Rechargeable lithium batteries have risen to prominence as key devices for green and sustainable energy development. Electric vehicles, which are not equipped with an internal combustion engine, have been launched in the market. Manganese-and iron-based positive-electrode materials(1,2), such as LiMn2O4 and LiFePO4, are used in large-scale batteries for electric vehicles. Manganese and iron are abundant elements in the Earth's crust, but lithium is not. In contrast to lithium, sodium is an attractive charge carrier on the basis of elemental abundance. Recently, some layered materials(3-6), where sodium can be electrochemically and reversibly extracted/inserted, have been reported. However, their reversible capacity is typically limited to 100 mAh g(-1). Herein, we report a new electrode material, P2-Na-2/3[Fe1/2Mn1/2]O-2, that delivers 190 m Ah g(-1) of reversible capacity in the sodium cells with the electrochemically active Fe3+/Fe4+ redox. These results will contribute to the development of rechargeable batteries from the earth-abundant elements operable at room temperature.