Dopant incorporation site in sodium cobaltate's host lattice: a critical factor for thermoelectric performance

Abstract

NaxCoO2 that comprises alternating Na and CoO layers has exotic magnetic and thermoelectric properties that could favorably be manipulated by adding dopants or varying Na concentration. In this work, we investigated the structural and electronic properties of Sr and Sb doped NaxCoO2 (x = 0.50, 0.625, 0.75 and 0.875) through comprehensive density functional calculations. We found that Sr dopants always occupy a site in the Na layer while Sb dopants always substitute a Co ion in the host lattice regardless of Na concentration. This conclusion withstood when either generalized gradient approximation (GGA) or GGA + U method was used. By residing on the Na layer, Sr dopants create charge and mass inertia against the liquid-like Na layer and, therefore, improve the crystallinity and decrease the electrical resistivity through better carrier mobility. On the other hand, by substituting Co ions, Sb dopants reduce the electrical conductivity and therefore decrease the Seebeck coefficient.

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.…”

“…occupying a vacant Na site. For x = 0.875, Sb's formation energy has been reported earlier [26], using the standard supercell defect formation energy approach [53] and the triangular method for obtaining the chemical potentials [54]. The reported values belonged to an O rich environment in which Sb 3 O 4 and CoO are the competing phases.…”

“…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.…”

Magnetic, electrochemical and thermoelectric properties of P2-Nax(Co7/8Assadi1, Li2, Zheng3 et al. 2017Chemical Physics Letters8130View full textAdd to dashboardCite

“…Furthermore, for x = 1.00 and 0.75, the substitution of Co with Sb adds elections to antibonding Co e g states which are ∼ 2.3 eV higher than the t 2g states. In this case, Sb Co dopant is, nonetheless, still more stable than Na substituting configurations as the formation energy of the latter is generally 2.2 to 2.8 eV higher than Co substituting configurations [35].…”

“…It has been established that Sb ions are always stabilized by substituting a Co ions for a wide range of x in the Na x CoO 2 host lattice [35]. Now, we examine how Sb behave electronically in Na x CoO 2 host lattice.…”