Transient Liquid Induced Hierarchical Structure Contributes to High Thermoelectric Performance in Ag₂Se

Owing to the intrinsic high thermoelectric performance, Ag₂Se is a promising alternative for traditional near-room temperature Bi₂Te₃-based materials. In this study, a Na₂SO₃ solution has been used as the transient liquid during the modified cold sintering process to induce a hierarchical structure, including micropores, nanopores, sub-nanopores, and additional nanoscale Na₂SO₃ residuals. Such a hierarchical structure contributes to an ultralow lattice thermal conductivity of 0.18 W m^-1 K^-1 at ≈300 K in the Ag₂Se-30%Na₂SO₃. Additionally, extra Se vacancies further optimize the carrier concentration to ≈5.6 × 10¹⁸ cm^-3, leading to a high power factor of ≈25 µW cm^-1 K^-2 at ≈300 K in the Ag₂Se-30%NS. Consequently, due to the synergistic effects of high power factor and low lattice thermal conductivity, an ultrahigh room-temperature figure of merit of 1.04 in the Ag₂Se-30%Na₂SO₃. The study demonstrates that introducing transient liquid solutions in the modified cold sintering process can effectively achieve specific structural engineering and high thermoelectric performance.