Normally, high temperatures are required for solid-state reactions to overcome energy barriers in the formation of lithium insertion materials. Consequently, conventional high-temperature lithiation reactions are very time- and energy-consuming and often accompanied by undesirable side reactions. Thus, how to synthesize Li-containing cathode materials with a desired structure under a short reaction time and low temperature is of paramount significance. Herein, layered sodium-deficient Na2/3□1/3(Ni0.25Mn0.75)­O2 (□ for vacancy) oxides with different oxygen stackings (P2 or P3 structure) were deployed in lithium ion batteries. An interesting Li+/Na+ ion-exchange reaction between the electrode material and LiPF6-based carbonate electrolyte was observed at room temperature for the first time. Such a reaction can produce the layered Li2/3□1/3(Ni0.25Mn0.75)­O2 compounds having the O2 or O3 structure, which show the ability to reversibly accommodate lithium ions over a relatively wide voltage range. Our experiments may open up a pathway toward the development of novel electrode materials.