High Thermoelectric Performance for n-Type K ₂ Bi ₈ S ₁₃ with Low Thermal Conductivity

The complex structure of bismuth chalcogenides shows promising prospects as a type of thermoelectric material with intrinsic low lattice thermal conductivity (κlat). The n-type K2Bi8S13 has the low κlat of 0.56–0.42 W m–1 K–1 from 300 to 823 K because of the large unit cell, complex structure, and rattling of K in tunnels composed of the NaCl-type and CdI2-type rod-shaped fragments. The power factor (PF) of K2Bi8S13 reaches a maximum value of 1.97 μW cm–1 K–2 at 823 K. With the low thermal conductivity of 0.50 W m–1 K–1, the pristine K2Bi8S13 achieves a dimensionless figure of merit ZT of ∼0.33 at 823 K. With a heavy conduction band minimum for K2Bi8S13, the extra Bi is introduced to enhance the carrier concentration, electrical conductivity (σ), and PF. Consequently, the σ and PF reveal an evident promotion from 34.85 S cm–1 and 1.97 μW cm–1 K–2 for K2Bi8S13 to 73.47 S cm–1 and 3.09 μW cm–1 K–2 for K2Bi8.003S13 at 823 K, respectively. Meanwhile, point defects introduced by extra Bi result in phonon scattering and decreased κlat to 0.38 W m–1 K–1 at 823 K. Finally, a maximum ZT ∼ 0.53 of K2Bi8.003S13 increases by 61% compared with 0.33 of intrinsic K2Bi8S13 at 823 K.