Lead-Doping Enabled Band Engineering and Phonon Transport Modulation: Key to High-Performance Thermoelectric AgSbTe₂

AgSbTe₂ is a promising mid-temperature thermoelectric (TE) material but has instability and limited performance from binary decomposition and vacancies. In this study, we report that Pb doping synergistically optimizes both electronic and phonon transport properties, resulting in a peak ZT of 2.0 at 623 K for p-type AgSb_0.96Pb_0.04Te₂-one of the highest values ever achieved for this material family. Pb²⁺ substituting Sb³⁺ induces lattice expansion and elevates Ag vacancy formation energy (from -0.133 to 0.024 eV), suppressing Ag₂Te precipitation and stabilizing the matrix. Electronically, Pb doping gives rise to a new valence band, pushing the Fermi level deeper into the valence band and enabling multiband transport. This combination leads to an enhanced electrical conductivity (276 S cm^-1 at 623 K), while retaining a moderate Seebeck coefficient (260 µV K^-1), yielding a maximum power factor of 18.14 µW cm^-1 K^-2. Phonon transport is inhibited by Pb-induced dislocations (enhancing phonon scattering) and lattice softening (lowering acoustic/optical phonon frequencies, promoting mode coupling), resulting in an ultralow lattice thermal conductivity (κ_L) of 0.27 W m^-1 K^-1 (near the diffusion limit). This work clarifies bivalent cation doping (Pb²⁺, Cd²⁺, Hg²⁺, Sn²⁺) tuning TE performance via crystal structure, band, and phonon modulation.