N-Alkyl-N-methylpyrrolidinium difluoro(oxalato)borate ionic liquids: Physical/electrochemical properties and Al corrosion

The synthesis and physical properties of difluoro(oxalato)borate (DFOB−)-based ionic liquids (ILs) are reported with N-alkyl-N  -methylpyrrolidinium (PY1R+)(PY1R+) cations with butyl, pentyl and hexyl alkyl chains. The PY14DFOB and PY16DFOB salts have a melting point (Tm) of −5 °C and 31 °C, respectively, whereas the PY15DFOB salt does not crystallize. Instead, this salt has a glass transition temperature (Tg) of −74 °C. Electrolytes composed of these ILs are noncorrosive toward Al when mixed with LiTFSI and LiFSI, as well as with LiPF6 or LiDFOB. The exceptional oxidative stability of these ILs suggests that they may be suitable for battery electrolyte applications.

► N-Alkyl-N-methylpyrrolidinium difluoro(oxalato)borate ILs have been synthesized.
► ILs are liquid at room temperature and have high thermal stabilities.
► ILs display high oxidative stability on Pt.
► ILs reversibly plate Li/Li+ when mixed with common lithium salts.
► Corrosion of Al is negligible, even in IL–LiTFSI and IL–LiFSI mixtures.