Variation of chemical bonding nature of layered LiMnO 2 upon delithiation/relithiation and Cr substitution

Abstract

X-ray absorption spectroscopic (XAS) and micro-Raman analyses have been performed to investigate the effect of delithiation/relithiation reaction on the electronic and crystal structures of layered LiMnO 2. From the spectroscopic results presented here, it becomes clear that the local structure around manganese in LiMnO 2 is changed from a layered α-NaFeO 2-type structure to a spinel-like one upon Li deintercalation–intercalation reaction. Such results highlight the lattice instability of layered manganese oxide for the extraction of interlayer lithium, which is responsible for the remarkable capacity fading. On the basis of the present experimental findings, we have prepared the chromium-substituted LiMn 1− x Cr x O 2 (0≤ x≤0.15) oxides to improve the electrochemical performance of layered lithium manganate. As expected, the electrochemical measurements clarify that the Cr-substituted compounds possess better electrochemical properties than the pristine LiMnO 2. According to the XAS and micro-Raman studies for these compounds, it is found that the trivalent chromium ions are fixed to the octahedral site of MnO 2 layer before and after electrochemical charge–discharge process, and that the manganese ion can be stabilized through the shortening of (MnO) bond distances. In this regard, the improved electrochemical performance of LiMn 1− x Cr x O 2 is surely attributed to the stabilization of layered lithium manganate lattice upon replacement of Mn with Cr.