From the above analysis, we know that both the activity. When the ratio of Mg/Ni is greater than 1 (P2-NaM0.20NMO in this study and P2-Na0.67Mg0.28Mn0.72O2 from the literature ), the Mg becomes responsible for activating oxygen redox, where the Mg-O bond length distortion becomes greater than that of Ni-O. This finding is well supported by DFT results that suggest a higher redox activity of the oxygen for the sample with the highest Mg content (Na0.22Mg0.22Ni0.13Mn0.67O2), see Figure . Due to this competing mechanism, the intensity of the oxygen redox becomes a function of the Mg content with a minimum at P2-NaMg0.15NMO, see Figure . Li, Cu, and Zn are also known to trigger oxygen redox in Ni free Mn-based layered oxides, and it is likely that these doping elements have similar concentration-dependent properties to that of Mg in Na0.67Ni0.33Mn0.67O2. P2-Na0.67(Li, Mg, Zn, Cu)0.10Ni0.23Mn0.67O2 samples were prepared with these dopants (10%), as shown in the Supporting Information (Figures and), and as predicted, the oxygen redox was also suppressed with the introduction of these dopants (see Figure ). This demonstrates that the changes in oxygen redox for doping with these elements are also suitable, and we can express this with the equation Na0.