Boosting High Thermoelectric Performance of Ni-Doped Cu1.9S by Significantly Reducing Thermal Conductivity.

At present, copper sulfide materials have been predicted as promising thermoelectric materials due to their inexpensiveness and nontoxicity property. Most researches on copper sulfide are focused on Cu2S and Cu1.8S because they are more easily synthesized into a single phase; however, the improper electrical conductivity greatly hindered their thermoelectric properties. In this work, a series of high-performance Cu1.9-xNixS (x = 0, 0.01, 0.015, and 0.02) bulk samples were fabricated by accurately manipulating the ratio of Cu/S with appropriate Ni-doping. The thermoelectric properties of Ni-doped Cu1.9S were explored in detail for the first time. It can be found that carrier thermal conductivity and lattice thermal conductivity of Cu1.9-xNixS were effectively reduced via Ni-doping, simultaneously, without the great influence on the power factor. Here, the carrier thermal conductivity (κe) was reduced due to the extreme reduction of Hall carrier concentration. In addition, amounts of nanopores introduced by Ni-doping and complex crystal structure from the phase transition of the second phase strengthen the phonon scattering and reduce lattice thermal conductivity (κl) remarkably. As a consequence, the lowest carrier thermal conductivity and lattice thermal conductivity reach 0.006 and 1.08 W m-1 K-1 for Cu1.88Ni0.02S at 773 K, and the average ZT is about 0.39 from 323 to 773 K (the ZTmax is about 0.9 at 773 K). This work demonstrates that low-cost and easily fabricated Ni-doped Cu1.9S is a pleasurable candidate for thermoelectric application despite it usually being treated as an ion conductor.