Chemical bonding engineering for high-symmetry Cu2S-based materials with high thermoelectric performance

Liquid-like materials have gained notable attention in thermoelectrics due to their ultralow lattice thermal conductivity and tunable electrical properties. Particularly, Cu2S-based materials have the advantages of earth abundant and environmentally friendly constituents, which have the great potential for industry applications. However, Cu2S undergoes a series of phase transitions above room temperature and only the high temperature cubic phase (above 700 K) exhibits desirable thermoelectric properties, leading to the low averaged thermoelectric performance. Herein we theoretically and experimentally demonstrated that the phase transition temperature can be strongly suppressed by tuning the chemical bonding in Mn-doped Cu2S, which effectively move the high-symmetry cubic phase to a much low-temperature regime. The weakened chemical bonding facilitates the formation of Cu vacancies and thus improves the hole concentrations for optimized electrical transports. Furthermore, the lattice thermal conductivity is suppressed due to the reduced sound velocity by lattice softening and the enhanced phonon scattering by point defects. As a total result, high average power factors of 6.13 μW cm−1 K−2 and average zT values of 0.44 are obtained in the temperature range of 423 to 723 K, both of which outperform most Cu2S-based materials reported so far. The present study corroborates that chemical bonding engineering can serve as a scaffold for regulating the phase structures and transport properties in thermoelectric materials.