High Thermoelectric Performance in Self‐Synthesized NiSe/AgBiSe ₂ Heterostructured Composites by Doping Ni in <i>n</i> ‐Type AgBiSe ₂

ABSTRACT AgBiSe 2 is a promising intermediate‐temperature n ‐type thermoelectric (TE) material. However, its performance enhancement has been constrained by limited modulation strategies. Here, the NiSe/AgBiSe 2 heterostructured composites were self‐synthesized via Ni incorporation, realizing a ∼129% enhancement in figure of merit ( ZT ∼1) at 783 K in Ag 0.985 Ni 0.015 BiSe 2 . The results demonstrated a significant improvement (∼250%) in the electrical conductivity ( σ ) for Ag 0.94 Ni 0.06 BiSe 2 , boosting the power factor PF by 98.6% to ∼4.9 µW cm −1 K −2 at 783 K. Simultaneously, ultralow lattice thermal conductivity ( κ L ) was realized, for example, ∼0.17 W m −1 K −1 for Ag 0.985 Ni 0.015 BiSe 2 and even ∼0.13 W m −1 K −1 for Ag 0.955 Ni 0.045 BiSe 2 at 783 K, representing a ∼60% reduction. Systematic investigations indicate that the optimization of electrical transport properties stems from the substantial increase in n H due to the combined contributions of selenium vacancy (V Se ), charge transfer, and Ni 2+ dopant. What is more intriguing, the in situ precipitated NiSe second phase effectively decouples σ and the S , enhancing PF . Additionally, the κ L suppression would be attributed to multiple phonon‐scattering mechanisms introduced by multidimensional defects. Furthermore, a maximum theoretical TE conversion efficiency (𝜂 max ) of ∼5.72% is attained. This work would offer a novel strategy for developing high‐performance n ‐type AgBiSe 2 ‐based TE materials by composite engineering.