NMR Relaxometry and Diffusometry Analysis of Dynamics in Ionic Liquids and Ionogels for Use in Lithium-Ion Batteries

The charge transport dynamics of a novel solid-like electrolyte material based on mixtures of the ionic liquid (IL) [BMIM] TFSI and various concentrations of lithium salt LiTFSI confined within a SiO₂ matrix has been investigated. The translational diffusion coefficients of BMIM⁺, TFSI^- and Li⁺ in ILs and confined ILs (ionogels, IGs) with different concentrations of lithium salt have been measured at variable temperatures (20-100^oC) using pulsed field gradient nuclear magnetic resonance (PFG-NMR) spectroscopy. Additionally, the effect of confinement on IL rotational dynamics has been analyzed by measuring ¹H, ¹⁹F, and ⁷Li spin-lattice relaxation rate dispersions of IGs at different temperatures, using fast field-cycling NMR relaxometry. The analysis of the experimental data was performed assuming the existence of two fractions of the liquid: a bulk fraction (at least several ionic radii from the silica particles) and a surface fraction (close to the silica particles). The results show that the ion dynamics slowed only modestly under confinement, which evidences that IGs preserve IL transport properties, and this behavior is an encouraging indication for using IGs as a solid electrolyte for Li⁺ batteries.