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.