Pyrrolidinium-based organic ionic plastic crystals: Relationship between side chain length and properties

Highlights: • Plastic crystals synthesized with different pyrrolidinium cations and FSA anion. • Ionic conductivity increased with increasing alkyl chain length on the cation. • Li transference number of LiFSA-doped pyrrolidinium salts was 0.27 at 60 °C. -- Abstract: Organic ionic plastic crystals (OIPCs) have been studied as solid-state electrolytes because of their desirable properties such as non-flammability, plasticity, and ionic conductivity. However, the temperature range of their conductive plastic crystal phase is narrow. We previously reported that N-ethyl-N-methylpyrrolidinium bis(fluorosulfonyl)amide ([C₂mpyr][FSA]) exhibited a wide temperature range of plastic crystal behavior and high ionic conductivity at room temperature. In this study, the relationship between the side chain structure of pyrrolidinium and the physicochemical properties of their compounds with FSA was investigated. With two introduced ethyl groups, N,N-diethylpyrrolidinium bis(fluorosulfonyl)amide ([C₂epyr][FSA]) showed higher ionic conductivity than [C₂mpyr][FSA], while maintaining a sufficient temperature range of plastic crystal phase. The ionic conductivity of [C₂epyr][FSA] was further improved by the addition of 5 mol% LiFSA. For the [C₂epyr][FSA] composited with 5 mol% LiFSA, the lithium transference number was determined to be 0.27 at 60 °C. Reversible lithium plating/stripping reaction was observed on a Ni electrode in the CV measurement. Symmetric pyrrolidinium cation with a longer alkyl chain (up to two carbons) showed conductive plastic solid phase in a wide temperature range and a higher lithium transference number.