Influence of nickel content on the chemical bonding character of LiMn2-xNixO4 spinel oxides

A relationship between the chemical bonding nature of LiMn2-xNixO4 and the Ni content has been systematically investigated. According to X-ray diffraction analysis, nickel-substituted lithium manganates crystallize with a cubic spinel structure without the formation of any impurity phase such as NiO in the present substitution range of 0 <= x <= 0.4. Micro-Raman analysis showed that Ni substitution gave rise to an increase in the intensities and energies of the two main phonon lines at similar to 580 and similar to 620 cm(-1), indicating enhancement of the average Mn oxidation states and reinforcement of the Mn-O bonds. This was confirmed by Mn K-edge X-ray absorption spectroscopic (XAS) analysis. Also, a new phonon line appeared at similar to 495 cm(-1) after the Ni substitution, which can be assigned as an Ni-O vibration mode rather than as a T-2g(2) mode of LiMn2O4. Ni K-edge XAS and micro-Raman analyses clearly demonstrate that divalent nickel ions existed in the octahedral sites of the cubic spinel lattice without a significant change in the chemical environment with the Ni content. In this regard, the poor electrochemical performance of heavily Ni-substituted compounds for the 3 V region is not attributable to the variation in the local environment of Ni ions with the Ni content, but to the low structural stability of the substituted NiO6 octahedra. (c) 2006 Elsevier B.V. All rights reserved.