Electrolyte Solvation and Ionic Association: Part IX. Structures and Raman Spectroscopic Characterization of LiFSI Solvates

Abstract

The bis(fluorosulfonyl)imide anion N(SO2F)2- (i.e., FSI-) (also referred to as bis(fluorosulfonyl)amide (i.e., FSA-) and imidodi(sulphuryl fluoride)) has attracted tremendous interest in recent years for its utility in both lithium salts and ionic liquids for battery electrolyte applications. To facilitate the understanding of the characteristics of this anion, crystal structures are reported here for the uncoordinated anion in LiFSI-based solvates with cryptand CRYPT-222 and tetraglyme (G4). These crystalline solvates were analyzed by Raman spectroscopy to aid in assigning the Raman bands to the modes of ion coordination found in liquid electrolytes. These structures, as well as a thorough review of other relevant crystallographic data, provide insights into the rather remarkable properties of the FSI- anion with regard to solvate formation and electrolyte properties.

The Effect of Boron Nitride Flakes on Sodium Ion Transport in PEO Electrolytes

“…We find that the addition of h-BN leads to a gradual increase in the percentage of CIPs from 79 to 93% whereas the percentage of both aggregate species (AGG-1 and AGG-2) goes from 20% to 6%. Although deconvolution of this peak is a challenge as also noted by Han and co-workers, 54 it is clear that CIPs dominate the local solvate structure in our P(EO) 4 :Na + SPEs. Further, given the breadth of this peak overlapping into the free anion region, we cannot rule out the presence of free anions at this salt concentration and possibly a corresponding increase in free anion percentage with increase in h-BN concentration.…”

“…858 cm −1 that represents both CH 2 symmetric rotational modes and (C−O−C) symmetric stretching modes. 53 Solvate structures of FSI − anion-based salts in various solvents have been studied widely, as recently summarized by Han et al 54 The deconvoluted data for the S−N−S moiety of the FSI − anion (690 to 790 cm −1 ) for both salt concentrations is shown in Figure 3. Incorporation of NaFSI into the PEO matrix results in the formation of a new, broad peak at ca.…”

A systematic study of solvation structure of asymmetric lithium salts in water

“…To elucidate the solvation structures, Raman analysis was performed at ambient temperature for aqueous electrolytes such as Li(FS-FS)I, Li(FS-PFS)I, and Li(FS-NFS)I at varying concentrations, as shown in figures 4(a)-(c). In general, the intense band that appears at about 720-780 cm −1 represents the S-N-S bending vibration of the anions [30,31]. As the electrolyte concentration increased from 0.5 to 20 m, the bending vibration (S-N-S) experienced a shift towards higher frequencies.…”