Expanded Structural Design of Organic Ionic Plastic Crystals Based on Linear Tris-Pyrrolidinium Salt

Organic ionic plastic crystal (OIPC) materials exhibit soft solid phases within a specific temperature range, making them promising candidates for various electrochemical devices due to their intrinsic ionic conductivity and electrochemical stability. In this study, a series of linear tris-pyrrolidinium salts with different alkyl side chain lengths and counter-anions were synthesized and systematically characterized in terms of their structural, thermal, and electrochemical properties. Among the synthesized compounds, N,N-bis(4-(N'-undecylpyrrolidinium)butyl)pyrrolidinium trishexafluorophosphate (11-PF₆) exhibited two solid-solid phase transitions and a low fusion entropy (ΔS_f) of 13 J·K^-1·mol^-1 at 210 °C. Similarly, N,N-bis(4-(N'-dodecylpyrrolidinium)butyl)pyrrolidinium tris-hexafluorophosphate (12-PF₆) showed multiple phase transitions with a low ΔS_f of 18 J·K^-1·mol^-1 at 206 °C. The temperature-dependent soft crystalline phases of 11-PF₆ were identified as plastic crystal phase using polarized optical microscopy and 1D wide-angle X-ray scattering (WAXS). The ionic conductivity of 11-PF₆ was measured to be 2.04 × 10^-6 S·cm^-1 at 70 °C. Upon blending with 80 mol% LiTf₂N, the ionic conductivity significantly increased to 1.16 × 10^-4 S·cm^-1. Electrochemical stability was evaluated via linear sweep voltammetry (LSV). The composite solid electrolyte consisting of 11-PF₆ and 80 mol% LiTf₂N exhibited electrochemical stability up to 5.4 V (versus Li/Li⁺), highlighting its potential as a high-voltage electrolyte component.