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 superpartial 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.

Lithium (Li) metal is promising for high energy density batteries due to its low electrochemical redox potential and high specific capacity. However, the formation of dendrites and its tendency for large volume expansion during plating/stripping restrict the application of Li metal in practical scenarios. In this work, we developed reduced graphene oxide-graphitic carbon nitride (rGO-C 3 N 4 , GCN) with highly elastic and wrinkled structure as the current collector. Lithiophilic site C 3 N 4 in GCN could reduce the nucleation overpotential. In addition, this material effectively inhibited electrode expansion during cycling. At the same time, due to its high elasticity, GCN could release the stress induced by Li deposition to maintain structural integrity of the electrode. Limetal anodes with GCN exhibited small volume expansion, high Coulombic efficiency (CE) of 98.6% within 300 cycles and long cycling life of more than 1700 h. This work described and demonstrated a new approach to construct flexible current collectors for stable lithium-metal anodes.