Na-ion batteries have become an important area of research in the last few years. Na2/3Ni1/3Mn2/3O2 with the P2-type structure is a promising cathode in Na-ion batteries due to its high capacity and stability. However, it experiences a structural transition at high voltages (by gliding of the transition metal layers) that causes a detrimental effect on the battery life. Our studies have shown that Mg-doped Na2/3Ni1/3Mn2/3O2 materials exhibit excellent performance with capacities greater than 130 mAh g-1 that remain stable up to 20 cycles. These results contrast to the undoped materials, where a capacity loss of 43.7% is observed. We propose a 5.5 day experiment on POLARIS to deliver a model of the long-range and short-range (using PDF analysis) to understand the role of Mg on the stabilisation of these compounds and thus its performance and further optimisation.