Layered transition metal oxides (Na x TMO 2 , where TM is a transition metal and x indicates the amount of Na per formula unit) are one of the most important classes of cathode materials for SIBs due to their compositional variety and their ability to provide high redox potentials combined with high capacities. In addition, previous studies have suggested that oxygen may also be actively involved in the electrochemical reaction between many layered oxides at high voltages, further improving the practical electrode capacity. Among the layered oxides, Ni-containing layered oxides for SIBs have attracted significant attention due to their high operating voltages and large capacities. In particular, NaNi 0.5 Mn 0.5 O 2 , referred to as the O3 phase according to the Delmas' notation, is attractive since it has a specific capacity of more than 200 mAh g -1 at a cut off voltage above 4.2 V. However, when charged to high voltages (deep desodiation), complex phase transitions occur that include a sudden reduction in the c lattice parameter of more than 20%. This behavior has been correlated with accelerated degradation of the material. To mitigate the degradation of O3-NaNi 0.5 Mn 0.5 O 2 at high voltages, many cations have been used as substitutional elements for Ni, including Li + , Mg 2+ , Al 3+ , Zn 2+ , Zr 4+ , Sn 4+ , etc. (with typical amounts of 2-10% with respect to the total amount of transition metals per formula unit). Interestingly, two distinctly different behaviors have been reported at high voltage. Substitution with Mg 2+ , Zn 2+ , Cu 2+ , and Li + can suppress the high voltage plateau and induce a sloping potential profile for the material. Meanwhile doping with Zr 4+ and Sn 4+ have been observed to preserve the high voltage plateau of the material. It is worth noting that sloping potential profiles are beneficial because they increase the average voltage compared to potential profiles showing similar capacities but based on plateaus, since the sloping profile demonstrates an increase in the energy density of a cell (see Figure ). A sloping potential profile in these types of materials has typically been explained as the suppression of the O3 to P3 phase transition above 4.0 V. However, the situation is more complex as there is always a combined effect of the dopant on the redox behavior and on the structural evolution. Moreover, the behavior can also change depending on the amount of dopant that is added, as recently shown for Mg doping in P2-Na 0.67 Ni 0.33 Mn 0.67 O 2 . Therefore, there is a great need to better understand the role of dopants on layered oxides in the high voltage region with the aim of mitigating failure mechanisms.