Na0.67Mn1-xMgxO2 (0

Earth-abundant Na-0.67[Mn1-xMgx] O-2 (0 <= x <= 0.2) cathode materials with the P2 structure have been synthesized as positive electrodes for sodium-ion batteries. Na0.67MnO2 exhibits a capacity of 175 mA h g(-1) with good capacity retention. A Mg content of 5% is sufficient to smooth the charge/ discharge profiles without affecting the capacity, whilst further increasing the Mg content improves the cycling stability, but at the expense of a lower discharge capacity (150 mA h g(-1) for Na0.67Mn0.8Mg0.2O2). It was observed that the cooling process during synthesis, as well as Mg content, have an influence on the structure.