Physical Properties, Electrochemical Stabilities and Molecular Dynamics Study of Novel Ionic Liquids Based Electrolytes for Electrochemical Application

Abstract

A series of novel ionic liquids (ILs) based on N-allyl-N,N-dimethylethylammonium (N112A +) are synthesized and characterized. Their physical properties and electrochemical stabilities are investigated and discussed toward the anion constituents. Herein, four anions are studied including one non-fluorinated and planar structured anion (DCA-) and three fluorinated anions (TFSI-, OTf-, TFA-). The experimental properties are compared to those predicted by molecular DFT dynamics simulations. All salts were found to be low melting compounds, while being all-liquid at room temperature. N112A-TFSI is found to be the most thermally stable (T d = 329°C) while N112A-TFA is the least stable (Td = 158°C). The ILs transport properties have been determined versus temperature and discussed using the ionicity and Angell's fragility, concepts. Among the good and fragile prepared ILs, N112A –DCA exhibits outstanding transport properties in terms of low dynamic viscosity (19.97 mPa.s) and high ionic conductivity (19.20 mS.cm−1) at 298 K. These suitable properties are explained, from DFT computational calculations, by the lower intermolecular interactions of ion-pairs in N112A –DCA, and quantified by the decreased value of dispersion ion-pairs interaction energy (-6 kJ.mol-1) compared to the others ones. The ILs electrochemical stability window (ESW) seems strongly affected by the cation structure, while their values are found to decreases in the following order N112A -TFSI (4.40 V) > N112A-OTf (3.80 V) > N112A-DCA (3.00 V) > N112A-TFA (2.18 V). Taking into account of the most likely oxidation and reduction reactions, these trends were discuss and validated by the SMD calculation model. The experimental values of ESWs are also very close to the predicted ones. Overall, the results show that, these ILs are suitable as electrolytes for electrochemical devices as battery systems and supercapacitors.