Advanced thermoelectric technology offers a potential for converting waste industrial heat into useful electricity, and an emission-free method for solid state cooling. Worldwide efforts to find materials with thermoelectric figure of merit, zT values significantly above unity, are frequently focused on crystalline semiconductors with low thermal conductivity. Here we report on Cu(2-x)Se, which reaches a zT of 1.5 at 1,000 K, among the highest values for any bulk materials. Whereas the Se atoms in Cu(2-x)Se form a rigid face-centred cubic lattice, providing a crystalline pathway for semiconducting electrons (or more precisely holes), the copper ions are highly disordered around the Se sublattice and are superionic with liquid-like mobility. This extraordinary 'liquid-like' behaviour of copper ions around a crystalline sublattice of Se in Cu(2-x)Se results in an intrinsically very low lattice thermal conductivity which enables high zT in this otherwise simple semiconductor. This unusual combination of properties leads to an ideal thermoelectric material. The results indicate a new strategy and direction for high-efficiency thermoelectric materials by exploring systems where there exists a crystalline sublattice for electronic conduction surrounded by liquid-like ions.

Iodine-doped Cu2 Se shows a significantly improved thermoelectric performance during phase transitions by electron and phonon critical scattering, leading to a dramatic increase in zT by a factor of 3-7 times culminating in zT values of 2.3 at 400 K.

Thermoelectric materials require an optimum carrier concentration to maximize electrical transport and thus thermoelectric performance. Element-doping and composition off-stoichiometry are the two general and effective approaches to optimize carrier concentrations, which have been successfully applied in almost all semiconductors. In this study, we propose a new strategy coined as bonding energy variation to tune the carrier concentrations in Cu2Se-based liquid-like thermoelectric compounds. By utilizing the different bond features in Cu2Se and Cu2S, alloying S at the Se-sites successfully increases the bonding energy to fix Cu atoms in the crystal lattice to suppress the formation of Cu vacancies, leading to much lowered carrier concentrations toward the optimum value. Combing the lowered electrical and lattice thermal conductivities, and the relatively good carrier mobility caused by the weak alloy scattering potential, ultrahigh zTs are achieved in slightly S doped Cu2Se with a maximum value of 2.0 at 1000 K, 30% higher than that in nominallystoichiometric Cu2Se.The table of contents entry: Beyond element-doping and composition off-stoichiometry, we propose a new strategy coined as bonding energy variation to tune the carrier concentrations in Cu2Se-based liquid-like thermoelectric compounds, leading to a maximum zT value of 2.0 at 1000 K, 30% higher than that in nominally-stoichiometric Cu2Se.