We synthesized a solid I-/I-3(-)-based redox-couple electrolyte using a mixture of succinonitrile, LiI and I-2. An increase of LiI to succinonitrile mole ratio (x) up to 2.5% led to a fast rise in the electrical conductivity (025 degrees C) of the electrolytes. Further increase in x slowly increased the sigma a25 degrees c-value. The composition with x = 5% achieved the highest sigma 25 degrees c-value (similar to 1.6 x 10(-3)S cm(-1)) and was referred to as the optimum conducting composition (OCC) of the electrolytes. Also, an increase of x led to a decrease of the melting temperature of the electrolytes as suggested by the log sigma-T-1 study and ascertained by the differential scanning calorimetry study. The log sigma-T-1 study showed a linear trend before and after the melting temperature, and the activation energy increased with increasing x-value. The electrical and thermal properties were explained using the vibrational spectroscopy study. The ionic transference number measurement showed that the electrolytes with x = 4 and 5% are predominantly ionic in nature.