Vibrational spectroscopic studies of monoclinic and rhombohedral Li3V2(PO4)3

Monoclinic (A−) and rhombohedral (B-) Li3V2(PO4)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 V2O5 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-LixV2(PO4)3 are recorded at different states of charge. The PO43− stretching and bending modes are very sensitive to the presence of Li+ ions and the oxidation state of the Vn+ ions. Changes in the frequencies and intensities of the PO43− 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 V3+ to V4+. The spectral changes observed in A-LixV2(PO4)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-Li3V2(PO4)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.