Two room temperature ionic liquids (RTIL) consisting of N-methyl-N-propyl(butyl)piperidinium [(PP13(4)] cation and bis(trifluoromethanesulfonyl)imide (TFSI) anion were newly synthesized, and their electrochemical properties were investigated in comparison with the other two RTIL, 1-butyl-3-methyl-imi- dazolium tetrafluoroborate (BMIBF₄) and 1-butyl-3-methyl-imidazolium hexafluorophosphate (BMIPF₆). The electrochemical window of PP13(4)-TFSI (5.8 V) is wider than those of BMIBF₄ (4.7 V) and BMIPF₆ (4.5 V). The cathodic limit of the PP13(4)-TFSI is about －0.3 V vs. Li/Li^＋, against 0.5 V for BMIPF₆ and 0.7 V for BMIBF₄, so it may be used as the electrolyte for second lithium batteries based on lithium anode. A Li/LiCoO₂ test cell using LiTFSI /PP14-TFSI as electrolyte exhibited very good cycle performance at constant current density of 0.05 mA•cm^－2. The capacity of the cell reached 150 mAh•g^－1 and the coulombic efficiency was close to 100% after the first several cycles. RC impedance measurement of the cell showed that the interfaces between electrode and electrolyte were stable with the time. It appeared that the interfacial passivation did not occur during the cell storage.

Key words: ionic liquid, room temperature molten salt, electrolyte, secondary lithium battery, lithium anode