Cation doping, or more specifically ion substitution, is an effective and practical strategy for achieving superior properties in cathode materials, which has been widely demonstrated in both laboratory experiments and commercial products. With respect to P2-Na0.67Ni0.33Mn0.67O2, the use of Li + , Mg 2+ , Cu 2+ , and Zn 2+ as substitutional elements has been found to be effective for tailoring the material properties. Suppression of the P2 to O2 phase transition is considered to be the most important reason for the better electrochemical properties by doping with these cations. In these systems, instead of a flat voltage plateau, a sloping potential profile can be observed for voltages above 4.0 V. This behavior has also been found in many systems where Ni and Li + coexist with Mg 2+ , Cu 2+ , or Zn 2+ . The origins of the sloping electrochemical behavior are still unknown, but they may be a result of a solid-solution type storage mechanism, kinetic effects, or different electrochemical contributions, among others. Similarly, the relationship between the sloping potential profiles and the P2 to O2 phase transition have not yet been well described, making further investigations necessary.