Evidence for structural defects in non-stoichiometric HT-LiCoO2 : electrochemical, electronic properties and 7Li NMR studies

HT-Lix0CoO2 phases were synthesised via a solid state reaction with nominal Li/Co ratios x0=0.90, 1.0, 1.05 and 1.10. These materials were studied by X-ray and neutron diffraction, galvanostatic lithium intercalation/deintercalation and electrical measurements as well as 7Li MAS NMR spectroscopy. While the galvanostatic charge curve of the stoichiometric material shows a voltage plateau at ca. 3.93 V for 0.75≤x≤0.94 and a particular feature for Li0.5CoO2 (x0=1.0) due to a monoclinic distortion, the charge curves of materials with x0=1.05 and 1.10 do not present the voltage plateau nor the monoclinic distortion. XRD studies show the occurrence of a solid solution in the 0.60≤x≤1.10 domain. 7Li MAS NMR spectra of Lix0CoO2 materials with x0>1 exhibit three new signals at 190, −18 and −40 ppm in addition to the signal at 0 ppm resulting from the presence of diamagnetic CoIII. A 7Li MAS NMR study of the materials obtained by electrochemical deintercalation and reintercalation of lithium from Li1.10CoO2 shows that these new features do not arise from lithiated surface species or from interstitial Li+. On the basis of our general knowledge of Li NMR in comparable materials with electron spins, we suggest the presence of paramagnetic low-spin CoII and of a simultaneous cobalt and oxygen deficiency in samples with x0>1.0.