Solvent Polarity Governs Ion Interactions and Transport in a Solvated

Letter pubs.acs.org/JPCL

Solvent Polarity Governs Ion Interactions and Transport in a Solvated Room-Temperature Ionic Liquid Naresh C. Osti,*,§ Katherine L. Van Aken,‡ Matthew W. Thompson,¶ Felix Tiet,¶ De-en Jiang,† Peter T. Cummings,¶ Yury Gogotsi,‡ and Eugene Mamontov§ §

Chemical and Engineering Materials Division, Oak Ridge National Laboratory, PO Box 2008 MS6455, Oak Ridge, Tennessee 37831, United States ‡ Department of Materials Science and Engineering, and A. J. Drexel Nanomaterials Institute, Drexel University, 3141 Chestnut Street, Philadelphia, Pennsylvania 19104, United States ¶ Department of Chemical and Biomolecular Engineering, Vanderbilt University, 2201 West End Avenue, Nashville, Tennessee 37235, United States † Department of Chemistry, University of California, 900 University Avenue, Riverside, California 92521, United States S Supporting Information *

ABSTRACT: We explore the influence of the solvent dipole moment on cation− anion interactions and transport in 1-butyl-3-methyl-imidazolium bis(trifluoromethylsulfonyl), [BMIM+][Tf2N−]. Free energy profiles derived from atomistic molecular dynamics (MD) simulations show a correlation of the cation− anion separation and the equilibrium depth of the potential of mean force with the dipole moment of the solvent. Correlations of the ion diffusivity with the dipole moment and the concentration of the solvent were further demonstrated by classical MD simulations. Quasi-elastic neutron scattering experiments with deuterated solvents reveal a complex picture of nanophase separation into the ionic liquid-rich and solvent-rich phases. The experiment corroborates the trend of concentration- and dipole moment-dependent enhancement of ion mobility by the solvent, as suggested by the simulations. Despite the considerable structural complexity of ionic liquid−solvent mixtures, we can rationalize and generalize the trends governing ionic transport in these complex electrolytes. parameters that allows flexible tuning of molecular dimensions, viscosities, mobility, and electrical properties of electrolytes, making energy storage devices based on them functional over variable operating conditions.18−20 A promising approach to enhance transport properties of ionic liquids is to mix them with a solvent, which improves physicochemical characteristics affecting their bulk and interfacial properties.21 Aprotic solvents solvate the ions in solution, disrupting the cation−anion interactions and resulting in improved viscosity and conductivity. Here we explore the impact of solvents with different dipole moments on the dynamics of an ionic liquid using neutron scattering and molecular dynamics (MD) simulations. We report dipole moment-dependent molecular interactions of solvents altering the microscopic dynamics of ionic liquids upon solvation. The correlation between the microscopic dynamics of an organic salt and the dipole moment of the solvent provides guidance in designing improved systems for energy applications.

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merging energy storage technologies mandate novel materials of improved capabilities and electrical energy storage mechanisms that increase the performances of electrochemical energy storage devices.1 Traditional capacitors and electrochemical batteries are not free of limitations. The low energy density of capacitors and low power density of batteries necessitate a different approach to formulate a system that could overcome those limitations.2 Supercapacitors,3−5 also called electric double layer capacitors (EDLCs), have shown great potential for storage and release of energy. Intrinsic properties of the electrode materials and electrolytes determine the performance of a supercapacitor. Energy density can be optimized using an electrolyte that withstands a high operational voltage window.6 Compared to aqueous and organic electrolytes, room-temperature ionic liquids (RTILs), which are also called designer solvents,7 have attracted much attention as electrolytes in EDLCs8 because of their large operational voltage window,6,9,10 high thermal stability,11 low volatility,12 and high conductivity, which provide increased device life expectancy.13−15 However, pure RTILs often exhibit high viscosity,16 resulting in low conductivity and diffusivity, which adversely affect the charging and discharging rates.17 Despite these drawbacks, RTILs offer variability in molecular © XXXX American Chemical Society

Received: November 4, 2016 Accepted: December 14, 2016 Published: December 14, 2016 167

DOI: 10.1021/acs.jpclett.6b02587 J. Phys. Chem. Lett. 2017, 8, 167−171

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simulation (details in the SI). The diffusivities of the cations in the presence of CH3CN and CH3OH are visibly higher than that in the presence of C4H8O and CH2Cl2. Cation diffusivity in the presence of solvents (Figure 2a) correlates inversely with the energy required to separate ions (Figure 1), thus showing direct correlation with the dipole moment of the solvent. We observed similar correlation with the solvent polarity of the anion diffusivity, as shown in Figure S2. Increasing the concentration of solvent sharply weakens the electrostatic force between the ions, thus increasing the screening and leading to enhancement of the cation diffusivity (Figure 2b). Solvent polarity also impacts the conductivity (details in the SI) of the ionic liquid mixture. As the concentration of the ionic liquid in each solvent is gradually increased from zero, the conductivity of the solution increases at first, reaches a maximum, and then starts to drop (Figure S3). This shows that ionic dissociation dominates the conductivity at lower concentrations. At higher concentrations, the viscosity of the ionic liquid becomes more dominant, leading to a decrease in the conductivity. Similar concentration dependence of conductivity has previously been attributed to competition between ionic dissociation and aggregation that leads to ion pair formation.24 Importantly, we have found a correlation of the measured conductivity with the dipole moment of the solvent. We next investigated25 the influence of solvent dipole moment and concentration on the cation dynamics using quasielastic neutron scattering, QENS (details in SI). Solvents were deuterated; therefore, the cation was the only hydrogen-bearing entity in the mixture (see the SI for a sample preparation description). The resulting QENS spectra (Figure 3a), showing [BMIM+] diffusivity, become broader in the presence of solvents, especially when plotted as a function of the solvent concentration (Figure 3c). The increase in quasi-elastic broadening results from enhancement of the diffusion mobility of the hydrogen-bearing cations. The solvent impact is further illustrated by plots of the dynamic susceptibility (Figure 3b,d) as a function of energy transfer, where the measured QENS spectra are converted to dynamic susceptibility using the Bose occupation factor. The susceptibility plot can visually separate the contribution of different relaxation processes to the overall dynamics of a complex system as distinct peaks.26 Interestingly, we have observed two distinct peaks in the susceptibility plots. The systematic variation of their relative intensities with the solvent concentration (Figure 3d) indicates that the low-energy peak originates from the dynamics of the cation in ionic liquid-

The complex nature of ionic liquids in solutions arises from the presence of various interactions (including hydrogen bonding, ionic, dispersive, polar, dipolar, and π−π).22 For aprotic solvents, dipole−dipole molecular interactions are often dominant. In this study, we consider the ionic liquid 1-butyl-3methyl-imidazolium bis(trifluoromethylsulfonyl) ([BMIM+][Tf2N−]) in four solvents with different dipole moments: acetonitrile (CH3CN, d = 3.92 D, dielectric constant ε = 37.5), methanol (CH3OH, d = 2.87 D, ε = 32.7), tetrahydrofuran (C4H8O, d = 1.75 D, ε = 7.58), and dichloromethane (CH2Cl2, d = 1.60 D, ε = 8.93). This RTIL mixed with a solvent acts as an organic salt in an aprotic solution. We begin by computing the free energy profiles derived from atomistic MD simulations (details in the Supporting Information (SI)). The potential of mean force (PMF) associated with pulling apart an ion pair23 in each solvent at infinite dilution (Figure 1) shows a trend with

Figure 1. Free energy profile as a function of the cation−anion separation for [BMIM+][Tf2N−] mixed with aprotic solvents at infinite dilution derived from atomistic MD simulations. The energy scale is defined such that the minimum energy is zero. The energy associated with separating an ion pair correlates with the dipole moment of the solvent.

dipole moment. This energy barrier in each solvent (CH3CN < CH3OH < C4H8O < CH2Cl2) correlates inversely with the solvent’s dipole moment, suggesting that a solvent with greater solvent polarity is better able to screen ion−ion interactions. Therefore, we expect ions in solvents with greater dipole moments to have higher mobility. Diffusivities of [BMIM+] (Figure 2a) in each solvent were calculated via classical MD

Figure 2. Cation diffusivity in solvated [BMIM+][Tf2N−] from MD simulation. (a) With solvents of different dipole moments at 50 wt % concentration. (b) With CH3CN, varying the ionic liquid concentration. Dotted lines are provided to guide the eye. Also included in both panels are MD snapshots of the ionic liquid structure at the corresponding concentrations of solvents. The snapshots are colored by molecule: [BMIM+] (blue), [Tf2N−] (red), CH2Cl2 (purple), C4H8O (green), CH3OH (pink), CH3CN (orange). 168

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Figure 3. (a,c) QENS profiles and (b,d) corresponding susceptibility plots at a representative Q of 0.7 Å−1. Top panels: [BMIM+][Tf2N−] mixed with different aprotic solvents in 1:1 ratio by mass. Bottom panels: The ionic liquid mixed with CD3CN at different mass concentrations. Symbols represent the data and the solid lines represent the model fits as described in the text.

rich phases, while its high-energy counterpart originates from the dynamics in solvent-rich phases. Thus, distinct peaks in the susceptibility plots indicate nanophase segregation into solventrich and solvent-poor phases with faster and slower diffusivity of cations, respectively. Two distinct peaks are present in the susceptibility plots for all solvent mixtures (Figure 3b). It has been shown that neat ionic liquids exhibit some nanoscale heterogeneity27,28 due to the presence of an alkyl chain and imidazolium ring within the cation.29 Introduction of solvents (such as water30) further facilitates the association processes, leading to clear nanophase separation31 and significantly impacting the physicochemical properties of ionic liquids.32 At the same time, the presence of two dynamic components even in pure RTILs has been attributed to the long-range and spatially localized diffusion processes.33−36 To elucidate the relaxation processes associated with phase separation, as evidenced by the susceptibility plots, and to extract the diffusivity of cations in those two phases, a twocomponent fit was used to analyze the QENS data (see the SI for details). From the fit (solid lines in Figure 3a,c), the halfwidth at half-maximum (hwhm) of the two peaks was extracted. A jump diffusion model fit shown by solid lines in Figure S4 gives the translational diffusivity of the cation in both ionic liquid-rich and solvent-rich phases. The diffusivity shows a clear dependence on the dipole moment (Figure 4a) and concentration (Figure 4b) of the solvent. Greater solvent polarity more effectively screens electrostatic interactions between ions. There is an increase in ion mobility as a function of dipole moment (Figure 4a) in both ionic liquid-rich and solvent-rich phases. However, the increase is more pronounced in the ionic liquid-rich phase. This can be attributed to progressively increased dissociation of the ion pairs due to a solvent introduced at low concentration (as in the ionic liquidrich phase), which is most effective for the solvent with the higher dipole moment (Figure 1). On the other hand, when the concentration of solvent is high (as in the solvent-rich phase), the complete dissociation of ion pairs is already achieved, and the dipole moment of the solvent has little further effect on the

Figure 4. Diffusivities extracted from jump diffusion model fit of QENS data (Figure 4S). Dependence of the diffusivity of a cation of [BMIM+][Tf2N−] on dipole moments of the aprotic solvents mixed with the ionic liquid in 1:1 ratio by mass (a). Dependence of the diffusivity of the cation on the concentration of CD3CN (b). Open symbols: Slow component; closed symbol: fast component.

cation diffusivity. It should be noted that the QENS experiment measures cation diffusivity in both ionic liquid-rich and solventrich phases (i.e., the latter are not composed of solvents alone). This is evidenced by the fact that the diffusivities of the pure solvents, which we measured for the reference, much exceed the solvent-rich phases diffusivities. In summary, the increase in diffusivity and conductivity of an ionic liquid in the presence of an aprotic solvent correlates with the dipole moment and concentration of the solvent. Atomistic MD simulations support these diffusivity trends and free energy calculations show that greater solvent polarity decreases the free energy of solvation. QENS experiments reveal that an ionic liquid mixed with solvents undergoes nanophase separation into ionic liquid-rich and solvent-rich phases, with slower and faster cation diffusivity, respectively. Furthermore, QENS experiments corroborate the greater enhancement of ion 169

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(7) Watkins, T.; Kumar, A.; Buttry, D. A. Designer Ionic Liquids for Reversible Electrochemical Deposition/Dissolution of Magnesium. J. Am. Chem. Soc. 2016, 138, 641−650. (8) Hao, L.; Ning, J.; Luo, B.; Wang, B.; Zhang, Y. B.; Tang, Z. H.; Yang, J. H.; Thomas, A.; Zhi, L. J. Structural Evolution of 2D Microporous Covalent Triazine-Based Framework toward the Study of High-Performance Supercapacitors. J. Am. Chem. Soc. 2015, 137, 219− 225. (9) Liu, C. G.; Yu, Z. N.; Neff, D.; Zhamu, A.; Jang, B. Z. GrapheneBased Supercapacitor with an Ultrahigh Energy Density. Nano Lett. 2010, 10, 4863−4868. (10) Lazzari, M.; Mastragostino, M.; Soavi, F. Capacitance Response of Carbons in Solvent-Free Ionic Liquid Electrolytes. Electrochem. Commun. 2007, 9, 1567−1572. (11) Babucci, M.; Akcay, A.; Balci, V.; Uzun, A. Thermal Stability Limits of Imidazolium Ionic Liquids Immobilized on Metal-Oxides. Langmuir 2015, 31, 9163−9176. (12) Earle, M. J.; Esperanca, J.; Gilea, M. A.; Canongia Lopes, J. N.; Rebelo, L. P. N.; Magee, J. W.; Seddon, K. R.; Widegren, J. A. The Distillation and Volatility of Ionic Liquids. Nature 2006, 439, 831− 834. (13) Mastragostino, M.; Soavi, F. Capacitors|Electrochemical Capacitors: Ionic Liquid Electrolytes. Encyclopedia of Electrochemical Power Sources; Elsevier: Amsterdam, The Netherlands, 2009. (14) Kowsari, E. Ionic Liquids  Current State of the Arts. HighPerformance Supercapacitors Based on Ionic Liquids and a Graphene Nanostructure; INTECH, 2015. (15) Dyatkin, B.; Mamontov, E.; Cook, K. M.; Gogotsi, Y. Capacitance, Charge Dynamics, and Electrolyte-Surface Interactions in Functionalized Carbide-Derived Carbon Electrodes. Prog. Nat. Sci. 2015, 25, 631−641. (16) Wilkes, J. S. Properties of Ionic Liquid Solvents for Catalysis. J. Mol. Catal. A: Chem. 2004, 214, 11−17. (17) Yoon, H.; Howlett, P. C.; Best, A. S.; Forsyth, M.; MacFarlane, D. R. Fast Charge/Discharge of Li Metal Batteries Using an Ionic Liquid Electrolyte. J. Electrochem. Soc. 2013, 160, A1629−A1637. (18) Vatamanu, J.; Hu, Z. Z.; Bedrov, D.; Perez, C.; Gogotsi, Y. Increasing Energy Storage in Electrochemical Capacitors with Ionic Liquid Electrolytes and Nanostructured Carbon Electrodes. J. Phys. Chem. Lett. 2013, 4, 2829−2837. (19) Tsai, W. Y.; Lin, R. Y.; Murali, S.; Zhang, L. L.; McDonough, J. K.; Ruoff, R. S.; Taberna, P. L.; Gogotsi, Y.; Simon, P. Outstanding Performance of Activated Graphene Based Supercapacitors in Ionic Liquid Electrolyte from -50 to 80 Degrees C. Nano Energy 2013, 2, 403−411. (20) Forse, A. C.; Griffin, J. M.; Merlet, C.; Bayley, P. M.; Wang, H.; Simon, P.; Grey, C. P. NMR Study of Ion Dynamics and Charge Storage in Ionic Liquid Supercapacitors. J. Am. Chem. Soc. 2015, 137, 7231−7242. (21) Lin, R.; Huang, P.; Segalini, J.; Largeot, C.; Taberna, P. L.; Chmiola, J.; Gogotsi, Y.; Simon, P. Solvent Effect on the Ion Adsorption from Ionic Liquid Electrolyte into Sub-Nanometer Carbon Pores. Electrochim. Acta 2009, 54, 7025−7032. (22) Anderson, J. L.; Ding, J.; Welton, T.; Armstrong, D. W. Characterizing Ionic Liquids on the Basis of Multiple Solvation Interactions. J. Am. Chem. Soc. 2002, 124, 14247−14254. (23) Yee, P.; Shah, J. K.; Maginn, E. J. State of Hydrophobic and Hydrophilic Ionic Liquids in Aqueous Solutions: Are the Ions Fully Dissociated? J. Phys. Chem. B 2013, 117, 12556−12566. (24) Li, W. J.; Zhang, Z. F.; Zhang, J. L.; Han, B. X.; Wang, B.; Hou, M. Q.; Xie, Y. Micropolarity and Aggregation Behavior in Ionic Liquid Plus Organic Solvent Solutions. Fluid Phase Equilib. 2006, 248, 211− 216. (25) Mamontov, E.; Herwig, K. W. A Time-of-Flight Backscattering Spectrometer at the Spallation Neutron Source, BASIS. Rev. Sci. Instrum. 2011, 82, 085109−10. (26) Roh, J. H.; Curtis, J. E.; Azzam, S.; Novikov, V. N.; Peral, I.; Chowdhuri, Z.; Gregory, R. B.; Sokolov, A. P. Influence of Hydration on the Dynamics of Lysozyme. Biophys. J. 2006, 91, 2573−2588.

mobility by the solvent with a higher dipole moment, as suggested by the simulations. A combination of molecular simulation, neutron scattering, and conductivity measurements thus reveals the general principles governing the performance of complex electrolyte mixtures of ionic liquids with aprotic solvents. Importantly, these principles, which take into consideration the solvent dipole moment and concentration, can guide the design of organic salt−solvent systems irrespective of the details of possible structural nanophase segregation.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.6b02587. Simulation details, experimental details, techniques used, conductivity measurements, and jump diffusion fit (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Naresh C. Osti: 0000-0002-0213-2299 Matthew W. Thompson: 0000-0002-1460-3983 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported as part of the Fluid Interface Reactions, Structures and Transport (FIRST) Center, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences. Work at ORNL’s Spallation Neutron Source was sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy. Oak Ridge National Laboratory is managed by UT-Battelle, LLC, for U.S. DOE under Contract No. DEAC05-00OR22725. This research used resources of the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC0205CH11231.

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