Enhanced Thermoelectric Efficiency in P-Type Mg3Sb2: Role of Monovalent Atoms Codoping at Mg sites

Due to natural abundance, low cost, and compatibility with sustainable green technology, Mg3Sb2-based Zintl compounds are comprehensively explored as potential thermoelectric materials for near-room temperature applications. The effective use of these materials in thermoelectric devices requires both p and n-type Mg3Sb2 having comparable thermoelectric efficiency. However, p-type Mg3Sb2 has inferior thermoelectric efficiency efficiency compared to its n-type counterpart due to low electrical conductivity (∼103Sm-1). Here, we show that codoping of monovalent atoms (Li-Ag, and Na-Ag) at the Mg site of Mg3Sb2 produces a synergistic effect and boosts the electrical conductivity, which enhances the thermoelectric properties of p-type Mg3Sb2. While, Ag prefers to occupy the Mg2 site, Li and Na are favorable at the Mg1 site of Mg3Sb2 lattice. Compared to Li-Ag codoping, Na-Ag codoping in Mg3Sb2 is found to be more effective for increasing the charge carrier concentration and significantly augmenting the electrical conductivity. The dominance of the three-phonon scattering mechanism in Li and Li-Ag doped Mg3Sb2 and the four-phonon scattering process for the Na and Na-Ag doped Mg3Sb2 are confirmed. Due to the simultaneous increase in electrical conductivity and decrease in thermal conductivity, the zT value ∼0.8 at 675 K achieved for Mg2.975Na0.02Ag0.005Sb2 is the highest value among p-type Mg3Sb2. Our work shows a constructive approach to enhance the zT of p-type Mg3Sb2 via monovalent atoms codoping at the Mg sites.