Electron Injection and Lattice Softening Lead to High Average ZT of n‐Type Mg ₃ (Sb, Bi) ₂ /Cr and Efficient Full‐Zintl Thermoelectric Module

Abstract Maximizing the average figure‐of‐merit ( ZT _avg ) of thermoelectric (TE) materials is crucial for optimizing the module performance. Herein, this work enhances the ZT _avg to 1.42 (298–673 K) in a n‐type Mg ₃ (Sb, Bi) ₂ ‐based material by a stepwise optimization strategy. Specifically, Cr injects electron into Mg ₃ Sb _0.8 Bi _1.19 Te _0.01 , which synergistically boosts carrier concentration and forms an electron accumulation layer at the interface. Meanwhile, lattice softening and interfacial scattering suppress lattice thermal conductivity. A peak ZT of 1.72 at 673 K and ZT _avg of 1.30 over 298–673 K are achieved for Mg ₃ Sb _0.8 Bi _1.19 Te _0.01 –1.5 wt.% Cr. Then, through grain refinement and Se doping instead of Te, a delicate equipoise is reached between the power factor and the electronic thermal conductivity. Eventually, the Mg ₃ Sb _0.8 Bi _1.19 Se _0.01 –1.5 wt.% Cr sample obtains an outstanding ZT of ≈1.9 at 573 K and ZT _avg of 1.42 over 298–673 K. The integrated two‐pair full‐Zintl YbZn ₂ Sb ₂ /Mg ₃ (Sb, Bi) ₂ module achieves a high conversion efficiency of 10.5% and power density of 0.37 W cm ⁻² simultaneously under a temperature difference of 370 K. More importantly, this module exhibits excellent thermal stability during a 10‐day in situ test. This work provides new ideas for applications of full‐Zintl modules to the recovery of waste heat.