Carrier and microstructure tuning for improving the thermoelectric properties of Ag₈SnSe₆ via introducing SnBr₂

The argyrodite compounds ( ${\text{A}}_{(12-n)/m}^{m+}{\text{B}}^n{}^{+}{\text{X}}_6^{2-}$(A^m+ = Li⁺, Cu⁺, and Ag⁺; Bⁿ⁺ = Ga³⁺, Si⁴⁺, Ge⁴⁺, Sn⁴⁺, P⁵⁺, and As⁵⁺; and X^2-= S^2-, Se^2-, or Te^2-)) have attracted great attention as excellent thermoelectric (TE) materials due to their extremely low lattice thermal conductivity (κ_l). Among them, Ag₈SnSe₆-based TE materials have high potential for TE applications. However, the pristine Ag₈SnSe₆ materials have low carrier concentration (< 10¹⁷ cm^-3), resulting in low power factors. In this study, a hydrothermal method was used to synthesize Ag₈SnSe₆ with high purity, and the introduction of SnBr₂ into the pristine Ag₈SnSe₆ powders has been used to simultaneously increase the power factor and decrease the thermal conductivity (κ). On the one hand, a portion of the Br^- ions acted as electrons to increase the carrier concentration, increasing the power factor to a value of ~698 μW·m^-1·K^-2 at 736 K. On the other hand, some of the dislocations and nanoprecipitates (SnBr₂) were generated, resulting in a decrease of κ_l (~0.13 W·m^-1·K^-1) at 578 K. As a result, the zT value reaches ~1.42 at 735 K for the sample Ag₈Sn_1.03Se_5.94Br_0.06, nearly 30% enhancement in contrast with that of the pristine sample (~1.09). The strategy of synergistic manipulation of carrier concentration and microstructure by introducing halogen compounds could be applied to the argyrodite compounds to improve the TE properties.