Improving thermal stability and revealing physical mechanism in n-type Mg3Sb2-Bi for practical applications

N-type Mg3Sb2-xBix alloys exhibit good potential for applications in waste-heat harvesting due to their high thermoelectric performance. However, recent studies have suggested that these materials suffer from high-temperature performance degradation, and the physical mechanism affecting their stability remains vague, both of which significantly limit their practicability. Here we focus on the thermal stability of Mg3Sb2-xBix compounds, as well as the underlying physical mechanism. By adding cationic Co and Er into the matrix materials, both the electrical conductivity and Seebeck coefficient of Mg3Sb2-xBix barely decline over 100 h of thermoelectric properties measurement at 673 K, indicating improved thermal stability, and the Mg3Sb2-xBix compounds doped with Co and Er demonstrate thermoelectric performance comparable to that of Mg3Sb2-xBix doped only with anionic Te. These findings indicate that enhancing the thermal stability of these materials does not result in their sacrificed thermoelectric performance. Additionally, the electron localization function is used to explore the underlying mechanism via density functional theory calculations, through which greater bonding strength is found between Co/Er and Sb/Bi in comparison with that between Mg and Sb/Bi, explaining the absence of high-temperature thermoelectric performance degradation for Mg3Sb2-xBix doped with Co and Er. This study provides a new viewpoint for investigating the thermal stability of Mg3Sb2-xBix materials, as well as for possible future studies on the thermal stability of other thermoelectric materials.