Boosting Thermoelectric Properties of BiSbSe₃ Through Ag-Induced Interstitial Occupancy and Multi-Phase Microstructure

Te-free BiSbSe₃ 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 BiSbSe₃ + x%Ag composites. Specifically, Ag compositing induces the generation of narrow bandgap AgBiSe₂ 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 BiSbSe₃ + 0.2%Ag at 723 K. Ultimately, a maximum zT value of ∼0.45 is obtained along the out-of-plane direction for BiSbSe₃ + 0.2%Ag composite at 723 K, corresponding to an approximately 7-fold enhancement over that of BiSbSe₃. 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.