shows the best stability, delivering a reversible capacity of 58 mAh g -1 after 500 cycles and 160 days. Under the same conditions, P2-NaS0.10NMO and undoped P2-NaNMO only deliver capacities of 38 and 3 mAh g -1 , respectively (see Figure , e and Figure ). Based on the excellent Na storage of P2-NaM0.10NMO, it can be concluded that suppressing the high voltage plateau is a promising strategy for enhancing the properties of P2-Na0.67Ni0.33Mn0.67O2. Oxygen redox can be triggered at high voltages, which has been shown for many layered oxides above 4.0 V. Here, we used Resonant Inelastic X-ray Scattering (RIXS) to identify differences in the oxygen redox behavior for the different compositions. Figure ) shows the results of RIXS measurements at the O K-edge for P2-NaNMO, P2-NaM0.10NMO, and P2-NaS0.10NMO at open-circuit voltage (OCV), when fully charged to 4.4 V, and when fully discharged to 2.5 V during the first cycle. The spectra for the OCV and the fully discharged state were similar for all samples. However, at the fully charged state, a new energy loss peak at 8 eV can be observed, which is consistent with the associated oxygen redox signal reported by The intensity of this peak is different among the three samples, with the highest intensity observed for P2-NaNMO followed by P2-NaS0.10NMO and P2-NaM0.10NMO. Figure highlights the region near 0 eV of the RIXS spectra of the three states for each material. A progression of sharp peaks can be observed only in the fully charged state, which are associated with vibrations arising from molecular O2 units. The distance between the first peak and 0 eV can be used to identify the oxygen species, where an energy of 0.194 eV was first reported as molecular O2 by Bruce et al. These molecular O2 units are not necessarily released from the host structure, but exist as trapped dimers within the crystal structure at the charged state. Among the three materials, the intensity of the vibration signals for molecular O2 in the P2-NaM0.10NMO material are the lowest and exhibit the fastest attenuation within the energy loss window. This is consistent with a lower intensity of the oxygen redox signal at an energy loss of 8 eV. The findings confirm that oxygen redox occurs for all samples at varying degrees and that the Mg-doped shows the smallest oxygen redox activity. The related mechanism is discussed in more detail below. In order to more clearly observe differences in the oxygen redox of the three samples, RIXS mapping was conducted on samples in the fully charged state with an excitation energy between 526 and 534 eV. The results are presented in Figure ), where it can be seen that there is a significant difference at the excitation energy between 530.5 and 531.5 eV (marked by blue dotted circles), which is where the features associated with the oxygen redox arise. The undoped material and Sc-doped material both show a bright spot in this region, but a similar feature is not found in the case of P2-NaM0.10NMO. This further supports that there is less oxygen redox in the Mg-doped material. Based on the Ni content in the three compositions, it is also expected that the intensity of the oxygen redox peak for the doped materials should be lower than that of the undoped P2-NaNMO material and also that the doped materials should demonstrate similar intensities for the oxygen redox. In contrast, according to the observed potential profiles and RIXS analysis, we find that the Mg-doped material has the lowest oxygen activity. These results are surprising, because Mg has been a popular element used to trigger oxygen redox in many Mn-based layered oxides such as Na0.67Mg0.28Mn0.72O2 and Na0.67Mg0.33Mn0.67O2, and one would therefore expect Mg doping to enhance the oxygen activity also in Ni/Mn-based layered oxides. The above analysis, however, shows that Mg-doping in Ni-Mn based cathode materials may also suppress the oxygen redox activity. The RIXS data is in line with the electrochemical data, where the Mg-doped sample showed a sloping potential profile in the high voltage region indicating a suppressed oxygen redox activity.