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.
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P.G.B. is indebted to the Engineering and Physical Sciences Research Council (EPSRC), including the SUPERGEN program, for financial support. We additionally thank the EPSRC for grant EP/K040375/1 for the ‘South of England Analytical Electron Microscope’. The authors thank N. Kumar, Max Planck Institute of Chemical Physics, for help with magnetic measurements. Synchrotron radiation experiments were performed at the ADRESS beamline of the Swiss Light Source at the Paul Scherrer Institute, Switzerland. We acknowledge technical and experimental support at the ADRESS beamline by L. Nue and M. Dantz. Part of this research was funded by the Swiss National Science Foundation through the Sinergia network Mott Physics Beyond the Heisenberg model and the NCCR MARVEL. The research leading to these results received funding from the European Community's Seventh Framework Programme (FP7/2007-2013) under Grant Agreement no. 290605 (CO-FUND: PSIFELLOW). The Advanced Light Source is supported by the Director, Office of Science, Office of Basic Energy Sciences, US Department of Energy, under Contract no. DE-AC02-05CH11231. The authors are also grateful to G. Cibin for contributing to the collection of hard XAS data.
The authors declare no competing financial interests.
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Maitra, U., House, R., Somerville, J. et al. Oxygen redox chemistry without excess alkali-metal ions in Na2/3[Mg0.28Mn0.72]O2. Nature Chem 10, 288–295 (2018). https://doi.org/10.1038/nchem.2923
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