Entropy-Induced Multivalley Band Structures Improve Thermoelectric Performance in <i>p</i>-Cu₇P(S_<i>x</i>Se_1–<i>x</i>)₆ Argyrodites

Abstract

Searching for novel low-cost and eco-friendly materials for energy conversion is a good way to provide widespread utilization of thermoelectric technologies. Herein, we report the thermal behavior, phase equilibria data, and thermoelectric properties for the promising argyrodite-based Cu₇P(S_<i>x</i>Se_1-<i>x</i>)₆ thermoelectrics. Alloying of Cu₇PSe₆ with Cu₇PS₆ provides a continuous solid solution over the whole compositional range, as shown in the proposed phase diagram for the Cu₇PS₆-Cu₇PSe₆ system. As a member of liquid-like materials, the investigated Cu₇P(S_<i>x</i>Se_1-<i>x</i>)₆ solid solutions possess a dramatically low lattice thermal conductivity, as low as ∼0.2-0.3 W m^-1 K^-1, over the entire temperature range. Engineering the configurational entropy of the material by introducing more elements stabilizes the thermoelectrically beneficial high-symmetry γ-phase and promotes the multivalley electronic structure of the valence band. As a result, a remarkable improvement of the Seebeck coefficient and a reduction of electrical resistivity were observed for the investigated alloys. The combined effect of the extremely low lattice thermal conductivity and enhanced power factor leads to the significant enhancement of the thermoelectric figure of merit <i>ZT</i> up to ∼0.75 at 673 K for the Cu₇P(S_<i>x</i>Se_1-<i>x</i>)₆ (<i>x</i> = 0.5) sample with the highest configurational entropy, which is around twice higher compared with the pure selenide and almost four times higher than sulfide. This work not only demonstrates the large potential of Cu₇P(S_<i>x</i>Se_1-<i>x</i>)₆ materials for energy conversion but also promotes sulfide argyrodites as earth-abundant and environmentally friendly materials for energy conversion.