Boosting High Thermoelectric Performance of Ni-Doped Cu_1.9S 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 Cu_1.8S 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 Cu_1.9-NiS (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 Cu_1.9S were explored in detail for the first time. It can be found that carrier thermal conductivity and lattice thermal conductivity of Cu_1.9-NiS were effectively reduced via Ni-doping, simultaneously, without the great influence on the power factor. Here, the carrier thermal conductivity (κ_e) 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 (κ_l) 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 Cu_1.88Ni_0.02S at 773 K, and the average ZT is about 0.39 from 323 to 773 K (the ZT_max is about 0.9 at 773 K). This work demonstrates that low-cost and easily fabricated Ni-doped Cu_1.9S is a pleasurable candidate for thermoelectric application despite it usually being treated as an ion conductor.