Electronic states and contact ion pair formation in lithium-ion electrolytes investigated by far-ultraviolet spectroscopy and quantum chemical calculations.

The electronic states of lithium-ion (Li⁺) electrolytes play a crucial role in understanding their solvation structures and electronic interactions. In this study, far-ultraviolet (FUV) spectroscopy and quantum chemical calculations were used to investigate lithium bis(trifluoromethanesulfonyl)imide (TFSI)-based electrolytes dissolved in dimethyl carbonate (DMC). At low Li⁺ concentrations, the absorption spectra exhibited a redshift, which was attributed to electronic interactions between Li⁺ and DMC in the solvation structure. In contrast, at high concentrations, a clear blueshift was observed. Quantum chemical calculations revealed that this blueshift originates from the formation of contact ion pairs, leading to intermolecular electronic excitation and electron transfer between the TFSI anion and DMC. These findings indicate that the nature of electronic transitions in the FUV region changes significantly with the ionic environment. The results demonstrate that FUV spectroscopy is a powerful technique for probing dynamic changes in solvation and ion association states in electrolytes, offering valuable guidance for the design and optimization of battery materials.