Recent Progress in the Potential of Ionic Liquids as High Temperature Electrolytes for Energy Storage Systems

Ionic liquids are low-temperature molten salts with unique properties such as low volatility, extensive liquid range, high thermal stability, and high solubility of organic, inorganic, and polymeric materials. Ionic liquids find applications both in industry and academia because of the flexibility to tune their properties according to the requirements. Ionic liquids are promising electrolytes for energy storage systems because they can address the safety concerns associated with using organic electrolytes. Because of the growing interest in ionic liquids-based electrolytes, many ionic liquids are available on the market. The electrochemical properties of ionic liquids are susceptible to their purity, and there is no systematic study on the electrochemical properties of commercially available ionic liquids.<br/><br/>In this MRS submission, we are going to present the recent progress of research and development of ionic liquids as high temperature electrolytes for energy storage. The progress will be based on the literature from other’s works as well as from our research group’s works. We have published an important milestone where the electrochemical stability of 22 commercially available and commonly used ionic liquids were studied using cyclic voltammetry in the temperature range 288.15 K to 358.15 K. Ionic liquids based on imidazolium, pyrrolidinium, piperidinium, and tetraalkylammonium cations combined with bis(fluorosulfonyl) imide and bis(trifluoromethanesulfonyl) imide were selected because of their hydrophobic nature. The temperature sensitivity of the anodic and cathodic potential limits was evaluated by combining the cyclic voltammetry experiments and a linear regression model. The ionic liquids under investigation displayed an electrochemical stability window of 4.1 V to 6.1 V. This study revealed that temperature had a mixed effect on the electrochemical stability of the ionic liquids. Despite the structural similarity of anions used in the study, bis(trifluoromethanesulfonyl) imide anion-based ionic liquids showed better electrochemical stability than the bis(fluorosulfonyl) imide anion analog due to the presence of more fluorine atoms. The increase in the alkyl chain length on the cation increased the electrochemical stability of the ionic liquids due to their ability to transfer electrons to the hetero atom on the cation. It was speculated that the presence of ether functionality could increase the electrochemical window of the ionic liquids because of the interaction between the oxygen atom on the ether group and the positively charged nitrogen atom on the cation. On the contrary, introducing the ether group to the cationic core reduced the electrochemical stability window. The inability of the ether functionality to donate electrons to the positively charged nitrogen atom and decrease in the electron density on the positively charged hetero atom due to its interaction with the ether group was responsible for the lower electrochemical stability of ether-containing ionic liquids. A similar effect was observed with the functionalization of the cation with aromatic groups. Pyrrolidinium and piperidinium-based ionic liquids showed higher electrochemical stability windows among the different cations studied. This might be because of the difference in their electrochemical degradation mechanism compared to imidazolium and tetraalkylammonium cations.