Ternary Solid Ionically Conductive Compositions and Method of Preparing the Same

The invention relates to solid ionic conductors and refers more particularly to ternary solid conductors in the system MAg(CN).sub.2 --MI--AgI and to a method of preparing them.

Recent work in the field of solid ionic conductors has produced materials having conductivities hig enough to make them useful in a number of solid state electrochemical devices including batteries, coulometers, and computer elements. Argue and Owens in U.S. Pat. No. 3,443,997 disclose one such composition, the binary material represented as MAg.sub.4 I.sub.5, M being either potassium, rubidium, ammonium (NH.sub.4) or cesium or mixtures thereof. Mellors in U.S. Pat. No. 3,582,291 and U.S. Pat. No. 2,723,185 discloses a composition represented as MCN--4AgI and xAgI--yMCN--zAgCN,; wherein M is potassium, rubidium or cesium or mixtures thereof, x ranges from 0.45 to 0.95 mole, y is 1 minus x mole and the ratio of y/z ranges from infinity to 1, or wherein x ranges from 0.45 to 0.95 mole, z is 1 minus x mole and the ratio of y/z ranges from 1 to 1/9. All of the above patents disclose compositions having good conductivities. However, the materials in the MAg.sub.4 I.sub.5 group have a tendency to thermodynamic instability at temperatures below about 20.degree. to 30.degree.C. The materials in the MCN--4AgI group and xAgI--yMCN--zAgCN group, while of greater stability, are prepared at quite high temperatures, of the order of 400.degree. to 500.degree.C., as disclosed in the Mellors patents. Heating at such temperatures is not only expensive but even more important may lead to decomposition of some constituents.

The present invention has for its principal object the provision of a stable, ionically conducting solid which can be prepared at relatively low temperatures, i.e., below about 250.degree.C. Another important object of the invention is a method of preparing at low temperatures solid compositions having ionic conductivities greater than 1 .times. 10.sup.-.sup.2 ohm.sup.-.sup.1 cm.sup.-.sup.1 at room temperature and which compositions are thermodynamically stable.

The invention, by means of which these objects are attained, comprises ternary compositions in the system represented by the notation MAg(CN).sub.2 --AgI--MI, wherein M is potassium, rubidium or mixtures thereof. The invention also comprises a method for producing such compositions which is based upon the discovery of low melting eutectic compositions in the system which permit the preparation of a number of specific compositions by heating the constituents together at a relatively low temperature, that is, below about 250.degree.C. The melting point of the eutectic compositions is below the decomposition point of desirable conducting compositions and, therefore, such compositions are of great practical value for use in battery applications or the like.

The compositions of the invention are more specifically defined as being ternary, containing 20% to 95% (molar percentages) silver iodide (AgI), the remainder being MI and MAg(CN).sub.2, M being potassium or rubidium or mixtures of these alkali metals and the molar proportion of MI to MAg(CN).sub.2 being greater than 1.5 to 1.0 and less than 7 to 1. A preferred ratio of MI to MAg(CN).sub.2 is 3 to 1. Compositions containing less than 20% silver iodide exhibit very poor conductivity, and compositions containing more than 95% silver iodide also exhibit poor conductivity. Compositions having a molar ratio of less than 1 to 1, as described in the Mellors patents, are prepared at quite high temperatures of the order of 400.degree.C. to 500.degree.C., and thus may lead to decomposition of some constituents. A specific composition, in the potassium system, which has excellent properties is represented by the molar formula KAg(CN).sub.2 --3KI--13AgI. In the rubidium system, a specific composition is RbAg(CN).sub.2 --3RbI--11AgI.

The ternary eutectic in the system KAg(CN).sub.2 --KI--AgI has the composition KAg(CN).sub.2 --3KI--5AgI and has a melting point of 200.degree.C. .+-. 2.degree.. In the rubidium system the ternary eutectic has the composition RbAg(CN).sub.2 --5.4RbI--9.6AgI and melts at 176.degree.C. .+-. 2.degree.. Because of the existence of these eutectics it is possible to produce the compositions of the invention by heating at temperatures not much above the eutectic melting points. This markedly lessens the possibility of decomposition of the materials present in the compositions and of course also provides the general advantages of low temperature heating against high temperature heating.

The composition are prepared by heating their constituents to a temperature below about 250.degree.C. Heating is continued under an inert atmosphere until all reactions are substantially complete. Two routes may be followed, the first involving the preparation of the low melting (below about 215.degree.C.) composition MAg(CN).sub.2 -- 3MI--5AgI and the addition to it of silver iodide followed by further heating, or simply by mixing the constituents of the composition in the desired molar proportions and heating until reaction is substantially complete. The compositions may be made from mixtures of MAg(CN).sub.2, MI and AgI or equivalent mixtures of MI, AgCN and AgI.

A process for preparing solid ionically conductive compositions in accordance with this invention comprises preparing a finely divided mixture in molar proportions of the composition of selected iodide and cyanide; heating the mixture at a temperature below about 250.degree.C. under inert atmosphere to produce at least a partially liquified mass; cooling the mass so produced to solidify the same, finely dividing the cooled mass; and then reheating the finely divided material to a temperature below about 215.degree.C. under inert atmosphere until all reactions are complete.

It is also possible to start with an eutectic composition, or some other composition of the MAg(CN).sub.2 --MI--AgI system, whereupon after the initial heat treatment step followed by cooling and finely dividing the solidified mass is completed, additional silver iodide may be added thereto prior to the reheating step so as to produce a composition having desired properties.

The sole drawing shows a ternary phase diagram of the system MI--AgI--MAg(CN).sub.2 wherein the trapezoidal area a-b-c-d defines the boundary of the compositions of the present invention. Line e-f on the ternary diagram defines the preferred compositions of the MI-AgI-MAg(CN).sub.2 system where the molar proportion of MI to MAg(CN).sub.2 is 3 to 1. The ternary eutectic in the system KAg(CN.sub.2)-- KI--AgI has the composition KAg(CN).sub.2 --3KI--5AgI and is designated on the ternary diagram as point g. The ternary eutectic in the system RbAg(CN).sub. 2 -RbI-AgI has the composition RbAg(CN).sub.2 --5.4RbI-9.6AgI and is designated on the ternary diagram as point h. The remaining points on the ternary diagram designate specific compositions in the MAg(CN).sub.2 --MI-AgI system which were prepared as illustrated in the following examples.

A finely divided mixture of 1.78 parts by weight of KI, 0.72 part AgCN and 2.51 parts AgI was prepared. It was heated to melting in the range of 205.degree. to 210.degree.C. for two hours. The melt was cooled and reground with an additional 5.02 parts by weight of AgI. This mixture, which yielded a molar proportion of KI to KAg(CN).sub.2 of 3 to 1, was then heated at 205.degree. to 210.degree.C. for 16 hours. The resultant material was cooled. The characteristic yellow color of AgI had substantially disappeared, indicating that there was little excess AgI. The specific conductance of the product was 1.9 .times. 10.sup.-.sup.1 ohm.sup.-.sup.1 cm.sup.-.sup.1 and is designated as point i on the ternary diagram.

A finely divided mixture containing in parts by weight 2.00 parts KI, 0.81 part AgCN and 8.48 parts AgI was prepared. This mixture yielded a molar proportion of KI to KAg(CN).sub.2 of 3 to 1. Heating at 200.degree.C. for 16 hours under inert atmosphere produced an incomplete reaction. The product was reground and heated at 236.degree.C. for 1/2 hour, reground and heated at 205.degree. to 210.degree.C. for 16 hours. The product had a specific conductance of 1.9 .times. 10.sup.-.sup.1 ohm.sup.-.sup.1 cm.sup.-.sup.1 and is designated as point i on the ternary diagram.

A mixture was prepared containing finely divided KAg(CN).sub.2, KI and AgI in the following molar proportions: 1 part KAg(CN).sub.2 ; 3 parts KI; 13 parts AgI. This mixture yielded a molar proportion of KI to KAg(CN).sub.2 of 3 to 1. The mixture was heated at 235.degree. to 250.degree.C. for 1 hour, cooled and ground. The finely divided mixture was then heated at 200.degree.C. for 16 hours. The product had a specific conductance of 1.8 .times. 10.sup.-.sup.1 ohm.sup.-.sup.1 cm.sup.-.sup.1 and is designated as point i on the ternary diagram.

From the above three examples we see that different routes may be followed to prepare a composition according to this invention.

A mixture containing in finely divided from 2.00 parts by weight RbI, 0.63 part AgCN and 6.63 parts AgI was prepared. This mixture yielded a molar proportion of RbI to RbAg(CN).sub.2 of 3 to 1. It was heated to 200.degree.C. for 16 hours. The product had a specific conductance of 1.9 .times. 10.sup.-.sup.1 ohm.sup.-.sup.1 cm.sup.-.sup.1 and is designated as point i on the ternary diagram.

A finely divided mixture of 6.34 parts by weight of RbI, 2.00 parts AgCN, and 7.02 parts AgI, was heated in inert atmosphere at 197.degree.-200.degree.C. for two hours yielding a generally liquid mixture. The latter was cooled and ground together with an additional 10.52 parts by weight of AgI which yielded a mixture having a molar proportion of RbI to RbAg(CN).sub.2 of 3 to 1. The resulting mixture was reheated at 200.degree.C. for 16 hours. The product had a specific conductance of 2.0 .times. 10.sup.-.sup.1 ohm.sup.-.sup.1 cm.sup.-.sup.1 and is designated as point j in the ternary diagram.

A finely divided mixture of 2.00 parts by weight of RbI, 0.42 part AgCN and 6.63 parts AgI, having a molar proportion of RbI to RbAg(CN).sub.2 of 5 to 1, was heated together in inert atmosphere at 220.degree.C. until the mix was generally molten. The melt was cooled, reground, and heated 16 hours at 204.degree.C. The specific conductance of the resulting material was 2.2 .times. 10.sup.-.sup.1 ohm.sup.-.sup.1 cm.sup.-.sup.1 and is designated as point 1 on the ternary diagram.

A finely divided mixture was prepared in the following molar proportions: 6.4 parts RbI, 2 parts AgCN, and 8.6 parts AgI. The mixture was heated in inert atmosphere at 220.degree.C. until the mix was generally molten. The mix was cooled, ground together with an additional 10.7 parts AgI and heated 16 hours at 204.degree.C. This mix yielded a molar proportion of RbI to RbAg(CN).sub.2 of 5.4 to 1. The product produced had a specific conductance of 2.2 .times. 10.sup.-.sup.1 ohm.sup.-.sup.1 cm.sup.-.sup.1 and is designated as point k on the ternary diagram.