On the dual effect of Mg doping in LiCoO2 and Li1+δCoO2:: Structural, electronic properties, and 7Li MAS NMR studies

HT-Lix0Co1-yMgyO2 (where x(0) is the Li/(Co + Mg) ratio; x(0) = 0.98, 1.0, 1.10; y = 0.0, 0.03, 0.06, and 0.10) materials were synthesized via a solid-state reaction. These samples were characterized by X-ray diffraction, Li-7 MAS NMR spectroscopy, and electrical properties measurements. The XRD study showed that pure phases are obtained for 0.0 less than or equal to y less than or equal to 0.10 and x(0) greater than or equal to 1.0. Li-7 MAS NMR spectra of the Mg-doped phases exhibit two types of new signals at 55 ppm and 325, 7, -9, and -27 ppm in addition to the signal at 0 ppm resulting from the presence of diamagnetic Co-III ions. On the basis of our general knowledge of Li NMR in layered oxides with electron spins, we suggest that Mg doping in LiCoO2 always leads to the simultaneous presence of Co-IV ions (sharing an itinerant electron hole with neighboring CoIII ions) and, to a smaller extent, of intermediate spin Co3+(Is) ions trapped in a square-based pyramidal environment because of an oxygen vacancy. This feature is further enhanced by lithium overstoichiometry.