A P2-type Na0.67Mn0.8Cu0.1Mg0.1O2 has been synthesized as a cathode material for sodium ion batteries. By utilizing the synergetic effects of Cu and Mg substitution, the prepared material delivers a discharge capacity of 84 mAh g−1 at 180 mA g−1, with a superior capacity retention of 93% after 500 cycles. The Mn, Cu and Mg ions are located in the transition metal sites, and a good structural reversibility during the charge and discharge process has been confirmed. In addition, the electrolyte additive fluoroethylene carbonate is shown to be effective for the formation of passivation layer at the electrode/electrolyte interface and improve the long-term cycling performance of the Na0.67Mn0.8Cu0.1Mg0.1O2 cathode material using a 1 M NaPF6 in ethylene carbonate and dimethyl carbonate electrolyte.