Abstract

Protic ionic liquids (PILs) represent a promising class of materials for numerous applications, including electrochemical ones, thanks to their high ionic conductivity, low volatility and tunable physicochemical properties1.

In recent years, increasing attention has been focused on mixtures of PILs with inorganic salts, since the addition of species such as LiNO3 can significantly alter the microscopic structure and transport properties of these systems, making them potentially attractive candidates as electrolyte for lithium devices2.

Despite this growing interest, the solvation environment of Li+ ions and their influence on the intermolecular interactions that determine the structure and properties of PILs are not yet fully understood3.

In this work, the structural organization and intermolecular interactions in mixtures of lithium nitrate (LiNO3) with ethylammonium nitrate (EAN) and propylammonium nitrate (PAN) at different compositions were investigated.

The characterization of these systems concerned the adoption of an integrated approach based on ATR-FTIR, Raman (532 and 785 nm excitation wavelength), and UV-Raman (213 and 248 nm excitation wavelength) spectroscopy.

The most innovative contribution concerns the application of a differential spectral analysis procedure to extract solute-correlated spectra, providing selective information on the lithium solvation shell in non-aqueous systems4.

Overall, the results obtained highlight how the addiction of LiNO3 leads to deep reorganization of the local structure of nitrate-based PILs.

References

[1] Gardiner, D. J., Hester, R. E. & Grossman, W. E. L. Infrared and Raman spectra of lithium and ammonium nitrate solutions in liquid ammonia. J. Chem. Phys. 59, 175–184 (1973).

[2] Hu, X. et al. Thermal stability of ionic liquids for lithium-ion batteries: A review. Renew. Sustain. Energy Rev. 207, 114949 (2025).

[3] O.Russina, R. Carminiti, T. Méndez-Morales, J. Carrete, O.Cabeza, L.J. Gallego, L.M. Varela, A.Triolo. How does lithium nitrate dissolve in a protic ionic liquid? Journal of Molecular Liquids 205, (2015)16-21.

[4] Ben-Amotz D. Hydration-Shell Vibrational Spectroscopy. J Am Chem Soc 2019;141(27):10569–10580.