7Li NMR Studies of Chemically-Delithiated Li1-xCoO2

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Journal Article

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7Li magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy and Rietveld analysis of X-ray diffraction data have been used to study chemically delithiated Li1-xCoO2. Samples of LiCoO2 were prepared at 400 (LT), 600 (MT), and 800 °C (HT), chemically delithitated to Li0.5CoO2 composition, and heated at 200 °C. It was found that all HT materials and the as-prepared MT-Li0.5CoO2 displayed layered structure, whereas all LT materials and the 200 °C 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 7Li MAS NMR results provide evidence for electronic phase segregation in layered and spinel Li1-xCoO2 materials. Several samples showed coexisting NMR peaks arising from lithium in a diamagnetic Co3+ environment and in a paramagnetic mixed-valence Co3+/4+ environment. Li0.5CoO2 samples derived from LiCoO2 fired at 600 °C and 800 °C 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 t2g holes (Co4+ 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 Li1-xCoO2, and chemical shift dispersion to be a probable secondary source


The Journal of Physical Chemistry B



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