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The filling fraction limit (FFL) of skutterudites, that is, the complex balance of formation enthalpies among different species, is an intricate but crucial parameter for achieving high thermoelectric performance. In this work, we synthesized a series of YbxCo4Sb12 samples with x=0.2–0.6 and systemically studied the FFL of Yb, which is still debated even though this system has been extensively investigated for decades. Our combined experimental efforts of X-ray diffraction, microstructural and quantitative compositional analyses clearly reveal a Yb FFL of ~0.29 in CoSb3, which is consistent with previous theoretical calculations. The excess Yb in samples with x>0.35 mainly form metallic YbSb2 precipitates, significantly raising the Fermi level and thus increasing the electrical conductivity and decreasing the Seebeck coefficient. This result is further corroborated by the numerical calculations based on the Bergman’s composite theory, which accurately reproduces the transport properties of the x>0.35 samples based on nominal Yb0.35Co4Sb12 and YbSb2 composites. A maximum ZT of 1.5 at 850 K is achieved for Yb0.3Co4Sb12, which is the highest value for a single-element-filled CoSb3. The high ZT originates from the high-power factor (in excess of 50 μW cm-K−2) and low lattice thermal conductivity (well below 1.0 W m-K−1). More importantly, the large average ZTs, for example, ~1.05 for 300–850 K and ~1.27 for 500–850 K, are comparable to the best values for n-type skutterudites. The high thermoelectric and thermomechanical performances and the relatively low air and moisture sensitivities of Yb make Yb-filled CoSb3, a promising candidate for large-scale power generation applications.

High-performance n-type YbxCo4Sb12: from partially filled skutterudites towards composite thermoelectrics