Mn K-edge X-ray absorption spectroscopic (XAS) analyses have been performed to probe the evolution of electronic and crystal structures of layered LiMnO2 upon chemical and electrochemical delithiation/relithiation. According to the X-ray absorption near-edge structure studies, it becomes clear that the trivalent manganese ion in LiMnO2 is significantly oxidized by acid treatment and is not fully recovered by subsequent lithiation reaction with n-BuLi. The extended X-ray absorption fine structure results presented here demonstrate that the local structure around manganese in LiMnO2 is changed from a layered α-NaFeO2-type structure to a spinel-like one upon chemical delithiation reaction. It is also found from the XAS analyses for the cycled LiMnO2 that the electrochemical charge−discharge process gives rise not only to the partial oxidation of manganese ion but also to the migration of Mn into the interlayer lithium site, resulting in the coexistence of the layered structure and the spinel one. Such results highlight the lattice instability of layered manganese oxide for the chemical and electrochemical extraction of lithium, which is responsible for the remarkable capacity fading and the formation of two plateaus at around the 3 and 4 V regions after the first electrochemical cycle. On the basis of the present experimental findings, we are now able to suggest that the electrochemical performance of layered LiMnO2 can be improved by blocking the Mn migration path through cationic substitution.

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/cm 2 , 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 MnO 6 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 L I -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.

Chromium-substituted LiMn 1-x Cr x O 2 (0 e x e 0.15) oxides have been prepared by the ion-exchange reaction between R-NaMn 1-x Cr x O 2 and LiBr. From the X-ray diffraction and infrared spectroscopic analyses, all of the present layered compounds are found to be crystallized with monoclinic structure. Additionally, the nitrogen adsorption-desorption isotherm measurements indicate a decrease in crystallite size induced by the replacement of Mn with Cr. According to the electrochemical measurements, the Cr-substituted compounds exhibit better electrochemical performance than the pristine LiMnO 2 . The effects of chromium substitution on the chemical bonding nature of LiMn 1-x Cr x O 2 have been investigated by performing X-ray absorption spectroscopic (XAS) analyses. The Cr K-edge XAS results presented here clarify that the trivalent chromium ions are stabilized in the octahedral site of the (Mn,Cr)O 2 layer before and after the electrochemical charge-discharge process. From the extended X-ray absorption fine structure analyses at the Mn K-edge, it becomes clear that the substitution of manganese with chromium gives rise to a shortening of the Mn-O bonds, leading to the stabilization of Mn in the octahedral site. On the basis of the present experimental findings, we suggest that the superior electrochemical performance of LiMn 1-x Cr x O 2 can be attributed to the enhanced stability of the layered manganese oxide lattice because of the presence of a chromium ion in the octahedral site of the transition metal oxide layer, which hinders the migration of manganese ions into the interlayer lithium sites.