Probing Ionic Liquid Electrolyte Structure via the Glassy State by

Feb 8, 2018 - Dynamic nuclear polarization (DNP)-enhanced solid-state NMR spectroscopy has been used to study an ionic liquid salt solution (N-methyl-...
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Letter Cite This: J. Phys. Chem. Lett. 2018, 9, 1007−1011

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Probing Ionic Liquid Electrolyte Structure via the Glassy State by Dynamic Nuclear Polarization NMR Spectroscopy Marc-Antoine Sani,† Pierre-Alexandre Martin,‡,§,∥ Ruhamah Yunis,‡ Fangfang Chen,‡ Maria Forsyth,‡ Michael̈ Deschamps,§,∥ and Luke A. O’Dell*,‡ †

School of Chemistry, Bio21 Institute, University of Melbourne, Melbourne, Victoria 3010, Australia Institute for Frontier Materials, Deakin University, Geelong, Victoria 3220, Australia § CEMHTI, CNRS UPR 3079, Université d’Orléans, F45071 Orléans, France ∥ RS2E, FR CNRS 3459, 80039 Amiens, France ‡

S Supporting Information *

ABSTRACT: Dynamic nuclear polarization (DNP)-enhanced solid-state NMR spectroscopy has been used to study an ionic liquid salt solution (N-methyl-Npropyl-pyrrolidinium bis(fluorosulfonyl)imide, C3mpyrFSI, containing 1.0 m lithium bis(fluorosulfonyl)imide, 6LiFSI) in its glassy state at a temperature of 92 K. The incorporation of a biradical to enable DNP signal enhancement allowed the proximities of the lithium to the individual carbon sites on the pyrrolidinium cation to be probed using a 13C−6Li REDOR pulse sequence. Distributions in Li−C distances were extracted and converted into a 3D map of the locations of the Li+ relative to the C3mpyr that shows remarkably good agreement with a liquid-phase molecular dynamics simulation.

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involves the observation of a relaxation-driven transfer of polarization from one nucleus to another, mediated by fluctuating dipolar interactions. This polarization transfer is quantified by the cross-relaxation rate σ (with dimensions of s−1), which in the intramolecular case scales as r−6, where r is the internuclear distance, and is only observable for close proximities (8 >8 >6 4.90 4.75 4.70

Δd2/Å

f2

1.20 1.15 1.40

0.68 0.53 0.28 0.78 0.82 0.79

Figure 3. Plots of Li+ spatial distribution around the C3mpyr cation constructed from (a) the 92 K solid-state 13C−6Li REDOR distance data in Table 2 and (b) a molecular dynamics simulation carried out on a liquid-phase [C3mpyrFSI]0.9[LiFSI]0.1 system with a simulated temperature of 298 K. Note that the two sets of images are not plotted on the same scale. The C2−C3 bond length on the C3mpyr cation is 1.53 Å. See the Supporting Information for more details.

Distribution widths Δd2 for C1, C2, and C3 are unspecified due to the 9 Å cut-off used in the fitting.

a

would then explain the apparent strong association of the Li+ to the C5 and C6 carbons on the heterocyclic ring, over which the positive charge of the C3mpyr cation will be delocalized. The shorter Li−C distance may be due to a smaller and less abundant Li(FSI)2− cluster. To visualize the distance distributions in Table 2, these data were converted into a 3D plot of Li+ spatial probability relative the C3mpyr cation. (See the Supporting Information for details.) This is shown in Figure 3a alongside similar plots generated from a liquid-phase MD simulation carried out on the system [C3mpyrFSI]0.9[LiFSI]0.1. The two methods show remarkable agreement and suggest that the Li+ ions are preferentially located above and below the pyrrolidinium ring,

with a higher probability to exist on the same side of the ring as the methyl group. In the MD simulations the Li+ locations are generally more distant from the cation, likely due to the dynamic nature of the liquid phase in which the ions will undergo various conformational changes and rotational and translational modes that are not present in the glass. Several assumptions made in this approach should be briefly addressed, specifically (1) that the glassy-state structure of the sample is the same as or very similar to the liquid phase structure, (2) that the sample does not undergo crystallization during the experiment, and (3) that the presence of the biradical molecule does not significantly alter the structure. It is generally accepted that supramolecular ordering in liquid-phase 1009

DOI: 10.1021/acs.jpclett.8b00022 J. Phys. Chem. Lett. 2018, 9, 1007−1011

Letter

The Journal of Physical Chemistry Letters ILs mimics the solid-state structure,1 and the good agreement between the experimental data obtained from the glassy system and the MD simulation data from the liquid phase backs this up. The very rapid cooling rate that occurs when the small volume of sample (∼20 μL) is inserted into the precooled NMR probe should ensure good glass formation. During our experiments, some evidence of partial crystallization was observed in the form of a slight narrowing of some of the 13 C NMR peaks, most noticeably for the methyl carbons, accompanied by an apparent increase in εDNP. This occurred over a time scale of several hours, and a melt−quench cycle returned the spectrum to its original appearance. (See the Supporting Information.) Finally, the low concentration of the biradical in combination with the paramagnetic quenching effect (whereby the NMR signals from the molecules in close proximity to the biradical are broadened beyond detection) should minimize the effects of this molecule on the observed structure, and it is possible that even lower biradical concentrations could still provide useful DNP enhancements. We also note that DNP is not, in principle, a necessary component of this approach, which could be carried out on a standard NMR spectrometer with a magic-angle spinning probe operating at a low enough temperature to achieve the ionic liquid’s glassy state. 13C enrichment of the ionic liquid cation would provide sensitivity boosts that match or exceed the DNP enhancement levels obtained herein, while 7Li−19F REDOR experiments would be feasible at natural abundance to study lithium−anion interactions. However, to test the potential sensitivity advantage of using DNP, a comparison of 13C CPMAS spectra was made for the IL discussed herein, with and without the presence of the biradical and DNP effect, and with all other experimental conditions the same. The pure IL without TEKPol exhibited longer 1H T1 relaxation times (∼20 s) as well as a weaker 13C CPMAS signal. For optimized experiments of the same total duration, the DNP resulted in a signal-to-noise ratio of 462 compared with 32 for the pure IL without DNP. (See the Supporting Information.) Finally, we note that while the strategy of characterizing ionic liquid structure via the glassy state permits a direct and quantitative probe of intermolecular distances, it cannot provide any insights into the dynamic nature of this structuring in the liquid phase, such as motional correlation times or life times of ion clusters. A more detailed understanding of IL structure will emerge from a combination of this approach with other methods such as HOESY and MD simulations. In summary, DNP-enhanced solid-state NMR at a temperature of 92 K has been shown to be a promising approach for probing intermolecular distances in ionic liquids via their glassy state, herein demonstrated for a lithium-containing IL electrolyte. Studying these materials in their glassy form opens up a broad suite of existing solid-state NMR pulse sequences for measuring through-bond and through-space correlations and distances, while the signal enhancement afforded by DNP permits less abundant or less sensitive nuclei to be utilized. The close agreement between our solid-state experimental results and liquid-state MD simulations provides strong evidence that the local structuring in glassy ILs mimics that of the liquid phase.

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Sample synthesis details, NMR experimental details, C−6Li REDOR dephasing plots fitted to a universal REDOR curve, BS-REDOR fitting details, evidence of IL crystallization, conversion of distance distributions to 3D plots (including Maple code), molecular dynamics simulation details, and sensitivity comparison with and without TEKPol/DNP. (PDF) Copy of the Maple code. (ZIP)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Marc-Antoine Sani: 0000-0003-3284-2176 Fangfang Chen: 0000-0002-8004-1720 Luke A. O’Dell: 0000-0002-7760-5417 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The Australian Research Council is acknowledged for funding of the DNP NMR instrument at the Bio21 Institute, University of Melbourne, through grant LE160100120.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.8b00022. 1010

DOI: 10.1021/acs.jpclett.8b00022 J. Phys. Chem. Lett. 2018, 9, 1007−1011

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The Journal of Physical Chemistry Letters

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