Developing sustainable batteries based on abundant elements such as sodium and manganese is very attractive. Thus, sodium-manganese oxides can be employed as electrodes for sodium-ion batteries. Herein, an NaxMnO2-yFy electrode material is investigated and optimized. Galvanostatic cycling and diffusion coefficient calculations have been employed. It is found that tailoring the stoichiometry using the sodium/manganese ratio and fluorine content in the synthesis can improve the electrochemical performance and achieve high capacity and superb cycling stability. An anion-doping strategy (F-doping) can significantly improve electrode stability, and greatly raise the maximum specific capacity from ca. 70 mA h g-1 for an F-free sample to ca. 120 mA h g-1 for an F-doped sample at a slow rate (10 mA g-1 of current intensity). The reversible capacity of the F-doped sample is stable for many cycles (around 40-45 mA h g-1 at 500 mA g-1 for 1000 cycles).