Quantitative\nAnalysis of the Li-Ion Solvation Structure\nin Li-Ion Battery Electrolytes Using ATR-FTIR Spectroscopy

Attenuated total reflection–Fourier transform\ninfrared (ATR-FTIR)\nspectroscopy is widely used to study condensed materials due to its\nconvenient sample preparation and ability to avoid absorption saturation.\nRecently, it has been applied to <i>in situ</i> and <i>in operando</i> observations of chemical reactions within electrochemical\ndevices, such as lithium-ion batteries. However, because ATR-FTIR\nspectroscopy relies on frequency-dependent attenuated reflectance,\nquantitative concentration measurements of chemical species using\nthe Beer–Lambert law are challenging. Despite the availability\nof several correction methods, discrepancies remain in the solvation\nstructures around Li<sup>+</sup> ions when comparing transmission-type\nFTIR and ATR-FTIR spectroscopy results, which complicate the determination\nof solvation and desolvation energies. In this study, we investigate\nATR-FTIR correction algorithms, elucidate the reasons for the discrepancies\nbetween ATR-FTIR and transmission FTIR spectroscopy results, and develop\na method to correct the ATR-FTIR spectrum to accurately determine\nthe solvation structures around Li<sup>+</sup> ions.