Local reaction-global diffusion unlocks high-performance Mg₃(Sb,Bi)₂-based thermoelectrics

Mg₃(Sb,Bi)₂-based thermoelectrics (TEs) show promise for near-room-temperature energy conversion and TE-cooling applications. However, further improvements in electrical power factors and figure-of-merits (zTs) are constrained by precise Mg-vacancy regulation and elucidation of underlying mechanisms. Herein, we report a novel in-situ Mg-vacancy engineering strategy in Mg₃(Sb,Bi)₂ where excess Mg is generated from local reactions between a selection of specific transition metals and the component anionic element(s) in Mg₃(Sb,Bi)₂ during spark-plasma-sintering. This process effectively refills matrix Mg-vacancies through the subsequent global diffusion of Mg cations in Mg₃(Sb,Bi)₂ lattices. This local-reaction-global-diffusion concept, contrasting with reported mechanisms associated with localized grain-boundary engineering, is elaborated through multiscale investigation. Vacancy-restrained Mg₃(Sb,Bi)₂ demonstrates remarkably enhanced carrier mobility and zTs, achieving record-high power factors. Our fabricated Mg₃Sb_0.5Bi_1.5/MgAgSb and Mg₃SbBi/MgAgSb modules achieve record-high dual-output performance with power-density/efficiency values of 1.23 W cm^-2/11.7% and 1.05 W cm^-2/12.8%, respectively, under a temperature difference (ΔT) of 315 K. The constructed Mg₃Sb_0.5Bi_1.5/Bi_0.5Sb_1.5Te₃ and Mg₃Sb_0.75Bi_1.25/Bi_0.5Sb_1.5Te₃ Peltier modules deliver competitive cooling ΔT_max exceeding 70 and 67 K, respectively, at 303 K. The concept is expected to extend to the defect engineering of other energy materials (e.g., SnTe and PbSe TEs), TE-interface materials, and metal-semiconductor interfaces with optimized functionalities.