Abstract:
An electrochemical storage cell or battery with one anode chamber containing molten alkali metal as the anolyte and one cathode chamber containing sulfur-containing catholyte substance, the anolyte and catholyte separated by an alkali-ion-conducting solid electrolyte. The cathode chamber is bounded by a cellwall having a protective layer of a light weight metal also serving as a current collector. The light weight metal in the regions in contact with the catholyte is coated with a metallic alloy which under conditions of operation has certain properties of adhesion, conduction, resistance, corrosion and solubility.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates an electrochemical storage cell or battery with at least one anode chamber for alkali metal as the anolyte and at least one cathode chamber for the sulfur-containing catholyte substance with the anolyte and catholyte separated from each other by an alkali-ion conducting solid electrolyte and bounded by a cellwall of a light metal which also functions as a current collector. 
     2. Description of the Prior Art 
     Storage cells of the above mentioned design present a considerable simplification as to their construction by using the cell wall as current collector. A further advantage results also from the use of light metals or the alloys of light metals for the cellwalls, which leads to high power-density of the storage cells or batteries. 
     However, a considerable disadvantage of such storage cells is due to the corrosion of the cellwall material by the catholyte substance, particularly by the sulfur or sodium polysulfide melt. 
     It was noted that light metal alloys corrode much faster in an electric field, particularly in the form of dangerous localized pitting corrosion. To overcome this problem, the suggestion was made (German Published Non-Prosecuted Application 2,457,418) to dispose a screening electrode made of a more corrosion resisting material, particularly graphite, in front of the cellwall. 
     Attempts were made to protect the light metal cellwalls of such storage cells from corrosion by means of a cover layer of MoS 2 , U.S. Pat. No. 3,749,603 and also to put layers of cobalt-or chromium sulfides onto the light-metal cellwalls. 
     The above mentioned procedures improved the behaviour with respect to corrosion for short periods, however, after longer time spans new disadvantages turned up. In this connection the rising of the interior resistance of the cell must be mentioned particularly. Also other negative symptoms appeared because of a lack of knowledge of the required material parameters necessary for the complex situation of this application. 
     SUMMARY OF THE INVENTION 
     An object of the instant invention is to provide a storage cell with a coating material for the cathode chamber cellwall, which in cells of the above mentioned construction would result in good operating conditions over a long time period, such as the electrical properties of the cell (particularly a constant lower electrical resistance), as well as operating life of the cell, i.e. the inhibition of the aging and corrosion processes. Another object is to provide a storage cell which can be efficiently and economically manufactured. 
     With the foregoing and other objects in view, there is provided in accordance with the invention an electrochemical storage cell or battery with at least one anode chamber containing alkali metal as the anolyte and at least one cathode chamber containing a sulfur-containing catholyte substance, with the anode chamber and the cathode chamber separated from each other by an alkali-ion conducting solid electrolyte, and with the cathode chamber bounded by a cellwall of a light-weight metal having a protective layer which wall also serves as current collector, including coating the light weight metal, at least in the region which is in contact with the catholyte substance in the cathode chamber, with a metallic alloy which, under the conditions of operation has properties 
     (a) of adhering to the light weight metal, 
     (b) of a good conductor for electric current, the alloy exhibiting, in the corroded condition resulting from conditions of operation, a minimum conductivity of ca. 0.03Ω -1  cm -1 , 
     (c) prevents the formation of a cohesive layer of aluminum sulfide and other corrosion products with a resistance greater than 2Ωcm 2  and 
     (d) the corrosion products resulting from the conditions of operation have a solubility of less than 10 mg/g melt in the catholyte substance. 
     Other features which are considered as characteristic for the invention are set forth in the appended claims. 
     Although the invention is illustrated and described herein as embodied in electrochemical storage cell or battery, it is nevertheless not intended to be limited to the details shown, since various modifications may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawing, in which is diagrammatically illustrated a vertical section of a cup-shaped storage cell in accordance with the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The improved results of the invention are attained by coating the light metal, at least in the region in contact with the catholyte substance, with a metallic alloy which has, in the operating state, strong adhesive properties and which is a good conductor for electric current. The metallic alloy exhibits a minimum conductivity of 0.03Ω -1  cm -1  in the corroded state and prevents the formation of a continuous layer of aluminum sulfide, or other corrosion products with a resistance over approximately 2Ωcm 2 . The corrosion products have a solubility of under approximately 10 mg/g in the catholyte substance. 
     The term light weight metals or light metals as used herein are the metals aluminum, magnesium, titanium, and the alloys of these metals. The preferred light weight metal is aluminum, with addition of a maximum of 5% magnesium. The properties of the coating of good adhesion and electrical conductivity, initially, is not surprising, but it should be remembered that under the conditions of operation, corrosion of the protective coating will occur. Herein the invention differs greatly from the previous attempts, where, for example, molybdenum-disulfide or graphite was used as a coating on the assumption that the protective coat itself would not corrode by contact with the sulfur containing catholyte substance. In the instant invention it is definitely assumed that the protective coat would corrode during operation, but nevertheless good results are obtained. 
     In many experiments it has been shown that the minimum-conductivity of 0.03Ω -1  cm 2  of the coating in the corroded condition is still quite sufficient to assure good electrical properties of the cell. 
     On the other hand, it is important that the corroded layer which is formed, sets up a sufficient large resistance against diffusion of aluminum, so that the aluminum is prevented from forming a significant amount of Al 2  S 3 , which, because of its high resistance, is particularly detrimental, or to form similar corrosion products with a resistance above 2Ωcm 2 . Finally, the alloy intended for the protective coating should be characterized by the property of the layer generated by the corrosion not dissolving in the catholyte substance, i.e. the corrosion products have a certain maximum solubility in the catholyte substance. Some of the previous known attempts failed by not meeting the above mentioned criterion. 
     Different types of alloys having the desired properties for use as a protective coating on cellwalls may be selected. In general, alloys are applicable which on a base of nickel or cobalt contain suitable amounts of chromium, cobalt, nickel, molybdenum or tungsten. Various high-grade steels and so called super-alloys also belong in this category. Some of these alloys have become known for housings of sodium sulfur cells, but are not known in connection with protective coatings on light metal, and not in regard to the special aspects of this invention. 
     During the investigations for the invention at hand, an alloy was found which proved to be particularly advantageous as protective layer. Such alloy contains at least one metal from the group iron, cobalt or nickel, and such alloy also contains 10 to 50 weight % chromium, preferably 20 to 30 weight % chromium. below 10 weight % chromium there seems to be no cohesive protective layer, while at chromium contents above 50 weight % the protective effect of the layer diminishes, apparently due to formation of chromium sulfides which exhibit a less advantageous behaviour. In the preferred region of 20 to 30% weight of chromium, chromium stratum and some sulfides seem to be formed which effect very dense, well protecting sub-layers. 
     The metallic alloys which are used as coating, according to the invention, are therefore at least partly sulfided by the melt of the cathode chamber. In contrast to the sulfidation of the light weight metals the hereby formed corrosion products such as heavy metal sulfides and thiospinel are not detrimental to the cell performance since they are not only of poorer solubility in the melt but also exhibit better conductivity for electric current than the corresponding products of the unprotected light metal. As experiments have shown, the coating must not form an absolute dense cover-layer since the light metal itself is also to a great extent corrosion resistant. 
     In the following, two embodiments are explained with the aid of the cup-shaped storage cell shown in the drawing. Referring to the drawing, a container made of aluminum serves as housing 1. The container was manufactured as follows: Before applying the coating, the individual parts were provided with an undercoating of 96% nickel and 4% aluminum, designated by the numeral 2. Thereafter, the coating with the protective layer is applied, indicated by the number 3, namely, with a super-alloy consisting of 56.71 weight % nickel, 18.4 weight % cobalt, 12.3 weight % chromium, 3.25 weight % molybdenum, 5 weight % aluminum and 4.33 weight % titanium. Both layers were applied by conventional plasma-spraying with a device called Plasmadyne, 36 kW. Argon was used as the plasma gas and a current strength of 500 A at 35 V was used. The parts, after being treated as described, are then combined by electron beam welding. The bottom designated by 4 in the drawing, was manufactured in the same way. 
     Obviously, the above-described manufacturing method can be replaced by more sophisticated methods. Between the cellwall or housing at its interior protective coating 3 and the solid electrolyte 5, is the cathode chamber, which, as usual, is filled with graphite felt 6. The short tube 7, serves for evacuation, if desired, of the catholyte substance in the cathode chamber. The sealing means 8 which is made of aluminum, and which is connected to the housing 1, is pressed onto the Al 2  O 3  ring 10 with the help of the retainer flange 9. In this way, the cathode chamber is sealed from the atmosphere. 
     In the following, a comparison is made between a sodium-sulfur cell with a housing consisting of steel, and one made according to the instant invention. Both cells follow generally the construction shown in the drawing. The electrolyte tube has a length of 220 mm, a diameter of 25 mm and a wall thickness of ca. 1 mm. The distance between the electrolyte tube and the housing wall is 7 mm. In both cells of the example, a coating of the same material whose composition was given above, is in contact with the melt. 
     The thickness of the housing wall is 1 mm. In the steel cell, in contrast to the drawing, the seal is constructed according to German Published Non-Prosecuted Application 24 59 530. 
     From such cells, one can take out 100 Wh during a discharge time of two hours. 
     The weight of the steel cell is ca. 1 kg, the weight of the coated aluminum cell is ca. 50 g. This clearly proves the advantage of using aluminum with respect to weight energy density. 
     Both cells reached a capacity of 78±2% at a charging current density of 75 mA/(cm 2  β Al 2  O 3 ), when treated by an addition of 0.8 mole % tetracyano-ethylene (German Published Non-Prosecuted Application 2 633 456 and U.S. Application Ser. No. 817,378 now abandoned, filed July 20, 1977, assigned to Brown, Boveri and Cie Aktiengesellschaft, the assignee of the present invention), which capacity remained constant within limits of error through 100 cycles. Thus, the two cells compared are equivalent with respect to capacity and aging. 
     In the case of an aluminum housing coated according to the state of the art, the results are inferior to that of the present invention. In a cell whose housing consisted of aluminum coated by plasma-spraying with molybdenum, whereby the molybdenum was then by contact with sulfur changed to MoS 2  at a discharge current of 75 mA/cm 2  (referred to the electrolyte surface) a cell potential of only 1.75 volt resulted, whereas in a cell constructed according to the invention, the cell potential was considerably higher at equal discharge current density, namely 1.88 volt. This potential remained constant over more than 100 cycles. 
     In a cell with an aluminum housing which had been coated with graphite by evaporation in a vacuum, the cell potential was 1.7 volt in the beginning at 75 mA/cm 2 , and after several cycles only a cell potential of 1.4 volt could be reached at 35 mA/cm 2 . 
     A comparison with a cellwall not coated with aluminum is not possible because when the cellwall is used as a current collector, the uncoated aluminum corrodes so rapidly and thereby the interior resistance of the cell rises so much due to the strongly insulating aluminum sulfide, that the cell as a practical matter is not operative.