Plastic crystal-based electrolytes using novel dicationic salts.

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(-ethylpyrrolidinium)ethane bis(trifluoromethanesulfonyl)imide ([C-Pyrr2][TFSI]) and 1,2-bis(-propylpyrrolidinium)ethane bis(trifluoromethanesulfonyl)imide ([C-Pyrr3][TFSI]). 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 [C-Pyrr3][TFSI], with the longer alkyl-side chain on the dication, showed improved transport properties compared to [C-Pyrr2][TFSI]. 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 [C-Pyrr3][TFSI]/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.