Fig. 2: Intermolecular structure of lithium bis(fluorosulfonyl)imide (LiFSI) in tetraglyme (G4), and correlation of 717 cm⁻¹ FSI⁻ peak area with [Li⁺], as determined by Raman microspectroscopy.

a Raman spectrum of 2 m LiFSI in G4 normalised to peak intensity at 1471 cm⁻¹; identifying the 717 cm⁻¹ FSI⁻ S–N–S bending mode peak, 868 cm⁻¹ [Li(G4)]⁺ crown ether-like breathing mode peak, 1471 cm⁻¹ –CH₂ bending/scissoring mode peak. b Normalised 717 cm⁻¹ peak, showing broadening of the peak with concentration, indicating the evolution of CIPs and AGGs, as illustrated. c Calibration curve equating normalised 717 cm⁻¹ peak area to concentration, which was used to determine the concentration gradient via operando Raman microspectroscopy 1D line scans. Error bars represent the propagated standard error from the balance and density measurements (x-axis) and area calculation from three separate Raman acquisitions (y-axis).