Development of New Plastic-Crystal Based Electrolytes using Pyrrolidinium- Bis(fluorosulfonyl)imide Dicationic Salts

Abstract

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.