Alkylimidazolium-based ionic liquids as lithium-ion battery electrolyte

The two alkylimidazolium-based ionic liquids of 1-methyl-3- methylimidazolium tetrafluoroborate (EMIBF₄) and 1-butyl-3- methylimidazolium tetrafluoroborate (BMIBF₄) are synthesized andtheir electrochemical properties are investigated by using cyclic voltammetry and galvanostatic charge-discharge techniques. The results reveal that the electrochemical windows of these two ionic liquids are both about 4.3 V and their ionic conductivity are 13.7 and 3.0 mS/cm respectively at 25°C. The demonstration battery with graphite anode and LiCoO₂ cathode are prepared by using EMIBF₄, BMIBF₄ and 1 mol L^-1 LiPF₆ as electrolyte and 5 % of vinylene carbonate (VC) as additive. The coulombic efficiency of the EMIBF₄-involved electrolyte is up to 90 % after 50 cycles and the capacity remaines above 91 mAhmg^-1 during the whole cycle, while the coulombic efficiency of the BMIBF ₄-based battery droppes directly after 10 cycles. The capacity of the EMIBF₄-based battery with C/5 rate is 80 % of C/15 rate in 25°C.