A combination technique of in situ synchrotron X-ray absorption spectroscopy (XAS) and X-ray diffraction (XRD) was employed to study the Li1-xNi0.5Co0.25Mn0.25O2 cathode material for Li-ion battery. The Li/Li1-xNi0.5Co0.25Mn0.25O2 cell with x= 0.82 charged to 4.5 V showed the first charge capacity of 225 mAh/g. 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. The main redox reaction during delithiation was achieved by Ni via the reaction Ni2+ -> Ni3+ followed by Ni3+ -> Ni4+. The oxidation states of Co and Mn remained Co3+ and Mn4+. The bond length of Ni-O decreased drastically, while the Co-O and Mn-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-M, Co-M and Mn-O) distances decreased due to the oxidation of metal ions. In situ XRD data showed that both a- and c-axes varied with different Li contents in this material system. At the beginning of charge, there was a contraction along the c-axis and a slight expansion along the a-axis. As x reached 0.57, the trend of the variation in c-axis was opposite. The changes of lattice parameters could be explained by the balance between ionic radius and the repulsive force of the layer-structured material. (c) 2007 Elsevier Ltd. All rights reserved.