Magnetic, electrochemical and thermoelectric properties of P2-Nax(Co7/8

Abstract

We theoretically investigated the electronic, electrochemical and magnetic properties of Sb doped Na x CoO 2 (x = 1, 0.75 and 0.50). Sb Co dopants adopt +5 oxidation state in Na x CoO 2 host lattice for all Na concentrations (x). Due to high oxidation states, Sb 5+ strongly repels Na ions and therefore it decreases the electrochemical potential (vs. Na/Na + ). The electrons introduced by Sb 5+ localize on nearby Co ions creating Co 2+ species which are absent in undoped Na x CoO 2 . Co 2+ ions reduce the spin entropy flow decreasing the Seebeck coefficient in the Sb doped compounds. The results can be generalized to other dopants with high oxidation state.

Suppression of magnetism and Seebeck effect in Na0.875CoO2 induced by SbCo dopants

“…The main findings of this investigation can be summarised as follow: (1) Sb Co is the most stable configuration Na 0.875 CoO 2 as it is in Na 1 CoO 2 as well. This trend is similar to the Sb dopant in other sodium cobaltates with smaller Na concentrations [26,27]. Moreover, as x increases the margin of the stabilisation of Sb Co configuration increases against those other configurations in which Sb is located in the Na layer; (2) the formation energy of Sb Co decreases with decreasing x-E f (Na 1 CoO 2 :Sb Co ) = 1.356 eV and E f-(Na 0.875 CoO 2 :Sb Co ) = 1.175 eV.…”

“…This is in contrast to the pristine Na 0.875 CoO 2 compounds in which the Co 4+ ions were antiferromagnetically aligned across the CoO 2 planes [56,57]. Moreover, we have earlier shown that in Na 1 CoO 2 and Na 0.75 CoO 2 , Sb Co dopant also adopts the higher oxidation state [27].…”

“…Our choice of Sb dopant was motivated by the successful synthesis of Sbdoped layered metal oxide compounds such as LiMn 2 O 4 / LiSbO 3 [23], Na 3 Ni 2 SbO 6 [24] and Na 3-x Sn 2-x Sb x NaO 6 [25], all of which share considerable structural features with Na x CoO 2 . Furthermore, although the Sb-doped Na 1 CoO 2 , Na 0.75 CoO 2 and Na 0.5 CoO 2 compounds have been previously examined [26,27], Sb doping in Na 0.875 CoO 2, which is thermoelectrically more critical has not yet been explored in details.…”

High-Performance Thermoelectric Oxides Based on Spinel Structure

“…Computational screening has recently emerged as a novel tool in the discovery of brand-new thermoelectric materials. [25][26][27][28][29][30] Subsequently, through computational survey into uncharted materials' territory, one may find a desirable oxide alternative to the common thermoelectric materials, at least for those applications where exposure to elements is inevitable. More specifically, our study was motivated by the recent prediction, 31 and observation 32 of spinel oxides with high Seebeck coefficient, and high-performance thermoelectric cubic oxides.…”