High Reversibility of Lattice Oxygen Redox Quantified by Direct Bulk Probes of Both Anionic and Cationic Redox Reactions

Summary The reversibility and cyclability of anionic redox in battery electrodes hold the key to its practical employments. Here, through mapping of resonant inelastic X-ray scattering (mRIXS), we have independently quantified the evolving redox states of both cations and anions in Na 2/3 Mg 1/3 Mn 2/3 O 2 . The bulk Mn redox emerges from initial discharge and is quantified by inverse partial fluorescence yield (iPFY) from Mn- L mRIXS. Bulk and surface Mn activities likely lead to the voltage fade. O -K super-partial fluorescence yield (sPFY) analysis of mRIXS shows 79% lattice oxygen redox reversibility during the initial cycle, with 87% capacity sustained after 100 cycles. In Li 1.17 Ni 0.21 Co 0.08 Mn 0.54 O 2 , lattice oxygen redox is 76% initial-cycle reversible but with only 44% capacity retention after 500 cycles. These results unambiguously show the high reversibility of lattice oxygen redox in both Li-ion and Na-ion systems. The contrast between Na 2/3 Mg 1/3 Mn 2/3 O 2 and Li 1.17 Ni 0.21 Co 0.08 Mn 0.54 O 2 systems suggests the importance of distinguishing lattice oxygen redox from other oxygen activities for clarifying its intrinsic properties.