Structural investigations of LiFePO₄ electrodes and in situ studies by Fe X-ray absorption spectroscopy

Fe K-edge X-ray absorption near edge spectroscopy (XANES) and extended X-ray absorption fine structure (EXAFS) have been performed on electrodes containing LiFePO₄ to determine the local atomic and electronic structure and their stability with electrochemical cycling. A versatile electrochemical in situ cell has been constructed for long-term soft and hard X-ray experiments for the structural investigation on battery electrodes during the lithium-insertion/extraction processes. The device is used here for an X-ray absorption spectroscopic study of lithium insertion/extraction in a LiFePO₄ electrode, where the electrode contained about 7.7 mg of LiFePO₄ on a 20 μm thick Al-foil. Fe K-edge X-ray absorption near edge spectroscopy (XANES) and extended X-ray absorption fine structure (EXAFS) have been performed on this electrode to determine the local atomic and electronic structure and their stability with electrochemical cycling. The initial state (LiFePO₄) showed iron to be in the Fe²⁺ state corresponding to the initial state (0.0 mAh) of the cell, whereas in the delithiated state (FePO₄) iron was found to be in the Fe³⁺ state corresponding to the final charged state (3 mAh). XANES region of the XAS spectra revealed a high spin configuration for the two states (Fe (II), d⁶ and Fe (III), d⁵). The results confirm that the olivine structure of the LiFePO₄ and FePO₄ is retained by the electrodes in agreement with the XRD observations reported previously. These results confirm that LiFePO₄ cathode material retains good structural short-range order leading to superior cycling capability.

Electrochemistry: from Nanostructures to Power Plants — Selection of papers from the 55th Annual Meeting of the International Society of Electrochemistry (ISE)19-24 September 2004, Thessaloniki, Greece