New insight into the atomic structure of electrochemically delithiated O3-Li_{(1- x)}CoO₂ (0 ≤ x ≤ 0.5) nanoparticles

Direct observation of delithiated structures of LiCoO₂ at atomic scale has been achieved using spherical aberration-corrected scanning transmission electron microscopy (STEM) with high-angle annular-dark-field (HAADF) and annular-bright-field (ABF) techniques. The ordered Li, Co, and O columns for LiCoO₂ nanoparticles are clearly identified in ABF micrographs. Upon the Li ions extraction from LiCoO₂, the Co-contained (003) planes distort from the bulk to the surface region and the c-axis is expanded significantly. Ordering of lithium ions and lithium vacancies has been observed directly and explained by first-principles simulation. On the basis of HAADF micrographs, it is found that the phase irreversibly changes from O3-type in pristine LiCoO₂ to O1-type Li_xCoO ₂ (x ≈ 0.50) after the first electrochemical Li extraction and back to O2-type Li_xCoO₂ (x ≈ 0.93) rather than to O3-stacking after the first electrochemical lithiation. This is the first report of finding O2-Li_xCoO₂ in the phase diagram of O3-LiCoO₂, through which the two previously separated LiCoO ₂ phases, i.e. O2 and O3 systems, are connected. These new investigations shed new insight into the lithium storage mechanism in this important cathode material for Li-ion batteries.