Ionic liquids containing an ester group as potential electrolytes

Various types of ionic liquids containing a methyl acetate group were synthesized and their physical and electrochemical properties were investigated. These ionic liquids exhibited wide electrochemical windows of at least 5.0 V and relatively high conductivities. Unlike the case of dialkyl-substituted ionic liquids, the conductivities of ionic liquids with methyl acetate group on the cations were reduced to a much smaller extent only by 30% upon mixed with a lithium salt. In addition, a severe deterioration of C-rate performance was not observed when an ionic liquid with a methyl acetate group was added to an electrolyte solution consisting of EC, DMC and LiPF6. Such favorable electrochemical properties of these ionic liquids are likely to be attributed to the interaction between methyl acetate group and lithium ions. FT-IR study clearly demonstrates that the methyl acetate group of the ionic liquids interacts with Li+ ions through carbonyl group. Upon interaction with LiPF6, the carbonyl stretching frequency shifted to a lower frequency, whereas the peak associated with C–O single bond shifted to a higher frequency.