Ionic thermoelectric materials can generate extraordinarily high thermal voltage under small temperature differences due to their orders of magnitude larger Seebeck coefficient than that of electronic materials. Together with their low-cost, environmentally friendly compositions and solution processability, electrolytes have brought renewed prosperity in thermoelectric fields. Despite the rapid growing number of good-performance materials, yet to be implemented in devices, the main challenge is the understanding of the mechanism of the large Seebeck coefficient in practical electrolytes. Here, we show that the ion/polymer interaction in PEG based electrolytes does not only affect the mobility of the ions, but also has a great impact on the Seebeck coefficient. By delicately varying the types of solvent and the concentration of the solute, we could tune the molar conductivity of the electrolytes and correlate with the Seebeck coefficient. The linear relation between the Seebeck coefficient and the logarithm of the molar conductivity is in agreement with the recently reported thermoelectric ratchet effect in ions with hopping dynamics. This could lead to new design rules for ionic thermoelectrics.Read less <