Plastic crystal-based electrolytes using novel dicationic salts

Abstract

The unique structures of dications increase the number of possible combinations of cations and anions that can be used to obtain new materials with a wide range of physicochemical properties. However, structure–property relationships related to dicationic organic salts are seldom explored. Here, we report the synthesis and characterization of two new dicationic salts, 1,2-bis(N-ethylpyrrolidinium)ethane bis(trifluoromethanesulfonyl)imide ([C₂-Pyrr2][TFSI]₂) and 1,2-bis(N-n-propylpyrrolidinium)ethane bis(trifluoromethanesulfonyl)imide ([C₂-Pyrr3][TFSI]₂). To investigate the physicochemical properties of the organic salts, local structure and dynamics were investigated by variable temperature solid-state NMR and correlated with the thermal analysis and ionic conductivity. These studies revealed that [C₂-Pyrr3][TFSI]₂, with the longer alkyl-side chain on the dication, showed improved transport properties compared to [C₂-Pyrr2][TFSI]₂. Further exploration of the organic salts as potential electrolyte materials was conducted by mixing with 10 mol% lithium bis(trifluoromethanesulfonyl)imide (LiTFSI). This study demonstrates the effect that lithium salt addition has on thermal and ionic conductivity properties, where the largest increase in conductivity was found for [C₂-Pyrr3][TFSI]₂/LiTFSI (10 mol% LiTFSI). Solid-state NMR analysis revealed that Li⁺ and [TFSI]⁻ ions acted as the major contributors to ionic conductivity while the dications in the bulk structure showed lower mobility.