from the carbon mat structure 14 over-compression and crushing of fibres 12 must be avoided as this will break the fibre filaments. This applies at all stages where the fibres 12 are cut and/or compressed. Contamination of the fibres 12 from oil, grease, dust and water etc must also be avoided as these will effect the performance of the resulting mat. The alumina material has a very low shear strength and only a low tension can be used in the handling thereof. The graphite yarn 16 is particularly poor at resisting abrasion.

Figs. 5 and 6 are schematic representations of the apparatus for manufacturing the cathode electrode structures 8 comprising a feeding and cutting station 20, a tying station 21 and mould cavity filling station 22 which are described in detail below. The product is transferred between the various stations by means of a transfer tool 23 which is shown in different orientations in Figs. 5 and 6 as 23a and 23b. Although Figs. 5 and 6 illustrate linear processing, a number of variations are envisaged which will be mentioned below.

Fig. 7 illustrates the feeding and positioning section 20a of the feeding and cutting station 20 and Fig. 3 illustrates the cutting section 20b.

Referring first to Fig. 7 a reel 26 of carbon mat 10 is rotatably mounted to the apparatus in a known manner on spindle 27. The mounting is positioned above a conveyor 28 such that the carbon reel 26 unwinds directly onto the conveyor 28 and such that the conveyor 28 drives the carbon mat reel 26 to unwind in the direction of arrow D. The alumina material 15 is also supplied on a reel 29 which is also rotatably mounted to the apparatus by means of spindle 30 and suitable known mounting means beneath the level of conveyor 28. The alumina material reel 29 is driven by a pair of drive wheels 31 which rotate to cause the alumina material 15 to unwind in the direction of arrow E. The actual mounting of drive wheels 31 is effected in a suitable known manner which is not relevant to the invention.

At point 32 (shown in Fig. 7) after the carbon mat 10 has left contact with conveyor 28 the carbon mat 10 and alumina material 15 meet to form a duplex layer 17 with carbon 10 on top and alumina material

15 underneath. The combined layer 17 is driven by a second driving conveyor 33 to the cutting station 20b. The second conveyor 33 is mounted at the same level as the first conveyor 28. The two conveyors 28, 33 and drive wheels 31 are all driven to rotate at the same surface speed and are driven in an indexing manner by suitable drive and control means. The length indexed varies on average between 6.7 mm and 12 mm depending on the dimensions of composite mat 14 being produced.

Means, not shown, detect the end or breakage of the mat 10, alumina material 15 and the combined layer 17 and provide a signal to the control means to stop the conveyors 28, 33 and drive wheels 31 until the problem is fixed.

To prevent the combined layers 17 from becoming displaced, it is necessary to constrain the mat 10 as it comes off the reel 26 and is fed to the cutting station 20b. The constraining means are not actually shown but may be of any suitable means. It is also necessary to control the tension in the mat 10 by ensuring that the mat 10 is unwound at a rate matched to its usage. The tension in the paper 15 must also be controlled and again the speed of unreeling of the paper 15 is matched to that of the conveyor 33.

Further detection means may be provided to detect faults during feeding of the various materials such as jamming, misalignment etc.

In an alternative embodiment of the invention the carbon mat 10 is cut into lengths of width 80mm and 40mm. Each length is split in half to form two lengths and a layer of alumina fibrous material inserted therebetween to form a sandwich. This process may be carried out manually or with appropriate apparatus. The lengths may either be mounted on reels and fed to the cutting part of the apparatus automatically or appropriate short lengths of strip can simply be placed manually on an indexing feeder for feeding to the cutting part of the apparatus. A length of 80mm width material and a length of 40mm width material are fed simultaneously to the cutting part 20b of the apparatus.

Any further reference to the combined layer of material 17 may refer to either the duplex or triplex arrangement. Referring now to Fig. 8, the cutting part 20b of the apparatus is illustrated. The combined layer of carbon/alumina 17 is fed to a stop 35 which is set to control the desired cut length of material i.e. the width of strips 11. A compression block 36 is positioned adjacent cutting blade 37 which is vertically movable such that the blade 37 is raised after each cut.

It has been found that improved cutting of the material 17 can be achieved when it is cut in a compressed state at approximately 20% of its natural thickness. The compression block 36 is therefore mounted so as to be movable vertically to enable compression of the material 17 by this preferred amount (see Fig. 9) . On an opposite side of the material 17 to the cutting blade 37 is located a shutter 39 which is movable in the direction of arrow F to open and close passage way 41, beneath which is located the transfer tool 23, as shown in Fig. 9. A feed blade 38 is positioned adjacent to cutting blade 37 above the cut strip(s) 11 and is vertically movable to push the cut strips 11 into the transfer tool 23 transversely to the plane of the sheet.

In operation the material 17 is fed beneath the compression block 36 and blades 37, 38 until it reaches stop 35. In one embodiment of the invention a single width of combined layer 17 is cut into strips. In an alternative embodiment where the two different width strips are fed simultaneously to the cutting apparatus, a pair of strips is cut in each operation. With shutter 39 closed the block 36 and blades 37, 38 are moved downwards together until the material 17 is suitably compressed. The cutting blade 37 is operated to move further downwards to cut the material 17 whilst the block 36 and feed blade 38 remain stationary. Then the cutting blade 37 and feed blade 38 are raised to allow the cut strip(s) 11 to de-compress. Shutter 39 is then operated to open passageway 41 and the feed blade 38 moved downwards to shunt the strip(s) 11 into the transfer tool 23, as shown in Fig. 9.

The blades 37, 38 and block 36 are automatically controlled, by pneumatic means or other suitable means. The transfer tool 23 holds and constrains the strips 11 after they have been cut. As can be seen in Figs. 10 and 11 the transfer tool 23 comprises a housing 44 having base shutter 45, slidable in the direction of arrow H to open the base of housing 44. At each end of the housing is affixed a lug piece 44a which is connected by means of a bolt or stud to shaft 42. Transfer tool 23 also comprises a pusher bar 43 which is slidably mounted with suitable bearings on the shafts 42 and connected by a shaft arrangement to pusher arms 46. As the pusher bar 43 is moved vertically upwards and downwards the pusher arms 46 move upwards and downwards within the housing 44. Positioned beneath the pusher bar 43 is a secondary bar 48 which is also slidable on the shafts 42. This secondary bar 48 is attached to knives 49 as shown in Fig. 11. The secondary bar 48 has a pusher knob 57 mounted thereon which allows some relative movement between the two bars 43 and 48, but is arranged such that when pushed to its full extent it also causes the pusher bar 43 to move with it.

Means are provided at the upper end of the shafts 42 to prevent the bars 43, 48 from passing over the ends.

On one side of the housing is a single or a pair of slots 47 (shown in Fig. 10 as a pair of slots) into which the cut strips 11 are pushed. When a pre-selected number of strips 11, usually 7, have been placed in the transfer tool 23, the secondary bar 48 is pushed downwards so that knive blades 49 close slot 47. This movement can be effective manually with the pusher knob 57 or the tool 23 can be connected to a pneumatic system which would operate it automatically.

It will be understood that in the embodiment of the invention which utilises pairs of strips being cut that the transfer tool will be as shown in Figs. 10, 10a and 11 having two slots 47 and pushers 46.

The transfer tool 23 can thus be loaded to move the material 17 between the feeding and cutting station 20 and the tying station 21. Figs. 5 and 6 illustrate a system in which a pick and place mechanism rotates the transfer tool 23 through 90° about a horizontal axis as it is moved between the two stations 20, 21 to correctly align the composite 14 therein for operation at the tying station 21. The transfer tool 23 need not necessarily be moved automatically but maybe transferred manually between the stations 20, 21 and again onto station 22 for further processing.

As an alternative to the linear processing mentioned above, the stations 20, 21 and 22 may be positioned so that the transfer tool 23 is moved therebetween on a rotable table 67. In this embodiment the orientations of the stations 20, 21 and 22 are correct so that transfer tool 23 remains in the same horizontal plane throughout operations. Referring now to Figs. 12a, b, c and d, the tying station 21 has a plurality of spools 50 of graphite yarn 16 rotably mounted adjacent to each other. Fig. 12a shows just one such spool 50. The composite 14 has a number of graphite ties 16 along its length (see Fig. 3) and as many spools 50 are provided as there are numbers of ties 16 required.

The same number of needles 51 are also provided, only one of which is shown in Figs. 12a to 12d. The needles 51 are all operable simultaneously to make the ties along the length of the composite 14 and the length of the composite 14 determines the number of ties 16 required. The operation of just one needle 51 and spool 50 will be described as they all operate in an identical manner. It should be noted that in the embodiment having two strips of material 17 being processed simultaneously the needles 51 are arranged to join the strips 11 of both simultaneously.

To prevent abrasion of the graphite yarn 16 all surfaces over which it passes must be smooth, highly polished, hard and free from sharp projections. Twisting and knotting of the yarn 16 is prevented by ensuring that it is under a light tension at all times. Breakage to each yarn 16 at the point of tying and de-spooling are detected by detector means (which have been omitted for clarity) . The yarn 16 passes from spool 50 through the centre of hollow needle 51 with a nominal 5mm protruding past the point of the needle 52. The needles 51 are mounted on a ram (not shown) and their movement is guided by bushes 54 which restrict the flexing of the needles 51.

The mouth of slot 47 in transfer tool 23 is closed with an insert 55 which has a series of holes 58 along its length, being the same number as ties required. Catch means (not shown) provided to hold the insert 55 in position and means may also be provided to facilitate its removal from slot 47. The protruding end _ yarn^' 16 is initially bent back over the needle point 52 as it passes through the hole 55 and remains in that position as the needle 51 passes through the material 17. When the end of the needle 52 exits therefrom through a gap 53 in the far side wall 44 of the transfer tool 23, the protruding end springs back to its original position.

A plurality of clamps 56 are positioned on an opposite side of the transfer tool 23 to the spools 50 which are operable to clamp the protruding end of the yarn 16 when positioned between the jaws of the clamp f:-, after the point of needle 51 emerges from the gar 53 in the housing 44, as shown in Fig. 12b. As the needle 51 is withdrawn from the transfer tool 23, as shown in Fig. 12c, the yarn 16 remains stationary. Suitable means are provided for detecting that the yarn 16 is gripped throughout the removal of the needle 51 are provided. The movement of the needles 51 and clamp 56 may be automatically controlled by pneumatic or other suitable means.

Once the needle 51 is detected as being fully withdrawn the blades 49 of the transfer tool 23 are operated either manually or automatically to cut the yarn 16. This operation separates the remaining ties 16 from the stock material and the needles 51 at the same time trap the prepared carbon alumina composite 14 into the transfer tool 23. If the graphite yarn 16 is not fully severed at the first cut the blades

49 are operable to provide more than one cutting action.

The transfer tool 23 containing the prepared composite 14 is moved either automatically or manually, from the tying station 21 to the mould cavity filing station 22 where it is positioned relative to a lower cavity mould 60 by means of suitable location features (not shown) . Detection means are also provided (not shown) to ensure that the correct positional alignment has been achieved before further action takes place.^' The lower cavity mould 60 is positioned in an injection moulding machine.

Fig. 13 shows the lower cavity mould 60 having two cavities of which the first is a base cavity 61 and a second is a side cavity 62 which form respectively base and side pieces for the cathode electrode.

Side cavity 62 has three lands 63 which define the three separate side pieces which will be required for the cathode electrode structure 8 which may be formed in a single side cavity 62.

A feed cavity 64 is provided to allow sulphur or sulphide to be fed in at entry point 65 to reach each of the cavities 61, 62. Ejector pins 66 are provided both in the feed cavity 64 and the base and side cavities 61, 62 to enable the resulting product to easily be removed from the mould 60. The layout of the mould 60 shown in Figs. 13-16 is one variation only and the shape of tool varies according to the resulting shapes and numbers of the components it is required to produce.

As can be seen from figures 17a-17e the steps for transfer of the composite 14 from the transfer tool 23 to the cavity mould 60 are as follows. The transfer tool shutter 45 is opened to create an exit for the composite fibre structure 14 (Fig 17b) . Pressure on the pusher knob 57 causes the knife blades 49 and pusher bar 46 to move together in downwards direction to push the composite 14 into the mould cavity, in this figure shown as side cavity 62 (Fig. 17c) . The knife blades 49 are withdrawn from the cavity 62 by pulling the knob 57 whilst the pusher 46 remains in position (Fig. 17d) so that as the composite 14 is released into the cavity 62 the position of the strips 11 is constrained. The transfer tool 23 is then reset by withdrawing the pusher 46 and the transfer tool 23 moved away from the cavity mould 60 prior to closing the shutter 45 which re-sets the transfer tool 23 ready for further fibre handling (Fig. 17e) . The injection moulding machine (not shown) covers and seals the lower cavity mould 60 with an upper cavity mould and then injects sulphur into the mould via port 65. The upper mould contacts the top of lands 63 in cavity 62 and either cuts the composite mat 14 to form three separate portions or provides areas of reduced thickness to enable the finished product to be easily broken into three separate portions.

After the injected sulphur has cooled and set the upper mould is removed and the side and base segments ejected from the lower mould 60 using ejector pins 66. A number of composite fibre manufacturing means may be used to interface with the injection moulding machine to speed up processing. The resulting components are shown in Figs.

18a, b and c and 19a and b. Figs. 18a, b and c show a side portion 70, three of which are placed with an end 72a adjacent end 72b of the next portion 70 to form a cylindrical ring. Figs. 19a and 19b show a base portion, the edges 73 of which co-operate with the lower surfaces 74 of the side portion 70.

The side portion 70 and base portions 71 can thus be assembled within the outer casing 9 of the sodium cell 5 as shown in Figure 1.

Claims

1. A method of forming a cathode structure for a sodium sulphur cell comprising the steps of cutting into a plurality of strips a sheet of electrically conductive fibrous material which has fibres predominantly extending parallel to one plane, assembling the strips to form at least one sheet-like structure in which the fibres predominantly have a component of direction normal to the plane of the sheet-like structure, shaping and compressing the structure to form at least one cathode component, which component comprises at least a section of the required shape of the cathode structure and impregnating the compressed component with a cathodic reactant.

2. A method as claimed in Claim 1 wherein the shaping is effected by compressing said sheet-like structure in a mould.

3. A method as claimed in Claim 2 wherein the mould is shaped so that the required number of components for at least one cathode structure are formed simultaneously.

4. A method as claimed in any one of the preceding claims further comprising the steps of compressing the sheet of fibrous material during the cutting step.

5. A method as claimed in Claim 4 in which the sheet is compressed to between 0.05 and 0.4 of its uncompressed thickness.

6. A method as claimed in Claim 5 in which the sheet is compressed to between 0.1 and 0.3 of its uncompressed thickness.

. A method as claimed in Claim 6 in which the sheet is compressed to 0.2 of its uncompressed thickness. 8. A method as claimed in any one of the preceding claims further comprising the step of successively pushing the cut strips of the sheet transversely to the plane of the sheet into a holding device to assemble the sheet like structure, which holding device holds the sheet like structure in a compressed state during further handling.

9. A method as claimed in any one of the preceding claims further comprising the step of tying the assembled strips together by passing at least one thread through the strips to join them together, the thread being parallel to one of the larger faces of the sheet-like structure.

10. A method as in claim 9 in which the thread is a plied yarn of twisted graphite.

11. A method as claimed in any one of the prececir.g claims wherein the sheet is predominantly formed or carbon fibre material.

₁2. A method as claimed in any one of the preceding claims further comprising the step of combining the sheet with a layer of fibrous alumina material before the cutting step.

13. A method of forming a cathode structure for a sodium sulphur cell comprising the steps of combining a sheet of electrically conductive fibrous material which has fibres predominantly parallel to one plane with a layer of fibrous alumina material, cutting the combined sheet into a plurality of strips, placing the strips into a holding device which maintain the strips under compression, threading at least one thread through the strips to join them together, transferring the joined strips under compression from the holding device to shaping means which shapes the combined material into side and base components, said components being sections of the required shape of the cathode structure, impregnating the shaped components with liquid sulphur or sulphide and maintaining compression until the sulphur or sulphide has solidified.

14. Apparatus for forming a cathode structure for a sodium sulphur cell comprising cutting means for cutting a sheet of electrically conductive fibrous material into strips, tying means for passing a thread through said strips to join them together injection moulding means for shaping the joined strips to form at least one component for a cathode structure and a holding device for compressing and transferring the strips from the cutting means to the tying means and from the tying means to the moulding means.

15. Apparatus as claimed in claim 14 further comprising feeding means for feeding the fibrous material to the cutting means in an indexing manner.

16. Apparatus as claimed in claim 15 in which the feeding means further comprise means for combining the fibrous alumina material with said sheet of fibrous material before cutting and feeding it to the cutting means in an indexing manner.

17. Apparatus as claimed in claims 15 or 16 i_n which the feeding means comprise at least one conveyor driven in an indexing manner. 18. Apparatus as claimed in claims 13 to 17 _n which the cutting means comprise a vertically movable cutting blade and a compression block also movable in a vertical direction, to enable the sheet of fibrous material to be compressed during cutting.

19. Apparatus as claimed in any one of claims ¹³ to

18 in which the cutting means further comprise an adjustable stop for determining the length of cut strips of fibrous material.

20. Apparatus as claimed in any one of claims 13 to

₁9 in which the cutting means further comprise a feed blade for pushing the cut strips into the holding device transversely to the plane of the sheet.

₂1. Apparatus as claimed in any one of claims 13 to

20 in which the tying means comprise at least one hollow needle carrying a continuous thread operable to pass through the strips held compressed in the holding device and to withdraw therefrom leaving the thread in position.

22. Apparatus as claimed in claim 21 in which the holding device has a pair of cutting blades operable to cut the thread at either end of a row of strips held therein after the needle has been withdrawn.

₂3. Apparatus as claimed in any one of claims 1 to 22 in which the holding device further comprises a shutter to open the holding device and pusher means operable to eject the sheet-like structure held therein.

₂₄. Apparatus as claimed in any one of claims 13 to 23 in which the moulding means comprises a cavity mould having a plurality of lands and valleys, the shape of which is such that on closure the mould shapes the joined strips into the required shapes.

25. Apparatus as claimed in claim 24 in which the moulding means further comprises means for injecting liquid sulphur or sulphide into the closed mould.

26. Apparatus as claimed in any one of claims 13 to 25 further comprising means for moving the holding device between the cutting, tying and moulding means and means locating the device relative thereto.

27. A cathode structure for a sodium sulphur cell formed by the method of any of claims 1 to 12.

28. A method of forming a cathode structure substantially as hereinabove described with relevance to or as shown in the accompanying drawings.

29. Apparatus for forming a cathode structure substantially as herein described with relevance to or as shown in the accompanying drawings.