Abstract
Layer-structured Li-Ni-based oxide has been studied extensively as the promising cathode material for lithium ion battery because of its lower cost and higher practice capacity compared with the currently used LiCoO 2 . In this study, the wide solid solutions of Mg-doped LiNi 0.6-y Mg y Co 0.25 Mn 0.15 O 2 (0 ≦ y ≦ 0.08) with the layered α-NaFeO 2 structure were synthesized by the mixing hydroxide method. The capacity retention of LiNi 0.57 Mg 0.03 Co 0.25 Mn 0.15 O 2 in both room temperature and 55 °C was increased to 93%. The oxidation state and local environment of transition metals (Ni, Co and Mn) were studied by the high energy synchrotron X-ray absorption spectroscopy (XAS). The X-ray absorption near edge structure (XANES) indicated that the initial valences were 2+/3+, 3+ and 4+ for Ni, Co, and Mn, respectively, in the pristine LiNi 0.57 Mg 0.03 Co 0.25 Mn 0.15 O 2 . The in situ XAS study with the cell charge to 5.2 V showed that the main redox reaction during delithiation was achieved by Ni (i.e. Ni 2+ /Ni 3+ → Ni 4+ ). The EXAFS data also exhibited that the bond length of Ni{single bond}O decreased drastically, while the Co{single bond}O and Mn{single bond}O distances exhibited a slight change with the decrease of Li content in the electrode. It was further revealed that all the second shell metal-metal (Ni{single bond}M, Co{single bond}M, and Mn{single bond}O) distances decreased due to the oxidation of metal ions and the contraction in a-axes of this structure. © 2009 Elsevier B.V. All rights reserved.