Effects of chromium substitution on the chemical bonding nature and electrochemical performance of layered lithium manganese oxide

Chromium-substituted LiMn_1-xCr^Ch (0 ≤ x ≤ 0.15) oxides have been prepared by the ion-exchange reaction between α-NaMn_1-xCr_xO₂ 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 LiMnO₂. The effects of chromium substitution on the chemical bonding nature of LiMn_1-xCr_xO₂ 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)O₂ 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-xCr_xO₂ 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.