Enhanced thermoelectric properties and electrical stability for Cu1.8S-based alloys: Entropy engineering and Cu vacancy engineering

Cu1.8S-based thermoelectric (TE) materials have garnered considerable interest due to their pollution-free, low-cost, and superior performance characteristics. However, high Cu vacancy and Cu migration inhibit their performance and electrical stability improvement. Through mechanical alloying and spark plasma sintering, a series of Cu1.8S and MnxCu1.8-S0.5Se0.5 (0.01 ≤ x ≤ 0.06) bulk samples were prepared in this study. With Se alloying and Mn doping, the configuration entropy of MnxCu1.8S0.5Se0.5 increases from low-entropy 0.4R* for pristine Cu1.8S to medium-entropy 1.2R* for MnxCu1.8S0.5-Se0.5. MnxCu1.8S0.5Se0.5 subsequently crystallized in a cubic phase with enhanced symmetry and Mn solid solubility. High solubility enables the filling of excessive Cu vacancies, the reduction of carrier concentration, the adjustment of band structure, the enhancement of the Cu migration energy barrier, and the inhibition of Cu migration. Even at current densities exceeding 25 A cm−2 at 750 K, the resistance of Mn0.03Cu1.8S0.5Se0.5 remained hardly changed, indicating a vastly improved electrical stability. In addition, the ultralow thermal conductivity of the lattice is achieved by decreasing the sound velocity and softening the lattice. At 773 K, the bulk ZT of Mn0.06Cu1.8S0.5Se0.5 reaches a maximum of 0.79, which is twice that of pure Cu1.8S. The results indicate that combining entropy engineering and Cu vacancy engineering is an effective strategy for developing high-performance Cu1.8S TE materials.