@article{27604,
  author = {Michael C Tucker and Jeffrey A Reimer and Elton J Cairns and Seungdon Choi and Arumugam Manthiram},
  title = {<sup>7</sup>Li NMR Studies of Chemically-Delithiated Li<sub>1</sub>-x</sub>CoO<sub>2</sub>},
  abstract = {<p><sup>7</sup>Li 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<sub>1</sub><sub>-</sub><em><sub>x</sub></em>CoO<sub>2</sub>. Samples of LiCoO<sub>2</sub> were prepared at 400 (LT), 600 (MT), and 800 °C (HT), chemically delithitated to Li<sub>0.5</sub>CoO<sub>2</sub> composition, and heated at 200 °C. It was found that all HT materials and the as-prepared MT-Li<sub>0.5</sub>CoO<sub>2</sub> displayed layered structure, whereas all LT materials and the 200 °C heated MT- Li<sub>0.5</sub>CoO<sub>2</sub> displayed spinel structure. The NMR results suggest that the local atomic and electronic structures of the as-prepared MT-Li<sub>0.5</sub>CoO<sub>2</sub> approach that of spinel phase although X-ray refinement results show the rhombohedral layer structure. The <sup>7</sup>Li MAS NMR results provide evidence for electronic phase segregation in layered and spinel Li<sub>1</sub><sub>-</sub><em><sub>x</sub></em>CoO<sub>2</sub> materials. Several samples showed coexisting NMR peaks arising from lithium in a diamagnetic Co<sup>3+</sup> environment and in a paramagnetic mixed-valence Co<sup>3+/4+</sup> environment. Li<sub>0.5</sub>CoO<sub>2</sub> samples derived from LiCoO<sub>2</sub> 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<sub>2g</sub> holes (Co<sup>4+</sup> 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<sub>0.5</sub>CoO<sub>2</sub> 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<sub>1</sub><sub>-</sub><em><sub>x</sub></em>CoO<sub>2</sub>, and chemical shift dispersion to be a probable secondary source</p>},
  year = {2002},
  journal = {The Journal of Physical Chemistry B},
  volume = {106},
  number = {15},
  pages = {3842-3847},
  month = {04/2002},
  doi = {10.1021/jp0133541},
  language = {eng},
}