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Realizing high‐temperature thermal stability in thermoelectric (TE) generators is a critical challenge. In this study, a synergistic interface and surface optimization strategy is implemented to enhance Mg3Sb1.5Bi0.5 TE generator performance by employing FeCrTiMnMg thermoelectric interface materials and the MgMn‐based alloy protective coating. The competitive output power density (ω) of 1.7 W cm−2 and a conversion efficiency (η) of 13% for the single‐leg device are achieved at hot‐side temperature (Th) and cold‐side temperature (Tc) of 500 and 5 °C, respectively. An ω of 0.8 W cm−2 and η of 6% for the two‐couple TE devices with p‐type commercial Bi2Te3 are also realized, values that are competitive with the commercial Bi2Te3 device. Additionally, the single‐leg device shows a high stable η for over 100 h when the Th and Tc are 400 and 5 °C, respectively, with an change rate (Δηmax/ηmax,o) of <3%. In situ transmission electron microscopy analysis further reveals that the high stability results from the effectively sluggish interdiffusion and reduced Mg evaporation that decrease the chemical potential gradient, reduce the saturated vapor pressure, and increase the diffusion activation energy barrier. This study provides a general technique route for boosting the high‐temperature thermal stability of TE generator.

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