A thermocell consisting of choline iodide/triiodide solid electrolyte is developed. Seebeck coefficient of the cell is -0.87 mV K-1 at ambient temperature, which sign is opposite to that of the aqueous I-1/I-3(-) cell. The ionic conductivity of I-3(-) doped choline iodide is higher than that of pure choline iodide by two to four orders of magnitude, reflecting high conductivity of I-3(1) in the solid electrolyte. The selective ionic conduction observed in the solid electrolytes provides a novel design principle for the thermoelectric conversion materials.