Origin of cycling stability in monoclinic- and orthorhombic-phase lithium manganese oxide cathodes

High-resolution electron microscopy of electrochemically cycled LiMnO2 and LiAlyMn1-yO2 shows that both the orthorhombic and monoclinic (layered) poly morphs form antiphase domain microstructures of 5-20 nm size upon transform ing to cubic spinel. During the subsequent cubic --> tetragonal transformat ion, the antiphase domains become ferroelastic domains that are able to acc ommodate the large Jahn-Teller induced strain. Electron diffraction pattern s show that much cubic spinel is retained at high overall lithiation. A par tially inverse structure with some Mn occupying 8a tetrahedral sites is lik ely. These features of the transformed spinels provide for improved capacit y retention upon cycling compared to conventional LiMn2O4. (C) 1999 The Ele ctrochemical Society. S1099-0062(99)05-060-9-X. All rights reserved.