Vibrational spectra and ion-pair properties of lithium hexafluorophosphate in ethylene carbonate based mixed-solvent systems for lithium batteries

The vibrational spectra of LiPF6 1M solutions formed in aprotic mixed solvents obtained by mixing ethylene carbonate with dimethyl carbonate and diethyl carbonate are discussed. The Raman and infrared spectra of lithium hexafluorophosphate (LiPF6) and the quantum chemical computations of the vibrational wavenumbers and intensities are reported. Due to the nature of the solutions, attenuated total internal reflectance spectroscopy was used to obtain the infrared spectra. The infrared active vibrational fundamentals of PF6- provided evidence for the anion-solvent interaction as well as ion-pair formation. Similarly, the infrared active modes of the solvent showed significant changes due to the cation-solvent interaction. The computations of the most energetically favorable geometry in the formation of the Li+ PF6- ion pair are also presented. Conductivity measurements carried out for the 1M solutions scanning a wide temperature interval (-30 to + 60 degreesC) confirm the viability of these electrolytes for practical applications.