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B18-Core EXAFS	Oxygen redox chemistry without excess alkali-metal ions in Na2/3[Mg0.28Mn0.72]O2 Urmimala Maitra , Robert A. House , James W. Somerville , Nuria Tapia-Ruiz , Juan G. Lozano , Niccolo Guerrini , Rong Hao , Kun Luo , Liyu Jin , Miguel A. Pérez-Osorio , Felix Massel , David M. Pickup , Silvia Ramos , Xingye Lu , Daniel E. Mcnally , Alan V. Chadwick , Feliciano Giustino , Thorsten Schmitt , Laurent C. Duda , Matthew R. Roberts , Peter G. Bruce Nature Chemistry 10, 288 - 295 DOI: 10.1038/nchem.2923 Diamond Proposal Number(s): [12559] Show Abstract ▼ Abstract: The search for improved energy-storage materials has revealed Li- and Na-rich intercalation compounds to have promise as a new class of high-capacity cathodes. They exhibit capacities in excess of what would be expected from alkali-ion removal/reinsertion charge compensated by the transition-metal ions. The additional capacity is provided through charge compensation by oxygen-redox chemistry and some oxygen loss. It has been reported previously that O-redox occurs in O-2p orbitals that interact with alkali-ions in the transition-metal and alkali-ion layers (i.e. O-redox occurs in compounds containing Li+ - O2p - Li+ interactions). Na2/3[Mg0.28Mn0.72]O2 exhibits excess capacity; here we show this is also due to O-redox, despite Mg2+ residing in the transition-metal (TM) layers rather than alkali-metal ions, demonstrating that excess alkali-metal ions are not required to activate O-redox. We also show that unlike the alkali-rich compounds, Na2/3[Mg0.28Mn0.72]O2 does not lose O. Extraction of alkali ions from the alkali and TM layers in the alkali-rich compounds results in severely underbonded oxygen promoting oxygen loss, whereas Mg2+ remains in Na2/3[Mg0.28Mn0.72]O2 stabilising oxygen.	Jan 2018

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B18-Core EXAFS

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