CuxS superionic compounds: Electronic structure and thermoelectric performance enhancement

In the present work, we have investigated the effect of Cu content (x) on the phase structure and thermoelectric transport properties of CuxS (x = 1.92, 1.94, 1.98) bulks fabricated by combining mechanical alloying and spark plasma sintering. From density functional theory calculations of the valence band energy levels, we hypothesized that favourable valence band alignments accelerate electrical transport and can be achieved by optimizing the molar ratio of Cu1.96S to Cu2S. Experimentally, the sample with x = 1.92 showed an improved electrical conductivity 538.7 Scm−1 at 623 K and retained a good Seebeck coefficient, synergistically leading to a broad, high power factor ranging from 105.9 to 900.3 μWm−1K−2. The highly-disturbed Cu ions give CuxS compounds an intrinsically low lattice thermal conductivity (∼0.5 Wm−1K−1), which endows a high ZT of 1.23 as x = 1.94 to this otherwise simple semiconductor. The results indicate that low-cost and non-toxic CuxS superionic compounds are promising thermoelectric materials.