Steric confinement for suppressing ion migration and boosting thermoelectric performance in Cu–S system via Ag and Se co-doping

The Cu-S superionic compound exhibits excellent thermoelectric (TE) performance in the mid-temperature region. However, obtaining a thermally and electrically stable material in this system is still challenging. Herein, motivated by the perspective of steric confinement for suppressing Cu⁺ migration, we designed an Ag and Ag-Se co-doping strategy on pristine Cu_1.93S to modulate the crystal structure. On one hand, Ag dopant is discovered to act as a vacancy filler, simultaneously optimizing carrier concentration and reducing thermal conductivity. Moreover, strategic Se co-substitution stabilizes Ag insertion within the Cu-S vacancy and induces non-negligible enhancement of phonon scattering to further reduce thermal conductivity. A competitive peak zT of 1.33@873 K was obtained for the Cu_1.93Ag_0.04S_0.9Se_0.1 sample at a relatively moderate temperature among current state-of-the-art Cu-S system. Moreover, thanks to the steric confinement, the Cu_1.93Ag_0.04S_0.9Se_0.1 composition exhibited superior electrical stability under dynamic DC-current compared to Cu_xS (with 1.93<x<1.97), indicating that Ag and Ag-Se co-doping are effective at suppressing the electromigration of Cu⁺. Furthermore, an improved thermal stability of Cu_1.93Ag_0.04S_0.9Se_0.1 was attributed to percolated Ag which confines sulfur evaporation by strengthening the chemical bonding. In addition, a first-principles study demonstrates that the addition of Ag and Se creates new electronic interactions with their nearest neighbors and also modifies the local atomic environment by increasing interatomic distances, which contributes to limiting Cu diffusion and improving thermoelectric stability. Through the method of steric confinement via Ag insertion and Se doping in the Cu_1.93Ag_xS_1-ySe_y system, we achieved a desirable combination of high TE performance and notable thermal and electrical stability.