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Spectral features of MnK-edge x-ray absorption near-edge structure (XANES) for Li2MnO3 were calculated using the first-principles full projector augmented wave method with the general gradient approximation plus U method. We demonstrated that the U parameter affects the spectral features in the pre-edge region while it does not affect those in the major absorption region. From the comparison with the experimental spectra and those of reference compounds, we showed that the spectral features of MnK-edge XANES and the differences in the valence state can be reproduced well. [ABSTRACT FROM AUTHOR]

Abstract: Although Li-rich solid-solution layered materials Li2MnO3–LiMO2 (M=Co, Ni, etc.) are expected as large capacity lithium insertion cathodes, the fundamental charge–discharge reaction mechanism of these materials is not clear. Therefore the change in valence states of Ni, Co and Mn of Li[Ni0.17Li0.2Co0.07Mn0.56]O2 during charge–discharge was examined in detail using in situ X-ray absorption spectroscopy (XAS), which includes both X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) measurements. Since the Mn K edge shift during charge–discharge was not clear to determine the valence change of Mn, the Mn K pre-edge shift was examined during charge–discharge. In our measurements, only a small shift of the Mn K pre-edge toward lower energy was observed on discharge from 4.8 to 2.0V. This corresponds to a decrease of the Mn valence from 4+ to approximately 3.6+. However, this shift cannot explain the large reversible capacity of this material and thus strongly suggests the participation of oxygen in the reversible charge–discharge reaction of this material. [Copyright &y& Elsevier]

Abstract: The structural changes upon the first charge and discharge of a Li-rich layered cathode material Li[Ni0.17Li0.2Co0.07Mn0.56]O2 were investigated using high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and selected-area electron diffraction (SAED). Atomic resolution STEM observations revealed that some of the transition metal (TM) atoms were transferred from the TM layers to the Li layers upon the first charge and discharge, leading to the partial formation of a framework structure. This framework structure was considered as a spinel structure based on our simulation results of the corresponding SAED pattern of the fully charged state. This framework structure was also recognized even at the early stage of the first charging process in the plateau region around 4.5V by using the SAED patterns, indicating that the formation of this framework structure started at the same time as the electrochemical activation. [Copyright &y& Elsevier]

Abstract: Transmission electron microscopy (TEM) studies were carried out to elucidate cyclic deterioration phenomena for Li-rich layered cathode material Li[Ni0.17Li0.2Co0.07Mn0.56]O2. The results obtained show that the deterioration starts during the initial charge/discharge to higher potential over 4.5V, and leads to the formation of micro-cracks at the crystal surface and the distortion of crystal periodicity. These two kinds of defects lead to further non-crystallization of the crystal surface and the emergence of a very small amount of another possible phase. Our stepwise pre-cycling treatment effectively depressed the formation of the former two kinds of defects, and could significantly improve cyclic durability. The observation of non-crystallization at the cathode crystal surface, which would diminish the battery performance, is consistent with our preliminary ac impedance results. [Copyright &y& Elsevier]