Effect of Chromium Substitution on the Local Structure and Insertion Chemistry of Spinel Lithium Manganates: Investigation by X-ray Absorption Fine Structure Spectroscopy

Abstract

Chromium substitution in lithium manganate spinels of composition LiCrxMn2-xO4 (0.2 ≤ x ≤ 1.0) is investigated with regard to lithium extraction and insertion reactions. The local structural environments and electronic states of chromium and manganese in the compounds following these reactions have been probed by X-ray absorption near-edge structure (XANES) and extended fine structure (EXAFS) spectroscopies at the Cr and Mn K edges. The data show that the replacement of d4 Mn3+ ions by d3 Cr3+ in the octahedral framework of the spinels eliminates the local disorder present in the lattice around [Mn3+O6] octahedra, which are distorted by Jahn−Teller stabilization of the electronic state. Reversible extraction and sorption of lithium, in HCl and LiOH solutions, respectively, take place with changes in the oxidation state of Mn. The Cr remains stable as the trivalent species for HCl concentrations ≤0.5 M. Additional insertion of lithium in LiCrxMn2-xO4 by reaction with butyllithium produces tetragonal Li2Crx...