Spectroscopic studies on cation-doped spinel LiMn₂O₄ for lithium ion batteries

Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) have been employed to characterize the structural and valence changes of cation-doped LiMn₂O₄ spinels. It is found that the spectral features of the cation-doped spinels are sensitive to the dopant content, the structure of the MO₆ (M=Li, Mn, Cr) groups, the charge distribution and the average oxidation state of Mn. Some new Raman bands have been observed in the low-frequency regions of the spectra of the heavy-doped Li_xMn_2-yCr_zO₄ (z≥0.2). These new bands are attributed to the vibrations of the Cr-O bonds. XPS study demonstrates a conversion of Cr³⁺ to Cr⁶⁺ with increasing Cr content in Li_xMn_2-yCr_zO₄. Based on the above results, the effects of cation doping on the structural variation of the spinels are discussed.