Enhanced thermoelectric properties of Mm-filled p-type (Fe, Co)Sb3 skutterudites via Co/Fe ratio regulation and extra Sb compensation

The band degeneracy thus thermoelectric performance of (Fe, Co)Sb3 based p-type skutterudites can be well modulated by adjusting Co/Fe ratio. Combining systematic experiments and density function theory (DFT) calculations, we revealed in this work that the variation of Co/Fe ratio could result in complicated and mutually coupled effects on the individual thermoelectric parameters (band degeneracy, band gap, Fermi level, filling limit of fillers, phonon spectrum, etc.) and that using Co/Fe ratio to improve the overall thermoelectric figure of merit is nontrivial. We eventually found that the optimal Co/Fe ratio for Mm (Misch metal) filled (Fe, Co)Sb3 lies at 1.3:2.7, which is somewhat off the Co/Fe = 1:3 point where the largest valence band degeneracy appears. Extra Sb was then introduced to compensate Sb volatilization for improved electrical transport performance. An overall peak figure of merit ZTmax of 1.06 was eventually achieved in the sample Mm0.8Fe2.7Co1.3Sb12.60, which is among the highest ones for all reported (Fe, Co)Sb3 based p-type skutterudites so far.