Spectroscopic investigations of solutions of lithium bis(fluorosulfonyl) imide (LiFSI) in valeronitrile

Abstract

Lithium bis(fluorosulfonyl)imide (LiFSI) is an interesting electrolyte for lithium ion batteries and valeronitrile is a good solvent for LiFSI as it forms complexes with the Li + -ions. In the present work, this complexation was studied by optical spectroscopy (Raman and IR), as well as by nuclear magnetic resonance (NMR) spectroscopy. Based on the spectroscopic information a chemical model for the complexation was developed and the equilibrium constants for the formation of the different species that were considered are reported.

Water/Ionic Liquid/Succinonitrile Hybrid Electrolytes for Aqueous Batteries

“…Dipole-dipole interactions, and in the salt containing electrolytes particularly ion-dipole interactions (e.g., Li + -nitrile coordination), transfer a significant amount of electron density from the nitrile carbon to the nitrogen, resulting in a higher chemical shift (see again discussion on 1 H NMR chemical shifts in Figure 2d above). [40] BSiS-A 0.5 shows the highest shift, reflecting strongest Li + -nitrile interaction, in agreement with strong Li + -AN coordination described in reference 4 and Figure S9 in the Supporting Information. The lower shift in the WISHEs suggests weaker Li + -nitrile interactions, which is plausible i) due to the larger size of SN compared to AN and ii) the presence of more TFSI (from the IL) that competes for a place in the Li + solvation shell.…”

“…The slight deshielding of H#2 in 1‐1‐1‐1 compared to pure SN hints at the following picture: The coordination of SN to Li + pulls electron density towards the nitrile nitrogen, thus de shielding the nitrile carbon, in turn resulting in shorter CH bonds and higher electron density (i.e., shielding) of the SN hydrogens. [ 40 ] The de shielding of the nitrile carbon upon Li + ‐SN interaction is documented in the corresponding 13 C NMR spectra (Figure 2e, discussed below), where a higher chemical shift is observed in case of 1‐1‐1‐1 compared to pure SN. This is in agreement with a study on valeronitrile‐LiFSI mixtures, where higher salt concentration and thus more Li + ‐nitrile coordination resulted in shielding of 1 H (valeronitrile hydrogens) and de shielding of 13 C (nitrile carbon) NMR resonances.…”

“…This is in agreement with a study on valeronitrile-LiFSI mixtures, where higher salt concentration and thus more Li + -nitrile coordination resulted in shielding of 1 H (valeronitrile hydrogens) and de shielding of 13 C (nitrile carbon) NMR resonances. [40] As the resonances of coordinated and "free" nitrile groups should be slightly different, one could expect to observe two different ( 1 H and 13 C) NMR signals when only one nitrile group coordinates to Li + , while if none or both nitriles coordinate, merely a single peak should be observed, as in this case the aliphatic CH 2 groups would be equivalent. However, the solvent exchange rate between the Li + solvation sheath and solution bulk often is too fast with respect to the NMR timescale to resolve the different states, yielding an averaging of the signals, hence one peak only.…”

“…The lower 13 C shifts of the aliphatic carbons in the salt containing electrolytes compared to the pure nitriles again fit the previously described picture (Figure 2d) where Li + -nitrile coordination results in de shielding of the nitrile carbon and shielding of the aliphatic carbons. [40] The high 1 H shift in BSiS-A 0.5 (Figure 2d), high 13 C shift for the nitrile carbon, and low 13 C shift of the aliphatic carbon compared to pure AN (Figure 2e) again fits the strong Li + -AN interaction, as described in reference [4]. 7 Li NMR chemical shifts, shown in Figure 2f, indicate increasing shielding, i.e., higher electron density around the lithium cation, in the order 21m LiTFSI<40/20-TFSI<BSiS-A 0.5 < 1-1-1-1 < 1-1-1-2.…”

Transport properties of lithium-ion in green nonaqueous solutions and polymer ionic exchange membranes: an attempt to elaboration of ecological Li-ion battery

“…The complexation was studied by various spectroscopies such as optical, Raman, infrared and nuclear magentic resonance. Based on the spectrocopic information, a chemical model for the complexation was developed and the equilibrium constants for the formation of the different species were obtained [7].…”