In situ x-ray absorption spectroscopic study of the Li[Ni1/3Co1/3Mn1/3]O2 cathode material -: art. no. 113523

The layered LiNi1/3Co1/3Mn1/3O2 system has recently drawn considerable interest for use as a cathode material for rechargeable lithium batteries. In order to investigate the charge-compensation mechanism and structural perturbations occurring in the system during cycling, in situ x-ray absorption spectroscopy (XAS) measurements were performed utilizing a novel electrochemical in situ cell specifically designed for long term x-ray experiments. The cell was cycled at a moderate rate through a typical Li-ion battery operating voltage range (2.9-4.7 V). The electrode contained 2.025 mg of LiNi1/3Co1/3Mn1/3O2 on a 25-mu m Al foil and had an area of 0.79 cm(2). The x-ray absorption spectroscopy (XAS) measurements were performed at the Ni, Co, and the Mn edges at different states of charge (SOC) during cycling, revealing details about the response of the cathode to Li insertion and extraction processes. Changes of bond distance and coordination number of Ni, Co, and Mn absorbers as a function of the state of charge of the material were obtained from the extended x-ray-absorption fine structure (EXAFS) region of the spectra. The x-ray absorption near-edge structure (XANES) region was studied in order to characterize the oxidation states of the 3d transition metals during cycling (Li extraction/insertion). We found that oxidation states of transition metals in LiNi1/3Co1/3Mn1/3O2 are Ni2+, Co3+, and Mn4+, whereas during charging Ni2+ is oxidized to Ni4+ through an intermediate stage of Ni3+, Co3+ is oxidized almost to Co4+ and, utilizing Faraday's calculation and XAS results, the Co was found to be at Co3.92+ at the end of the charge, while Mn was found to be electrochemically inactive and remains as Mn4+. The EXAFS data that were collected continuously during cycling revealed details about the response of the cathode to Li insertion and extraction. These measurements on the LiNi1/3Co1/3Mn1/3O2 cathode confirmed that the material retains its symmetry and good structural short-range order leading to superior cycling. (C) 2005 American Institute of Physics.