Ultralow Lattice Thermal Conductivity and High <i>ZT</i> of n-Type Polycrystalline SnSe Realized by Liquid Phase Sintering

SnSe has drawn increasing attention in thermoelectric applications because of its exceptional n/p-type characteristics. Although recent studies have reported an excellent figure of merit (<i>ZT</i>) value in p-type polycrystalline SnSe, achieving a breakthrough in thermoelectric performance for its n-type counterpart SnSe remains a critical challenge. The presence of V_Sn imposes a critical constraint on the synergistic optimization of carrier transport and phonon scattering in n-type SnSe. In this study, liquid phase sintering introduces high-density dislocations into n-type SnSe polycrystals, effectively scattering mid-frequency phonons. Huge lattice strain fluctuations caused by the defects enable an ultralow lattice thermal conductivity (0.21 W m^-1 K^-1) at 793 K. In addition, part of the liquid phase Sn tends to penetrate into the matrix, which leads to a higher carrier concentration and considerable enhancement in electrical properties. Consequently, a superior <i>ZT</i> (~1.9, 793 K) and an outstanding average <i>ZT</i> (<i>ZT</i> _ave) (~0.72, 300 to 873 K) are achieved in polycrystalline SnSe, which rank at the top level reported for SnSe-based n-type thermoelectric materials, exceeding those of most n-type thermoelectric systems for mid-temperature applications.