Observation of partial reduction of manganese in the lithium rich layered oxides, 0.4Li(2)MnO(3)-0.6LiNi(1/3)Co(1/3)Mn(1/3)O(2), during the first charge

Abstract

Lithium-rich layered oxides show promise as high-energy harvesting materials due to their large capacities. However, questions remain regarding the large irreversible loss in capacities for the first charge-discharge cycle due to oxygen removal in lattices related to layered Li2MnO3. Herein we present detailed studies on Li-rich Mn-based layered oxides of 0.4Li(2)MnO(3)-0.6LiNi(1/3)Co(1/3)Mn(1/3)O(2) (Li-rich NCM) electrochemically activated between 2.5 V and 4.3 or 4.7 V vs. Li+/Li. Electron energy loss spectroscopy (EELS) and X-ray absorption spectroscopy (XAS) revealed unusual manganese reduction after the first charge up to a high voltage of 4.7 V. Moreover, the electronic structure did not fully recover to the original pristine of Mn4+ state after discharge. Interestingly, these phenomena were not limited to a single particle, but were observed across the entire electrode. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images and electron dispersive spectra (EDS) also showed a dramatic decline in oxygen content with highly porous morphologies, associated with oxygen vacancy formation following oxidation of O2- ions to O-2. Our analysis suggests that an unstable manganese valence state with severe defects due to oxygen vacancies may lead to large irreversible capacity loss during the first charge-discharge cycle of Li-rich layered oxides.