Chemical stability and degradation mechanism of Mg3Sb2-Bi thermoelectrics towards room-temperature applications

Mg3Sb2-xBix compounds have recently attracted surging research attention owing to their low cost and high thermoelectric performance that is comparable with the commercialized Bi2Te3-xSex alloys. For practical applications, chemical stability is another key factor that determines the lifetime of the materials, which is rarely investigated for Mg3Sb2-xBix. Here we show that Mg3Sb2-xBix is more sensitive to moisture, but not to the conventionally considered oxygen. A spontaneous decomposition to crystalline Bi/Sb and amorphous Mg(OH)2 is thermodynamically favorable in the moist ambient air, leading to catastrophically degraded thermoelectric performance. Moreover, such an irreversible degradation was also found to occur in other Bi-contained Zintl phase compounds. Thankfully, if moisture is eliminated, the thermoelectric properties of Mg3Sb2-xBix keep nearly unchanged after two months’ exposure to the dry air. This work provides insight into the degradation mechanism of Mg3Sb2-xBix and other Bi-contained thermoelectric materials, paving the way for their protection in future applications.