Investigation of the Physicochemical Properties of Pyrrolidinium-Based Mixed Plastic Crystal Electrolytes

Abstract

Organic ionic plastic crystals (OIPCs) are promising candidates for solid-state electrolyte materials for energy storage applications. Mixing of two OIPCs to produce new solid-state electrolyte materials is proposed to be a route to increasing defects/disorder in the materials, which may in turn promote ion transport. In this work, the thermal phase behavior and transport properties of two different pyrrolidinium-based binary OIPC mixtures were investigated. The most promising was the mixture of N,N-diethylpyrrolidinium bis(fluorosulfonyl)imide ([C2epyr][FSI]) and N-isopropyl-N-methylpyrrolidinium bis(fluorosulfonyl)imide ([C(i3)mpyr][FSI]), studied across the entire composition range, where the 10 mol % [C(i3)mpyr][FSI] mixture showed the highest ionic conductivity of 2 × 10-5 S cm-1 at 30 °C, consistent with the increased ion dynamics indicated by solid-state NMR analysis. Synchrotron XRD analysis revealed that the addition of 10 mol % [C(i3)mpyr][FSI] to [C2epyr][FSI] contributed to lattice expansion, hinting at increased defect volume and/or rotational disorder that assists with improved transport properties. Additionally, 10 mol % LiFSI was added to the chosen binary OIPC mixtures to investigate their potential use as electrolytes. The 10 mol % binary mixture with 10 mol % LiFSI showed the highest ionic conductivity (1.8 × 10-3 S cm-1 at 30 °C), while PFG analysis showed that the [FSI]− anions in the 10 mol % mixture with Li-salt have the highest diffusivity compared to other binary mixtures with Li-salt. Analysis of the structure-dynamics of mixed pyrrolidinium-based binary OIPCs provides insights into this scarcely explored strategy for improving the physicochemical properties of plastic crystal systems and toward the development of improved solid-state electrolytes for battery applications.