⁷Li NMR Studies of Chemically-Delithiated Li₁-xCoO₂

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 Li_1-xCoO₂. Samples of LiCoO₂ were prepared at 400 (LT), 600 (MT), and 800 °C (HT), chemically delithitated to Li_0.5CoO₂ composition, and heated at 200 °C. It was found that all HT materials and the as-prepared MT-Li_0.5CoO₂ displayed layered structure, whereas all LT materials and the 200 °C heated MT- Li_0.5CoO₂ displayed spinel structure. The NMR results suggest that the local atomic and electronic structures of the as-prepared MT-Li_0.5CoO₂ 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 Li_1-xCoO₂ materials. Several samples showed coexisting NMR peaks arising from lithium in a diamagnetic Co³⁺ environment and in a paramagnetic mixed-valence Co^3+/4+ environment. Li_0.5CoO₂ 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 t_2g 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-Li_0.5CoO₂ 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 Li_1-xCoO₂, and chemical shift dispersion to be a probable secondary source