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