Possible high-potential ilmenite type Na1MO3 (M=V–Ni) cathodes realized by dominant oxygen redox reaction

Identifying high-voltage cathode materials is critically important for increasing the energy density of Na ion batteries. Through a comprehensive density-functional survey, we demonstrate that oxygen redox in $R\overline{3}$ (ilmenite structure) $\mathrm{N}{\mathrm{a}}_{1}M{\mathrm{O}}_{3}$ generates high operating voltage upon extraction and insertion of a Na ion. In the $R\overline{3}$ structure, O ions are undercoordinated by two $M$ and two Na ions and two vacant sites, creating unhybridized O $2p$ states with a nonbonding character that are lifted closer to the Fermi level. Since O $2p$ and $M {t}_{2g}$ states do not significantly overlap at the top of the valence band, the redox reaction is mainly borne on O ions. We also show that, in general, higher covalent bonding between the transition metal and oxygen results in higher voltage in this class of materials in which O redox is dominant. Furthermore, a thorough examination of the phase stability of $R\overline{3} {\mathrm{Na}}_{1}M{\mathrm{O}}_{3}$ compounds reveals that $\mathrm{N}{\mathrm{a}}_{1}\mathrm{V}{\mathrm{O}}_{3}$ is an economical high-voltage (5.907 V) cathode with robust cyclability for Na ion batteries. Finally, although the crystal overlap Hamilton population does not indicate any significant bonding between oxidized O ions upon desodiation in $\mathrm{N}{\mathrm{a}}_{x}M{\mathrm{O}}_{3}$ compounds, we predict that gaseous ${\mathrm{O}}_{2}$ may still develop through thermodynamic decomposition of $\mathrm{N}{\mathrm{a}}_{1}M{\mathrm{O}}_{3}$ to $\mathrm{N}{\mathrm{a}}_{1}M{\mathrm{O}}_{2}$ in some compounds.