Reinforced bond covalency and multiscale hierarchical architecture to high performance eco-friendly MnTe-based thermoelectric materials

Highlights: • The simultaneous improvement of carrier concentration and mobility. • The bipolar effects were effectively suppressed by the reinforced bond covalency. • A great reduction in k_lat has been obtained by the multiscale hierarchical architecture. • A record-high peak zT ~ 1.3 has been achieved in the Li/S co-doped sample. -- Abstract: Pb-free MnTe has recently been discovered to be a promising thermoelectric material because of its low toxicity and eco-friendly nature. Here, we have proposed and demonstrated an effective approach to boost the electrical transport of MnTe compound via reinforcing bond covalency through M/S (alkaline dopants M = Li, Na, and K) co-doping. By means of this strategy, the electrical conductivity was significantly improved owing to the increasing carrier concentration and mobility, which is attributed to the decreasing electronegativity difference |χ_Te− χ_M| as M going from K to Na to Li. The single Kane band model enables a reliable assessment of their temperature-dependent electrical properties, further suggesting that the bipolar effects at high temperature can be effectively suppressed by reinforcing bond covalency. Moreover, beneficial from alkali doping and sulfur substitution, the lattice thermal conductivities have been sharply reduced to amorphous limit through intensive phonon scattering induced by the multiscale hierarchical architecture such as the nanostructures, coherent grain boundary and high-density dislocations, etc. As a result, a record-high peak zT of ~1.3 @ 873 K, corresponding to a calculated engineering output power density ~1.46 Wcm² and leg efficiency η ~8.4%, has been achieved in the Li/S co-doped (Mn_1.04Li_0.02Te_0.99S_0.01) sample. This work provides a referential route to enhance electrical properties via synergistically improving carrier concentration and mobility by reinforcing bond covalency, impelling the potential applications of MnTe-based thermoelectric materials as a robust candidate for waste heat recovery.