Controlled In Situ Precipitation of Yb ₂ O ₃ for Synergistic Electron–Phonon Optimization in Yb-Filled Skutterudites

CoSb₃-based skutterudites have emerged as promising candidates for midtemperature thermoelectric (TE) applications. However, their further performance enhancement is hindered by intrinsically high lattice thermal conductivity and strong intercoupling among TE transport parameters. In this work, we introduce low-melting-point Sb₂O₃ as an oxidizing agent into the classical Yb_0.3Co₄Sb₁₂ system via a melt-spinning technique to optimize its TE transport properties. In situ oxidation generates high-melting-point Yb₂O₃ nanoprecipitates within the matrix while simultaneously reducing the Yb filling fraction. To address this, we employ a stoichiometric compensation strategy by incorporating additional Yb:Sb₂O₃ in a 2:1 ratio, thereby achieving controlled Yb₂O₃ nanoparticle precipitation while maintaining optimal Yb filling. The engineered oxide nanophase exhibits dual functionality: (1) carrier energy filtering enhances the power factor to 61 μW cm^-1 K^-2, and (2) multiscale phonon scattering suppresses the lattice thermal conductivity to 0.4 W m^-1 K^-1. These synergistic effects yield a record-high peak <i>ZT</i> of 1.58 at 873 K (a 32% enhancement) and an exceptional average <i>ZT</i> of 1.05 across 300-873 K in the Yb_0.36Co₄Sb₁₂/0.03 Sb₂O₃ composite. This work established a paradigm for decoupling electronic and phonon transport through nanophase precision oxide nanophase engineering.