In summary, doping with both Mg and Sc significantly improves the cycling stability of Na0.67Ni0.33Mn0.67O2, but two different behaviors are induced in the high voltage region with the different dopants. Table provides a comparison of the electrochemical, structural, and redox behavior observed for the samples with different dopants. While the Sc-doped sample showed the smallest change in lattice parameters, it also presented the highest structural irreversibility during cycling. This demonstrates that it is the redox behavior and not the structural reversibility, as normally understood, that dominates the cycle life in these materials. Through RIXS measurements, we found that both dopants successfully decrease the degree of oxygen redox, but the suppression is strongest in the case of Mg. This result seems to contradict other studies as Mg is well-known to trigger oxygen redox for Ni-free Mnbased layered oxides. In order to determine the origin of the reduced oxygen redox in the Mg-