Improved electrical transport properties and optimized thermoelectric figure of merit in lithium-doped copper sulfides.

Copper sulfide Cu₂S is a p-type semiconducting compound that has attracted great attentions in the thermoelectric (TE) community most recently. Considering the intrinsic ultralow lattice thermal conductivity, the enhancement of TE performance in Cu₂S should be achieved through improving its electrical transport properties. To achieve this goal, lithium element was doped into Cu₂S in this study. A series of Cu_{2−<italic>x</italic>}Li_{<italic>x</italic>}S samples with different Li contents (<italic>x</italic> = 0, 0.005, 0.010, 0.050, and 0.100) was synthesized by the melting-annealing method. When <italic>x</italic> ≤ 0.05, the Cu_{2−<italic>x</italic>}Li_{<italic>x</italic>}S samples are stable and pure phases, having the same monoclinic structure with the pristine Cu₂S at room temperature. The electrical conductivities in the Cu_{2−<italic>x</italic>}Li_{<italic>x</italic>}S samples are greatly improved with the Li-doping content increasing due to the enhanced carrier concentrations. Meanwhile, doping Li into Cu₂S increases the ionic activation energy and lessens the influence of mobile Cu ions on the heat-carrying phonons. Thus, the thermal conductivities of the Li-doped Cu₂S samples increase. A maximal figure of merit (<italic>zT</italic>) of 0.84 at 900 K is obtained in Cu_1.99Li_0.01S, about 133% improvement as compared with that in Cu₂S matrix. [ABSTRACT FROM AUTHOR]