NMR Diffusion Study of Ionic Liquids and Ionogels for Use in Lithium Ion Batteries

Abstract

Solid-state electrolytes have been explored increasingly for use in Li+ batteries on account of their ability to address the safety issues, packaging constraints, and volumetric design concerns associated with liquid electrolytes. Recently, ionogels, pseudo-solid-state electrolytes consisting of an ionic liquid electrolyte confined in a mesoporous inorganic matrix, have attracted interest because of their high ionic conductivity, stability, and solution processability. Due to the complexity of the trapped liquid, the nature of the ion interaction and movement is not well established. Understanding how the ions behave inside the matrix is paramount for increasing the ionic conductivity and Li+ transference number. The literature for these characteristics is lacking in several key areas including, but not limited to, variations in the electrolyte’s Li+ concentration, the choice of silica precursors, and diffusion under different conditions. The ionogel matrix was synthesized using a non-aqueous organic acid catalyzed sol-gel process to form a nanoporous silica matrix around an ionic liquid, BMIM TFSI with variable amounts of LiTFSI added, thus confining the electrolyte. The choice of silica precursors and use of an organic acid allows for a crack free monolith to form under rapid gelation conditions. The silica component maintains a highly porous network with surface areas up to 1300 m2/g and an average pore size of 20nm as measured through N2 adsorption. The ionic liquid (IL) electrolyte component constitutes 75 vol% of the ionogel. Thermal gravimetric analysis shows that the ionogel is thermally stable to 400 oC. Due to the interconnected porosity and large percentage of liquid, conductivities of ̴10-3 S/cm were measured at room temperature for the prepared monoliths using electrochemical impedance spectroscopy (EIS). The degree to which each ion contributes to the total conductivity can be determined by combining the conductivity measurements with self-diffusion measurements obtained by pulsed field gradient nuclear magnetic resonance (PFG NMR) methods. Furthermore the presence of ion pairing or aggregation can be ascertained by direct comparison between the NMR-predicted and EIS-measured conductivity. In this work, the PFG NMR stimulated echo sequence was used to measure the self-diffusion coefficients of the IL cation, Li+, and anion via 1H, 7Li and 19F, respectively, in the IL and ionogel with a concentration of 1M LiTFSI, at variable temperature covering the 0o – 100oC range. Two main confinement effects of all mobile ions in the ionogel are observed (i) a reduction in self-diffusion values of all ions compared to the pure IL; (ii) the presence of a relatively immobile fraction of the ions. Interestingly the effective Li transference numbers defined as D(Li)/[D(Li) + D(BMIM) + D(TFSI)] in the liquid electrolyte and the and ionogel are comparable, with a value of approximately 0.20.