LITHIUM INTERCALATION INTO LAYERED LIMNO2

Recently Armstrong and Bruce(1) reported a layered modification of lit hium manganese oxide, LiMnO2, isostructural with LiCoO2. LiMnO2 obtain ed by ion exchange from alpha-NaMnO2 synthesized in air is characteriz ed by x-ray diffraction and by electrochemical insertion and extractio n of lithium in a series of voltage ranges between 1.5 and 4.5 V relat ive 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 ther modynamics, insertion of larger amounts of lithium leads to a more com plete conversion. We suggest that a relatively high mobility of mangan ese leaves Li and Mn randomly distributed in the close-packed oxygen l attice after a deep discharge. This isotropic Mn distribution can rela tively easily relax to the Mn distribution characteristic of spinels w hereas the anisotropic distribution characteristic of layered structur es is not reformed when excess lithium is extracted.