Inhibiting Mg Diffusion and Evaporation by Forming Mg-Rich Reservoir at Grain Boundaries Improves the Thermal Stability of N-Type Mg₃Sb₂ Thermoelectrics

N-type Mg3Sb2-based thermoelectric materials show great promise in power generation due to their mechanical robustness, low cost of Mg, and high figure of merit (ZT) over a wide range of temperatures. However, their poor thermal stability hinders their practical applications. Here, MgB2 is introduced to improve the thermal stability of n-type Mg3Sb2. Enabled by MgB2 decomposition, extra Mg can be released into the matrix for Mg compensation thermodynamically, and secondary phases of Mg-B compounds can kinetically prevent Mg diffusion along grain boundaries. These synergetic effects inhibit the formation of Mg vacancies at elevated temperatures, thereby enhancing the thermal stability of n-type Mg3Sb2. Consequently, the Mg-3.05(Sb0.75Bi0.25)(1.99)Te-0.01(MgB2) (0.03) sample exhibits negligible variation in thermoelectric performance during the 120-hour continuous measurement at 673 K. Moreover, the ZT of n-type Mg3Sb2 can be maintained by adding MgB2, reaching a high average ZT of approximate to 1.1 within 300-723 K. An eight-pair Mg3Sb2-GeTe-based thermoelectric device is also fabricated, achieving an energy conversion efficiency of approximate to 5.7% at a temperature difference of 438 K with good thermal stability. This work paves a new way to enhance the long-term thermal stability of n-type Mg3Sb2-based alloys and other thermoelectrics for practical applications.