Surface structure of LiMn2O4 electrodes

The chemical composition of LiMn2O4 electrode surfaces has been determined by X-ray photoelectron spectroscopy and compared with the results of X-ray diffraction studies to expose differences between bulk and surface structure in electrodes cycled or stored at room temperature. The surface of electrodes discharged to Li2Mn2O4 contains only Mn3+ although the bulk phase also contains a component of LiMn2O4 phase. On the other hand, both Mn3+ and Mn4+ are found oil the surface of an electrode delithiated to phase-pure lambda-MnO2. Electrodes cycled or stored exhibit a Mn3+-rich surface, contrary to the bulk structure. It is concluded that Li2Mn2O4 or some defect spinel-type structure with averaged Mn oxidation state below 3.5 is formed on the electrode surface. (C) 2001 The Electrochemical Society.