Lithium Electrochemical Deintercalation from O2-LiCoO[sub 2]

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Abstract
Electrochemical deintercalation of Li from the metastable O2­LiCoO2O2­LiCoO2 phase has been investigated up to the composition Li0.15CoO2.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, T#2′,T#2′, O6, O2, O2. In particular, the T#2T#2 stacking, similar to the T2 phase reported by Dahn and co-workers for the Li2/3Ni1/3Mn2/3O2Li2/3Ni1/3Mn2/3O2 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 CoO6CoO6 octahedra, which might allow Co3+/Co4+Co3+/Co4+ ordering in alternate sheets. The most deintercalated O2­Li0.15CoO2O2­Li0.15CoO2 phase has never been reported before. Electronic properties and 7Li7Li magic-angle spinning nuclear magnetic resonance show a transition to a metallic state for x<0.94xT#2T#2 phase with x=0.72).x=0.72). These stacking changes are proposed to result from the minimization of electrostatic repulsion, except for T#2′T#2′ (x=0.50),(x=0.50), which is believed to result from a Li/vacancy ordering. © 2002 The Electrochemical Society. All rights reserved.