P2-structure sodium manganese oxide is synthesized by a facile reduction reaction. We provide new data on the direct use of sodium manganese oxide instead of lithium analogues as a low-cost electrode for lithium/sodium hybrid ion batteries after an ion-exchange process. Ion exchange occurs in sodium manganese oxide through the electrolyte during cell stabilization, as observed by X-ray diffraction analysis. The self ion-exchanged material delivers a high discharge capacity of 195 mA h g−1 and a stable plateau at 3.0 V for 50 cycles with stable cyclability. In-situ X-ray diffraction and ex-situ X-ray absorption near edge structure studies also confirm reversible redox reactions and stable structural changes. The ex-situ inductively coupled plasma atomic emission spectroscopy analysis verifies that residual sodium ions in the structure act as pillars during cycling.