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 R3 (ilmenite structure) Na1MO3 generates high operating voltage upon extraction and insertion of a Na ion. In the R3 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 t2g 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 R3 Na1MO3 compounds reveals that Na1VO3 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 NaxMO3 compounds, we predict that gaseous O2 may still develop through thermodynamic decomposition of Na1MO3 to Na1MO2 in some compounds.
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