Enhancement of Thermoelectric Performance in Mg ₃ (Sb, Bi) ₂ via Dual Doping of Cations and Anions

N‐type Mg 3 (Bi, Sb) 2 alloys exhibit great potential for waste‐heat harvesting applications due to their excellent thermoelectric performance, lightweight nature, and the abundance and low cost of their constituent elements. This work presents a cation‐anion (Lu‐Se) co‐doping strategy in Mg 3 (Bi, Sb) 2 materials, which effectively enhances their thermoelectric properties. The electrical conductivity is significantly increased by the dual doping effect, resulting in a high power factor of 27 µW cm −1 K −2 . Meanwhile, the lattice thermal conductivity is significantly reduced due to the enhanced phonon scattering caused by mass fluctuations and stress‐field fluctuations, resulting from the two types of atoms, as well as the reduced grain size. Ultimately, a high thermoelectric figure of merit ( zT ) of ≈1.06 is achieved in (Lu, Se)‐co‐doped Mg 3 Sb 1.5 Bi 0.5 at 498 K. Furthermore, a module composed of n‐type (Lu, Se)‐co‐doped Mg 3 Sb 1.5 Bi 0.5 and p‐type BiSbTe exhibits superior power generation performance, achieving a maximum efficiency of 8% and a maximum output power of 0.46 W at a temperature difference of 280 K, which is at the forefront level. This work provides an effective and low‐cost route to harnessing the potential applications of n‐type Mg 3 (Bi, Sb) 2 thermoelectric materials.