Enhancement of thermoelectric properties in n-type Bi₂Te₃ through defect engineering via in situ NiTe₂ precipitates

Bi₂Te₃ composition serves as the parent compound of the highest performing thermoelectric materials at room temperature, including (Bi,Sb)₂Te₃, and Bi₂(Te,Se)₃. Herein, the thermoelectric transport properties of (Bi_1-xNi_x)₂Te₃ (x = 0, 0.0075, 0.015, 0.03, 0.045, 0.06 and 0.075) compositions were systematically investigated by introducing Ni into the Bi₂Te₃ composition, which results in in-situ NiTe₂ precipitates and Te-vacancies during synthesis. At low Ni contents (x = 0.0075 and 0.015), the electrical conductivity and power factor (PF) increase, while the lattice thermal conductivity (κ_latt) concurrently decreases, yielding a broad enhancement of zT across 375–425 K with a peak near ∼450 K, compared with pristine Bi₂Te₃. Mechanistically, slight Te deficiency associated with NiTe₂ formation introduces donor-type Te vacancies, increasing the carrier concentration (n_H); a concurrent moderate rise in the density-of-states effective mass (m_d∗) supports higher weighted mobility and PF, whereas fine and numerous NiTe₂ precipitates induce strong phonon scattering that sustains low κ_latt. Consequently, both the thermoelectric quality factor (B) and zT are significantly elevated for x = 0.0075 and 0.015. In contrast, at higher Ni contents (x ≥ 0.03), self-compensation lowers n_H, alloy/defect disorder grows, and NiTe₂ coarsening partially recovers thermal conductivity and flattens the PF, shifting the zT maximum toward lower temperatures (∼350 K). This study demonstrates that combining reservoir-controlled defect chemistry with in-situ nanoscale precipitates is an effective strategy for improving Bi₂Te₃-based n-type thermoelectrics in the near-room-temperature regime.