Magnetic Resonance Imaging and Molecular Dynamics Characterization of Ionic Liquid in Poly(ethylene oxide)-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 ¹H microimaging [magnetic resonance imaging (MRI)] techniques and molecular dynamics (MD) simulations to study the translational and rotational dynamics of the N-butyl-N-methylpyrrolidinium (PYR₁₄) cation in poly(ethylene oxide) (PEO) matrices containing the lithium bis(trifluoromethanesulfonyl) imide salt (LiTFSI) and the PYR₁₄TFSI IL. The analysis of diffusion-weighted images in PEO/LiTFSI/PYR₁₄TFSI samples with varying mole ratios (10:1:x, with x = 1, 2, 3, and 4) shows, in a wide range of temperatures, a spatially heterogeneous distribution of PYR₁₄ diffusion coefficients. Their weight-averaged values increase with IL content but remain well below the values estimated for the neat IL. The analysis of T₂ (spin-spin relaxation) parametric images shows that the PEO matrix significantly hinders PYR₁₄ 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 PYR₁₄ diffusion is mainly affected by Li/TFSI coordination within the IL phase. In agreement with MRI experiments, increasing the IL content increases the PYR₁₄ 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.