Atomic Layer Deposited Cu Incorporation Enables Enhanced Thermoelectric Performance in n‐Type Mg 3 (Sb, Bi) 2

Thermoelectric (TE) materials with high efficiency near room temperature are crucial for low‐temperature waste‐heat recovery and solid‐state cooling applications. Among them, Mg 3 (Sb, Bi) 2 ‐based Zintl compounds have emerged as leading n‐type materials due to their intrinsically low lattice thermal conductivity and favorable electronic structure. However, their performance is still constrained by the relatively low carrier mobility in the low‐temperature regime because of the dominated grain‐boundary charge scattering, which limits the achievable power factor and thus the overall figure of merit ( zT ). Hereby, we proposed an interface modification strategy based on powder atomic layer deposition (pALD) of metallic Cu to enhance the carrier mobility of n‐type Mg 3 (Sb, Bi) 2 . The ALD‐Cu addition is found to be beneficial for improving carrier mobility by promoting grain growth, compensating Mg deficiencies at some of the grain boundaries (particle boundaries), and thereby mitigating the interfacial transport barriers. Metallic Cu was precisely deposited on TE powders without introducing oxygen‐ or water‐based precursors, thereby avoiding surface oxidation and degradation of the powders. Simultaneously, despite the more than twofold increase in grain size, the formation of Cu‐rich domains near particle boundaries acts as effective phonon‐scattering centers, leading to a lower lattice thermal conductivity. Benefiting from this synergistic modification of electrical and thermal transport, both peak and average zT over 303–573 K were enhanced by 14% and 13.3%, respectively. This work demonstrates a feasible metallic pALD approach for interface modification and establishes a new strategy to decouple transport parameters in n‐type Mg 3 (Sb, Bi) 2 ‐based alloys for near room temperature applications.