Structure and dynamics of an ionic liquid-based electrolyte confined in porous materials
Janis HESSLING1, Monika SCHÖNHOFF1
1University of Münster, Münster, Germany
Ionic Liquids (ILs) are promising electrolyte components for electrochemical energy storage devices due to their high electrochemical stability and low volatility and flammability. Especially in Li-ion batteries, these favourable properties represent an advantage over commonly used organic electrolytes. However, the poor Li-conductivity, high viscosity and unfavorable Lithium-anion clusters are major drawbacks of ILs as battery electrolytes. The confinement of ILs in porous materials could solve these problems, due to altered structural and physiochemical properties of the confined ILs.
The aim of this work is to investigate the modified dynamics of a confined IL/Li-salt mixture and reveal whether the confinement has positive effects on ion clustering in IL/Li-salt systems. To this end, a mixture of 1-butyl-1-methylpyrrolidinium bis(trifluormethyl-sulfonyl)imide (Pyr14TFSA) and LiTFSA is confined in the silica SBA?15 with a pore diameter of 4 nm and the metal-organic frameworks (MOFs) ZIF-8 and MOF-177 with pore diameters of 1.16 nm and 1.08 nm. Resulting samples are investigated by NMR methods, mainly temperature-dependent 7Li spin-lattice (T1) relaxation rates, which are analyzed using the Bloembergen, Purcell, and Pound (BPP) model. Furthermore, Raman spectra elucidate the Li-anion coordination in detail, providing information about the fraction of Li-coordinated anions.
The BPP-model provides a very good fit of measured 7Li spin-lattice relaxation rates, enabling statements on the local environment, the local dynamics and its activation. In SBA-15 and MOF-177, the local dynamics of Li+ is reduced upon confinement, suggesting an overall reduction of liquid dynamics, as expected due to geometrical restrictions. The local Li+ environment is unchanged in SBA-15 with persisting Li-TFSA clusters and only slightly more asymmetric in MOF-177. Surprisingly, however, upon confinement in ZIF-8 Li+ displays faster local dynamics and a more asymmetric environment, suggesting the breakup of Li-TFSA clusters. These results are supported by Raman spectroscopy showing that Li+ confined in SBA-15 is experiencing nearly the same coordination as in the neat IL/Li-salt system, whereas Li+ confined in ZIF-8 shows a different coordination to TFSA-. Raman spectroscopy furthermore suggests that in SBA-15 the TFSA- ion is interacting with the pore walls, whereas in ZIF-8 the Pyr14+ ion is immobilized by the pore walls. These results demonstrate a strong influence of internal interfaces on IL structure and dynamics and might bear potential for tailoring ion dynamics.