Magnetic, electrochemical and thermoelectric properties of $P2 - Na_x(Co_{7/8}Sb_{1/8})O_2$

TL;DR

This paper theoretically explores how Sb doping affects the electronic, electrochemical, and magnetic properties of Na_xCoO_2, revealing changes in oxidation states, ion interactions, and thermoelectric behavior.

Contribution

It provides new insights into the effects of high oxidation state dopants on Na_xCoO_2's properties, including electrochemical potential and spin entropy.

Findings

Sb^{5+} ions repel Na ions, lowering electrochemical potential.

Sb doping induces Co^{2+} formation, affecting magnetic and thermoelectric properties.

Doping reduces Seebeck coefficient by decreasing spin entropy flow.

Abstract

We theoretically investigated the electronic, electrochemical and magnetic properties of Sb doped $Na_xCoO_2$ ($x = 1, 0.75$ and $0.50$). $Sb_{Co}$ dopants adopt +5 oxidation state in $Na_xCoO_2$ 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 $Na_xCoO_2$. $Co^{2+}$ 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.