Polymer-Coupled Local Dynamics Enhances Conductivity of Ionic Liquids

Abstract

Solvent and polymer-grafted nanoparticle addition to 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (HMIM-TFSI) was investigated by measuring the translational diffusion of cationic species in quasi-elastic neutron scattering experiments and ionic conductivity by electrochemical impedance spectroscopy. Adding polymers or nanoparticles to neat ionic liquids generally increases their viscosity and lowers the ionic diffusivity. In this work, we added two different solvents (acetonitrile and methanol) to HMIM-TFSI/grafted particle mixtures to understand the interplay between polymer–ionic liquid interactions and conformational state of brushes, which governs the transport properties (diffusivity and conductivity) of ionic liquids. Our results showed that with grafted chains swollen in acetonitrile, the cationic mobility and ionic conductivity were improved with the number of grafted chains on particles. The unusual high diffusivity attained with the addition of PMMA-grafted magnetic nanoparticles suggests that polymer-coupled ionic liquid dynamics can be effective in increasing the free cation amount and, therefore, ionic conductivity in particle-based electrolytes.