Application and Characterization of Pyrrolinium-based Ionic liquids as Electrolytes for Lithium Ion Batteries

Abstract

Lithium ion batteries (LIBs) have been used as a battery of portable electronic device and concerned as a one of the most promising energy conversion/storage systems for large-scale devices such as electric vehicles (EVs) and energy storage systems (ESSs). However, the need for the high energy density of current LIBs have been increased, which results that many research have been focus on the advanced electrode materials for high working voltage, a large specific capacity to enhance overall energy density of the lithium ion cells. In other words, the relatively low safety of current organic electrolyte should be overcome to catch up with the enhancement of the energy density for reaching the high level of LIBs. For increasing the safety performance of the conventional electrolyte, many attempts have been studied to apply ionic liquids as alternative electrolyte or additive. Ionic liquids are ionic species that remain in liquid phase at room temperature even if they are composed of ionic species, cations and anions. Due to ionic interaction, they have unique properties such as high ionic conductivity, non-volatility, and non-flammability as well as a wide liquid range and wide electrochemical stability window. In spite of these advantages, the ILs are not useful as electrolytes for LIBs owing to somewhat defect such as decomposition or high viscosity. As already reported, the well-known ILs such as imidazolium-based ILs have been investigated to allow them apply the LIBs, which is not carried on its acidic proton of C-2 position in imidazolium structure. To exclude the active proton, pyrrolidnium and piperidinium-based ILs have been researched

however, their relatively high viscosity is the obstacle to facilitate in LIBs. To enhance both the mobility of Li+ and the safety performance of LIBs, we suggest the pyrrolinium-based ILs with multifunctional groups such as a planar sp2 carbon of C=N double bond, a C-O ether linkage, and no unstable C-H bond, which would be beneficial to improve the physical properties as well as electrochemical performances. Among the prepared pyrrolinium-based ILs, the N-ethyl-2-methoxypyrrolinium bis(fluorosulfonyl)imide (E(OMe)Pyrl-FSI) exhibit the highest value of ionic conductivity, and the N-allyl-2-methoxypyrrolinium bis(fluorosulfonyl)imide (A(OMe)Pyrl-FSI) shows the best electrochemically performance and lithium ion mobility. To obtain the synergetic effect of pyrrolinium-based ILs and carbonate solution, we prepared the binary electrolytes systems composed of two type electrolytes. Among the prepared pyrrolinium-based ILs, the E(OMe)Pyrl-FSI is chosen as the one of the binary electrolytes systems due to its highest value of ionic conductivity. In ionic conductivity of these binary electrolytes, the optimum compositions are 40 wt% and 60 wt% of pyrrolinium-based ILs. Additionally, when the 60 wt% of E(OMe)Pyrl-FSI (E60) is in binary system, the SET (self extinguish time, s g-1) is outstandingly reduced. In electrochemical performance, all of the binary electrolytes exhibit the similar results that of the conventional carbonate. It should be noted that the E 60 binary system is the best synergetic effect on both the electrochemical performance and fire-retarding characteristic.