United States Patent (19) (11) 3,719,746 Johnston (45) March 6, 1973

(54) AQUEOUS SYNTHESIS OF IONICALLY Vol. 5, pp. 433-436 CONDUCTIVE COMPOSITIONS OF Jacobson: Encycopedia of Chemical Reactions, Vol. MATTER VI, 1956, p. 162. 75 Inventor: William V. Johnston, Camarillo, Primary Examiner-George F. Lesmes Calif. Assistant Examiner-J. P. Brammer (73) Assignee: North American Rockwell Corpora Attorney-L. Lee Humphries, Thomas S. MacDonald tion and Henry Kolin 22 Filed: Dec. 8, 1970 57 ABSTRACT (21) Appl. No.: 96,229 An aqueous process for the preparation of a solid ioni cally conductive composition of matter having the for (52) U.S. Cl...... 423/463,252/518 mula MAgas in which M is Rb, K, NH4, Cs, or a com 51 Int. Cl...... H01b. 1706, C0 lb 9/00, C01g 1/06, bination of these, Cs being present only as a minor C01.g3/04, C01g 7/00, C01g 9/04 constituent of M, comprising preparing a substantially 58) Field of Search...... 252/518; 23/87, 89,367 saturated solution of MI in water, intimately admixing 56) References Cited four molar equivalents of AgI with the MI solution to effect reaction between the MI and AgI, and removing UNITED STATES PATENTS the water to recover MAgals as the synthesized ioni 3,519,404 7/1970 Argue et al...... 252/518 cally conductive composition. 2,944,975 7/1960 Folberth...... 23/204 9 Claims, No Drawings OTHER PUBLICATIONS C. Brink, et al., Acta Crystallegraphica July 1952, 3,719,746 2 AQUEOUSSYNTHESIS OF IONICALLY According to the process of this invention, an aque CONDUCTIVE COMPOSITIONS OF MATTER ous essentially saturated solution of MI is prepared. This is prepared conveniently and preferably at room BACKGROUND OF THE INVENTION temperature, although any temperature up to the boil This invention relates to a process for the prepara 5 ing point of the saturated aqueous alkali metal iodide tion of ionically conductive compositions of matter solution may be used. Four molar equivalents of silver having unusually high ionic conductivity. It particularly iodide are then intimately admixed with this saturated relates to an aqueous process for the preparation of solution to effect the desired reaction. The water is compounds having the formula MAgis in which M IS then removed so as to recover the formed MAgis ioni Rb, NH, K, Cs, or a combination of these, Cs being 10 cally conductive composition therefrom. present only as a minor constituent of M. Such com C. Brink et al. in Acta Cryst. 5, 433-36, (1952) have positions are particularly useful as electrolytes in solid described the preparation of the nonconductive com state electric cells. Such compositions are shown and positions KAgla, Rb,Agla, and (NH),Aga by saturat described in U. S. Pat. No. 3,519,404; electric cells 5 ing a warm concentrated solution of the alkali metal containing these electrolyte materials are described in iodide with silver iodide. They found that one mole of U. S. Pat. No. 3,443,997. KI, RbI, or NHI would react with only half a mole of Compositions of the foregoing type, MAgIs, as well AgI, thereby forming the nonconductive compounds as certain organic ammonium silver iodide composi shown. tions have recently been shown to have ionic conduc 20 It has now been discovered that it is feasible to react tivity in the solid state over a wide range of tempera four molar equivalents of AgI for each molar tures. The known methods of preparing the ionically equivalent of MI present in a saturated aqueous solu conductive alkali metal silver iodides involve mixing tion of MI so that the reaction that occurs between the appropriate amounts of the solid reactants MI and AgI silver iodide and the alkali metal iodide results in for in the desired 1:4 molar ratios, and either heating the 25 mation of the desired ionically conductive alkali metal mixture until molten, followed by a holding period at a tetrasilver pentaiodide composition as the final product lower temperature to form the desired compound obtained, substantially free of the Brink et al com (melt-anneal technique), or reacting the solid reactants pound. Continuous stirring is preferably employed as in a time-consuming solid state reaction at 200°C or the entire calculated amount of silver iodide is higher to form the compound. In an alternative method 30 of preparation of MAgis, the constituents are dissolved gradually added as a fine powder to the saturated aque in acetone and precipitated by evaporation. However, ous alkali metal iodide solution. Initially, the resulting this method results in a mixture of the desired con mixture rapidly thickens, suggesting possible formation pound and another compound, generally a high re of the Brink et al compound as an intermediate. Then, sistivity compound having the formula MAgia(2 35 with continued addition of Agi, the mixture begins to MI:AgI) rather than the pure ionically conductive com thin out to a more fluid paste, suggesting that formation position. For preparing the organic silver ammonium of the desired compound has occurred. While not es iodide conductive compositions, a melt-anneal sential, it is preferable to facilitate the mixing and reac technique is generally used. A paste preparation tion by warming the solution. Further, by the addition technique has also been used in which a slurry or paste 40 of a slight excess of water or by raising the temperature, is prepared of the silver iodide and the organic am a clear amber-colored solution is obtained. After reac monium iodide, followed by a multiple annealing tion has occurred, as indicated by formation of the technique. Synthesis in an aqueous medium in which paste or of the clear solution, the water present is the silver iodide and organic ammonium iodide reac removed by evaporation at room temperature or by tants and the formed product exhibit only a limited 45 drying in an oven. The process results in the formation solubility has also been suggested as feasible. of MAgis and the relative absence of the nonconduc The present invention provides a simple and con tive M2Ags. venient method for the preparation of solid ionically conductive alkali metal silver iodides from selected DESCRIPTION OF THE PREFERRED solutions. The process of this invention is advantageous 50 EMBODIMENTS compared with the methods known to the prior art in that it is more easily adapted to large-scale production, In the practice of the process of the present invention does not require the use of a high temperature, and in its preferred aspects, a minimum amount of water is may be used to produce large crystals or thin films of utilized for dissolving the appropriate amount of the al the desired compound. 55 kali metal iodide, particularly rubidium iodide. While a The compositions prepared by the process of this in near-saturated solution may be used initially, depend vention have a high ionic conductivity and may be used ing in part on the reaction temperature, it is preferred in a variety of electrochemical devices. The process is to use a saturated solution preferably maintained at a of particular utility for preparing compositions for use temperature of not less than 40°C, generally between 60 40' and 100°C. The calculated amount of silver iodide in thin-film devices. is gradually added as a solid to the solution with con SUMMARY OF THE INVENTION stant stirring. Although an apparent initial reaction The solid ionically conductive compositions which seems to occur in that the mixture rapidly thickens, it may be prepared by the process of this invention have 65 has been found that by continued addition of the silver the general formula MAgis in which M is Rb, K, NH, iodide accompanied by stirring, this thickened solution Cs, or combinations thereof, Cs being present only as a thins out to a paste as formation of the desired com minor constituent of M. pound occurs. 3,719,746 3 4 While the process may be carried out at any suitable Following the general procedure shown in Example temperature, for example, from room temperature to 1, NH4AgIs is prepared by reacting 0.29 g. NHI dis the boiling point of the saturated alkali metal iodide solved in 0.3 g water with 1.88 g. Ag. solution, a preferred reaction temperature is above it will of course be realized that various modifica 40°C, generally between 40 and 100°C, and particu tions can be made in the design and operation of the larly between 40 and 70°C. After reaction occurs, the present invention without departing from the spirit paste solution or amber-colored solution is evaporated thereof. Thus, while the principle, preferred construc to dryness. The solid which remains is the ionically con tion and mode of operation of the invention have been ductive material MAgs substantially free from the explained and what is now considered to represent its nonconductive material MAga. The solid is generally O best embodiment has been illustrated and described, it further dried by heating it at a temperature of about should be understood that within the scope of the ap 50°-150°C for 0.5 to 5 hours, preferably between 50° pended claims the invention may be practiced other and 100°C. By suitable control of the conditions of wise than as specifically illustrated and described. precipitation and evaporation, thin films or crystals of I claim: the ionically conductive compositions may be 15 1. A process for preparing an ionically conductive composition of matter pf the general formula MAgs produced in a relatively simple manner, since removal where M is Rb, K, NH, Cs, or dombinations thereof, Cs of the water by evaporation leaves the compound be being present only in combination and as a minor con hind. stituent of M, comprising preparing a saturated aque The following examples serve to illustrate the inven 20 ous solution of MI, intimately admixing four molar tion, but are in no way considered limiting thereto: equivalents of AgI with said solution to effect reaction between the MI and Ag, and removing the water to EXAMPLE 1 recover MAgs as the synthesized ionically conductive A saturated solution of Rb1 in water is prepared by composition. dissolving 0.44 g. Rb in a minimal quantity of water at 25 2. The process according to claim 1 wherein M is room temperature. Silver iodide is gradually added to potassium. the solution, and a clear orange-tan liquid results. On 3. The process according to claim 1 wherein M is ru further addition of silver iodide, precipitation occurs. bidium. Continued addition of silver iodide results in the solu 4. The process according to claim 1 wherein M is am tion becoming clear. Continued further addition of 30 monium. silver iodide to a total weight of 1.90 g. results in forma 5. The process according to claim 1 wherein the tem tion of a precipitate. The solution containing the perature of the reaction is between about 40°C and the precipitate is warmed to a temperature above 40°C, boiling point of the solution. and a clear orange-tan solution is obtained. Upon 6. The process according to claim 1 wherein the satu evaporation of this solution, a white precipitate 35 rated solution of MI is prepared at room temperature, remains as residue. This is further dried on a hot plate. the silver iodide is gradually added to the saturated X-ray diffraction of the precipitate confirmed that it solution with constant stirring while raising the solution was RbAgis and that Rb,Agis was absent. Additional temperature to between about 40°C and the boiling confirmation of the formation of the desired compound point of the solution, the solution is cooled following was obtained by observing the known solid state trans 40 reaction to effect precipitation of MAgis, and the formation temperature of 207K using differential water is removed so that the so-prepared MAgis is scanning colorimetry. thereby recovered. 7. The process according to claim 6 wherein M i EXAMPLE 2 potassium. 45 8. The process according to claim 6 wherein M is ru Following the general procedure shown in Example bidium. 1, KAgs is prepared by reacting 0.33 g. KI dissolved in 9. The process according to claim 6 wherein M is am 0.3 g water with 1.88 g. AgI. monium. EXAMPLE 3 50

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