Co-ionic conductivity in some silver solid electrolytes

SHORT

CO-IONIC SILVER

COMMUNICATION

CONDUCTIVITY IN SOME SOLID ELECTROLYTES M. LAZZARI

Centro Studio Processi

Elettrodici

de1 C.N.R.,

Polytechnic

of Milan, Italy

and Institute of Physical

Chemistry

B. %ROSATl and Electrochemistry,

(Received

1 February

(I)

showed* that only 0.35% and 0.70°/0 of silver by weight, respectively at 25” and 70°, can be replaced. But more sigmficantly it was observed that after the ‘saturation’ was reached, the behaviour of the material was typical of a copper electrolyte. In fact the cells which had been electrolysed below the saturation, showed at the cathodic interphase a silver deposit, while the cells which had been electrolysed above the saturation, showed at the cathode copper and silver dendrites. Moreover, the migration of copper ions through the material did not alter significantly the Ag/Cu ratio, as shown by the analyses of samples submitted to the circulation of different charges. Unfortunately the dendritic growth of the deposits did not allow to perform very long electrolyses. The observed, progressive increase in cell voltage (at constant current) as a function of time[2] could be attributed to the increased resistance at the cathode/electrolyte interphase, but partly also to an increased bulk resistance of the material when it changes from silver to copper conduction. The silver solid electrolytes tetramethylammonium iodide+ilver iodide (CH,),NI. 6AgI[3] and hexamethonium iodide-silver iodide (CH,),N-(CH2)6-N(CHj)sI,.

*by Tubandt experiments tion analyses.

1977)

12AgI[4], though not studied in detail, showed, with respect to the silver substitution by copper, a behaviour similar to RbAg+I,. Also cells of type I employing Ag,SBr as electrolyte showed[5J, after prolonged electrolyses, copper dendrites at the cathode. A behaviour strictly similar to that of RbAgJ, has been recently observed by Bazti and Schmidt[B] in alfa-Agl in which silver is partially substituted by copper. In this case the ‘mixed’ compound can be represented by the formula Ag,,,,&uO_+~~I at 255” and Ag,.,,,Cu,.,,,l at 320”. Copper deposition on the cathode was observed when the substitution was complete, and mixed silver and copper deposition when the substitution was below the maximum at the chosen temperature. The Cu+ transport number in the Cu substituted AgI is close to the unity and the electronic conductivity is negligible in the temperature range studied. These results therefore confirm the prediction by Farrington and Roth that the phenomenon of co-ionic conductivity is common, besides the beta-aluminas, to typical tridimensional solid electrolytes.

Recently Fatington and RothEll reported that sodiumbeta alumina in which more than 50% of the Na+ content has been replaced by Li+, behaves as a pure Li* conductor. The phenomenon, by which Li+ ions can move through the material without altering the Na/Li ratio, has been termed ‘co-ionic conductivity’. In the present communication further examples of this behaviour are quoted. In a research on the electrochemical substitution of silver in RbAg,& and related compounds by copper[Z], experiments performed on solid state cells of the type: CulRbAgJ,lAg

University of Rome, Italy

REFERENCES

1. G. C. Farrington and W. L. Roth, International Sym posium on Solid Ionic and Ionic-Electronic Conductors, l-3 Sept. Rome, Italy (1976). 2. B. Scrosati, G. Pistoia, M. Lauari, and L. PeraId Bicelti, J. appl. Electmchem 4, 201 (1974). 3. B. B. Owens, J. electrochem. Sac. 117, 1536 (1970). 4. M. L. Berardelli, C Biondi, M. De Rossi, G. Fonseca and M. Giomini, J. electrochem Sot. 119, 114 (1972). 5. M. Lazzari and B. Scrosati, unpublished results. 6. J. C. Badn and J. A. Schmidf J. appl. Electrochem. 6,411 (1976).

confirmed by atomic absorp-

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