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Energies 2020, 13, 5729 9 of 12 of resonant inelastic X-ray scattering (mRIXS) is a powerful tool to explore the lattice oxygen redox by disentangling the intrinsic oxygen redox from the TM-O hybridization [41]. In mRIXS, the lattice oxygen redox can be confirmed through the emergence of a specific feature at 531 eV excitation energy and 523.7 eV emission energy [44,45]. The position of this feature lies exactly between t2g and eg features in the excitation energy coordinate system. Hence, this filled dip can indirectly indicate the presence of lattice oxygen redox during the first charge process. It should be noted that the filling of this dip may also be related to the reduction of manganese, as proposed previously [46]. However, the exact explanation for this phenomenon remains ambiguously, which will be investigated in detail for our future study. Upon subsequent discharge to 2.5 V, the integrated intensity decreases continuously, with the reappearance of the dip between t2g and eg states. In combination with the Mn L2-edge TFY spectra and the electrochemical analysis discussed above, the reappearance of the dip should be related to the reduction of oxygen in this voltage regime. The further decrease of the pre-edge intensity after discharge to 1.5 V is related to the reduction of Mn and thus the decreased hybridization strength between Mn and O, in good accordance with the downward shift of the peak centroid of Mn L2-edge. The spectral evolution of O K-edge XAS in TEY mode shows a similar trend as that in TFY mode. For the TEY results, the slope from point I to point II is higher than that of the TFY results, which is related to the surface activity at charged state. More specifically, after discharge to 2 V, the intensity ratio of t2g and eg states (t2g/eg) dramatically decreases as a consequence of electron filling into the t2g energy level. The different t2g/eg ratios between bulk and surface once again suggest that manganese is more reduced on the electrode surface compared with that in the bulk, indicating a gradient distribution of the valence state of Mn from the surface to the bulk. This phenomenon is related to the surface densification and/or the surface side reaction with organic electrolyte and the formation of oxygen vacancies on the surface of the cycled electrode due to the irreversible oxygen activity, which has been reported for the related Na-deficient SIB cathodes and Li-excess LIB cathodes [8,33,43]. The pre-edge intensity is continuously reduced after 2 and 10 cycles, further demonstrating the continuous reduction of Mn as the cycle number increases, which leads to the inferior electrochemical performance upon extended cycling. 4. Conclusions In summary, we have successfully designed and synthesized P2-type NaMMO electrode materials for advanced SIBs. The charge compensation and capacity fading mechanisms of NaMMO have been systematically investigated by a combination of sXAS and electroanalytical methods. The results provide solid evidence that the initial charge capacity is solely provided by the anion oxidation, leading to a long charge voltage plateau. While for the following discharge process, the capacity from the high voltage region is provided by anion reduction, and the capacity is mainly contributed by manganese reduction at the low voltage region. Combined with the surface-sensitive and bulk-sensitive sXAS spectra, a gradient distribution of Mn valence states from the surface to the bulk of the cycled electrodes is disclosed, which is highly associated with the irreversible oxygen activity and surface reaction with electrolyte. A decrease in the average valence of Mn upon extended cycles, which may be caused by irreversible oxygen activity, is observed, which results in gradual capacity fading. We believe that this work can provide a better understanding of cationic and anionic redox behaviors in Na-deficient layered cathodes, which may offer a promising perspective for developing high-performance P2-type cathode materials for SIBs. Supplementary Materials: The following are available online at http://www.mdpi.com/1996-1073/13/21/5729/s1, Figure S1: The dQ/dV curves for the corresponding charge/discharge curves as a function of charging time. Figure S2: charge/discharge curves for the first two cycles at C/20 between 4.5 and 1.5 V (vs. Na+/Na). Figure S3: (a) The spectral difference between Mn L-edge TEY of 1 Ch 4.5 V and that of 2 Ch 4.5 V. (b) The spectral difference between Mn L-edge TEY of 1 Ch 4.5 V and that of 10 Ch 4.5 V. (c) The spectral derivative of 1 Ch 4.5 V, 2 Ch 4.5 V and 10 Ch 4.5 V, respectively. Figure S4: (a) The Mn L-edge XAS spectra (TFY mode) collected on NaMMOPDF Image | Cathode Materials for Advanced Sodium-Ion Batteries
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