Similar to our recent work on P2-Na 0.67 Ni 0.33 Mn 0.67 O 2 doped with Mg 2+ and Sc 3+ , these two cations were doped into O3-NaNi 0.5 Mn 0.5 O 2 to systematically study their impact on the resulting potential profiles. In this work, the dopants substituted Ni sites and had different effects in the high voltage region (above 4.0 V). Mg 2+ (0.072 nm) and Sc 3+ (0.075 nm) were chosen because they have a similar radius to Ni 2+ (0.069 nm), but Sc 3+ requires charge balancing by Mn due to its higher charge. For the first time, Sc doping is demonstrated to maintain the high voltage plateau while Mg doping is found to suppress the high voltage plateau and induce a sloping potential profile. Through synchrotron-based operando XRD and ex-situ pair distribution function (PDF) analysis, it is found that doping reduces the number and type of phase transitions. Notably, the typical formation of a hexagonal O3-phase combined with a defined high voltage plateau is prevented and instead the monoclinic distorted O'3 phase or an "X-phase" with smaller changes in the c-lattice parameter form at high voltage. Moreover, the results of resonant inelastic X-ray scattering (RIXS), X-ray absorption spectroscopy (XAS), and density functional theory (DFT) calculations reveal that both Mg and Sc doping can increase the Ni redox activity, but at the same time they both suppress the O redox activity because of the reduced Ni content. However, it is found that the 'O bound to Mg' acts as an additional redox center, which is triggered at high voltages.