Abstract
A layered compound, NaxMn2/3Co1/6Ni1/6O2 (NMCN), prepared by a solid-state reaction, served as a high capacity positive electrode material with the reversible capacity of ca. 200 mA · h · g− 1using a Na foil as the counter electrode. A high-resolution synchrotron X-ray diffraction (XRD) analysis revealed that, whereas the NMCN sample had the P3 structure (R3m) when the precursor was heated below 700°C, the P2 structure (P63/mmc) was obtained when heated above 800°C. The latter excels over the former in both capacity and cycle stability; i.e., the initial discharge capacities and potentials of the P2 and P3-NMCN were 216 mA · h · g− 1, 2.87 V vs. Na+/Na and 206 mA · h · g− 1, 2.78 V vs. Na+/Na, respectively. The P2-MNCN retained 80% of its initial discharge capacity while the P3-NMCN dropped to 69% after 30 cycles in the potential range of 1.0–4.5 V vs. Na+/Na. An in-situ XRD analysis revealed that the lattice volume of the P2-NMCN contracts and expands by 20% during the charge and discharge (i.e., Na extraction and insertion) without drastically changing its crystallographic structure.
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