High Capacity Positive Electrode Material for Room Temperature Na Ion Battery: NaxMn2/3Co1/6Ni1/6O₂

A layered compound, Na x Mn 2/3 Co 1/6 Ni 1/6 O 2 (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 − 1 using 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 ( R 3 m ) when the precursor was heated below 700°C, the P2 structure ( P 6 3 / 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.