Optimizing the thermoelectric transport properties of fast ionic conductor β -Ag2S under high pressure and high temperature

Inorganic fast ionic thermoelectric (TE) materials (IFITEMs) exhibit excellent TE capabilities due to the special carrier of delocalized ions. Optimization of the TE performance of an IFITEM, however, is limited by a conflict between its electrical conductivity (σ) and its Seebeck coefficient (S). It remains challenging to regulate σ and S in IFITEMs because they are mainly only stable under high temperature. In this work, σ and S of α-Ag2S (semiconductor) and β-Ag2S (fast ionic conductor) are modulated by the in situ measurement under high pressure and high temperature. It uncovered that pressure increases the electrical conductivity with improving the carrier concentration in α-Ag2S, but increased pressure hinders ion transfer and thus reduces conductivity in β-Ag2S. These results show that the pressure responses of σ and S in α-Ag2S and β-Ag2S are distinctly opposite. Nevertheless, pressure can optimize the power factor (PF) and estimated thermoelectric figure of merit (ZT) in both α-Ag2S and β-Ag2S, with optimum values of 1.97 × 10−4 W/m K2 and 0.122 (3.3 GPa, 447 K), and 2.93 × 10−4 W/m K2 and 0.18 (2.2 GPa, 574 K), respectively. The pressure effect has improved about 4.5 and 3.6 times in PF and ZT of β-Ag2S comparing with α-Ag2S at 0.8 GPa 436 K. This work provides a way to optimize TE performance in fast ionic conductors by altering the pressure, which will help in the production of high-powered TE materials.