TEMPO-Substituted Ionic Liquids As Redox Mediators for High Performance Lithium-Oxygen Batteries

Abstract

The performance of lithium-oxygen (Li-O2) batteries can be improved using redox mediators (RMs) to promote the decomposition of the insoluble, electronically insulating discharge product Li2O2 during the charging process. Homogenous RMs are expected to have a better performance in Li-O2 batteries than heterogeneous catalysts due to a larger contact area.1 Several redox mediators have been tested, including TEMPO and its derivatives.2 However, the use of Li-O2 batteries in many applications such as electric vehicles is still restricted due to electrolyte decomposition, flammability and volatility. In order to overcome these issues, ionic liquids have been successfully used as solvents for Li-O2 batteries.3, 4 In this project, we present TEMPO-substituted imidazolium ionic liquids that can perform the dual roles of solvent and redox mediator for Li-O2batteries (Figure 1). Figure 1. Structure and cyclic voltammograms of TEMPO-substituted imidazolium ionic liquids.Ionic liquids were prepared using modified literature procedures.5-7 Cyclic voltammetry of the ionic liquids in dimethylacetamide with 0.1 M lithium bis(trifluoromethane)sulfonimide was performed using a CHI660D electrochemical workstation in combination with a conventional three-electrode cell. Figure 1 shows cyclic voltammograms for compounds with 4, 5, and 8 carbon atoms in the chains linking the TEMPO and imidazolium moieties. The ionic liquids undergo a reversible electrochemical oxidation reaction at ~0.5 V vs Ag/AgCl (~3.8 V vs Li/Li+) which means that they should be capable of oxidizing the discharge product Li2O2 upon charging to give molecular oxygen and lithium ions. The facilitation of decomposition of Li2O2 can substantially reduce the charge overpotential, thus improving the energy efficiency of Li-O2batteries. References Feng, N.; He, P.; Zhou, H. (2016) Critical Challenges in Rechargeable Aprotic Li-O2 Batteries. Advanced Energy Materials 6:1502303-152326.Bergner, B. J., Hofmann, C., Schürmann, A., Schröder, D., Peppler, K., Schreiner, P. R., Janek, J. (2015) Understanding the fundamentals of redox mediators in Li–O2 batteries: a case study on nitroxides. Physical Chemistry Chemical Physics 17:31769-31779.Zhang, T., Zhou, H. (2012) From Li-O2 to Li-Air Batteries: Carbon Nanotubes/Ionic Liquid Gels with a Tricontinuous Passage of Electrons, Ions and Oxygen. Angewante Chemie, International Edition 51:11062−11067.Zhang, T., Wen, Z.-Y. (2017) A High-Rate Ionic Liquid Lithium-O2 Battery with LiOH Product. The Journal of Physical Chemistry C, doi: 10.1021/acs.jpcc.7b00336.Qian, W., Jin, E., Bao, W., Zhang, Y. (2006) Clean and selective oxidation of alcohols catalyzed by ion-supported TEMPO in water. Tetrahedron 62:556-562.Zheng, Z., Wang, J., Chen, H., Feng, L., Jing, R., Lu, M., Hu, B., Ji, J. (2014) Magnetic Superhydrophobic Polymer Nanosphere Cage as a Framework for Miceller Catalysis in Biphasic Media. ChemCatChem 6:1626-1634.Bonhôte, P., Dias, A.-P., Papageorgiou, N., Kalyanasundaram, K., Grätzel, M. (1996) Hydrophobic, Highly Conductive Ambient-Temperature Molten Salts. Inorganic Chemistry 35:1168-1178. Figure 1