Two-dimensional Li-based ternary chalcogenides LiMTe2 (M = Al, Ga, and In): Promising high-temperature thermoelectric materials

High-efficiency thermoelectric materials require two essential characteristics; low lattice thermal conductivity (κl) and high power factor (PF). Although group-III chalcogenides exhibit extremely high PF, their high κl hinders their practical applications. Through first-principles calculations, we have discovered that the LiMTe2 (M = Al, Ga, and In) monolayers, which have a similar structure stacking as the group-III chalcogenides, simultaneously possess the two key characteristics mentioned above. The κl values of the LiMTe2 monolayers are as low as 1.4−2.0 W m−1 K−1 at room temperature. Detailed analysis of bonding characteristics and phonon scattering rates suggests that the ultralow κl can be mainly attributed to the strong bond anharmonicity, which is a result of the cooperative endeavor of bond heterogeneity (coexistence of strong covalent bonds and weak ionic bonds) and the existence of lone-pair electrons. Additionally, the "pudding-mold" type of the conduction band near the Fermi level contributes to the high power factor of LiMTe2 monolayers. Therefore, the ZT values of n-type doped LiMTe2 monolayers exceed 1 at T = 800 K and carrier conentration of 1.71 × 1012–3.32 × 1012 cm−2, indicating the great potential of the 2D LiMTe2-family under n-type doping as high-temperature thermoelectric materials. The current work suggests that increasing bond complexity and the introduction of lone-pair electrons are strategic pathways for designing high-performance thermoelectric materials.