Exceptional thermoelectric performance in Mg3Sb0.6Bi1.4 for low-grade waste heat recovery

Bi2Te3 alloys have been the most widely used n-type material for low temperature thermoelectric power generation for over 50 years, thanks to the highest efficiency in the 300-500 K temperature range relevant for low-grade waste-heat recovery. Here we show that n-type Mg3Sb0.6Bi1.4, with a thermoelectric figure-of-merit zT of 1.0-1.2 at 400-500 K, finally surpasses n-type Bi2Te3. This exceptional performance is achieved by tuning the alloy composition of Mg-3(Sb1-xBix)(2). The two primary mechanisms of the improvement are the band effective-mass reduction and grain size enhancement as the Mg3Bi2 content increases. The benefit of the effective-mass reduction is only effective up to the optimum composition Mg3Sb0.6Bi1.4, after which a different band dominates charge transport. The larger grains are important for minimizing grain-boundary electrical resistance. Considering the limited choice for low temperature n-type thermoelectric materials, the development of Mg3Sb0.6Bi1.4 is a significant advancement towards sustainable heat recovery technology.