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.