Crystal structure and spectroscopic properties of LixNi1-yTiyO2 and their electrochemical behavior

The LixNi1-yTiyO2 (0.1 less than or equal to y less than or equal to 0.5) solid solution has been prepared by solid state reaction. From the X-ray diffraction analysis, these materials were found to have rhombohedral layered structure and an isostructure of LiNiO2. The partial disordering between transition metal (Ni and Ti) layer and lithium was detected by the Rietveld refinement. According to the X-ray photoelectron spectroscopic studies, the titanium and nickel in the lattice was found to be trivalent and bivalent, respectively. From the Ni K-edge X-ray absorption spectroscopic analysis, the presence of the mixed Valence state of Ni(II) and Ni(III) was observed, and the increase of the coordination number of long Ni-O bond from 2 (for LiNiO2) to 3 was determined, indicating the reduction of ca. 30% Ni(III) to Ni(II), By considering the ionic radius and the Ni-O bond length, the Ni(II) ions are concluded to be partially stabilized in the lithium site. Li/LiAsF6 in EC, DEC/LixNi1-yTiyO2 (y = 0.1, 0.3) cell showed far larger charging rates (Delta x = 0.25 similar to 0.4) than discharging one in a high charge cut-off voltage of 4.5 V.