Insights of the anionic redox in P2-Na0.67Ni0.33Mn0.67O2

The search for high-energy batteries has promoted intense attention to anionic redox in layered transition metal oxides because of their ability of delivering much higher capacity than traditional cathodes. P2-Na0.67Ni0.33Mn0.67O2 electrode exhibits outstanding air-stability and high average potential. Very recently, the anionic redox in this promising sodium cathode has been evidenced by mapping of resonant inelastic X-ray scattering. However, the origin of this oxygen redox has not been recognized yet. Here, based on the combination of X-ray absorption spectroscopy and density functional theory (DFT) calculations, we demonstrate that with the remove of Na+, the Ni2+ oxidized to Ni3+ and followed by the oxidation of lattice oxygen. Our DFT calculation further confirms that the oxygen redox in Na0.67Ni0.33Mn0.67O2 is rooted from Ni-O anti-bonding (eg*) state rather than the non-bonding O-2p band and result in the highly reversible oxygen redox reactions without O-2 loss. Moreover, at very low sodium contents, it is highly possible that a charge redistribution process between the Ni and O ions occurs, which results in the inconsistent experimental observations in previous references.