Hexagonal P2-structure sodium manganese-based NaLi0.2Mn0.8O2 oxide is successfully synthesized by a simple conventional solid-state reaction method. XRD analysis reveals that the NaLi0.2Mn0.8O2 material has layered structure, space group P63/mmc, and lattice parameters of a =2.8616Å, and c =11.0659Å. Lithium ions can stabilize the P2-structure of sodium manganese-based NaxMnO2 oxides even though the mole ratio of alkali metals to transition metal is as high as 1.5. The NaLi0.2Mn0.8O2 material has a high initial discharge capacity of 241mAhg−1 between 1.5–4.5V at a current density of 26.6mAg−1. The structure of the NaLi0.2Mn0.8O2 material is stable over a wide range of sodium concentrations during the charge-discharge process, and capacity retention is 78% after 40 cycles.