The thermal and structural properties of novel pyrrolidinium dicationic salts with varying alkyl-chain lengths are investigated, neat and in combination with lithium salts.

Dicationic organic salts are an interesting class of solid‐state electrolyte materials due to their unique structure. Here we present, for the first time, the synthesis and characterization of three dicationic‐FSI salts, 1,2‐bis(N‐methylpyrrolidinium)ethane bi(bis(fluorosulfonyl)imide) ([C2‐Pyrr1][FSI]2), 1,2‐bis(N‐ethylpyrrolidinium)ethane bi(bis(fluorosulfonyl)imide) ([C2‐Pyrr2][FSI]2) and 1,2‐bis(N‐n‐propylpyrrolidinium)ethane bi(bis(fluorosulfonyl)imide) ([C2‐Pyrr3][FSI]2). The structure and dynamics of the organic salts were probed using variable temperature solid‐state NMR and were compared with the thermal and transport properties. The investigation revealed that [C2‐Pyrr1][FSI]2, with shorter alkyl‐side chains on the dication, displayed increased transport properties compared to [C2‐Pyrr2][FSI]2 and [C2‐Pyrr3][FSI]2. To determine the proficiency of these dicationic‐FSI salts as electrolyte materials for battery applications, 10 mol% and 50 mol% lithium bis(fluorosulfonyl)imide (LiFSI) was mixed with [C2‐Pyrr1][FSI]2 and [C2‐Pyrr2][FSI]2. Increased transport properties were observed for [C2‐Pyrr1][FSI]2/10 mol % LiFSI in comparison to [C2‐Pyrr2][FSI]2/10 % LiFSI, while pulse field gradient NMR analysis revealed the highest Li+ self‐diffusion ratio for [C2‐Pyrr1][FSI]2/50 % LiFSI out of the four Li‐salt‐containing mixtures.

Organic ionic plastic crystals (OIPCs) are promising candidates for solid-state electrolyte materials for energy storage applications. Mixing of two OIPCs to produce new solidstate 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), 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 [C 2 epyr][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.