Far-ultraviolet spectroscopic investigation of electronic interactions between lithium ions and organic solvents

In fifteen words or less, explain the significance of this contribution (Novel Aspect).: Electronic interactions in Li-ion electrolytes revealed by far-UV spectroscopy and calculations.

Abstract Text:

Understanding the electronic structure of lithium-ion battery electrolytes is essential for enhancing battery performance. In this study, we employed attenuated total reflectance far-ultraviolet (ATR-FUV) spectroscopy to investigate the interaction between lithium ions (Li⁺) and dimethyl carbonate (DMC), a widely used electrolyte solvent. Conventional UV spectroscopy fails to access the sub-200 nm region where critical electronic transitions occur in liquid-phase systems. Our ATR-FUV method overcomes this limitation by eliminating atmospheric interference, allowing direct measurement of FUV absorption without the need for vacuum conditions.

We observed a concentration-dependent redshift in the absorption spectra with increasing Li⁺, indicating the formation of Li⁺–DMC complexes. Time-dependent density functional theory (TD-DFT) calculations supported the spectral assignments, revealing that the redshift arises from a change in transition origin: from intramolecular excitations in free DMC to charge-transfer transitions in Li⁺–coordinated DMC. Furthermore, multivariate curve resolution–alternating least squares (MCR-ALS) analysis successfully decomposed the spectra into two distinct components, corresponding to free DMC and Li⁺–DMC.

These findings were consistent across different Li salts, including Li[BF₄] and Li[TFSI], although subtle spectral variations reflected the influence of the counter anions. Our results demonstrate, for the first time, the liquid-phase FUV absorption behavior of Li⁺–DMC complexes and provide experimental validation of computational predictions. This work not only expands the analytical capabilities for probing solvation structure in nonaqueous electrolytes but also offers a valuable foundation for designing advanced electrolyte systems.