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 LixCoO2 (x approximate to 0.50) after the first electrochemical Li extraction and back to O2-type LixCoO2 (x approximate to 0.93) rather than to O3-stacking after the first electrochemical lithiation. This is the first report of finding O2-LixCoO2 in the phase diagram of O3-LiCoO2, through which the two previously separated LiCoO2 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.