Extraordinary thermoelectric performance of NaBaBi with degenerate and highly non-parabolic bands compared to LiBaSb and Bi2Te3

Abstract

Thermoelectric materials can generate electricity directly utilizing heat and thus, they are considered to be eco-friendly energy resources. The thermoelectric efficiency at low temperatures is impractically small, except for only a few materials (e.g., Bi₂Te₃ and its alloys). Here, I predict two new thermoelectric materials, NaBaBi and LiBaSb, the former with excellent transport properties compared to LiBaSb and Bi₂Te₃ at low temperature, by using the first-principles method. The twofold degenerate bands of NaBaBi, instead of the valley degeneracy (one-fold) of Bi₂Te₃, and relatively low DOS near the Fermi level induce high electrical conductivity, despite its bandgap being two times wider compared to Bi₂Te₃, and the highly non-parabolic bands generate larger thermopower, leading to an almost two times higher anisotropic power factor at 300 K compared with Bi₂Te₃. On the other hand, the similar phononic structure and anharmonicity of NaBaBi cause almost identical lattice thermal conductivity, but it is much higher in LiBaSb. Such electronic and phononic features make it a superior thermoelectric material, with a predicted cross-plane (in-plane) ZT ≈ 2 (≈1) at 300 K for both n- and p-type carriers, and the ZT is even higher (≈2.5 for p-type carriers) at 350 K. On the other hand, the isotropic maximum ZT of NaBaBi is ≈1.2 and 1.6 at 350 K for n- and p-type carriers, respectively. However, LiBaSb is less suitable for low-temperature TE applications, because of its wider bandgap and high lattice thermal conductivity.