X-ray absorption spectroscopy study of the Li_xFePO₄ cathode during cycling using a novel electrochemical in situ reaction cell

The extraction and insertion of lithium in LiFePO₄ has been investigated in practical Li-ion intercalation electrodes for Li-ion batteries using Fe K-edge X-ray absorption spectroscopy (XAS). A versatile electrochemical in situ reaction cell was utilized, specifically designed for long-term X-ray experiments on battery electrodes during the lithium-extraction/insertion process in electrode materials for Li-ion batteries. The electrode contained about 7.7 mg of LiFePO₄ on a 20 µm-thick Al foil. In order to determine the charge compensation mechanism and structural perturbations occurring in the system during cycling, in situ X-ray absorption fine-structure spectroscopy (XAFS) measurements were conducted on the cell at a moderate rate using typical Li-ion battery operating voltages (3.0-4.1 V versus Li/Li⁺). XAS studies of the LiFePO₄ electrode measured at the initial state (LiFePO₄) showed iron to be in the Fe(II) state corresponding to the initial state (0.0 mAh) of the battery, whereas in the delithiated state (FePO₄) iron was found to be in the Fe(III) state corresponding to the final charged state (3 mAh) of the battery. The X-ray absorption near-edge structure (XANES) region of the XAS spectra revealed a high-spin configuration for the two states [Fe(II), d⁶ and Fe(III), d⁵]. The XAFS data analysis confirmed that the olivine structure of the LiFePO₄ and FePO₄ is retained by the electrodes, which is in agreement with the X-ray diffraction observations on these compounds. The XAFS data that were collected continuously during cycling revealed details about the response of the cathode to Li insertion and extraction. These measurements on the LiFePO₄ cathode show that the material retains good structural short-range order leading to superior cycling.