decreases the oxygen activity for P2-Na0.67Ni0.33Mn0.67O2. This can be considered contradictory to other reports on Mn-based layered oxides such as P2-Na0.67Mg0.28Mn0.72O2 that have shown that Mg triggers oxygen redox. To further understand the role of Mg in suppressing the oxygen redox activity, a series of materials with different Mg contents (5% to 20%, replacing Ni) were synthesized and investigated by XRD, RIXS, EXAFS, and PDF analysis. The XRD patterns in Figure indicate that all materials have a P2 layered structure with a space group of P63/mmc, but they exhibit different electrochemical behavior, especially in the high voltage region (grey region in Figure ). With increasing Mg-content, the charge/discharge curves become smoother and the corresponding capacity was found to first decrease before increasing again. It was also found that this resulted in increased long-term cycling stability for the 10% Mg-doped sample relative to the other Mg doping concentrations, as shown in Figure . The high voltage plateau gradually becomes sloping and then a new flat high voltage plateau reappears as the doping content reaches 20%. The corresponding dQ/dV curves in Figure show that the intensity of the peak in the high voltage region also gradually decreases, followed by the appearance of a new and high intensity peak with higher potential.