Abstract:
A slurry for use in metal-air batteries comprises an admixture of particulate porous zinc, an aqueous solution of at least one Group Ia metal hydroxide, an inorganic inhibitor ingredient effective to inhibit the interaction of porous zinc with the Group Ia metal hydroxide which would otherwise result in the evolution of hydrogen gas, a gelling agent, a particulate and/or fibrous filler, a labelling agent, and a dissolved electrolyte extender.

Description:
FIELD AND BACKGROUND OF THE INVENTION 
     The present invention relates to a slurry for use in rechargeable metal-air batteries generally, and more particularly, to rechargeable electric batteries useful in electric vehicles and energy storage systems. 
     Various proposals have been made in the past for electric powered vehicles. To date, for a number of reasons, electric vehicle systems have yet to become commercially viable generally, for urban and highway applications. 
     There have been proposals to employ zinc/air batteries for urban vehicle propulsion. An example is the following publication: Improved slurry zinc/air systems as batteries for urban vehicle propulsion, by P. C. Foller, Journal of Applied Electrochemistry 16 (19860, 527-543. 
     Metal/air battery structures are described in the following publications: U.S. Pat. Nos. 4,842,963, entitled Zinc Electrode and Rechargeable Zinc-Air Battery; 4,147,839, entitled Electrochemical Cell with Stirred Slurry; 4,908,281, entitled Metal/air Battery with Recirculating Electrolyte; 3,847,671, entitled Hydraulically-Refuelable Metal-Gas Depolarized Battery System; 4,925,744, entitled Primary Aluminum-Air Battery; 3,716,413, entitled Rechargeable Electrochemical Power Supply; 4,925,744, entitled Primary Aluminum-Air Battery. In U.S. Pat. No. 3,592,698, entitled Metal Fuel Battery with Fuel Suspended in Electrolyte, there is described inter alia a method for circulating an electrolyte/metal fuel powder mixture through the batteries; U.S. Pat. No. 4,126,733, entitled Electrochemical Generator Comprising an Electrode in the Form of a Suspension, relates to a similar subject using a circulated suspension of inert cores coated with an electrochemically active material. 
     Electrical energy storage systems are described in the following publications: U.S. Pat. Nos. 4,843,251 entitled Energy Storage and Supply Recirculating Electrolyte; Energy on Call by John A. Casazza et al, IEEE Spectrum June, 1976, pp 44-47; 4,275,310, entitled Peak Power Generation; 4,124,805, entitled Pollution-Free Power Generating and Peak Power Load Shaving System; 4,797,566, entitled Energy Storing Apparatus. 
     The disclosures of the foregoing publications (including patents) are explicitly incorporated herein by reference. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to make possible from a practical point of view, the general commercial viability of metal-air batteries, more particularly for use in electric vehicle propulsion and energy storage systems. 
     A more specific object of the invention is to provide a rechargeable slurry for use in metal-air batteries. 
     Other objects of the invention will become apparent from the description which follows. 
     The present invention accordingly provides a slurry for use in metal-air batteries which comprises an admixture of at least components (a), (b) and (c), of the following components (a), (b), (c), (d), (e), (f) and (g): 
     (a) particles comprising a metal, preferably porous, selected from aluminum, iron and zinc; 
     (b) an aqueous solution of at least one Group Ia metal hydroxide; 
     (c) an inorganic inhibitor ingredient effective to inhibit the interaction of components (a) and (b) which would otherwise result in the evolution of hydrogen gas; 
     (d) a gelling agent; 
     (e) a particulate and/or fibrous filler; 
     (f) a labelling agent; 
     (g) a dissolved electrolyte extender. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the slurry of the invention, the weight ratio of the essential components (a): (b): (c) is preferably 1:0.5-2.0:0.0005-0.04, and optional components (d), (e), (f) and (g), if any or all of these are present in the slurry, are preferably present within the following weight percentages based on the weight of the total slurry, namely, (d) 0.3-3.0%, (e) 1.0-10.0%, (f) 0.001-1.0% and (g) 0.1-10.0%, provided that the percentage of zinc in the slurry is within the range of 33.3-67.0 wt. %, preferably 45.0-60.0 wt. %. 
     In this connection, it is noted that zinc slurries as used in conventional alkaline batteries contain more than 70 wt. % zinc typically 76 wt. % zinc, see e.g. &#34;Handbook of Batteries and Fuel Cells&#34; (David Linden, editor-in-chief), McGraw-Hill Book Company, 1984, at paragraph 7.3.2, the sub-paragraph headed &#34;Gelled Anodes&#34;. In U.S. Pat. No. 4,341,841 there is described a circulating alkaline electrolyte containing 20-30 wt. % zinc particles. The contents of the cited Handbook and of U.S. Pat. No. 4,341,841 are explicitly incorporated herein by reference. 
     In a particular embodiment, the particles comprising a metal are selected from porous metal particles and a particulate inert substrate coated first with a conductive coating and then with the porous metal. Exemplary conductive coatings are of copper, nickel, cadmium, tin, silver or lead. 
     The particles comprising a preferably porous metal may have a particle size falling within the range of 100-500 microns, and in the case of a particulate inert substrate coated first with a conductive coating (e.g. of thickness between 0.5 and 5 microns) and then with porous metal, the substrate may have a particle size falling within the range of 10-250 microns. The material of the particulate inert substrate may be selected, for example, from polypropylene, polyethylene, nylon, glass and polyvinyl chloride, desirably in the form of microspheres. 
     When the particulate metal is porous zinc, this will preferably have a density which is 5-20% of the density of non-porous zinc, e.g. it may have a density within the range of 0.3-1.4 g./cc, preferably 0.3-1.1 g./cc, and a surface area within the range 0.5-6.0, preferably 0.75-5.0 m 2  /g. It may be noted that zinc powder as used in conventional alkaline batteries is less porous, having a density of at least 2.5 g./cc., e.g. ≈3.5 g./cc, and having a very limited surface area, generally about 0.2-0.4 m 2  /g (see &#34;Handbook of Batteries and Fuel Cells&#34;, loc cit). 
     The aqueous solution is preferably a 4-12 molar aqueous solution in terms of Group Ia metal hydroxide; the at least one Group Ia metal hydroxide is preferably selected from LiOH, NaOH and KOH. 
     The inorganic inhibitor ingredient is preferably selected from at least one of the following, namely, oxides of antimony, bismuth, cadmium, gallium, indium, lead, mercury (including the red and yellow varieties), thallium and tin; sulfides of cadmium, iron, lead and mercury. In addition, organic inhibitors such as phosphate esters, surfactants (e.g. ethylene oxide polymers and derivatives thereof) and/or quaternary ammonium compounds (e.g. tetraethylammonium hydroxide) may also be optionally present. Metal oxide inhibitors for zinc (in practice zinc oxide) electrodes are described in U.S. Pat. No. 4,084,047, in which the inhibitors are mixed thoroughly into the zinc oxide. Although the present invention departs from the teaching of this patent, insofar as in the present slurries the electrode material cannot be zinc oxide and the inhibitors are not mixed with the anode material (i.e. the porous metal), it is presently contemplated that the metal oxide inhibitors and their mixtures as disclosed therein will be useful in the present invention; accordingly, the disclosure of this U.S. patent is explicitly incorporated herein by reference. 
     As regards the optional organic inhibitor, a suitable phosphate ester is e.g., that sold under the trade designation of GAFAC RA600 (BASF Corporation). In this connection, it may be noted that U.S. Pat. No. 4,195,120 teaches alkaline cells containing a predominantly zinc anode and an organic phosphate ester of the ethylene oxide adduct type; the disclosure of this patent is incorporated herein by reference. Reference may also be made to U.S. Pat. Nos. 4,112,205 which utilizes double salts containing both mercuric ion and quaternary ammonium ion as inhibitors in galvanic cells comprising zinc anodes, as well as to 3,945,849, which employs quaternary ammonium halides as inhibitor for zinc anodes; the disclosures of these U.S. patents are incorporated herein by reference. 
     The gelling agent is preferably selected from polyacrylic acid (e.g. Carbopol from B.F. Goodrich Co.), carboxymethylcellulose (CMC) and partially hydrolyzed polyacrylonitrile (e.g. Waterlock A-400 from Grain Processing Corporation). A method of gelling anodes for alkaline cells using hydrolyzed polyacrylonitrile is taught in U.S. Pat. No. 4,563,404, the disclosure of which is incorporated herein by reference. Other gelling agents are, for example, starch graft copolymers such as Waterlock A-221 and cross-linked polyacrylamides (e.g. as described in U.S. Pat. No. 3,884,721, the disclosure of which is incorporated herein by reference), as well as cross-linked CMC (e.g. as described in U.S. Pat. No. 4,435,488, the disclosure of which is incorporated herein by reference). 
     The filler may be selected from (for example) powdered graphite, carbon fibers, cellulose fibers, calcium hydroxide, silicon dioxide, titanium dioxide, titanium sub-oxide, polymeric microspheres, glass microspheres, and any such microspheres coated with a metal selected from Cu, Ni, Cd, Sn, Ag and Pb. The filler is preferably a conductive filler, particularly (but not exclusively) when the slurries contain an amount of zinc at the lower end of the 33.3-67.0 wt. % range. The filler is used in order to bulk out the slurries of the invention so as to avoid collapse during discharge. Persons skilled in the art will appreciate that a filler is not required in slurries for conventional alkaline batteries, because in this case they are so viscous that they usually must be extruded into the cells (see &#34;Handbook of Batteries and Fuel Cells&#34;, loc cit), and the possibility of collapse in this prior instance is probably negligible. 
     The labelling agent, the purpose of which is to permit the ready identification of the slurry of the present invention, may be selected from, e.g., visibly colored dyes, fluorescent dyes and encapsulated magnetic particles, such as Cresol Red, sodium fluorescein or nylon-, polypropylene-, polyvinyl chloride- or polytetrafluoroethylene-encapsulated Fe 3  O 4 . 
     The slurry of the invention may also contain 0.1-10.0 wt. % in total, based on the total slurry, of a dissolved electrolyte extender ingredient selected from at least one of barium hydroxide, strontium hydroxide, potassium silicate, sodium silicate, sorbitol, potassium borate and potassium phosphates. 
     It will be appreciated that while the present invention requires that all of the components (a), (b) and (c) are present in the inventive slurry, the latter need not contain any of the optional components (d), (e), (f) and (g). In alternative embodiments of the invention, however, the slurry may contain, in addition to components (a), (b) and (c), any one, two, three or all four, of optional components (d), (e), (f) and (g). 
     The invention will be illustrated by the following nonlimitative example. 
     EXAMPLE 
     Zinc powder (50 g., 30 mesh), having a density and surface area, respectively, of approximately 0.6 g./cc. and 1.0 m 2  /g., and potassium hydroxide solution (50 g., 8 molar) were stirred together, and the following ingredients were then stirred into the initial admixture: as inorganic inhibitor, yellow lead oxide (1 g.); as conductive filler, graphite (5 g., Acheson); as gelling agent, carboxymethyl cellulose (1 g.); and as indicator, Cresol Red (0.01 g.). The product was a gel-like slurry of density about 2 g./cc. The zinc did not segregate, nor was the generation of hydrogen bubbles visible. Also, the characteristic red shade of the dye could be readily identified by reflected light. 
     The discharge characteristics of this slurry were then tested in a zinc-air cell, which was made up of two commercial air electrodes (Electromedia Type AE 20) of active dimensions 7.1×7.1 cm. per electrode, for a total area of 100 cm 2 . Each electrode was fitted with a current collecting tab (the positive) of nickel foil. The two electrodes were glued to a U-shaped polyvinyl chloride structure which held them parallel and about 3 mm. apart. This arrangement made up a watertight compartment of 25 ml. volume into which the slurry could be introduced. To prevent shorting between the zinc powder and the air electrodes, a layer of porous polyamide separator was glued to the electrolyte side of each air electrode inside the cell. Air was freely available to the air side of the air electrodes external to the cell. 20 ml. slurry was introduced into the space between the two separators. A copper foil (0.2 mm. thickness), tabbed with a copper strip, was inserted into the slurry as a negative current collector. 
     After wet-in of the separators, an open circuit voltage of 1.45 V was measured. The cell then delivered a steady current of 3 A (30 mA/cm 2 ) at an average voltage of 1.2 V for three hours to a final cut-off voltage of 1 V. The cell capacity was thus 9 Ahr. Since there were about 18 g. zinc originally in the cell, the zinc utilization was about 60%. Following discharge of the cell, the spent slurry was rinsed out with alkali. A fresh portion of slurry was introduced into the cell, giving a similar discharge performance. The spent slurry could be recharged, for example, as described in our copending application (11785), the disclosure of which is incorporated herein by reference. 
     While the invention has been particularly described, it will be appreciated by persons skilled in the art that many modifications and variations are possible. The invention is accordingly not to be construed as limited to the particularly described embodiments, rather its concept, scope and spirit are to be understood in the light of the claims which follow.