Raman and FTIR Spectroscopic Study of Li[sub x]FePO[sub 4] (0≤x≤1)

Abstract

A series of compounds, LixFePO4 (0 ≤ x ≤ 1), was prepared by chemical delithiation and investigated with Fourier transform infrared (FTIR) and Raman spectroscopy for the first time. Sample homogeneity with respect to Fe and P was analyzed with an electron microprobe. The imaging capabilities of the electron microprobe reveal larger-sized particles (10-20 μm diam) and fine intergranular particles (≤1 μm). Elemental analysis with the electron microprobe demonstrates that the LixFePO 4 particles are relatively homogeneous. Thus, the electron microprobe may be used in conjunction with other instruments that are more commonly used to investigate particle morphology, such as scanning or transmission electron microscopes. Isotopic substitution experiments show that the symmetric and antisymmetric bending vibrations of the PO43- anion are highly mixed with Li+ translatory vibrations. Consequently, no band in the IR spectrum of LiFePO4 may be assigned solely as a lithium ion "cage" mode. Spectroscopic measurements of the LxFePO 4 series demonstrate that the intramolecular modes of PO 43- are particularly sensitive to the extraction of Li- ions from LiFePO4 and the accompanying oxidation of Fe2+ to Fe3+. Spectroscopic data suggest that the PO 43- effective force constants (band frequencies), dipole moment derivatives (FTIR intensities), and polarizability derivatives (Raman intensities) change as LiFePO4 is converted into FePO4. Together, the data strongly support the two-phase shell model of LiFePO 4 delithiation. © 2004 The Electrochemical Society. All rights reserved.