Solvation Structure and Dynamics of Li+ in Ternary Ionic Liquid

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Solvation Structure and Dynamics of Li in Ternary Ionic Liquid – Lithium Salt Electrolytes Qianwen Huang, Tuanan Costa Lourenço, Luciano T. Costa, Yong Zhang, Edward J. Maginn, and Burcu Gurkan J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.8b08859 • Publication Date (Web): 13 Dec 2018 Downloaded from http://pubs.acs.org on December 15, 2018

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The Journal of Physical Chemistry

Solvation Structure and Dynamics of Li+ in Ternary Ionic Liquid – Lithium Salt Electrolytes Qianwen Huang 1,a, Tuanan C. Lourenço 2,3,a, Luciano T. Costa3, Yong Zhang 2, Edward Maginn 2, Burcu Gurkan 1,* 1 Department

of Chemical and Biomolecular Engineering, Case Western Reserve University, Cleveland, OH 44106, USA

2

Department of Chemical and Biomolecular Engineering, University of Notre Dame, Notre Dame, IN 46556-5637, USA

3

Department of Physical Chemistry, Fluminense Federal University, Outeiro de São João Batista s/n, CEP 24020-141, Niterói-RJ, Brazil

a Contributed

*

equally

Corresponding author: [email protected]

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Abstract The structural and dynamical changes in the solvation shell surrounding Li+ in a multi-anion environment are studied by Raman spectroscopy and molecular dynamics (MD) simulations. The ternary electrolyte is composed of a mixture of two ionic liquids (ILs) n-methyl-npropylpyrrolidinium

bis(trifluoromethylsulfonyl)

imide

([PYR13][TFSI]),

1-ethyl-3-

methylimidazolium dicyanamide ([EMIM][DCA]) and a lithium bis(trifluoromethylsulfonyl) imide ([Li][TFSI]) salt (0 – 1 M). A 1:9 volumetric mixture of [PYR13][TFSI]:[EMIM][DCA] formed an eutectic that exhibited a lower melting point than that of either parent IL. The local structure of Li+ in this eutectic is found to be heterogenous and preferentially solvated by [DCA], which is the smaller and more abundant anion. While [TFSI] is able to bridge multiple Li+ at high salt concentrations and form both monodentate and bidentate conformations through its oxygen atoms, [DCA] is capable of forming only monodentate coordination with Li+ through either of its end nitrogen atoms. The Raman and MD analysis suggest a wide distribution of solvation structures in the form of [Li(TFSI)m(DCA)n]-(m+n-1) where m = 0 – 1 and n = 3 - 4. The computations showed increased ion pair lifetime for Li+ - [DCA] and decreased lifetimes for Li+ - [TFSI] in the ternary mixture with the increase in [Li][TFSI] concentration. These results show that the solvation and transport properties of charge carriers in ILs can be modified via the presence of multiple ions with varying degree of coordination, which provides an approach to impact the performance in electrochemical processes.


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Introduction Ionic liquids (ILs) are salts that are liquid below 100 °C, an arbitrary temperature adopted to distinguish them from molten salts.1 They are versatile liquids that are alternatives to conventional molecular solvents in many applications including separations,2–4 energy storage,5–7 catalysis8 and sensors.9 Previous studies utilizing ionic liquids as electrolytes in energy storage devices report improved cyclability in lithium-ion10–12, sodium-ion6,13,14 and lithium-metal batteries15–17, and increased power density in supercapacitors18–20. Despite the increasing interest in ionic liquids for high-energy density batteries, the achievable C-rates (rate of capacity discharge) are still lower compared to conventional organic solvent based electrolytes. Similar to many applications, viscosity and conductivity are important physical properties that influence the reaction kinetics and charge transport in electrochemical energy storage devices. Therefore, understanding the solvation structure and dynamics in ILs, and how to tune ion-ion interactions to improve viscosity and conductivity (especially of the charge carrier), are of great interest. For energy storage devices, ILs enable unique capabilities such as lack of flammability, increased capacity due to their wide electrochemical windows, and increased energy density due to the increased solubility of redox active species (e.g., in flow batteries).15,21–25 However, the high viscosity and low conductivity of ILs restrict their realization in practical devices. The conductivity of ILs is typically in the range of ~1 mS/cm at 25 ˚C, which is about one order of magnitude lower than conventional carbonate-based organic electrolytes used in Li-ion batteries.26 Small charged ions such as Li+ are solvated by anions present in ILs and this ion clustering inhibits the mobility of Li+. In the context of IL electrolytes, we refer to an “ion cluster”



as a microstructure that consists of counter ion aggregates around the charge carrier of interest such as Li+. Several studies have focused on understanding the solvation of charged ions such as Li+, Na+ and Mg+2 in ILs with or without chelating agents (e.g., glymes).27–34 Solvation structure and dynamics impact the rates of charge/discharge and thus the rate of the redox reactions at the solid-electrolyte interfaces in batteries. The most commonly investigated Li-salt is lithium bis(trifluoromethanesulfonyl) imide ([Li][TFSI]) due to its electrochemical stability and high solubility, specifically in TFSI-based ILs with imidazolium and pyrrolidinium cations. It has been well established both experimentally and computationally that at low [Li][TFSI] concentrations (e.g., the mole fraction of Li+ is less than 0.2), Li+ is solvated by two TFSI anions through the bidentate ligand conformation, [Li(TFSI)2]‒.33 Pitawala et al.35 used Raman spectroscopy to show that Li+ coordination number (the number of anions in its solvation shell) and the final complex structure are highly dependent on the lithium salt concentration. As the Li-salt concentration increases from 0.2 to 0.4 mole fraction, clusters form with both monodentate and bidentate ligands. Lesch et al.36 confirmed that Li+ coordinates with 4 oxygen atoms on [TFSI], irrespective of the cation, according to monodentate and bidentate conformations. More complex structures coexisting in the form of [Li(TFSI)n]‒(n-1) with n = 3, 4 were also noted by the Passerini and Henderson groups.37 These highly coordinated nano- to micro-scale phases result in increased macroscopic viscosities. Ab-initio calculations at the density functional theory (DFT) level in support of experimental Raman analysis by Umebayashi, Watanabe and coworkers on lithium bis(fluorosulfonyl)imide ([Li][FSI]) and [Li][TFSI] reported Li-coordination of 3 for [FSI] as opposed to 4 for [TFSI] with monodentate or 2 for bidentate interpretations.31 In contrast to the [Li(TFSI)2]‒ 4 ACS Paragon Plus Environment

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complex, Monti et al.38 predicted a coordination number of 3 for Na+ with [TFSI] containing ILs through a combination of DFT computations and Raman analysis. The differences between Li+ and Na+ coordination can be explained by the larger ion size for Na+ and steric effects. There are two mechanisms that can explain the Li+ transport in ILs: (i) the vehicular mechanism in which the Li+ moves as a solvated species with anion neighbors and (ii) the exchange mechanism where Li+ moves by jumping between the solvation shells in the system.39– 41

Borodin et al.42 showed by MD simulations that Li+ transport in pyrrolidinium based ILs with

the [TFSI] anion occurred mainly via exchange of the anion in the first solvation shell while the diffusion of Li+ with the entire coordination shell was minor. Later, the same group modified their force fields and concluded the exchange mechanism did not have a significant influence on the Li+ transference, specifically when a mixture of [TFSI] and [FSI] anions are considered with the 1ethyl-3-methylimidazolium ([EMIM])] cation.41 Niu et al.43 explained that the molecular environment of Li+ was dependent on the ligand exchange rate, which is related to the stability of the first coordination shell that allows Li+ hopping. Haskins et al.40 performed a systematic study of the contributions of vehicular and exchange mechanisms on Li+ transport in three different ILs: [PYR13][TFSI], [PYR14][TFSI], and [EMIM][BF4] at Li-salt mole fractions of 0.05 – 0.33. It was found that the exchange mechanism contributes less than 40% to the Li+ diffusion. Lesch and coworkers41 also performed MD simulations using a polarizable force field for mixtures of [EMIM][TFSI] and [Li][FSI], with a variable mole fraction of the anions. Through the analysis of different contributions of ion-pair (IP) lifetimes for single or dimer Li+-anion, they showed that the exchange rates of [TFSI] and [FSI] depend on the different anion ratios, but there is a minimal influence on the Li+ transport rate. 5 ACS Paragon Plus Environment


The Passerini and Griffin groups44 examined Mg+2 solvation by pyrrolidinium [TFSI] ionic liquids. They explained the Mg+2 solvation via a fast process that involves exchange between the “free” and bridging TFSI anions, and a slow process that involves the transformation of bidentate to bridging coordination of Mg+2 with the surrounding [TFSI] anions. The term “free” is often used to describe the [TFSI] anion that is coordinated with IL cation such as pyrrolidinium while “bound” refers to the anion coordinated with the charged ion of interest such as Mg+2. Bridging implies that [TFSI] is interacting with multiple Mg+2 via monodentate configuration with each cation through different oxygen atoms on [TFSI]. These studies showed that the diffusion of the aggregates is correlated with the bound bidentate anions, suggesting a vehicular mechanism for Mg2+ transport. Increase in ion aggregates increases the viscosity and hinders the vehicular movement. While increasing temperature reduces viscosity and generally results in an increase in conductivity, it is only tolerable for certain applications. One strategy to reduce viscosity is to break apart aggregates by the use of additives such as water, chelating agents or neutral organic solvents.45–47 Such additives displace anions within the solvation shell of Li+ or Mg+, particularly in the case of glyme additives, thus reducing the formation of ion clusters. Unfortunately, some additives are known to reduce the electrochemical stability of ILs. Another strategy that shows promise for altering the coordination environment around charge carriers, and thus improving ion mobility, is to use mixtures of ILs.48,49 The previous work described above clearly demonstrates that the solvation behavior of salts in ILs is complex and depends on the nature and concentration of the various ions present. There are no simple “design rules” that one can employ to readily predict how a given salt mixture 6 ACS Paragon Plus Environment

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will behave. In the present work, we use a combination of Raman spectroscopy, physical property measurement and MD simulations to investigate the properties of a mixture of two ILs: n-methyln-propylpyrrolidinium bis(trifluoromethanesulfonyl) imide ([PYR13][TFSI]) and 1-ethyl-3methylimidaziolium dicyanamide ([EMIM][DCA]). We chose [PYR13][TFSI] because it is one of the most electrochemically stable ILs, with an electrochemical window of 5.9 V,50 compared to a typical value of about 3 V for Li+ salts dissolved in conventional organic electrolytes.51 It also has a high thermal stability of 400 °C.52 The drawback of [PYR13][TFSI] is that it has a relatively high viscosity (70 cP at 25 °C) and low conductivity (5 mS/cm at 25 °C). In contrast, [EMIM][DCA] is a relatively low-viscosity IL (15 cP at 25 °C) with high conductivity (27 mS/cm at 25 °C).53 However, it has relatively low thermal stability54 and narrower electrochemical window (about 3.4 V).55 One goal of this work is to see if favorable electrolyte properties can be obtained by mixing these two ILs in different ratios. We find that an eutectic forms at a 1:9 (v/v) ratio of [PYR13][TFSI] to [EMIM][DCA]. We then investigate the solvation structure and key physical properties of [Li][TFSI] at various concentrations in the eutectic mixture.

Methods Materials. High purity [PYR13][TFSI] (99.5 %, referred to as IL 1) and [EMIM][DCA] (>99 % referred to as IL 2) were purchased from Iolitec (Tuscaloosa, Alabama). The structures and formal chemical names of the ILs are listed in Table 1. The binary mixtures of IL 1: IL 2 with volume ratios of 1: 9, 1: 1, and 9: 1 were prepared in an argon-purged glovebox (VTI, < 0.1 ppm O2, < 0.1 ppm H2O). The lithium salt, [Li][TFSI], (99.95 %) was purchased from Sigma Aldrich. The ternary



[Li][TFSI]/[PYR13][TFSI]/[EMIM][DCA] mixtures were prepared by dissolving the appropriate amount of [Li][TFSI] (0 – 1 M) in [PYR13][TFSI]/[EMIM][DCA] (1:9). The samples were allowed to equilibrate at 50 °C to facilitate the dissolution of [Li][TFSI]. The water content of the samples was measured by Karl Fischer titrator (Metrohm Coulometric KF 899 D), and they were between 100 and 1330 ppm as reported previously56.

Table 1. Molecular structures and abbreviation of ILs studied

Thermal Measurements. The decomposition temperatures of the eutectic mixture and parent ILs were measured by Thermal Gravimetric Analysis (TGA, TA Q500). Approximately 5-10 mg of sample was heated from 25 to 500°C at a heating rate of 10°C/min under nitrogen, similar to the previously reported methods.57 The weight loss was recorded as a function of temperature. The temperature corresponding to the fastest weight loss was recorded as the decomposition temperature, Td (max of the second derivative of the weight loss curve). The onset temperature, Ton, was determined where 5 w% loss was observed. Differential scanning calorimeter (DSC, TA Q100) was used to detect the crystallization and melting temperatures. The IL sample (5 – 10 mg) was cooled to -70 °C at 5°C/min from its initial temperature at 40 °C, then heated up to 50°C at 8 ACS Paragon Plus Environment

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5°C/min, similar to the technique reported in literature.58 The peaks associated with adsorbed and released heat were assigned to melting and crystallization points, respectively. Viscosity. Viscosities of the ILs were measured by a microchannel viscometer (MicroVISC, Rheosense) at a temperature range of 22 – 55°C with an uncertainty of ± 0.15°C inside the argon filled glove box. Approximately 10 -20 μL was injected into the microchannel at a flow rate of 1 – 23 μL/min for [PYR13][TFSI] and 7 – 139 μL/min for [EMIM][DCA]. At each temperature, viscosity measurements were repeated five times followed by a repeated measurement at 30 °C in order to verify repeatability and confirm that there is no hysteresis. The Vogel-Fulcher-Tamman (VFT) model expressed in eq. 1 was fitted to the viscosity data, similar to the previous literature:59 𝑏

Eq. 1

𝜇 = 𝜇0 ∙ 𝑒𝑥𝑝𝑇 ― 𝑇0

where 𝜇 is the dynamic viscosity (cP) at temperature 𝑇 (K). 𝜇0, 𝑏 and 𝑇0 are the fitted parameters. 𝑇0 is fitted close to the reported glass transition temperature of ILs. Conductivity. The ionic conductivity was measured by Electrochemical Impedance Spectroscopy (EIS) using a potentiostat (BioLogic SP 240) inside the argon filled glove box. A two-electrode conductivity cell with parallel Pt electrodes (MMA 500, Materials Mates Italia) with a cell constant of 1.41 cm-1 was used for the measurement. The voltage amplitude was set at 10 mV, and the frequency range was 2 MHz to 100 Hz, within the temperature range of 25 – 50°C. Each measurement was repeated five times followed by a repeated measurement at 30°C. The VFT model was fitted as expressed in eq. 2: 𝑘

Eq. 2

𝜎 = 𝜎0 ∙ 𝑒𝑥𝑝𝑇 ― 𝑇0 9 ACS Paragon Plus Environment


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where 𝜎 is the conductivity (mS/cm) at temperature 𝑇 (K). 𝜎0, 𝑘 and 𝑇0 are the fitted parameters. Electrochemical Window. The electrochemical stability was measured by cyclic voltammetry (CV) using a potentiostat (BioLogic SP 240) inside an argon filled glove box. A screen printed electrochemical cell with Au working electrode (1.6 mm diameter), Au counter electrode, and Ag quasi-reference electrode (DropSens C223AT) was used. A drop of IL (~ 20 μL) was placed on top of the electrodes to wet the entire surface. The CV scan rate was 20 mV/s. The cut-off current density for the reported electrochemical windows (EWs) was 1 mA·cm-2. Raman Spectroscopy. The ion coordination and specifically the Li+ solvation structure in [Li][TFSI]/IL binary and [Li][TFSI]/[PYR13][TFSI]/[EMIM][DCA] eutectic ternary mixtures were obtained with Raman spectroscopy (Xplore Horiba, NJ; 785 nm, 9 mW power) on rough copper substrate (3M 1190). The spectra were collected within the range of 500 to 2500 cm-1 with a spectral resolution of 6.5 cm-1. The local spectra of the peaks of interest were collected with a spectral resolution of 4.2 cm-1. Recorded Raman spectra was corrected with baseline subtraction using NGSLabSpec (HORIBA). After determining peak locations from second derivatives, width at half-maximum and amplitude for each peak were obtained by multi-peak fitting program in Igor Prof 7.08 (WaveMetrics). The reported fit results have a χ2/I < 170 where I is the maximum Raman intensity recorded. The local spectra showed in the figures were normalized according to the highest intensity. The fraction of coordinated anions with Li+ was calculated by the areal fraction of the fitted peaks from the local spectrum. The coordination number was then calculated by eq. 3. 𝑁=

𝑐𝑜𝑜𝑟𝑑𝑖𝑛𝑎𝑡𝑒𝑑 𝑎𝑛𝑖𝑜𝑛 𝑓𝑟𝑎𝑐𝑡𝑖𝑜𝑛 𝐿𝑖𝑇𝐹𝑆𝐼 𝑚𝑜𝑙𝑒 𝑓𝑟𝑎𝑐𝑡𝑖𝑜𝑛

∙ 𝑎𝑛𝑖𝑜𝑛 𝑐𝑜𝑛𝑐𝑒𝑛𝑡𝑟𝑎𝑡𝑖𝑜𝑛 10 ACS Paragon Plus Environment

Eq. 3

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For the [Li][TFSI] in [PYR13][TFSI] and in [EMIM][DCA] binary mixtures, eq. 4 was used: 𝐴𝑏𝑜𝑢𝑛𝑑

𝑁𝑖 = ( 𝐴

𝑙𝑜𝑐𝑎𝑙

𝑥𝐿𝑖𝑇𝐹𝑆𝐼) ∙

𝑛𝑎𝑛𝑖𝑜𝑛𝑖

Eq. 4

𝑛𝑎𝑛𝑖𝑜𝑛

where 𝐴𝑏𝑜𝑢𝑛𝑑 is the area of the Li+ coordinated peak in the local spectrum of [TFSI] and [DCA]. 𝐴𝑙𝑜𝑐𝑎𝑙 is the total area of the anion peak in the local spectrum. 𝑛𝑎𝑛𝑖𝑜𝑛𝑖 is the mole of the anion of interest. 𝑛𝑎𝑛𝑖𝑜𝑛 is the mole of all the anions in the bulk. 𝑥𝐿𝑖𝑇𝐹𝑆𝐼 is the mole fraction of [Li][TFSI]. For the [Li][TFSI] in [PYR13][TFSI]/[EMIM][DCA] (1: 9) ternary mixtures, eq. 5 was used to find the total coordination number:

𝑁𝑡𝑜𝑡𝑎𝑙 =

𝑛𝑇𝐹𝑆𝐼 𝑛𝑇𝐹𝑆𝐼 + 𝐷𝐶𝐴

𝐴

∙

𝐿𝑖 + ― 𝑇𝐹𝑆𝐼 𝐴𝑙𝑜𝑐𝑎𝑙 𝑇𝐹𝑆𝐼

+𝑛

𝑛𝐷𝐶𝐴

𝑇𝐹𝑆𝐼 + 𝐷𝐶𝐴

𝐴

∙

𝐿𝑖 + ― 𝐷𝐶𝐴 𝐴𝑙𝑜𝑐𝑎𝑙 𝐷𝐶𝐴

Eq. 5

𝑥𝐿𝑖𝑇𝐹𝑆𝐼

where 𝑛𝑇𝐹𝑆𝐼 is the mole of [TFSI], 𝑛𝐷𝐶𝐴is the mole of [DCA], 𝑛𝑇𝐹𝑆𝐼 + 𝐷𝐶𝐴 is the sum of mole of [TFSI] and [DCA]. 𝐴𝐿𝑖 + ― 𝑇𝐹𝑆𝐼 and 𝐴𝐿𝑖 + ― 𝐷𝐶𝐴 are the areas of Li+ coordinated anion peaks in the local spectrum of [TFSI] and [DCA], respectively. MD Simulations. Simulations were performed using the LAMMPS package.60 The neat ILs, [PYR13][TFSI], [EMIM][DCA], and their binary mixtures (IL1/IL2) were simulated in the same range as the experimental compositions. The Li-salt mixtures with [PYR13][TFSI], [EMIM][DCA] and the eutectic system were simulated at 0.2, 0.5 and 1 M [Li][TFSI] concentrations. Due to the high viscosity in the 1 M [Li][TFSI]/[PYR13][TFSI] mixture, 0.1 M was simulated instead in order to capture the dynamical properties and the trends in the mixtures. The [Li][TFSI] mole fractions used for both experimental and simulated systems are given in Tables S1 and S2.



The initial configurations were randomly built in a cubic box with 600 ions using the PACKMOL package,61 followed by a structure minimization and equilibration for 12 ns via an annealing simulation from 700 K to 400 K in the isothermal-isobaric (NPT) ensemble and 1 atm pressure. The densities given in Table S3 were obtained based on the last 4 ns of the NPT simulations. Production runs were carried out in the canonical ensemble (NVT) at 400 K for 46 ns for [Li][TFSI] in [EMIM][DCA] and the IL1/IL2 mixtures. For [Li][TFSI]/[PYR13][TFSI], 92 ns were needed due to the high viscosity. The Lagrangian62 approach and the Nosé-Hoover63 thermostat were used to control pressure and temperature, respectively. The equations of motion were integrated using a time step of 1 fs, and periodic boundary conditions were applied. The General Amber Force Field (GAFF)64 was used to describe the interactions between the atoms. The structures of each isolated ion were optimized at the B3LYP/6-311++g(d,p) level using the Gaussian 09 Package65, and the partial charges were obtained by the Restrained Electrostatic Potential (RESP) method.66 To capture the effect of charge transfer and the polarizability in the ions, the RESP charges were uniformly scaled by a factor of 0.8.67–69 The Particle-Particle Particle-Mesh method70 was used to treat the electrostatic and long-range pair interactions. The real space Coulombic and van der Waals interactions were calculated using a cutoff distance of 12 Å. The effect of Li-salt addition on the liquid structure was analyzed in terms of the center of mass radial distribution function, g(r), between Li+-anion. To establish a direct correlation between the structure from MD and the structure observed in the Raman measurements, the coordination numbers (CN) for Li+ solvation shell were obtained using eq. 3. Note that application of eq. 3 requires that a cutoff distance be defined; if two ions are closer than this distance then they are assumed to be coordinated, but if they are 12 ACS Paragon Plus Environment

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farther away than the cutoff distance they are assumed to be uncoordinated. The numerical value of the CN depends strongly on this cutoff distance. While the cutoff distance is often assumed to be equal to the distance where the first minimum in the ion-ion radial distribution function is located,39 this definition is somewhat arbitrary and does not necessarily correspond to the coordination distance observed in Raman experiments. Therefore, we have computed CNs as a function of cutoff distance in order to better compare simulation results with experiment. To analyze the solvation dynamics of the Li+ anion environment, IP lifetimes were calculated. Note that the IP lifetime has a few different definitions in the literature.36,39–41 The definitions of IP lifetime used in this work is consistent with ref 71. The IP correlation functions were calculated by eq. 6: 𝐶(𝑡)≅

〈ℎ(0)ℎ(𝑡)〉

Eq. 6

〈ℎ〉2

where h=1 when the Li+-anion interaction is formed at time t and h=0 when this interaction is over. The IP lifetime considers the dynamics between Li+ and the closest anion in the solvation shell. The IP correlation functions were fit to a multi-exponential function as shown in eq. 7 and the lifetimes were obtained by the integral of this function: 𝐶(𝑡) = ∑𝑎𝑖𝑒 ―𝑡/𝑏𝑖

Eq. 7

where the bi and ai are the fitting parameters.



Results and Discussions Thermal analysis Figure 1 shows the TGA and DSC traces for IL1, IL2 and the binary mixture (1:9 IL1:IL2, v/v). The measured decomposition temperatures for IL1 is 460 ˚C and IL2 is 325 ˚C, consistent with previous reports.54,72 These temperatures correspond to the inflection points observed on the TGA curves, as indicated by the dashed lines in Figure 1b. The measured onset temperatures are 408 and 278 ˚C for IL1 and IL2, respectively. The peak of melting transition is at 8.9 ˚C for IL1 (Figure 1a), similar to previous publication (12 ˚C).73 No glass transition was observed within the temperature range examined. However, the existence of a cold crystallization at -20.5 ˚C possibly suggests a preexisting glassy state in [PYR13][TFSI]. The melting transition of IL2 occurs at -17˚C with a maximum peak at -9.5 ˚C, consistent with the melting temperature of -12 ˚C from a previous study.74 A single cold crystallization peak temperature is observed for [EMIM][DCA] at -52˚C, while Fletcher et al.75 reports three cold crystallization transitions at a similar temperature that extends to the onset of melting. The small differences in the thermal behavior observed and those of previous publications for the neat ILs can be attributed to the difference in impurities, heating rates, and lack of annealing step in our measurements. For the IL1/IL2 (1:9, v/v) mixture, crystallization is observed upon heating at -52 ˚C that peaks at -42 ˚C followed by a melting transition that starts at -22 ˚C and peaks at -13 ˚C. These results indicate a eutectic behavior for the 1:9 mixture whose melting temperature is lower than its parent ILs. No phase transformation were observed within the temperature range investigated for the 1M [Li][TFSI] containing IL1/IL2 (1:9) mixture (Figure S.1); possibly due to the small sample mass (~5 mg) and scan rate of 10 ˚C/min that were adapted to be consistent with the rest of the measurements. 14 ACS Paragon Plus Environment

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Figure 1. (a) DSC and (b) TGA curves of IL1: [PYR13][TFSI] (red), IL2: [EMIM][DCA] (green), and [PYR13][TFSI]/[EMIM][DCA] mixture in volumetric ratios of 1:1 and 1:9 (blue).

Viscosity and Conductivity To examine the impact of the eutectic behavior of the 1:9 binary mixture on the transport properties, the conductivity and viscosity were measured as a function of temperature and [Li][TFSI] concentration and fit to the VFT equation (Figures 2 and 3). IL1 has a viscosity of 70 cP at 25°C and IL2 has a viscosity of 15 cP at 25°C; both values are consistent with previous literature values.50,76 The measured conductivities for IL1 and IL2 are 5 and 27 mS/cm at 25 ˚C, respectively. These are also consistent with previous reports.50,77 The viscosity of IL1/IL2 (1:9, v/v) mixture is about the same as IL2 within the uncertainty of our measurements. The conductivity of the 1:9 binary is similar to that of IL2. MD simulations predict similar trends (Figures S2-S4). VFT parameters for conductivity and viscosity are listed in Table 2.



Figure 2. Temperature and composition dependence of viscosities for (a) IL1/IL2 binary and (b) [Li][TFSI] in IL1/IL2 (1:9, v/v) ternary mixtures.

Figure 3. Temperature and composition dependence of conductivities for (a) IL1/IL2 binary and (b) [Li][TFSI] in IL1/IL2 (1:9, v/v) ternary mixtures.


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Table 2. VFT parameters of conductivity (𝜎0, 𝑘, 𝑇0) and viscosity (𝜇0, 𝑏, 𝑇0) for IL1/IL2 binary and [Li][TFSI]/IL1/IL2 ternary mixtures. 𝑇0 was set the same for both conductivity and viscosity. IL1 = [PYR13][TFSI] and IL2 = [EMIM][DCA]; binary mixtures are abbreviated according to volumetric ratios of IL1:IL2. Ternary mixture contains [Li][TFSI] in 1:9 binary. 𝝈𝟎 (mS/cm) IL1 IL2 9:1 1:1 1:9 1 M in IL1 1 M in IL2 0.2 M in 1:9 0.5 M in 1:9 1 M in 1:9

𝑻𝟎 (K) 𝝁𝟎 (mPa s) 𝒌 (K) Neat ILs 266.491 -473.255 183.000 0.400 753.042 -440.644 165.000 0.207 Binary mixtures: IL1/IL2 (v:v) and [Li][TFSI]/IL 470.777 -569.139 168.751 0.206 1146.75 -729.1 136.945 0.096 1732.87 -659.653 143.936 0.143 121.779 -376.199 218.997 0.391 1031.36 -613.849 161.912 0.261 Ternary mixtures: [Li][TFSI]/IL1/IL2 723.957 -434.5 177.958 0.389 486.851 -473.841 169.711 0.284 1134.55 -659.946 154.460 0.181

𝒃 (K) 594.889 572.669 699.405 901.681 728.337 556.524 686.089 469.376 584.058 800.051

The viscosity of the 1:9 binary mixture is very similar to that of IL2, Figure 2a. According to the ideal mixing law of Katti and Chaudhri,78 the viscosity of the 1:9 mixture is expected to be 17 cP at 25 ˚C using the mole fraction and viscosities (Tables S2 and S3) of the parent ILs measured at 25 ˚C. According to the Grunberg and Nissan mixing law,79 the viscosity of the eutectic mixture is also predicted to be 17 cP at the same temperature. The measured viscosity of the 1:9 mixture at 25 ˚C is 16 cP, in comparison to 70 cP for IL1 and 15 cP for IL2. This suggests that the eutectic mixture (1:9) conforms to these mixing laws. The measured conductivities for the binary mixtures are also observed to follow the empirical mixing laws (Figure 3a). Viscosities of ILs were observed to increase with increasing concentration of [Li][TFSI] (Figure 2b), while the conductivities were



observed to decrease with increasing concentration of [Li][TFSI] (Figure 3b). With 1 M [Li][TFSI] (xLiTFSI = 0.145), the viscosity of the 1:9 mixture increased by ~3 fold at 25 ˚C relative to the saltfree mixture, slightly higher than 1 M [Li][TFSI] in IL2 (xLiTFSI = 0.139) (Figure S5). This increase is significantly smaller when compared to the ~6 fold increase of the IL1 viscosity with 1 M [Li][TFSI] (xLiTFSI = 0.231) at 25 ˚C (Figure S5). The dependence of conductivity on [Li][TFSI] concentration is not as dramatic as the viscosity. In fact, increasing the [Li][TFSI] concentration from 0.5 (xLiTFSI = 0.078) to 1 M (xLiTFSI = 0.145) has little effect on the conductivity of the ternary 1:9 eutectic mixture. MD simulations predicted a similar increase in viscosity (Figure S3) but did not capture the constant conductivity behavior between 0.5 and 1 M [Li][TFSI] (Figure S4). Note that the conductivities obtained from MD were calculated using the Nernst-Einstein relations which assumes no correlation between ions, a significantly different picture from that observed in experiments. Therefore, it is essential to understand the ion coordination as investigated experimentally by Raman spectroscopy as well as MD simulations.

Electrochemical Stability The cyclic voltammograms of the binary mixtures in comparison to neat ILs are shown in Figure 4a. EW of IL1 is measured to be 5.79 V, while IL2 is 3.08 V with a cutoff current density of 1 mA/cm2; both are consistent with previous reports.50,80 Regardless of the compositional range of the binary IL mixtures of IL1 and IL2, all of the EWs measured are close to the value of IL2 which has the narrower window. Unlike viscosities and conductivities, electrochemical stability is not an additive property and mixtures present no significant advantage over the parent IL with the


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lower stability. This restricts the use of these mixtures with high-voltage electrode materials in energy storage. In the broader context of electrochemistry, binary mixtures may allow tuning of process rates and affect interfacial phenomena. On the other hand, with the addition of the Lisalt, the stability improves, as shown in Figure 4b.

Figure 4. Electrochemical window of (a) neat ILs and IL1/IL2 binary mixtures; (b) Eutectic IL mixture with [Li][TFSI] salt: [Li][TFSI]/IL1/IL2 ternary. Cyclic voltammetry was performed with a scanning rate of 20 mV/s; results from 1 mV/s rate are similar as shown in Figure S6. Au working and counter electrodes, and Ag quasi reference electrode.

Li+ Solvation Structure The coordination structure in the studied ternary mixture, [Li][TFSI]/IL1/IL2, is more complex than the previously investigated Li-salt/IL binaries as it involves not only different anions but also



different cations. A systematic Raman analysis was performed to distinguish the solvation structure of Li+ between binary and ternary mixtures. Figure 5a shows the local Raman spectra for [TFSI] in [Li][TFSI]/IL1; Figures 5d and 5g show the [DCA] region in [Li][TFSI]/IL2 and [Li][TFSI]/IL1/IL2, respectively, (full Raman spectra are in Figures S7-9). Cis and trans conformers of the CF3-SNS-CF3 skeleton of [TFSI] could not be distinguished and overlapped at 741 cm-1 (referred to as ‘free’ [TFSI]) in Figure 5b. The corresponding vibrations in the 710-765 cm-1 region are the CF3 bending 𝛿(CF3) coupled with S-N stretching 𝜐𝑠(SNS). With 1 M [Li][TFSI] (xLiTFSI = 0.231) salt, which shares the same anion with IL1, a new peak corresponding to Li+ coordinated [TFSI] (referred to as ‘bound’ [TFSI]) is seen at 748 cm-1, Figure 5c. Peak locations (743 and 748 cm-1) were obtained from the second derivative analysis. The observance of two peaks allows the categorization of [TFSI] as ‘free’ and ‘bound’, however does not enable differentiation between monodentate and bidentate confirmations.


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Figure 5: Local Raman spectra of [Li][TFSI]/IL1 (a-c), [Li][TFSI]/IL2 (d-f) and [Li][TFSI]/IL1/IL2 (IL1:IL2 is 1:9) (g-i). 741 cm-1 corresponds to the expansion and contraction of [TFSI] that is coordinated with the [PYR13] cation. 748 cm-1 is due to the Li+ coordinated with TFSI. 2180-2240 cm-1 region corresponds to the symmetrical stretches and combination bands of [DCA] in IL2 and IL1/IL2 mixtures. 2194 and 2209 cm-1 peaks are due to symmetric stretch of ‘free’ and ‘bound’ [DCA], respectively. Red markers are the experimental data. Black lines are fitted peaks and blue lines are fitted curves in the lower two rows. No water-specific influences are observed in solvation in these samples (100-1330 ppm water) by Raman spectra as evident from Figure S10.

The Li+ coordinated [TFSI] was not observed in [Li][TFSI]/IL2 since the concentration of [TFSI] was very low and below the detection limit. However, spectral changes are observed for the [DCA] region in Figures 5d-f. The peaks at 2194 and 2223 cm-1 in Figure 5e are the symmetrical stretch 𝜈𝑠(𝐶 ≡ 𝑁) mode and the 𝜈𝑠(𝐶 ― 𝑁) + 𝜈𝑠(𝐶 ― 𝑁) combination bands of [DCA]81,82. Upon dissolving 1 M of [Li][TFSI], a new peak at 2209 cm-1 corresponding to the symmetrical stretch of Li+ coordinated with [DCA] is seen in Figure 5f. Accordingly, the ‘bound’ [DCA] has a blue shift of 15 cm-1 from the ‘free’ [DCA]. As a result, the combination bands become broader around 2223 cm-1. In the [Li][TFSI]/IL1/IL2 mixture, minimal changes are observed in [TFSI] region (Figure S11) indicating very small contribution to Li+ solvation by [TFSI]. On the other hand, rich spectral changes were observed for [DCA] with increasing [Li][TFSI] concentration (Figures 5g-i). In principle, [DCA] can coordinate with all three cations: [EMIM], [PYR13] and Li+ in the ternary mixture. For simplicity, we did not attempt to differentiate between [EMIM] and [PYR13] coordination in Figure 5h where the observed peaks are identified as “free” modes. The 21 ACS Paragon Plus Environment


“bound” modes that correspond to Li+-[DCA] are identified as the 2209 and 2226 cm-1 peaks in Figure 5i.

Li+ Coordination Number The experimental and simulated coordination numbers of Li+ in both [Li][TFSI]/IL1 and [Li][TFSI]/IL2 are shown in Figures 6a and b, respectively. The radial distribution function, g(r), for Li+-anion for each system are shown in Figures S12-14. Those for Li+-Li+ in IL1 and IL2 are shown in Figures S15 and S16. There is reasonable agreement for CN between the simulations and experiments. Both show that there are around 1-3 [TFSI] anions surrounding Li+ in [Li][TFSI]/IL1 and 2-4 [DCA] anions surrounding Li+ in [Li][TFSI]/IL2. Both CNs also show that there is a weak concentration dependence. As the Li+ concentration increases in [Li][TFSI]/IL1, [TFSI] can coordinate with multiple lithium ions forming bridges, which results in a slight decrease in the CN. Such bridge structure can be seen in Figure 7a as well as in the Li+-Li+ g(r) shown in Figure S16. The peaks at around 489 pm in Figure S16 are an evidence that the lithium ions are close to each other, which is possible only when they are shared by the same anion. The discrepancy between the experimental and simulated CN at high [Li][TFSI] concentrations in [Li][TFSI]/IL1 can be attributed to the fact that we cannot isolate the monodentate Raman peak from the free peak, thus artificially suppressing the measured CN. The bidentate binding-based coordination numbers in [Li][TFSI]/ IL1 were calculated from MD simulations and the results are provided in Figure S17. Although the calculated CNs are sensitive to the cutoff distances, the values agree with the experimental results very well especially at high [Li][TFSI] concentrations.


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It has been reported that the monodentate conformation of [TFSI] with Li+ in acetonitrile has a very small blue shift (0-3 cm-1)83. Therefore, it is possible that monodentate conformation has an unresolvable shift in the collected spectra since a third peak in 710 – 760 cm-1 region does not emerge from the second derivative analysis. If the monodentate [TFSI] has similarly small shifts in ILs, it is possible that it is counted as ‘free’ [TFSI] in the Raman analysis. This observation is in agreement with literature.35 For the analysis of the [DCA] region, only the fitted symmetrical stretch modes of ‘free’ and bound’ [DCA] are considered in the calculations of Li+ coordination in [Li][TFSI]/IL2 and [Li][TFSI]/IL1/IL2 systems, while the changes in the combination bands are neglected. This prevents the double counting of different [DCA] modes of the same anion. Lower resolution Raman spectra predict similar blue shifts with Li+ coordination of both [TFSI] and [DCA] modes in the ternary mixture (Figure S18). However, the fitted peaks lead to slightly different Li+[DCA] coordination numbers. The location of each fitted peak is determined from the second derivatives and also guidance from prior literature on the expected modes of [TFSI] and [DCA]27,30,37,44,81,82. Figure 7 shows representative solvation structures in IL1 and IL1:IL2 obtained from MD. Figure 7a shows that even at a concentration of 1M [Li][TFSI], there are both monodentate and bidentate Li+ - [TFSI] conformations. Figures 7b and 7c demonstrate that there is a heterogenous coordination environment. Li+ prefers to be solvated by [DCA] over [TFSI] and [DCA] coordinates only in a monodentate manner. In the presence of [DCA], the Li+-[TFSI] coordination is primarily monodentate.



Figure 6: Experimentally and computationally determined coordination numbers for Li+ in (a) Li+ – [TFSI] and (b) Li+ – [DCA]. The blue region encompasses the computed coordination numbers obtained using a cutoff distance determined from the first minimum in the radial distribution function (see Figs. S13-S14) plus 100 pm (inverted triangles) and minus 100 pm (triangles). For a) these distances are 332 pm and 532 pm while for b) these distances are 415 pm and 615 pm.


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Figure 7: MD simulation snapshots of Li+ solvation structures of 1M [Li][TFSI] in IL1 (a) and IL1/IL2 (1:9) mixture (b and c). In the ternary mixture, a heterogeneous solvation structure is observed. Two representative structures are shown in b and c. The Li+ solvates in the [Li][TFSI]/IL1/IL2 system may exist in the form of [Li(TFSI)m(DCA)n](m+n-1) where m = 0 to 1 and n = 3 to 4. The most probable structure suggested by Raman is [Li][DCA]3]-2 (Figure 8). The averaged total coordination number is about 3 at the [Li][TFSI] concentration range investigated as seen in Figure 8b with mostly Li+-[DCA]. Note that because almost all of the [TFSI] coordination is monodentate in the ternary mixture across all [Li][TFSI] concentrations, the Raman data underestimates CN because the monodentate peak cannot be decoupled from the ‘free’ [TFSI] peak. Additionally, the [TFSI] concentration is very low, making it difficult detect (Figure S10). The computed CN is insensitive to the coordination number cutoff distance for Li+-[TFSI]. This is because the small amount of [TFSI] anions in the system are bound to a single Li+.



Figure 8: Experimentally and computationally determined coordination numbers for Li+ in [Li][TFSI]/IL1/IL2. Partial coordination numbers are shown for (a) Li+-TFSI and (b) Li+-DCA. The blue region in b) encompasses the computed coordination numbers obtained using a cutoff distance determined from the first minimum in the radial distribution function (see Figs. S15) plus 100 pm (inverted triangles) and minus 100 pm (triangles). For a) these distances are 332 and 532 pm while for b) these distances are 415 pm and 615 pm. Note that in a) the CNs are so similar that there is no blue area.

Li+ Solvation Dynamics In general for ionic systems, the IP lifetime correlates with other dynamical properties, such as diffusivity and viscosity.71,84,85 For example, IP lifetimes typically increase when the viscosity of the system increases. Figure 9 shows the IP lifetimes for the anions around Li+ in the simulated systems. As expected, in IL2 and the eutectic mixture (Figures 9a and b), the [DCA] lifetimes around Li+ increase with the addition of [Li][TFSI], which is consistent with the increase in viscosity. Interestingly, the lifetimes of [TFSI] around Li+ in the eutectic mixture and also in the [EMIM][DCA] system (Figures 9a and b) decrease with the addition of more [Li][TFSI]. This unexpected behavior is due to the small number of [TFSI] in the system. When the [Li][TFSI] concentration is increased in the eutectic mixture, the anion exchange rate of [TFSI] increases because there are now more [TFSI] anions to exchange with Li+. That is, at low Li+ concentrations, the [TFSI] anions are more tightly bound to Li+ and so remain associated with Li+ for a long time. As the number of [TFSI] anions increase in the system, more of the [TFSI] anions are bound in weaker configurations around Li+, and so the IP lifetimes decrease. The decrease in IP lifetime is 26 ACS Paragon Plus Environment

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expected to create fluctuations in the Li+ solvation structure, and tends to increase the conductivity as suggested by Figure 3b even as the viscosity increases as seen in Figure 2b between 0.5 and 1 M [Li][TFSI]. We believe this effect is related to the compensation of the higher viscosity with the shorter IP lifetime for [TFSI]. In the [Li][TFSI]/IL1 system (Figure 9c), the IP lifetime of [TFSI] about Li+ continuously increases as the Li+ concentration increases. This is correlated with the abundance of strongly bound [TFSI] anions in this system. Here, the increase in viscosity causes the IP lifetime to increase, just like that of [DCA] in Figures 9a and b.

Figure 9. The IP lifetimes as a function of [Li][TFSI] concentration for the [DCA] (black circles) and the [TFSI] around lithium (red squares) in (a) [Li][TFSI]/IL2, (b) [Li][TFSI]/IL1/IL2 (v:v = 1:9) and (c) [Li][TFSI]/IL1 at 400 K.



Conclusions Ternary mixtures of [Li][TFSI]/[PYR13][TFSI]/[EMIM][DCA] electrolytes are investigated in terms of physicochemical and electrochemical properties and compared to binary systems of lithium salt and IL. Li+ solvation structure and dynamics are elucidated by Raman spectroscopy and MD simulations. It is observed that increased heterogeneity within the Li+ solvate structure by IL mixtures and the concentration of the lithium salt influences the viscosity and conductivity dramatically. Specifically, Li+ prefers to be solvated by the smaller [DCA] anion over [TFSI]. The calculated decrease in ion pair lifetime of [TFSI] with Li+ at higher concentrations of the lithium salt is believed to depress the decrease in conductivity while the viscosity continues to increase with increasing [Li][TFSI] concentration. Specifically, the increased connectivity of solvate clusters at low to moderate [Li][TFSI] concentrations inhibit conductivity. Diluting a high viscosity IL with a lower viscosity IL promotes heterogeneity in Li+ solvate structure which impacts dynamics. In the studied ternary electrolyte system, the combination of [TFSI] and [DCA] anions of different sizes and coordination chemistries demonstrate the ability to modify Li+ solvate charge carrier structure and dynamics.


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Supporting Information Experimental and computational details of the electrolyte compositions, viscosity and conductivity. Full Raman spectra, supporting analysis and computational radial distribution functions.

Author Contribution QH performed the experiments and Raman analysis. TCL carried out MD simulations. YZ and LTC oversee the simulation analysis. EM supervised the simulation work. BG designed the experiments and supervised the work. All authors contributed intellectually and to writing of the manuscript.

ACKNOWLEDGEMENTS QH and BG would like to thank Prof. Ozan Akkus for the many useful discussions on Raman Spectroscopy. TLC and LTC acknowledge FAPERJ for the fellowships 201.995/2016, JCNE No.214996/E06/2015 and CAPES. YZ and EM appreciate the support by the U.S. Department of Energy, Basic Energy Science, Joint Center for Energy Storage Research under contract no. DEAC0206CH11357. Computational resources were provided by the Center for Research Computing (CRC) at the University of Notre Dame.



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This study demonstrates how partial solvation of lithium ion in mixtures of ionic liquids can impact solvation structure and dynamics that are relevant to energy applications. Macroscopic properties such as viscosity and conductivity as a function of lithium salt in binary ionic liquid mixtures are investigated. To further correlate these to ion coordination and understand the complex ion solvation environment, Raman spectroscopy and molecular dynamics simulations are carried out. 82x44mm (300 x 300 DPI)

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Solvation Structure and Dynamics of Li+ in Ternary Ionic Liquid

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