  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 Na2/3Mg1/3Mn2/3O2. 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 initial cycle, with 87% capacity sustained after 100 cycles. In
Li1.17Ni0.21Co0.08Mn0.54O2, 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 Na2/3Mg1/3Mn2/3O2 and
Li1.17Ni0.21Co0.08Mn0.54O2 systems suggests the importance of distinguishing
lattice-oxygen redox from other oxygen activities for clarifying its intrinsic
properties.
