Vibrational spectroscopic studies of monoclinic and rhombohedral Li 3V 2(PO 4) 3

Abstract

Monoclinic (A−) and rhombohedral (B-) Li 3V 2(PO 4) 3 cathodes are investigated with X-ray diffraction (XRD), vibrational spectroscopy, and wavelength dispersive spectrometry using an electron microprobe. The latter technique is used to quantify the V:P ratio and detect small V 2O 5 impurities within the samples that are not observed with XRD or vibrational spectroscopy. Additionally, the imaging capabilities of the electron microprobe are very useful in probing the particle sizes and morphologies of these materials. Infrared spectra of A- and B-Li x V 2(PO 4) 3 are recorded at different states of charge. The PO 4 3− stretching and bending modes are very sensitive to the presence of Li + ions and the oxidation state of the V n+ ions. Changes in the frequencies and intensities of the PO 4 3− modes are due to subtle changes in the effective force constants and dipole moment derivatives of the P–O bonds accompanying the removal of Li + ions and the subsequent oxidation of V 3+ to V 4+. The spectral changes observed in A-Li x V 2(PO 4) 3 (0 ≤ × ≤ 3) are consistent with the structural changes that Yin et al. described with neutron diffraction and 7Li NMR [J. Am. Chem. Soc., 125, (2003) 10402]. The 490 cm − 1 band of B-Li 3V 2(PO 4) 3 is assigned to Li + ions occupying the M(3) sites. This band disappears between 3.92 and 4.30 V; simultaneously the cells exhibit significant polarization. We propose that these two effects are caused by the migration of Li + ions from the M(3) sites to M(1) sites.