Sodium layered oxides as cathode for Na-ion battery could exceed the energy density offered by Li-ion cathode thanks to two main advantages: (i) the activation of Fe3+/Fe4+ in Na2/3Fe1/2Mn1/2O2 and (ii) Oxygen redox without Oxygen loss in Na2/3Mn2/3Mg1/3O2. However those materials suffer from capacity/voltage fade or huge voltage hysteresis that limit their practical interest. We managed to combine those advantages while preventing their drawbacks in Na2/3(Mg,Mn,Fe)O2. The origin of such improvements is probably nested in the transition metal migration during (dis)charge. As neutron source offers a very good contrast between Fe/Mn/Mg and Na (contrarily to X-rays) and PDF is able to solve the short/medium range structure of those defected materials, neutron PDF appears to the only technique able to unambiguously reveal the transition metal migration during cycling.