Study of the local structure of LiNi0.33+δMn0.33+δCo0.33−2δO2 (0.025 ≤ δ ≤ 0.075) oxides

Abstract

A series of oxides of the composition LiNiy+δMny+δCoy−2δO2 (y=0.33; δ=0.025, 0.05, 0.075) were synthesized using the sol–gel method assisted by citric acid as chelating agent. The resulting nano-sized particles crystallize with a layered structure in the rhombohedral system. They exhibit an ordered rock-salt α-NaFeO2-type structure (R3¯m space group) having an interlayer spacing close to 0.47nm. Long- and short-range structural properties were characterized using several techniques such as X-ray diffraction (XRD), Raman scattering (RS), Fourier transform infrared (FTIR) and 7Li magic angle spinning (MAS) NMR spectroscopy, high-resolution transmission electron microscopy (HRTEM) and energy dispersive analysis of X-rays (EDAX). Rietveld refinements of the XRD diagrams and NMR resonance integration show that the concentration of Ni2+ ions located onto Li-3b sites is lower than 3% for the three samples. HRTEM images show layered structure without dislocations. The role of the cobalt is investigated quantitatively. The results show that the increasing cobalt content induces better crystallinity, smaller cation mixing, and a change of particles morphology, from octahedron- to spherical-like shape with size in the range 200–350nm.