Ni-Mn based layered cathode materials have attracted much attention in both LIBs and SIBs due to the high operating voltage of the Ni redox and its multiple electron couples (Ni 2+ →Ni 3+ →Ni 4+ ). In these materials, Ni is not only an active element that provides capacity, but it can also trigger oxygen redox when the voltage is above 4.1 V. This has been found for a variety of different Ni/Mn ratios, e.g. compositions of Na0.78Ni0.23Mn0.69O2, Na0.67Ni0.2Mn0.8O2, Na0.5Ni0.25Mn0.75O2, and Na0.67Ni0.33Mn0.67O2. Among these materials, Na0.67Ni0.33Mn0.67O2 is one of the most common compositions, and it is known to have a P2 phase in SIBs (following Delmas' notation). Although oxygen redox in the high voltage region is known to initially improve the storage capacity in layered oxides, it ultimately can result in capacity fade due to a combination of oxygen loss and cation migration. Another process that causes capacity fade is the phase transition from P2 to O2 which leads to significant volume changes and has been reported to start when exceeding approximately 4.1 V. Therefore, finding a balance between stability and high capacity is required for future battery applications.