Development of Plastic-Crystal Based Electrolytes Using Novel Dicationic Salts

Shanika Abeysooriya – @IamShanika_SL

Organic ionic plastic crystals (OIPCs) are promising candidates as solid-state electrolyte materials for Li battery applications. OIPCs consist entirely of ions and possess a long-range ordered crystalline lattice but short-range disorder. They possess advantageous properties such as non-volatility, non-flammability, high ionic conductivity, and wide electrochemical window. This has prompted researchers to investigate different combinations of cations and anions to produce new OIPCs. Many mono-cation and anion combinations have been investigated to develop new plastic crystal systems. However, the unique structure of dications increase the number of possible combinations, for new materials with wider range of physicochemical properties. As structure-property relationships related to dicationic organic salts are seldom explored, this study reports 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). The physicochemical properties and local structure dynamics were investigated by solid-state NMR and correlated with thermal analysis and the ionic conductivity. [C2-Pyrr3][TFSI]2, with the longer alkyl-side chain, showed improved transport properties compared to [C2-Pyrr2][TFSI]2. Further exploration of the organic salts as potential electrolyte material was conducted by mixing with 10 mol% lithium TFSI, where highest increase in conductivity was recorded for 10 mol% LiTFSI in [C2-Pyrr3][TFSI]2. Solid-state NMR analysis revealed that Li+ and [TFSI]- ions acted as the major contributors for ionic conductivity, while the dications in the bulk structure were less mobile.