Additive-containing ionic liquid electrolytes for secondary lithium battery

Room temperature ionic liquid (RTIL) consisting of N-methyl-N-propylpiperidinium (PP13) cation and bis(trifluoromethanesulfonyl)imide (TFSI) anion was synthesized and its electrochemical stability was investigated in comparison with 1-butyl-3-methylimidazolium tetrafluoroborate (BMIBF4) and 1-butyl-3-methylimidazolium hexafluorophosphate (BMIPF6). The electrochemical window of PP13-TFSI (5.8 V versus Li/Li+) is wider than that of BMIBF4 (4.7 V) and BMIPF6 (4.5 V). The cathodic limit of the PP13-TFSI is about −0.3 V versus Li/Li+, against 0.7 V for BMIPF6 and BMIBF4, so it may be used as the electrolyte for second lithium batteries based on lithium anode. In this work, charge efficiency of lithium plating/striping on nickel substrate and the cycle life have been measured using 0.4 M LiTFSI/PP13-TFSI electrolyte both without and with additives such as vinyl acetate (VA), ethylene sulfite (ES), and ethylene carbonate (EC). Remarkable improvement in cycling efficiency and cycle life was found for EC as additive.