Narrowing of the Co-3d band related to the order–disorder phase transition in LiCoO2

The d–d transition related to the Co-3d bands in LiCoO2 films shows a drastic change by the crystal-structural transformation from layered-rock-salt (hexagonal structure, ordered-distribution of Li and Co) to modified-spinel (cubic, random-distribution) structures. In the ordered-phase, a broad optical absorbance peak with a shoulder is observed around 2.1 eV, reflecting the density of states for the conduction-band related to two Co-3d(eg) bands. In the disordered-phase, the broad peak observed in the ordered phase sharpens and the shoulder disappears, showing the narrowing of Co-eg bands attributed to the higher lattice-symmetry of the cubic structure. These characteristics reflect the partial (complete) isolations of CoO6 clusters arising from the random-distribution of Li (Co) atoms in Co-rich (Li-rich) layers.