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 LiCr xMn2-xO4 (0.2 less than or equal to x less than or equal to 1.0) is in vestigated with regard to lithium extraction and insertion reactions. The local structural environments and electronic states of chromium an d manganese in the compounds following these reactions have been probe d by X-ray absorption near-edge structure (XANES) and extended fine st ructure (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 sorp tion of lithium, in HCl and LiOH solutions, respectively, take place w ith 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 buty llithium 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 ph ases are elongated along one axis, [Cr3+O6] octahedra retain a regular symmetry with six equal Cr-O distances.