Se-doped Mg 3.2 Sb 1.5 Bi 0.5 -based thermoelectric materials are revisited in this study. An increased ZT value ≈1.4 at about 723 K is obtained in Mg 3.2 Sb 1.5 Bi 0.49 Se 0.01 with optimized carrier concentration ≈1.9 × 10 19 cm −3 . Based on this composition, Co and Mn are incorporated for the manipulation of the carrier scattering mechanism, which are beneficial to the dramatically enhanced electrical conductivity and power factor around room temperature range. Combined with the lowered lattice thermal conductivity due to the introduction of effective phonon scattering centers in Se&Mn-codoped sample, a highest room temperature ZT value ≈0.63 and a peak ZT value ≈1.70 at 623 K are achieved for Mg 3.15 Mn 0.05 Sb 1.5 Bi 0.49 Se 0.01 , leading to a high average ZT ≈1.33 from 323 to 673 K. In particular, a remarkable average ZT ≈1.18 between the temperature of 323 and 523 K is achieved, suggesting the competitive substitution for the commercialized n-type Bi 2 Te 3 -based thermoelectric materials.The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adfm.201906143. helps to promote the utilization of energy for the sustainable development. [1][2][3][4][5][6][7] The conversion efficiency of TE materials is determined by the dimensionless figureof-merit ZT = σS 2 T/κ, where σ is the electrical conductivity, S is the Seebeck coefficient, T is the absolute temperature, and κ is the thermal conductivity. High TE performance needs high electrical conductivity, high Seebeck coefficient, and low thermal conductivity. However, it is hard to independently optimize them because they are interconnected. [8][9][10][11][12][13][14] Zintl compounds have complex crystal structure desired for high TE performance. [15,16] The anions provide the "electron-crystal" structure through the covalent bonding network, and Zintl cations play the role of the "phonon scattering center" for extremely low lattice thermal conductivity. [5,[17][18][19] Most of Zintl phases are intrinsically p-type and difficult to be doped to n-type semiconductors. While Mg 3 Sb 2 -based Zintl-phase materials show outstanding n-type TE performance by chalcogens or rare earth doping. [20][21][22] The maximum peak ZT value of Te-doped Mg 3+δ Sb 1.5 Bi 0.5 is higher than ≈1.5 at 723 K. [21][22][23][24][25][26] Especially, a prominent average ZT value has recently been obtained at around room temperature range, which is close to that of the commercialized Bi 2 Te 3 -based TE material. Considering the scarcity of Te, Mg 3 Sb 2 -based materials are more promising for large-scale applications. The high performance at lower temperature is considered the results of the enhanced carrier mobility, which is ascribed to the Sb/Bi alloying for band structure tailoring [27][28][29] and transition metal doping (Nb, Ta, Co, Mn, Hf, etc.) or grain size increment for scattering mechanism manipulation. [24,25,[30][31][32][33][34] The average ZT ≈ 1.05 was obtained for Mn&Te-codoped Mg 3+δ Sb 1.5 Bi 0.5[35] and ≈1.08 was obt...