The rising demand of clean energy in the global market is a hot topic and one of the greatest challenge in our generation. The transfer from fossil fuels to rechargeable electrochemical batteries is a first step towards a greener planet. Currently Li-ion batteries are the dominant technology in the battery market. However, the limited amount of Li as raw material leads to an increasing demand of alternative materials. Na-ion batteries are quite promising due to the abundancy of the raw materials, the similar working principle, the usage of cheaper Al as current collector and the similar electrochemical potential window of Na in comparison with Li. One of the main challenge of Na-ion batteries is to find high-performance cathode materials that can make them competitive against the Li system. Among the promising cathode candidates the layered Mn/Ni oxide family with a 3/1 ratio shows good energy densities and capacities.[1]  However, the layered P2 type oxides display limited cycling stability due to an undesired P2-O2 phase transition. According to literature, doping with different transition metals (Fe, Cu, Al,…) is an effective way to suppress this phase transition into an intermediate phase, called OP4.[2] In this work, Mg has been incorporated as a dopant into P2-Na0.67Mn0.75Ni0.25O2  synthesized via a combination of co-precipitation and solid-state synthesis.[3] Dense spherical particles have been obtained, in which Mg is homogeneously distributed within the polycrystalline particles. The successful Mg-doping has been further confirmed by electrochemical measurements, which “stabilize” the high voltage plateau. By using synchrotron operando XRD measurements light is shed on the nature of the phase transition occurring at high voltages. The analysis shows, unlike the expectations, multiple possible intermediate phases (“Z-phase”) as it is already shown for a Fe doping, rather than a pure OP4 like structure.[4] Finally, the results of the P2, O2 and OP4 phase for P2-Na0.67Mn0.75Ni0.25O2 (P2-MNO) and P2-Na0.67Mn0.75Ni0.20Mg0.05O2 (P2-MNMO) have been investigated with periodic density functional theory (DFT) calculations to investigate the stability of those phases with respect to the degree of Mg doping.