A Comparison among Viscosity, Density, Conductivity, and Electrochemical Windows of N‑n‑Butyl‑N‑methylpyrrolidinium and Triethyl‑n‑pentylphosphonium Bis(fluorosulfonyl imide) Ionic Liquids and Their Analogues Containing Bis(trifluoromethylsulfonyl) Imide Anion

Ionic liquid consists of an organic cation and generally an inorganic anion. The properties of liquids can be modulated by changing the anion or the cation or both. Two ionic liquids derived from the anion [FSI] were synthesized and characterized: (N-n-butyl-N-methylpyrrolidinium bis­(fluorosulfonyl imide), [BMPYR]­[FSI]) and triethyl-n-pentylphosphonium bis­(fluorosulfonyl)­imide), [P2225]­[FSI]). We also report the comparison with [TFSI] based ionic liquids, namely: (N-n-butyl-N-methylpyrrolidinium bis­(trifluoromethylsulfonyl)­imide), [BMPYR]­[TFSI]), (N-n-butyl-N-methylpiperidinium bis­(trifluoromethylsulfonyl)­imide), [BMP]­[TFSI]), triethyl-n-pentylphosphonium bis­(trifluoromethylsulfonyl)­imide), [P2225]­[TFSI]) and (triethyl­(2-methoxyethyl) phosphonium bis­(trifluoromethylsulfonyl imide), [P222201]­[TFSI]. Thermophysical properties, such as viscosity (298.15–373.15 K), density (298.15–373.15 K), conductivity (298.15–361.15 K), electrochemical (cutoff density current within 150 μA·cm–2), and thermal stability (303.15–973.15 K) were experimentally determined. The results showed that [FSI] derivatives exhibit better transport properties and lower thermal and electrochemical stability when compared with their [TFSI] counterparts.