Structural investigation of Li1−xNi0.5Co0.25Mn0.25O2 by in situ XAS and XRD measurements

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.