Ionic Liquids

Ionic liquids are an exciting class of materials with important energy applications. Ionic systems which are liquid at or near room temperature are attracting a great deal of attention in applied physics and chemistry as novel, and more importantly, green solvents suitable as active media in batteries [12]. This is a rich field of research potential.  These materials are often built up from a bulky asymmetric cation and an inorganic anion and their versatility derives from the fact that there are hundreds of cations and anions producing tens of thousands of mixing combinations alone.  This mixing produces a diversity of properties relevant to battery design including conductivity, density and cost [13].  The most fundamental elements of a battery are a set of anode and cathode materials and between them some electrolyte materials for ion conduction.  One of the most interesting and exciting application for ionic liquids is as an electrolytic medium in Lithium batteries.  The common lithium battery electrolyte medium actually doesn’t conduct very well [14] and is prone to oxidation and device failure under conditions of modest overcharge.  In order for ionic liquid replacement of lithium ion battery electrolytic media, our fundamental understanding of microstructure must improve.  Their low melting points and low vapor pressures make them particularly easy to work and perfect for an undergraduate laboratory.  Over the past three years with and my students and I conducted a series of x-ray diffraction, EXAFS, and Anomalous x-ray diffractions measurements at the APS of imidazolium- and pyrrilidium- based ionic liquids at various temperatures.   We worked with Doug Robinson at Sector 6 to modify a low temperature furnace to take diffraction measurements at room temperature up to nearly 250 °C.

We recently published a paper [15] suggesting that, not only does extended order form in several ionic liquid systems with various cation alkyl chain lengths, but that order demonstrates an anomalous temperature dependency.  As shown below in Fig. 3, for an imidazolium-based ionic liquid, as the temperature is raised, the order is enhanced, counter to expected trends.  This result is important because it provides insight into the dominant bonding motifs in the liquid but it also provides a quantitative validative mechanism for molecular dynamics simulations of these ionic liquids, which are ongoing with my collaborator.

Figure 3—Comparison between the experimental structure factors of C4MIM/TF at different temperature, spanning the range of acquired data.  The high-q data have no temperature dependence while the low-q data (inset) show anomalous correlation enhancement with increasing temperature.  (Figure taken from Ref [15])

 

The microscopic structure of these liquids is a subject of great current interest and intense debate and it is clear that our current understanding on liquid structure is insufficient to describe the observed behavior.  How and why does the conductivity display non-monotonic behavior with temperature and composition?

It has also been observed that conductivity in classes of ionic liquids based on 1-Butyl-1-methylpyrrolidinium triflate demonstrates a non-monatomic behavior with temperature [16]. We postulate that the extended structures observed in the ionic liquids dictate the conductivity and diffusivity.  We have some initial results from experiments at Sector 12 at the APS suggested extended order (20-30 nanometers) where we took small angle x-ray diffraction data (shown below in Fig. 4) and found a distinct difference between different chain lengths.  Essentially a peak at a specific momentum transfer (q) means there are structures at that length scale. We see that the shortest length (red circles) shows very little structure, while the longest chain length (black triangles) shows enhanced correlations.  Further, the peak position shifts to larger length scales with increasing chain length, perfectly consistent with the notion that these correlation indicators are based in aggregate structures.

 

Figure 4-Initial Small Angle X-ray Diffraction (SAXS) data on imidazolium-based ionic liquids with varying cation alkyl chain lengths.  The shortest length (red circles) shows very little structure, while the longest chain length (black triangles) shows enhanced correlations. (Unpublished at this time)

 

 

These compounds are technologically interesting, have proven challenging to synthesize (yet possible), and are systems that demonstrate ordering on multiple length scales.   In order to begin to address these fundamental question about the connection between structure and transport, I’ve been working to utilize resonant scattering experiments that overcome some inherent challenges presented by standard elastic x-ray diffraction.  Specifically, with high energy x-ray scattering, everything scatters, thus distinguishing between cation and anion correlations becomes difficult.  There are two experimental resonant absorption and scattering techniques I’ve been investigating in these systems, both showing promise as providing information about component-specific order.

 

1. Extended X-ray Absorption Fine Structure

In this technique, a variable energy x-ray beam is directed through a sample and the relative absorption is measured as a function of energy.  When that energy range is tuned to around the absorption k-edge of an element, the attenuation fluctuates because the resultant wave function of the photoelectron is a standing wave whose wavelength is dictated by the incident energy.  The local coordination around an absorber influences the boundary conditions to which Schrödinger’s equation is solved, affecting the amplitude of the wave function and thus the absorption itself is 1.) a function of energy and 2.) dictated by the local environment.  Of interest in ionic liquids is the transport of charged ions in electrolytes in batteries or sensors.  As such, there is interest in determining the local environment around those ions and how we can change that environment.  As a test of this techniques efficacy, we doped a series of ionic liquids in the imidazolium TFSI family with potassium and conducted EXAFS experiments this past summer at Sector 20 with my students and Dr. Mali Balasubramanian.  We overcame a number of technical challenges associated with these samples and obtained in initial results, yet to be fully analyzed, showing that the technique is feasible.  Below (Figs. 5-8) describe the initial results presented at a poster session this fall.  Our work will continue to analyze these results and use them as the basis for further beamtime proposals.

Figures 5-8.  Initial results presented at the North Central College Summer Undergraduate Research Poster Session by Sven Marnauzs and Nicholas Mauro.  (Unpublished at present)

 

2. Anomalous X-ray Scattering

In this technique, we utilize the fact that a specific atom absorbs x-rays differently as the energy of those x-rays changes a few electron volts around the absorption k-edge.  For certain Transition Metal-containing ionic liquids, when we take x-ray diffraction patterns at different energies on either side of the absorption edge, the transition metal atoms preferentially absorb x-rays incident on them when the energy is just higher than the k-edge, while the absorption from the other elements isn’t changed.  By taking the difference between these patterns and taking a Fourier transform, we can determine the real space distribution function of just the transition metal.  This is extremely useful because it provides chemically specific structural information that, combined with simulations, helps to determine dominant structures.  This is, to my knowledge, the first time this technique has been used in these systems.  My students and I spent two days at the Advanced Photon Source this summer at Sector 33 with Dr. Zhan Zhang in a proof of concept experiment.  We overcame a number of challenges associated with sample preparation and mounting and were successful in obtaining energy-dependent structural data on Co-, Cu-, and Zn-based ionic liquid complexes, one of which is shown below in Fig. 9. Moving forward, we must proceed with a complicated analysis of the data, however, this may prove to be a powerful technique, particularly now that we have compound syntheses determined and sample preparation issues identified and mitigated. My goal is to run a series of temperature-dependent investigations at while we work through the analysis of the data.

Figure 9-Integrated intensity for a Zn-Butyl ionic liquid at room temperature at multiple energies near the absorption edge (E  = 9.663 keV). (Unpublished at present.)

 

It has been observed that doping these ionic liquids with lithium, sodium, magnesium and potassium results in dramatically different conductivity [16], but the effect of the doping on atomic and molecular structure remains to be seen. We suspect that changes in conductivity will strongly correlate with the structural evolution of the liquids, however, it remains an interesting and open question what the nature of that structural ordering is. These investigations will begin to address this important question and will lead to a better understanding of how to tailor ionic liquids for battery applications.

References:

[12] C.S. Santos et al.  The Journal of Chemical Physics, 134 (2011) 064501.

[13] E. Bodo et al.  The Journal of Physical Chemisty, 114 (2010) 16398.

[14] A.P Abbott and K.S. Ryder, Annual Review of Materials Research, 43 (2013) 335-358.

[15] D. Edson, C. Pueblo, M.E. Blodgett, K.F. Kelton, and N.A.Mauro.  Journal of Molecular Liquids. 242 (2017) 807.
[16] A. Fleshman, Private Communication, 2014.
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