Improving Electrochemical Activity of P2-type Na2/3Mn2/3Ni1/3O2 by Controlling its Crystallinity

Layered P2-type Na containing manganese-nickel oxide (Na-2/3(Mn2/3Ni1/3)O-2) materials show relatively higher electrochemical capacity. The electrochemical performance of Na-2/3(Mn2/3Ni1/3)O-2 decreases during cycling when more than 1/2 of Na was extracted owing to the structural change with large volume change and oxygen emission. We found that smaller crystallite size and introduction of the defects at transition metal site of Na-2/3(Mn2/3Ni1/3)O-2 plays a key role to improve its cycling performance. The structural analysis observations using X-ray total scattering analysis, X-ray absorption fine structure (XAFS) and transmission electron microscopy (TEM), revealed that the length and numbers of stacks of the MO2 layers were shortened and decreased, respectively, compared to that of P2-type layered Na-2/3(Mn2/3Ni1/3)O-2. This short-range ordering of the layered structure restricts the structural change during cycling resulting in mitigating large volume changes. Moreover, the metal defects at the transition metal site of the Na-2/3(Mn2/3Ni1/3)O-2 increases the contribution of the oxygen redox which confirmed by the density functional theory calculation, without showing significant overpotential and capacity fading during cycling.