Electrodes made from mixed silver-silver oxides

#The present invention relates to electrodes made from particles of silver and silver oxide, containing a nickel or nickel-plated metal support-current collector, which may be used in silver-metal batteries, particularly silver-iron batteries.

#Silver-iron batteries are well known in the art, and are taught by Brown, in U.S. Pat. No. 4,078,125, and Buzzelli, in U.S. Pat. No. 4,383,015. These patents teach the use of perforated silver sheet or expanded silver screen supports containing active silver material for positive plates, multiply-microporous separators, and either sulfurized iron oxide and/or iron oxide hydrate negative plates according to the teachings of Jackovitz et al., U.S. Pat. No. 4,356,101, or sintered metallic iron negative plates.

#The silver-iron battery is now generally considered more stable than the silver-zinc battery. The silver-zinc battery has always presented major problems of internal electrical shorts due to zinc dendritic growth from the negative plates through the separator system. Both battery systems are quite expensive, and are usually restricted to applications where the energy density of the battery is critical to the total system mission. An example of such an application is the propulsion system power source for underseas vehicles.

#The objects and advantages of this invention will become more apparent by reading the following detailed description in conjunction with the drawings, which are shown by way of example only, wherein:

#FIG. 1 shows one embodiment of the silver electrode of this invention;

#FIG. 2 shows a magnified cross-section of the electrode of FIG. 1, showing a possible idealized configuration of large and small particles;

#Referring now to FIG. 1 of the Drawings, a positive silver electrode plate 10, is shown in accordance with this invention, also containing positive electrical lead tab 12. This silver electrode is made by: (1) mixing 1 part by weight of dry elemental silver particles, i.e., Ag.degree. of about 99+% purity, preferably greater than 99.7% purity, having a particle size of from 47 microns diameter to 175 microns diameter, preferably from 55 microns to 175 microns diameter, with from about 0.05 to 0.40 part by weight, preferably 0.15 to 0.30 part by weight, of dry silver (I) oxide i.e., Ag.sub.2 O or argentous oxide, particles having a particle size of from 3 microns diameter to 45 microns diameter, no other additives being necessary or desired; (2) pressing the mixture into contact with a support which is either nickel or nickel coated, by loading some of the mixture into a die cavity and pressing lightly to densify, followed by pressing the support into the modestly densified mixture, then adding additional mixture on top of the support, and finally, pressing to densify and consolidate the entire structure; (3) low temperature sintering the consolidated structure, at from about 350.degree. C. to about 550.degree. C. in a reducing atmosphere, preferably hydrogen gas, for from about 10 minutes to about 90 minutes, to form an electrode plate. During sintering, the Ag.sub.2 O particles lose oxygen from their lattice structure, gaining porosity, and bonding to the original silver particles, to provide a sintered-pore structure having minute electrolyte permeable interstices throughout the silver matrix; and (4) applying a suitable electrical lead tab to the top of the electrode plate, preferably a silver or annealed nickel sheet.

#The Ag.sub.2 O particles are more stable in storage and less expensive than divalent silver oxide, i.e., AgO, and when reduced, provide a porous silver form. By using Ag.degree. and Ag.sub.2 O particles within the described particle size ranges and weight ratio, a silver electrode is formed after sintering that has an optimum amount of high surface area and superior internal electrolyte access. Within these parameters, the pores are not oversized at the expense of surface area, and are uniformly distributed. After sintering, the particles in the silver body attached to the support retain substantially their original shape, with a certain amount of deformation, and with interdiffusion of metal between particles to effect diffusion bonding.

#The sintered silver particles are the sintered product of particles having the dual unsintered particle size ranges previously described. Low sintering temperatures must be used to prevent formation of agglomerated, non-porous silver masses. The sintered particles substantially retain their original size, so that even after sintering, the major portion is between 47 microns and 175 microns and the minor portion is between 3 microns and 45 microns. If over 0.40 part by weight of Ag.sub.2 O is used per part Ag.degree. or if the Ag.sub.2 O particles are over 45 microns diameter, too much internal porosity of inadequate size pores will result. Under the specified ranges of both Ag.degree. and AgO.sub.2, too little porosity will result. Under 47 microns for the Ag.degree. particles and porosity may begin to suffer.