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Energies 2020, 13, 5729 6 of 12 Energies 2020, 13, x FOR PEER REVIEW 6 of 12 The above results clearly indicate that the charge capacity of the first cycle mainly originates The above results clearly indicate that the charge capacity of the first cycle mainly originates from the oxidation of oxygen. In order to understand the contributions of oxygen and manganese from the oxidation of oxygen. In order to understand the contributions of oxygen and manganese reduction to the discharge capacity, controllable charge/discharge profiles were obtained by controlling reduction to the discharge capacity, controllable charge/discharge profiles were obtained by the charging time [30]. As shown in Figure 4a, with increasing the charging time from 5 h to 20 h, controlling the charging time [30]. As shown in Figure 4a, with increasing the charging time from 5 the charge capacity continuously increases from 42 mAh/g to 170 mAh/g accompanied by the growth h to 20 h, the charge capacity continuously increases from 42 mAh/g to 170 mAh/g accompanied by of the discharge capacity above 2 V. By setting specific cut-off voltages to exactly match the discharge the growth of the discharge capacity above 2 V. By setting specific cut-off voltages to exactly match capacity with the charge capacity, it is found that these cut-off voltages vary around a small range the discharge capacity with the charge capacity, it is found that these cut-off voltages vary around a (1.96 to 2.17 V), which may result from irreversible oxygen activity and structural distortion during small range (1.96 to 2.17 V), which may result from irreversible oxygen activity and structural the charge process. This part of the released oxygen cannot be reduced during discharge, so certain distortion during the charge process. This part of the released oxygen cannot be reduced during manganese is required to participate in the charge compensation mechanism to provide capacity, discharge, so certain manganese is required to participate in the charge compensation mechanism to causing a slight decrease in the low cut-off voltage. Similar phenomenon has also been observed for provide capacity, causing a slight decrease in the low cut-off voltage. Similar phenomenon has also the electrode material Na2/3Mg1/3Mn2/3O2 [30]. Interestingly, in addition to the growth of the discharge been observed for the electrode material Na2/3Mg1/3Mn2/3O2 [30]. Interestingly, in addition to the capacity above 2 V, with the increase of charging time, the dQ/dV curve of the corresponding discharge growth of the discharge capacity above 2 V, with the increase of charging time, the dQ/dV curve of curve (Figure 4b and Figure S1 in Supplementary Materials) also shows a continuous increase of the the corresponding discharge curve (Figure 4b and Figure S1 in Supplementary Materials) also shows oxygen reduction peak at ~2.75 V, explicitly revealing that the discharge capacity above 2 V is mainly a continuous increase of the oxygen reduction peak at ~2.75 V, explicitly revealing that the discharge provided by oxygen reduction. capacity above 2 V is mainly provided by oxygen reduction. Figure 4. (a) The controllable charge/discharge curve as a function of charging time. (b) The dQ/dV Figure 4. (a) The controllable charge/discharge curve as a function of charging time. (b) The dQ/dV curves for the corresponding discharge curves. curves for the corresponding discharge curves. To further investigate the charge compensation mechanism of P2-type NaMMO upon To further investigate the charge compensation mechanism of P2-type NaMMO upon sodiation/desodiation, we used soft X-ray absorption spectroscopy (sXAS) for NaMMO electrodes sodiation/desodiation, we used soft X-ray absorption spectroscopy (sXAS) for NaMMO electrodes at at different states of charge (SOCs) using the surface-sensitive total electron yield (TEY) mode and different states of charge (SOCs) using the surface-sensitive total electron yield (TEY) mode and bulk- bulk-sensitive total fluorescence yield (TFY) mode. sensitive total fluorescence yield (TFY) mode. Mn L-edge sXAS spectra are first measured to elucidate the valence state evolution of manganese Mn L-edge sXAS spectra are first measured to elucidate the valence state evolution of manganese as a function of SOCs. Note that TM K-edge hard XAS (hXAS) has been widely applied to electrode as a function of SOCs. Note that TM K-edge hard XAS (hXAS) has been widely applied to electrode materials for exploring the oxidation states of different metal elements. However, the K-edge materials for exploring the oxidation states of different metal elements. However, the K-edge corresponds to the 1s-np transitions, which cannot effectively fingerprint the TM valence states corresponds to the 1s-np transitions, which cannot effectively fingerprint the TM valence states (i.e., (i.e., d states). In contrast, because of the dipole-allowed excitations from 2p to nd states, TM L-edge d states). In contrast, because of the dipole-allowed excitations from 2p to nd states, TM L-edge sXAS sXAS can directly probe the empty d orbitals, which are extremely sensitive to the formal valence can directly probe the empty d orbitals, which are extremely sensitive to the formal valence states of Figure 5a shows the Mn L-edge sXAS of NaMMO at different SOCs of the first cycle. In general, Figure 5a shows the Mn L-edge sXAS of NaMMO at different SOCs of the first cycle. In general, there are mainly two features located at 638–643 eV and 649–657 eV, which are assigned to the L3 there are mainly two features located at 638–643 eV and 649–657 eV, which are assigned to the L3 (Mn (Mn 2p3/2 to Mn 3d transition) and L2 edges (Mn 2p1/2 to Mn 3d transition), respectively. Note that 2p3/2 to Mn 3d transition) and L2 edges (Mn 2p1/2 to Mn 3d transition), respectively. Note that the 2p1/2 the 2p1/2 core hole has a shorter lifetime, leading to a broad feature for L2 edges [34], and therefore, core hole has a shorter lifetime, leading to a broad feature for L2 edges [34], and therefore, we will states of TMs [31–33]. TMs [31–33]. we will mainly focus on the Mn L3-edge features in the following discussions. mainly focus on the Mn L3-edge features in the following discussions.PDF Image | Cathode Materials for Advanced Sodium-Ion Batteries
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