GB1595767A

GB1595767A - Sodium-sulphur cells

Info

Publication number: GB1595767A
Application number: GB12764/80A
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 compries 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 from the sulphur electrode (according as to whether the cell is discharging or recharging) via the solid electrolyte. The sulphur electrode is a region, typicaly 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 eletcrode is inside the tube, then the current collector may be a rod axialy 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.

According to one aspect of the present invention, a sodium sulphur cell has a cathode electrode comprising a porous matrix containing sulphur/polysulphides wherein the matrix is formed of a helically wound strip of matrix material.

The invention also includes within its scope a method of forming a cathode structure for a sodium sulphur cell comprising helicaly winding a strip of porous matrix material to form an annular cathode structure, and wherein the matrix material is partially or completely impregnated with sulphur before assembly in a cell.

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 some parts than in 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. If on the other hand the resistance of the matrix material is high, the current will transfer to the lower resistance 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 much lower than that of the sodium polysulphide and also the transfer resistance is low. This means that, on recharge of a sodiumsulphur cell, the non-uniform 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 a preferred embodiment of the present invention, in a sodiumsulphur cell having a cathode electrode comprising a porous matrix containing sulphur/polysulphides, the matrix is formed of a helically-wound strip of matrix material and has 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 neighbourhood of the electrolyte material with the consequent production of the nonconductive sulphur. By decreasing the conductivity of the matrix material and/or 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 con ductivity, 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. It is thus possible to provide a graded electronic con ductivity and/or surface area per unit volume which decreases from the current collector towards the region of the electrolyte. 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.

It may, in some cases, be preferred to use concentric layers of matrix material having the required different properties, i.e. with electronic conductivity and/or surface area per unit volume which, for a central sul phur 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 matrix material helically wound to have concentric layers of matrix material having different electronic conductivity and/or surface area per unit volume. The separate layers may be of different materials and/or many be differentially compressed to obtain the required different properties. More than two layers may be employed.

In one form of construction a multilayered strip of matrix material may be wrapped around a current collector rod.

For example a multi-layered, e.g. a twolayered strip may be wrapped helically around the current collector rod.

In another construction the concentric layers of matrix may each comprise a helically wound strip of matrix material.

In the following description, reference will be made to the accompanying drawings in which: Figures 1 and 2 illustrate a method of forming a graded cathode matrix assembly for a sodium-sulphur cell.

Figures 1 and 2 illustrate the forming of a cathode structure for a sodium-sulphur cell. In this embodiment, the cell is of the central sulphur type and has an electrolyte tube typically formed of beta-alumina. The outside of this tube is covered with liquid sodium. The cathodic reactant which comprises sulphur and sodium polysulphides, is inside the electrolyte tube and there is an axially-located cathode current collector, for example a rod of metal with a protective impermeable sheath of graphite or molybdenum or other material chemically inert to the sulphur/polysulphide cathodic reactant material. The present invention is concerned more particularly with a porous matrix which is provided in the cathodic region.

The cathode structure, in this embodiment, is formed by wrapping graphite felt around the cathode current collector rod.

A method of grading the conductivity or surface activity is to form the electrode from wrapped layers of material as shown in Figures 1 and 2. Two or more layers of material 40, 41 are stacked together as shown in Figure 1 to form a strip and wrapped helically around the current collector 42 so that the material with the higher conductivity or surface activity, or both, is adjacent to the collector. The pitch of the helix is such that the current collector 42, at least in the region to be immersed in the cathodic reactant, is completely covered by the composite strip 40, 41. The assembly is then inserted in the electrolyte tube. The thickness of the composite strip 40, 41 is such that, when in position on the current collector and within the electrolyte tube, it fills or substantially fills the annular region between the current collector and electrolyte tube.

The assembly may be facilitated by impregnation of the components with sulphur at an intermediate stage.

As previously explained, by decreasing the conductivity and the surface area of the porous matrix towards the region of the electrolyte, the high reaction rate in that region is reduced.

In the above-described embodiment, 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.

This application is divided out of Application No. 37088/76 (Serial No. 1595764.

Also divided out of No. 37088/76 is Application No. 8012762 (Serial No. 1595765) which describes and claims a sodiumsulphur cell having a cathode electrode between a solid electrolyte and a cathode cvurrent collector, the cathode electrode comprising a porous matrix containing sulphur/polysulphides where in the matrix in the region adjacent the electrolyte comprises mixed conductive and non-conductive fibres and, in the region adjacent the current collector, comprises only conductive maetrial whereby the electronic conductivity of the matrix decreases from the current collector towards the region of the electrolyte.

Also divided out of Application No.

37088/76 is Application No. 8012763 (Serial No. 1595766) which describes and claims 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.

WHAT WE CLAIM IS: - 1. A sodium-sulphur cell having a cathode electrode comprising a porous matrix containing sulphur/polysulphides wherein the matrix is formed of a helically wound strip of matrix material.

2. A sodium-sulphur cell having a cathode electrode comprising a porous matrix containing sulphur/polysulphides wherein the matrix is formed of a helically wound strip of matrix material and 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 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 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 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 matrix material helically wound to have concentric layers of matrix material having different electronic conductivity and/or surface area per unit volume.

6. A sodium-sulphur cell as claimed in claim 5 wherein the concentric layers are differentially compressed.

7. A sodium-sulphur cell as claimed in claim 5 wherein the matrix comprises a multi-layered strip wrapped around a cur

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Claims

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molybdenum or other material chemically inert to the sulphur/polysulphide cathodic reactant material. The present invention is concerned more particularly with a porous matrix which is provided in the cathodic region.

The cathode structure, in this embodiment, is formed by wrapping graphite felt around the cathode current collector rod.

A method of grading the conductivity or surface activity is to form the electrode from wrapped layers of material as shown in Figures 1 and 2. Two or more layers of material 40, 41 are stacked together as shown in Figure 1 to form a strip and wrapped helically around the current collector 42 so that the material with the higher conductivity or surface activity, or both, is adjacent to the collector. The pitch of the helix is such that the current collector 42, at least in the region to be immersed in the cathodic reactant, is completely covered by the composite strip 40, 41. The assembly is then inserted in the electrolyte tube. The thickness of the composite strip 40, 41 is such that, when in position on the current collector and within the electrolyte tube, it fills or substantially fills the annular region between the current collector and electrolyte tube.

The assembly may be facilitated by impregnation of the components with sulphur at an intermediate stage.

As previously explained, by decreasing the conductivity and the surface area of the porous matrix towards the region of the electrolyte, the high reaction rate in that region is reduced.

In the above-described embodiment, 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.

This application is divided out of Application No. 37088/76 (Serial No. 1595764.

Also divided out of No. 37088/76 is Application No. 8012762 (Serial No. 1595765) which describes and claims a sodiumsulphur cell having a cathode electrode between a solid electrolyte and a cathode cvurrent collector, the cathode electrode comprising a porous matrix containing sulphur/polysulphides where in the matrix in the region adjacent the electrolyte comprises mixed conductive and non-conductive fibres and, in the region adjacent the current collector, comprises only conductive maetrial whereby the electronic conductivity of the matrix decreases from the current collector towards the region of the electrolyte.

Also divided out of Application No.

37088/76 is Application No. 8012763 (Serial No. 1595766) which describes and claims 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.

WHAT WE CLAIM IS: - 1. A sodium-sulphur cell having a cathode electrode comprising a porous matrix containing sulphur/polysulphides wherein the matrix is formed of a helically wound strip of matrix material.
2. A sodium-sulphur cell having a cathode electrode comprising a porous matrix containing sulphur/polysulphides wherein the matrix is formed of a helically wound strip of matrix material and 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 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 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 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 matrix material helically wound to have concentric layers of matrix material having different electronic conductivity and/or surface area per unit volume.
6. A sodium-sulphur cell as claimed in claim 5 wherein the concentric layers are differentially compressed.
7. A sodium-sulphur cell as claimed in claim 5 wherein the matrix comprises a multi-layered strip wrapped around a cur

rent collector rod.
8. A sodium-sulphur cell as claimed in claim 7 wherein the multi-layered strip is wrapped helically around the rod.
9. A sodium-sulphur cell as claimed in claim 5 wherein the concentric layers of the matrix each comprise a helically wound strip of matrix material.
10. A method of forming a cathode structure for a sodium-sulphur cell comprising helically winding a strip of porous matrix material to form an annular cathode structure, and wherein the matrix material is partially or completely impregnated with sulphur before assembly in a cell.
11. A method as claimed in claim 10 wherein the matrix material is carbon felt.
12. A method of forming a cathode structure for a sodium-sulphur cell substantially as hereinbefore described with reference to the accompanying drawings.
13. A cathode structure for a sodiumsulphur cell made by the method of any of claims 10 to 12.