Temperature effect on redox voltage in Li_xCo[Fe(CN)₆]_y

The electrochemical thermoelectric (TE) coefficient (SEC≡∂V∂T; V and T are the redox potential and temperature, respectively) is a significant material parameter, because it enable us to convert heat into electricity. Here, we systematically investigated the TE properties of cobalt hexacyanoferrate (Co-HCF), Li_xCo[Fe(CN)₆]_y, against the Li concentration (x). |S_EC| is higher than the Seebeck coefficient (= 0.2 mV/K at room temperature) of Bi₂Te₃ and distributes from 0.2 to 0.8 mV/K. We further observed a sign reversal behavior of S_EC: S_EC is negative at y =0.71 while it is negative (positive) at x≤0.3 (x≥0.6) at y =0.90. Based on the ionic model, we qualitatively reproduced the sign reversal behavior by including the volume expansion effect. These arguments suggest that S_EC in solid is mainly governed by the electrostatic energy.