Evidence for structural defects in non-stoichiometric HT-LiCoO 2: electrochemical, electronic properties and 7Li NMR studies

Abstract

HT-Li x 0 CoO 2 phases were synthesised via a solid state reaction with nominal Li/Co ratios x 0=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 Li 0.5CoO 2 ( x 0=1.0) due to a monoclinic distortion, the charge curves of materials with x 0=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 Li x 0 CoO 2 materials with x 0>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 Co III. A 7Li MAS NMR study of the materials obtained by electrochemical deintercalation and reintercalation of lithium from Li 1.10CoO 2 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 Co II and of a simultaneous cobalt and oxygen deficiency in samples with x 0>1.0.