Quantitative Analysis of the Li-Ion Solvation Structure in Li-Ion Battery Electrolytes Using ATR-FTIR Spectroscopy

Attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectroscopy is widely used to study condensed materials due to its convenient sample preparation and ability to avoid absorption saturation. Recently, it has been applied to in situ and in operando observations of chemical reactions within electrochemical devices, such as lithium-ion batteries. However, because ATR-FTIR spectroscopy relies on frequency-dependent attenuated reflectance, quantitative concentration measurements of chemical species using the Beer-Lambert law are challenging. Despite the availability of several correction methods, discrepancies remain in the solvation structures around Li⁺ ions when comparing transmission-type FTIR and ATR-FTIR spectroscopy results, which complicate the determination of solvation and desolvation energies. In this study, we investigate ATR-FTIR correction algorithms, elucidate the reasons for the discrepancies between ATR-FTIR and transmission FTIR spectroscopy results, and develop a method to correct the ATR-FTIR spectrum to accurately determine the solvation structures around Li⁺ ions.