Interactions in sodium bis(fluorosulfonyl)imide/1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide electrolytes for Na-ion batteries: Insights from molecular dynamics simulations

Abstract

Classical and ab initio molecular dynamics simulations have been performed for a series of electrolytes for Na-ion devices based on sodium bis(fluorosulfonyl)imide (NaFSI) solutions in 1-ethyl-3-methylimidazolium FSI (EMIM-FSI) ionic liquid. Viscosities and conductivities estimated in a polarizable force field agree with experimental data. Fourier transform of the autocorrelation function of the dipole moment obtained in ab initio simulations has been used to calculate the IR spectra of electrolytes. Computed shifts of the vibrational band originating from the S-F stretch of the FSI anion are in very good agreement with experimental data measured for electrolytes with increasing NaFSI mole fraction. Fourier transform of geometrical parameters of FSI anions has been applied to probe locally the frequency changes. Results show the correlation between the local environment of the FSI ion and vibrational frequencies, corroborating the deconvolution of the experimental spectrum into contributions from "free" and "bound" ions.