Investigations on electrical and electrochemical properties of Ag+ ion conducting quaternary solid electrolyte systems: x[0.75 AgI : 0.25 AgCl] : (1−x)RbI

Abstract

Investigations on ion transport behaviour of new Ag+ ion conducting quaternary solid electrolyte systems: x[0.75AgI : 0.25AgCl] : (1 − x)RbI, where 0.1 ≤ x ≤ 1 mol.wt fractions, are reported. The solid electrolyte systems were synthesized adopting different routes of solid solution reactions. An alternative host salt, ‘a quenched [0.75AgI : 0.25AgCl] mixed system solid solution’, discovered at the present laboratory, has been used in place of AgI. The compositional dependent conductivity studies at room temperature revealed the quaternary systems 0.7[0.75AgI : 0.25AgCl] : 0.3RbI as the optimum conducting composition (OCC). AgI has been traditionally and most widely used as host salt in the past to prepare the majority of well-known fast Ag+ ion conductors including the ternary superionic solids MAg4I5 (M = Rb, K, NH4). Also, RbAg4I5 has been recognized as one of the highest conducting superionic solids with conductivity σrt ∼ 10−1 S cm−1 at room temperature. However, it has been observed that RbAg4I5 is thermodynamically unstable, particularly in the open ambient conditions, while the newly synthesized quaternary solid electrolyte, having OCC 0.7[0.75AgI : 0.25AgCl] : 0.3RbI, has been observed to be much more stable than RbAg4I5. For direct comparison of the ion conduction behaviour of the newly synthesized OCC, the ternary superionic solid RbAg4I5 (0.8AgI : 0.2RbI) has also been synthesized in an identical manner. The room temperature conductivity (σrt) of the newly synthesized quaternary OCC was slightly lower than that of RbAg4I5. However, σrt remained practically stable for a fairly long time in the open ambient conditions as compared with that of RbAg4I5. The mechanism of ion transport in OCC has been characterized on the basis of experimental studies on various ionic parameters namely conductivity (σ), ionic mobility (μ), mobile ion concentration (n), ionic drift velocity (vd), ionic transference number (tion) etc. The temperature dependent studies on these ionic parameters provided information regarding various energies involved in the respective thermally activated processes. Solid state batteries have been fabricated using OCC as electrolyte in the cell configuration Ag-metal (anode)/0.7[0.75AgI : 0.25AgCl] : 0.3RbI (solid electrolyte)/C+I2 (cathode) and the cell potential discharge characteristics have been studied under various load conditions.