Thorough XPS analyses on overlithiated manganese spinel cycled around the 3V plateau

Lithium-rich spinel Li1.2Mn1.8O4 thin film electrodes operated at 3 V/Li+/Li are studied by means of X-ray photoelectron spectroscopy (XPS), mainly on the basis of the evolution of the Mn2p XPS peak during the electrode cycling. The analysis of this core peak has long been debated in literature given its complex character. Based on manganese oxide references, MnO (Mn2+), Mn2O3(Mn3+) and Li2MnO3(Mn4+), we propose a deconvolution method to identify each Mn oxidation state. This method is then used for the deconvolution of Mn2p XPS peaks of bulk lithium-rich spinels Li1+xMn2-xO4 (0 ≤ x ≤ 0.25) for validation before proceeding to the study of cycled Li1.2Mn1.8O4 thin film electrodes. Electrochemical measurements exhibit significant capacity loss during the first cycle. Based on XPS analyses, this phenomenon could be explained by mechanical breakup of parts of the electrode. A stable behavior during subsequent cycles is then observed. The presence of Mn2+ species (XPS) at the most top surface of the electrode and the significant polarization observed during the discharge illustrate the kinetical limitation of the two-phase reaction, despite the reduced thickness of the electrode material.