[Introduction] Ionic liquids (ILs) have unique properties (incombustibility, high electrical conductivity and wide potential window) and are expected to be used as electrolytes for safe and high-performance Li-ion batteries. To improve the performance, it is essential to elucidate the solvation structures in the IL electrolyte. In 2007, a new spectroscopy (ATR-FUV spectroscopy) was introduced, which measures electronic absorption in the FUV (< 200 nm) region.^[1] Recently, we have reported ATR-FUV investigations of ILs.^[2] In this study, we investigated influences of solute Li⁺ on the electronic states of ILs and their solvation structure by ATR-FUV spectroscopy and multivariate analyses.

[Experimental] [BMP][DCA] and Li[TFSI] were used as ILs and solute respectively. Electronic transition spectra in 180-300 nm region were measured by ATR-FUV spectroscopy with changing the concentrations of Li⁺. Band assignments of obtained spectra were carried out by time-dependent (TD) DFT calculations using Gaussian16.

[Results and Discussion] Absorption spectra of [BMP][DCA] with Li⁺ are shown in Figure. The absorption bands around 200 nm shift to a shorter wavelength by Li⁺ additions. These mechanisms can be explained by energy diagrams of ground and excited states calculated by TD-DFT. Using multi curve resolution – alternating least square (MCR-ALS) algorithm, these spectra were decomposed into 2 types of [DCA]^－ on the level of electronic states which are categorized as pure [BMP][DCA] and [DCA]^－ affected by Li⁺. As a result, the number of [DCA]^－ whose electronic states are influenced by a Li⁺ can be estimated as ~ 5, which is larger than the coordination number of [DCA]^－ around a Li⁺ ; 3 ~ 4^[3]. Detail models will be discussed in my presentation.

[References]
[1] N. Higashi et al., Rev. Sci. Instrument., 78, 103107 (2007).
[2] I. Tanabe et al., Analyst, 143, 2539 (2018); Anal. Chem., 91, 3436 (2019).
[3] Q. Huang et al., J. Phys. Chem. B, 123 516 (2019).

Figure ATR-absorbance spectra of pure [BMP][DCA] (black line) and Li-IL under various ionic concentration of Li[TFSI] (colored lines) between 180 – 300 nm obtained by ATR-FUV spectroscopy. Molecular structures of [BMP]⁺ and [DCA]^－are shown in the figure.

Ichiro Tanabe

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