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 Mg3.05(Sb0.75Bi0.25)1.99Te0.01(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 ≈1.1 within 300–723 K. An eight‐pair Mg3Sb2‐GeTe‐based thermoelectric device is also fabricated, achieving an energy conversion efficiency of ≈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.