Cation Code: Designing Bis(trifluoromethylsulfonyl)imide-Based Ionic Liquids for Electrochemical Applications

The choice of the core cation structure can have significant effects on the suitability of a bis(trifluoromethylsulfonyl)imide ([Tf₂N]⁻)-based room temperature ionic liquid (RTIL) for a particular electrochemical application. While bulkier aliphatic cations (such as ammonium, phosphonium, piperidinium and pyrrolidinium) exhibit wider electrochemical windows (ECWs > 6 V) than sulfur containing cations and many aromatic cations, they are more viscous and, subsequently, have lower molar conductivities. Among RTILs with wide ECWs, pyrrolidiniums have the best molar conductivities (0.81 S cm² mol^–1 for butylmethylpyrrolidinium [Tf₂N] at 298.15 K). Triethylsulfonium [Tf₂N] has the highest molar conductivity of the RTILs tested, followed by three ILs with aromatic cations (dialkylimidazolium, dialkyl-3-triazolium, and pyridinium). Triethylsulfonium [Tf₂N] and butylpyridinium [Tf₂N] also have high degrees of dissociation (0.67 and 0.71, respectively). Superbase derived RTILs are largely unsuitable, due to high melting points, small ECWs and low molar conductivity. While pyrrolidinium and imidazolium ILs are popular choices for electrochemical applications, pyridinium and, especially, triethylsulfonium, ILs have been largely overlooked as RTIL candidates for electrochemical applications.