Electrochemical cell

Abstract

A molten sodium / metal chloride electrochemical cell comprises an anode compartment (14) and a cathode compartment (15), each enclosed in part by a respective metal plate (11, 12), the two compartments being separated by an impermeable, sodium-ion-conducting electrolyte element. The electrolyte element comprises an impermeable glass-ceramic layer (23b) bonded by a porous and permeable glass-ceramic layer (23a) to a perforated metal sheet (16), wherein the materials of the glass-ceramic layers comprise Na5RSi4O12, where R is a rare-earth element. The element R may be Sc, Er, Y, Ho, Dy, Gd, Eu, or Sm. The porous layer (23a) and the impermeable layer (23b) may be of different chemical compositions.

Description

[0001] Electrochemical Cell

[0002] The present invention relates to an electrochemical cell with a solid ionically- conducting electrolyte / separator, and to such an electrolyte / separator.

[0003] 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 270° and 300°C. During discharge the normal reactions are as follows:

[0004] Cathode (positive electrode): NiCh + 2 Na++ 2 e‘ -> Ni + 2 NaCI

[0005] Anode (negative electrode): Na -> 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 V at 300°C.

[0006] 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. The ion-conducting ceramic may be P"-alumina; other options are Nai+xZ^SixPs- xOn or combinations such as NasPC - Na2SC>4, or glass ceramics such as NasPS^ 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 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.

[0007] However it has been found that the difference in thermal expansivity between the metal of the perforated metal sheet and the ceramic of the ion-conducting layer can lead to permanent deformation formed during manufacture, so an initia lly-planar perforated metal sheet may produce an electrolyte element that is not uniformly planar. This may make it more difficult to seal the edges of the electrolyte element to adjacent cell components. The deformation may be reduced to some extent by placing a frame-like weight on the periphery of the metal sheet, surrounding the ceramic coating during the sintering process, but this process only partly mitigates the deformation and does not alleviate sintering stresses generated from the mis-match in thermal expansion coefficients. It will be appreciated that this is a time-consuming, and labour-intensive process.

[0008] According to the present invention there is provided a 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 electrolyte element comprising an impermeable glass-ceramic layer bonded by a porous and permeable glass-ceramic layer to a perforated metal sheet, wherein the materials of the glass-ceramic layers comprise NasRSi^ , where R is a rare-earth element.

[0009] In this document the term glass-ceramic means a polycrystalline material produced through crystallisation of a glass, so producing a uniform dispersion of fine crystals. A glass is an amorphous material that has been molten and has then hardened and become rigid without crystallising. The rare earth element R may be Sc, Er, Y, Ho, Dy, Gd, Eu, or Sm. It has been established that ionic conductivity increases with increasing radii of trivalent cations from Sc3+ to Sm3+, ranging from 3 x 10-3 S / cm (Sc3+) to 0.1 S / cm (Sm3+) at 200°C. It has also been found that the higher the ionic conductivity, the lower the sintering temperature required to achieve a fully dense layer (see for example Materials 2022, 15, 1104, available In this document the term "N5-ceramic" means a glass-ceramic that comprises NasRSi^i?, where R may be Sc, Er, Y, Ho, Dy, Gd, Eu, or Sm.

[0010] In another aspect, the present invention provides a method of making an electrolyte element for such a cell. The electrolyte element is made by coating the perforated metal sheet with a first N5-ceramic precursor, for forming a porous and permeable layer, and then coating that with a second N5-ceramic precursor for forming an impermeable layer; the first layer may be dried or may be sintered before the second layer is deposited. The structure would then be sintered to form the required layers and to bond them together and to the metal sheet. The thermal expansivity of the metal sheet is typically about 14 x 10‘6 / K whereas the thermal expansivity of the N5-ceramic is about 13 x 10‘6 / K. The expansivity of the N5-ceramic is much closer to that of the metal sheet than is the case for the prior-art ceramics such as Nasicon or (3"-alumina, so there is negligible thermally induced strain after sintering, resulting in a uniformly flat and planar electrolyte element. Nevertheless, as the expansivity of the metal is slightly greater than that of the N5-ceramic, the N5-ceramic layer is held under slight compression when at the cell's operating temperature, which is advantageous. Hence it is possible to sinter the structure and produce an undistorted planar structure without having to use the previously mentioned frame-like weight on the periphery of the metal sheet.

[0011] In a third aspect, the invention provides such an electrolyte element, that is to say an impermeable, sodium-ion-conducting electrolyte element comprising an impermeable glassceramic layer bonded by a porous and permeable glass-ceramic layer to a perforated metal sheet, wherein the materials of the glass-ceramic layers comprise NasRSi^n, where R is a rare-earth element.

[0012] As explained above, both the porous layer and the impermeable layer are of N5- ceramic, but they are made from different precursors. For example the precursors may both be of the same chemical composition, but differ in the particle sizes; or may be of different chemical compositions. For example the porous layer may be of the version of NasRSi40i2 in which R is Y (yttrium), whereas the dense and impermeable layer is the version of NasRSi40i2 in which R is Sm (samarium) or Gd (gadolinium). These N5-ceramics may be referred to as N5-Y, N5-Sm and N5-Gd. As mentioned above, the higher the ionic conductivity, the lower the sintering temperature needed to achieve a fully dense layer. So the sintering can be carried out at a temperature that ensures the impermeable layer is fully dense while the other layer remains porous; and this also optimises the ionic conductivity of the impermeable layer.

[0013] Typically each layer of N5-ceramic is made from a powder of the desired composition, the powder being mixed with a liquid to form a slurry, and used to coat a perforated metal sheet, for example by spraying or screen-printing, before being dried and sintered to form a layer of N5-ceramic. The liquid may be water or an alcohol. The slurry may also comprise a dispersant to ensure the powder remains dispersed in the liquid (the water or the alcohol) as the liquid is evaporated during the drying step. The dispersant may be a polymer-based dispersant, for example a non-ionic polymeric dispersant, or an anionic polymeric dispersant, or a cationic polymeric dispersant. One suitable dispersant is Croda Hypermer KD-7 (trade mark), which is an anionic polymer-based dispersant.

[0014] Furthermore, to suppress the formation of cracks, it is desirable to ensure the N5- ceramic is formed of very small crystals (preferably of size less than 1 micron) that are randomly oriented. The layer of N5-ceramic may also include a small proportion of a toughening ceramic, to prevent the formation of cracks. A suitable toughening ceramic is 3- YSZ (which is 3 mole% yttria stabilised zirconia); the proportion of 3-YSZ may be less than 10% by weight, and may be less than 7% by weight, for example 5% by weight. This is particularly advantageous where the rare earth is samarium (Sm), but is applicable with other N5-ceramic compositions too. The 3-YSZ suppresses the formation of cracks in the ceramic layer during firing, while also inhibiting crystal growth by pinning the crystal boundary locations.

[0015] In such a cell the cathode compartment in its uncharged state typically contains a cathodic mixture comprising metal powder, sodium chloride, and sodium aluminium chloride (sodium tetrachloroaluminate, NaAICU). For manufacturing the cell, the cathodic mixture may first be pre-formed into a free-standing 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, NaAICU), 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 NaAICU.

[0016] The cell is thus a modified ZEBRA cell, such that the anode compartment of the cell when charged contains sodium metal, which will be molten at the cell's operating temperature. 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.

[0017] The impermeable N5-ceramic layer 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.

[0018] The impermeable N5-ceramic layer of the electrolyte element separates the anode compartment from the cathode compartment. The perforated metal sheet of the electrolyte element, which supports the N5-ceramic layers, is preferably in the anode compartment where it will help wick the molten sodium towards or away from the surface of the N5-ceramic. The impermeable N5-ceramic layer 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.

[0019] 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 perforated 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), which contains chromium and aluminium, and forms an alumina scale when heated in air), or a steel that forms an electronically-conductive and adherent scale, for example a CrMn oxide scale, when heated in air. 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 pm, for example 30 pm or less, or of mean diameter between 50 pm and 300 pm, 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 pm and 500 pm, for example 150 pm.

[0020] The perforated metal sheet may have a margin around its periphery that is not perforated; this margin makes 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.

[0021] Where the metal of the perforated plate is a steel alloy that forms an alumina surface on initial heating, the subsequently-deposited ceramic coating bonds to the alumina; the alumina is a good diffusion barrier. Similarly, if the metal forms a CrMn oxide scale, the subsequently-deposited ceramic coating bonds to that oxide scale. However in each case it has surprisingly been found that some transition elements (such as Fe and Cr) can diffuse out of the metal into the ceramic, where they act as sintering aids, and give a loss of phase purity. Considering the glass-ceramics N5-R, as we change R from Y to Gd to Sm the tendency to be affected in this way by the steel increases, which is detrimental to the properties of the glass-ceramic. Consequently N5-Y is the preferred material for the porous ceramic layer.

[0022] The consistency of the cell performance may be improved by a pretreatment to 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 layer of glass-ceramic. 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. There are benefits from using lead acetate, with the baking being in an oxygen-free atmosphere, as the resulting Pb / Pb(IV) oxide mixture forms an electronically conductive structure that extends into the pores of the porous layer of N5-ceramic.

[0023] 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.

[0024] Such a cell operates at an elevated temperature. The operating temperature depends in part on the nature of the electrolyte and its ionic conductivity; a cell with a thin layer of N5-ceramic as the electrolyte may have an operating temperature 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 an inorganic material of an electrical insulator, such as mica or vermiculite, or a glass. The anode plate may be sealed to the edge portion of the perforated metal sheet with a carbon-based gasket, or indeed they may be welded together.

[0025] 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 electrolyte element; and cell performance can also be enhanced by pre-treating the surface of the electrolyte element facing the anode compartment to enhance wetting by molten sodium. These modifications may be utilised separately, but are preferably both used.

[0026] In such a cell, the anode compartment and the cathode compartment may each sealed by a weld line. In particular, a weld line may join the periphery of the perforated metal sheet to the metal plate enclosing the anode compartment. In addition an electrically insulating sealing element must separate the periphery of the perforated metal sheet from the periphery of the metal plate enclosing the cathode compartment, but the sealing element may be bonded to a flat metal ring, so a weld line can join the flat metal ring to the metal plate enclosing the cathode compartment. Such a sealing element may be of glass, which may be cast (when molten) into position on the perforated metal sheet. Any such weld line may be a weld formed by a laser.

[0027] In addition to weld lines to hold the components together, the cell may also be provided with a clamping ring of L-shaped cross-section, and a compression element that locates within the clamping ring, joined together and arranged to apply a clamping force to the peripheries of the plates enclosing the anode compartment and the cathode. This reduces any stresses on the weld lines during operation of the cell.

[0028] 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 la shows part of figure 1 at a larger scale;

[0029] Figure 2 shows a cross-sectional view of an alternative cell of the invention; and Figure 3 shows a cross-sectional view of a modification to the cell of figure 2.

[0030] The invention envisages use of one or more glass-ceramics of the formula NasRSi40i2, where R is a rare-earth element, and may be Sc, Er, Y, Ho, Dy, Gd, Eu, or Sm. As explained above, a glass-ceramic means a polycrystalline material produced through controlled crystallisation of a glass, so producing a uniform dispersion of fine crystals. Such a material may be referred to as an N5-ceramic. Such a material may be made by thoroughly mixing powders of sodium carbonate, silica, and one such rare earth oxide in the proportions corresponding to the NasRSi^n stoichiometry, and then melted, for example in a platinum crucible, at 1350°C. The mixture can then be quenched to room temperature, for example on a metal block, producing a glass frit. The glass frit is then milled to form a powder, and heated to bring about crystallisation, with the formation of a powder of ceramic particles. This involves heating the powder gradually up to a temperature which is at least 100°C below its melting point, and holding it at that temperature for a prolonged time (such as 1 hour), before gradually cooling back to room temperature; a heating and cooling rate of 5K / min is suitable. The Table shows suitable crystallisation temperatures, sintering temperatures, and melting points. Table of Temperatures

[0031] The resulting powder can be mixed with water or alcohol to form a slurry, and used to coat a perforated metal sheet, for example by spraying or screen-printing, before being dried and sintered to form a layer of N5-ceramic. The slurry may also comprise a dispersant to ensure the powder remains dispersed in the liquid (the water or the alcohol) as the liquid is evaporated during the drying step. The dispersant may be a polymer-based dispersant, for example a non-ionic polymeric dispersant, or an anionic polymeric dispersant, or a cationic polymeric dispersant. One suitable dispersant is Croda Hypermer KD-7 (trade mark), which is an anionic polymer-based dispersant. The dispersant is converted to gases during firing.

[0032] Where the N5-ceramic has Sm (samarium) as the rare earth it has been found to be beneficial to include 3-YSZ ceramic powder in the slurry, at 5% of the total weight of the ceramic powder.

[0033] As mentioned above, it is desirable to ensure the ceramic is formed of very small crystals (preferably of size less than 1 micron) that are randomly oriented. This ensures that any anisotropy in the thermal expansivity of the crystals is not a feature of the resultant ceramic layer. It is achieved by forming the slurry of particles that are small, and depositing the slurry using processes which don't result in any preferential orientations. In addition excessive recrystallisation and crystal growth must be avoided during sintering, so sintering must be carried out without raising the temperature too high, or too high for too long.

[0034] These N5-ceramics have a larger average thermal expansivity than Nasicon ceramics over the temperature range between ambient temperature and the sintering temperature, so their expansivity is much closer to that of the metal sheet. The average expansivity of Fecralloy metal (over the range 25°C - 1200°C) is about 12-14 xl0‘6K1. The expansivity of N5-ceramics is shown in the following table: Table of Average Thermal Expansivities (100°C to 1050°C)

[0035] (In the Table, the term "Fe(nit) Nasicon refers to Nasicon with Fe oxide as a sintering aid, provided in the form of ferrous nitrate; it is essentially just Nasicon of the formula Na3Zr2(SiO4)2PO4 with 2% Fe2Os by wt as a sintering aid, the iron oxide being in the form of nano-sized particles that are formed during heating.)

[0036] Referring to figure 1, there is shown an electric cell 20 of the present invention. The cell 20 operates at an elevated temperature, and comprises dish-shaped metal electrode plates 11 and 12 that may be of stainless steel and that define flat peripheral rims 82, 81, 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 embodiment 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. A carbon felt 83 may be enclosed in the anode compartment 14.

[0037] The perforated metal sheet 16 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. In this example it is of aluminium-bearing ferritic steel that has been heat treated to have an alumina surface.

[0038] As shown in figure la, a margin 16a around the periphery of the metal sheet 16 is not perforated; the remainder has multiple perforations 18. The margin 16a is typically of width 5 mm. The perforated portion of the sheet 16 is covered by the electrolyte layer 23. This layer 23 consists of a porous and permeable N5-ceramic sub-layer 23a which is itself covered by a non-permeable N5-ceramic layer 23b, the non-permeable ceramic layer 23b being sodium-ion-conducting. In this example the non-permeable N5-ceramic layer 23b is of the glass-ceramic NasSmSi^i?, with 5% (by weight) of 3-YSZ, whereas the porous and permeable sub-layer 23a is of the glass-ceramic NasYSi^ .

[0039] The layer 23b is not 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 la). The electrolyte sheet 23 thus consists of the combination of the porous and permeable N5-ceramic sub-layer 23a and the non-permeable N5-ceramic layer 23b; these N5-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 and permeable N5-ceramic sub-layer 23a may be of thickness between 10 pm and 100 pm, while the non-permeable N5-ceramic layer 23b may be of thickness in the range 5 pm to 50 pm, for example 20 pm, 30 pm or 40 pm.

[0040] In its charged state the cell 20 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, NaAICU) and preferably also a small proportion other ingredients such as iron sulphide, sodium iodide, sodium fluoride, and aluminium powder. (In an alternative the cell might instead be a sodium / iron chloride cell, if iron powder is used instead of nickel powder.)

[0041] In particular, for ease of assembly, the powder mixture including nickel powder and sodium chloride, but without the sodium aluminium chloride, may 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. Referring again to figure 1, 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. The cell 20 is assembled in a dry atmosphere, so humidity does not interact with the chemicals in the biscuit 25.

[0042] 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 sublayer 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(ll) 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. Good wetting by sodium may also be achieved by coating with an 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.

[0043] 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 N5-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 compartment 14, where sodium metal is formed, while within the cathode compartment 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.

[0044] It will be appreciated that the electrical capacity of the cell 20 is dependent on the quantity of the active materials that are initially provided in the cathode compartment 15, in the form of the biscuit 25 described above, and the space available for molten sodium in the anode compartment 14. A cell 20 of greater capacity can be provided by increasing the volume of the biscuit 25 in the cathode compartment 15, for example by increasing the thickness of the biscuit 25. The Table shows the properties of cells 20 with different thicknesses of biscuit 25 (and correspondingly different thicknesses of the cathode compartment 15 and of the anode compartment 14). Thicker biscuits 25 give greater capacity, but lower charging and discharging rates; hence cells 20 of different thicknesses may suit different applications. In the Table, a rate of 1C means that the cell's total capacity is charged (or discharged) in 1 hour.

[0045] Table of Different Cathode Thicknesses

[0046] The cell 20 is 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 margin 16a of the metal plate 16 may differ from that shown in the drawings. For example the margin 16a may rest directly on the flat rim 82 of the anode plate 11, and indeed can be welded directly to the flat rim 82. Referring now to figure 2 there is shown a cross-sectional view to show one side of a cell 20a which is a modification to the cell 20 of figure 1; identical components are referred to by the same reference numbers. Thus the cell 20a comprises dish-shaped metal electrode plates 11 and 12 that may be of stainless steel and that define flat peripheral rims 82, 81, between which is a perforated metal sheet 16 to which is bonded a layer of impermeable sodium-ion-conducting N5-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. As explained above, the materials may be provided as a biscuit 25 in the cathode compartment 15, during assembly of the cell 20a, and the cell 20a is assembled in a dry atmosphere.

[0047] A glass seal 74 covers the periphery of the electrolyte layer 23 and is also bonded to the peripheral margin 16a of the perforated metal plate 16, and the opposite face (the upper face as shown) of the glass seal 74 is bonded to a flat metal ring 75. During assembly of the cell 20a the flat rim 82 of the anode plate 11 is laser welded to the peripheral margin 16a of the perforated metal sheet 16 along a peripheral line corresponding to the location A, and the flat rim 81 of the cathode plate 12 is laser welded to the flat metal ring 75 along a peripheral line corresponding to the location B. Thus the glass seal 74 seals the contents of the cell 20a; and sealing of the cell 20a is simplified by use of laser welding around the periphery. Laser welding enables very precise welding seams to be formed, and minimises the heat-affected zone such that the mechanical properties of the welded metals are unaffected. It can ensure a hermetic seal, while minimising the part-count and weight of the cell 20a, so it provides both technical and commercial advantages.

[0048] Referring now to figure 3 there is shown a cross-sectional view to show one side of a cell 20b which is a modification to the cell 20a of figure 2; identical components are referred to by the same reference numbers. In this cell 20b, instead of the glass seal 74, there is a two-part seal: an inner seal 67 of glass or of an ultra-high molecular weight polyethylene (UHMWPE) or another suitable polymer, and an outer air seal 66 of vermiculite or mica. At least the inner seal 67 is bonded to a flat peripheral ring 75. Hence during initial assembly of the cell 20b the flat rim 82 of the anode plate 11 is laser welded to the peripheral margin 16a of the perforated metal sheet 16 along a peripheral line corresponding to the location A, and the flat rim 81 of the cathode plate 12 is laser welded to the flat metal ring 75 along a peripheral line corresponding to the location B. These assembly steps are carried out in a dry atmosphere, as moisture may react with materials in the biscuit 25. The cell 20b can be safely handled in this state and does not need to be kept in a dry atmosphere as it is securely sealed.

[0049] However before heating the cell 20b to its operating temperature, additional features are provided to ensure the seals 66 and 67 remain under compression at all times. These comprise an L-shaped peripheral clamping ring 65 which has one part that engages the undersurface (as shown) of the flat rim 82 and another part that projects above the flat rim 81, and an L-shaped compression ring 64 that locates above the flat rim 81 and inside the projecting part of the clamping ring 65. The compression ring 64 is separated from the flat rim 81 by an electrically insulating gasket 63. The clamping ring 65 and the compression ring 64 are placed as shown, and pressure is applied between the compression ring 64 and the part of the clamping ring 65 that engages the undersurface of the flat rim 82; while applying that pressure the compression ring 64 is then welded to the clamping ring 65 at the location marked C, for example by resistance welding or laser welding.

[0050] The cell 20b can then be raised to its operating temperature, and can be operated. The compression ring 64 and the clamping ring 65 ensure that the seals 66 and 67 remain under compression at all times.

[0051] It will be appreciated that the cell 20a may also be provided with a compression ring 64 and a clamping ring 65, to ensure the glass seal 74 is always held under compression.

Claims

Claims1. An electrolyte element for use in a sodium / metal chloride electrochemical cel I, comprising an impermeable glass-ceramic layer bonded by a porous and permeable glassceramic layer to a perforated metal sheet, wherein the materials of the glass-ceramic layers comprise NasRSi40i2, where R is a rare-earth element.

2. An electrolyte element as claimed in claim 1 wherein the rare earth element R is Sc, Er, Y, Ho, Dy, Gd, Eu, or Sm.

3. An electrolyte element as claimed in claim 1 or claim 2 wherein the porous layer and the impermeable layer are of different chemical compositions.

4. An electrolyte element as claimed in claim 3 wherein the porous layer is of NasRSi40i2 in which R is Y (yttrium), whereas the dense and impermeable layer is of NasRSi40i2 in which R is Sm (samarium) or Gd (gadolinium).

5. A 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 electrolyte element as claimed in any one of the preceding claims.

6. A sodium / metal chloride cell as claimed in claim 5 wherein the metal chloride is nickel chloride.

7. A sodium / metal chloride electrochemical cell as claimed in claim 5 or claim 6 wherein the anode compartment and the cathode compartment are each sealed by a weld line.

8. A sodium / metal chloride electrochemical cell as claimed in claim 7 wherein a weld line joins the periphery of the perforated metal sheet to the metal plate enclosing the anode compartment.

9. A sodium / metal chloride electrochemical cell as claimed in claim 7 or claim 8 wherein an electrically insulating sealing element separates the periphery of the perforated metal sheet from the periphery of the metal plate enclosing the cathode compartment, the sealing element being bonded to a flat metal ring, and a weld line joins the flat metal ring to the metal plate enclosing the cathode compartment.

10. A sodium / metal chloride electrochemical cell as claimed in claim 9 wherein the sealing element is of glass.

11. A sodium / metal chloride electrochemical cell as claimed in any one of claims 7 to 10 wherein each weld line is a weld formed by a laser.

12. A sodium / metal chloride electrochemical cell as claimed in any one of claims 7 to 11 also comprising a clamping ring of L-shaped cross-section, and a compression element that locates within the clamping ring, joined together and arranged to apply a clamping force to the peripheries of the plates enclosing the anode compartment and the cathode compartment.

13. A sodium / metal chloride electrochemical cell as claimed in any one of claims 5 to 12. wherein the thickness of the cathode compartment and the thickness of the anode compartment are selected to achieve a cell adapted for a particular use.

14. A method of making an electrolyte element for a cell as claimed in any one of claims 5 to 12, the method comprising forming a structure by depositing on a perforated metal sheet a first coating of a first N5-ceramic precursor, for forming a porous and permeable layer, and then depositing on the first coating a second coating of a second N5-ceramic precursor for forming an impermeable layer; the first coating may be dried or may be sintered before the second coating is deposited; the method then comprising the steps of drying and sintering the structure to form the required layers and to bond them together and to the metal sheet.

15. A method as claimed in claim 14 wherein each coating of N5-ceramic precursor is made from a powder of the desired composition, the powder being mixed with a liquid to form a slurry, the slurry also comprising a dispersant to ensure the powder remains dispersed as the liquid evaporates during the drying step.

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