Na/Bi-co-doping and heterogeneous interfaces leading to enhanced thermoelectric performance of p-type Mg3Sb2-based Zintls

[Display omitted] •A high ZT value of 0.73 at 773 K is achieved in p-type Mg3Sb2.•Bi-doping leads to intensive Bi-rich nanoprecipitates with quazi-quantum sizes.•Ag2Ni3 is chosen to be the barrier layer with low contact resistivity.•An n-p uni-couple with all-Mg3Sb2 legs shows 4.1 % efficiency with a ΔT of 390 K. Owing to the non-toxicity and low cost, Mg3Sb2-based Zintls are promising candidates for mid-temperature thermoelectric applications. However, the thermoelectric performance of p-type Mg3Sb2 is significantly lower than that of n-type ones. Here, we achieve simultaneous optimization of the band structure and enhanced phonon scattering through the combined doping of Na at the cation site and Bi at the anion site, leading to a competitive figure of merit, ZT of 0.73 at 773 K and a maximum theoretical energy conversion efficiency of 6.5 % at a temperature difference of 450 K. First-principles calculations indicate that Na and Bi co-doping effectively narrows the band gap and shifts the Fermi level into the valence band, thereby increasing the hole carrier concentration. This ensures a relatively high Seebeck coefficient and improved electrical conductivity, resulting in an excellent power factor of 7.5 µW cm−1 K−2. Additionally, by adjusting the amount of Bi nanoprecipitates in the matrix, the heterogeneous interfaces significantly enhance phonon scattering and markedly reduce the lattice thermal conductivity (0.52 W m−1 K−1), collectively leading to the high thermoelectric performance. Furthermore, by pairing n-type Mg3.18Y0.02Sb1.5Bi0.49Se0.01, a double-leg thermoelectric device achieves an energy conversion efficiency of 4.1 % with a temperature difference of 390 K.