TOPICAL DAY - Ionic liquids under nanometric unidimensional confinement : a route for a better electrolyte ?

Orateur : Filippo FERDEGHINI (Laboratoire des Interfaces Complexes et de l’Organisation Nanométrique, ECE-Paris Ecole d’Ingénieurs, 37 Quai de Grenelle, 75015 Paris, France)

This work is a contribution to a global worldwide effort to the development of new safe and sustainable systems for the energy storage. Due to their electrochemical and physical/chemical stability, we have focused our efforts on a particular class of electrolytes : Ionic Liquids (ILs)[1]. The competition between electrostatic and van der Waals interactions of the charged and a-polar alkyl side-chain(s) moieties of their cations drive a specificity of IL : a nanometric structuration[2-3]. We identify confinement as a way to circumvent the nanostructuracion and hence turn the IL to a homogeneous liquid (with no density fluctuation) showing improved transport properties. We have confined imidazolium-based ILs in carbon nanotube (CNTs) membranes. These systems show a macroscopic orientation of their cylindrical pore network, with a hydrophobic porous surface. We study the self-organization/dynamics correlation in the case of bulk ILs. We investigate the nanometer/nanosecond dynamics of bulk OMIM-BF4 by Quasi-Elastic Neutron Scattering (QENS) and Neutron Spin-Echo (NSE). We also probe the same IL on the microscopic (μm and ms) scale by Pulsed Field Gradient NMR[4]. To interpret the neutron data, we introduce a new physical appealing model to account for the dynamics of the side-chains and for the diffusion of the whole molecule, which is able to describe the observables over the whole and unprecedented investigated Q ([0.15 -2.5] Å-1) and time ([0.5 -2000] ps) ranges. When the OMIM-BF4 is confined in CNT membranes (tubes’ diameter ≈ 2 nm), no strong dynamical effect of confinement has been detected by QENS at the molecular scale (ps-ns) both for the radial and longitudinal dynamics of IL. But we have been able to show that at the microscopic (by Pulsed Field Gradient NMR) and macroscopic scale (by impedance spectroscopy), we can obtain a remarkable enhancement of factor 3 of the electrolyte’s transport properties, due the severe (one dimensional) confinement conditions[5]. A patent has been filed[6] on the use of CNT membranes as a possible solution to boost the transport properties and hence the specific power of lithium batteries.

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