Oxygen redox in hexagonal layered Na_{: X}TMO₃ (TM = 4d elements) for high capacity Na ion batteries

Through comprehensive density functional calculations, we demonstrate oxygen's significant participation in the redox reaction in a Na excess Na_xRuO₃ cathode material. The availability of O electrons for the redox reaction originates from the local coordination environment. For high sodium content (x ≈ 2), O ions in the layered hexagonal Na₂RuO₃ compound are coordinated by four Na ions and consequently have their 2p electrons lifted closer to the Fermi level. For lower Na content (x ≈ 1), Na₁RuO₃ adopts an ilmenite type R3 structure in which O ions are coordinated by two Ru and two Na ions. In this case, O under-coordination further elevates O 2p states closer to the Fermi level. In both cases, high O electronic population near the Fermi level facilitates continuous participation of O in the redox reaction over a wide range of Na concentrations. Based on this concept, we also predict that Na₁NbO₃ with an ilmenite framework is a suitable and economical candidate for high voltage and high capacity cathodes for Na ion batteries.