In situ X-ray absorption spectroscopic study of Li-rich layered cathode material Li[Ni0.17Li0.2Co0.07Mn0.56]O2

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 2.0 V. 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.