Catching TFSI: A Computational-Experimental Approach to Β-Cyclodextrin-Based Host-Guest Systems as electrolytes for Li-Ion Batteries

Cyclodextrins (CDs) are pyranoside-based macromolecules with a hydrophobic cavity to encapsulate small molecules. They are used as molecular vehicles, for instance in pharmaceutical drug delivery or as solubility enhancer of monomers for their polymerization in aqueous solution. In this context, it was discovered about 10 years ago that the bis(trifluoromethylsulonyl)imide (TFSI) anion forms host-guest complexes with beta CD in aqueous media. This sparked interest in using the TFSI anion in lithium-based battery electrolytes open for its encapsulation by beta CD as an attractive approach to increase the contribution of the cation to the total ion conductivity. By using semi-empirical quantum mechanical (SQM) methods and the conductor-like screening model for a real solvent (COSMO-RS), a randomly methylated beta CD (RM beta CD) is here identified as a suitable host for TFSI when using organic solvents often used in battery technology. By combining molecular dynamics (MD) simulations with different NMR and FTIR experiments, the formation of the corresponding RM beta CD-TFSI complex was investigated. Finally, the effects of the addition RM beta CD to a set of electrolytes on the ion conductivity are measured and explained using three distinct scenarios.