Variation of chemical bonding nature of layered LiMnO₂ upon delithiation/relithiation and Cr substitution

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 LiMnO₂. From the spectroscopic results presented here, it becomes clear that the local structure around manganese in LiMnO₂ is changed from a layered α-NaFeO₂-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-xCr_xO₂ (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 LiMnO₂. 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 MnO₂ 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-xCr_xO₂ is surely attributed to the stabilization of layered lithium manganate lattice upon replacement of Mn with Cr.