United States Patent (19) 11, 3,943,001 Miles (45) Mar. 9, 1976

(54) SILVER SULFIDE CATHODE FOR LIQUID 58 Field of Search ...... 136/86 A, 6 LN, 86 R AMMONIA BATTERES AND FUEL CELLS CONTAINING SULFUR AND HS IN THE 56) References Cited ELECTROLYTE UNITED STATES PATENTS 75) Inventor: Melvin H. Miles, Murfreesboro, 2,937,219 5/1960 Minnicket al...... 36/6 LN Tenn. 2,996,562 8/1961 Meyers...... 136/6 LN 3, 21,028 2/964 Story...... 136/6

(73) Assignee: The United States of America as 3.248,265 4/1966. Herbert...... 136/6 LN represented by the Secretary of the 3,408,229 10/1968 Posey et al...... 13676 LN Navy, Washington, D.C. G . Primary Examiner-G. L. Kaplan 22 Filed: July 5, 1973 Assistant Examiner-H. A. Feeley (21) Appl. No.: 376,784 Attorney, Agent, or Firm-Richard S. Sciascia; Joseph Related U.S. Application Data M. St. Amand (63) Continuation-in-part of Ser. No. 63,638, July 19, 57 ABSTRACT 1971, abandoned. A rechargeable silver sulfide cathode for batteries and 52) U.S. Cl...... 1366 LN 13686 R fuel cells using liquid ammonia electrolytes. (51) int. Cl.'...... H01M 10/00 5 Claims, 3 Drawing Figures U.S. Patent March 9, 1976 3,943,001

Afg. J.

2O c RESIDUAL E CURRENT . T = -50° C O 1. 4. 3 -2O

- 4 O

-O-O.8-O.6 -O-4-O,2 O O.2 O,4 POTENTIAL VS Pb/Pb (NO3)2.V

O > N Afg. 2. g-O. 3. d l -0.2 f s 3. E -O.3 2 H - O.4

O O.5 O .5 2.O 2,5 YELD, ELECTRONS/ATOM

O), Afg. 3. i REDUCTION o Ag TEST ELECTRODE OPEN CIRCUIT A Ag CONTROELECTRODE - O,3 (NO OX DATION) CURRENT = O,5OOn A O. 4. T = 50 C - O, 5 REDUCTION 76 N ------1A

O. 8 O 2O 4O 6O 8O OO 2O TIME, min 3,943,001 1. 2 useful cathode material at low temperatures. The acid SILVER SULFIDE CATHODE FOR LIQUID ity of the liquid ammonia electrolyte is determined by AMMONA BATTERIES AND FUEL CELLS the ammonia (NH") ion concentration. The NH' ion CONTAINING SULFUR AND HS IN THE is a proton donor and hence acts as an acid in liquid ELECTROLYTE ammonia. NH" in liquid ammonia corresponds to HO" in water. The maximum theoretical coulombic efficiency of two electrons per silver sulfide molecule CROSS REFERENCE TO RELATED APPLICATION can be obtained as predicted from the reaction This is a continuation in part of copending patent application, Ser. No. 163,638 filed July 19, 1971 now O AgS + 2e 2Ag+S- abandoned for Silver Sulfide Cathode for Liquid Am monia Batteries and Fuel Cells. Reference is also made where each monovalent silver atom is reduced to the to related copending patent application, Ser. No. zero oxidation state. The electrochemical reduction of 98,117 filed Dec. 14, 1970, now abandoned for Hydra silver sulfide in liquid ammonia solutions shows that 15 even at subzero temperatures the reactions rates are zine Anode in Liquid Ammonia Electrolytes. high. By contrast, the direct reduction of elemental BACKGROUND OF THE INVENTION sulfur on platinum and many other metals which do not This present invention relates to the practical use of readily form the metallic sulfides becomes too slow at silver sulfide as a rechargeable cathode for batteries subzero temperatures due to mass transport and kinetic and fuel cells utilizing ammonia electrolytes. 20 limitations. Insoluble, electronically conducting silver The attractiveness of liquid ammonia electrolytes for sulfide is therefore unique as a useful, rechargeable batteries is largely due to the comparatively low freez cathode material for use in low temperature batteries ing point of ammonia and to the high conductivity of and fuel cells. electrolytes in this solvent. Such properties provide for The high solubility of sulfur in ammonia makes it efficient operation of liquid ammonia batteries at sub 25 convenient to produce silver sulfide by the direct zero temperatures, whereas the performance of batter chemical reaction in solution ies utilizing aqueous electrolytes greatly deteriorates at 2Ag -- S - AgS 2} subzero temperatures due to increased viscosity or since silver combines directly with the dissolved sulfur, freezing of the solvent. even in the cold to form silver sulfide cathode material. The use of cathodes where sulfur species undergo 30 Addition of HSaids the rate of solution of the sulfur in changes in oxidation states by acting as electron-accep liquid ammonia, renders the sulfur in a reactive form tors in the cathode reaction are known in the prior art and also makes it possible to produce silver sulfide as shown by U.S. Pat. Nos. 2,689,876; 3,082,284 and electrochemically by the reverse of reaction (l). 3,121,028. However, the use of sulfur species as elec Anodes that can be used with the silver sulfide cath tron-acceptors is quite different from the present in 35 ode in liquid ammonia at low temperatures include vention, using a silver sulfide cathode, where monova magnesium and other active metals such as lithium. A lent silver atoms act as electron-acceptors. Also, the concentrated solution of lithium in ammonia consisting use of silver sulfide as a consumable cathode in liquid primarily of the compound tetrammine-lithium and ammonia batteries is in direct contrast with the use of represented by Li(NH) can also be used as the anode AgO and AgS as catalysts for other electrode reac 40 material. Near room temperatures or above, hydrazine tions in aqueous electrolytes as can be found in U.S. can be used as the anode material oxidized at the an Pat. No. 3,386,859. Materials which function as cata ode. A hydrazine anode is disclosed in the aforemen lysts do not get consumed by the reaction or undergo tioned copending patent application, Ser. No. 98,117, any net changes in oxidation states. The silver sulfide or which teaches the electrochemical oxidation of hydra silver polysulfide in the instant invention in consumed 45 zine in liquid ammonia. in the cathodic reaction and has no catalytic function as such. Other metallic sulfide compounds do not have STATEMENT OF THE OBJECTS OF THE the unique properties in liquid ammonia as do silver INVENTION sulfides. The primary object of the present invention is to Liquid ammonia batteries generally use soluble cath provide a rechargeable silver sulfide cathode for liquid ode materials such as m-dinitrobenzene. The perfor ammonia batteries and fuel cells. mance of these batteries is limited by the rate and ex A further object is to show that properties such as the tent of the solubility of the cathode material. Also, insolubility of silver sulfide in liquid ammonia, the solu some of the dissolved cathode material may be lost by bility of sulfur and the effect of adding HS, the elec undesired side reactions or by chemical reaction with 55 tronic conductivity of silver sulfide, and the reversibil the anode material. Furthermore, the performance of a ity of the electrochemical reaction involving silver sul soluble cathode material can be limited by mass move fide give the silver sulfide cathode of this invention ment to the electrode surface, by the active electrode unique advantages over the prior type cathode materi surface area available and by electrosorption onto the als being used in liquid ammonia batteries. Another electrode. These limitations do not exist when the in 60 object of this invention is to show that the present silver soluble, electronically conducting silver sulfide cath sulfide cathode can indeed be used at the subzero tem ode of this invention is used in liquid ammonia batter peratures made possible by the use of liquid ammonia S. electrolytes. Other objects, advantages and novel features of the SUMMARY OF THE INVENTION 65 invention will become apparent from the following Electrochemical studies of silver sulfide in acid liquid detailed description of the invention when considered ammonia electrolyte solutions over the temperature in conjunction with the accompanying drawings range of +20 to -50°C show that silver sulfide is a wherein: 3,943,001 3 sented above and reduction below the center horizon BRIEF DESCRIPTION OF THE DRAWINGS tal line. For a fuller understanding of the invention, the fol lowing description should be read in conjunction with Table the drawings in which: 5 Sulfur reactivity on various mctals in FIG. 1 shows cyclic voltammograms for 0.1M sulfur, NH-NHNO at 20°C from cyclic voltametric experiments. Oxidation HS solution in NH-1M NHNO at -50°C on silver wire electrode as a function of the anodic potential sweep limit. Silver sulfide is formed during the anodic sweep and reduced during the cathodic sweep. 10 FIG. 2 shows a typical curve for the constant current reduction of sulfur, HS solutions in stirred NH-NH NO at 15°C on silver electrodes of about 50cm geo metrical area, ( = 1.00mA). FIG. 3 shows constant current coulometry experi 15 ments at 15°C on silver in NH-NHNO solutions containing HS using constant currents of 0.500 mA; 2cm electrode area. Silver sulfide is formed during oxidation and is then later quantitatively reduced. Reduction DESCRIPTION OF THE PREFERRED -6 -).5 -0.4 -O3 - 0.2 -0. O O. 2 0. EMBOOMENT Potential vs. Pb/Pb(NO), W The chemicals used for experimental purposes in clude Matheson N.F. sublimed powder sulfur, Baker 25 FIG. 1 shows cyclic voltammograms for a 0.1 M reagent NHNO and LiNO, and Matheson anhydrous sulfur, HS solution in NH-1M NHNO at -50°C, ammonia (99.99%), each used without further purifi cation. using a silver electrode of 0.3 cm geometrical area. Constant current experiments Rate of Solution 30 A typical curve for the constant current reduction of Although sulfur is soluble in ammonia solutions in sulfur in stirred acid ammonia solutions at 15°C on excess of 30% by weight (10 M), the rate of solution is silver electrodes is shown in FIG. 2. A sharp inflection often slow; for example, in attempting to prepare a 0. in potential is observed when 2 electrons are consumed M sulfur solution in concentrated NH-LiNO, a di in the reaction per atom of sulfur initially present. This chroic blue-red solution formed which changed gradu 35 indicates that there are no detrimental side reactions or ally to a yellow solution, but some of the sulfur re decomposition of the reactant. Similar studies on silver mained undissolved even after ten days. During poten at -10°C gave approximately the same results as ob tiostatic reduction, the color of this solution changed to tained at +15°C, showing that this high coulombic effi a deep brownish red, and the excess sulfur dissolved. ciency does not decrease appreciably with decreasing When HS is present, even in low concentrations, how 40 temperature. ever, the rate of solution of sulfur in ammonia is very rapid. Neutral solutions The foregoing results favor acid (NHNOs) ammonia Screening of Materials solutions to which HS was added to accelerate the rate The electrochemical reactions of sulfur-HS solutions 45 of solution of the sulfur. Acidity is determined by the in acid liquid ammonia depend markedly on the nature ammonium (NH) ion concentration in liquid ammo of the metal used. Many metals tested (Ti, Zr, Hf, Th, nia. The results are quite different for sulfur dissolved Nb, Ta, Al, Ga., and In) do not form the sulfides in the in neutral (LiNO) ammonia solutions where no HS is sulfur-HS solutions and are completely useless for any used. Cyclic voltametric experiments on the sulfur electrochemical reaction of the sulfur solutions. Many 50 dissolved in neutral liquid ammonia showed little evi other metals (Pt, Pd, Ir, Rh, Re, Au, W, Mo, V, Co, Ni) dence for any electrochemical oxidation or reduction can be used as electrodes for electrochemical reaction of sulfur or sulfide compounds. Constant current and of the sulfur solutions, but they do not form the desired potentiostatic experiments in stirred neutral ammonia sulfide compounds in the ammonia solutions. Although solutions showed similar negative results. Addition of insoluble sulfide compounds are formed by the reac 55 LiS produced no change in these experiments, but the tions of Hg, Cu, Pb, and Sn with the sulfur solutions, addition of HS quickly rendered the sulfur reactive. these metallic sulfides do not have the attractive prop These very different effects of Li,S and H.S on the erties claimed for silver sulfide. These other metallic sulfur solution in neutral ammonia supports the con sulfides do not adhere as well to the surface of the cept that HS in ammonia forms a hydrogen-bonded metal as does silver sulfide, they are not as good as 60 species, SHNH), but produces no free sulfide ions, silver sulfide as electrical conductors, and their electro while sulfide ions added to ammonia do not undergo chemical potentials and reaction rates are less favor ammonolysis to HS. Apparently HS forms the species able than those observed for silver sulfide. SHNH) and NH which are necessary to render the The results for the various metals are summarized in dissolved sulfur in a reactive form. Dissolved sulfur Table I by listing the metal at the potential (to the 65 species in liquid ammonia apparently differ greatly in nearest O. l. V) where the oxidation and reduction cur properties from dissolved sulfur species in aqueous rents due to sulfur or HS attain a value of 2 mA/cm solutions. The reactive forms of sulfur in the liquid when the sweep rate is 125 mV/sec. Oxidation is repre ammonia containing HS probably are complex poly 3,943,001 5 6 sulfides since solutions of sulfur and HS in liquid am On the silver electrode, formation of the electronically monia have been shown to produce polysulfides, and conducting AgS greatly increases the effective elec the compound (NH),S has been isolated. trode area, resulting in higher currents. The peak cur rents on the silver electrode in sulfur-HS solutions are Experimental Tests quite insensitive to the bulk sulfur concentration; also Both AgS and PtS, like most sulfides of the electrode the number of coulombs involved in the reaction peaks metals tested, are black compounds which are insolu are independent of the potential sweep rate, indicating ble in liquid ammonia and therefore readily detectable that we are essentially observing reaction involving if formed in significant amounts. During the clectro insoluble AgS. The reversibility of this reaction indi chemical studies on platinum of the sulfur-HS solu O cates use of AgS as a rechargeable cathode in liquid tions in liquid ammonia, the platinum electrodes main ammonia batteries or fuel cells. FIG. indicates that tain a bright, metallic luster indicating very little build reaction 1 occurs readily even at -50'C, indicating up of any metallic sulfide compounds. However, when good performance of the AgS cathode at subzero tem a silver electrode remains at open circuit in the sulfur peratures. HS solution, or during electrochemical oxidation of 5 As aforementioned, FIG. 1 shows cyclic voltammo the solution on silver, the silver electrode becomes grams for a 0.1 M sulfur, HS solution in NH-1M black, indicating formation of AgS from the chemical NHNO at -50°C, using a silver electrode of 0.3 cm reaction geometrical area. A potential sweep rate of 100 2Ag+ S - AgS 2 mV/sec was used. Positive current represents oxidation 20 and negative current reduction. The numbered vertical or from the electrochcmical reaction lines represent the potential at which the anodic sweep was reversed, while the numbered curves indicate the 2Ag - S are AgS +2e. corresponding cathodic sweep. The resulting curves 25 show that the amount of material reduced corresponds Constant current coulometry experiments show that to the amount of material oxidized. During the anodic the electrochemical oxidation of ammonia-HS solu sweep, a blackening of the electrode is observed (silver tions on silver produces insoluble products which can sulfides forming) which lightens during the cathodic be quantitatively reduced. An acid ammonia solution sweep (silver sulfide being reduced). This experiment Saturated with HS was oxidized on silver for 1,800 sec demonstrates that silver sulfide can be reduced at tem at 0.500mA, turning the electrode black. The solution 30 was then stirred for 30 minto allow any soluble sulfur peratures as low as -50'C, and that even at this ex products to dissolve, then the electrode was transferred treme subzero temperature the electrochemical reac to a separate compartment with fresh electrolyte. Re tion is reversible. Therefore, this invention can be prac duction at the same current, 0.500 mA, produced a ticed in liquid ammonia batteries operating at subzero gradual fading of the black color with a sharp inflection 35 temperatures. in potential after 1,815 sec, indicating that the silver This invention is the first demonstration that silver was first oxidized to insoluble AgS and then the insolu sulfide has attractive applications as the cathode mate ble sulfide formed was quantitatively reduced. rial in liquid ammonia batteries. The principal advan FIG. 3 shows the results of the constant current cou tages of this invention over cathode materials now in 40 See ae lometry experiment at 5°C in an ammonia solution Silver sulfide is insoluble in liquid ammonia. The containing HS. Silver electrodes of 2 cm geometrical silver sulfide cathode material can therefore be area were used, the reference electrode was retained on the electrode surface where it is most Pb/Pb(NO), saturated, and NHNO was used as the useful. This eliminates the need of diaphragms, electolyte salt. The control electrode showed that no 45 separators, binders, or grids to confine the active electrochemical reduction is observed without first material, and also prevents loss of the material by forming silver sulfide. This important experiment undesired reactions such as chemical reaction with shows that the electrochemical reaction the fuel or anode. Furthermore, mass transport and AgS +2e are 2Ag+ S. 1 electrosorption limitations are avoided with this SO insoluble silver cathode. is completely reversible, demonstrates that silver sul As indicated above, silver sulfide can be formed fide can be quantitatively reduced in liquid ammonia, under proper conditions by direct reaction of silver and shows that silver sulfide is insoluble in liquid am with sulfur dissolved in liquid ammonia. The blacken monia. These are very important characteristics of this ing of silver due to AgS formation is readily observed invention. Similar tests on platinum showed oxidation 55 when it is immersed in a sulfur-ammonia solution con of the ammonia-HS solution at about 0.0V with the taining HS. This would not be possible in many other electrode remaining bright. Reduction in a separate solvents in which sulfur is not soluble, compartment showed only hydrogen evolution, indicat Silver sulfide is electronically conducting, hence, the ing that no insoluble platinum sulfide compounds were cathode material itself serves to greatly increase formed on the platinum electrode during oxidation of 60 the effective surface area of reaction. the ammonia-HS solution. The electrochemical reaction of silver sulfide is re The very different peak potentials and peak currents versible. This cathode material, therefore, can be observed in cyclic voltametric experiments on platinum used in rechargeable batteries or fuel cells. and silver reflect the different nature of the sulfur reac The silver sulfide cathode in liquid ammonia can be tions with these two metals. Sulfur atoms undergo the 65 used at subzero temperatures. changes in oxidation states during the electrochemical Present cathode materials do not have all five of reaction on platinum, while silver atoms undergo the these distinct advantages; in fact, many have only one changes in oxidation states when silver metal is used. or two of the advantages listed above. 3,943,001 7 8 Obviously many modifications and variations of the Modifications present invention are possible in the light of the above Another embodiment of this invention is to use silver teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be polysulfide rather than silver sulfide. Solutions of sulfur practiced other than as specifically described. and HS in liquid ammonia form polysulfides, S. , I claim: where x represents the number of sulfur atoms con 1. An electrochemical cell having an anode and a tained in the polysulfide ion. FIG. 1 actually shows cathode and using acid acting liquid ammonia electro results for the silver polysulfide cathode, AgSc, since lytes, comprising: the ammonia solution contained both HS and sulfur. O a. an insoluble, reducible cathode maerial consisting The results are practically identical to those obtained of a silver sulfide compound, using only HS, in fact, polysulfides are probably b. said acid acting liquid ammonia electrolyte con formed during the reduction of the silver sufide cath taining sulfur and HS in solution, ode. An attraction of a liquid ammonia solution con c. said silver sulfide compound being consumed in taining sulfur is that silver sulfide is produced chemi 15 the cathodic reaction with the silver undergoing cally by the spontaneous reaction 2Ag-- S --> AgS. the change in oxidation state in accordance with The blackening of a silver electrode due to AgS forma the completely reversible reaction tion is readily observed when it is immersed into a sulfur ammonia solution. AgS +2e R. 2Ag+ ST A further modification of this invention is the use of 20 d, said electronically conducting silver sulfide cath solvents other than ammonia. The performance of this ode material which is insoluble in the liquid ammo silver sulfide cathode will be independent of the solvent nia being quantitatively reduced therein. or electrolyte salt used where the silver sulfide is insolu 2. In the cell of claim 1 wherein said silver sulfide ble in the solvent and where the electrolytic conductiv compound is in the form of a silver polysulfide. ity level is retained. Aqueous solutions, although being 25 3. In the cell of claim 1 wherein the anode is magne inferior for subzero temperature applications due to sium. reduced electrolytic conductivity, would be an example 4. In the cell of claim 1 wherein the material oxidized of other solvents in which to use the silver sulfide cath at the anode is hydrazine. ode. Using mixed solvents, such as water-ammonia 5. In the cell of claim 1 wherein the material oxidized mixtures, is another example of a modification involv 30 at the anode is Li (NH). ing the solvent. k sk sk : xk

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