Thermoelectric properties of AgBiS2: Unveiling the origin of ultra-low lattice thermal conductivity and optimization strategies for electrical performance

Abstract

AgBiS2, a typical I–V–VI2 compound, has attracted widespread attention in the thermoelectric community due to its inherently ultra-low-thermal conductivity. However, its practical applications are limited by inferior electrical transport properties. Herein, we systematically investigate the origin of its low lattice thermal conductivity and propose effective strategies to enhance its thermoelectric performance. Theoretical calculations reveal that the intrinsic ultra-low lattice thermal conductivity of AgBiS2 arises from multiple phonon transport features, including low-lying optical branches, rattler-like local vibration, and low phonon group velocity. Combining theoretical and experimental analyses, it is found that cation disorder suppresses cation doping effects, whereas anion doping, especially with Cl doping, is more effective in optimizing electrical performance. Furthermore, introducing additional Cu, unaffected by cation disorder, not only generates extra electrons to optimize electrical transport but also reduces thermal conductivity. Notably, Cu-doped sample achieves a significantly improved power factor of ∼3.64 μW cm−1 K−2 at 823 K, with a maximum ZT of ∼0.53, a ∼22% enhancement compared to the pristine AgBiS2. This study not only uncovers the ultra-low lattice thermal conductivity mechanisms but also validates synergistic strategies to optimize thermoelectric properties, positioning AgBiS2-based materials as promising candidates for thermoelectrics.