Lithium-7 NMR Studies of Li1−xCoO2 Battery Cathodes

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Abstract
Lithium-deficient cathode materials Li1-xCoO2 where x = 0.1, 0.4 and 0.6 were prepared electrochemically from the stoichiometric parent compound (x = 0.0).The materials were observed to be air-stable, and x-ray diffraction characterization yielded good agreement with the in situ studies of Dahn and co-workers, regarding changes in lattice parameters. In addition to both static and magic angle spinning (MAS) 7Li NMR, measurements, the samples were investigated by EPR and cobalt K-edge NEXAFS. The removal of Li is accompanied by compensating electrons from the Co d-orbitals, asevidenced by both shifts in the NEXAFS peak and the observation of EPR signals due to spins localized on the Co ions. These spins, in turn, result in dramatic 7Li chemical shifts (89 ppm for x = 0.6) and line broadening. Whereas MAS analysis of Li0.9CoO2 indicates two magnetically inequivalent Li sites, the spectra becometoo broad to resolve different sites for higher values of x. Finally NMR linewidth and spinlattice relaxation measurements as a function of temperature suggest a modest increase in Li+ ion mobility for Li-deficient samples as compared to the parent compound.