Sodium-ion batteries (SIBs) are establishing themselves as a low-cost alternative to the widespread lithium-ion technology, a trend that is exemplified by the use of aluminium as anode current collector. In order to be in line with this philosophy, environmentally friendly, abundant and cheap materials need to be used in order to provide a complementary rather than competing battery technology other than lithium-ion. With the same scope in mind, herein we present the structural and electrochemical characterization of P2-type NaxMg0.11Mn0.89O2 material to demonstrate the effectiveness of Mg-doping for the development of future layered cathode materials. Of particular interest is the effect on the long-term cyclability (200 cycles), which has not been reported, yet. As shown in the manuscript, a Mg content as low as 11% in the MO2 layer leads to a smoothing of the potential profile, very high coulombic efficiencies exceeding 99.5% at 12mAg−1 and a stable long-term cycling behaviour.