Lithium intercalation into layered LiMnO2

Recently Armstrong and Bruce(1) reported a layered modification of lithium manganese oxide, LiMnO2, isostructural with LiCoO2. LiMnO2 obtained by ion exchange from alpha-NaMnO2 synthesized in air is characterized by x-ray diffraction and by electrochemical insertion and extraction of lithium in a series of voltage ranges between 1.5 and 4.5 V relative to a lithium electrode. During cycling voltage plateaus at 3.0 and 4.0 V vs. Li develop, indicating that the material is converted from its original layered structure to a spinel structure. This finding is confirmed by x-ray diffraction. Contrary to expectations based on thermodynamics, insertion of larger amounts of lithium leads to a more complete conversion. We suggest that a relatively high mobility of manganese leaves Li and Mn randomly distributed in the close-packed oxygen lattice after a deep discharge. This isotropic Mn distribution can relatively easily relax to the Mn distribution characteristic of spinels whereas the anisotropic distribution characteristic of layered structures is not reformed when excess lithium is extracted.