title: 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 0.4Li2MnO3-0.6LiNi1/3Co1/3Mn1/3O2(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.
authors.affiliation: Department of Nano-Mechanics, Korea Institute of Machinery & Materials (KIMM), 156, Gajeongbuk-ro, Yuseong-gu, Daejeon, 34103, Republic of Korea.
authors.forename: Hyung Cheoul
authors.initials: HC
authors.lastname: Shim
authors.affiliation: Energy Material Lab., Material Research Center, Samsung Advanced Institute of Technology, Samsung Electronics, Co., Ltd., 130, Samsung-ro, Yeongtong-gu, Suwon-si, Gyeonggi-do 16678, Republic of Korea. taehwan.yu@samsung.com.
authors.forename: Donghan
authors.initials: D
authors.lastname: Kim
authors.affiliation: Energy Material Lab., Material Research Center, Samsung Advanced Institute of Technology, Samsung Electronics, Co., Ltd., 130, Samsung-ro, Yeongtong-gu, Suwon-si, Gyeonggi-do 16678, Republic of Korea. taehwan.yu@samsung.com.
authors.forename: Dongwook
authors.identifier: http://orcid.org/0000-0002-5660-3789
authors.initials: D
authors.lastname: Shin
authors.affiliation: Department of Nano-Mechanics, Korea Institute of Machinery & Materials (KIMM), 156, Gajeongbuk-ro, Yuseong-gu, Daejeon, 34103, Republic of Korea.
authors.forename: Seungmin
authors.initials: S
authors.lastname: Hyun
authors.affiliation: Department of Nano-Mechanics, Korea Institute of Machinery & Materials (KIMM), 156, Gajeongbuk-ro, Yuseong-gu, Daejeon, 34103, Republic of Korea.
authors.forename: Chang-Su
authors.initials: CS
authors.lastname: Woo
authors.affiliation: Energy Material Lab., Material Research Center, Samsung Advanced Institute of Technology, Samsung Electronics, Co., Ltd., 130, Samsung-ro, Yeongtong-gu, Suwon-si, Gyeonggi-do 16678, Republic of Korea. taehwan.yu@samsung.com.
authors.forename: Taehwan
authors.initials: T
authors.lastname: Yu
authors.affiliation: Advanced Analysis Center (AAC), Korea Institute of Science and Technology (KIST), 5, Hwarang-ro, 14-gil, Seongbuk-guSeoul, 02792, Republic of Korea. jpahn@kist.re.kr.
authors.forename: Jae-Pyoung
authors.initials: JP
authors.lastname: Ahn