We report a new layered Na(Mn[subscript 0.25]Fe[subscript 0.25]Co[subscript 0.25]Ni[subscript 0.25])O[subscript 2] compound with O3 oxygen stacking. It delivers 180 mAh/g initial discharge capacity and 578 Wh/kg specific energy density with good cycling capability at high cutoff voltage. In situ X-ray diffraction (XRD) shows a reversible structure evolution of O3-P3-O3′-O3″ upon Na de-intercalation. The excellent capacity and cycling performance at high cutoff voltage make it an important model system for studying the general issue of capacity fading in layered Na cathode compounds.Samsung Advanced Institute of Technology