Synergistic Approach Toward a Reproducible High zT in n-Type and p-Type Superionic Thermoelectric Ag₂Te

Recently, superionic thermoelectrics 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 Cu₂X (X: chalcogen atom) family despite a relatively low superionic transition temperature of ∼400 K. At such high temperatures, the liquid-like flow of the metal ions results in material's degradation. Here, we present thermoelectric properties of superionic Ag₂Te synthesized by various methods. The sintered Ag₂Te samples are shown to exhibit an unpredictable behavior with respect to the sign of thermopower (S) in the superionic phase and the magnitude of electrical conductivity (σ). We overcome this issue using an all-room-temperature fabrication technique leading to an excellent reproducibility from one sample to another. To improve the zT of Ag₂Te beyond the phonon-liquid electron-crystal limit (∼0.64 at 575 K in the ingot samples), we adopted a heirarchical nanostructuring technique, which effectively suppressed the thermal conductivity, leading to a significant improvement in the zT values for both n-type and p-type samples. We obtained zT of 1.2 in the n-type and 0.64 in the p-type Ag₂Te at 570 K. These values supersede the zT of any Ag₂Te previously reported. At 570 K, for our ball-milled/cold-pressed samples, the critical current density for metal-ion migration exceeds 15 A cm^-2, which further confirms that Ag₂Te is a promising thermoelectric material. Our results are supported by first-principles density functional theory calculations of the electronic and thermal properties.