Electrochemical cell production

Abstract

A process for producing biscuits (25) of a cathode mixture, which involves introducing into an extruder (42) all the materials (41) required in the cathode mixture, including the secondary electrolyte, all the materials being in particulate form, and operating the extruder (42) while heating the mixture to above the melting point of the secondary electrolyte. The materials required typically comprise a metal powder such as nickel or iron, sodium chloride, and aluminium powder; and the secondary electrolyte is sodium aluminium chloride (NaAlCl4). These materials may all be in powder form. The extruded mixture is used to form biscuits (25). Such biscuits may be used in an electrochemical cell (10).

Description

[0001] Electrochemical Cell Production

[0002] The present invention relates to process for making an electrode of an electrochemical cell, to the electrode so produced, and the resulting cell.

[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. 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. As described in WO 2022 / 123246 the planar ceramic sheet that acts as the electrolyte may be formed separately from the perforated planar sheet, rather than being formed by deposition onto it. Such a cell includes a metal case, which may have a peripheral flange.

[0007] In each case the cell is typically produced in its uncharged state, so each cell comprises two electrode compartments, one being an anode compartment and the other being a cathode compartment, the electrode compartments being enclosed in part by metal plates that define end faces of the cell, the two compartments being separated by an impermeable, sodium-ion-conducting ceramic electrolyte element. The cathode compartment in its uncharged state contains a cathodic mixture comprising metal powder, sodium chloride, and sodium aluminium chloride (sodium tetrachloroaluminate, NaAICU), and the anode compartment in its uncharged state is empty. The metal powder in the cathodic mixture may be nickel or iron.

[0008] Prior to operation the cell is uncharged and at ambient temperature. In order for the cell to function the cell is warmed to above the melting point of the NaAICU, for example to 180°C. The sodium aluminium chloride may be referred to as the secondary electrolyte, as when it is molten it allows sodium ions to move through it during charging and discharging of the cell. The cell is thus a modified ZEBRA cell, such that the anode compartment of the cell when charged contains sodium metal.

[0009] In the manufacture of 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 nickel 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 (NaAICk), 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. However it is not straightforward to scale this process up to produce large numbers of cells.

[0010] According to the present invention there is provided a process for producing biscuits of a cathode mixture, the process comprising introducing into an extruder all the materials required in the cathode mixture, including the secondary electrolyte, all the materials being in particulate form, and operating the extruder while heating the mixture to above the melting point of the secondary electrolyte. The materials required comprise a metal powder such as nickel or iron, sodium chloride, and aluminium powder; and the secondary electrolyte is sodium aluminium chloride (NaAICk). These materials should be kept dry during the process.

[0011] Although operation of the extruder mixes the mixture together, the materials in powder form may, in addition, be mixed together before being introduced into the extruder to ensure that the correct ratio of materials is fed into the extruder if introduced from a single source. Such pre-mixing is advantageous, in ensuring the mixture is of uniform and consistent composition even before extrusion.

[0012] The extruder may be a twin screw extruder. The extruder must operate such that the mixture reaches a temperature above the melting point of the secondary electrolyte, for example above 160°C, for example more than half the length of the extruder may be at a temperature of at least 170°C. The extruder may be heated along its entire length, or only a portion leading to the outlet may be heated. At least the portion of the extruder that includes the outlet should be above the melting point of the secondary electrolyte. The extruder may incorporate a venting port near the outlet, to allow any air or gases to be removed from the mixture; this may be combined with an applied vacuum source.

[0013] It will be appreciated that the extruder intimately mixes all the various materials that constitute the cathode mixture, including the secondary electrolyte. Importantly, at least parts of the extruder are above the melting point of the secondary electrolyte. The molten secondary electrolyte gets intimately mixed with the cathode components of the mix, which is highly beneficial, and also lubricates the extrusion process, so reducing torque and degradation of equipment. Importantly, use of a ratio by weight (cathode material / secondary electrolyte) of between 1.5 to 1 and 3 to 1, particularly between 1.7 to 1 and 2.4 to 1, for example 1.8 to 1 results in a slurry composition that is optimal to extrude through a die and to form a sheet and also forms a high-performance electrode structure. When the secondary electrolyte cools below its melting point it solidifies the electrode mixture. This means that not only does the secondary electrolyte act to lubricate the extrusion process and to ensure all the constituent powders are thoroughly mixed, it enables an electrode biscuit of desired dimensions to be created without the need for additional binders, solvents and other agents typically added to the extrusion process (if added, they would need to be removed otherwise they would adversely affect the performance of the resulting cell).

[0014] The mixture that emerges from the extruder may be formed into biscuits in a variety of ways.

[0015] A: The extruded material is collected in a container. The resulting mixture can subsequently be broken into small parts (attrition milling) and then packed into appropriate moulds; reheating the moulds melts the material to form a desired net shape, and the mould and its contents are then cooled to set again, forming the biscuit of the desired shape. This process is not ideal for mass manufacture, but is a useful method at a smaller laboratory scale.

[0016] B: The extruded material is collected in a heated container (above the melting point of secondary electrolyte) of desired dimensions for final net shape. A hot press is then used to press the layer to desired thickness. Applying a hot press prevents contact cooling, and removes any porosity to maximise density, at the same time as forming the correct dimensions. This step could be combined with an appropriate vacuum to ensure all porosity is removed.

[0017] C: The material is extruded as a sheet, and the sheet is collected on a heated conveyor and subsequently cut into desired shapes downstream. It may be beneficial to section the sheet when at an elevated temperature (when the material is softer), or after it has cooled down (when solid).

[0018] D: Similar to C, however the extruded material comes out of the extruder from a die with a larger orifice, so the sheet is thicker than the desired final thickness of the biscuit. This reduces the likelihood of blocking the die or the extruder. The sheet is then conveyed through a heated roller (could be multiple rollers) to achieve a desired thickness and width. Possible advantages of the rollers are to remove any residual porosity and increase density. Downstream, the material is then cut into the required shape as with process (C). E: The extruder is combined with a hot melt injection moulder (equivalent to that used in the plastics industry). Injection moulding is an excellent technique for rapidly producing complex shapes with high quality and precision. Advantages are minimal downstream processes for shaping the sheet (cutting etc) and it can also be performed at pressure to maximise the amount of material in each mould (desirable for maximising performance of the resulting cells and batteries).

[0019] F: The extruder feeds directly into a mould with an outlet valve; when the mould is full the valve opens to let the resulting sheet emerge onto a conveyor, and then the valve closes to cut through the sheet. These steps are repeated.

[0020] G: The extruder may feed material directly into the cathode compartment of a cathode electrode plate, acting as a mould, so when the material cools it forms the biscuit in situ.

[0021] It will be appreciated that a cathode biscuit may be a laminated structure, formed by stacking sheets from different extrusion processes. For example a first extrusion process may have a different composition to a second extrusion process. It is hence possible to produce cathode biscuits with specific desirable technical properties. For example:

[0022] 1. A layer that contains iron powder may be provided at the surface of the biscuit that contacts the ceramic electrolyte; this would give the lowest impedance discharge pulses at any state of charge. In one option all the layers of the biscuit contain nickel powder (along with the other cathode materials and the secondary electrolyte), so only the layer at that surface of the biscuit contains both iron and nickel powders. In an alternative, the layer at the surface of the biscuit that contacts the ceramic electrolyte does not contain nickel, but contains iron powder; the other layers contain nickel powder and may also contain iron powder, and the ratio of iron to nickel may vary from layer to layer.

[0023] 2. A layer or layers with a higher proportion of the secondary electrolyte may be provided in the part of the biscuit closer to the ceramic electrolyte, and with a layer or layers with a lower proportion of secondary electrolyte in the region at the rear of the biscuit and so furthest from the ceramic electrolyte. This would maximise electronic conduction to the front of the electrode during charging at low states of charge and discharging at high states of charge and ionic conduction to the back of the electrode when charging at high states of charge and discharging at low states of charge. The result of which is to achieve good overall density and capacity whilst keeping the impedance lower for wider states of charge.

[0024] 3. One or more layers may differ in their proportions of aluminium powder; this may give similar benefits for similar reasons.

[0025] 4. One or more layers may differ in the proportion of sodium chloride; this may give benefits for similar reasons.

[0026] The powders of the different materials that are supplied to the extruder are typically of somewhat different sizes, and indeed the particles of any one material are typically of a range of sizes. By way of example some suitable particle sizes are shown in the Table:

[0027] Table of Particle Sizes

[0028] The particles are of very different densities and have a wide range of sizes, so it is possible to have poor mixing and a tendency for separation of phases, if a mix is left for a time and not re-agitated. Particle size and morphology is important in achieving good results. The nickel particles are desirably small, but with a high surface area, for example having a jagged, filamentous or spiky surface. The presence of such a surface on nickel powder is evident at a macro level as the powder does not flow well and has a fairly low tap density. Overall this is good as the jagged, filamentous or spiky nickel forms a good interconnected network for high electronic conductivity, as the particles become entangled; and as it forms a fairly physically strong network the electrode does not collapse and is stable as the sodium chloride dissolves in the melt during charging.

[0029] The significant difference in particle size between the nickel and the sodium chloride is important to get both a high surface area nickel and also a good void space for the ionic conductive paths in the secondary electrolyte whilst the sodium chloride is consumed during charging. Large nickel particles would have a low surface area, and so produce a cell with low capacity. A suitable type of nickel is Ni 525B, which is a nickel particle product from Novamet; the material is made by the thermal decomposition of nickel carbonyl, and has the form of filaments and clumps of tiny particles bonded together. Another choice is Vale 255 which is also made by the nickel-carbonyl refining process. If the intention is to make a cell based on iron instead of nickel, it may be appropriate to use the corresponding iron powder produced from iron carbonyl.

[0030] As indicated previously, a cell using a cathode biscuit made in such a way has a ceramic sheet, typically a planar sheet, as its electrolyte, to separate the anode compartment from the cathode compartment. The electrolyte can take one of two forms, the first is sheet of ceramic, typically termed 'self-supporting'. Alternatively, the second is a layer of ceramic that is deposited onto a perforated metal sheet (metal-supported). The second approach allows for a much thinner layer to be fabricated. In the case of the self- supporting ceramic electrolyte, the cell may also comprise, preferably in the anode compartment, a perforated or non-perforated sheet of metal adjacent to the sheet of ceramic, to provide support to the 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.

[0031] 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 highly porous, and preferably graphitic; it may be a paper-like sheet of initial thickness about 1.5 mm, and may for example have an area density of less than 200 g / m2, for example 100 g / m2. Indeed such a layer of carbon felt may be provided as a wicking layer between a metal sheet and the ceramic electrolyte sheet, if the metal sheet and the ceramic electrolyte sheet are not bonded together.

[0032] The provision of a perforated metal plate or a carbon felt adjacent to the anodic face of the ceramic electrolyte, these being electrically connected to the anode plate, enables sodium metal to be formed at that face of the electrolyte during charging of the cell, and sodium to ionise at that face during discharge. As an alternative, the surface of the ceramic electrolyte may be coated, on the surface facing the anode compartment, with an electronically conductive coating. Such a coating may be of graphite, or of a polyphosphate glass containing particles of electronically conductive material. The polyphosphate glass is a sodium-ion conductor. The particles may be carbon powder, tin powder, and / or aluminium flake or powder.

[0033] 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), or a steel that forms an electronically-conductive and adherent scale, for example a CrMn oxide scale, when heated in air. If it is separate from the ceramic electrolyte, it may be of another type of steel, such as stainless steel. The perforated 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.

[0034] The perforated 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 perforated sheet is preferably in the anode compartment, where it will help wick the molten sodium towards the surface of the ceramic sheet. 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.

[0035] The metal plates that enclose 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.

[0036] 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 (metal-supported) 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. The seal of the anode compartment may use a carbon-based gasket, or indeed the anode plate may be welded to the periphery of the perforated sheet, if the ceramic is deposited on the perforated sheet. High-temperature polymers are preferably not used to seal the anode compartment, as they may interact with the molten sodium.

[0037] 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 perspective view, partly broken away, of the cell of figure 1; and Figure 3 shows a diagrammatic view of a process for producing a cathodic biscuit for use in the cell of figure 1;

[0038] Figures 4a to 4f show different ways of completing the process of figure 3;

[0039] Figure 5 shows an SEM image of a mixture of sodium chloride particles and 525B nickel particles; and

[0040] Figure 6 shows graphically the variation of cell capacity with cycle number for a cell whose biscuit is made by the standard route, and cells whose biscuit is made by extrusion.

[0041] 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 a planar impermeable sodium- ion-conducting electrolyte sheet 13. The anode electrolyte plate 11 has a flat peripheral rim 18 which is slightly wider than the rim of the cathode electrode plate 12, from which a short sleeve portion 19 extends upward (as shown). The cell 10 is square in plan view. 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 example the cell components are held together by crimping the upstanding edge of the sleeve portion 19 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 is a sodium / nickel chloride cell. The electrode plates 11 and 12 may be of stainless steel. 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 initially containing a biscuit 25 made from 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 and sodium fluoride, and aluminium powder. This biscuit 25 is placed in the cathode compartment 15, occupying most of the space; a layer of carbon felt (not shown) may be placed above the biscuit 25, between the biscuit 25 and the cathode plate 12.

[0042] The impermeable sodium-ion-conducting electrolyte sheet 13 may be of NaSICON. A perforated metal sheet 24 is adjacent to the electrolyte sheet 13, but spaced apart from it by a layer of carbon felt 82. The perforated metal sheet 24 provides mechanical support to the electrolyte sheet 13. 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.

[0043] 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, so it can act as a secondary electrolyte, 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 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.

[0044] 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 electrically-conductive and adherent scale, for example a CrMn oxide scale, when heated in air. 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.

[0045] The plate 12 has a central circular region 22 which is depressed so it forms a slightly concave outer face as the cell is initially assembled and prior to operation, that is to say when the cell is uncharged and at ambient temperature. The central region 22 is surrounded by a substantially flat rim 23. When the cell 10 is heated to its operating temperature, the increase of volume of the materials in the cathodic compartment 15 is accommodated by the concave surface of the central region 22 becoming flatter. This doesn't alter the thickness of the cell 10.

[0046] Referring also to figure 2, which shows a perspective view of the cell 10 with an edge portion broken away, the plate 12 also defines a protrusion or bump 30 near each corner of the cell 10; the plate 11 defines recesses 32 in the directly opposite positions. The bumps 30 and the recesses 32 are of complementary shapes. Consequently when two or more cells 10 are assembled into a stack, the bumps 30 of each cell 10 mate with the recesses 32 of the adjacent cell 10. Not only do the bumps 30 and the recesses 32 ensure the cells 10 remain correctly aligned, but the regions of the plates 12 and 11 that define the bumps 30 and the recesses 32 are also stiff enough to ensure good electrical connection between successive cells in the stack, despite changes in temperature of the cells 10, and changes in the state of charge of the cells 10. (The carbon felt 82 and the perforated metal sheet 24 are not shown in figure 2 for clarity.)

[0047] As an alternative, instead of providing the electrolyte in the form of the discrete sheet 13 of ceramic, the electrolyte may instead be in the form of one or more layers of ceramic deposited onto a perforated metal sheet. The metal sheet may be substantially equivalent to the metal sheet 24, but without the curved springs 80, so it is planar. Preferably a porous and permeable layer of ceramic would first be deposited and bonded to the metal sheet, and then an impermeable layer of sodium-ion-conducting ceramic deposited and bonded to the porous layer, completely covering the porous layer. The ceramic layers would cover all the perforations through the perforated metal sheet, but the edges of the sheet would not be perforated. The edges may be in electrical contact with the anode plate 11, for example they may rest on the peripheral rim 18, to which they may be welded to provide a seal, or they may be sealed to the rim 18 by a conductive gasket. Electrical contact may also be provided by carbon felt within the anode compartment 14.

[0048] Referring now to figure 3, a powder mixture 40 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 and sodium fluoride, and aluminium powder is introduced through a feeder 41 into a twin- screw extruder 42. This feeder 41 may be a gravimetric or volumetric single or twin screw feeder. The powders are provided in a ratio of cathode material to secondary electrolyte (by weight) of between 1.5 to 1 and 3 to 1, particularly between 1.7 to 1 and 2.4 to 1, for example 1.8 to 1, so there's just less than twice as much cathode material as secondary electrolyte. Optionally the powder mixture 40 may be heated to about 150°C before being introduced into the extruder 42. The extruder 42 is of the type used with polymers, including regions for kneading and regions for conveying, and is surrounded by a heating jacket 44. The heating jacket 44 is arranged such that the temperature within the extruder 42 starts at about 160°C where the powders are introduced, and increases gradually to about 200°C, 220°C, or 250°C at the outlet end. Alternatively the extruder 42 may be heated to a substantially uniform temperature along its length, of 190°C, 200°C or 205°C. The outlet from the extruder 42 is through an extrusion die 46. Near the outlet end of the extruder 42 is a venting port 48 through which any gases such as air, nitrogen or other gases can escape; this may be connected to a vacuum system (not shown).

[0049] As previously mentioned, the nickel powder may be particles of size 1 - 10 microns, produced by the thermal decomposition of nickel carbonyl (for example 525B); the sodium chloride particles may be significantly larger, 10 to 40 microns in size. These two materials are shown in figure 5, which is a scanning electron microscope image of a mixture: the sodium chloride particles are the large particles, whereas the nickel powder is in the form of filaments or clumps of tiny particles, so it has a very irregular surface that provides a large surface area. In the final biscuit made by extrusion the multiple nickel filaments and clumps often contact each other and become entangled, so the nickel forms a network throughout the biscuit.

[0050] The extruder 42 produces an output stream of cathodic material at a temperature above the melting point of the sodium aluminium chloride. There are a number of different ways in which this output stream can be used to form the required biscuit 25. In each case it is important to ensure the material remains dry, so it does not come into contact with water or humidity, so operation in a suitable containment space, such as a glove box or antechamber is essential. Some methods to form biscuits 25 are illustrated in Figures 4a to 4f. Those in figures 4a, 4b and 4e essentially use the extruder 42 only to heat and mix the mixture.

[0051] As shown in figure 4a, the extruded material is collected in a container 50. The resulting mixture can subsequently be broken into small parts (attrition milling) and then packed into appropriate moulds; the moulds; can then be reheated to melt the material to form desired net shape, and then cooled so the material sets again to form a biscuit. This approach may not be ideal for mass manufacture, but is a useful method at a smaller laboratory scale.

[0052] As shown in figure 4b the extruded material is collected in a heated container 52 (above the melting point of the secondary electrolyte) of desired dimensions for final net shape. A hot press 53 is then used to press the layer to a desired thickness. Applying a hot press 53 prevents contact cooling, and removes any porosity to maximise density as well as forming the required dimensions. This step could be combined with an appropriate vacuum to ensure all porosity is removed. The heating of the container 52 is then ended, and when the material has cooled down and set, the resulting biscuit can then be removed from the container 52.

[0053] As shown in figure 4c, the extruder 42 may incorporate a slot-shaped extrusion die 46, so the material is extruded as a sheet 54. The sheet is collected on a heated conveyor 55 and subsequently cut into desired shapes downstream. The temperature of the conveyor 55 decreases as it gets further from the extruder 42. It may be beneficial to cut up the conveyed sheet 54 when at an elevated temperature (when the material is softer), or after it has cooled down (when solid).

[0054] As shown in figure 4d, the arrangement is similar to that of figure 4c in that the extruder 42 incorporates a slot-shaped extrusion die 46, so the material is extruded as a sheet 54a and collected on a heated conveyor 55a, but in this case the slot is wider, so the sheet 54a is thicker than the desired final thickness. This reduces the likelihood of there being a blockage of the extrusion die 46. The sheet 54a is then conveyed through a heated roller 56 (or multiple heated rollers) to form a sheet 54 of a desired thickness and width carried by a second conveyor 55. Possible advantages of the rollers 56 are to remove any residual porosity and increase density. Downstream of the rollers 56, the temperature of the second conveyor 55 decreases; the material is then cut into the required shape, and so to form the required biscuits when the material has set.

[0055] As shown in figure 4e, the extruder 42 is combined with a hot melt injection moulder 58 (equivalent to that used in the plastics industry), incorporating a number of moulds 59 for individual cathodic biscuits 25. Injection moulding is an excellent technique for rapidly producing complex shapes with high quality and precision. Advantages are minimal downstream processes for shaping the sheet (cutting etc) and it can also be performed at pressure to maximise the amount of material in each mould (desirable for maximising performance of the resulting cells and batteries).

[0056] As shown in figure 4f, the extruder 42 is combined with a die / mould 60 using a back plate or valve 61 that forces the cathode material 62 to fill all the available space. As indicated in diagram (ii) when the pressure measured with a pressure sensor 63 near the inlet to the die / mould 60 reaches a preset value, or a preset time has elapsed, the back plate or valve 61 is opened to allow flow out of the die / mould 61 and on to a conveyor (not shown), as indicated in diagram (iii). When a second specified time has elapsed, or a preset length of sheet has come out of the die-mould 60, as indicated in diagram (iv), the valve 61 is used to cut the projecting length 64 of the sheet, and the process is then repeated. The conveyor allows the cut projecting lengths 64 to cool to below the melting point of the secondary electrolyte. This produces rectangular lengths of material of a consistent width, corresponding to the width of the die / mould 60.

[0057] It will be appreciated that the processes of figures 4a, 4b, 4e and 4f can be arranged to form biscuits 25 of a required size and shape, so once the biscuit has cooled it is ready for insertion into the cathode compartment 15 of a cell 10. The processes of figures 4c and 4d produce a sheet of material of the desired thickness, from which biscuits 25 of the desired shape can then be cut out. Any excess / waste material produced by the extruder 42 can be collected, broken up and powdered, and returned to the feed hopper 40 to be extruded again, as its chemical content has not been altered. So such material is not wasted.

[0058] As another alternative, rather than forming a biscuit 25 and then inserting it into a cathode compartment 15, the extruder 42 may be arranged to inject cathode material directly into the cathode compartment 15 of a cathode electrode plate 12 held with its rim uppermost and at temperature above the melting point of the secondary electrolyte. The cathode compartment 15 acts as a mould. Any excess material may be removed by a doctor blade. The electrode plate 12 is then cooled so the material sets to form the desired biscuit 25 in situ in the cathode compartment 15.

[0059] Referring now to figure 6, this shows graphically the variation of cell capacity with cycle number (or "cycle index") for a cell whose biscuit was made by the standard method, and for two cells whose biscuits were made by extrusion. All these cells were made using the same materials. (As explained earlier, the standard method introduces a powder mixture into a mould in which the mixture is compacted, the mixture including all the required ingredients apart from the sodium tetrachloroaluminate (NaAICU); the compacted mixture is then infiltrated with molten sodium tetrachloroaluminate (NaAICU), preferably under vacuum.) It will be appreciated that in each case the cell capacity (as a proportion of the initial capacity) is initially 1, and decreases as the number of charge / discharge cycles is increased, but that the decrease is markedly less for the cells with a cathode biscuit made by extrusion, than for the cell with a cathode biscuit made conventionally. It is thus clear that the extrusion process produces a better cathode biscuit that provides a more long- lasting cell.

[0060] The procedures described above with reference to figure 4a to 4f have been described as producing biscuits 25 for use in a cathode compartment 15, but it will be appreciated that if a laminated biscuit 25 is to be produced, that is to say a biscuit 25 made of a plurality of layers, each layer of the laminated biscuit may be made by such a procedure, and the separate layers then stacked together. The stack of layers may be inserted into the cathode compartment 15 without further treatment, as the secondary electrolyte will melt when the cell 10 is heated to its operating temperature. Alternatively the layers of the stack may be joined together, for example by stacking them in a container of the desired dimensions for final net shape. This may be like the heated container 52, and may be heated to above the melting point of the secondary electrolyte. A hot press 53 may also be used to press the stack of layers to a desired thickness, and to ensure the layers bond together, before cooling to below the melting point of the secondary electrolyte so the laminated biscuit can be removed. When producing laminated biscuits in high volume, it can be envisaged that the manufacturing process would consist of multiple extruders arranged to produce different layers that interlace to form a final desired sheet. This sheet may be further processed using the processes described above, that is to say heated rollers, conveyors, and cutting mechanisms etc.

Claims

Claims1. A process for producing biscuits of a cathode mixture, the process comprising introducing into an extruder all the materials required in the cathode mixture, including the secondary electrolyte, all the materials being in particulate form, and operating the extruder while heating the mixture to above the melting point of the secondary electrolyte.

2. A process as claimed in claim 1 wherein the materials required comprise a metal powder such as nickel or iron, sodium chloride, and aluminium powder; and the secondary electrolyte is sodium aluminium chloride (NaAICU).

3. A process as claimed in claim 1 or claim 2 wherein all the materials are in powder form and are mixed together before being introduced into the extruder.

4. A process as claimed in any one of the preceding claims wherein the extruder is a twin screw extruder.

5. A process as claimed in any one of the preceding claims wherein more than half the length of the of the extruder is at a temperature of at least 170°C.

6. A process as claimed in any one of the preceding claims wherein the extruder is heated along its entire length.

7. A process as claimed in any one of the preceding claims wherein the extruder incorporates a venting port near the outlet, to allow any air, nitrogen or other gases to be removed from the mixture.

8. A process as claimed in any one of the preceding claims wherein the extruded material is collected, allowed to set and then broken up and packed into moulds; this is then reheated to melt and again cooled to set again to provide a biscuit or biscuit layer.

9. A process as claimed in any one of claims 1 to 7 wherein the extruded material is collected in a container that is above the melting point of the secondary electrolyte and is of desired dimensions for final net shape, and the collected material is then subjected to a hot press to produce a biscuit or biscuit layer of a desired thickness.

10. A process as claimed in any one of claims 1 to 7 wherein the extruded material is extruded through a slot to form a sheet, and the sheet is collected on a heated conveyor and subsequently cut into desired shapes downstream to form biscuits or biscuit layers.

11. A process as claimed in claim 10 wherein the sheet, as extruded, is thicker than the desired final thickness, and the sheet is then compressed by a heated roller to achieve a desired thickness before being subsequently cut into the desired shapes downstream to form biscuits or biscuit layers.

12. A process as claimed in any one of claims 1 to 7 wherein the extruded material is fed into a hot melt injection mould to produce biscuits or biscuit layers of the desired shape and size.

13. A cathode biscuit that is a laminated structure, formed by stacking biscuits or biscuit layers that have different compositions and that were produced by extrusion processes as claimed in any one of the preceding claims.

14. A rechargeable electrochemical cell comprising two electrode compartments, one being an anode compartment and the other being a cathode compartment, the electrode compartments being enclosed in part by metal plates that define end faces of the cell, the two compartments being separated by an impermeable, ion-conducting electrolyte element, the cell being a molten sodium / metal chloride cell, the electrolyte element being a sodium-ion-conducting ceramic; and the cathode compartment in its uncharged state containing a cathodic biscuit produced by a process as claimed in any one of claims 1 to 12.

15. A rechargeable electrochemical cell comprising two electrode compartments, one being an anode compartment and the other being a cathode compartment, the electrode compartments being enclosed in part by metal plates that define end faces of the cell, the two compartments being separated by an impermeable, ion-conducting electrolyte element, the cell being a molten sodium / metal chloride cell, the electrolyte element being a sodium-ion-conducting ceramic; and the cathode compartment in its uncharged state containing a cathodic biscuit as claimed in claim 13.

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