New organic ionic plastic crystals based on pyrrolidinium dication for a solid‐phase electrolyte

Abstract A series of 1,4‐bis( N ‐alkylpyrrolidinium)butane salts combined with Br − , I − , PF 6 − , and Tf 2 N − anions were newly designed and synthesized. In nonpolar CDCl 3 solution, the axial and equatorial proton peaks of the bis‐pyrrolidinium cation ring are distinguished in 1 H NMR. Also, the strengths of ion paring could be estimated from the 1 H NMR chemical shift changes of the bridge and pyrrolidinium proton peaks. 1,4‐Bis( N ‐hepthylpyrrolidinium)butane I − is an organic ionic plastic crystal showing the lowest fusion entropy value ( Δ S f = 3.4 J K·mol −1 ). Softer crystalline morphologies were found by polarized optical microscope images at plastic crystal phase of each sample and the results were corresponding to differential scanning calorimetry results. 1,4‐Bis( N ‐ethylpyrrolidinium)butane Tf 2 N − showed the softest crystal phase. Ionic conductivity changes and electrochemical stabilities of the bis‐pyrrolidinium PF 6 − and Tf 2 N − salts were also measured, and they were stable up to 4.3 V (vs. Li/Li + ) and expected to be applied as an electrolyte component.