Synergistic Grain Boundary and Phonon Engineering Enables Ultrahigh Thermoelectric Performance in Mg₃(Sb, Bi)₂-Based...

Thermoelectric Mg₃(Sb, Bi)₂ has emerged as a rising star in virtue of its promising thermoelectric performance and abundant constituent elements. However, its practical application is hindered by challenges in synthesis and the strong coupling between electrical and thermal transport properties. Herein, we propose a dual-pronged strategy that combines grain boundary engineering and phonon engineering to decouple electron and phonon transport in Mg₃(Sb, Bi)₂. By employing a tantalum-tube encapsulated melting technique together with Cu doping, the grain size is markedly increased to ∼25 µm, effectively suppressing carrier grain boundary scattering and enabling single-crystal-like electrical transport with a high carrier mobility exceeding 200 cm² V^-1 s^-1. Meanwhile, atomic-scale alloy disorder and nanoscale Mg₂Cu precipitates introduce strong phonon scattering, leading to an ultralow lattice thermal conductivity of 0.36 W m^-1 K^-1. Eventually, a peak zT of 1.9 at 750 K and a high conversion efficiency of 10.6% at ΔT = 488 K are achieved, demonstrating the competitive performance of these materials and devices. This study not only provides new insights into the manipulation of grain boundary scattering, but also demonstrates the great promise of Mg₃(Sb, Bi)₂ for waste-heat harvesting.