Improving thermoelectric performance of caged compounds through light-element filling

Heavy elements filling have been considered the most effective way to improve the thermoelectric performance of caged compounds such as CoSb3 by reducing kL. Here, we show an opposite example of filling a light element, Na, into CoSb3 for obtaining high thermoelectric figure of merit ZT.ZT=1.25 at 850 K for Na0.48Co4Sb12 is one of the highest values among all reported single-element-filled CoSb3. The Na-filling scatters phonons less effectively but it results in relatively high mobility thus large power factor. This most likely comes from the extra electronic states near the Fermi level induced by Na.