Grain boundary complexions stabilize Mg3Sb2-based thermoelectric devices with superior performance

The practical application of Mg 3 Sb 2 -based thermoelectrics has been largely retarded by their poor thermal stability, mainly due to the rapid loss of Mg at elevated temperatures. Here, we prove that grain boundaries are fast diffusion channels for Mg, and we block these channels by forming Ga-rich grain boundary complexions. This design suppresses the formation of Mg vacancies at grain boundaries and inhibits the outward diffusion of Mg and the inward growth of MgO-related phases. Consequently, the thermal stability of Mg 3 Sb 2 -based materials is significantly improved at a high temperature of 718 K for at least 80 h. The corresponding single-leg Mg 3 Sb 2 -based device can maintain a conversion efficiency of 12.5% ± 0.6% for 7 days at a temperature difference of 423 K during the cycle test. Our findings provide an atomic-scale grain boundary engineering approach to enhance the thermal stability of thermoelectric devices and other functional materials operating at elevated temperatures.