The OP4 phase can be formed when full slab gliding is prohibited and is related to the reported "Z-phase" which consists of a continuously evolving intergrowth structure arising from P2 and O2 domains that go through an intermediate OP4 structure. Materials that undergo a P2 to OP4 phase transition will have a much smaller volume change than the P2 to O2 phase transition of the parent material. In the case of P2-NaS0.10NMO, no clear phase change is observed in the high voltage region, although there is a change in lattice parameters which indicate a solid solution-type reaction. While the O2 phase of P2-NaNMO and the OP4 phase of P2-NaM0.10NMO return to the P2 phase, the structural changes for all samples are not fully reversible, where the lattice parameters are found to not fully recover to their initial state at the beginning of charge. In contrast, all three samples exhibit significant differences in the behavior of the c lattice parameter. The c lattice parameters in the investigated materials decrease by 21.8% and 4.6% for P2-NaNMO and P2-NaM0.10NMO, respectively, while P2-NaS0.10NMO demonstrated a 0.3% expansion. Although P2-NaS0.10NMO shows the smallest changes in lattice parameters, it also shows the highest structural irreversibility. This is surprising at first because it indicates that it is not the structural changes that dominate cycle life in these materials, but instead the redox behavior is more important. These parameter changes are in line with the observation from pair distribution function (PDF) measurements (see Figure ). Overall, Mg is found to be the better dopant for Na0.67Ni0.33Mn0.67O2 compared Sc. While Sc-doping is more effective in minimizing the lattice shrinkage during charging, it is less effective in mitigating oxygen redox and shows only a limited structural reversibility during subsequent charging. As a consequence, cycle life is better compared to the parent material but poorer compared to the Mg-doped sample. This shows that the capacity fade is not linked to only volume and structural changes, but instead redox chemistry becomes dominant for dictating cyclability in this case.