DOI: 10.5061/dryad.66t1g1kdx
title: Disentangling cation effects on ion mobility and structure in ionic liquid electrolytes

description: Ionic liquids (ILs) are low-temperature molten salts, and therefore the
 transport of ions within ILs is dominated by ion-ion interactions.
 However, the influence of organic IL cations on key electrolyte
 properties, such as ion dissociation and overall transport behavior, in
 lithium-salt-doped ILs remains poorly understood. Moreover, despite their
 critical role in designing IL-based electrolytes for energy storage
 applications, ion-ion interactions and ion-specific transport under an
 applied electrical potential are seldom quantified, largely due to the
 unique experimental and computational challenges involved. Herein, we
 compare transport properties obtained using 1H, 7Li, and 19F pulsed-field
 gradient nuclear magnetic resonance (PFG NMR) and electrophoretic NMR
 (eNMR) with those measured by electrochemical impedance spectroscopy
 (EIS). Non-equilibrium molecular dynamics (MD) simulations and eNMR
 confirm the presence of negatively charged [Li(TFSI)n](1-n) aggregates
 that migrate towards the positive electrode, resulting in negative lithium
 transference numbers. Equilibrium MD simulations reveal a vehicular Li ion
 transport mechanism facilitated by long-lived aggregates with Li+ cations
 strongly bound to multiple TFSI– anions. Finally, we observe an inverse
 relationship between the apparent charge of the TFSI– anion in the neat
 IL, which is dictated by the IL cation, and Li+ transport in the
 salt-doped systems. This highlights the opportunity to tune electrolyte
 performance by tailoring cation chemistry.
description: PFG NMR: All NMR measurements were performed
 on a Bruker Avance III 300 spectrometer with a 7.05 T super-wide bore (150
 mm) superconducting magnet equipped with a Bruker Diff50 probe (maximum
 gradient strength of 28.98 T m<sup>–1</sup>), operating at
 300.15 MHz for <sup>1</sup>H, 116.65 MHz for
 <sup>7</sup>Li, and 282.40 MHz for <sup>19</sup>F.
 All measurements were performed at 313.15 K following at least 30 minutes
 of equilibration with the sample temperature controlled using dry
 N<sub>2</sub> at a flow rate of 800 L
 h<sup>–1</sup>. 313.15 K was chosen as an application-relevant
 temperature that would allow for faster ionic mobility through a decreased
 viscosity and, therefore, more accurate measurements. Self-diffusion
 coefficients of the IL (C4C1im, C4C1pyrr, and C4C1pip) and
 Li<sup>+</sup> cations, and TFSI<sup>–</sup>
 anion, were obtained from <sup>1</sup>H,
 <sup>7</sup>Li, and <sup>19</sup>F PFG NMR
 experiments, respectively. Diffusivities were determined using a variable
 gradient strength stimulated echo pulse sequence designed to minimize
 signal loss from short transverse relaxation times
 (T<sub>2</sub>). The self-diffusion coefficient of species i
 was determined by fitting the observed signal intensity as a function of
 the variable gradient strength using the Stejskal−Tanner
 equation. eNMR: eNMR measurements
 were performed using a double stimulated echo
 convection-compensating pulse sequence, in which the
 polarity of the DC potential pulse was reversed halfway through the pulse
 sequence. Applied potentials and gradient strengths were varied from 0 to
 90 V, and from 0.2 to 3 T m<sup>–1</sup>,
 respectively. Equilibrium MD simulations:
 Equilibrium atomistic molecular dynamics (MD) simulations were
 performed for C4C1im, C4C1pyrr, and C4C1pip ionic liquids at 313 K, each
 containing LiTFSI at a molar fraction of 0.1 (defined as the ratio of the
 number of Li<sup>+</sup> ions to the total number of cations).
 Bonded (bond, angle, dihedral) and nonbonded interactions were described
 using the CL&P force field for C4C1im and C4C1pyrr, and the
 OPLS-AA force field for C4C1pip. Full methods can be
 found in the Supplementary Information of the published paper.
 Non-equilibrium MD simulations:
 Non-equilibrium molecular dynamics (NEMD) simulations were
 performed to evaluate ionic drift and transport under
 an applied external electric field. External electric fields ranging from
 0.02 to 0.10 V nm<sup>–1</sup> were applied along a fixed
 direction, with increments of 0.02 V nm<sup>–1</sup>. All
 parameters, including force fields, cutoffs, and thermostat/barostat
 settings, were identical to those used in the equilibrium
 simulations.
description: # Disentangling cation effects on ion mobility and structure in ionic
 liquid electrolytes This README.md file was generated on 2026-01-22 by Leo
 W. Gordon. **GENERAL INFORMATION** Date of data collection: 2022-2024
 Geographic location of data collection: Santa Barbara, CA, USA; Austin,
 TX, USA **METHODOLOGICAL INFORMATION** 1. Description of methods used for
 collection/generation of data: See supporting information from published
 main text: [10.1088/2515-7655/ae3c3f](10.1088/2515-7655/ae3c3f). 2.
 Methods for processing the data: See supporting information from published
 text: [10.1088/2515-7655/ae3c3f](10.1088/2515-7655/ae3c3f). 3. Instrument-
 or software-specific information needed to interpret the data: N/A 4.
 Standards and calibration information, if appropriate: N/A 5.
 Environmental/experimental conditions: See supporting information from
 published text: [10.1088/2515-7655/ae3c3f](10.1088/2515-7655/ae3c3f). 6.
 Describe any quality-assurance procedures performed on the data: See
 supporting information from published text:
 [10.1088/2515-7655/ae3c3f](10.1088/2515-7655/ae3c3f). 7. People involved
 with sample collection, processing, analysis and/or submission: Oscar A.
 Nordness, Leo W. Gordon, Zidan Zhang, Venkat Ganesan, Raphaële J. Clément
 **DATA & FILE OVERVIEW** File List: Sub-files are contained within
 Nordness_et_al_2026_raw_data.zip and are discretized by result. Structure
 is as follows: upper-case letter = description and figure reference;
 lower-case letter = file name; roman numerals = variables and column
 names. A. Mobilities determined by eNMR and PFG NMR, with associated
 errors (Data for Figures 1, S7, S9) a. Experimental Mobilities.csv i.
 Variables: * Ionic liquid (units=none); * Mobility
 (units=meters^2/volt/second); * Mobility error
 (units=meters^2/volt/second) ii. Column Names: * IL; * μ_eNMR_cat_neat /
 m$^2$ V$^{-1}$ s$^{-1}$; * μ_eNMR_cat_neat_err / m$2$ V$^{-1}$ s${-1}$; *
 μ_eNMR_an_neat / m$^2$ V$^{-1}$ s$^{-1}$; * μ_eNMR_an_neat_err / m$2$
 V$^{-1}$ s${-1}$; * μ_eNMR_cat / m$^2$ V$^{-1}$ s$^{-1}$; * μ_eNMR_cat_err
 / m$2$ V$^{-1}$ s${-1}$; * μ_eNMR_an / m$^2$ V$^{-1}$ s$^{-1}$;\ *
 μ_eNMR_an_err / m$2$ V$^{-1}$ s${-1}$; * μ_eNMR_Li / m$^2$ V$^{-1}$
 s$^{-1}$; * μ_eNMR_Li_err / m$2$ V$^{-1}$ s${-1}$; * μ_PFG_cat_neat /
 m$^2$ V$^{-1}$ s$^{-1}$; * μ_PFG_cat_neat_err / m$2$ V$^{-1}$ s${-1}$; *
 μ_PFG_an_neat / m$^2$ V$^{-1}$ s$^{-1}$; * μ_PFG_an_neat_err / m$2$
 V$^{-1}$ s${-1}$; * μ_PFG_cat / m$^2$ V$^{-1}$ s$^{-1}$; * μ_PFG_cat_err /
 m$2$ V$^{-1}$ s${-1}$; * μ_PFG_an / m$^2$ V$^{-1}$ s$^{-1}$; *
 μ_PFG_an_err / m$2$ V$^{-1}$ s${-1}$; * μ_PFG_Li / m$^2$ V$^{-1}$
 s$^{-1}$; * μ_PFG_Li_err / m$2$ V$^{-1}$ s${-1}$ B. Self diffusivities
 determined by PFG NMR, with associated errors (Data for Figures 4, S7,
 S8). a. Experimental Self-diffusivities.csv i. Variables: * Ionic liquid
 (units=none); * Diffusivity (units=meters^2/second); * Diffusivity error
 (units=meters^2/second) ii. Column Names: * IL; *
 D_Cation_Neat_PFG_($m^2s^{–1}$); * D_Cation_Neat_PFG_err_($m^2s^{–1}$); *
 D_TFSI_Neat_PFG_($m^2s^{–1}$); * D_TFSI_Neat_PFG_err_($m^2s^{–1}$); *
 D_Cation_PFG_($m^2s^{–1}$); * D_Cation_PFG_err_($m^2s^{–1}$); *
 D_TFSI_PFG_($m^2s^{–1}$); * D_TFSI_PFG_err_($m^2s^{–1}$); *
 D_Li_PFG_($m^2s^{–1}$); * D_Li_PFG_err_($m^2s^{–1}$) C. Experimental and
 simulated transference numbers, with associated errors (Data for Figure
 2). a. Transference.csv i. Variables: * Ionic liquid (units=none); *
 Transference number (units=none) ii. Column Names: * ILs; * Cation_PFG; *
 Cation_PFG_err; * Anion_PFG; * Anion_PFG_err; * Li_PFG; * Li_PFG_err; *
 Cation_eNMR; * Cation_eNMR_err; * Anion_eNMR; * Anion_eNMR_err; * Li_eNMR;
 * Li_eNMR_err; * Cation_MD; * Cation_MD_err; * Anion_MD; * Anion_MD_err; *
 Li_MD; * Li_MD_err D. Inverse Haven ratios for neat and salt-doped
 electrolytes determined from eNMR and PFG NMR measurements, with
 associated errors (Data for Figure 3). a. Inverse Haven Ratios.csv i.
 Variables: * Ionic liquid (units=none); * inverse Haven ratio (units=none)
 ii. Column Names: * IL; * H_inv_neat; * H_inv_neat_err; * H_inv; *
 H_inv_err E. Experimentally determined apparent charges of ions, with
 associated errors (Data for Figure 3). a. Apparent Charge.csv i.
 Variables: * Ionic liquid (units=none); * Apparent charge (units=none) ii.
 Column Names: * IL; * Apparent charge cation neat; * Apparent charge
 cation neat err; * Apparent charge anion neat; * Apparent charge anion
 neat err; * Apparent charge cation binary; * Apparent charge cation binary
 err; * Apparent charge anion binary; * Apparent charge anion binary err F.
 Probability distribution for the number of TFSI^– ions coordinated to
 Li^+, with associated errors (Data for Figure 4). a. P_n.csv i. Variables:
 Probability distribution (units=none) ii. Column Names: * n; * C_4C_1pyrr;
 * C_4C_1pyrr_std; * C_4C_1im; * C_4C_1im_std; * C_4C_1pip; * C_4C_1pip_std
 G. Liquid-state 1H NMR spectra of neat ILs (Data for Figure S5). a.
 Neat_1H_ILs.csv i. Variables: * shift (units=ppm); * Normalized intensity
 (units=none) ii. Column Names: * Im_shift / ppm; * Im_Normalized
 intensity; * Pyrr_shift / ppm; * Pyrr_Normalized intensity; * Pip_shift /
 ppm; * Pip_Normalized intensity H. Liquid-state 19F NMR spectra of neat
 ILs (Data for Figure S5). a. Neat_19F_ILs.csv i. Variables: * shift
 (units=ppm); * Normalized intensity (units=none) ii. Column Names: *
 Im_shift / ppm; * Im_Normalized intensity; * Pyrr_shift / ppm; *
 Pyrr_Normalized intensity; * Pip_shift / ppm; * Pip_Normalized intensity
 I. Liquid-state 1H NMR spectra of salt-doped ILs (Data for Figure S6). a.
 Doped_1H_ILs.csv i. Variables: * shift (units=ppm); * Normalized intensity
 (units=none) ii. Column Names: * Im_shift / ppm; * Im_Normalized
 intensity; * Pyrr_shift / ppm; * Pyrr_Normalized intensity; * Pip_shift /
 ppm; * Pip_Normalized intensity J. Liquid-state 19F NMR spectra of
 salt-doped ILs (Data for Figure S6). a. Doped_19F_ILs.csv i. Variables: *
 shift (units=ppm); * Normalized intensity (units=none) ii. Column Names: *
 Im_shift / ppm; * Im_Normalized intensity; * Pyrr_shift / ppm; *
 Pyrr_Normalized intensity; * Pip_shift / ppm; * Pip_Normalized intensity
 K. Liquid-state 7Li NMR spectra of salt-doped ILs (Data for Figure S6). a.
 Doped_7Li_ILs.csv i. Variables: * shift (units=ppm); * Normalized
 intensity (units=none) ii. Column Names: * Im_shift / ppm; * Im_Normalized
 intensity; * Pyrr_shift / ppm; * Pyrr_Normalized intensity; * Pip_shift /
 ppm; * Pip_Normalized intensity L. Non-equilibrium MD simulation results,
 with associated errors (Data for Figures S7, S12). a. Simulated
 Mobilities.csv i. Variables: * Ionic liquid (units=none); * Mobility
 (units=meters^2/volt/second); * Mobility error
 (units=meters^2/volt/second) ii. Column Names: * ILs; *
 Cation_sim_(m$^2$V$^{-1}$s$^{-1}$); *
 Cation_sim_err_(m$2$V$^{-1}$s${-1}$); * TFSI_sim_(m$^2$V$^{-1}$s$^{-1}$);
 * TFSI_sim_err_(m$2$V$^{-1}$s${-1}$); * Li_sim_(m$^2$V$^{-1}$s$^{-1}$); *
 Li_sim_err_(m$2$V$^{-1}$s${-1}$) M. Equilibrium MD simulation results,
 with associated errors (Data for Figures S8, S12). a. Simulated
 Diffusivities.csv i. Variables: * Ionic liquid (units=none); * Diffusivity
 (units=meters^2/second); * Diffusivity error (units=meters^2/second) ii.
 Column Names: * ILs; * D_Cation_($m^2s^{–1}$); *
 D_Cation_err_($m^2s^{–1}$); * D_TFSI_($m^2s^{–1}$); *
 D_TFSI_err_($m^2s^{–1}$); * D_Li_($m^2s^{–1}$); * D_Li_err_($m^2s^{–1}$)
 N. Persistence times for Li-TFSI solvation structures, with associated
 errors (Data for Figure S10). a. Persistence Times.csv i. Variables: *
 Ionic liquid (units=none); * Persistence time, intermittent
 (units=nanoseconds); * Persistence time, intermittent error
 (units=nanosecond) * Persistence time, Li[TFSI]2 vehicle
 (units=nanoseconds); * Persistence time, Li[TFSI]2 vehicle error
 (units=nanosecond) ii. Column Names: * IL; * tau_intermittent_(ns); *
 err_tau_intermittent_(ns); * tau_Li_TFSI_2_remaining_(ns); *
 err_tau_Li_TFSI_2_remaining_(ns) O. Persistence lengths for Li-TFSI
 solvation structures, with associated errors (Data for Figure S10). a.
 Persistence Lengths.csv i. Variables: * Ionic liquid (units=none); *
 Persistence length, intermittent (units=Angstroms); * Persistence length,
 intermittent error (units=Angstroms) * Persistence length, Li[TFSI]2
 vehicle (units=Angstroms); * Persistence length, Li[TFSI]2 vehicle error
 (units=Angstroms) ii. Column Names: * IL; *
 Persistence_length_intermittent_(Angstroms); *
 err_Persistence_length_intermittent_(Angstroms); *
 Persistence_length_continuous_(Angstroms); *
 err_Persistence_length_continuous_(Angstroms); *
 Persistence_length_Li_TFSI_2_remaining_(Angstroms); *
 err_Persistence_length_Li_TFSI_2_remaining_(Angstroms) P. Persistence
 ratios for Li-TFSI solvation structures, with associated errors (Data for
 Figure S10). a. Persistence Ratios.csv i. Variables: * Ionic liquid
 (units=none); * Persistence ratio, intermittent (units=none); *
 Persistence ratio, intermittent error (units=none) * Persistence ratio,
 Li[TFSI]2 vehicle (units=none); * Persistence ratio, Li[TFSI]2 vehicle
 error (units=none) ii. Column Names: * IL; *
 Persistence_ratio_intermittent; * err_Persistence_ratio_intermittent; *
 Persistence_ratio_Li_TFSI_2_remaining; *
 err_Persistence_ratio_Li_TFSI_2_remaining Q. Radial distribution functions
 for Li-TFSI interactions (Data for Figure S11). a. G_r_Li-TFSI.csv i.
 Variables: Interspecies distance (units=nanometers) ii. Column Names: *
 r/nm; * c4c1im; * c4c1pyrr; * c4c1pip R. Radial distribution functions for
 Li-Cation interactions (Data for Figure S11). a. G_r_Li-Cation.csv i.
 Variables: Interspecies distance (units=nanometers) ii. Column Names: *
 r/nm; * c4c1im; * c4c1pyrr; * c4c1pip S. Radial distribution functions for
 Cation-TFSI interactions (Data for Figure S11). a. G_r_Cation-Anion.csv i.
 Variables: Interspecies distance (units=nanometers) ii. Column Names: *
 r/nm; * c4c1im; * c4c1pyrr; * c4c1pip Relationship between files, if
 important: Data in file C was calculated using values from files A, B, L,
 and M. Data in files D and E was calculated using values from files A and
 B.