Ionic Liquids: nanostructuration and multiscale transport properties. Technological consequences for batteries.

The remarkable chemical and electrochemical stability of Ionic Liquids (ILs) makes them excellent candidates for the development of energy storage systems meeting strict criteria to battery safety in particular combustion. Bringing together three independent probes (neutrons, NMR and light-scattering), we conduct a detailed multiscale analysis of the cation self-diffusion process from the molecular to the mesoscopic scale. We show i) that nanometric transient self-association is a limiting factor to the electrochemical conductivity and ii) that the frustration of the formation of these aggregates by unidimensional nanometric confinement is a promising way to turn ILs into prime competitors against less stable benchmark electrolytes.

Ionic Liquids (IL) are electrolytes which are molten at room temperature and exclusively consist of organic anions and cations in direct interaction i.e. with no solvent. These systems are currently attracting a dual interest. First in technological terms in the field of electrochemical energy storage equipment, since their high chemical and electrochemical stability would enable them to meet major concerns about the pyrotechnic safety of batteries. But they also raise questions of a very fundamental nature. A vigorous debate is under way to define their exact physicochemical nature: strong (strongly dissociated) or weak (weakly dissociated) electrolytes.

Figure 1: a) Space and time mapping achieved over the present multi-scale study of the diffusion processes of BMIM-TFSI ionic liquid. b) Despite a difference in order of magnitude between the two quantities, a rheology experiment by DLS (red curve) makes it possible to bridge the large scale translational diffusion coefficients of the cations as measured on a molecular scale by neutron scattering (Dlr NSE) to the one derived at the mesoscopic scale by PFG-NMR (Dt NMR). From [4].

In a first approach, the coexistence of anions and cations in the absence of any co-solvent may lead to the conclusion that the system being characterized by a high concentration of charges is a strong electrolyte. However, anions and cations pairing could also lead to the formation of electrically neutral entities which, since they can be assimilated to an uncharged solvent, would greatly reduce the effective concentration of charges. We would then have to deal with a weak electrolyte. The puzzling aspect of ionic liquids is that if they are probed at the nanoscale (by Surface Force-SFA, for example) experimental data lead to define IL as weak electrolytes [1]. Conversely, measurements that probe the system at a macroscopic scale (electrochemical impedance measurements, for example) lead to an opposite conclusion: the system shows the characteristics of a strong electrolyte [2]. 
A key to solve the controversy would be to probe the displacement of charges from the molecular (nanoscale) scale to the macroscopic scale (a few microns). To date, no technique allows such a direct multiscale measurement. However, since electrical conductivity is controlled by the IL components mobility (Nernst-Einstein relation), measurement of the processes of translational diffusion of species can provide important information. Neutrons Scattering (time-of-flight Quasi-Elastic-QENS and Spin-Echo-NSE experiments) is an appealing technique for this type of multi-scale measurements from the very local scale (ps and Å) to a scale of a few nanometers and a few tens of nanoseconds. The scattering vector dependence (Q) of the correlation times also makes it possible to ensure that the processes are indeed of a diffusional nature. Field gradient NMR can then take over to reach micrometers and milliseconds. We have adopted this experimental strategy (Fig. 1a) and have focused on two imidazolium-based ILs complementary by respect to their self-structuring propensity: 1-octyl-3-methylimidazolium tetrafluoroborate (OMIM-BF4 [3], strongly nanostructured) and 1-butyl-3-methyl imidazolium bis(trifluoromethanesulfonyl)imide (BMIM-TFSI [4], moderately nanostructured).

Figure 2: Artist's view illustrating that the large-scale diffusion process is limited by the presence of transient nanometric aggregates in constant formation/breaking within the liquid. From [4]. Image courtesy of Adrian van Leen,

While we study by both neutron and NMR the diffusion processes of cations (as probed by the dynamics of their protons) we find a difference of two orders of magnitude between the diffusion coefficients measured at the molecular and mesoscopic scales. A particle probe rheology experiment based on nanometric tracers dispersed within the IL and followed by Dynamic Light Scattering (DLS) shows that these differences (Fig.1b) are due to the presence of nanometric aggregates (Fig. 2). This observation is an important element in clarifying the debate about the exact nature of ionic liquids. It further suggests that the frustration of the formation of nano-structuring of ILs by unidimensional confinement [5,6] is a promising way to significantly increase the conductivity performances of these systems.

Contact: J.-M. Zanotti (LLB, France)
Contact at the ILL: O. Czakkel

[1] M.A. Gebbie, M. Valtiner, X. Banquy, E.T. Fox, W. A. Henderson and J. N. Israelachvili, Ionic liquids behave as dilute electrolyte solutions, PNAS, 110, 9674 (2013).
[2] A. A. Lee, D. Vella, S. Perkin and A. Goriely, Are Room-Temperature Ionic Liquids Dilute Electrolytes, J. Phys. Chem. Lett., 6, 159 (2015).
[3] F. Ferdeghini, Q. Berrod, J. -M. Zanotti, P. Judeinstein, V. Garcia Sakai, O. Czakkel, P. Fouquet and D. Constantin, Nanostructuration of Ionic Liquids: impact on the cation mobility. A multi-scale study, Nanoscale, 9, 1901 (2017).
[4] Q. Berrod, F. Ferdeghini, J.-M. Zanotti, P. Judeinstein, D. Lairez, V. García Sakai, O. Czakkel, P. Fouquet and D. Constantin, Ionic Liquids: evidence of the viscosity scale-dependence , Scientific Reports, 7, 2241 (2017).
[5] Q. Berrod, F. Ferdeghini, P. Judeinstein and J.-M. Zanotti, Patent FR1552572 (2016).
[6] Q. Berrod, F. Ferdeghini, P. Judeinstein, N. Genevaz, R. Ramos, A. Fournier, J. Dijon, J. Ollivier, S. Rols, D. Yu and J.-M. Zanotti, Enhanced ionic liquid mobility induced by confinement in 1D CNT membrane, Nanoscale, 8, 7845 (2016).