GB2621656A - Electrochemical cell - Google Patents

Abstract

A modified ZEBRA molten sodium/metal chloride cell with anode and cathode compartments 14, 15 (partly enclosed by metal plate 11, 12) separated by impermeable, sodium-ion-conducting ceramic sheet 13, 23b, preferably NaSICON, adjacent, deposited and/or bonded on a metal sheet 24 (16, fig 2), which in the latter case is perforated; the cathode compartment containing a cathodic mixture 25 e.g. biscuit, comprising metal powder (e.g. Iron or Nickel), NaCl, and sodium aluminium chloride (sodium-tetrachloroaluminate, NaAlCl4). The cathode compartment 15 may include a carbon felt layer 26, e.g. between plate 12 and the cathodic mixture face remote from the ceramic. Carbon felt 82, (83, fig 2) may be provided on the anode side between ceramic sheet and metal plate or in the anode molten sodium. The supporting metal sheet allows a thinner ceramic sheet, allowing lower operating temperature. The carbon felt improves cell performance consistency. Performance can also be enhanced by pre-treating the surface of the electrolyte/separator 13, (16, fig 2) facing the anode compartment (14) to enhance wetting by molten sodium, e.g. by coating or painting the surface with sodium phosphate containing carbon, tin, or aluminium powder; or lead acetate, or, in ethanol, manganese nitrate or iron chloride or tin chloride.

Description

Electrochemical Cell The present invention relates to an electrochemical cell with a solid ionicallyconducting electrolyte/separator.

A number of different types of electrochemical cell are known that require an elevated temperature to operate. These include cells in which an electrolyte must be at elevated temperature to provide adequate conductivity; and cells in which an electrode must be at elevated temperature for an electrode component to be liquid. One such type of cell is a molten sodium/metal halide rechargeable battery, such as the sodium/nickel chloride cell which may be referred to as a ZEBRA cell (see for example J.L. Sudworth, "The Sodium/Nickel Chloride (ZEBRA) Battery (J. Power Sources 100 (2001) 149-163). A sodium/nickel chloride cell incorporates a liquid sodium negative electrode separated from a positive electrode by a solid electrolyte which conducts sodium ions. The solid electrolyte may for example consist of beta alumina. The positive electrode includes nickel, nickel chloride and sodium tetrachloroaluminate which is liquid during use and acts as a secondary electrolyte to allow transport of sodium ions from the nickel chloride to the solid electrolyte. The positive electrode also incorporates aluminium powder. Partial replacement of the nickel with other transition metals such as iron can result in additional discharge voltage levels. The cell operates at a temperature which is typically below 350°C, but must be above the melting point of the sodium tetrachloroaluminate, which is 157°C, and the operating temperature is typically between 2700 and 300°C. During discharge the normal reactions are as follows: Cathode (positive electrode): NiCl2 + 2 Na t+ 2 e 4 Ni + 2 NaCI Anode (negative electrode): Na 4 Na + e the overall result being that anhydrous nickel chloride (in the cathode) reacts with metallic sodium (in the anode) to produce sodium chloride and nickel metal; and the cell voltage is 2.58 Vat 300°C.

A modified type of a ZEBRA cell, that is to say a molten sodium-nickel chloride rechargeable cell, is described in WO 2019/073260. This uses an electrolyte element that comprises a perforated sheet of non-reactive metal, and a non-permeable layer of sodium-ion-conducting ceramic bonded to one face of the perforated sheet. In this electrolyte element the strength can therefore be provided by the metal sheet, and this enables the electrolyte thickness to be significantly reduced as compared to that required in a conventional ZEBRA cell. This results in a cell or a battery that can perform adequately at significantly lower temperatures, for example less than 200°C. Furthermore, a significantly thinner layer of ceramic also significantly reduces stresses induced by heating from ambient, so start-up times from ambient can be just a few minutes. These are both commercially advantageous benefits. The non-permeable layer is bonded to the perforated metal sheet, and this bonding may be by a porous ceramic sub-layer. Such a cell includes a metal case, which may have a peripheral flange.

An alternative form of molten sodium/nickel chloride cell is described in WO 2022/123246 (International application PCT/6B2021/053215) in which the electrolyte is again a planar sheet of ceramic that can conduct ions of the alkali metal, and a perforated planar sheet of an inert metal is immediately adjacent to the sheet of ceramic and in contact with the sheet of ceramic over substantially its entire area, to provide support to the sheet of ceramic. The ceramic sheet is formed separately from the perforated planar sheet, rather than being formed by deposition onto it.

In each of these cells the electrolyte separates the anode chamber from the cathode chamber, and in use at the operating temperature there are liquid phases in each chamber. It has nevertheless been found that the cells may perform inconsistently; this appears to be due to problems at the interfaces between the electrolyte and the materials in those chambers.

According to the present invention there is provided a molten sodium/metal chloride electrochemical cell comprising two electrode compartments, one being an anode compartment and the other being a cathode compartment, each enclosed in part by a respective metal plate, the two compartments being separated by an impermeable, sodium-ion-conducting ceramic sheet, wherein the cathode compartment in its uncharged state contains a cathodic mixture comprising metal powder, sodium chloride, and sodium aluminium chloride (sodium tetrachloroaluminate, NaAIC14); wherein the cathode compartment also comprises a resilient element between the cathodic metal plate and the face of the cathodic mixture remote from the ceramic sheet.

The provision of this resilient element has been found to significantly improve the consistency of cell performance. This is because the resilient element allows the cathodic material to have better contact with the ceramic sheet. The resilient element may be a layer of carbon felt, or a resilent metal item such as a wave washer.

For manufacturing the cell, the cathodic mixture may first be preformed into a freestanding structure (referred to as a "biscuit"). For example, a powder mixture containing cathodic metal powder, sodium chloride, and aluminium powder and preferably also a small proportion other ingredients such as iron sulphide, sodium iodide and sodium fluoride, may be introduced into a mould, compacted, and then infiltrated with molten sodium aluminium chloride (sodium tetrachloroaluminate, NaAIC14), preferably under vacuum. When cooled to room temperature the resulting biscuit is strong enough to be handled, and can be assembled with the other cell components. During subsequent operation of the cell, the operating temperature is above the melting point of the NaAIC14. The provision of the resilient element urges the biscuit towards the sodium-ion-conducting ceramic sheet. It has been found that if the resilient element is a layer of carbon felt, the molten NaAIC14does not migrate far into the layer of carbon felt.

The cell is thus a modified ZEBRA cell, such that the anode compartment of the cell when charged contains sodium metal. The cathodic metal powder in the cathode compartment may be nickel, or may be iron. Iron provides a cell with an open circuit voltage about 10% lower than that with nickel.

The ceramic sheet may be planar and may for example be rectangular, square, or any other polygonal shape; it may have rounded corners; or it may be circular or elliptical. It determines the area of the cell through which ionic conduction occurs between the two electrode compartments.

The cell has the ceramic sheet as its electrolyte, to separate the anode compartment from the cathode compartment. The cell also comprises, preferably in the anode compartment, a sheet of metal adjacent to the sheet of ceramic, to provide support to the sheet of ceramic, the ceramic sheet either being formed by deposition as a layer onto the metal sheet, in the case that the metal sheet is perforated, or alternatively being formed separately from the metal sheet. The ceramic electrolyte sheet must be non-permeable to gases or liquids, although it is a conductor of the sodium ions that must pass between the anode and cathode compartments during operation.

The anode compartment may also comprise a carbon felt, preferably highly porous, for example of long carbon fibres, to assist in the transfer of sodium metal away from or towards the sheet of ceramic, during charging and discharging of the cell. The carbon felt is in the form of a paper-like sheet, initially about 1.5 mm thick, that is highly porous, fibrous, and preferably graphitic, and may for example have an area density of less than 200 g/m2, for example 100 g/m2. This porous element forms part of a capillary wick system that ensures that sodium uniformly wets the ceramic electrolyte sheet at all states of charge.

The metal of which the metal sheet is formed is "inert" in the sense that it does not react chemically with components of the cell with which it is in contact during use; it may for example be a metal such as nickel, or aluminium-bearing ferritic steel (such as the type known as Fecralloy (TM), or a steel that forms an electronically-conductive and adherent scale, for example a CrMn oxide scale, when heated in air. If the metal sheet is separate from the ceramic sheet then it may be of a steel such as stainless steel. The metal sheet may be of thickness no more than 1.0 mm, or no more than 0.5 mm, for example 0.1 mm or 0.2 mm. The sheet is perforated so it has a very large number of through holes, and the perforations or holes may be of mean diameter less than 50 jtm, for example 30 p.m or less, or of mean diameter between 50 jtm and 300 km, and may for example be produced by a laser drilling process or by chemical etching. The through holes may have their centres spaced apart at between 100 km and 500 km, for example 150 p.m. The holes help the distribution of sodium.

The perforated metal sheet may have a margin around its periphery that is not perforated; this margin may make it easier to seal the periphery of the perforated plate to adjacent components of the cell. This margin may be of width no more than 15 mm, for example 10 mm or 5 mm or 3 mm. The edge of the perforated sheet may be welded to a lip of an adjacent cell component, for example a plate that forms the anode compartment.

The metal sheet is preferably in the anode compartment, where it will help wick the molten sodium towards the surface of the ceramic sheet. If the metal sheet is in the anode compartment but spaced away from the ceramic sheet, then it does not need to be perforated.

Where the ceramic sheet is separate and spaced away from the metal sheet, or if there is no metal sheet the invention may also involve pretreating the surface of the ceramic sheet facing the anode compartment, the pretreatment being such as to improve wetting of the surface by molten sodium. For example the surface may be painted with an aqueous solution of sodium polyphosphate containing carbon powder, tin powder, and/or aluminium flake or powder, and then dried and baked. The sodium polyphosphate forms a sodium-ion-conducting glass, and the metal or carbon particles assist wetting by sodium. Other options are to coat or paint the surface with lead acetate in aqueous solution, or manganese nitrate or iron chloride or tin (II) chloride in solution in ethanol, the surface then being dried and baked; in the case of lead acetate or tin chloride the baking is performed in an oxygen-free atmosphere to produce a mixture of Pb(IV) oxide and Pb metal, or Sn(IV) oxide/chloride and Sn metal, all of which are conductive. The resulting small particles of metal, metal oxides or metal chlorides on the surface improve wetting by molten sodium and so improve the consistency of cell operation.

A preferred coating is formed from an aqueous solution of lead acetate and tin acetate. This mixture can be painted on and dried. The cell can then be assembled, and the decomposition will then occur subsequently in situ in the cell. Decomposition in the absence of air initially produces a mixture of oxides which may include different oxidation states -Pb02 for example is electronically conductive -but once the cell is charged the oxides are reduced to the metals. Alternatively the coating may be dried and heated in an inert atmosphere to bring about the decomposition, before being assembled into the cell. If the compounds in the aqueous solution are in proportions that correspond to the eutectic composition, 63%Sn and 37%Pb, then the metallic particles on the surface will be molten at a typical cell operating temperature. Good results can also be obtained by painting or spraying an aqueous solution of sodium polyphosphate containing tin powder; this is then dried and then heat treated to 275°C to dewater the polyphosphate. Where the anode compartment contains a metal shim or foil rather than carbon felt adjacent to the ceramic sheet, to spread the molten sodium by capillarity, such a metal shim or foil is desirably also coated with a sodium-wetting material; for example a thin steel foil may be coated with tin.

Where the ceramic sheet is bonded to the perforated metal sheet to form an electrolyte element, the metal sheet would be on the side facing the anode compartment.

The ceramic sheet is typically formed by deposition onto the perforated metal sheet, preferably with at least one porous liquid-permeable ceramic sub-layer between the perforated metal sheet and the non-permeable ion-conducting ceramic layer that is the electrolyte. In this case too the consistency of the cell performance may be improved by such a pretreatment to further improve wetting of the surface by molten sodium. The perforated metal surface of the electrolyte element may be painted with a solution, such as lead acetate in aqueous solution, or manganese nitrate or iron chloride or tin chloride in solution in ethanol, so the solution soaks through the perforations and into the porous portions of the ceramic sheet. Drying and baking the electrolyte element produces very small particles of oxide, metal or chloride on the surfaces of the perforations and the pores, which enhance wetting by molten sodium in the operating cell. In this case there are benefits from using lead acetate, with the baking being in an oxygen-free atmosphere as mentioned above, as the resulting Pb/Pb(IV) oxide mixture forms an electronically conductive structure that extends into the pores of the ceramic sheet.

If the ceramic sheet electrolyte is formed separately, the perforated sheet may be held up against the ceramic sheet electrolyte by resilient elements in the anode compartment, for example edge portions of the metal sheet may be curved back to form C-shaped or V-shaped springs that contact the anodic plate on the opposite side of the anode compartment. In addition there may be a highly porous or permeable layer between the ceramic sheet and the metal sheet, such as a layer of carbon felt, in which case the metal sheet does not need to be perforated.

The metal plates that define in part the anode compartment and the cathode compartment are also of inert metal, in the sense that they do not react with the contents of the respective compartments during use. They may be of stainless steel, or the metals mentioned above as suitable for the perforated sheet.

Such a cell operates at an elevated temperature. A conventional sodium/nickel chloride cell, or ZEBRA cell, operates at 280°C or 300°C. The operating temperature depends in part on the nature of the electrolyte and its ionic conductivity; a cell with a thin layer of ceramic as the electrolyte may have a lower operating temperature, for example in the range 175°C to 225°C. In any event the sealing between the cell components must remain tight at the elevated temperature of operation. The sealing may utilise a high-temperature polymer such as PTFE, or an inorganic material of an electrical insulator, such as mica or vermiculite. High-temperature polymers are preferably not used to seal the anode compartment, as they may interact with the molten sodium. The anode plate may be sealed to the edge portion of a perforated sheet with a carbon-based gasket, or indeed they may be welded together.

It will be appreciated that, as described above, cell performance may be enhanced by providing a resilient element such as a layer of carbon fibre felt in the cathode compartment, between the cathodic metal plate and the face of the cathodic mixture remote from the ceramic sheet; and that it can also be enhanced by pre-treating the surface of the separator/electrolyte facing the anode compartment to enhance wetting by molten sodium. These modifications may be utilised separately, but are preferably both used.

The invention will now be further and more particularly described, by way of example only, and with reference to the accompanying drawings in which: Figure 1 shows a cross-sectional view through an electrical cell of the invention; Figure 2 shows a cross-sectional view of an alternative cell of the invention; and Figure 2a shows part of figure 2 at a larger scale.

Referring to figure 1, this shows a cell 10 of the present invention. The cell 10 operates at an elevated temperature, and comprises dish-shaped metal electrode plates 11 and 12 each defining a flat peripheral rim, between which is an impermeable sodium-ion-conducting electrolyte sheet 13. The electrode plates 11 and 12, together with the electrolyte sheet 13, define an anode compartment 14 on one side of the electrolyte sheet 13 and a cathode compartment 15 on the other side of the electrolyte sheet 13, which contain chemicals that interact as a consequence of the passage of ions through the electrolyte sheet 13 to generate electricity. Around their periphery the rims of the electrode plates 11 and 12 are sealed to the sheet of electrolyte 13 by a heat-resistant electrically-insulating sealant 17. In this case the cell components are held together by crimping the edge of the electrode plate 11 around the edges of the electrolyte sheet 13 and the electrode plate 12, all of which are separated by the insulating sealant 17; this sealing and crimping arrangement is represented diagrammatically.

The cell 10 in this example is a sodium/nickel chloride cell. The electrode plates 11 and 12 may be of stainless steel, and of dished form to define the anode compartment 14 and the cathode compartment 15, with a flat peripheral rim. In its charged state the cell 10 would contain sodium metal in the anode compartment 14 and nickel chloride in the cathode compartment 15. However, the cell would typically be assembled in a completely discharged state, the cathode compartment 15 being initially filled with a powder mixture containing nickel powder, sodium chloride, and sodium aluminium chloride (sodium tetrachloroaluminate, NaAIC14) and preferably also a small proportion other ingredients such as iron sulphide, sodium iodide, sodium fluoride, and aluminium powder. (As mentioned above, in an alternative the cell might instead be a sodium/iron chloride cell, if iron powder is used instead of nickel powder.) In particular, for ease of assembly, the powder mixture including nickel powder and sodium chloride, but without the sodium aluminium chloride, would be placed in a mould, and compressed; the compressed powder mixture is then infiltrated with molten sodium aluminium chloride under vacuum, and then cooled, to form a coherent biscuit 25. This biscuit 25 is placed in the cathode compartment 15, occupying most of the space, and a layer 26 of carbon felt is placed above the biscuit 25, between the biscuit 25 and the cathode plate 12.

The impermeable sodium-ion-conducting electrolyte sheet 13 may be of NaSICON, that is to say a sodium ion conductor of the general formula Nato( Zr2 Six P3-x 012, 0< x < 3; more broadly it refers to a sodium ion conductor which is a similar compound in which Na, Zr and/or Si are replaced by isovalent elements. A perforated metal sheet 24 is adjacent to the electrolyte sheet 13, spaced apart from it by a layer of carbon felt 82. The perforated metal sheet 24 provides mechanical support to the electrolyte sheet 13, and may be of thickness in the range 100 to 200 pm. Along two opposite sides, peripheral portions of the perforated metal sheet 24 are curved in to form springs 80 which contact the inner face of the anode plate 11 in the assembled cell 10, so those springs 80 push the perforated metal sheet 24 resiliently towards the electrolyte sheet 13.

The carbon felt 82 is readily wetted by molten sodium, so it helps to wick molten sodium towards or away from the face of the electrolyte sheet 13. The wicking layer of carbon felt 82 helps utilise the full charge of the cell 10, and that the full rate charge can be provided by or supplied to the cell 10. It has nevertheless been found that more consistent cell performance can be achieved by treating the surface of the electrolyte sheet 13 itself so it is more readily wetted by molten sodium. This may be achieved by painting that surface with 10 pm aluminium powder or flake suspended in an aqueous sodium polyphosphate solution, or 10 km tin powder suspended in the sodium polyphosphate solution; the painted layer is dried, and baked. The sodium polyphosphate forms a sodium-ion-conducting glass, while the particles of aluminium or tin improve the wetting.

By way of example the suspension may be in the proportions 10 parts (by weight) water, 4 parts sodium polyphosphate and 4 parts metal powder, with a small amount of acetone. This is thoroughly mixed, to generate the suspension. It can then be painted onto the ceramic with a brush; or with a higher proportion of water it can be sprayed on (preferably onto a heated surface). The treated ceramic is then dried at room temperature for a few hours or at an elevated temperature for a shorter time; and then baked at 275°C to dewater the polyphosphate and form the glass.

Another satisfactory coating is formed from an aqueous solution of lead acetate and tin acetate in proportions that correspond to the eutectic composition, 63%Sn and 37%Pb. This mixture can be painted on, or sprayed on, and then dried. The cell can then be assembled, and the decomposition and reduction to SnPb occurs subsequently in situ in the cell. The small particles (or droplets, if the cell is at above 183°C) of SnPb on the surface of the ceramic enhance the degree to which molten sodium wets the surface.

For the cell 10 to operate, it must first be heated to a temperature above 157°C, such as 200°C, at which the sodium aluminium chloride is molten, and at such a temperature the non-permeable ceramic electrolyte sheet 13 will conduct sodium ions sufficiently. The molten sodium aluminium chloride enables sodium ions to diffuse between the sodium chloride and the electrolyte sheet 13. The cell can therefore be charged by applying a voltage from an external power supply between the two electrode plates 11 and 12, so sodium ions pass through the electrolyte sheet 13 into contact with the carbon felt 82 and the perforated metal sheet 24 in the anode space 14, where sodium metal is formed, while within the cathode space 15 the remaining chloride ions react with the nickel to form nickel chloride. The charge transfer reaction occurs at the interface where the ceramic electrolyte sheet 13 contacts an electronically conductive element; during charging this is initially the carbon felt 82, but once a layer of molten sodium has been produced on the surface of the ceramic sheet 13 the charge transfer and metallic sodium formation will take place at the interface between the ceramic sheet 13 and the sodium. The cell 10 is readily reversible, so it can be charged and discharged multiple times.

The perforated metal sheet 24 which provides support to the electrolyte sheet 13 may be of a metal such as nickel, or aluminium-bearing ferritic steel (such as the type known as Fecralloy (TM)), or a steel that forms an electronically-conductive and adherent scale, for example a CrMn oxide scale, when heated in air, or indeed any other type of steel with a clean metal surface. Most of the sheet 24 is perforated to produce a very large number of through holes, but the peripheral portions that form the springs 80 do not require perforations.

Referring to figure 2, there is shown an alternative electric cell 20 of the present invention; it has many features in common with the cell 10, identical components being referred to by the same reference numbers. The cell 20 operates at an elevated temperature, and comprises dish-shaped metal electrode plates 11 and 12 each defining a flat peripheral rim, between which is a perforated metal sheet 16 to which is bonded a layer of impermeable sodium-ion-conducting ceramic electrolyte 23. The electrode plates 11 and 12, together with the electrolyte layer 23, define an anode compartment 14 on one side of the electrolyte layer 23 and a cathode compartment 15 on the other side of the electrolyte layer 23, which contain chemicals that interact as a consequence of the passage of ions through the electrolyte layer 23 to generate electricity. Around their periphery the rims of the electrode plates 11 and 12 are sealed to the metal sheet 16 by a heat-resistant electrically-insulating sealant 17. In this case the cell components are held together by crimping the edge of the electrode plate 11 around the edges of the metal sheet 16 and the electrode plate 12, all of which are separated by the insulating sealant 17; this sealing and crimping arrangement is represented diagrammatically.

As shown in figure 2a, a margin 16a around the periphery of the metal sheet 16, typically of width 5 mm, is not perforated; the remainder has multiple perforations 18. The perforated portion of the sheet 16 is covered by the electrolyte layer 23. This layer 23 consists of a porous and permeable ceramic sub-layer 23a which is itself covered by a non-permeable ceramic layer 23b, the non-permeable ceramic layer 23b being of a sodium-ion-conducting ceramic. The non-permeable ceramic layer 23b is preferably of NaSICON, though it may instead comprise beta alumina; furthermore it may contain a material that forms a glass during the sintering process. Thus although it is referred to as a ceramic layer, the term "ceramic" in this context includes combinations of ceramic and glass, as long as the layer is conductive to sodium ions during operation. The non-permeable ceramic layer 23b must not be permeable, that is to say it would be impermeable to gases, and consequently impermeable to liquids during operation. The non-permeable layer 23b also covers the edges of the sub-layer 23a (as shown in figure 2). The electrolyte sheet 23 thus consists of the combination of the permeable ceramic sub-layer 23a and the non-permeable ceramic layer 23h; these ceramic layers may be formed by deposition onto the metal sheet 16, so they are integral with each other, and bonded to the metal sheet 16.

The porous sub-layer 23a may be of the same sodium-ion-conducting ceramic as the non-permeable ceramic layer 23b. The porous and permeable ceramic sub-layer 23a may be of thickness between 10 pm and 100 pm, while the non-permeable layer 23b may be of thickness in the range 5 pm to 50 pm, for example 20 km, 30 pm or 40 pm.

Referring again to figure 2, the cell 20 is shown in its initially assembled state. The cathode compartment 15 encloses a biscuit 25 made of the powder mixture including nickel powder and sodium chloride, held together with sodium aluminium chloride as described above, and occupying most of the space, and a layer 26 of carbon felt above the biscuit 25, between the biscuit 25 and the cathode plate 12.

Both the perforated metal sheet 16 and the carbon felt 83 are readily wetted by molten sodium, so helping wick molten sodium towards or away from the electrolyte layer 23. It has nevertheless been found that more consistent cell performance can be achieved by treating the metal sheet 16 so it is more readily wetted by molten sodium. This may be achieved by painting that surface with a solution of manganese nitrate in ethanol, or of lead acetate in water so the solution soaks through the perforations and into the porous sub-layer 23a. Drying and baking at a temperature in the range 250° to 300°C then produces very small particles of oxide or metal on the surfaces of the perforations and the pores, which enhance wetting by molten sodium in the operating cell. Alternative solutions that may be used for this purpose include tin chloride in ethanol, and ferrous chloride in ethanol; with ferrous chloride solution the resulting small particles are of ferrous chloride, whereas with Sn(II) chloride solution if baked in an atmosphere without oxygen it appears to produce a mixture of tin metal and Sn(IV) chloride and oxide which provides a surface that is both conductive and readily wetted. As mentioned above, good wetting by sodium may be achieved by coating with aqueous solution of lead acetate and tin acetate in proportions that correspond to the eutectic composition, 63%Sn and 37%Pb; the surface is dried, and subsequent heating produces particles of SnPb on the surface.

For the cell 20 to operate, it must first be heated to a temperature above 157°C, such as 200°C, at which the sodium aluminium chloride is molten, and at such a temperature the non-permeable ceramic electrolyte layer 23 will conduct sodium ions sufficiently. The molten sodium aluminium chloride enables sodium ions to diffuse between the sodium chloride and the electrolyte sheet 13. The cell can therefore be charged by applying a voltage from an external power supply between the two electrode plates 11 and 12, so sodium ions pass through the electrolyte layer 23 into contact with the perforated metal sheet 16 and the carbon fibre felt 83 in the anode space 14, where sodium metal is formed, while within the cathode space 15 the remaining chloride ions react with the nickel to form nickel chloride. The cell 10 is readily reversible, so it can be charged and discharged multiple times.

The cells 10 and 20 are shown only by way of example, and can be modified in various ways while remaining within the scope of the claims. For example the sealing of the edges of the plates 11, 12 to the metal plate 16 or to the electrolyte sheet 13 may differ from that shown in the drawings.

Claims (1)

1. Claims 1. A molten sodium/metal chloride electrochemical cell comprising two electrode compartments, one being an anode compartment and the other being a cathode compartment, each enclosed in part by a respective metal plate, the two compartments being separated by an electrolyte element comprising an impermeable, sodium-ionconducting ceramic sheet either bonded to a perforated metal sheet or adjacent to but not bonded to a metal sheet, wherein the cathode compartment in its uncharged state contains a cathodic mixture comprising metal powder, sodium chloride, and sodium aluminium chloride (sodium tetrachloroaluminate, NaAIC14); wherein: - the cathode compartment also comprises a resilient element between the cathodic metal plate and the face of the cathodic mixture remote from the ceramic sheet; and/or - the surface of the electrolyte element facing the anode compartment has been pretreated to enhance wetting of the surface by molten sodium. 15 2. A cell as claimed in claim 1 wherein the ceramic sheet is separate from the metal sheet, wherein the surface of the ceramic sheet facing the anode compartment has been pretreated to improve wetting of the surface by molten sodium.3. A cell as claimed in claim 2 wherein the pretreatment is coating or painting the surface with a solution of sodium polyphosphate containing carbon powder, tin powder, or aluminium flake or powder, and then dried and baked.4. A cell as claimed in claim 2 wherein the pretreatment is coating or painting the surface with a solution, the solution being lead acetate in aqueous solution, or manganese nitrate or iron chloride or tin chloride in solution in ethanol, or an aqueous solution of lead acetate and tin acetate in proportions that correspond to the eutectic composition, 63%Sn and 37%Pb, the surface then being dried and baked to produce small particles on the surface.5. A cell as claimed in claim 4 wherein the small particles include metallic lead and/or metallic tin.6. A cell as claimed in claim 1 wherein the ceramic sheet is bonded to the perforated metal sheet to form the electrolyte element, the metal sheet being on the side facing the anode compartment, wherein the surface of the electrolyte element facing the anode compartment has been pretreated to improve wetting of the surface by molten sodium.7. A cell as claimed in claim 6 wherein the ceramic sheet is formed by deposition onto the perforated metal sheet, with at least one porous liquid-permeable ceramic sub-layer between the perforated metal sheet and the non-permeable ion-conducting ceramic electrolyte sheet.8. A cell as claimed in claim 6 or claim 7 wherein the pretreatment is painting or coating the perforated metal surface of the electrolyte element with a solution, such as lead acetate in aqueous solution, or manganese nitrate or iron chloride or tin chloride in solution in ethanol, or an aqueous solution of lead acetate and tin acetate in proportions that correspond to the eutectic composition, 63%Sn and 37%Pb, so the solution soaks through the perforations, the electrolyte element then being dried and baked to form small particles on the surface of the perforations that enhance wetting.9. A cell as claimed in claim 8 wherein the small particles include metallic lead and/or metallic tin.10. A cell as claimed in any one of claims 6 to 9 wherein the anode compartment also comprises a carbon felt, preferably highly porous, of long carbon fibres, to assist in the transfer of sodium metal away from or towards the sheet of ceramic, during charging and discharging of the cell.11. A cell as claimed in claim 10 wherein the carbon felt in the anode compartment has an area density of less than 200 g/m2, for example 100 g/m2.