It is worth noting that the same plateau has been reported in literature for the Ni-free compound P2-Na0.67Mg0.28Mn0.72O2, indicating that the oxygen redox may change from Niderived to Mg-derived for P2-NaM0.20NMO. The results of the RIXS measurements in distance increases with Mg doping content from 0 to 15% and then decreases again for P2-NaM0.20NMO (see Figure ). All of the above results reveal that the Mg-O bond length becomes shortened with increasing Mg doping content, which can therefore activate the oxygen redox in P2-NaM0.20NMO. This is also reasonable since the bond length is similarly decreased in P2-Na0.67Mg0.28Mn0.72O2 in the fully charged state.