GB1586659A

GB1586659A - Electrochemical cells

Info

Publication number: GB1586659A
Application number: GB39934/76A
Authority: GB
Original assignee: Chloride Silent Power Ltd
Current assignee: Chloride Silent Power Ltd
Priority date: 1977-09-29
Filing date: 1977-09-29
Publication date: 1981-03-25

Description

(54) IMPROVEMENTS IN OR RELATING TO ELECTRO CHEMICAL CELLS (71) We, CHLORIDE SILENT POWER LIMITED, a British Company, of 52 Grosvenor Gardens, London, SWiW 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 electrochemical cells of the kind having a solid electrolyte separating an alkali metal, constituting the anode, from a cathodic reactant. The invention is applicable more particularly but not exclusively to sodium-sulphur cells.

According to the present invention an electrochemical cell of the kind having a solid electrolyte separating alkali metal, which is liquid at the operating temperature, from a cathodic reactant, comprises a plurality of tubes (referred to hereinafter as electrolyte tubes) formed wholly or in part of said solid electrolyte, which tubes contain said cathodic reactant and are arranged parallel to one another in a tubular container, and an apertured closure plate sealed to the container at one end thereof and to each of the electrolyte tubes.

The electrolyte tubes may be closed at one end. At the open end, the tubes may each extend through a separate aperture in the closure plate and the tubes may be externally sealed around a current collector protruding from the tube. Alternatively the closure plate may extend across and be sealed to the open ends of each of the electrolyte tubes and sealed around current collectors which are arranged, one in each electrolyte tube, to protrude through apertures in the closure plate.

The electrolyte tubes conveniently however are open at both ends and a current collector in each electrolyte tube extends outwardly from the tube at each end, the electrolyte tubes being assembled in said outer container with apertured closure plates at both ends of the tubes, each closure plate being sealed to the outer container and to one end of each electrolyte tube. Each closure plate might be sealed around the electrolyte tubes, which thus have to be separately sealed externally of the container.

Preferably the closure plates each extend across the end of each of the electrolyte tubes to close those ends, apertures in the closure plates being sealed around the current collectors.

As previously indicated, the present invention has particular application to sodiumsulphur cells. In such a case, the sulphur polysulphide material forming a cathodic reactant is put inside the electrolyte tubes and the sodium around the outside of the tubes. These electrolyte tubes for sodiumsulphur cells have to be of a material permitting the flow of sodium ions and typically a beta-alumina ceramic material is employed.

It is readily possible to extend the length of beta-alumina electrolyte tubes by sealing to one or to each end a cylinder of alphaalumina. Such sealing may be readily effected with a glass seal. If the tubes are to extend through an end plate or end plates of the container, conveniently the electrolyte tubes have such alpha-alumina extensions and the end plate or each end plate may be sealed around the extended tube at an appropriate point along its length. In such an arrangement, each tube may be separately closed to seal the sulphur electrode within the tube, with a current collector extending through the tube seal or, if the tube is sealed at both ends, through at least one tube seal but preferably through both tube seals.

As previously explained however the closure plates may extend across the ends of the electrolyte tubes and be sealed around current collectors. Such an arrangement simplifies the construction. Since the whole length of the electrolyte tube in this case is within the outer container, it is preferably formed wholly of the electrolyte material, e.g. wholly of beta-alumina.

The current collectors have to be of a material which is resistant to attack by the hot sulphur/polysulphide material forming the cathodic reactant. Each current collector may be formed of impermeable carbon or graphite or of molybdenum or it may be a composite structure with a core of metal to form a good conductor and an outer protective sheath, e.g. of carbon or graphite.

It is preferred to use a double-ended current collector, that is to say a current collector extending out of both ends of the electrolyte tube since, assuming all other requirements are equal, this reduces the required crosssectional area of the current collector for a given electrical resistance by a factor of 4.

With the multitube cell arrangement described above, the required cross-sectional area is further reduced compared with a single tube cell of the same electrolyte diameter and of the same electrical capacity as the multitube cell. For example, if there are six electrolyte tubes in an outer container and double-ended current collectors are employed, the required cross-sectional area of the current collector is reduced by a factor of about 100 compared with that for a single tube cell of the same electrical capacity having a single ended current collector. Such a reduction in the cross-sectional area permits the use of metals such as molybdenum as the current collector.

It becomes possible to utilise molybdenum sheet which can be fabricated into a tube or more conveniently into an elongate element of C-shape in cross-section or molybdenum wires. Such a C-shaped molybdenum current collector may be secured, e.g. welded, at its ends to lengths of tube, for example Nilo tube (Nilo is a Registered Trade Mark), this tube forming the part of the current collector which extends through the seal of the container. Such a tubular extension to the current collector enables the sulphur to be introduced into the cell after sealing, molten sulphur being poured through the tube onto the current collector and passing outwardly through the lengthwise gap in the C-shaped molybdenum sheet to fill the region between the current collector and the electrolyte tube. This region may be filled in the known manner with a matrix material such as graphite felt.

In a sodium-sulphur cell having the sodium around the outside of the electrolyte tubes, the aforesaid outer container, which is in contact with the sodium, may be formed of metal and may be utilised as the anode current collector. It is preferred however in the present construction to form this outer container of ceramic material, e.g. alphaalumina. It is possible to use alpha-alumina closure plates and the alpha-alumina material may be selected to have a coefficient of thermal expansion matching that of the betaalumina. This enables simple glass seals to be employed.

If the outer container is of ceramic material, at least one anode current collector must be provided. This may be a metal element, e.g. a rod or tube passing through one or through both end closure plates.

The anode current collector may be of tubular form at its ends when it passes through the end closure plate or each end closure plate, being apertured or C-shaped in cross-section within the container where it extends into the sodium within the container. This tubular form enables the current collector to be used for putting molten sodium into the cell. Molten sodium is a good conductor and it is merely necessary to have a single tube making electrical contact therewith. Such a tube may be formed of a nickel-iron corrosion resistant alloy, for example of Nilo K, this tube preferably being coated with niobium to prevent attack where it is sealed in the closure member so enabling a glass seal to be used.

In a sodium-sulphur cell, it is convenient to make the aforementioned outer container of alpha-alumina. Such a container may be a circular-section cylinder or it may have flat sides; for example a cell to contain six electrolyte tubes might have a hexagonal outer container, preferably with rounded edges between the flat sides. It is readily possible to make the end closure or each end closure of a plate of alpha-alumina of the required shape, the plate being machined and drilled to form the required apertures for the current collectors, or electrolyte tubes if the latter are to pass through the closure plate. The alpha-alumina for the closure plates and for the casing are chosen to have the required coefficient of expansion corresponding to that of the electrolyte tubes.

Such an assembly may readily be sealed with the closure plate secured to the electrolyte tubes and to the outer casing, for example using barium alumino-borate glass. Butt seals to the ends of the electrolyte tubes may be used. Conveniently the end closure plate fits over the end surface of the alpha-alumina container and is sealed thereto with a butt joint using glass. All the necessary sealing operations may therefore be effected using glass in a single firing operation.

The invention furthermore includes within its scope a method of fabricating a sodiumsulphur cell comprising the steps of machining a number of beta-alumina tubes to the required length, machining an outer alphaalumina container to the required length and assembling a plurality of the beta-alumina electrolyte tubes parallel to one another in a single alpha-alumina outer container with tubular current collectors extending into each of the electrolyte tubes and into the container region between the electrolyte tubes and sealing the assembly at one end or at each end with an alpha-alumina end plate having apertures for the current collector tubes, the alpha-alumina plate being secured by glass to the end surfaces of the outer alphaalumina tube and the end surfaces of the electrolyte tubes and wherein sulphur electrode assemblies are put inside the electrolyte tubes with the current collectors before the tubes are put in the container or, at least, before the end plates are put in position.

As previously indicated, such end closures may be provided at both ends of the assembly enabling open-ended electrolyte tubes to be employed with the current collectors extending outwardly through the closure plate at each end. Wicking means may be provided around the electrolyte tubes before they are assembled in the outer container. Preferably the electrolyte tubes are proof-tested by applying an internal pressure before they are assembled in the container. The abovedescribed construction permits the sealing to be effected using only alpha-alumina and beta-alumina components apart from the current collectors. The alpha-alumina outer casing and end plates may be chosen to have the required coefficients of thermal expansion corresponding to those of the beta-alumina electrolyte tubes so as to avoid stress on temperature cycling.

After the electrolyte tubes and current collectors have been secured in position in the alpha-alumina container, an electric heater may be put around the container, for example a helical nickel-chromium alloy wire and the assembly then secured in an outer housing, for example an aluminium housing. Conveniently the end closure plates extend outwardly beyond the alpha-alumina tube and the housing may be secured onto the peripheral surfaces of these end plates.

After assembly and sealing, the cell can be filled with molten sodium and with molten sulphur through the tubular current collectors and then these tubes are closed after the cell is filled.

The following is a description of one embodiment of the invention, reference being made to the accompanying drawings, in which: Figure 1 is a transverse section through a multitube cell and Figure 2 is a part longitudinal section along the line 2-2 of Figure 1.

The cell shown in the drawings is a sodiumsulphur cell comprising, in this embodiment, six solid electrolyte tubes 10 formed of betaalumina. These tubes are open-ended and the two ends of the completed assembly may be similar. In this particular embodiment six such electrolyte tubes, after proof pressure testing to a hoop stress level of 20 MNm-2 or greater, are put within an outer alphaalumina container 11. This container is formed of an alpha-alumina selected to have a similar coefficient of expansion to that of the beta-alumina. The alpha-alumina and beta-alumina tubes can be machined to the required length prior to assembly. In operation the cell contains molten sodium which is arranged in the area outside the electrolyte tubes but within the alpha-alumina container 11. For effecting electrical connection to the sodium, there is a current collector 12 formed of a controlled expansion alloy such as Nilo K (Nilo is a Registered Trade Mark) tube which is coated externally with niobium to prevent attack when it passes through a glass seal to be described later. Two such current collectors may be provided one at each end of the assembly or a single current collector may extend through the assembly if connections at both ends of the assembly are required. Capillary means 14 are provided around each of the electrolyte tubes so as to ensure that the external surfaces of the tubes remain covered with sodium despite the fall in the sodium level within the container as the cell discharges.

Within the electrolyte tubes 10 are the sulphur electrodes which, in this embodiment, comprise, for each electrolyte tube, a central C-shaped molybdenum sheet current collector 15 together with an electronically conductive matrix material, for example carbon felt, 16 between the current collector and the inner surface of the electrolyte tube. The molybdenum sheet, at each end of the electrolyte tube, is electron beam-welded to a nickel-iron corrosion resistant alloy tube 18 (Figure 2) forming the external connection for the current collector and also constituting a filler tube by which molten sulphur can be introduced into the electrolyte tube after the assembly has been completed. The cathode current collectors may be coated, for example using a plasma spray technique, with a conductive coating material to give further protection against possible corrosion.

At each end, the cell is closed by means of an alpha-alumina plate 20 which has holes drilled through it to receive and locate the current collector tubes 12, 18 and which is sealed at 21 to the end surfaces of each of the electrolyte tubes 10 and at 22 to the end surface of the alpha-alumina tube 11 using barium alumino-borate glass (a sodiumresistant glass) seal. The current collector tubes are also sealed into the alpha-alumina plate 20 as shown at 23 using barium alumino-borate glass and the whole of the sealing operation can be effected in a single firing.

Heating of the cell to raise it to the operating temperature, typically 350"C, is effected by means of an electrical heater which, in this embodiment comprises a nichrome wire 24 wound as a helix around the alpha-alumina tube 11. An outer aluminium container 25 is provided around the outside of the assembly and is secured to the peripheral surfaces of the two end closure plates 20.

It will be seen that, using this construction, the assembly can readily and simply be effected. In assembling a cell, the betaalumina tubes, after machining to length and having the wick material put around their outside, are assembled, with the cathode current collectors and cathode electrode matrix material, in the outer container.

Then the end plates are put on with the anode current collector and are sealed in position using glass in a single firing operation which seals the end plates to the alphaalumina container 11, to the electrolyte tubes 10 and to the current collector tubes 16, 18.

The end closures 20 may be sealed, using glass seals, by passing the cell through a furnace and heating it from above so that the closures 20 are raised to the sealing temperature, typically 850"C, while the main body of the cell is retained at a temperature which is considerably less than this and preferably below 100 C. Fan cooling and appropriately disposed thermal insulation may be used to maintain this temperature difference.

After the sealing of the end plates, the heater element 24 can be put around the outside of the alpha-alumina container 11 and the assembly then put in the outer aluminium can 25. The sulphur electrodes can then be filled through the current collector tubes and the sodium electrode filled through the anode current collector. A vacuum filling technique may be employed for the sulphur electrodes or alternatively air can be extracted from the sulphur electrodes after filling. The current collector tubes can then be closed to complete the assembly. In some cases it may be preferred, before the end closure plates are fitted, to fill the sulphur electrodes using pre-forms of sulphur and the porous matrix made by injection moulding.

This construction, using a number of electrolyte tubes in a single container, enables a very rugged cell to be built having a high charge capacity. It enables this high capacity to be obtained using smaller electrolyte tubes than in a single tube cell of similar capacity.

This gives improved reliability as there is less risk of large fracture stresses developing in a small ceramic body than in a large one.

WHAT WE CLAIM IS: 1. An electrochemical cell of the kind having a solid electrolyte separating alkali metal, which is liquid at the operating temperature, from a cathodic reactant, said cell comprising a plurality of tubes formed wholly or in part of said solid electrolyte, which tubes contain said cathodic reactant and are arranged parallel to one another in a tubular container, and an apertured closure plate sealed to the container at one end thereof and to each of the electrolyte tubes.

2. A cell as claimed in claim 1 wherein the electrolyte tubes are closed at one end and wherein each electrolyte tube, at its open end, extends through a separate aperture in the closure plate, each tube being externally sealed around a current collector protruding from the tube.

3. A cell as claimed in claim 1 wherein the electrolyte tubes are closed at one end and wherein said closure plate extends across and is sealed to the open ends of each of the electrolyte tubes and are sealed around current collectors which are arranged, one in each electrolyte tube, to protrude through apertures in the closure plate.

4. A cell as claimed in claim 1 wherein the electrolyte tubes are open at both ends and wherein a current collector in each electrolyte tube extends outwardly from the tube at each end, the electrolyte tubes being assembled in said outer container with apertured closure plates at both ends of the tubes, each closure plate being sealed to the outer container and to one end of each electrolyte tube.

5. A cell as claimed in claim 4 wherein each closure plate is sealed around the electrolyte tubes.

6. A cell as claimed in claim 4 wherein each closure plate extends across the end of each of the electrolyte tubes to close those ends, apertures in the closure plates being sealed around the current collectors.

7. A cell as claimed in any of the preceding claims and having sulphur/polysulphide material forming the cathodic reactant inside the electrolyte tubes and sodium around the outside of the tubes to constitute thereby a sodium-sulphur cell.

8. A cell as claimed in claim 7 wherein the electrolyte is a beta-alumina ceramic material and wherein said tubes each comprise a beta-alumina electrolyte tube having sealed to one or to each end a cylinder of alpha-alumina.

9. A cell as claimed in claim 8 and having said tube extending through an end plate or end plates of the container, wherein the end plate or each end plate is sealed around said alpha-alumina cylinder forming an extension of the beta-alumina tube.

10. A cell as claimed in claim 9 wherein each tube is separately closed to seal the sulphur electrode within the tube and with a current collector extending through the tube seal.

11. A cell as claimed in claim 9 wherein each tube is sealed at both ends and wherein current collectors extend through one tube seal or through both tube seals.

12. A cell as claimed in claim 7 wherein the tubes are formed wholly of beta-alumina, with closure plates sealed to the tubes at each end thereof.

13. A cell as claimed in any of claims 7 to 12 wherein the current collectors are

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

Claims

**WARNING** start of CLMS field may overlap end of DESC **. the assembly can readily and simply be effected. In assembling a cell, the betaalumina tubes, after machining to length and having the wick material put around their outside, are assembled, with the cathode current collectors and cathode electrode matrix material, in the outer container. Then the end plates are put on with the anode current collector and are sealed in position using glass in a single firing operation which seals the end plates to the alphaalumina container 11, to the electrolyte tubes 10 and to the current collector tubes 16, 18. The end closures 20 may be sealed, using glass seals, by passing the cell through a furnace and heating it from above so that the closures 20 are raised to the sealing temperature, typically 850"C, while the main body of the cell is retained at a temperature which is considerably less than this and preferably below 100 C. Fan cooling and appropriately disposed thermal insulation may be used to maintain this temperature difference. After the sealing of the end plates, the heater element 24 can be put around the outside of the alpha-alumina container 11 and the assembly then put in the outer aluminium can 25. The sulphur electrodes can then be filled through the current collector tubes and the sodium electrode filled through the anode current collector. A vacuum filling technique may be employed for the sulphur electrodes or alternatively air can be extracted from the sulphur electrodes after filling. The current collector tubes can then be closed to complete the assembly. In some cases it may be preferred, before the end closure plates are fitted, to fill the sulphur electrodes using pre-forms of sulphur and the porous matrix made by injection moulding. This construction, using a number of electrolyte tubes in a single container, enables a very rugged cell to be built having a high charge capacity. It enables this high capacity to be obtained using smaller electrolyte tubes than in a single tube cell of similar capacity. This gives improved reliability as there is less risk of large fracture stresses developing in a small ceramic body than in a large one. WHAT WE CLAIM IS:

1. An electrochemical cell of the kind having a solid electrolyte separating alkali metal, which is liquid at the operating temperature, from a cathodic reactant, said cell comprising a plurality of tubes formed wholly or in part of said solid electrolyte, which tubes contain said cathodic reactant and are arranged parallel to one another in a tubular container, and an apertured closure plate sealed to the container at one end thereof and to each of the electrolyte tubes.

formed of impermeable carbon or graphite.

14. A cell as claimed in any of claims 7 to 12 wherein the current collectors are formed of molybdenum.

15. A cell as claimed in any of claims 7 to 12 wherein the current collectors are each a composite structure with a core of metal to form a good conductor and an outer protective sheath.

16. A cell as claimed in claim 14 wherein each current collector is a molybdenum sheet fabricated into a tube or into an elongate element of C-shape in cross-section.

17. A cell as claimed in claim 14 wherein the current collectors are formed of molybdenum wires.

18. A cell as claimed in any of claims 7 to 17 wherein said container is formed of metal and is utilised as the anode current collector.

19. A cell as claimed in any of claims 7 to 17 wherein said container is formed of ceramic material.

20. A cell as claimed in claim 19 wherein said container is formed of alpha-alumina and wherein alpha-alumina closure plates are provided at the ends of the container.

21. A cell as claimed in either claim 19 or claim 20 wherein at least one anode current collector is provided comprising a metal element passing through one or through both end closure plates.

22. A cell as claimed in claim 21 wherein the anode current collector is of tubular form and is apertured or of C-shaped section within the container where it extends into the sodium.

23. A cell as claimed in claim 20 wherein the or each end closure plate fits over the end surface of the alpha-alumina container and is sealed thereto with a butt joint using glass.

24. A method of fabricating a sodiumsulphur cell comprising the steps of machining a number of beta-alumina tubes to the required length, machining an outer alphaalumina container to the required length and assembling a plurality of the beta-alumina electrolyte tubes parallel to one another in a single alpha-alumina outer container with tubular current collectors extending into each of the electrolyte tubes and into the container region between the electrolyte tubes and sealing the assembly at one end or at each end with an alpha-alumina end plate having apertures for the current collector tubes, the alpha-alumina plate being secured by glass to the end surfaces of the outer alphaalumina tube and the end surfaces of the electrolyte tubes and wherein sulphur electrode assemblies are put inside the electrolyte tubes with the current collectors before the tubes are put in the container or, at least, before the end plates are put in position.

25. A method as claimed in claim 24 wherein end closures are provided at both ends of the assembly, the electrolyte tubes being open-ended with the current collectors extending outwardly through the closure plate at each end.

26. A method as claimed in either claim 24 or claim 25 wherein wicking means are provided around the electrolyte tubes before they are assembled in the outer container.

27. A sodium-sulphur cell substantially as hereinbefore described with reference to the accompanying drawings.

28. A method of fabricating a sodiumsulphur cell substantially as hereinbefore described with reference to the accompanying drawings.

29. A sodium-sulphur cell made by the method of any of claims 24 to 26 or claim 28.