Effect of chromium substitution on the local structure and insertion chemistry of spinel lithium manganates:: Investigation by X-ray absorption fine structure spectroscopy

Chromium substitution in lithium manganate spinels of composition LiCrxMn2-xO4 (0.2 less than or equal to x less than or equal to 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 d(4) Mn3+ ions by d(3) 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 less than or equal to 0.5 M. Additional insertion of lithium in LiCrxMn2-xO4 by reaction with butyllithium produces tetragonal Li2CrxMn2-xO4 spinels, but the degree of tetragonal distortion is significantly reduced by Cr substitution. The XAFS data show that while the [Mn3+O6] octahedra in the tetragonal phases are elongated along one axis, [Cr3+O6] octahedra retain a regular symmetry with six equal Cr-O distances.