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

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 Mg₃(Sb, Bi)₂-based material by a stepwise optimization strategy. Specifically, Cr injects electron into Mg₃Sb_0.8Bi_1.19Te_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 Mg₃Sb_0.8Bi_1.19Te_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 Mg₃Sb_0.8Bi_1.19Se_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 YbZn₂Sb₂/Mg₃(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.