GB2294803A - High-temperature cell having curved solid electrolyte separator and flexible anode cover to accommodate volume changes during charging/discharging - Google Patents

Description

1 ELECTROCEEKICAL CELL 2294803 THIS INVENTION relates to electrochemical

cells. More particularly, the invention relates to a high temperature rechargeable electrochemical cell.

According to the invention there is provided a high temperature rechargeable electrochemical cell which comprises a cell casing defining an anode compartment for containing an alkali metal anode and a cathode compartment containing a liquid electrolyte, the cell having an operating temperature at which the anode and liquid electrolyte are molten and the anode compartment being separated from the cathode compartment by a separator comprising a solid electrolyte which is a conductor of ions of the alkali metal of the anode at the operating temperature of the cell, the casing comprising a cathode cover of sheet material enclosing the cathode-side surface of the separator and an anode cover of sheet material enclosing the anode-side surface of the separator, the cathode cover and anode cover being electronically conductive and electronically insulated from each other and respectively forming a cathode terminal and an anode terminal for the cell, the separator being in the form of a sheet or plate and having a curved concave anodeside surface facing into the anode compartment, the cathode cover being relatively inflexible and being spaced from the cathode-side surface of the separator and the anode cover being relatively flexible, and the cell having a fully discharged or overdischarged state in which the anode compartment is substantially empty of alkali metal and the anode cover has a convex surface which nests in the concave surface of the separator to reduce the volume of the anode compartment, the anode cover being capable of flexing away from the separator in response to Ionic passage of alkali metal into the anode compartment through the separator upon charging of the cell, to Increase the volume of the anode compartment.

Preferably the anode cover and shape of the concave surface of the separator are selected or constructed so that the anode cover is capable of 2 providing a said convex surface which has a shape complementary to that of the concave surface of the separator, so that the anode cover can nest in said concave surface in abutment therewith or closely spaced therefrom.

with regard to the relative inflexibility of the cathode cover and the relative flexibility of the anode cover, these should be such that, when there is passage of alkali metal in ionic form through the separator from the cathode compartment and in metallic form into the anode compartment in response to charging of the cell from its fully discharged or overdischarged state, so that the anode cover flexes away from the separator, there is no more flexing than that which is allowed, during normal cell operation, by the requirement to preserve a gap between the anode cover and cathode cover respectively of adjacent cells in series-connected stacks of the type described hereunder.

Typically, the sheet material of the anode cover and of the cathode cover will be sheet metal.

The anode cover will thus typically be in a non-planar state, and it preferably nests in the separator so that the anode cover is in contact, over at least a major proportion of the area of the concave surface of the separator, with said surface of the separator in the fully discharged or overdischarged state of the cell.

The cell may be circular in outline, the separator being circular; and the separator may have a bead of electronically insulating material such as a glass extending along its periphery, to which bead it is sealingly connected, eg by being integral therewith or by being co-sintered therewith, or by having its periphery cast into said bead, which acts as an insulating ring. The peripheries of the anode cover and the cathode cover may respectively be sealingly connected to opposite sides of this bead, being electronically insulated from each other thereby. Accordingly, the cell may be circular in outline, the separator also being circular in outline, the cell having a fully discharged or overdischarged state in which the convex surface of the anode cover is in face-to-face contact with a major proportion of the 3 concave surface of the separator, the separator having a bead of electronically insulating material extending along its periphery, to which bead the separator is sealingly connected, the cathode cover and anode cover having peripheries which are sealingly connected to opposite sides of the bead.

Thus, opposite sides of the bead may have peripherally extending metallic rings hermetically sealed or bonded thereto, the rings being electronically insulated from each other and the peripheries of the anode cover and the cathode cover being welded respectively to said rings.

The separator may be in the form of a disc, which is part-spherical in shape, the rings being in the form of flattened metal strips. Instead, the separator may be of composite construction, comprising a hydraulically impermeable solid electrolyte layer supported on a porous supprt layer, which support layer is located between the cathode compartment and the solid electrolyte layer and is hydraulically permeable by and impregnated with the liquid electrolyte. In a particular embodiment the separator may have a flat surface facing the cathode compartment and a curved concave surface facing the anode compartment, the metallic rings being of circular cross-section. Conveniently, the separator has both said composite construction and said flat and concave surfaces facing respectively into the cathode compartment and into the anode compartment, in which case, if the hydraulically impermeable solid electrolyte layer is of constant thickness and has a convex curved surface via which it is supported by a concave surface on the porous layer,the porous layer will typically be of minimum thickness at its centre and maximum thickness at its periphery, and in this case the porosity of the porous layer may increase progressively in a radial direction from a minimum at its centre to a maximum at its periphery, so that the composite separator has substantially the same ionic conductivity or Ionic resistance/unit area over its whole surface. Accordingly, in a particular construction, the separator may be of composite construction, comprising a hydraulically impermeable solid electrolyte layer supported on a porous support layer which is located between the cathode compartment and the solid electrolyte layer and is hydraulically permeable by and 4 impregnated with the liquid electrolyte, the porous layer having a flat surface facing the cathode compartment and the hydraulically impermeable electrolyte layer having a curved concave surface facing the anode compartment, the porous layer having a concave curved surface via which it supports the electrolyte layer and the electrolyte layer having a convex curved surface via which it is supported by the porous layer, the porous layer having a porosity which increases in a radial direction from a minimum at a central position on the porous layer, to a maximum at the periphery of the porous layer.

It should be noted that, while the cell cathode compartment will usually contain molten liquid electrolyte together with a solid active cathode material, the electrolyte may be in the form of a catholyte, whereby it acts both as liquid electrolyte and as liquid active cathode material. Preferably, the cell cathode compartment contains, in additio n to the liquid electrolyte, a solid active cathode material.

Various other possibilities exist for the present invention, with regard to the detailed construction of the cell. Thus, as indicated above, the separator may comprise a layer, preferably as thin as feasible, of hydraulically impermeable solid electrolyte material, supported on a porous support layer, for example as described for sodium/sulphur cells in European patent application EP 0 543 796, which describes an electronically conductive porous support plate, eg of reduced Ti02. Naturally, the thermal expansion properties of the porous support layer should be compatible with those of the solid electrolyte layer, and the material of the porous support layer should be chemically compatible with the molten liquid electrolyte and other cathode compartment contents. The material of the porous support layer may be electronically conductive or electronically non-conductive, and may be ionically conductive or lonically non-conductive, and in a convenient embodiment the porous support layer may be of the same material as the solid electrolyte layer.

In this embodiment the separator can be regarded as a composite bilayer separator, having a thin, continuous, pore-free hydraulically impermeable solid electrolyte layer supported by a porous layer which is hydraulically permeable to the molten liquid electrolyte, so that the molten electrolyte can impregnate and saturate the porous layer, thereby coming into contact with the hydraulically impermeable but ion-permeable and ion- conductive continuous solid electrolyte layer, which electrolyte layer isolates the interior of the cathode compartment from the interior of the anode compartment. Preferably the solid electrolyte layer and the porous support layer are sintered together to form a coherent bilayer ceramic of unitary, monolithic construction, the separator being used in the cell with the porous layer facing the interior of the cathode compartment and the impermeable electrolyte layer facing the interior of the anode compartment.

As also indicated above, the composite separator is conveniently flat and planar on its porous support layer surface which faces into the cathode compartment, being convexly crved on its solid electrolyte surface which faces into the anode compartment. The concavity acts to accommodate flexing of the metallic anode cover as described above. The thin impermeable solid electrolyte layer may be of more or less constant thickness, being convexly curved on its side bonded to the porous layer, and concavely curved on its side facing the anode compartment so that, correspondingly, the porous layer may have a concavely curved side where it is bonded to the solid electrolyte layer, being flat on its -side facing to the cathode compartment. This feature, in cells of the present invention, permits the use of a flat, planar cathode matrix, as described hereunder.

The composite separator may have its bead or insulating ring formed of a material other than the cast glass described herein. Thus, its insulating ring may be made of a ceramic such as a-alumina or a suitable P-alumina compound from the family of fl-alumina compounds, including P"-alumina compounds. Conveniently, the insulating ring is made of the same material which forms the porous support layer of the composite separator, which porous support layer will thus be electronically insulating. Indeed, in a particular embodiment, the porous support layer, the insulating ring and the non-porous ion-conductive solid electrolyte layer may all consist of the same material, such as P"-alumina, forming a unitary 6 monolithic ceramic sintered body. If desired, however, the insulating ring may be made of a fl-alumina type ceramic of relatively reduced ionic conductivity compared with the solid electrolyte layer of the separator. A suitably debased fl-alumina may be used for this purpose, having its conductivity with regard to the alkali metal cations of the anode reduced by appropriate doping, eg doping with calcium ions, and/or by substitution, for alkali metal cations forming part of the R-alumina, with other metal cations.

The insulating ring, which forms a sealing rim for the separator, may have its two metal rings, when they are of circular cross-section, bonded thereto by active brazing. These metal rings, conveniently of nickel or a nickel alloy, may be located respectively in each of two grooves or recesses provided therefor, on opposite sides of the insulating ring or sealing rim. These metal rings will thus be electronically insulated from eacl other, and will be welded sealingly respectively to the anode cover and cathode cover. Furthermore, these metal rings may be prof lied, so that they are somewhat non-circular in cross-section, to facilitate exact positioning of the anode cover and cathode cover thereon for easy welding, and the profiling thereof can be carried out after the active brazing, eg by stamping, or by machining, such as by turning the separator on a lathe.

Advantageously, an additional groove or rebate may be provided on the anode side of the insulating ring or sealing rim, into which a circumferentially extending peripherally located fold in the form of a rib or ridge on the side of the anode cover facing the separator fits, this fold being engaged by the radially inner wail of said groove or rebate. This feature can facilitate flexing of the anode cover to allow the anode cover easily to flex in response to volume changes of the anode compartment arising from volume changes of the anode in response to charge/discharge cycling of the cell. Preferably, the fold in the anode cover is shaped to fit especially closely and most snugly into its groove or rebate when the anode compartment is empty of anode material, eg when the cell is completely discharged or overdischarged.

7 The sealing rim or insulating ring may project radially outwardly beyond the metal rings, to act as a spacer to centre the cell in a tubular battery housing or casing, abutting against the housing or against tubular insulating material forming an internal lining for the housing, or against a tubular temperature control means which can form an internal lining for the casing or an internal lining for the insulating material. The outer periphery of the sealing rim or insulating ring, where it projects radially outwardly from between the metal rings, may be constructed to facilitate transport of temperature control fluids, or to facilitate the housing of temperature control elements, such as electrical heating elements or cooling coils, by providing grooves or channels therefor. For example, the sealing rim or insulating ring of each cell may be provided with a plurality of radially outwardly projecting formations of the nature of teeth, separated by indentations, so that in plan view its edge looks similar to that of a gear wheel. It will be appreciated that, if a number of such cells are stacked in a stack with their teeth and indentations circumf erenti ally in register, the indentations of the various cells can combine to form longitudinally extending grooves or channels extending longitudinally along the outside of the stack. Instead, a circumferentially extending spacer may extend around the sealing rim or insulating ring of each cell, such spacer being optionally in the form of a heating element or cooling coil.

The curvature of the concave side of the composite separator which faces the anode compartment is preferably chosen to correspond in shape to the convex curvature which will be assumed by the anode cover when the anode compartment is empty of anode material. This shape can be determined by trial and error, and can be approximated by the shape assumed by an ideal flat elastic plate which is freely supported at its periphery or rim, and has been subjected to a pressure providing a load or force acting perpendicular to the elastic plate, with the elastic plate being supported by a flat supporting plate, the supporting plate having a diameter of 60 90% of the diameter of the elastic plate, and being concentrically aligned therewith. The anode cover can be regarded as corresponding with such elastic plate, the supporting plate corresponding to the anode-side surface of the separator. In this case the curved concave shape of the separator anode-side 8 surface can be regarded as somewhat meniscus-like, being more or less flat over its central portion, and curved at its periphery. Instead, however, the curvature may naturally be different, eg part-spherical, as described above.

The porosity of the support layer of the bilayer separator may be provided 5 by any convenient known means, such as incorporating volatile or decomposable constituents (pore-forming agents or blowing agents) in green ceramic material prior to sintering, by using granules or coarse- grain particulate material for the support layer so as to leave a network of open pores after sintering, or by mechanical action such as punching the green ceramic or by extruding it through an orifice arranged to provide it with a pattern of holes. As indicated above, particularly in the case where holes are formed mechanically, the pattern/number/surf ace density of the holes, and/or the diameter of the holes, can be selected to equalise the iniernal resistance provided for the cell by the separator. Thus, the arrangement and size of the holes can be selected to compensate for variable thickness of the separator, which variable thickness arises from the fact that the separator has a flat cathode-side surface, and a concave anode-side surface. This compensation for the variable thickness can act to provide the separator with internal gonic) resistance/unit area which is more or less constant, porosity being at a minimum towards the centre of the separator where the separator is at its thinnest, the porosity, as indicated above, increasing progressively in a radial direction towards the periphery of the separator where it is at this thickest. The proportion of the area of the support layer formed by voids should preferably amount to at least 40% of the area of the cathode-side surface of the non-porous solid electrolyte layer. Instead or in addition, to compensate for the fact that the porous support layer decreases progressively in thickness in a radially inward direction from its periphery where it is at its thickest towards its centre where it is at its thinnest, the hydraulically impermeable solid electrolyte layer may also be non-uniform in thickness, progressively decreasing in thickness in a radially outward direction from its centre R where it is of maximum thickness towards its periphery where it is of minimum thickness, to promote constant cell internal resistance arising from the separator, over the full area of the separator.

The composite bilayer separator may be made by pressing in the green state, eg by pressing in a die which shapes the support layer and the solid electrolyte layer respectively, or the bilayer separator may instead be made by first making the support layer, eg in final form, followed by coating the support layer with the solid electrolyte layer, and then sintering. Various methods of coating can in principle be used, such as rolling, spraying or slip-casting suitable slips or paste to preparations. Preferably, the coating is done on a green support layer, the green composite then being consolidated by firing and sintering, optionally after debonding and decomposition of any pore-forming agents used in the porous layer and other organic matter, such as binders, if used.

If solid electrolyte material, such as that used for the electrolyte layer, is used for the support layer, a further and different process is possible, whereby a green porous support layer is made in a plastic state, and is then subjected to mechanical deformation on its anode-side surface, the mechanical deformation effecting closure of pores in said anode-side surface. Thus, the same material can be used for the support layer as for the electrolyte layer, and no different material need be used. The porous plastic green support layer may thus be obtained by extruding a suitable porous plastic body through a die, followed by mechanical shaping thereof.

According to another aspect of the invention there is provided a battery of 15 electrochemical cells, the battery comprising a plurality of cells in accordance with the present invention as described above, stacked in series, one upon another, to form a stack of said cells, the cells of each adjacent pair of cells of the stack being in contact with each other via contact between the anode cover of one cell of said pair and the cathode cover of the other cell of said pair.

One of said anode and cathode covers, for example the cathode cover, Of each said cell may have a recessed rim providing a peripherally extending rebate in its outer surface adjacent its periphery, the other cover, for example the anode cover, of said cell having a raised rim providing a peripherally extending step in its outer surface adjacent its periphery, the step and the rebate being complementarily shaped and the cells being stacked one upon another, with the step of one of the cells of each adjacent pair of cells nestingly received in the rebate of the other cell of said pair.

1 k Heating and/or thermal cooling means may be provided in the housing, for thermal management of the battery. Accordingly, a thermal control means, selected eg from heating means and/or cooling means, may be provided in the housing. The thermal control means may concentrically surround the stack.

The stack of cells may be contained in a battery housing, which may comprise thermal insulation; and cell anode and cathode terminals may extend through respective insulated feedthroughs provided therefor at opposite ends of the housing. In other words, the stack of cells may be contained in a tubular battery housing comprising thermal insulation, the stack of cells being provided with respective anode and cathode battery terminals and the housing having ends provided with electronically insulating terminal feedthroughs, the terminals extending respectively through the feedthroughs.

The battery terminals may respectively engage opposite ends of the stack and urge the cells in the stack together to keep them in place in the stack, the terminals in turn being held in place by the feedthroughs..

The invention extends to a method of making a cell as described above, which method includes the step of sealingly connecting the periphery of the separator to the bead by moulding or casting the periphery of the separator into a bead of insulating material, and causing the bead to adhere to said periphery to form an artifact therefrom while simultaneously hermetically sealing the bead to the periphery of the separator.

While the periphery of the separator may be moulded into a bead of particulate sinterable material, followed by sintering the bead to the separator periphery to form a unitary sintered artifact, it is preferred to cast the bead, for example centrifugally, in molten form along said periphery, the bead, upon cooling, adhering to said periphery and forming the hermetic seal.

I1 Typically the separator will be formed by shaping it into its curved shape while in a particulate green state, and sintering it prior to the moulding or casting of the bead. After or during the construction of the separator to the bead, the strips may be bonded to the bead, for example by casting them in position, and the covers, after shaping thereof, for example by punching them from sheet material followed by deep-drawing, forming, stamping and/or pressing, may be welded respectively to the strips.

Conveniently the welding is carried out with the anode compartment empty and with a fully discharged cathode, or a precursor thereof which may amount to an overdischarged cathode or its equivalent, contained in the cathode compartment. If desired, a heat sink, which may be in contact with at least the cathode cover, may be employed to cool the contents of the cathode compartment during welding.

The invention will now be described, by way of example, with reference to the accompanying diagrammatic drawings, in which:

Figure 1 shows an axial side elevation of a cell in accordance with the present invention; Figure 2 shows an exploded view of Figure 1; Figure 2A shows an enlarged axial side elevation of another embodiment of part of Figure 2 indicated by circle 'W'; Figure 2B shows a further embodiment of the part of Figure 2 indicated by circle "X"; Figure 3 shows an axial sectional side elevation of a battery in accordance with the present invention; Figure 4 shows a schematic sectional side elevation of a device for connecting together the solid electrolyte separator of the cell of Figure 1 and its peripheral bead; Figure 5 shows an axial side elevation of another embodiment of a cell in accordance with the present invention; 13 Figure 6 shows an enlarged axial side elevation of part of Figure 5 indicated by circle "Y"; and Figure 7 shows a plan or end-on view of part of a further embodiment of a cell in accordance with the invention.

Referring first to Figures 1 and 2 of the drawings, reference numeral 10 generally designates an electrochemical cell in accordance with the present invention. The cell 10 comprises a metallic cell casing made up of a cathode cover 12 and an anode cover 14. The cell 10 is circular in plan view outline, although it may, naturally, instead have a plan view outline which is square, hexagonal or elongate rectangular, for enhanced space utilization for cells in batteries, to permit close packing for improved volumetric energy density.

Between the covers 12 arid 14, which are typically of a nickel alloy, mild steel or the like, are shown a solid electrolyte separator 16, which may be of fl- or preferably P"-alumina, and an insulating ring 18, conveniently of a suitable glass.

The ring 18 forms a peripheral bead extending circumferentially along the periphery of the separator 16, and is hermetically bonded thereto as described hereunder. The ring 18 has flat oppositely outwardly facing axially directed faces on opposite sides thereof, to which are respectively connected peripherally extending met-allic strips 20 and 22, as described hereunder, the strips 20 and 22 being in the form of flat annular metal rings, and standing radially proud of the bead forming the ring 18. In Figures 2A and 2B the same reference numerals are used to designate the same parts as in Figure 2, unless otherwise specified. Figures 2A and 2B illustrate altered embodiments of the attachment of the strips 20, 22 to the ring 18. In Figure 2A the strips 20, 22 are also in the form of annular rings but each strip 20, 22 has, at its radially inner periphery, an axially inwardly projecting flange. The flanges of the strips 20, 22 are respectively designated 21 and 23 and project perpendicularly to the strips 20, 22. The strips 20, 22 are connected to the ring 18 by bonding, eg by shrinking, the flanges 21, 23 on to the radially outer surface of the ring 18. In Figure 213, each of the flanges 21, 23 is at an angle A of about 1351 to its associated ring 20, 22 and is connected to the ring 18 by sealingly 14 embedding the flanges 21, 23 and the radially inner edges of the rings 20, 22 in the ring 18, the rings 20, 22 entering the radially outer face of the ring 18. Instead of being flat, the radially outer peripheries of the rings formed by the strips 20 and 22 may each, if desired, have a raised outer peripheral rim or rebate (not shown), to facilitate welding to one of the covers 12, 14 and/or stacking of cell parts before welding, as described hereunder. In another embodiment, each ring formed by the strips 20, 22 may have two peripheral rims or rebates provided thereon (not shown), with one said rebate being at the radially inner periphery of the ring 18 and bent axially inwardly to be securable or anchorable to the ring 18, and the other said rebate being at the radially outer periphery as described above and bent axially outwardly to facilitate welding thereof to the cathode cover 12 or the anode cover 14, as required. The cathode cover 12 has a recessed rim providing a circumferentially extending peripheral rebate 24 and a sealing flange 26; and the anode cover 14 has a raised rim providing a circumferentially extending peripheral step 28 and a sealing flange 30. The flange 26 is hermetically welded face-to-face to the ring 20; and the flange 30 is hermetically welded to the ring 22.

The separator 16 is circular in outline and curved to be part-spherical in shape, having a convex surface 32 facing towards the cathode cover 12 and a concave surface 34, facing towards the anode cover 14. The anode cover 14 is relatively flexible, whereas the cathode cover 12 is

relatively rigid and inflexible. In Figure 1 the anode cover 14 is shown flexed upwardly so that it conforms complementarily in shape with the shape of the separator 16 and is in face-to-face abutment with the separator 16, over substantially the full area of both the separator 16 and central portion of the anode cover 14 radially inwardly of the step 28, the cell 10 of Figure 1 being shown in its fully discharged or overdischarged condition, with substantially no anode material in the anode compartment which is defined between the separator 16 and anode cover 14. The cathode compartment of the cell 10, which is defined between the separator 16 and the cathode cover 12, is shown containing active cathode material and molten electrolyte, the cathode compartment contents being generally designated 36.

In Figure 2 it should be noted that the anode cover 14 is shown in an unstressed flat or planar condition, being manufactured with a planar shape, corresponding to the shape assumed thereby when the cell is in its fully charged condition, in which fully charged condition active anode material (not shown) will be contained in the anode compartment between the separator 16 and anode cover 14.

To make the cell 10, the separator 16 is pressed to have its circular part- spherical shape as shown in Figures 1 and 2 in a green state from particulate solid electrolyte material or from a particulate precursor thereof, which particulate material or precursor is then sintred to produce the separator 16 in the form of a dished unitary polycrystalline artifact. The peripheral glass electronically insulating bead forming the ring 18 is then connected to the periphery of the separator 16, as described hereunder with reference to Figure 4, during which connecting the rings 20 and 22 are respectively hermetically bonded to opposite sides of the ring 18. If the rings 20, 21 of Figure 2A are employed, the rings 20, 21 are fixed or bonded to the outer circumference of the ring 18 by shrinking the flanges 21, 23 on to the ring 18. If the rings illustrated in Figure 2B are employed, the flanges 21, 23 and the radially inner peripheries of the rings 20, 22 are embedded in the bead forming the ring 18.

The covers 12 and 14 can be cut or punched from sheet metal and simultaneously deep-drawn, stamped shaped and/or formed to have the profiles shown in Figures 1 and 2, whereby the cathode cover 12 is provided with the rebate 24 and flange 26, and the anode cover 14 is provided with the step 28 and flange 30. The flange 26 is then welded to the ring 20, while the ring 22 is welded to the flange 30.

16- It should be noted that, contrary to what has been described above, the anode cover 14 can be instead formed to be curved, so that it has a convex inner surface and a concave outer surface in its unstressed condition as shown in Figure 1, but is flexible, and so that it can be stressed by pressure on its inner surface to be essentially flat and planar as shown at 38 in Figure 2. It is also to be noted that the cathode cover 12 is welded to the ring 20 with the cathode compartment contents 36 in position.

To provide the flexibility of the anode cover 14, and to provide it with the desired degree of elasticity so that it is resiliently flexible, appropriate selection can be made of the sheet metal from which it is made, and by designing it to have appropriate metal thicknesses at various parts and to have an appropriate profile. Thus it may be profiled to have radial zones of different metal thicknesses, and indeed of different curvatures, oother approaches known in the art can be used, selected with a view to ensuring that, when the anode compartment is empty of anode material, the convex side of the anode cover 14 fits flush against the concave surface of the separator 16 as far as possible.

As far as the cathode contents 36 are concerned, these will typically be, when the cell is assembled, a cell catholyte or a mixture of active cathode material and a molten salt electrolyte, and these cathode compartment contents 36 are contoured to fit closely against the inside of the cathode cover 12 and against the convex surface 32 of the solid electrolyte separator 16.

Conveniently the various cell components shown in Figure 2 are assembled by stacking together the components shown in Figure 2, namely the anode cover 14 lowermost, the prefabricated central part comprising the sub-assembly of the separator 16, insulating ring 18 and metal rings 20, 22 stacked on the anode cover 14, and the sub-assembly comprising the cathode cover 12 with the cathode compartment contents 36 as a solid mass nesting therein stacked on the central part. The stack can then be placed in a welding fixture, for welding the flange 26 to the ring 20 and welding the ring 22 to the flange 30. This welding can be 1-1 carried out under vacuum or under inert gas. If desired, the stacking can be inverted, with the cathode cover 14 containing the cathode compartment contents 36 lowermost and upside down, and the central part stacked thereon with the anode cover uppermost in the stack.

The cell can be cooled during the welding, for example by placing a heat sink (not shown) in thermal contact against at least the cathode cover 12 and optionally against the anode cover 14.

Typically by way of example the solid electrolyte separator is,6"-alumina, the insulating ring 18 being a glass compatible with 8"-alumina, compatible with the anode such as sodium and compatible with the molten salt electrolyte used; and the molten salt electrolyte can be sodium chloroaluminate, the active cathode material being ferrous chloride (FeC'2). The metals of the rings 20 and 22 will be selected for compatibility as regards thermal expansion with the insulating ring 18, being alloys known in the art and selected for good metallglass adhesion. The covers 12, 14 can be steel, nickel-plated steel or nickel alloy.

A number of options in fact exist for obtaining a close face-to-face fit between the convex side of the anode cover 14 as shown in Figure 1 and_ the concave surface 34 of the separator 16. Thus, the anode cover 14 can initially be flat and planar (see Figure 2) and the anode compartment can be evacuated after the welding by way of a porthole to bring the cover 14 into contact with the separator 16, after which the porthole is sealed. Instead, the welding of the flange 30 to the ring 32 can be under vacuum, after which atmospheric pressure on the outside of the anode cover 14 can urge it against the separator 16. A further possibility is by forming the cover 14, for example by stamping, deep-drawing or hydro-forming, so that it has the necessary curved shape, complementary to the curve of the separator 16.

It is contemplated that the method involving evacuation via a porthole will be suitable for cell systems such as those in sodium/sulphur cells which are assembled in a charged condition with sodium in the anode compartment and a sulphur catholyte 36 in the cathode compartment, and in this case molten sodium can be injected under vacuum through a porthole or opening provided therefor, which is subsequently closed. The remaining methods are expected to be more suitable for systems which are assembled in the discharged or overdischarged state, such as those systems where the anode is an alkali metal such as sodium, and the cathode is a transition metal chloride in porous matrix form or particulate form impregnated with a sodium chloroaluminate-based molten salt electrolyte. Examples are ferrous chloride and nickel chloride active cathode materials.

By way of a variation of the above procedure, it should be noted that the glass insulating ring or bead 18 can be replaced by a thickened peripheral portion of the solid electrolyte of the separator 16, which thickened portion can be sintered integrally with the remainder th6eof; or the ring 18 can be made from a material such as a-alumina which is co- sinterable together with the solid electrolyte of the separator 16 to form a peripheral ring-like bead or zone to which the metal rings 20, 22 can be bonded or sealed, eg by active brazing.

Referring now to Figure 3, a plurality of cells of the type shown in Figure 1 are shown stacked in series to form a vertically extending stack 40, forming part of a battery generally designated 42.

In this regard and with reference to Figure 1 it should be noted that the rebate 24 is shaped such that the step 28 of an adjacent cell can nest snugly therein, to facilitate this stacking. It should be noted that there is no hermetic seal between the anode covers 14 and the cathode covers 12 of adjacent cells because gas exchange due to pressure imbalances and heating/cooling of the stack 14 must be provided for. Radial grooves (not shown) can be provided along the radially outer periphery of the cathode cover 12 to facilitate this gas exchange.

To promote good electrical contact under all temperature conditions which are expected for the system, the cells in the stack are preferably springloaded Ok together under compression, which compression is provided in Figure 3 by an annular spring 44 engaging the uppermost cell 10 along its rebate 24 and urging it resiliently downwardly. Instead, or in addition, the step 28 of each cell can be welded in position in the rebate 24 of the adjacent cell.

The stack 40 is housed in a housing 46, conveniently of mild steel, whose interior surface is provided with a thermally insulating lining 48; and the battery of cells constituted by the stack 40 is provided with respective positive and negative battery terminals 50 and 52 passing through respective electronically insulating feedthroughs 54 which in turn pass through the roof and floor of the housing 46 respectively.

Hollow-cylindrical heating/cooling means in the form of a cylinder 56, which may be appropriately electronicaily insulated and may comprise electrical heating means, is shown provided in the housing 46, between the insulated lining 48 and the stack 40, concentrically embracing the stack 40.

It should be noted that, instead, smaller stacks of cells, in the form of modules, can be housed in housings which are metal cans such as mild steel cans, omitting the thermally insulating material 48 shown in Figure 3, and such modules can be connected in series to form a battery.

When the housing 46 is in the form of a metal can, it can replace one of the terminals 50, 52 which can accordingly be omitted. In this regard it will be noted that each terminal 50, 52 comprises a terminal rod passing through the associated feedthrough 54, the inner end of the terminal rod 52 being integral with a circular flat flange, the lowermost cell 10 in the stack 40 in Figure 3 resting on the flange of the terminal 52, and the flange of the terminal 50 bearing downwardly on the spring 44 to keep it under compression.

With reference to Figure 4, a device for joining the insulating ring 18 to the separator 16 is generally designated 57. This device 57 is intended for an insulating ring 18 of a glass compatible with the solid electrolyte of the separator 16, and the device 57 is also capable of bonding the rings 20, 22 to the ring 18.

In Figure 4 there is shown a centrifugal casting table 58 rotatable in the direction of arrow 60, on which table 58 is concentrically mounted a mould 62 of 5 graphite or of a suitable metal.

The mould 62 has a base part 64 having a raised portion 66. The base part 64 and raised portion 66 are shown in the form of a unitary ring, but may instead be made up of several components. The mould further has a middle or central part 68, and an upper part 70, the parts 64, 68 and 70 being annular in shape.

After optional pre-heating of the separator 16, it is positioned on the raised portion 66 of the mould base part 64 and the mould parts 68 and 70 are stacked in position as shown, partially to enclose and clamp the metal rings 20, 22 in position as shown, while leaving an annular space 72, bounded by the metal rings 20 and 22 and by the central part 68, into which the peripheral edge of the separator 16 intrudes, to be filled with glass melt as described hereunder.

The device is clamped together by means of an outer peripheral clamp_74, which may be of split construction and made up of clamp portions, and the table 58 is spun in the direction of arrow 60 about its axis provided by a vertical support shaft 76 for the table 58.

After heating of the assembly to the glassing temperature, molten glass 78 is fed from a heated vessel 80 after opening a feed valve 82, to fill the annular space or gap 72, the glass which enters said space 72 flowing around and enclosing the radially inner portions of the rings 20, 22, and the radially outer periphery of the separator 16. When the space 72 has been filled with the glass 76, the glass 78 in the space 72 can be allowed to cool and solidify, after which the sub-assembly so formed is taken from the mould 62, the sub-assembly constituting the solid electrolyte separator 16 in the form of a disc, whose it periphery is set into the insulating bead ring 18 (see Figures 1 and 2) with the two metal rings 20, 22 protruding radially therefrom, ready for welding as described above.

Referring to Figure 5, the same reference numerals are used to designate the same parts in Figures 1 - 4 unless otherwise specified. In Figure 5, the separator 16 is of composite construction. The separator 16 is circular in outline and comprises a continuous pore-free solid electrolyte layer 84 supported by a porous support layer 86, the porous support layer 86 and the solid electrolyte layer 84 being integrally fast with each another. The support layer 86 is shaped to conform to the shape or curvature of the solid electrolyte layer 84, the support layer 86 having a flat surface which faces the cathode compartment contents 36 and a concave surface which is sintered to a convex surface of the solid electrolyte layer 84 as described hereunder. The cathode compartment of the cell 10, which is defined between the porous support layer 86 and the cathode cover 12, contaifis the active cathode contents 36, the porous support layer 86 being saturated with liquid molten salt electrolyte. Because the layer 86 has a flat surface facing the cover 12, the cathode contents 36 can comprise a matrix which is flat and planar in shape.

The support layer 86 has thermal expansion proper-ties which are compatible with the thermal expansion properties of the solid electrolyte layer 84, the support layer 86 being of the same material as the solid electrolyte layer 84 and being chemically compatible with the cathode contents 36. Thus, the support layer 86 is electronically non-conductive and ionically conductive. The support layer 86 is (hydraulically) permeable to the molten liquid electrolyte, and the liquid molten salt electrolyte impregnates and saturates the support layer 86, so that the liquid electrolyte is in contact with the solid electrolyte layer 84. The solid electrolyte layer 84 is (hydraulically) impermeable to the liquid electrolyte but is ion-permeable, through ionic conductivity, with regard to mobile cations of the molten electrolyte.

?-I- The layer 86 has a flat surface facing the cathode contents 36, to allow a flat, planar cathode matrix to be used, and has a concave surface sintered integrally with a convex surface 88 of the solid electrolyte layer 84. The solid electrolyte layer 84 is in turn curved to provide said convex surface 88 and to provide a concave surface 90, facing away from the support layer 86, to accommodate flexing of the anode cover 14 as described above.

Referring to both Figures 5 and 6, the insulating ring 18 is made from the same P"-alumina as the separator 16, so that the support layer 86, the solid electrolyte layer 84 and the ring 18 are all of 6"-alumina, forming a single continuous sintered ceramic body. The ring 18 has two peripheral grooves or recesses 92, 94 (Figure 5) extending circumferentially along the periphery of the separator 16 as shown. A pair of nickel rings 96, 98 (Figure 6), of circular crosssection, are bonded to the insulaing ring 18 by active brazing, being located in the grooves 92, 94 respectively. The metal rings 96, 98 are electronically insulated from each other by the ring 18, with the rings 96, 98 being sealingly welded to the cathode cover 12 and the anode cover 14 respectively.

The ring 18 also has a rebate or groove 100 (as particularly illustrated in Figure 6) in which a peripherally extending fold or rib 102 on the surface of-the anode cover 14 facing the electrolyte layer 84 fits (see Figure 5), a circumferential protrusion or rib 104 of the ring 18, together with the periphery of the electrolyte layer 84, being nestingly received in a complementary rebate or groove formation 105 (Figure 5) formed in the surface of the anode cover 14 facing the electrolyte layer 84. The rebate 100 and the protrusion 104 on the ring 18, in conjunction with the fold 102 and the groove formation 105 on the anode cover 14, act to improve and enhance the flexibility of the anode cover 14 as the volume in the anode compartment varies, while facilitating location of the cover 14 in place on the ring 18 prior to welding of the cover 14 to the ring 98. Thus, the fold 102 conforms complementarily in shape with the shape of the rebate 100 and i's in face-to-face abutment with the rebate 100 when the cell 10 is in its fully Z3 discharged or overdischarged condition with the cover 14 abutting at least part of the surface 90 of the layer 84.

A portion of the ring 18 in use projects radially outward between the rings 96 and 98, to abut against the tubular battery housing 46 or its lining 48 or the cylinder 56 (see Figure 3), thereby to centre (and electrically insulate) the cell 10 in the battery 42 when the cell stack 40 is formed.

In Figure 7, part of the radially outer edge surface of the ring 18 is shown, having projections 106 in the shape of teeth provided thereon, the projections 106 having gaps or indentations 108 therebetween. When stack 40 of such cells 10 is formed, the projections 106 of all the cells 10 will be arranged to be in register, to define extended more or less continuous channels (made up by the indentations 108) extending along the length of the stack 40, the channels acting to facilitate transport of heating/cooling fluids along the stack and/or acting to house electrical heating elements. Instead, heating/cooling means in the form of a tube or coil 110 (Figure 5) can be connected to the ring 18 to extend circurnferentially along the outer periphery of the ring 18.

While the concave face 90 of the layer 84 can be shaped as described above with reference to Figures 1 - 4 it is, in Figure 5, shaped to conform with the convex side of an ideal elastic plate freely supported at its rim and subjected to a pressure which applies a force or load perpendicular thereto in a direction towards the layer 84, so that the central 60 - 90% of the area of such ideal plate (corresponding to the anode cover 14) would abut and be supported by the surface 90.

Porosity of the support layer 86 is varied over its area to compensate for varying thickness of the support layer 86 to equalise the ionic resistance between the cathode and the anode over the full solid electrolyte separator area. In other words, ionic resistance/unit area of the separator 16 is kept constant by grading the porosity of the support layer 86, porosity being greater when the support layer 2, k 86 is thicker and porosity being less when the support layer 86 is thinner. The pore area of the support layer 86, when sintered, is at least 40% of the area of the surface of solid electrolyte layer 84 which is in contact with the layer 86.

The separator 16 of Figure 5 is typically made by pressing, in a die, a green ceramic paste, to shape the support layer 86 and the solid electrolyte layer 84. Instead, the separator 16 can be made by forming the support layer 86 and then coating the support layer 86 with the solid electrolyte layer 84, the coating being carried out on a green ceramic support layer 86 to form a green composite separator 16, the green composite separator 16 then being consolidated by firing, after debonding of the green material and decomposition of any pore- forming agents and/or other organic matter used to form pores in the support layer 86.

Advantages of the inveniion as shown in the drawings, and advances constituted thereby in the art, are described hereunder.

The present invention can be applied to electrochemical cells having cell systems with electrochemistry whereby, in particular, a liquid sodium anode is combined with a sodium ion- conducting solid electrolyte of the 6-alumina structural family of compounds such as 6- or fl"-alumina (6- or 6"- sodium polyaluminate) and a liquid molten salt sodium ion-conductive electrolyte in hydraulic and ionic contact with both the solid electrolyte and with a mass of active cathode material. Instead, the cathode may be provided by a catholyte.

The liquid molten salt electrolyte may be a chloroaluminate electrolyte having a composition selected to achieve both a low melting point and a high sodium ion conductivity. The electrolyte may be a low melting chloroaluminate electrolyte having quaternary ammonium or imidazolinium compounds or sulphur dioxide as constituents, so that it is liquid even at ambient temperatures or below. When a catholyte is employed such as in sodium/sulphur cells, the catholyte will be molten sulphur/sodium sulphidelsodium polysulphide which acts as electrolyte and active cathode material.

Z5 In further variations the active cathode material may contain, instead, phosphorus, a phosphide and/or a polyphosphide, and halides or polyhalides may also be employed. Many active cathode materials can potentially be used, including metallic compounds, non-metallic compounds and organic compounds such as, for example, redox-active polymeric compounds containing disulphide bridges. Furthermore, precursors of the active cathode material may be used in cell assembly, being intended to be activated to form cathodes by heating and/or charging. Thus, by precursors are meant chemical compositions or mixtures having constituents which can be caused to react chemically and/or electrochemically after cell closure in situ, by heating and/or by applying a charging potential to the cell to form the eventual active cathode material of the cell, which active cathode material is periodically charged and discharged during cell cycling.

Thus, the active cathode raterial or its precursor may be impregnated into or otherwise compounded with an electronically conducting current collecting body or mass such as graphite or metal felts or foams, expanded metal screens or metal powders, said mass or body serving as a cathode current collector in electronic contact with the positive pole or terminal of the cell. This current collector such as a graphite felt may extend throughout the active cathode material as a threedimensional current collector, both holding and caging both the solid and liquid constituents of the cathode, including the molten salt liquid electrolyte.

It is expected that the present invention will have particular application to sodium,Isulphur cells, and sodium/transition metal chloride cells, such as sodium/ferrous chloride or sodium/nickel chloride cells.

Sodium/sulphur cells or sodium,Itransition metal chloride cells known to the Applicant having tubular solid electrolyte ceramic separators have certain disadvantages, and substantially flat, non-tubular and more or less planar solid electrolyte ceramic separators are desirable for the construction of more or less planar or flattened cells having high energy densities. While the Applicant is aware that it is possible to build flattened or planar cells having flattened hollow solid 2& electrolyte ceramic separators constructed to serve as electrode holders, it can be desirable instead to employ flattened or planar cells having a solid electrolyte in the form of a single more or less flattened sheet or layer in each cell, such planar cell being particularly suitable for stacking to form a stack of series-connected cells, for 5 example as electric vehicle batteries.

In the context of a sodium/sulphur cell system, various possible uses and advantages of such flattened planar cells are given in United States Patent US 5053 294.

For low cell internal electrical resistance, solid electrolyte separators should be as thin as possible, but with thicknesses compatible with the strength required for the typically brittle ceramics in question which constitute such solid electrolytes, to resist the stresse which are generated in the cells during heating and cooling, and during charge/discharge cycling.

In such cells the quantity of molten alkali metal anode material, such as sodium, in the anode compartment can vary considerably with the state of charge of the cell, which is aggravated by the fact that alkali metals expand substantially during melting. In a planar cells having a more or less planar separators, these factors endanger the integrity of flat solid electrolyte ceramic separators or membranes which separate the molten alkali metal liquid on the anode side thereof, and the molten electrolyte or catholyte liquid on the cathode side thereof.

In US 5 053 294 it is proposed to strengthen the solid electrolyte separator by means of a supporting grid or mesh, whereas the Applicant Is aware that provision has been made for flexible bipolar end plates separating neighbouring cells and shared by the neighbouring cells In question.

However, both of these solutions have shortcomings. A supporting mesh or grid diminishes the active surface of the separator available for ion transport, and bipolar flexible end plates will compress the cathode contents when the cell is 2--t charged, because during charging liquid alkali metal is generated in the anode compartment, and causes the associated end plate to bulge outwardly away from the alkali metal anode, thereby compressing the cathode in the cathode compartment of the neighbouring cell.

Conversely, during discharge, each bipolar plate will move away from the associated cathode and bulge into the neighbouring anode compartment. These continuous movements of the bipolar end plates can continuously change the degree of electronic contact between the end plate and the solid parts of the cathode, which solid parts usually comprise a current collecting matrixes of the type mentioned above.

The present invention as described with reference to the drawings provides a solution for at least some of th problems mentioned above, and provides a cell having, at least potentially, enhanced energy density.

Thus, an advantage of the construction shown in the drawings emerges from the fact that the curved (convex/concave) shape of the solid electrolyte separator 16 has in principle a higher bending strength compared with a flat or planar disc, and accordingly can be made relatively thinner and/or of a larger diameter than.a flat or planar disc, while being able to resist pressure differences across it of equal or greater magnitude. Furthermore, the flexible and conveniently elastic or resilient metal construction of the anode cover 14 allows it to flex during charge/discharge cycling, to accommodate any and all volume changes of the molten alkali metal anode, which can, at least in principle, eliminate the need for alkali metal-retaining and - wicking structures in the anode compartment.

In particular, in contrast to the elastic flexible bipolar end plates mentioned above, the anode cover 14 of the present invention does not form an end plate of a neighbouring cell, and does not form a wall of a neighbouring cell cathode compartment. Its flexing will thus not compress or otherwise influence the contents of the cathode compartment of the neighbouring cell.

Z-5 The flexible or elastic anode cover 14, by containing the contents of the anode compartment at at least atmospheric pressure, can by means of this pressure assist in providing mechanical support for the solidelectrolyte separator which, as mentioned above, is relative fragile.

It should be noted that, if desired, the separator panel 16 may be strengthened and otherwise improved by joining a porous, electrolytepermeable supporting layer to a relatively thin, pore-free alkali metal ion- permeable conducting layer, the porous layer being provided on the cathode side of the separator, and the ion-conductive layer being provided on its anode side.

Furthermore, the separator can be shaped to have a thickness which gradually increases from a minimum at its apex or centre, in a radially outward I direction, so that it has a cross-section which resembles that of a concave optical lens, this radially outwardly decreasing thickness gradient acting to provide the separator with adequate strength and acceptable average thinness for good ion conduction.

The present Invention, as described with reference to Figures 5 - 7, by virtue of the active brazing avoids the necessity for any glass sealing. Provision is made for a cathode matrix of constant thickness; and the insulating ring 18, where it projects radially outwardly between the nickel rings 96, 98, eliminates the need for separate electrical insulation around the periphery of the cell. The method of connection of the periphery of the anode cover 14 to the ring 18 reduces stress in the anode cover, upon flexing thereof, and provision is made for relatively increased volume of the charged anode compartment, compared with the construction of Figures 1 - 4.

Claims (16)

2CA CLAIMS

1. A high temperature rechargeable electrochemical cell which comprises a cell casing defining an anode compartment for containing an alkali metal anode and cathode compartment containing a liquid electrolyte, the cell having an operating temperature at which the anode and liquid electrolyte are molten and the anode compartment being separated from the cathode compartment by a separator comprising a solid electrolyte which is a conductor of ions of the alkali metal of the anode at the operating temperature of the cell, the casing comprising a cathode cover of sheet material enclosing the cathode-side surface of the separator and an anode cover of sheet material enclosing the anode-side surface of the separator, the cathode cover and the anode cover being electronically conductive and electronically insulated from each other and respectively forming a cathode terminal and an anode terminal for the cll, the separator being in the form of a sheet or plate and having a curved concave anode-side surface facing into the anode compartment, the cathode cover being relatively inflexible and being spaced from the cathode-side surface of the separator and the anode cover being relatively flexible, and the cell having a fully discharged or overdischarged state in which the anode compartment is substantially empty of alkali metal and the anode cover has a convex surface which nests in the concave surface of the separator to reduce the volume of the anode compartment, the anode cover being capable of flexing away from the separator in response to ionic passage of alkali metal into the anode compartment through the separator upon charging of the cell, to increase the volume of the anode compartment.

2. A cell as claimed in claim 1, in which the cell is circular in outline, the separator also being circular in outline, the cell having a fully discharged or overdischarged state in which the convex surface of the anode cover is in face-toface contact with a major proportion of the concave surface of the separator, and the separator having a bead of electronically insulating material extending along its periphery, to which bead the separator is sealingly connected, the cathode cover and anode cover having peripheries which are sealingly connected to opposite sides of the bead.

3. A cell as claimed in claim 2, in which opposite sides of the bead have peripherally extending metallic rings hermetically sealed thereto, the rings being electronically insulated from each other and the peripheries of the anode cover and the cathode cover being welded respectively to said rings.

4. A cell as claimed in claim 3, in which the separator is in the form of a disc which is part-spherical in shape, the rings being in the form of flattened metal strips.

5. A cell as claimed in claim 3, in which the separator is of composite construction, comprising a hyaraulically impermeable solid electrolyte layer supported on a porous support layer, which support layer is located between the cathode compartment and the solid electrolyte layer and is hydraulically permeable by and impregnated with the liquid electrolyte.

6. A cell as claimed in claim 3 or claim 5, in which the separator has a flat surface facing the cathode compartment and a curved concave surface facing-the anode compartment, the metallic rings being of circular crosssection.

7. A cell as claimed in claim 3, in which the separator is of composite construction, comprising a hydraulically impermeable solid electrolyte layer supported on a porous support layer which is located between the cathode compartment and the solid electrolyte layer and is hydraulically permeable by and impregnated with the liquid electrolyte, the porous layer having a flat surface facing the cathode compartment and the hydraulically impermeable electrolyte layer having a curved concave surface facing the anode compartment, the porous layer having a concave curved surface via which it supports the electrolyte layer and the electrolyte layer having a convex curved surface via which it is supported by the porous layer, the porous layer having a porosity which increases in a radial 3 direction from a minimum at a central position on the porous layer, to a maximum at the periphery of the porous layer.

8. A cell as claimed in any one of the preceding claims, in which the cell cathode compartment contains, in addition to the liquid electrolyte, a solid active 5 cathode material.

9. A battery of electrochemical cells, the battery comprising a plurality of cells as claimed in any one of claims 1 - 8 or 15, stacked in series, one upon another, to form a stack of said cells, the cells of each adjacent pair of cells of the stack being in contact with each other via contact between the anode cover of one cell of said pair and the cathode cover of the other cell of said pair.

10. A battery as claimed in clim 9, in which one of said anode and cathode covers of each said cell has a recessed rim providing a peripherally extending rebate in its outer surface adjacent its periphery, the other cover of said cell having a raised rim providing a peripherally extending step in its outer surface adjacent its periphery, the step and the rebate being complementarily shaped and the cells being stacked one upon another, with the step of one of the cells of each adjacent pair of cells nestingly received in the rebate of the other cell of said pair.

11. A battery as claimed in claim 9 or claim 10, in which a thermal control means is provided in the housing.

12. A battery as claimed in claim 11, in which the thermal control means concentrically surrounds the stack.

13. A battery as claimed in any one of claims 9 - 12 inclusive, in which the stack of cells is contained in a tubular battery housing comprising thermal insulation, the stack of cells being provided with respective anode and cathode battery terminals and the housing having ends provided with electronically insulating terminal feedthroughs, the terminals extending respectively through the feedthroughs.

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14. A battery as claimed in claim 13, in which the battery terminals respectively engage opposite ends of the stack and urge the cells in the stack together to keep them in place in the stack, the terminals in turn being held in place by the feedthroughs.

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

1 '

16. A battery of electrochemical cells as claimed in claim 9, substantially as herein described and illustrated.