Observation of partial reduction of manganese in the lithium rich layered oxides, 0.4Li2MnO3–0.6LiNi1/3Co1/3Mn1/3O2, 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 0.4Li₂MnO₃–0.6LiNi_1/3Co_1/3Mn_1/3O₂ (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 O₂. 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.