Lithium electrochemical deintercalation from O2-LiCoO2 -: Structure and physical properties

Electrochemical deintercalation of Li from the metastable O2-LiCoO2 phase has been investigated up to the composition Li0.15CoO2. The single-phase domains that separate the voltage plateaus observed have been characterized by X-ray diffraction. The succession of phases observed upon deintercalation results from reversible sheet gliding or lithium/vacancy ordering, leading to the sequence O2, T(#)2, T(#)2', O6, O2, O2. In particular, the T(#)2 stacking, similar to the T2 phase reported by Dahn and co-workers for the Li2/3Ni1/3Mn2/3O2 phase, corresponds to oxygen ions not sitting on the three positions of a triangular lattice, hence the # character is used. It exhibits very distorted tetrahedral sites for Li. The O6 stacking exhibits two kinds of CoO6 octahedra, which might allow Co3+/Co4+ ordering in alternate sheets. The most deintercalated O2-Li0.15CoO2 phase has never been reported before. Electronic properties and Li-7 magic-angle spinning nuclear magnetic resonance show a transition to a metallic state for x< 0.94 (appearance of the T(#)2 phase with x 5 0.72). These stacking changes are proposed to result from the minimization of electrostatic repulsion, except for T(#)2' (x = 0.50), which is believed to result from a Li/vacancy ordering. (C) 2002 The Electrochemical Society.