Nonlinear Vibrational Study of Solvent Adsorption on the Cathode Surface of Li-Ion Batteries

To improve the performance of Li-ion batteries, it is important to understand the structures on the electrode/solution interface. In the present study, a surface sensitive nonlinear vibrational spectroscopic technique, sum frequency generation (SFG),¹ was employed to evaluate the molecular structures of the solvent adsorption at the LiCoO₂ cathode surface of the Li-ion battery, rather than those of the solvent in the bulk solution obtained by previous studies.

A LiCoO₂ thin film was deposited on the CaF₂ substrate surface. A broadband SFG system using a tunable infrared pulse (2.5~10mm) and a visible pulse (800nm) was used to characterize LiCoO₂ surface in contact with electrolyte solutions consist of carbonate solvents, such as ethylene carbonate (EC), propylene carbonates (PC), diethyl carbonate (DEC) and  dimethyl carbonate (DMC).^2,3

Figure 1a shows the sps-polarized SFG spectrum at the LiCoO₂ contacting with EC (middle panel, circles) and the fitting result (solid trace) in the infrared frequency region between 1700 and 1850 cm⁻¹. As SFG does not occur in a homogenous bulk solution with inversion symmetry, the present result suggests that the EC molecules align on the LiCoO₂ surface with a certain structure. The bottom panels are deconvoluted bands for the SFG spectrum. The upward and downward peaks represent the modes with reversed orientations as the schematic model shown in inset. These modes optically interfere with nonresonant signal c_NR⁽²⁾ as well as themselves, generating constructive (upward) and destructive (downward) peaks depending on the adsorption geometry of the molecules. The top panel in Fig. 1a shows SFG spectrum after removing contribution from c_NR⁽²⁾. Based on the spectral analyses, two adsorption geometries of EC molecules on LiCoO₂ surface have been observed by the present SFG measurement, by pointing the C=O groups away from or towards the electrode surface, respectively.

Solvent adsorption has also been investigated in the other solvent solvents or their mixtures. Figure 1b shows the SFG spectra of different carbonate solutions after removal of c_NR⁽²⁾.  It is found that the SFG spectra of the mixed solvents containing EC molecule are quite similar to that of EC solvent. This implies that the preferential adsorption of EC molecules on the LiCoO₂ surface compared to the linear carbonates of DEC and DMC, with a largely different molecular proportion from that in the mixed solution bulk.³ The present results is expected be useful to understand the initial stage for the solid electrolyte interphase (SEI) formation on the cathode surface.

References

[1] Shen, Y. R. The Principles of Nonlinear Optics; Wiley: N. Y., 1984; [2] Liu, H.; Tong, Y.; Kuwata, N.; Osawa, M.; Kawamura, J.; Ye, S. J. Phys. Chem. C 2009, 113, 20531. [3] Yu, L.; Liu, H. J.; Wang, Y.; Kuwata, N.; Osawa, M.; Kawamura, J.; Ye, S. Angew. Chem. Int. Ed. 2013, 52, 5753.