Synchrotron-based Operando XRD contour plots for the first charge/discharge cycle are shown in Figure (waterfall XRD plots are shown in Figures and). Each material shows a unique structural evolution in the high voltage region, which can be attributed to the influence of the different dopants. It can be observed that the intensity of the main (002) reflection of P2-NaNMO gradually decreased upon charging in the high voltage region, and a new phase appears, as indicated by the appearance of a new reflection at 2.7° (2θ) (see Figure for enlarged view). This phase becomes gradually more dominant and is consistent with the previously reported O2 phase. Unlike the clear phase change exhibited by P2-NaNMO, the doped materials show more continuous structural evolution in the high voltage region, as can be seen in Figure and. The main (002) reflection of the Mg-doped material can be seen to gradually shift to higher angles until the OP4 phase is formed in the fully charged state. The OP4 phase contains layers with alternating octahedral and prismatic stacking of Na in the c-axis, leading to the presence of oxygen in four different environments.