6Li MAS NMR study of stoichiometric and chemically delithiated LixMn2O4 spinels

Abstract

⁶Li magic angle spinning (MAS) nuclear magnetic resonance spectroscopy has been used to study the structure of a series of LiMn₂O₄ spinel samples prepared using different annealing methods. The results show that the annealing conditions affect the number of defect sites within the structure; higher annealing temperatures leading to a spinel sample with fewer defects. Analysis of the peak intensities in the NMR spectra revealed that the structural evolution of the spinel samples with temperature is dependent upon particle size. A series of chemically delithiated samples have also been studied using a combination of ⁶Li MAS NMR, X-ray diffraction and infrared spectroscopy in order to follow the structural changes that occur upon delithiation. The NMR spectra of the delithiated samples, Li_xMn₂O₄, exhibit a gradual shift in the position of main spinel resonances (i.e. Li in 8a tetrahedral sites) with decreasing lithium content for 0.3 < x < 1.0. For x < 0.3, the delithiated samples exhibit a new ⁶Li resonance at ∼950 ppm. This resonance is assigned to lithium present in 8a sites in a Mn⁴⁺-rich phase at the centre of the spinel particles. This assignment is supported by FT-IR data and crystallite size measurements (XRD), which suggest that HMn₂O₄ forms on the surface of the particles in the heavily delithiated samples.