In Situ Engineering of Grain Boundary Phase toward Superior Thermoelectric Performance in Mg₃(Sb,Bi)₂

As a promising thermoelectric material for electronic cooling and power generation, Mg₃(Sb,Bi)₂ has received extensive attention. Despite efforts to enhance its performance through composite modulation, challenges such as secondary phase refinement, dispersion, and interfacial mismatch, particularly at grain boundaries, remain critical. In this work, by incorporating TiO_2-n into the Mg₃(Sb,Bi)₂-based matrix, the grain boundary phases are in situ engineered, yielding a superior figure of merit (zT) exceeding 2 at 798 K. The electrical conductivity is significantly enhanced with only slight changes to the Seebeck coefficient over the entire temperature range, mainly due to the contribution to carrier concentration and mobility from the newly generated Ti₃Sb at grain boundaries. Benefiting from the remarkably enhanced power factor and the diminished lattice thermal conductivity, the zT value shows an overall increase within the temperature range of 300–798 K, leading to a considerable conversion efficiency of 15% for the single-leg device.