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Energies 2020, 13, 5729 7 of 12 Energies 2020, 13, x FOR PEER REVIEW 7 of 12 Figure 5. Mn L-edge sXAS (TEY mode) collected at different SOCs of the initial cycle (a) and at fully Figure 5. Mn L-edge sXAS (TEY mode) collected at different SOCs of the initial cycle (a) and at fully charged and discharged states after 2 and 10 cycles (b). charged and discharged states after 2 and 10 cycles (b). For the spectrum of pristine electrode, it exhibits an obvious Mn4+ feature with two well-defined For the spectrum of pristine electrode, it exhibits an obvious Mn4+ feature with two well-defined peaks at 640.3 and 642.8 eV, suggesting that the pristine electrode mainly contains Mn4+ as expected. peaks at 640.3 and 642.8 eV, suggesting that the pristine electrode mainly contains Mn4+ as expected. Upon charging to 4.25 V, a little hump at around 642 eV appears, representing the emergence of the Upon charging to 4.25 V, a little hump at around 642 eV appears, representing the emergence of the trace amount of Mn3+ on the electrode surface. This counterintuitive phenomenon could be related to trace amount of Mn3+ on the electrode surface. This counterintuitive phenomenon could be related to the electrolyte degrades in high voltage region, which produces a surface parasitic reaction to reduce the electrolyte degrades in high voltage region, which produces a surface parasitic reaction to reduce the Mn on the surface [35]. The spectral shape remains invariant with the discharge voltage above the Mn on the surface [35]. The spectral shape remains invariant with the discharge voltage above 3.1 3.1 V, indicating that manganese does not participate in the charge compensation during this part V, indicating that manganese does not participate in the charge compensation during this part of the of the electrochemical process, in good accordance with the above electroanalytical measurements. electrochemical process, in good accordance with the above electroanalytical measurements. With With continuous discharge to 2.5 V, a weak shoulder peak at 639.9 eV related to Mn2+ and a broad continuous discharge to 2.5 V, a weak shoulder peak at 639.9 eV related to Mn2+ and a broad hump hump at 642 eV attributed to Mn3+ emerge, implying the gradual reduction of Mn. When discharging at 642 eV attributed to Mn3+ emerge, implying the gradual reduction of Mn. When discharging to 2 to 2 V, the spectral lineshape displays a substantial difference compared with the spectrum discharged V, the spectral lineshape displays a substantial difference compared with the spectrum discharged to to 2.5 V. The intensities of the characteristic features of Mn3+ and Mn2+ largely increase accompanied 2.5 V. The intensities of the characteristic features of Mn3+ and Mn2+ largely increase accompanied by by a further decrease of the M4n+ 4+ features. At fully desodiated state (1.5 V), this trend is further a further decrease of the Mn features. At fully desodiated state (1.5 V), this trend is further 4+ 2+ strengthenedbythenearobliterationoftheMn4+ features,whilethecharacteristicfeaturesofMn2+ strengthened by the near obliteration of the Mn features, while the characteristic features of Mn 3+ 2+ 3+ andMn3+reachtheirmaxima.Obviously,theconcentrationsofM2+nand3M+naremuchhigher and Mn reach their maxima. Obviously, the concentrations of Mn and Mn are much higher than than that of the pristine electrode, which implies a dramatic change in the chemical environment and that of the pristine electrode, which implies a dramatic change in the chemical environment and structure of the electrode surface after just one cycle. The high concentration of Mn2+ 2o+n the surface at structure of the electrode surface after just one cycle. The high concentration of Mn on the surface discharged state is ascribed to the surface reaction with electrolyte and/or the surface densification at discharged state is ascribed to the surface reaction with electrolyte and/or the surface densification induced by oxygen release, as proposed previously [36]. induced by oxygen release, as proposed previously [36]. To further explore the surface evolution and the capacity fading mechanism of NaMMO electrode To further explore the surface evolution and the capacity fading mechanism of NaMMO upon extended cycles, we further record the Mn L-edge TEY spectra of the fully charged/discharged electrode upon extended cycles, we further record the Mn L-edge TEY spectra of the fully electrodes after 2 and 10 cycles, as shown in Figure 5b. The spectral shape of NaMMO electrode after charged/discharged electrodes after 2 and 10 cycles, as shown in Figure 5b. The spectral shape of second charge is identical to that of the first charge, indicative of the recovery of Mn4+. A similar NaMMO electrode after second charge is identical to that of the first charge, indicative of the recovery pheno4m+ enon is also observed for NaMMO after second discharge, indicating the high reversibility of of Mn . A similar phenomenon is also observed for NaMMO after second discharge, indicating the Mn redox for the first two cycles. Therefore, the capacity fading (Figure S2 in Supplementary Materials) high reversibility of Mn redox for the first two cycles. Therefore, the capacity fading (Figure S2 in should be mainly ascribed to the possibly irreversible oxygen activity. Interestingly, the reversibility of Supplementary Materials) should be mainly ascribed to the possibly irreversible oxygen activity. Mn redox decreases and the amount of Mn2+ on the electrode surface gradual2l+y increases after 10 cycles, Interestingly, the reversibility of Mn redox decreases and the amount of Mn on the electrode surface as shown in Figure 5b. To better visualize the reversibility of manganese redox upon extended cycles, gradually increases after 10 cycles, as shown in Figure 5b. To better visualize the reversibility of the spectral difference and spectral derivative for the 1st, 2nd and 10th cycles are shown in the Figure manganese redox upon extended cycles, the spectral difference and spectral derivative for the 1st, S3 (See Supplementary Materials), which demonstrate that the Mn redox reversibility is relatively 2nd and 10th cycles are shown in the Figure S3 (See Supplementary Materials), which demonstrate high for the first two cycles compared with that of the tenth cycle. The above data indicate that the that the Mn redox reversibility is relatively high for the first two cycles compared with that of the evolution of manganese valence may experience a main reversible cycling reaction (Mn4+ → Mn4+/3+ tenth cycle. The above data indicate that the evolution of manganese valence may experience a main ↔Mn3+/2+)followedbyaprogres4s+ivelyirr4+e/v3+ersibled3+e/2g+radationprocess(Mn4+→Mn2+,irreversible) reversible cycling reaction (Mn → Mn2+ ↔ Mn ) followed by a progressively irreversible during extended cycles. N4+ote tha2t+ the Mn on the electrode surface may dissolve from the2+cathode degradation process (Mn → Mn , irreversible) during extended cycles. Note that the Mn on the electrode surface may dissolve from the cathode and deposit on the anode, which is detrimental toPDF Image | Cathode Materials for Advanced Sodium-Ion Batteries
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