Hf doping for enhancing the thermoelectric performance in layered Na0.75CoO2

Abstract

Abstract Doping has been extensively used as the main strategy for improving the thermoelectric performance of sodium cobaltate (NaxCoO2). Here, by examining the formation energy of Hf dopant in Na0.75CoO2, we demonstrate that under Na and O rich environments, Hf replaces a Co while in all other conditions Hf replaces a Na. We then discuss the critical dependence of the thermoelectric performance of Na0.75CoO2:Hf on the Hf location in the host lattice by examining how the spin entropy flow is influenced by Hf doping. Our results indicate that Hf dopant at Co site increase the Seebeck coefficient to 274 μV/K while Hf dopant at Na site increases the Seebeck coefficient to 388 μV/K.