GB1595192A - Alkali metal/sulphur cells - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO ALKALI METAL/SULPHUR CELLS (71) We, CHLORIDE SILENT POWER LIMITED, a British Company, of 52 Grosvenor Gardens, London, SW1W OAU, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to alkali metal/sulphur cells such as for example sodiumsulphur cells.

In a sodium-sulphur cell, liquid sodium metal forming the anode is separated from the cathodic reactant by a solid electrolyte typically of beta alumina. The cathodic reactant which is liquid at the cell operating temperature comprises sulphur and sodium polysulphides, the composition depending on the state of charge or discharge of the cell. As the cell discharges, sodium ions pass through the electrolyte into the cathodic reactant to combine with negatively charged sulphide ions to form the sodium polysul hides. On charge, the reverse action takes place. It is necessary to inject and extract electrons from the cathode electrode and this is done by means of a porous conductive body such as a fibrous graphite or carbon matrix. This matrix extends through the cathodic electrode region and enables electrons to be supplied to and removed from the cathodic reactant. The electrical conductivity of such a porous material however is low and a current collector has to be electrically connected to this matrix to enable an external circuit to be connected to the cathode of the cell.

It is the usual practice to make sodiumsulphur cells with the solid electrolyte in the form of a tube. The cathodic reactant may be in the region outside this tube and the sodium inside. In this case, it is usual to utilise a cylindrical metallic housing as the cathode current collector, the cathodic reactant and the matrix material being in the annular space between the electrolyte and the housing. Although stainless steel is commonly suggested as the housing for such a cell, this material is rapidly corroded if there should be any fracture in the electrolyte permitting the sodium to mix with the cathodic reactant, the resultant reaction being exothermic and producing material which is highly corrosive to stainless steel. It has therefore been proposed to coat the internal surface of such a housing with carbon or molybdenum. With such coatings however, spalling can readily occur because of the differing coefficients of thermal expansion.

In a preferred construction of tubular cell, the cathodic reactant is put inside the electrolyte tube and in this case a current collector in the form of a rod or tube may be located axially within the cathodic region.

Thus, in this construction also the matrix material lies in the annular space between the current collector and the electrolyte tube. With a central cathodic system, the current collector may be a graphite rod or tube. Graphite will withstand the corrosive conditions but it does not have a very high conductivity. One of the particular advantages of sodium sulphur cells compared with more conventional batteries is the high electrical storage capacity per unit volume and the high rate of current discharge which can be obtained. Full advantage of these features can only be obtained if the cathode current collector has a low electrical resistance. In order to reduce the electrical resistance of the current collector in such an arrangement, a core of high conductivity metal, for example copper or aluminium may be provided within a graphite tube.

There are problems however in maintaining the core in electrical contact with such a graphite tube because of the differing coefficients of thermal expansion. These problems may be overcome by the provision of a deformable interface between the core and the graphite tube, e.g. a liquid metal interface as described and claimed in the specification No. 1 513 681 or a graphite felt or similar deformable interface as described and claimed in the specification No.

1 513 685.

It is one of the objects of the present invention to provide, in an alkali metal/sulphur cell, an improved protective coating on a metal member such as will withstand the conditions in the cathodic region of an alkali metal/sulphur cell. Such a coating, if it is electrically conductive, may be used for coating a current collector. This method of protecting a metal member however may also be used for the protection of metal parts which are not used as a current collector, for example a metal seal across the end of the cell. In this case the protective coating need not necessarily be electrically conductive.

According to one aspect of the present invention in an alkali metal/sulphur cell, a metal member in the cathodic region has a coating formed of at least two different materials, the proportions ot which are not the same through the thickness of the coating, the outer surface being of a material known to be resistant to corrosion in the cathodic reactant. if the coating is applied directly on the metal substrate, the materials applied may include the same metal as the substrate and, in this case. the coating may be 100% of the same metal as the substrate metal at the surface of the substrate changing gradually to 100% of the corrosion resistant material at the outer surface. In some cases however, two or more such coatings may be applied. Each such layer may have a gradual transition in composition from being the same as that of the underlying surface or substrate to a different composition.

In a preferred construction, a coating on a substrate metal such as aluminium or copper in a current collector or stainless steel or nickel iron or nickel cobalt iron alloy such as might be used for a housing or seal, has a composition which is 100% of the same metal as the substrate at the metal surface and changes gradually to be 100% of carbon, or molybdenum, or chromium, or nickel chromium or iron chromium alloy, or aluminium oxide, or chromium oxide, or molybdenum disulphide, or nickel oxide, or titanium dioxide, or titanium dioxide doped with tantalum or niobium, at the outer surface.

The protective coating may be formed on a metal member for use in an alkali metal/sulphur cell by a programmed deposition, for example vacuum deposition, of two or more materials upon a metal substrate so as to provide, through the thickness of the coating, a gradual transformation from the composition of the metal substrate to the required corrosion-resistant composition on the outer surface of the coating.

For a current collector, the substrate may conveniently comprise aluminium or copper. For other purposes, for example for a cell housing, other materials might be employed such as steel as the substrate. For a metal seal, nickel iron alloys and nickel iron cobalt alloys may be employed. As previously mentioned, the corrosion-resistant coating on its outer surface might comprise carbon, or molybdenum, or chromium, or a nickel chromium or iron chromium alloy or aluminium oxide, or chromium oxide, or nickel oxide, or molybdenum disulphide, or titanium dioxide, or titanium dioxide doped with tantalum or niobium.

As an example of the invention, a cathode current collector might be formed of a rod or tube of aluminium which is coated with molybdenum the composition of the coating gradually changing from 100% aluminium adjacent the surface of the aluminium to 1 00 S, molybdenum on the outer surface. A further coating of carbon might be provided over the molybdenum again with a gradual change of composition from 100% molybdenum to 100% carbon.

Carbon may be deposited using known carbon vapour deposition techniques.

In another example, carbon is coated directly onto an aluminium rod or tube to form a cathode current collector for a sodium-sulphur cell again with the gradual change in composition from the aluminium of the substrate to the 100% carbon at the outer surface.

In the above examples the gradual change in composition is achieved by depositing two materials simultaneously as previously described.

Conveniently an electron beam gun is used to heat the evaporant source for the vapour deposition, an ion plating technique being employed.

In the following description, reference will be made to the accompanying drawings in which: Figures 1 and 2 each illustrate a form of ion plating apparatus for the production of a cathode current collector for a sodiumsulphur cell; and Figure 3 illustrates a plasma-activated vapour deposition apparatus for the production of a cathode current collector for a sodium-sulphur cell.

Referring to Figure 1, there is illustrated diagrammatically an ion-plating apparatus comprising an outer metal housing 10 which is electrically earthed at 11 and which is evacuated via an outlet 12 connected to a gas pump (not shown). Within the outer housing 10 is an inner casing 13 in which is supported the article to be plated, which, in the embodiment illustrated, is an aluminium rod 14 which constitutes the substrate of a cathode current collector for a sodiumsulphur cell. In the casing 13 the rod 14 is supported on a negative potential supply lead 15 which extends through an insulating bush 16 sealed into the inner casing 13 and outer housing 10. An inlet 17 is provided for an inert gas, e.g. argon at a low pressure to provide the required atmosphere within the casing 13, in this case, argon a vent 18 enabling the apparatus to be flushed with the argon.

For producing the required coating with a graded interface, there are provided two cathodes 20, 21 made of aluminium and of nickel-chromium alloy respectively. The current supplies to the two cathodes 20, 21 are individually regulated by current adjusters 22, 23 respectively which are controlled by a controller 24.

In making the current collector, the current is fed initially to the aluminium cathode 20 so that the sputtered material is aluminium; the current to this cathode 20 is gradually reduced to zero and simultaneously a gradually increasing current is fed to the other cathode 21. Thus the deposited coating is a mixture of aluminium and nickel-chromium alloy, with the amount of aluminium falling gradually to zero whilst simultaneously the amount of nickelchromium alloy is increased. By this means the composition of the coating is gradually varied as the coating thickness builds up, so producing the required graded interface.

More than two cathodes may be provided if it is required to deposit more than one layer. The cathodes would be made of the required materials and the currents to the cathodes would be gradually changed to produce a first coating having a composition changing gradually, as the coating is built up from the substrate material to a first coating material and then changing gradually from the first coating material to a second coating material.

Figure 2 illustrates an electron beam ion-plating apparatus for coating chromium metal onto an aluminium substrate. This apparatus comprises a chamber 30 which may be evacuated via an outlet 31 connected to a pump (not shown) and into which argon is admitted at 32 at a suitable low pressure. The current collector 33 which is to be coated is connected to a D.C. voltage supply 34. An electron beam gun 35 directs a beam of electrons, indicated at 36, onto aluminium 37 in a crucible 38 so as to evaporate aluminium and a similar electron beam gun 39 provides an electron beam 40 to evaporate chromiuin from a chromium source 41 in a crucible 42. The metal ions are electrically attracted to the article 33 which is to be coated. The two electron beams 36, 40 are controlled by current regulators 43, 44 with a controller 45 for automatically regulating the relative amounts of aluminium and chromium which are evaporated. Thus the deposited material has a composition which can be gradually changed from solely aluminium through a mixture of aluminium and chromium and thence to solely chromium.

More than two materials may be deposited using this technique, the composition being changed gradually from the substrate to a first material and then changed gradually to a second material.

Instead of using a separate electron beam gun for each material to be deposited, a single electron beam may be scanned across two melting crucibles containing the different materials to be employed, the dwell periods on the crucibles being controlled so that the composition of the plasma gradually changes with time and hence the composition of the coating changes as the coating builds up. It will be immediately apparent that more than two melting crucibles may be employed to give deposits containing more than two constituents. It is thus readily possible in this way to deposit metal alloys such as nickel chromium or iron chromium alloys on a metal substrate which may itself be an alloy.

In the arrangement of Figure 2, the appropriate materials for the coating are evaporated by an electron beam. Other forms of vapour deposition may be used, for example a plasma activated vapour deposition process such as is described in our copendingAplication No. 21709/77 (Serial No. 1 592 063). Figure 3 illustrates an apparatus for the simultaneous deposition of two materials using such a technique. In that figure, a current collector rod 50 to be coated is suspended within a chamber 51 which is surrounded by a helical watercooled copper coil 52 energised by a highfrequency generator 53. The chamber is evacuated by a pump 54 through an outlet 55 and is initially filled with argon, through an inlet 56, at a low pressure, e.g. 2.0 torr.

Energisation of the coil causes not only induction heating of the rod 50 but also ion bombardment of the rod 50 with ions of the inert gas, cleaning the surface of the rod; a further gas or gases is or are then fed into the chamber 51 through inlets 57, 58, the pump 54 being operated to maintain the pressure constant. These further gases may be vaporised metal or metals or gaseous material, e.g. metal carbonyls which dissociate to deposit metal on the collector rod.

Flow control means are provided for controlling the various gas flows, preferably in an automatically controlled program. The collector rod 50 is biased negatively so that the metal ions move towards it and the metal is deposited on it. It will be immediately apparent that, by controlling the proportions of the different gases or vapours fed into the chamber 51, the composition of the deposited coating may be controlled to provide a required graded composition coating.

This technique is particularly suitable for producing a carbon coating, using a carbon-containing gas, e.g. ethylene, which dissociates in the plasma to deposit carbon on the rod 50. It is thus readily possible to produce a graded coating in which the amount of carbon is gradually increased as the coating is built up, leaving an external surface solely of carbon.

Although in the above examples, reference has been made more particularly to a current collector rod, the technique may be applied to other metal components in a sodium-sulphur cell. e.g. metal closure members, where it is required to produce a firmly adherent coating of a material which is chemically and electro-chemically inert to the reactant materials of the cell.

WHAT WE CLAIM IS: 1. An alkali metal/sulphur cell wherein a metal member in the cathodic region has a coating formed of at least two different materials, the proportions of which are not the same through the thickness of the coating, the outer surface being of a material known to be resistant to corrosion in the cathodic reactant.

2. A cell as claimed in claim 1 and having the coating applied directly on the metal substrate, wherein the coating is 100% of the same metal as the substrate at the surface of the substrate changing gradually to 100% of the corrosion resistant material at the outer surface.

3. An alkali metal/sulphur cell having two or more superimposed coatings on a metal member. each coating being formed of at least two different materials. the proportions of which are not the same through the thickness of the coating, the first coating having a composition changing from being the same as that of the underlying surface or substrate to a different composition, and the outer surface of the outermost coating being of a material known to be resistant to corrosion in the cathodic reactant.

4. A sodium-sulphur cell as claimed in any of claims l to 3 wherein the metal member is a cathode current collector.

5. A sodium-sulphur cell as claimed in claim 4 wherein the substrate material is aluminium.

6. An alkali metal/sulphur cell as claimed in any of the preceding claims wherein the coating is formed by a programmed deposition of two or more materials upon a metal substrate so as to provide, through the thickness of the coating, a gradual transformation from the composition of the metal substrate to the required corrosion-resistant composition on the outer surface of the coating.

7. An alkali metal/sulphur cell as claimed in claim 6 wherein the deposition is effected by a vapour deposition technique.

8. An alkali metal/sulphur cell as claimed in claim 6 wherein the deposition is effected by ion plating.

9. An alkali metal/sulphur cell having, in the cell, a metal member with a coating of graded composition such that the outer surface is corrosion-resistant to the reactants in the cell substantially as hereinbefore described.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (9)

**WARNING** start of CLMS field may overlap end of DESC **. dissociates in the plasma to deposit carbon on the rod 50. It is thus readily possible to produce a graded coating in which the amount of carbon is gradually increased as the coating is built up, leaving an external surface solely of carbon. Although in the above examples, reference has been made more particularly to a current collector rod, the technique may be applied to other metal components in a sodium-sulphur cell. e.g. metal closure members, where it is required to produce a firmly adherent coating of a material which is chemically and electro-chemically inert to the reactant materials of the cell. WHAT WE CLAIM IS:

1. An alkali metal/sulphur cell wherein a metal member in the cathodic region has a coating formed of at least two different materials, the proportions of which are not the same through the thickness of the coating, the outer surface being of a material known to be resistant to corrosion in the cathodic reactant.

2. A cell as claimed in claim 1 and having the coating applied directly on the metal substrate, wherein the coating is 100% of the same metal as the substrate at the surface of the substrate changing gradually to 100% of the corrosion resistant material at the outer surface.

3. An alkali metal/sulphur cell having two or more superimposed coatings on a metal member. each coating being formed of at least two different materials. the proportions of which are not the same through the thickness of the coating, the first coating having a composition changing from being the same as that of the underlying surface or substrate to a different composition, and the outer surface of the outermost coating being of a material known to be resistant to corrosion in the cathodic reactant.

4. A sodium-sulphur cell as claimed in any of claims l to 3 wherein the metal member is a cathode current collector.

5. A sodium-sulphur cell as claimed in claim 4 wherein the substrate material is aluminium.

6. An alkali metal/sulphur cell as claimed in any of the preceding claims wherein the coating is formed by a programmed deposition of two or more materials upon a metal substrate so as to provide, through the thickness of the coating, a gradual transformation from the composition of the metal substrate to the required corrosion-resistant composition on the outer surface of the coating.

7. An alkali metal/sulphur cell as claimed in claim 6 wherein the deposition is effected by a vapour deposition technique.

8. An alkali metal/sulphur cell as claimed in claim 6 wherein the deposition is effected by ion plating.

9. An alkali metal/sulphur cell having, in the cell, a metal member with a coating of graded composition such that the outer surface is corrosion-resistant to the reactants in the cell substantially as hereinbefore described.