7Li NMR studies of chemically-delithiated Li1-xCoO2

Li-7 magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy and Rietveld analysis of X-ray diffraction data have been used to study chemically delithiated Lil-xCoO2. Samples of LiCoO2 were prepared at 400 (LT), 600 (MT), and 800 degreesC (HT), chemically delithitated to Li0.5CoO2 composition, and heated at 200 degreesC. It was found that all HT materials and the as-prepared MT-Li0.5CoO2 displayed layered structure, whereas all LT materials and the 200 degreesC heated MT- Li0.5CoO2 displayed spinel structure. The NMR results suggest that the local atomic and electronic structures of the as-prepared MT-Li0.5CoO2 approach that of spinel phase although X-ray refinement results show the rhombohedral layer structure. The Li-7 MAS NMR results provide evidence for electronic phase segregation in layered and spinel Lil-xCoO2 materials. Several samples showed coexisting NMR peaks arising from lithium in a diamagnetic Co environment and in a paramagnetic mixed-valence Co3+/4+ environment. Li0.5CoO2 samples derived from LiCoO2 fired at 600 degreesC and 800 degreesC gave rise to only one NMR peak, associated with a mixed-valence cobalt environment, and produced a low NMR signal intensity, arising from an environment containing localized t(2g), holes (Co ions). A large NMR shift was observed for lithium in the mixed-valence environment, attributed to a Knight shift for the as-prepared HT-Li0.5CoO2 and to a hyperfine shift in the other samples, Variable-field studies showed homonuclear dipolar coupling to be the dominant source of residual line broadening in Lil-xCoO2, and chemical shift dispersion to be a probable secondary source.