Ester Modified Pyrrolidinium Based Ionic Liquids as Electrolyte Component Candidates in Rechargeable Lithium Batteries

Room temperature ionic liquids (RTILs), especially pyrrolidinium based RTILs with bis(trifluoromethane-sulfonyl)imide (TFSI) as counterion, are frequently proposed as promising electrolyte component candidates thanks to their high thermal as well as high oxidation stability. In order to avoid a resource intensive experimental approach, mainly based on trial and error experiments, a computational screening method for pre-selecting suitable candidate molecules was adopted and three homologous series compounds were synthesized by modifying the cation structure of pyrrolidinium RTILs. The obtained high purity RTILs: methyl-methylcarboxymethyl pyrrolidinium TFSI (MMMPyrTFSI), methyl-ethylcarboxymethyl pyrrolidinium TFSI (MEMPyrTFSI) and methylpropylcarboxymethyl pyrrolidinium TFSI (MPMPyrTFSI) revealed excellent thermal stabilities higher than 300 °C. Furthermore, MMMPyrTFSI and MPMPyrTFSI exhibit high oxidation stability up to 5.4 V vs. Li/Li+. No aluminum corrosion of current collector was observed at 5 V vs. Li/Li+. In addition to that, these RTILs display a superior salt (LiTFSI) solubility (3.0–3.5 M), compared to the unmodified RTIL 1-butyl-1-methylpyrrolidinium TFSI (Pyr14TFSI) (1.5–2.0 M) at room temperature. All these properties make novel ester modified RTILs promising and interesting candidates for application in rechargeable lithium batteries