All Cubic‐Phase δ‐TAGS Thermoelectrics Over the Entire Mid‐Temperature Range

Abstract GeTe‐based pseudo‐binary (GeTe)_x(AgSbTe₂)_100−x (TAGS–x) is recognized as a promising p‐type mid‐temperature thermoelectric material with outstanding thermoelectric performance; nevertheless, its intrinsic structural transition and metastable microstructure (due to Ag/Sb/Ge localization) restrict the long‐time application of TAGS‐x in practical thermoelectric devices. In this work, a series of non‐stoichiometric (GeTe)_x(Ag_1‐δSb_1+δTe_2+δ)_100−x (x = 85∼50; δ = ≈0.20–0.23), referred to as δ‐TAGS‐x, with all cubic phase over the entire testing temperature range (300‐773 K), is synthesized. Through optimization of crystal symmetry and microstructure, a state‐of‐the‐art ZT_max of 1.86 at 673 K and average ZT_avg of 1.43 at ≈323–773 K are realized in δ‐TAGS‐75 (δ = 0.21), which is the highest value among all reported cubic‐phase GeTe‐based thermoelectric systems so far. As compared with stoichiometric TAGS‐x, the remarkable thermoelectric achieved in cubic δ‐TAGS‐x can be attributed to the alleviation of highly (electrical and thermal) resistive grain boundary Ag₈GeTe₆ phase. Moreover, δ‐TAGS‐x exhibits much better mechanical properties than stoichiometric TAGS‐x, together with the outstanding thermoelectric performance, leading to a robust single‐leg thermoelectric module with η_max of ≈10.2% and P_max of ≈0.191 W. The finding in this work indicates the great application potential of non‐stoichiometric δ‐TAGS‐x in the field of mid‐temperature waste heat harvesting.