Electron Microscopy Study of Surface Reconstruction and Its Evolution in P2-Type Na0.66Mn0.675Ni0.1625Co0.1625O2 for Sodium-Ion Batteries

We performed an aberration-corrected scanning transmission electron microscopy (STEM) and energy-dispersive X-ray spectroscopy (EDS) study of Na0.66Mn0.675Ni0.1625Co0.1625O2, which was prepared via a solid-state reaction for sodium-ion battery applications. Powder X-ray diffraction (XRD) showed that the material had a well-crystallized P2-type layered structure (P6(3)/mmc). Results from further STEM and EDS analyses showed the presence of reconstructed surface layers of thickness about 1-2 nm, which contained a large amount of antisite defects and obvious lattice distortions. Detailed chemical analysis showed an inhomogeneous elemental distribution inside these reconstructed surface layers; they were cobalt rich and nickel deficient. These surface layers further evolved into thicker regions of width 5-10 nm, accompanied by a spinel (Fd (3) over barm) phase to rocksalt phase (Fm (3) over barm) transition.