Atomic Structure of Surface-Densified Phases in Ni-Rich Layered Compounds

In this work, we report the presence of surface-densified phases (β-Ni5O8, γ-Ni3O4, and δ-Ni7O8) in LiNiO2 (LNO)- LiNi0.8Al0.2O2 (LNA)-layered compounds by combined atomic level scanning transmission electron microscopy (STEM) energy loss spectroscopy (EELS). These surface form upon electrochemical aging at high state charge corresponding to a fully delithiated state. A unique feature these is periodic occupancy Ni2+ Li layer. This Ni gives rise extra diffraction reflections, which are qualitatively similar those LiNi2O4 spinel structure, but have lower valence cation content than spinel. experimental results confirm thermodynamically stable provide new insights into phenomena phase formation Ni-rich layered structures.