Stoichiometric spinel refers to a lithium manganese oxide with a cation/anion substance ratio, that is, nM/nO of 3/4. In the range of 0≤x≤1, lithium is inserted into the compound at about 3V to generate Li1+x(Mn2-δLiδ)O4 configuration species. When lithium ions are inserted into LiMn2O4, due to the Jahn-Teller effect, the symmetry of the electrode material decreases to the tetragonal symmetry of LiMn2O4 (I4I/amd space group); when δ=0.33, that is, Li4Mn5O12, all manganese ions in the cubic spinel structure (Fd3m space group) are +4 valence. In the compound, with the increase in the valence of manganese ions, the lithium ions partially occupy the 16d position of the manganese octahedron. As a result, the cubic lattice parameter shrinks to a=0.8137nm.
Figure 1 shows the Raman scattering spectra (RS) of several stoichiometric spinels such as Li4Mn5O12, λ-LiMn2O4 and Li2Mn2O4. The spinel ( space group) compound is treated with group theory, and 9 spectral peaks are obtained in the Brillouin zone, of which 5 (A1g+Eg+3F2g) are Raman active vibrations, and 4 (4F1u) are infrared active vibrations. It can be seen from Figure 1 that the Raman activity of LiMn2O4 is low, which is mainly caused by its polarization characteristics. The weakening of the Raman band is due to the strong absorption in the visible light region caused by the d→d transition. The Raman spectrum of λ-LiMn2O4 is mainly in the broad peak near 625cm-1 and the shoulder peak at 583cm-1, consisting of a medium-intensity peak near 483 cm-1 and two low-intensity peaks near 382 cm-1 and 295 cm-1. The peak near 625cm-1 can be regarded as caused by Mn-O stretching vibration, and this frequency band is marked as A1g symmetry in the spectroscopy space group.
Figure 1 - Raman scattering spectrum of Li-Mn-O compound
The position and half-width of this peak hardly change with the extraction of lithium ions. The broadening of the peak is related to the anion-cation bond length and the polyhedral deformation that occurs in λ-LiMn2O4. The position and intensity of the shoulder peak at about 583 cm-1 increase with the increase of the amount of lithium extraction. When lithium is released, the position of this shoulder moves to the high-frequency direction, which is mainly caused by the contraction of the MnⅣ-O bond. When lithium is completely extracted into λ-MnO4 (a=0.8029nm), the position of this shoulder shifts to 597cm-1. 295cm-1 is the Eg symmetrical peak, and 382cm-1 is the F2g symmetrical peak.
As shown in Figure 1(a), the central region of the main frequency band of the Raman scattering spectrum (RS) of spinel Li4Mn5O12 is 630~650cm-1, which corresponds to the stretching vibration of the Mn-O bond. Carefully distinguish, there are two peaks in this area. Located at 634cm-1 and 653cm-1, respectively. Compared with λ-LiMn2O4, the increase in the stretching vibration frequency is due to the shrinkage of the Mn-O bond. Except for the obvious increase in the intensity of the peak at 300cm-1, the spectrum of Li4Mn5O12 in the low frequency range is similar to that of λ-LiMn2O4. The appearance of this band peak is related to the position of lithium ion in the octahedral coordination position, because as lithium occupies the 16d position, the intensity of this peak increases.
Further lithiation of LiMn2O4 will cause Jahn-Teller deformation and the crystal symmetry will change from vertical to tetragonal. This can be seen in the split peaks of some X-ray diffraction peaks in Figure 2. The tetragonal Li2Mn2O4 is a deformed spinel, the manganese atom occupies the 8d position, and the lithium atom occupies the tetrahedral 8a and octahedral 8c positions. The 4a, 8c, 8d, and 16h bits in the tetragonal structure correspond to the 8a, 16c, 16d, and 32e bits in the cubic structure. As shown in Figure 1(c), the Raman spectrum of tetragonal Li2Mn2O4 mainly includes four peaks at 607 cm-1, 398 cm-1, 279 cm-1 and 258 cm-1. The first peak is the strongest, with a half-width of 42 cm-1, which is caused by Mn-O stretching vibration, which is consistent with X-ray diffraction and electrochemical data. The tetragonal Li2Mn2O4 belongs to the I4I/amd (D194h) space group, in which Mn3+ occupies 8d position, O2- is at 16h position, and the inserted lithium ion occupies 8c position. It is easy to distinguish tetragonal Li2Mn2O4 and cubic Li2Mn2O4 from the Raman spectrum. The former has four spectral lines. At the same time, due to the difference in electronic conductivity, the peak of the former is sharper and stronger. Due to the lack of single crystal diffraction pattern data, it is difficult to determine its active frequency band in the Raman spectrum. The 607cm-1 frequency band can be regarded as AlgO-Mn-O stretching vibration. The relatively strong low-wavenumber frequency band of 398cm-1 is classified as Li-O stretching.
Figure 2 - X-ray powder diffraction patterns of several Li-Mn-O compounds
In spinel LiMn2O4 doped with lithium, the main peak of the Raman spectrum in the low-frequency region is located near 258cm-1, which is mainly caused by the Ba-symmetric Li-O bond. The appearance of this peak is consistent with the tetragonal lithium manganese oxide obtained from the improved spinel with lithium ions occupying 16c position.
Figure 3 shows the room temperature Raman and infrared absorption spectra of spinel LiMn2O4. The Raman spectrum of spinel LiMn2O4 consists of broad and strong peaks near 625cm-1 (a shoulder peak near 580cm-1) broad medium-strong peaks near 483cm-1, and three weaker peaks near 426 cm-1, 382 cm-1, and 300 cm-1, respectively. Its infrared spectrum consists of two broad strong absorption peaks near 615cm-1 and 513cm-1, and four low-frequency weak absorption peaks near 420cm-1, 355cm-1, 262cm-1 and 225cm-1.
Figure 3 - Room temperature vibration spectrum of spinel LiMn2O4
According to the calculation of lattice dynamics and the general spectral characteristics of spinel oxide, the assignment is as follows. The spectral band near 625 cm-1 belongs to the symmetric stretching vibration of MnO6, and the high wave number band belongs to the A1g type in the O7h spectral symmetry. The broadening is caused by the anion-cation bond length and the polymorphic deformation in the spinel LiMn2O4. Due to the uneven charge of manganese cations in spinel LiMn2O4, such as LiMn3+Mn4+O4, there are isotropic Mn4+O6 octahedrons and locally deformed Mn3+O6 octahedrons caused by the Jahnr-Teller effect. Therefore, MnO69- and MnO68- stretching vibrations can be observed, which causes a broadening of the A1g waveform. Due to its weak strength, the shoulder peak near 583cm-1 cannot be separated. The localized vibration method infers that the intensity of the shoulder peak is closely related to the average oxidation state of manganese in the spinel. Since the intensity of the shoulder peak is very sensitive to the stoichiometric value of lithium, it can be considered that this peak is caused by Mn4+-O stretching vibration.
The electrical properties of LiMn2O4 reduce its Raman spectral efficiency. LiMn2O4 is a small polaron semiconductor. Electrons transition between manganese ions in two different oxidation states. The increase in electrical conductivity means a higher concentration of carriers, which increases the absorption intensity of incident laser light in the visible light region, and consequently reduces the intensity of the Raman spectrum. The medium-intensity peaks near 483cm-1 and the low-intensity peaks near 426cm-1 and 382cm-1 correspond to F2g(2), Eg and F2g(3) symmetry, respectively, and the low wavenumber band near 300cm-1 may be caused by the disorder of cations. In the ideal cubic spinel structure, Mn3+ and Mn4+ are usually regarded as crystallographically equivalent, both located at the 16d position, that is, the occupancy ratio of the two ions at the 16d position is 1:1, which is also consistent with the x-ray diffraction data. But in fact, due to the Jahn-Teller effect of Mn3+, the crystal lattice is deformed in a local area. At the same time, Mn3+ has a larger ion radius than Mn4+, and this balance is destroyed. As a result, more vibration peaks than expected can be observed in the infrared spectrum.
All infrared spectral bands are F1u symmetric. The high-frequency infrared absorption spectra of the spinel located near 615cm-1 and 513cm-1 are caused by the antisymmetric stretching of the MnO6 group, while the low frequency bands near 225cm-1, 262cm-1, 355cm-1 and 420cm-1 are caused by the bending of O-Mn-O and the vibration of LiO4 group. Since infrared waves are very sensitive to the oxidation state of cations, when lithium ions are removed from λ-LiMn2O4, significant migration occurs in the high wavenumber frequency band. Experimental studies have shown that the high frequency band of λ-MnO4 with +4 valence of manganese ion migrates to 610cm-1 (the corresponding peak of λ-LiMn2O4 is at 615cm-1).