(rhombohedral) during the first charge process; these phase transitions are reversible during the following discharge process. These findings are in line with previous reports by Meanwhile, it is worth noting that the rhombohedral O3 phase at the fully charged state shows a decrease in the c lattice parameter of about 17% compared to the parent O3 phase before charging, see Figure , which is in line with previous studies by Komaba et. al. Although the phase transition from P3 to O'3 also occurs at about 4.0 V during charging for the Mg-doped material (Figure ), only a small and gradual shift in reflections can be observed during the continuous charging process (4.0 V→ 4.4 V), indicating a solid solution-type reaction in the high voltage region. During discharging, the characteristic 111 O'3 reflection for O3-NaMNMO (marked by the white dashed box in Figure ) disappears until reaching a voltage of about 3.2 V, indicating that the O'3 phase exists only up to this voltage. Upon further discharging, the new 015 P3 reflection gradually becomes stronger but the main reflection located at 2.1° shows a small shift to a lower angle (see inset in Figure ). This indicates that the Mg-doped material shows only an O'3 to P3 phase transition during the discharge process. The c lattice parameter shows only a 5.69% expansion during charging, but poor reversibility during discharging, which is in line with the lower Coulomb efficiency of the first cycle for the Mg-doped material.