Magnetic, electrochemical and thermoelectric properties of P 2 - Na x ( Co 7 / 8 Sb 1 / 8 ) O 2

Highlights: • Sb doped Na_xCoO₂ compounds were studied with GGA + U density functional theory. • Sb dopants reduce the Na⁺/Na electrochemical potential. • Sb⁵⁺ dopant's electrons create Co²⁺ species in Na₁CoO₂ and Na_0.75CoO₂. • Sb_Co decreases the Seebeck coefficient in Na_0.75CoO₂ and Na_0.5CoO₂. We theoretically investigated the electronic, electrochemical and magnetic properties of Sb doped Na_xCoO₂ (x = 1, 0.75 and 0.50). Sb_Co dopants adopt 5+ oxidation state in Na_xCoO₂ host lattice for all Na concentrations (x). Due to high oxidation states, Sb⁵⁺ strongly repels Na ions and therefore it decreases the Na⁺/Na electrochemical potential. The electrons introduced by Sb⁵⁺ localize on nearby Co ions creating Co²⁺ species which are absent in undoped Na_xCoO₂. Co²⁺ 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.