MAGNETIC RESONANCE IMAGING AND MOLECULAR DYNAMICS CHARACTERIZATION OF IONIC LIQUID IN PEO-BASED POLYMER ELECTROLYTES.

Ternary systems consisting of polymers, lithium salts, and ionic liquids (ILs) are promising materials for the development of next generation lithium batteries. The ternary systems combine the advantages of polymer-salt and IL-salt systems, thus providing media with high ionic conductivity and solid-like mechanical properties. In this work, we apply nuclear magnetic resonance H microimaging (MRI) techniques and molecular dynamics (MD) simulations to study the translational and rotational dynamics of the N-butyl-N-methylpyrrolidinium (PYR) cation in poly(ethylene oxide) (PEO) matrices containing the lithium bis(trifluoromethanesulfonyl) imide salt (LiTFSI) and the PYRTFSI IL. The analysis of diffusion-weighted images in PEO-LiTFSI-PYRTFSI samples with varying mole ratios (10:1:x, with x=1, 2, 3, 4) shows, in a wide range of temperatures, a spatially heterogeneous distribution of PYR diffusion coefficients. Their weight-averaged values increase with IL content, but remain well below the values estimated for the neat IL. The analysis of T (spin-spin relaxation) parametric images shows that the PEO matrix significantly hinders PYR rotational freedom, which is only partially restored by increasing the IL content. The MD simulations, performed on IL-filled cavities within the PEO matrix, reveal that PYR diffusion is mainly affected by Li-TFSI coordination within the IL phase. In agreement with MRI experiments, increasing the IL content increases the PYR diffusion coefficients. Finally, the analysis of MD trajectories suggests that Li diffusion mostly develops within the IL phase, although a fraction of Li cations is strongly coordinated by PEO oxygen atoms.