Boosting Thermoelectric Properties of BiSbSe ₃ Through Ag‐Induced Interstitial Occupancy and Multi‐Phase Microstructure

ABSTRACT Te‐free BiSbSe 3 is a promising medium‐temperature thermoelectric (TE) material owing to its cost‐effectiveness and intrinsically low lattice thermal conductivity. However, its comparatively low electrical conductivity has severely hindered the optimization of TE performance. Herein, Ag compositing enables the simultaneous optimization of electrical and thermal transport performance for BiSbSe 3 + x %Ag composites. Specifically, Ag compositing induces the generation of narrow bandgap AgBiSe 2 and BiSe precipitates, which optimize carrier transport through interfacial potential barrier engineering. Meanwhile, a fraction of Ag atoms occupies the van der Waals gaps, effectively increasing the carrier concentration and establishing supplementary conductive pathways. Additionally, the incorporation of multi‐scale defects, including point defects, dislocations, nanoclusters, heterogeneous interfaces, and elemental segregation, enables full‐frequency phonons scattering, thereby yielding a low lattice thermal conductivity of ∼0.21 W m −1 K −1 along the out‐of‐plane direction for BiSbSe 3 + 0.2%Ag at 723 K. Ultimately, a maximum zT value of ∼0.45 is obtained along the out‐of‐plane direction for BiSbSe 3 + 0.2%Ag composite at 723 K, corresponding to an approximately 7‐fold enhancement over that of BiSbSe 3 . This study elucidates the underlying mechanisms by which interstitial occupancy and multiphase engineering boost the TE performance, and establishes a practical design strategy and solid theoretical foundation for developing high‐performance TE materials.