Ionic Seebeck coefficient and figure of merit in ionic thermoelectric materials

Ionic thermoelectric (i-TE) materials have attracted much attention due to their large ionic Seebeck coefficients, but the basic relations between the property parameters and operation performances are still open questions. We derive the ionic Seebeck coefficient, Std, and the dimensionless ionic figure of merit, ZtdT, from the view of field evolutions of temperature, voltage, and ionic concentration. Different from traditional knowledge, only ionic Eastman entropies determine Std, while ionic diffusion coefficients can only affect the speed of the voltage to reach the steady state. The figure of merit for electronic thermoelectric materials is inapplicable for i-TE materials as the voltage build-up is dominated by the temperature field evolution instead of the capacitor constant. By combining the net charge storage and dimensionless response time, the proposed ZtdT can embody the transient and average efficiencies in i-TE materials, which also opens new windows for the optimization of i-TE materials.