Vibrational spectroscopic study of lithium intercalation into LiTi2(PO4)3

Ex situ vibrational spectra are recorded during the first discharge of LiTi2(PO4)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 LiTi2(PO4)3 and Li3Ti2(PO4)3 bands are due to changes in the effective force constants, dipole moment derivatives, and polarizability derivatives as Li+ is inserted into LiTi2(PO4)3. The intramolecular PO43− bending modes (ν2 and ν4) are found to be more sensitive to Li+ insertion than the intramolecular PO43− stretching modes (ν1 and ν3). This is because ν2 and ν4 are less localized than ν1 or ν3 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 Li3Ti2(PO4)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 Li3Ti2(PO4)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.