Electronic structure and lattice dynamics of Li_xCoO₂single crystals

Abstract

Spectroscopic ellipsometry and Raman scattering measurements of single-crystal Li x CoO 2 (x = 0.33, 0.43, 0.50, 0.53, 0.72, and 0.87) are reported. The room temperature optical absorption spectra for x values in the range of 0.33-0.72 exhibit three bands near 1.60, 3.35, and 5.20 eV. On the basis of firstprinciples calculations, the observed optical excitations were appropriately assigned. The chargetransfer absorption bands shift to higher energies in Li 0.87 CoO 2 because of symmetry-breakinginduced distortions of the hybridized Co-O orbitals with shifted oxygen 2p states. Furthermore, two Raman-active phonon modes, which display E g and A 1g symmetry, are sensitive to lithium doping. Upon cooling across 200 K, which is the antiferromagnetic phase transition temperature of Li 0.50 CoO 2 , large splitting of the E g mode and a discontinuous change in the frequency of the A 1g mode were observed. These results highlight the importance of spin-phonon coupling in Li 0.50 CoO 2 .

Electronic structure and polar catastrophe at the surface of LixCoO2 studied by angle-resolved photoemission spectroscopy

“…The dispersion of the incoherent component would be related to incoherent hopping transport of Co 4+ species in background of low-spin Co 3+ as suggested in other triangular-lattice Co oxides. [11] This picture is consistent with the observation of Co 4+ and Co 3+ peaks in the O 1s XAS spectra [32] as well as the strong charge fluctuation suggested by recent theoretical [37] and optical [38] studies. The present ARPRES study shows that the x=0.46 sample exhibits homogeneous electronic states compared to the in homogeneous…”

Measuring Li+Inventory Losses in LiCoO2/Graphite Cells Using Raman Microscopy

“…For cycled particles at lower PSOCs, an additional shoulder was observed and an additional broad Lorentzian was necessary to fit the response. At present, we do not specify the origin of this shoulder but report its consistent emergence across several samples along with its similarity to the spectra of Liu et al 18 The resulting fits of the six states of charge served as the matrix S in Equation 1. Having S, the Raman derived PSOC, x R , is acquired by calculating a weighted average according to:…”

“…12,15,18 Second, there is little spectral change between the first and second cycle in the charged state (100% ESOC) suggesting that the structure returns to Li 0.5 CoO 2 after each cycle. Quantifiably, Figure 4 shows the near invariance in spectral position in each mode at 100% ESOC.…”

“…15 In addition, Figure 7 demonstrates that the E g :A 1g peak intensity ratio increases with ESOC, which is similar to that observed by Liu et al as a function of delithiation. 18 The change of intensity ratio is attributed to expansion of the crystal's c-axis. Wang et al, for instance, reported a decrease in the E g :A 1g ratio for upon the application of compressive strain.…”

In situ and Operando Raman Spectroscopy of Layered Transition Metal Oxides for Li-ion Battery Cathodes

“…Pristine LiCoO 2 powder displays two Raman bands as expected, which are sharp and intense ( Figure 4A). The intensity of the band at 596 cm −1 has a strong dependence on the polarization of the incident radiation so it has been assigned to the A 1g mode while the E g mode is assigned to the band at 486 cm −1 (Inaba et al, 1997;Perkins et al, 1997;Liu et al, 2015a). Despite similar compositions, the spectra of LCO below illustrate that the experimentally measured position and relative intensity of the E g and A 1g bands differ between literature reports, which is as discussed above related to slightly differing experimental condition during oxide synthesis, electrode preparation, and measurement configuration.…”