Recent developments in Earth-abundant copper-sulfide thermoelectric materials

The ability of thermoelectric devices to convert waste heat into useful electrical power has stimulated a remarkable growth in research into thermoelectric materials. There is, however, a growing recognition that limited reserves of tellurium, together with the reduction in performance that occurs at elevated temperatures, places constraints on the widespread implementation of thermoelectric technology based on the current generation of telluride-based devices. Metal sulfides have attracted considerable attention as potential tellurium-free alternatives. This perspective provides an overview of the key characteristics of sulfide thermoelectrics and the advantages they offer in the development of devices for energy recovery in the temperature range 373 ≤ T/K ≤ 773. The structures and properties of a group of synthetic materials, related to the minerals chalcocite (Cu2S), stannite (Cu2FeSnS4)/kesterite (Cu2SnS4), chalcopyrite (CuFeS2), bornite (Cu5FeS4), colusite [Cu26V2(As,Sn,Sb)6S32], and tetrahedrite [(Cu,Fe)12Sb4S13], are discussed. In addition to all being composed of Earth-abundant elements, these sulfides share a common tetrahedral CuS4 structural building block. The use of chemical substitution to manipulate electrical and thermal transport properties is described, and common features are identified. This includes the presence of low-energy vibrational modes, the onset of copper-ion mobility, and the emergence of a liquid-like sublattice, which serve to reduce thermal conductivity. Issues associated with materials' stability during synthesis, consolidation, and device operation due to sulfur volatilization and migration of mobile copper ions are also highlighted. Future prospects for sulfide thermoelectrics are discussed in the light of the performance of materials investigated to date.