Fire-Retardant Pyrrolinium-Based Ionic Liquids As Electrolytes for Lithium Ion Batteries

Lithium ion batteries (LIBs) have some advantages such as a high energy density, negligible memory effect and a less self-discharge. Consequently, they have been applied to not only small electronic devices but also large energy storage systems. Although LIBs have been considered as one the most promising energy conversion/storage systems, the safety issue of LIBs must be overcome to realize large, high-power, and high-voltage LIBs. To get over the limitation of safety, many kinds of approach have been attempted to enhance the safety of the LIBs and one of the efficient ways is partial or total replacement of combustible organic electrolytes with a new class viscous materials, acting as poorly flammable and/or flame retardant components. In this respect, ionic liquids (ILs) have been considered as attractive electrolyte candidates because they are non-flammable owing to its unique chemical nature.

 The ILs are salts that remain as liquid at room temperature even if they are composed of ionic species, cations and anions. Due to the ionic nature in bonding, ILs show favorable physicochemical properties such as high ionic conductivity, non-volatility, and non-flammability as well as a wide liquid range and wide electrochemical stability window. Previously reported imidazolium-based ILs with low viscosity and high ionic conductivity have some defects such as decomposition of an acidic proton at the C-2 position^[1]. To overcome the decomposition imidazolium ring, we have studied saturated cyclic ammonium-based ILs based on pyrrolidinium^[2] or piperidinium^[2], which showed more stable electrochemical behavior compared to the imidazolium-based ILs, but less favored physicochemical properties such as high viscosity and lower ionic conductivity.

 In this work, some novel pyrrolinium-based ionic liquids have been prepared and studied as electrolytes for LIBs to address those problems. Introduction of a beneficial functional group such as an oxygen atom into the partially saturated pyrrolinium ring was expected to give some positive effects on both the electrochemical stability and the physicochemical properties. Therefore, several target ILs that were synthesized in the present study showed higher thermal stability than the combustible organic solvent such carbonates. We will present the thermal, physicochemical, and electrochemical properties of several pyrrolinium-based ionic liquids as electrolytes, which are compared to those of the conventional carbonate.

[1] Bull. Korean Chem. Soc., 2006, 27, 847

[2] Bull. Korean Chem. Soc., 2007, 28, 1567