Vibrational spectroscopic study of lithium intercalation into LiTi 2(PO 4) 3

Abstract

Ex situ vibrational spectra are recorded during the first discharge of LiTi 2(PO 4) 3. Spectral changes are consistent with a two-phase model for the electrochemical insertion of Li + ions. Differences in the frequencies and relative intensities of the LiTi 2(PO 4) 3 and Li 3Ti 2(PO 4) 3 bands are due to changes in the effective force constants, dipole moment derivatives, and polarizability derivatives as Li + is inserted into LiTi 2(PO 4) 3. The intramolecular PO 4 3− bending modes (ν 2 and ν 4) are found to be more sensitive to Li + insertion than the intramolecular PO 4 3− stretching modes (ν 1 and ν 3). This is because ν 2 and ν 4 are less localized than ν 1 or ν 3 and are more susceptible to small structural changes in the unit cell. Furthermore, a band at 487 cm − 1 appears in the infrared spectrum of Li 3Ti 2(PO 4) 3. This band is assigned as a Li + ion cage mode and is due to Li + ions that occupy the M(3) and M′(3) sites in the Li 3Ti 2(PO 4) 3 structure. A small degree of band broadening is also detected in the vibrational spectra when Li + ions are inserted, which might indicate some disordering in the cathode material.