Synchrotron X-Ray Absorption Study of LiFePO₄ Electrodes

LiFePO₄ was investigated in practical Li-ion intercalation electrodes used for Li-ion batteries by applying X-ray absorption spectroscopy (XAS). A special in situ XAS cell was constructed to charge and discharge the LiFePO₄ electrodes at current densities typically used in Li-ion batteries. Ex situ XAS measurements were made with technically relevant electrodes developed by Hydro-Québec power research using LiPF₆ as an electrolyte and ethylene carbonate/dimethyl carbonate as a solvent. The X-ray absorption spectra were measured at the initial state [containing mainly LiFePO₄, Fe(II)], which corresponds to the uncharged state of the battery and at the delithiated state [FePO₄, Fe(III)], which corresponds to the battery’s charged state. The X-ray absorption near-edge structure region of the X-ray absorption spectra showed a substantial Fe K-edge shift after the electrode was electrochemically oxidized. The pre-edge features of the X-ray absorption spectra revealed high spin configuration for the two oxidation states [Fe(II), d⁶ and Fe(III), d⁵]. Reasonable ligand field stabilization energies could be extracted from the pre-edge features. Using the data analysis program EXAFSPAK with the FEFF8 code provided in this program, the EXAFS spectrum and its Fourier transformed radial structure function could be simulated satisfactorily, where only single scattering paths were used for this simulation.