New Layered Cathode Compounds with High Energy Density for Sodium Ion Batteries

We report a new layered Na(Mn_0.25Fe_0.25Co_0.25Ni_0.25)O₂ 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 up to 4.5V. In situ X-ray diffraction (XRD) shows a reversible structure evolution of O3-P3-O’3-O’’3 upon Na de-intercalation. The atomic scale mechanism for its higher capacity is systematically studied by a combination of DFT simulation, TEM and in situ XRD experiments, through the comparison with all the previously reported layered cathode compounds. We further design new compound based on this understanding that pushes the initial discharge energy density beyond 600 Wh/kg.