Synergistic approach towards reproducible high zT in superionic thermoelectric Ag2Te

Abstract: Recently, the superionic thermoelectrics, which typify the novel `phonon-liquid electron-crystal' concept, have attracted enormous attention due to their ultralow thermal conductivity and high figure-of-merit (zT). However, their high zT is generally obtained deep inside the superionic phase, e.g., near 1000~K in the Cu$_2$X (X: chalcogen atom) family where the superionic transition is close to 400~K. At such high temperatures, the liquid-like flow of the metal ions under an electric field or a temperature gradient, both of which are integral to the working of a thermoelectric device, results in device degradation. To harness the full potential of the superionic thermoelectrics, it is, therefore, necessary to reach high zT at low temperatures where the metal-ion diffusion is not an issue. Here, we present a novel all-room-temperature route to fabricate 100\% dense, nanostructured Ag$_2$Te with highly reproducible thermoelectric properties and a high zT of 1.2 at 570~K, i.e., merely 150~K above its superionic transition. The samples show a broad particle-size distribution ranging from a few nm to a few $\mu$m. This hierarchical nanostructuring is shown to suppress the thermal conductivity of Ag$_2$Te beyond the phonon-liquid electron-crystal limit to ultralow values, leading to a remarkable enhancement of 87\% in the zT over that of the ingot sample. These values supersede the zT of any Ag$_2$Te previously reported. Our results are supported by first-principles density functional theory calculations of the electronic and thermal properties.