Oxygen redox chemistry without excess alkali-metal ions in Na2/3_{2/3}[Mg0.28_{0.28}Mn0.72_{0.72}]O2_2

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 2$p$ orbitals that interact with alkali ions in the TM and alkali-ion layers (that is, oxygen redox occurs in compounds containing Li$^+$–O(2$p$)–Li$^+$ interactions). Na$_{2/3}$[Mg$_{0.28}$Mn$_{0.72}$]O$_2$ exhibits an excess capacity and here we show that this is caused by oxygen redox, even though Mg$^{2+}$ 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, Na$_{2/3}$[Mg$_{0.28}$Mn$_{0.72}$]O$_2$ 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 Mg$^{2+}$ remains in Na$_{2/3}$[Mg$_{0.28}$Mn$_{0.72}$]O$_2$, which stabilizes oxygen