In this work is described an improved synthesis of hydrophobic room-temperature ionic liquids ͑RTIL͒ composed of N-methyl-N-alkylpyrrolidinium ͑or piperidinium͒ cations and ͑perfluoroalkylsulfonyl͒imide, ͓͑C n F 2n+1 SO 2 ͒͑C m F 2m+1 SO 2 ͒N − ͔, anions. The procedure described allows the synthesis of hydrophobic ionic liquids with the purity required for electrochemical applications such as high-voltage supercapacitors and lithium batteries. This new synthesis does not require the use of environmentally unfriendly solvents such as acetone, acetonitrile, and alogen-containing solvents that are not suitable for industrial applications. Only water and ethyl acetate are used throughout the entire process. The effect of the reaction temperature, time, and stoichiometry in the various steps of the synthesis has been investigated. With an iterative purification step performed at the end of the synthesis, ultrapure, clear, colorless, inodorous RTILs were obtained. The final vacuum drying at 120°C gave RTILs with a moisture content below 10 ppm. Details for the synthesis of N-butyl-N-methylpyrrolidinium bis͑trifluoromethansulfonyl͒imide ͑PYR 14 TFSI͒ are reported. The overall yield for the synthesis of this ionic liquid was above 86 wt %. Electrochemical tests performed on this material are also reported.

The physical properties and electrochemical performance of solvent-free polymer electrolytes, consisting of poly͑ethylene oxide͒ ͓P͑EO͔͒ with Li + and N-butyl-N-methylpyrrolidinium ͑PYR 14 + ͒ salts of bis͑trifluoromethanesulfonyl͒imide ͑TFSI͒, are reported. The addition of the PYR 14 TFSI room-temperature molten salt to the conventional P͑EO͒ 10 LiTFSI polymer electrolyte results in a significant enhancement of the ionic conductivity ͑to 2.6 ϫ 10 −4 S/cm at 20°C͒. Li metal/LiFePO 4 batteries making use of this new family of solid polymer electrolytes have shown a very promising low-temperature cycle life. It was found that a Li metal/LiFePO 4 cell at 20°C can deliver 138 mAh/g during the first discharge, corresponding to 90% of the nominal specific capacity ͑154 mAh/g͒, and a promisingly low capacity fading of about 0.2%/cycle over 500 cycles.

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The electrochemical properties of ternary systems that consist of a polymer, a lithium salt, and an ionic liquid that shares the same anion (TFSI, bis(trifluoromethansulfonyl)imide) are reported and compared. The investigation involved two different polymers (PVdF-HFP and PTFE) that were selected because of their common use in lithium-based electrochemical devices. It was found that PVdF-HFP swelled by the ionic liquid used in the work while porous PTFE remained inert. The ternary electrolytes showed interesting ionic conductivities. However, the presence of fluorinated polymers resulted in poor interfacial properties with lithium metal electrodes.

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