WO1999066569A1

WO1999066569A1 - Electrochemical cell

Info

Publication number: WO1999066569A1
Application number: PCT/IB1999/001111
Authority: WO
Inventors: Bernd Wegner
Original assignee: Bi-Patent Holding S.A.; Van Der Walt, Louis, Stephanus
Priority date: 1998-06-15
Filing date: 1999-06-15
Publication date: 1999-12-23
Also published as: AU4054699A

Abstract

A high temperature rechargeable electrochemical cell includes a housing containing an anode and a cathode, the housing having an interior divided by a solid electrolyte separator into an anode compartment and a cathode compartment. The cell has a charged state in which the anode includes an alkali metal or alkali metal alloy, and an operating temperature at which the anode is molten. The separator comprises a conductor of alkali metal ions, and the cathode comprises, at said operating temperature and in said charged state, an electronically conductive porous electrolyte-permeable matrix having a porous interior impregnated with a molten salt electrolyte. The housing is in the form of a polygonal metal canister having a closed off lower end and an open upper end which is welded to a header, i.e. a polygonal outer metal collar (100) sealing member which is sealed in a first sealing zone to an electrically insulating collar (181), which insulating collar in turn is sealed in a second sealing zone to an inner metal collar (102) sealing member in electrical contact with a current collector for an electrode of the cell. The metal collar materials are selected from the group consisting of nickel and nickel-alloys. Said electrochemical cell comprises, in combination: first and second coaxial radially outwardly facing truncated conical sealing zones (106) on the insulating collar joined by active brazing (116, 112) to radially inwardly facing coaxial truncated conical sealing zones (112) respectively of the inner and outer metal collars; an active braze comprising nickel, niobium and titanium alloying components being used to seal the collars in place.

Description

ELECTROCHEMICAL CELL

THIS INVENTION relates to electrochemical cells. More particularly, the invention relates to seals for electrochemical cells. Still more particularly, the invention relates to metal-ceramic seals and headers for rechargeable high temperature electrochemical cells having molten alkali metal anodes and ceramic solid electrolyte separators, and to precursors of such cells.

According to one aspect of the invention, there is provided, a high temperature rechargeable electrochemical cell which includes a housing containing an anode and a cathode, the housing having an interior divided by a solid electrolyte separator into an anode compartment and a cathode compartment, the anode compartment and the cathode compartment containing respectively the anode and the cathode, the cell having a charged state in which the anode includes an alkali metal or alkali metal alloy, and the cell having an operating temperature at which the anode is molten, the separator comprising a conductor of alkali metal ions, and the cathode comprising, at said operating temperature and in said charged state, an electronically conductive porous electrolyte-permeable matrix having a porous interior impregnated with a molten salt electrolyte, the matrix containing, dispersed in its porous interior, active cathode material, and the housing being in the form of a polygonal metal canister having a closed off lower end and an open upper end which is welded to a polygonal outer metal collar sealing member which is sealed in a first sealing zone to an electrically insulating collar, which insulating collar in turn is sealed in a second sealing zone to an inner metal collar sealing member in electrical contact with a current collector for an electrode of the cell, the metal collar materials being selected from the group consisting of nickel and nickel-alloys and said electrochemical cell comprising, in combination: first and second coaxial radially outwardly facing truncated conical sealing zones on the insulating collar joined by active brazing to radially inwardly facing coaxial truncated conical sealing zones respectively of the inner and outer metal collars; the inner and outer metal collars having their mass distributed in rotationally symmetrical fashion around the cone axes of their sealing zones when sealing is effected; and an active braze comprising nickel, niobium and titanium alloying components being used to seal the collars in place.

According to another aspect of the invention, there is provided a header for sealing a polygonal metal canister of a high temperature rechargeable electrochemical cell, the header including an insulating collar having two radially outwardly facing concentric truncated conical sealing zones for press-fitting and joining to metal collars, the insulating collar also having a sealing zone for joining to a solid electrolyte separator; an outer metal collar having a perimeter which has the same polygonal outline as the canister and which is press-fitted into the canister opening before being welded into the canister opening, the outer metal collar having an upper part consisting of a number of lobes obtained by folding and deep-drawing a circular disk of a metal alloy to give said disk the polygonal outline of the canister while retaining the rotationally symmetrical mass distribution of the disk, and the outer metal collar having a lower part comprising a cylindrical portion, said lower part being joined to said insulating collar at a radially outer and wider of its said conical sealing zones by active brazing; an inner metal collar having an essentially cylindrical or truncated conical port or portion joined to said insulating collar at an inner and narrower of its said conical sealing zones; and a metal closure device or lid joined to said inner metal collar and provided with a closable filling opening for admitting reactants or other cell materials into the cell interior.

According to still another aspect of the invention, there is provided a method of sealing a high temperature rechargeable cell, the method including making an outer metal collar by folding and deep-drawing a metal disk to form a plane having a polygonal outline and to form lobes which project essentially at right angles to said plane from one side of said plane, one lobe at each of the edges of said plane; perforating the polygonal plane to form a circular hole in said plane; deep-drawing the periphery of said hole to form a collar protruding away from or towards the direction in which the lobes project; folding the lobes radially inwardly over the polygonal plane; and back-folding radially inner folded parts of said lobes to be essentially at right angles to the plane; making an inner metal collar having a cylindrical or truncated conical part or portion; making an insulating collar having two concentrictruncated conical radially outwardly directed sealing faces respectively dimensioned to nest respectively inside at least part of the outer metal collar and the truncated conical part or portion of the inner metal collar, the insulating collar having a sealing face for sealing to the solid electrolyte separator; press-fitting at least part of each metal collar to the associated concentric conical sealing face of the insulating collar to form seals respectively between the metal collars and the insulating collar; joining the metal collars to the insulating collar by active brazing with an active braze based on nickel, niobium and titanium alloy components at the press-fitted seals between the metal collars and the insulating collar to form a header; joining the header by glass welding to the solid electrolyte ceramic separator by means of said sealing face provided for the separator on the insulating collar; and welding a metal canister forming a cell housing to the polygonal, diametrically widest part of the outer metal collar.

Preferably, the collar formed around the hole in the polygonal plane protrudes away from, i.e. opposite to, the direction in which the lobes project.

Usually, the material of the solid electrolyte separator will be a ceramic comprising at least one member of the ?-alumina class of compounds, and the insulating collar may be σ-alumina, which may be modified to have a coefficient of expansion which approaches as closely as possible the coefficient of expansion of nickel/iron or nickel/iron/cobalt alloys suitable for metal- ceramic or metal-glass joining. Such alloys are commercially available and examples include alloys of the Vacovit , Vacon ^M and Dilver ^M series. Alternatively to said alloys, thin nickel collars may be used .

Suitable glasses for glassing the insulating member to the solid electrolyte are also known and include commercially available glasses such as Schott glass No. 8245, or Corning glass No. 7059.

The active braze material may have alloy components based on nickel, niobium and titanium, and optionally iron; the preferred composition is the subject of a co-pending application and is characterized by a titanium content of less than 1 0% and at least 3% by mass, and a niobium content in a proportion of 1 0% to 70% by mass relative to the combined nickel and niobium contents.

In rechargeable alkali metal anode high temperature electrochemical cells having alkali metal anodes and solid electrolyte separators, seals are necessary for: mechanical separation of cathode space, anode space and the ambient environment; electrical feedthroughs from cathode space and/or anode space to cell terminals outside the cell; joining of the solid ceramic electrolyte to the cell canister or housing; and/or providing a sealable filling opening for loading active electrode masses or precursors thereof into the anode space and/or into the cathode space.

Cells of the type described above having liquid sodium as the alkali metal anode and molten sodium aluminium chloride as the molten salt metal electrolyte have become known as ZEBRA cells. As with the related sodium/sulphur cells, the solid electrolyte separator is usually a /?-alumina type of ceramic, usually in the shape of a tube closed off at one end, the other end being open and joined to a ceramic insulating collar, usually of α-alumina, to which in turn are joined both an outer- and an inner metal collar. The artefact (alumina insulating collar together with the two metal collars) is known as a header in the art.

Both metal collars are, after joining thereof to the insulating member, welded or brazed, the outer collar to a metal canister serving as a cell housing, and the inner metal collar to a current collector extending into the separator tube and in contact with the active electrode mass contained therein, the inner metal collar being welded or brazed shut to close the separator tube and hence to close the cell, after assembly of the cell.

The joint between the solid electrolyte and the ceramic insulating collar or header is usually made by glassing or glass welding after making the header, suitable glass compositions being known in the art. However, the required metal- ceramic seals are difficult to make reliably by glassing. Seals in high temperature cells must withstand both thermal cycling and corrosive media, which in the case of ZEBRA-type cells are liquid sodium and the sodium aluminium chloride melt, including additives which may comprise sulphur or sulphur compounds. Thus, thermocompression bonded (TCB) seals have usually been used in such cells and their precursors, eg as described in US 5 009 357.

The TCB seals must be designed for bi-directional pressure application during bonding at about 1000°C. To compensate for the mechanical stresses following cooling down to ambient temperature, only thin-walled soft nickel is used for the metal collar(s) employed in the headers. If a cell canister of mild steel is used, the quality of a direct weld of the outer nickel collar to said canister may suffer from embrittlement owing to formation of a brittle alloy in the weld .

Another problem involves the need to provide terminals for electrical connection, since TCB seals must have flat joining areas on both upper and lower faces thereof. Therefore, the space for filling openings, for feedthroughs and for electrical connections is restricted, because a significant space is taken up by the TCB seal.

The invention provides a simpler header design by substituting active brazing for thermocompression bonding . A novel active braze particularly suited for this is the subject of a co-pending application.

The invention also provides a header design which reduces the particular difficulties associated with using a polygonal, eg square, cell canister. The header design provides the required square outline of the metal collar by folding and/or deep drawing of a circular metal disk, thus conserving the rotational symmetry of mass distribution of the metal and avoiding stresses resulting from uneven metal mass distribution. The use of thin nickel for the collars is no longer required, although it can still be used, if desired .

The above use of the terms sealing face and sealing zone does not mean that the active braze is required completely to fill the interfacial space arising from press-fitting the metal collars to the respective truncated conical sealing zones of the insulating collar. Each press-fit, which may be controlled so as not to exceed the elastic limit of the metal, is intended to centre and seat the metal collars firmly on the ceramic insulating collar, thereby promoting narrow gaps between these components of the header. Subsequent active brazing after press-fitting may completely fill or partially fill any gap remaining or generated by thermal expansion at the interfaces between these components, but it is sufficient that complete active braze seals are achieved in annular zones bordering the ceramic insulating collar and bordering the respective metal collars.

Press-fitting is meant to include any method of generating a mechanical force having a compressive component acting on the interface between a said metal collar and the insulating collar, thus including, for example, shrink-fitting a heated metal collar to the insulating collar. To increase the press-fitted contact area, the cylindrical parts of the metal collars may be given conical shapes at the contact areas, either by deep-drawing or by the press-fitting itself.

The lobes generated on the outer and wider metal collar may be used as tabs for brazing or welding single or multiple cell connectors thereon. Each lobe may have at least one connector, eg a wire, brazed, soldered or welded thereon. This permits the possibility of connecting each lobe to individual current collectors of opposite polarity in another cell or cells, which current collectors extend through the inner collar of each said other cell into the interior of that cell, and which current collectors are welded, brazed or soldered sealingly to said inner collar or conveniently to a lid joined thereto. In a simple example, these individual current collectors may be in the form of wires brazed to the lobes of the outer collar of a first cell and led through feedthrough openings in a lid in the inner collar of a second cell into the interior of said second cell, serving as current collectors therein for the associated electrode. The design of the outer collar may also be used advantageously in cells wherein a TCB seal is used to seal said metal collar to the insulating collar. A cylindrical or conical die is conveniently used in the final backfolding of the outer collar lobes, giving middle portions of said lobes an arcuate shape and locating the arcuate middle portions equidistant from one another and closer to the inner metal collar than to the corners of the polygon formed by the widest (equatorial) part of the outer metal collar. This shape facilitates welding of said widest part to the cell canister and reduces the risk of accidental short circuits.

Welding of the outer collar to join said collar to the cell canister may be in a top-down direction, eg by laser welding, and is normally performed after glassing the solid electrolyte separator to the insulating collar. The insulating collar and/or the metal collars may be shaped to provide annular zones for application of the braze which adjoin the conical sealing zones. In the sealing zones, heating for brazing can create narrow gaps between ceramic and metal collars by differential thermal expansion of ceramic and metal, and said gaps can be filled by capillary action from said annular application zones during brazing . The annular application zones may be parallel to the polygonal plane of the disk from which the outer metal collar is formed, or they may be inclined inwardly to form an annular groove for receiving the active braze material prior to brazing . In a preferred design, the application zone for the brazing material for the conical portion of the outer and wider metal collar is radially outward of said conical portion of the first outer and wider metal collar, and the application zone for the brazing material for the conical portion of the inner metal collar is radially inwardly of the conical portion of the inner metal collar.

After active brazing and sealing of the canister to the outer metal collar, the lobes of said metal collar may be altered by cutting, drilling or welding to provide the final cell design. Furthermore the requirement of even and symmetrical metal mass distribution around the cell central axis may be violated to a degree permitted by the stresses arising from, and tolerated by, the thermal management of the cell. The invention will now be described, by way of example, with reference to the accompanying diagrammatic drawings, in which:

Figure 1 shows a schematic sectional side elevation of a prior art high temperature electrochemical cell provided with a header comprising thermocompression-bonded seals to illustrate the prior art and to illustrate general cell design of alkali metal/transition metal halide-type (ZEBRA) cells;

Figure 2 shows in plan view a metal disk for making an outer collar for a metal/ceramic header according to the present invention, the disk being subjected to initial shaping steps; Figure 3 shows a three-dimensional view of the outer collar before deep drawing and resulting from the steps illustrated in Figure 2;

Figures 4 - 6 show in schematic sectional side elevation successive steps in the further shaping of the outer collar of Figures 2 and 3;

Figure 7 shows in schematic sectional side elevation a header in accordance with the present invention with two metal collars joined to a solid electrolyte separator;

Figure 8 shows in schematic sectional side elevation a variation of the insulating collar of the header of Figure 7;

Figure 9 shows in schematic sectional side elevation another variation of the insulating collar of the header of Figure 7 and the preferred mode of application of active braze material;

Figure 1 0 shows in plan view the upper ends of two cells according to the invention interconnected together by means of cell connectors; and

Figure 1 1 shows a three-dimensional view of the upper end of a cell according to the invention.

In Figure 1 of the drawings a cell of the alkali metal/transition metal halide (ZEBRA) type having thermocompression-bonded seals is generally designated 1 0. The cell 10 has a casing 1 2 of square cross-section made of mild steel containing, centrally suspended therein, a sodium /?-alumina separator tube 1 4. The tube 14 is glass-welded at 1 6 to an σ-alumina insulating collar 1 8. A pressed nickel lid 20 for the casing 1 2 is sealingly thermocompression bonded to the upper surface of the collar 1 8 at 22, the lid 20 having an upstanding rim 24 welded into the open end of the casing 1 2. A nickel tube 26 having a radially projecting circumferentially extending flange 28 is sealingly thermocompression bonded to the collar 1 8 via the flange 28 to the lower surface of the collar 1 8 at 32. The upper edge of the tube 26 stands proud of the upper surface of the collar 1 8 at 32. The upper edge of the tube 26 stands proud of the upper surface of the collar 1 8 and is separated from the inner edge of the lid 20 by an insulating spaced at 34. A cup-shaped nickel pressing 36 is welded sealingly on to the upper edge of the tube 26. The pressing 36 has a cental upstanding filler tube 38 whose upper end 40 (seen edge-on in the drawing) is crimped closed so that it is chisel-shaped, and is welded into a slot in a square mild steel cathode terminal plate 42 having upstanding rims 44 at its edges.

The cell 1 0 includes a nickel cathode current collector 46 comprising two elongate current collector sections 48, 50, joined together and defining between them, a hollow or cavity (not shown) . Wicking material 56, in the form of carbon felt, is provided between the portions 48, 50. The wicking material 56 is exposed along its length to the interior of the separator tube 1 4. Each of the sections 48, 50 has, at its upper end, a limb 58 by means of which it is attached, by welding, to a floor 60 of the pressing 26. Thus, the current collector 46 is suspended by the pressing 26. The current collector 46 extends downwardly from the pressing 26 to a lower end (not shown) spaced closely above the lower end of the separator tube 1 4, which lower end is closed .

The casing 1 8 is provided with a mild steel anode terminal 70, welded to the lid 20. A cathode (not shown) comprising a porous, electrolyte-permeable electronically conductive matrix having a transition metal halide active cathode material evenly dispersed therein and impregnated with a molten sodium aluminium chloride salt electrolyte is located in the tube 1 4. The molten salt electrolyte fills the tube 1 4 up to the level 74 and molten sodium (not shown) anode material fills the space between the tube 1 4 and casing 1 2 up to the level 76. Referring now to Figure 2, 78 represents in plan view a circular metal disk of metal starting material for fabrication of an outer collar 80 as shown in Figure 6. The disk is folded and deep-drawn in a combined folding and deep-drawing step to yield a flat-bottomed shallow dish having a square floor and four upstanding lobes as side walls. A central hole is punched into the floor, either subsequent to folding and deep-drawing or simultaneously therewith, thus yielding the component shown in Figure 3.

In Figure 3 the floor is shown at 82 and the upstanding side walls at 84, with the central hole at 86. Numerals 82 - 86 are used to indicate the equivalent parts of the disk 78 of Figure 2.

Figures 4 - 6 shows in schematic side elevation the steps of :

(a) expanding the periphery 88 of the hole 86 (shown in Figure 3) outwardly and downwardly in a direction away from the lobes 84 in the direction of the arrows 90, and creating a lower tubular truncated-conical part 92 of the outer collar 80 by deep-drawing;

(b) folding the lobes 84 inwardly in the direction of arrows 94; and

(c) back-folding the radially innermost parts 96 of said lobes 84 to leave a rim zone 98 which is square in plan view and forms an outermost part of the outer collar, suitable for welding; thus completing the outer metal collar 80 of the header, the collar 80 being, in terms of function, a combination of the lid 20 and the terminal 70 of Figure 1

Figure 7 shows in schematic side elevation the lower conical part 92 (cf 92 of Figures 4- 6) of the outer metal collar 80 joined to an σ-alumina ceramic insulating collar 1 8 by active brazing along a first outer and wider radially outwardly directed conical sealing zone 1 00 on the collar 1 8. An inner conical metal collar 102 (corresponding to the nickel tube 26 of Figure 1 ) is likewise joined via tubular truncated-conical lower part 1 04 thereof to a second inner and narrower conical sealing zone 106 of the collar 1 8, said second sealing zone 1 06 also being directed radially outwardly. A /?-alumιna solid electrolyte separator tube 1 4 is joined by glassing to a third sealing zone of the ceramic in a recess or rebate 1 6 of the ceramic insulating member 1 8.

The other parts of the construction, namely the lid and filling tube and the terminal for the inner cell electrode, are omitted from Figure 7 and may be taken, together with the inner current collector which is welded to the lid, from Figure

1 , for completion of the picture, the outer cell electrode terminal already being provided by at least one of the lobes (84, 96) of the outer metal collar 80.

In the filling procedure of the cell, the top surface 1 08 at the upper end of the inner conical part of the ceramic 1 8 may be temporarily protected during the filling operation, eg by means of an elastomer lip, to protect the header sealing zones from contamination by the materials loaded into the cell interior.

Although the first and second sealing zones 1 00 and 1 06 may have different solid angles and heights, the preferred design for the ceramic insulator 1 8 provides essentially equal angles and heights for these zones.

The square cell canister or casing 1 2 (shown in Figure 1 ) is welded sealingly to the header of Figure 7 along rim 98 after the ?-alumina tube 1 4 has been glassed to the σ-alumina insulating member 1 8.

Figure 8 shows a variation of the header design of Figure 7 with diminished height and mass compared with that shown in Figure 7, and an arrangement of the inner and narrower sealing zone 106 whereby the outwardly facing truncated conical sealing zone 106 faces a radially inwardly facing truncated conical wall. This forms a groove or recess 1 1 0 in the ceramic insulating member 1 8, giving a serrated profile of the upper surface of said insulating member, as shown in Figure 8.

Figure 9 shows a similar variation of the arrangement of the outer and wider sealing zone 1 00 which forms a wall of an annular groove or recess 1 1 2, and, in addition, shows the preferred application of an active braze relative to the sealing zones. Active braze 1 1 4 sealing the outer metal collar 80 is applied circumferentially outwardly of the outer metal collar 80 at its widest portion which is press-fitted to the ceramic insulator 1 8. Active braze 1 1 6 sealing the smaller inner metal collar 1 02 is applied circumferentially inwardly of said collar

1 02 in the corner formed between the ceramic insulator and said collar. The inner collar 1 02 is right-cylindrical with no conical part 1 04 (see Figure 7) . Collar 102 is press-fitted to the conical sealing zone 106 and defines a gap between itself and the conical sealing zone 1 06 of the ceramic 1 8, the gap widening upwards and becoming filled with braze in the process of brazing . The brazing material may be applied as a paste with binder, or as a solid, eg in the form of wire, as rings or other pre-fabricated shapes.

Figure 1 0 shows schematically in plan view two cell tops 1 1 8, 1 20 of square canisters 1 2, the square outline metal collar parts 98 (see Figures 6 and 7) being welded sealingly to said canisters. Lobes 84 are bent radially inwardly and have arcuate portions 1 22. The upper rims of the conical inner metal collars 1 02 are closed by closure members or lids 1 24 welded sealingly into said inner metal collars. Four intercell connectors 1 26 are welded into said lids, said connectors 1 26 being brazed to the arcuate portions 1 22 of four separate lobes 84 of the second cell top 1 20, and extend as current collectors into the interior of the cell 1 8 through brazing joins 1 28. The filling tube is omitted.

In a particular arrangement (not illustrated in this schematic plan view) of the intercell connectors 1 26 it is possible to reduce cross-overs thereof, and to provide a safe distance between said connectors 1 26 if non-contacting crossovers of connectors 1 26 with different potentials occur. Also, a certain elasticity or give can be designed into the intercell connectors to compensate for thermal expansion and contraction and vibration stresses, eg by using curved intercell connectors 1 26. One possible design comprises connecting the connectors to the inner terminals at a level higher than the connections of the connectors to the outer terminals formed by the lobes 84. Figure 1 1 shows a pictorial drawing of a square cell top with canister 1 2 welded on to the outer metal collar and a circular opening in the collar intended to be fitted with a cell cover or lid .

Claims

1 . A high temperature rechargeable electrochemical cell which includes a housing containing an anode and a cathode, the housing having an interior divided by a solid electrolyte separator into an anode compartment and a cathode compartment, the anode compartment and the cathode compartment containing respectively the anode and the cathode, the cell having a charged state in which the anode includes an alkali metal or alkali metal alloy, and the cell having an operating temperature at which the anode is molten, the separator comprising a conductor of alkali metal ions, and the cathode comprising, at said operating temperature and in said charged state, an electronically conductive porous electrolyte-permeable matrix having a porous interior impregnated with a molten salt electrolyte, the matrix containing, dispersed in its porous interior, active cathode material, and the housing being in the form of a polygonal metal canister having a closed off lower end and an open upper end which is welded to a polygonal outer metal collar sealing member which is sealed in a first sealing zone to an electrically insulating collar, which insulating collar in turn is sealed in a second sealing zone to an inner metal collar sealing member in electrical contact with a current collector for an electrode of the cell, the metal collar materials being selected from the group consisting of nickel and nickel-alloys and said electrochemical cell comprising, in combination: first and second coaxial radially outwardly facing truncated conical sealing zones on the insulating collar joined by active brazing to radially inwardly facing coaxial truncated conical sealing zones respectively of the inner and outer metal collars; the inner and outer metal collars having their mass distributed in rotationally symmetrical fashion around the cone axes of their sealing zones when sealing is effected; and an active braze comprising nickel, niobium and titanium alloying components being used to seal the collars in place.

2. A high temperature rechargeable electrochemical cell as claimed in claim 1 , in which the active braze is characterized by a titanium content of less than 1 0% and at least 3% by mass, and a niobium content in a proportion of 1 0% to 70% by mass relative to the combined nickel and niobium contents.

3. A high temperature rechargeable electrochemical cell as claimed in claim

1 or claim 2, in which the active braze includes iron.

4. A header for sealing a polygonal metal canister of a high temperature rechargeable electrochemical cell, the header including an insulating collar having two radially outwardly facing concentric truncated conical sealing zones for press-fitting and joining to metal collars, the insulating collar also having a sealing zone for joining to a solid electrolyte separator; an outer metal collar having a perimeter which has the same polygonal outline as the canister and which is press-fitted into the canister opening before being welded into the canister opening, the outer metal collar having an upper part consisting of a number of lobes obtained by folding and deep-drawing a circular disk of a metal alloy to give said disk the polygonal outline of the canister while retaining the rotationally symmetrical mass distribution of the disk, and the outer metal collar having a lower part comprising a cylindrical portion, said lower part being joined to said insulating collar at a radially outer and wider of its said conical sealing zones by active brazing; an inner metal collar having an essentially cylindrical or truncated conical port or portion joined to said insulating collar at an inner and narrower of its said conical sealing zones; and a metal closure device or lid joined to said inner metal collar and provided with a closable filling opening for admitting reactants or other cell materials into the cell interior.

5. A method of sealing a high temperature rechargeable cell, the method including making an outer metal collar by folding and deep-drawing a metal disk to form a plane having a polygonal outline and to form lobes which project essentially at right angles to said plane from one side of said plane, one lobe at each of the edges of said plane; perforating the polygonal plane to form a circular hole in said plane; deep-drawing the periphery of said hole to form a collar protruding away from or towards the direction in which the lobes project; folding the lobes radially inwardly over the polygonal plane; and back-folding radially inner folded parts of said lobes to be essentially at right angles to the plane; making an inner metal collar having a cylindrical or truncated conical part or portion; making an insulating collar having two concentrictruncated conical radially outwardly directed sealing faces respectively dimensioned to nest respectively inside at least part of the ouer metal collar and the truncated conical part or portion of the inner metal collar, the insulating collar having a sealing face for sealing to the solid electrolyte separator; press-fitting at least part of each metal collar to the associated concentric conical sealing face of the insulating collar to form seals respectively between the metal collars and the insulating collar; joining the metal collars to the insulating collar by active brazing with an active braze based on nickel, niobium and titanium alloy components at the press-fitted seals between the metal collars and the insulating collar to form a header; joining the header by glass welding to the solid electrolyte ceramic separator by means of said sealing face provided for the separator on the insulating collar; and welding a metal canister forming a cell housing to the polygonal, diametrically widest part of the outer metal collar.

6. A method as claimed in claim 5, in which the active braze is characterized by a titanium content of less than 1 0% and at least 3% by mass, and a niobium content in a proportion of 1 0% to 70% by mass relative to the combined nickel and niobium contents.

7. A method as claimed in claim 5 or claim 6, in which the active braze includes iron.

8. A method as claimed in any one of claims 5 to 7 inclusive, in which the cylindrical parts of the metal collars are provided with conical shapes at the contact areas, to increase the press-fitted contact area between the metal collars and the insulating collar.

9 A method as claimed in any one of claims 5 to 8 inclusive, which includes electrically connecting at least one cell connector to each lobe generated on the outer and wider metal collar.

1 0. A method as claimed in any one of claims 5 to 9 inclusive, which includes using a cylindrical or conical die in the final backfolding of the outer collar lobes, giving middle portions of said lobes an arcuate shape and locating the arcuate middle portions equidistant from one another and closer to the inner metal collar than to the corners of the polygon form by the widest part of the outer metal collar

1 1 . A method as claimed in any one of claims 5 to 1 0 inclusive, in which the insulating collar and/or the metal collars are shaped to provide annular zones for application of the braze which adjoin the conical sealing zones.

1 2. A high temperature rechargeable electrochemical cell as claimed in claim 1 , substantially as herein described and illustrated .

1 3 A header for sealing a polygonal metal canister of a high temperature rechargeable electrochemical cell as claimed in claim 4, substantially as herein described and illustrated

1 4. A method of sealing a high temperature rechargeable cell as claimed in claim 5, substantially as herein described and illustrated .

1 5. A new cell, a new header for a cell, or a new method of sealing a cell, substantially as herein described .