Oxygen redox chemistry without excess alkali-metal ions in Na2/3[Mg0.28Mn0.72]O2

The search for improved energy-storage materials has revealed Li- and Na-rich intercalation compounds as promising high-capacity cathodes. They exhibit capacities in excess of what would be expected from alkali-ion removal/reinsertion and charge compensation by transition-metal (TM) ions. The additional capacity is provided through charge compensation by oxygen redox chemistry and some oxygen loss. It has been reported previously that oxygen redox occurs in O 2p orbitals that interact with alkali ions in the TM and alkali-ion layers (that is, oxygen redox occurs in compounds containing Li+–O(2p)–Li+ interactions). Na2/3[Mg0.28Mn0.72]O2 exhibits an excess capacity and here we show that this is caused by oxygen redox, even though Mg2+ resides in the TM layers rather than alkali-metal (AM) ions, which demonstrates that excess AM ions are not required to activate oxygen redox. We also show that, unlike the alkali-rich compounds, Na2/3[Mg0.28Mn0.72]O2 does not lose oxygen. The extraction of alkali ions from the alkali and TM layers in the alkali-rich compounds results in severely underbonded oxygen, which promotes oxygen loss, whereas Mg2+ remains in Na2/3[Mg0.28Mn0.72]O2, which stabilizes oxygen. Alkali-metal-rich compositions (for example, Li[LixM1–x]O2) are promising battery cathode materials that exhibit oxygen redox, which provides additional charge capacity. It is thought to occur in compounds containing alkali ions in the transition metal layers and featuring Li+–O(2p)–Li+ interactions; however, now it is observed in Na2/3[Mg0.28Mn0.72]O2, in which Mg2+ ions are present in the transition metal layer.