Combined effects of Ni and Li doping on the phase transitions in LixCoO2: electrochemical and 7Li nuclear magnetic resonance studies

High temp. Lix0Co1-yNiyO2 (x0=1.0, 1.10; yr=0.0, 0.03, 0.06, and 0.10) phases were synthesized by solid-state chem. Their characterization by X-ray diffraction and galvanostatic measurements shows that 3% of Ni ions substituted for Co in the LiCoO2 lattice suppress the two-phase domain, related to the semiconductor-to-metal transition, that is obsd. at the beginning of the charge process in LixCoO2. These ions, trapped in the lattice, prevent the phase sepn. On the other hand, more than 10% of Ni ions need to be substituted for Co in order to inhibit the monoclinic distortion due to a lithium/vacancy ordering in the interslab for Li0.50Co1-yNiyO2. Besides, a Li/(Ni+Co) ratio (x0) strictly higher than one in Lix0Co0.97Ni0.03O2 leads, as in the case of the unsubstituted Li1.10CoO2 phase, to the disappearance of all the phase transitions upon deintercalation. 7Li magic angle spinning NMR measurements show that NiIII ions are the only paramagnetic species in the LiCo1-yNiyO2 phases while in the overlithiated Lix0Co1-yNiyO2 (x0>1.0) phases, NiIII and intermediate spin Co3+(IS) are present. This suggests the existence of structural defects assocd. with O vacancies which are responsible for the suppression of the electronic delocalization and of the lithium/vacancy ordering upon lithium deintercalation