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

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_2/3Mg_1/3Mn_2/3O₂. 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 the initial cycle, with 87% capacity sustained after 100 cycles. In Li_1.17Ni_0.21Co_0.08Mn_0.54O₂, 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 Na_2/3Mg_1/3Mn_2/3O₂ and Li_1.17Ni_0.21Co_0.08Mn_0.54O₂ systems suggests the importance of distinguishing lattice oxygen redox from other oxygen activities for clarifying its intrinsic properties. Battery cathodes based on 3d-transition-metal oxides need viable improvements in their energy density. Recent proposals of O redox have enabled conceptual possibilities, although the assessment of its reversibility remains elusive. This work reports independent and direct quantifications of the evolving O-2p and Mn-3d redox states through O-K and Mn-L mapping of resonant inelastic X-ray scattering (mRIXS). The high reversibility of the lattice O redox in Na_2/3Mg_1/3Mn_2/3O₂ (79%) and Li_1.17Ni_0.21Co_0.08Mn_0.54O₂ (76%) is revealed during the initial cycle. While Na_2/3Mg_1/3Mn_2/3O₂ displays decent O-redox capacity retention (87% after 100 cycles), the Li-rich system shows significant decay (44% after 500 cycles). We demonstrate direct quantifications of the reversibility of lattice O redox through photon-in-photon-out bulk-sensitive mRIXS. The quantification results directly show that the reversibility of lattice O redox could be very high in both Li-ion and Na-ion batteries. Reversibility of lattice oxygen redox holds the key to its practical employments, especially in 3d transition-metal compounds. Here, direct, independent, and quantitative bulk probes of both cationic and lattice anionic redox chemistry are achieved through mapping of resonant inelastic X-ray scattering. We found highly reversible lattice oxygen redox in Na_2/3Mg_1/3Mn_2/3O₂ (79%) and Li_1.17Ni_0.21Co_0.08Mn_0.54O₂ (76%) during the initial cycle, with 87% and 44% capacity retention after 100 and 500 cycles, respectively. Therefore, aside from other oxygen activities, lattice oxygen redox could be highly reversible in batteries.