The compounds NaNi 0.5 Mn 0.5 O 2 (O3-NaNMO), NaMg 0.1 Ni 0.4 Mn 0.5 O 2 (O3-NaMNMO), and NaSc 0.1 Ni 0.4 Mn 0.5 O 2 (O3-NaSNMO) investigated in this study were produced by a solid-state synthesis method. X-ray diffraction (XRD) patterns of all three materials evidencing pure O3 structures and scanning electron microscopy (SEM) images of the µm-sized particles are shown in Figures and. The electrochemical properties of the three samples were evaluated in coin cells (half-cell geometry with sodium foil as the counter electrode) over a voltage range of 2.5-4.4 V at a current density of 20 mA g -1 . Assuming that all Na can be extracted from the structure, the theoretical capacities of these compounds would amount to about 240 mAh g -1 . As shown in Figures and-S4, the first discharge capacities of the Mg-and Sc-doped materials are 177 and 158 mAh g -1 , respectively, which are lower than that of the undoped material (198 mAh g -1 ), especially for the Sc-doped material. This is expected due to replacement of some of the electrochemically active Ni sites with electrochemically inactive dopants (Sc or Mg). However, the three materials exhibit significant differences in their potential profiles, especially in the high voltage region above 4.0 V, see Figure .