Local crystal structure around manganese in new potassium-based nanocrystalline manganese oxyiodide
Résumé
A new nanocrystalline potassium-based lithium manganese oxyiodide has been prepared by using Chimie Douce route at room temperature. According to the electrochemical measurements, this nanocrystalline sample shows a large initial capacity up to 340 mAh/g at a constant current density of 0.2 mA/cm2, which is much larger than that of sodium-based homologue. The X-ray diffraction analysis demonstrates that the amorphous character of the nanocrystalline compounds is maintained before and after chemical lithiation reaction. The local crystal structure around manganese in these materials has been determined by performing the combinative micro-Raman and X-ray absorption spectroscopy. From the Mn K-edge X-ray absorption near-edge structure and micro-Raman results, it becomes certain that manganese ions are stabilized in the rhombohedral layered lattice consisting of edge-shared MnO6 octahedra, and the crystal symmetry is changed into a monoclinic symmetry upon reaction with n-BuLi. The Mn K-edge extended X-ray fine structure analysis reveals that the structural distortion caused by lithiation process is less significant for these nanocrystalline compounds than for the spinel lithium manganate. In this context, the great discharge capacity of the nanocrystalline materials is attributable for the pillaring effect of larger alkali metal ion than lithium ion, providing an expanded interlayer space available for Li insertion. In addition, the I LI-edge X-ray absorption near-edge structure results presented here make it clear that iodine is stabilized as iodate species on the grain boundary or the surface of the nanocrystalline manganese oxyiodide, which helps to maintain the nanocrystalline nature of the present materials before and after Li insertion.