Local ionic enthalpy mismatch as the origin of high thermoelectric performance in superionic crystals

Superionic crystal possesses a crystalline sublattice and diffusive ions, which is therefore assumed to be a thermal insulator while an electrical conductor, i.e., an ideal thermoelectric. Indeed, some of them, such as Cu2⁢Se, show excellent thermoelectric performance, while some superionic crystals, such as Ag2⁢Te, only possess a modest thermoelectric performance. In this work, we demonstrate that the thermoelectric performance of superionic crystals is significantly affected by the Soret effect, which originates from the local enthalpy mismatch between cations and anions. Our results show that the negative Soret effect transport coefficient, characterized by antialigned heat and enthalpy currents as shown in Cu2⁢Se, hinders the energy transfer channels of both conduction and mass diffusion, while the positive Soret effect transport coefficient with aligned heat and enthalpy currents as observed in Ag2⁢Te, promotes both these energy transfer channels. Meanwhile, the Soret effect in superionic crystals generates the ionic Seebeck effect. The ionic Seebeck coefficient stemming from the Soret effect can be considerable. The thermoelectric performance of a material is inversely (directly) proportional to the energy transfer coefficient (the Seebeck coefficient). Therefore, while superionic crystals possess the same crystalline structures, their thermoelectric performance may vary significantly, e.g., the figure of merit of Cu2⁢Se and Ag2⁢Te is ∼3.0 and ∼0.2 at 900 K, respectively.