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.