Synergistic Suppression of Bipolar Effect and Lattice Thermal Conductivity Leading to High Average Figure of Merit in Bi_0.4Sb_1.6Te₃ Materials through Alloying with AgSbTe₂

Bismuth telluride-based materials have been widely used in commercial thermoelectric applications due to their excellent thermoelectric performance in the near-room-temperature range, yet further improvement of their thermoelectric properties is still necessary. Moreover, the narrow band gap of these materials results in a bipolar effect at elevated temperatures, which causes severe degradation of the thermoelectric performance. In this work, the commercial Bi_0.4Sb_1.6Te₃ was alloyed with AgSbTe₂ by using high-energy ball milling method combined with spark plasma sintering. It was found that ball milling can effectively reduce the lattice thermal conductivity of the samples. The alloying of AgSbTe₂ leads to a gradual increase in hole carrier concentration, resulting in an enhanced electrical conductivity and optimized power factor. Additionally, the bipolar effect is also weakened due to the increased hole carrier concentration. Furthermore, the substitution of Ag in the Bi/Sb sublattice causes further reduction in the lattice thermal conductivity. Ultimately, the sample alloyed with 0.15 wt % AgSbTe₂ demonstrates its best thermoelectric performance with a maximum <i>zT</i> of 1.35 at 393 K, showing a 20.5% improvement compared to the commercial sample. Besides, its average <i>zT</i> reaches a high value of 1.25 between 303 and 483 K, with a 27.6% improvement compared to that of the commercial sample.