GB1595766A

GB1595766A - Sodium-sulphur cells

Info

Publication number: GB1595766A
Application number: GB12763/80A
Authority: GB
Original assignee: Chloride Silent Power Ltd
Current assignee: Chloride Silent Power Ltd
Priority date: 1977-11-30
Filing date: 1977-11-30
Publication date: 1981-08-19

Description

(54) IMPROVEMENTS IN OR RELATING TO SODIUM-SULPHUR CELLS (71) We, CHLORIDE SILENT POWER LIMITED, a British Company, of 52 Grosvenor Gardens, London, SW1W OAU, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to sodium-sulphur cells and is concerned more particularly with the cathode structure of such a cell.

In a sodium-sulphur cell, solid electrolyte material, typically beta-alumina, separates an anode electrode comprising sodium, which is molten at the operating temperature of the cell, from the cathode electrode which comprises sulphur and polysulphides. During operation of the cell, electrons flow in an external circuit between the sodium electrode and a current collector which is connected to the sulphur electrode. Sodium ions flow to or frorn the sulphur electrode (according as to whether the cell is discharged of recharging) via the solid electrolyte. The sulphur electrode is a region, typically several millimetres thick, in which the current is progressively transformed from electronic current at the current collector to ionic current at the electrolyte surface.

Because of the poor electronic conductivity of the sulphur/polysulphides, it is the usual practice to pack the sulphur electrode region with a porous electronically conductive matrix, such as a carbon felt. The solid material of the felt typically occupies a few percent of the electrode volume. The rest of the volume is partly filled with molten sulphur when the cell is fully charged and almost completely filled with sodium polysulphide when the cell is discharged. The sulphur electrode region may be a rectangular prism bounded by a planar electrolyte and a planar current collector but more commonly it is an annular region between concentric cylinders. The electrolyte is commonly a tube. If the sulphur electrode is inside the tube, then the current collector may be a rod axially located within the electrolyte tube. If the sulphur electrode is on the outside of the electrolyte tube, then the current collector may be a cylinder, e.g.

an outer metallic housing, which is concentric with and around the electrolyte tube.

When the cell is operating, considering the conditions during re-charging, electronic current flows through the cathode current collector and is dispersed through the sulphur electrode by flowing through the fibres of the porous matrix. As the current passes through the electrode, it is gradually transferred from those fibres to the molten reactant (sulphur/sodium polysulphide). This transfer occurs by means of an electrochemical reaction which takes place at the surface of the fibres and which has a net effect of transferring the current from electrons to sodium ions. It is necessary that the transfer must be completed by the time the current reaches the electrolyte surface since only sodium ions will flow in the electrolyte.

The transfer will not, in general, occur at a uniform rate throughout the sulphur electrode but will be more rapid in sorne parts than others. The distribution of the reaction rate is determined by, amongst other things, the electrode thickness, the resistance of the porous matrix, the resistance of the molten polysulphides and the resistance involved in transferring current from one to the other.

Consider for example the case where the resistance of the fibres in the matrix material and the transfer resistance (referred to unit volume of felt) are low compared with the resistance of the polysulphides. The current will then flow mostly through the felt and can transfer, using only a small portion of the electrode, to the polysulphide close to the electrolyte surface, travelling only a short distance in the polysulphide. lf on the other hand the resistance of the matrix material is high, the current will transfer to the lower resistence polysulphides close to the current collector. If the transfer resistance is high, the reaction rate will be more uniformly distributed, compared with the case where the matrix resistance and transfer resistance are low, so that the maximum area of fibre surface of the matrix material can be used.

However considering the circumstances on discharge of the cell, increase of transfer resistance or of felt resistance increases the impedance of the cell and so reduces the power output.

In the case of sodium-sulphur cells, for the felt materials commonly used, the resistance of the porous matrix is such lower than that of the sodium polysulphide and also the transfer resistance is low. This means that, on recharge of a sodium-sulphur cell, the nonuniform distribution of transfer described above will occur with the reaction concentrated close to the electrolyte surface. The product of the reaction is sulphur which is an electrical insulator and which can therefore inhibit the recharge process in the rest of the electrode by preventing access of sodium ions to the electrolyte surface. If this occurs, the cell cannot be fully recharged and therefore will no longer have its full capacity on subsequent discharge.

According to one aspect of the present invention, a sodium-sulphur cell has a cathode current electrode between a solid electrolyte and a cathode current collector, the cathode structure comprising a porous graphite or carbon felt matrix containing sulphur/polysulphides, the part of the felt adjacent the current collector surface being more highly compressed than the part adjacent the electrolyte surface. The matrix may be formed to have a graded electronic conductivity and/or surface area per unit volume which decreases from the current collector towards the region of the electrolyte. The grading may be a change occurring gradually across the electrode region or it may occur in one or more steps.

As explained above, the inability to recharge a cell fully can arise from the high reaction rate in the immediate neighbour- hood of the electrolyte material with the consequent production of the non-conductive sulphur. By decreasing the conductivity of the matrix material andíor its surface area in the region of the electrolyte, the reaction rate in that region is decreased. It is desirable to maintain or even enhance the reaction rate (compared with a cell using a uniform matrix material) in the regions further away from the electrolyte, e.g. by increase of conductivity, to minimise the impedance penalty.

One convenient way of grading both the conductivity and the surface area per unit volume of the matrix is to use a matrix material which is packed more tightly in the region where the higher conductivity and surface area is required. In the case for example of a cell of cylindrical form and having a central cathode currant collector axially located within a cylindrical electrolyte tube with a cathodic reactant between the cathode current collector and the electrolyte tube, a convenient form of construction is to use washers of an electronically conductive matrix material chemically inert to the cathodic reactant, e.g. carbon or graphite felt, on the current collector, a plurality of washers of two or more different external diameters being provided, the washers being arranged so that some of them extend radially outwardly as far as the electrolyte tube and others only extend part of the way from the current collector towards the electrolyte tube to provide a graded electronic conductivity and/or surface area per unit volume which decreases from the current collector towards the region of the electrolyte. Preferably the washers are arranged to fit tightly around the current collector. If only two sizes of washer are used, conveniently these are put on alternately. The washers would normally be tightly packed along the length of the current collector. With this arrangement, the effect is to have a denser matrix in the immediate region of the current collector and a less dense matrix in the region of the electrolyte tube. If graphite felt is used for the matrix material, in the neighbourhood of the electrolyte, the felt material would, by its resilience, spring out and hence would extend substantially along the whole length of the electrolyte tube. It will readily be appreciated that the ratio of the diameters of the washers of different sizes may be chosen to give a desired variation in conductivity and surface activity appropriate to the electrode thickness and the cell application.

In the preceding paragraph, reference has been made more particularly to a central sulphur cell, i.e. a cell with the current collector located axially within an electrolyte tube. It will be immediately apparent that if the sulphur electrode is around the outside of a tubular electrolyte, then it may be formed in exactly the same way with annular washers around the electrolyte tube and, if desired, annular conductive elements e.g. graphite foil washers. In this case the washers would be made to extend inwardly from the current collector and would preferably be a tight fit on their external diameter to ensure that they are in contact with the current collector. The grading would, in this case, be arranged to decrease the conductivity and/or surface area radially inwardly towards the electrolyte.

Thus the invention furthermore includes within its scope a sodium-sulphur cell of cylindrical form and having a cylindrical electrolyte tube arranged within a cylindrical cathode current collector with a cathodic reactant in the annular region between the electrolyte tube and the current collector wherein an electronically conductive matrix of material chemically inert to the cathodic reactant is provided in the annular region between the electrolyte tube and current collector, which matrix comprises a plurality of washers of two or more different internal diameters but fitting closely within the current collector, the assernbly of washers being axially compressed to provide a graded elec tronic conductivity and/or surface area per unit volume which decreases from the current collector towards the region of the electrolyte.

The washers put on the current collector need not necessarily have the same intrinsic structure or be formed of the same material.

Thus washers of different sizes may be formed of different materials. For example the smaller washers may have a conventional felt whereas the larger washers or at least some of the larger washers may be comprised of a material containing broken fibres or adulterated fibres (that is to say comprising fibres partly of conducting material and partly of non-conducting material). By breaking the fibres or by incorporating nonconducting fibres, the matrix material may retain its wicking properties but it has reduced conductivity.

As explained above, it may in some cases be desirable to enhance the electronic conductivity in the neighbourhood of the current collector. In so far as the reduction in the density of the matrix material in the neighbourhood of the electrolyte leads to an increase in the cell impedance, it may be desirable to enhance the conductivity all the way across the region between the current collector and the electrolyte. Enhancement of electronic conductivity may readily be effected., in a construction such as has been described above with washers of matrix material on a current collector within an electrolyte tube, by providing elements of electronically conductive material between the washers of the matrix material. For example washers of graphite foil may be provided between felt matrix washers. The graphite foil washers would normally be very thin (in the axial direction of the cell) compared with the washers of matrix material. These graphite foil washers may extend completely across the electrode region from the current collector to the electrolyte if the conductance is to be increased all the way across that region or they may extend only part of the way from the current collector towards the electrolyte if the conductance is to be locallv increased. Such graphite foil washers, if they extend completely across the cathiodic region, may be perforated to permit of flow of the molten cathodic reactant material in the longitudinal direction of the cell. Instead of using graphite foil, other materials may be employed to increase the conductance, for example metal washers coated with a material such as molybdenum or carbon which is chemically inert to the cathodic reactant or graphite flakes may be provided between the washers of matrix material or elongate current conductors, for example metal current conductors or coated current conductors, may be provided between the washers of matrix material and extending in the direction between the current collector and the electrolyte.

In order to enhance the conductance, the axial spacing of graphite foil washers or other electronically conductive elements should preferably be made equal to or less than the electrode thickness, that is to say the distance between the current collector and the electrolyte. Thus preferably the axial thickness of the washers of matrix material is equal to or less than the radial distance between the cathode current collector and the electrolyte tube. The low resistance path to the current collector through the graphite foil thus ensures that the electrochemical activity is more evenly distributed through the matrix material. This provision of enhanced conductivity is particularly useful for thick electrodes. In thin electrodes, for example of two millimetres thickness or less, the main contribution to electrode impedance is the transfer resistance whereas, for thick electrodes, the resistance of the matrix material in practise is dominant while the transfer resistance is low. Thus for electrodes of the ordcr of 1 centimetre thickness and greater, it may be beneficial to increase the transfer resistance to induce more uniform activity while providing bulk conductors to reduce the electronic impedance.

Instead of using coaxial washers all fitting closely around the current collector, it may, in some cases, be preferred to use concentric layers of matrix material having the required different properties, i.e. with electronic conductivity andíor surface area per unit volume which, for a central sulphur cell, is lower in the outer layer than in the inner layer. More generally in a cell in which the electrolyte is a tube and in which the porous matrix is in an annular region between the cathode current collector and the electrolyte, the matrix may comprise concentriclayers of matrix material having different electronic conductivity and/or surface area per unit volume. The separate layers may be of different materials and are differentially compressed to obtain the required different properties. More than two layers may be employed. Each layer may be formed of coaxial washers. In one convenient form of construction for a cell having a central cathode current collector, a first set of washers of matrix material is put on the current collector, these washers all being the same size and another set is then put ill the annular region between the first set and the electrolyte tube. The required difference in properties is obtained by compressing the first set more than the second set. It may be more convenient to form the outer matrix layer or layers of semi-cylindrical elements which may extend for the whole of the length of the assembly. More generally one or more layers may be formed of segments of cylindrical matrix elements.

One method of assembling a cathode assembly for a sodium sulphur cell comprises the steps of assembling washers of matrix material on a current collector rod, the washers being either of two of more different outer diameters but fitting closely on the current collector rod or being of uniform diameter and having further matrix material put concentrically around the washers.

In any of these methods of construction, the matrix material may be wholly or partly impregnated with sulphur before assembly.

In the following description, reference will be made to the accompanying drawings in which: Figure 1 is part of a longitudinal section through a cathode structure of a sodiumsulphur cell; Figure 2 is a cross-section through the cathode region of another form of cell; and Figure 3 shows an alternative form of matrix elemcnt for use in the assembly of Figure 2.

Referring to Figure 1 there is shown diagrammatically part of the cathode structure of a sodium-sulphur cell of the central sulphur type and having an electrolyte tube 1 0, typically formed of beta-alumina. Only part of the length of the tube is shown. The outside of this tube is covered with liquid sodium (not shown). The cathodic reactant which comprises sulphur and sodium polysulphides, is inside the electrolyte tube and there is an axially-located cathode current collector 11, for example a rod of metal with protective impermeable sheath of graphite omolybdenum or other material chemically inert to the sulphuWpolysulphide cathodic reactant material. The present invention is concerned more particularly with a porous matrix which is provided in the cathodic region. In the construction of Figure 1, this matrix is formed by putting washers of graphite felt on the current collector rod 11. Two sizes of washer are used, smaller washers 12 and larger diameter washers 13, the two sizes of washers being arranged alternately on the current collector rod 11. All washers 12, 13 have an internal diameter such that they fit tightly on the current collector rod 11 so as to make good electrical contact therewith but only the larger washers 13 extend out as far as the electrolyte material 10. The washers are tightly packed along the length of the collector rod and are thus compressed along the rod but the resilience of the felt material causes the larger washers 13 to spring out where they extend beyond the smaller washers 12 and hence the felt material at the outer diameter, substantially covers the whole surface of the electrolyte tube 10. By this construction, the carbon felt material is more tightly packed around the current collector thereby giving a greater surface area of fibre per unit volume and hence promoting electrochemical transfer by increase of surface area. The increased density of the fibre material in this region also gives an increased electronic conductivity per unit volume.

The decrease in surface activity in the neighbourhood of the electrolyte material can be further enhanced by using a suitably different material for the larger washers, for example an intricsically more porous matrix or by using a material in which the fibres are of larger diameter and hence which gives a lower surface area per unit volume of material, for example a reticulated vitreous carbon foam material. Alternatively or additionally the transfer resistance might be increased in the neighbourhood of the electrolyte 10 by deliberately impairing the surface activity of the fibers of the larger washers, for example by a chemical surface treatment.

The larger washers 13 might be made of fibre material having shorter fibres than the smaller washers thereby reducing the electronic conductivity. This can be achieved by deliberately breaking the fibres of a felt material. Electronic conductivity of the larger washers 13 may be made lower by using a material partly of insulating fibres and partly of conducting fibres.

In the case, for example, of a cell having a central cathode current collector 20 axially located within a cylindrical electrolyte tube 21 the desired grading of the properties of the cathode matrix assembly can be achieved by using concentric washers of felt as shown in Figure 2. The washers may be of two or more sizes: for example, one set of washers 22 may fit closely on the current collector, but do not extend to the electrolyie surface, while another set 23 may fill the region between the first set 22 and the electrolyte surface. The density of the first set 22 may be greater than that of the second, outer, set of washers. This density gradation may be obtained by rnaking the two sets of washers from the same felt material but by inserting more of the smaller diameter washers in a given length of electrode. This would have the effect of increasing both the electrical conductivity and the surface activity of the felt in the regions remote from the electrolyte surface. The two, or more, sets of washers may have a different intrinsic structure: for example, the diameter of the fibres making up the felt in one set of washers may be greater than that in the other set, so that the surface activity may be varied independently of the conductivity. In yet another arrangement the fibres in one set of washers may be broken, or adulterated with insulating fibres, or both.

Such an assembly of concentric washers may be difficult to construct from the fibrous materials commonly used at present, and construction may be facilitated by partial or complete impregnation of the materials with sulphur before assembly. Thus the outer annulus of Figure 2 may be prefabricated as a half-cylinder as shown at 26 in Figure 3 and impregnated with molten sulphur by vacuum impregnation or injection moulding. When the sullhur is cooled, the outer annulus may be assembled from two such half-cylinders, which are rigid and may be easily handled.

The outer diameter of the half-cylinders is such as to give a close fit within the electrolyte tube 31. The half-cylinders may be made from a stack of washers or, more simply, by forming a layer of felt. With the outer layer so formed and assembled within the electrolyte tube, the inner set of washers can be inserted in the inner region to the required density. A second sulphur filling may then be carried out to impregnate the inner region, but in certain cases this is not necessary; that volume may serve as the expansion volume.

It is understood that the pre-impregnation could equally be carried out for the inner felt layer or for both regions.

In all the above-described embodiments, the cathodic reactant is contained within the electrolyte tube. The sodium forming the anode would be around the outside of the electrolyte tube. Such cells are referred to as central sulphur cells. The invention is equally applicable to central sodium cells in which the cathodic reactant is in an annular region between the outer surface of an electrolyte tube and the inner surface of a surrounding tubular cathodic current collector. If the washers of differing diameters are employed, these would be made to fit closely within the current collector, i.e. they would have differing internal diameters. It will be immediately apparent that constructions analogous to those of Figures 1 to 7 may be employed with central sodium cells.

In Application No. 37088/76 (Serial No. 1 595 764) out of which the present application is divided, there is described and claimed a sodium-sulphur cell of cylindrical form having an annular cathode electrode between a surface of a cylindrical electrolyte tube and a cathode current collector, comprising a porous matrix containing sulphur/polysulphides wherein the matrix is formed of a plurality of annular elements of each of at least two different sizes differing in radial dimensions, each of the elements of at least one size but not all the sizes having a cylindrical surface adjacent the electrolyte, and the matrix of elements being formed to have a graded electronic conductivity and/or surface area per unit volume which decreases from the current collector towards the region of the electrolyte.

WHAT WE CLAIM IS: 1. A sodium-sulphur cell having a cathode electrode between a solid electrolyte and a cathode current collector, the cathode electrode comprising a porous graphite or carbon felt matrix containing sulphur/polysulphides, the part of the felt adjacent the current collector surface being more highly compressed than the part adjacent the electrolyte surface.

2. A sodium sulphur cell as claimed in claim 1 wherein the compression is such that the matrix has a graded electronic conductivity and/or surface area per unit volume which decreases from the current collector towards the region of the electrolyte.

3. A sodium-sulphur cell as claimed in either claim 1 or claim 2 wherein the conductivity and/or surface area per unit volume changes gradually across the electrode region.

4. A sodium-sulphur cell as claimed in either claim 1 or or claim 2 wherein the conductivity and/or surface area per unit volume changes in one or more steps across the electrode region.

5. A sodium-sulphur cell of cylindrical form and having a central cathode current collector axially located within a cylindrical electrolyte tube with a cathodic reactant between the cathode current collector and the electrolyte tube wherein washers of an electronically conductive matrix material chemically inert to the cathodic reactant are provided on the current collector, a plurality of washers of two or more different external diameters being provided, the washers being arranged so that some of them extend radially outwardly as far as the electrolyte tube and others only extend part of the way from the current collector towards the electrolyte tube, and the assembly of washers being axially compressed to provide a graded electronic conductivity and/or surface area per unit volume which decreases from the current collector towards the region of the electrolyte.

6. A sodium-sulphur cell as claimed in claim 5 wherein all the washers fit tightly around the current collector.

7. A sodium-sulphur cell of cylindrical form and having a cylindrical electrolyte tube arranged within a cylindrical cathode current collector with a cathodic reactant in the annular region between the electrolyte tube and the current collector wherein an electronically conductive matrix of material chemically inert to the cathodic reactant is provided in the annular region between the electrolyte tube and current collector, which matrix comprises a plurality of washers of two or more different internal diameters but fitting closely within the current collector, the assembly of washers being axially compressed to provide a graded electronic conductivity and/or surface area per unit volume which decreases from the current collector towards the region of the electrolyte.

8. A sodium-sulphur cell as claimed in either claim 5 or claim 6 wherein at least

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. sulphur before assembly. Thus the outer annulus of Figure 2 may be prefabricated as a half-cylinder as shown at 26 in Figure 3 and impregnated with molten sulphur by vacuum impregnation or injection moulding. When the sullhur is cooled, the outer annulus may be assembled from two such half-cylinders, which are rigid and may be easily handled. The outer diameter of the half-cylinders is such as to give a close fit within the electrolyte tube 31. The half-cylinders may be made from a stack of washers or, more simply, by forming a layer of felt. With the outer layer so formed and assembled within the electrolyte tube, the inner set of washers can be inserted in the inner region to the required density. A second sulphur filling may then be carried out to impregnate the inner region, but in certain cases this is not necessary; that volume may serve as the expansion volume. It is understood that the pre-impregnation could equally be carried out for the inner felt layer or for both regions. In all the above-described embodiments, the cathodic reactant is contained within the electrolyte tube. The sodium forming the anode would be around the outside of the electrolyte tube. Such cells are referred to as central sulphur cells. The invention is equally applicable to central sodium cells in which the cathodic reactant is in an annular region between the outer surface of an electrolyte tube and the inner surface of a surrounding tubular cathodic current collector. If the washers of differing diameters are employed, these would be made to fit closely within the current collector, i.e. they would have differing internal diameters. It will be immediately apparent that constructions analogous to those of Figures 1 to 7 may be employed with central sodium cells. In Application No. 37088/76 (Serial No. 1 595 764) out of which the present application is divided, there is described and claimed a sodium-sulphur cell of cylindrical form having an annular cathode electrode between a surface of a cylindrical electrolyte tube and a cathode current collector, comprising a porous matrix containing sulphur/polysulphides wherein the matrix is formed of a plurality of annular elements of each of at least two different sizes differing in radial dimensions, each of the elements of at least one size but not all the sizes having a cylindrical surface adjacent the electrolyte, and the matrix of elements being formed to have a graded electronic conductivity and/or surface area per unit volume which decreases from the current collector towards the region of the electrolyte. WHAT WE CLAIM IS:

1. A sodium-sulphur cell having a cathode electrode between a solid electrolyte and a cathode current collector, the cathode electrode comprising a porous graphite or carbon felt matrix containing sulphur/polysulphides, the part of the felt adjacent the current collector surface being more highly compressed than the part adjacent the electrolyte surface.

some of the washers are formed of carbon or graphite felt.

9. A sodium-sulphur cell as claimed in any of claims 5 to 8 wherein two sizes of washer are used, these being arranged alternately along the length of the current collector.

10. A sodium-sulphur cell as claimed in any of claims 5 to 9 wherein the washers are tightly packed along the length of the current collector.

11. A sodium-sulphur cell as claimed in claim 10 and having graphite felt matrix material, wherein the washers are so arranged that, in the neighbourhood of the electrolyte, the felt material, by its resilience, springs out to extend substantially along the whole length of the electrolyte tube.

12. A sodium-sulphur cell as claimed in any of claims 5 to 11 wherein washers of different sizes are formed of different materials.

13. A sodium-sulphur cell as claimed in claim 12 wherein the larger washers or at least some larger washers are formed partly of non-conductive material.

14. A sodium-sulphur cell as claimed in claim 13 wherein at least some larger washers are formed of mixed conductive and non-conductive fibres.

15. A sodium-sulphur cell as claimed in any of claims 5 to 14 wherein washers of different sizes are formed of materials of differing intrinsic structure.

16. A sodium-sulphur cell as claimed in any of claims 5 to 15 wherein elements of electronically conductive material are provided between washers of the matrix material.

17. A sodium-sulphur cell as claimed in claim 16 wherein said elements comprise washers of graphite foil between matrix washers, the graphite foil washers being thin compared with the felt matrix washers.

18. A sodium-sulphur cell as claimed in either claim 16 or claim 17 wherein the axial thickness of the washers of matrix material is equal to or less than the radial distance between the cathode current collector and the electrolyte tube.

19. A sodium-sulphur cell as claimed in claim 1 wherein the electrolyte is a tube and wherein the porous matrix is in an annular region between the cathode current collector and the electrolyte tube, the matrix comprising concentric layers of matrix material having different electronic conductivity and/or surface area per unit volume and wherein the concentric layers are differentially compressed, the part adjacent the current collector surface being more highly compressed than the part adjacent the electrolyte surface.

26. A sodium-sulphur cell as claimed in claim 19 wherein each of said concentric layers is formed of coaxial washers.

21. A sodium-sulphur cell as claimed in claim 19 wherein one or more of said concentric layers is formed of segments of cylindrical matrix elements.