Electrochemical Stability Window of Imidazolium-Based Ionic Liquids

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Electrochemical Stability Window of ImidazoliumBased Ionic Liquids as Electrolytes for Lithium Batteries Saeed Kazemiabnavi, Zhengcheng Zhang, Katsuyo Thornton, and Soumik Banerjee J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.6b03433 • Publication Date (Web): 06 Jun 2016 Downloaded from http://pubs.acs.org on June 8, 2016

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The Journal of Physical Chemistry

Electrochemical Stability Window of Imidazolium-Based Ionic Liquids as Electrolytes for Lithium Batteries Saeed Kazemiabnavi1,2, Zhengcheng Zhang3, Katsuyo Thornton1,4 and Soumik Banerjee5,∗ 1

Joint Center for Energy Storage Research, University of Michigan Ann Arbor, MI 48109 USA 2

Department of Mechanical Engineering, University of Michigan Ann Arbor, MI, 48109-2136, U.S.A

3

Chemical Sciences and Engineering Division, Argonne National Laboratory 9700 South Cass Avenue, Argonne, IL 60439-4837, U.S.A

4

Department of Materials Science and Engineering, University of Michigan Ann Arbor, MI, 48109-2136, U.S.A

5

School of Mechanical and Materials Engineering, Washington State University Pullman, WA, 99164-2920, U.S.A

ABSTRACT. This paper presents the computational assessment of the electrochemical stability of a series of alkyl methylimidazolium-based ionic liquids for their use as lithium battery electrolytes. The oxidation and reduction potentials of the constituent cation and anion of each ionic liquid with respect to a Li /Li reference electrode were calculated using density functional theory following the method of thermodynamic cycles and the electrochemical stability window (ESW)s of these ionic liquids were obtained. The effect of varying the length of alkyl side chains of the methylimidazolium-based cations on the redox potentials and ESWs were investigated. The results show that the limits of the ESWs of these methylimidazolium-based ionic liquids are defined by the oxidation potential of the anions and the reduction potential of alkyl∗

Corresponding Author, Tel: +1 509 3350294, E-mail: [email protected] 1 ACS Paragon Plus Environment


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methylimidazolium cations. Moreover, ionic liquids with [PF ] anion have a wider ESW. In addition to characterizing structure-function relationships, the accuracy of the computational approach was assessed through comparisons of the data against experimental measurements of ESWs. The potentials calculated by the thermodynamic cycle method are in good agreement with the experimental data while the HOMO/LUMO method overestimates the redox potentials. This work demonstrates that these approaches can provide guidance in selecting ionic liquid electrolytes when designing high-voltage rechargeable batteries.

2 ACS Paragon Plus Environment

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1. INTRODUCTION During the past several years, rechargeable batteries have gathered significant attention for applications ranging from plug-in hybrid vehicles to power supplies in aerospace systems.1-4 Improving the specific energy density of lithium batteries while maintaining high safety standards for large-scale applications has always been a challenge due to the high vapor pressure and flammability of conventional organic electrolytes. Room-temperature ionic liquids, which have low vapor pressure, high thermal stability and low flammability, are considered as potential alternative due to remarkable safety advantages over conventional organic electrolytes.5-8 In fact, studies investigating potential application of room-temperature ionic liquid electrolytes in lithium-ion and lithium-sulfur batteries have demonstrated cyclability and capacity improvement over the batteries operating with conventional electrolytes.9-11 In our recent studies, we have investigated the thermodynamics and kinetics of cathodic and anodic reactions in lithium-air batteries with ionic liquid electrolytes.12-15 In general, the electrochemical stability window of the electrolyte, defined as the difference between the solvent’s reduction and oxidation potentials, plays a crucial role in large-scale applications of lithium batteries from pure electric and/or hybrid electric vehicles to grid energy storage. Recent attempts in improving the energy and power density of lithium-ion batteries have focused on improving the voltage of individual cells, successful implementation of which hinges on the electrochemical stability of the electrolyte. The electrochemical reduction of the electrolyte by lithium metal depends on the potential of the cathode relative to a Li+/Li electrode. On the contrary, the electrolyte is oxidized if the cathode potential exceeds the electrolyte’s oxidation potential.16 The selection of cathode and anode materials for use in high-voltage batteries with high power density is limited due to the relatively narrow electrochemical stability window of



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conventional electrolytes, which results in the oxidation or reduction of electrolyte within the potential window of the anode and cathode electrodes. This issue can be resolved by utilizing electrolytes that are electrochemically more stable. Experimental studies have shown that many ionic liquids exhibit high electrochemical stability windows ranging between 4-7 V,6 which makes them promising electrolytes for use in batteries with high voltage cathode materials such as LiNiPO with a predicted potential of more than 5 V17 and Li(Ni. Mn. )O with a discharge potential of 4.7 V vs. a Li /Li reference electrode.18, 19 Experimental approach to determine the electrochemical window of an electrolyte is based on the i-V polarization curve generated by cyclic voltammetry using a non-porous working electrodes, such as platinum (Pt) and glassy carbon (GC).20 Several experimental studies have been performed to determine the electrochemical stability window of ionic liquids with respect to various reference electrodes.6, 20-26 However, the individual oxidation and reduction potentials of the constituent ions have not been reported in experimental literature. In most ionic liquids, it is assumed that the cathodic limit is set by the reduction potential of cation, and the anodic limit is defined by the oxidation potential of anion; however, experimental and theoretical studies have shown that this statement may not always hold. For instance, experimental study by Howlett et al.27 as well as a computational study by Ong et al.16 showed that the widely used bis(trifluoromethylsulfonyl)imide ([TFSI] ) anion undergoes electrochemical reduction more easily than the N,N-propylmethylpyrrolidinium cation. Therefore, it is necessary to obtain both reduction and oxidation potentials of the constituent ions in an ionic liquid in order to determine the electroactive species that define the electrochemical stability window of the ionic liquid. Furthermore, the electrochemical properties of ionic liquids can be tuned by modifying the chemical structure of the constituent ions. It is possible to synthesize ionic liquids with a wide


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variety of tunable physicochemical and electrochemical properties by combining ionic liquid cations such as tetralkylammonium ([R N] ), aromatic and saturated cyclic amines, sulfonium ([R S] ) and phosphonium ([R P] ), where R represents an alkyl group, with both inorganic and organic anions such as [PF ] , [BF ] , [CF CONCF SO ] and [N(CF SO ) ] .6 Therefore it is important to develop structure-function relationships in order to efficiently screen ionic liquids and choose the electrolytes that meet our desired properties. Quantum mechanical calculations can be used to shed light on the relation between the chemical structure and electrochemical stability windows of electrolytes, as well as to predict potential candidates for new electrolytes with desired electrochemical stability, significantly reducing the number of time-consuming exploratory experiments. Ong et al.16 have previously studied the electrochemical stability window of six methylimidazolium-based (imidazolium-based for short) ionic liquids as well as pyrrolidinium-based ionic liquids using a combination of classical molecular dynamics (MD) simulations and density functional theory (DFT) calculations. Recently, Borodin et al.28 implemented the distributed multi-scale computing framework, developed by Knap et al.,29 to screen the electrochemical stability of carbonate and phosphate molecules that find potential application in lithium batteries. Their work employed DFT calculations on isolated solvent molecules surrounded by an implicit solvent to determine solvent stability towards first and second reduction and oxidation. In an effort to estimate the oxidation potential of sulfone-based electrolytes in high-voltage lithium-ion batteries, Shao et al.30,

31

applied the DFT-based

thermodynamic cycle method.32, 33 Their results were in good agreement with the experimentally measured oxidation potentials. While the previous studies have investigated various modeling approaches for evaluating the electrochemical stability of specific chemical species, the effect of structural changes and cation-



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anion pairing in ionic liquids has not been reported. Moreover, considering the large number of possible combinations of cations and anions that form ionic liquids with an extensive range of electrochemical properties, applying an efficient method such as the well-known thermodynamic cycle approach will significantly reduce the time and cost associated with the selection of an appropriate ionic liquid based on their electrochemical stability. The objective of the present study is to use a computationally effective approach to screen from a wide range of imidazolium ionic liquids, as potential battery electrolytes, based on their electrochemical stability window. The oxidation and reduction potential limits were calculated for 1-alkyl-3-methylimidazolium ([C MIM] ) cations, in which the alkyl group was varied from ethyl to hexyl (n = 2 to n = 6), as well as bis(trifluoromethylsulfonyl)imide ([TFSI] ), hexafluorophosphate ([PF ] ), tetrafluoroborate ([BF ] ) and trifluoromethanesulfonate ([TfO] ) anions using the DFT-based thermodynamic cycle method.30, 32, 33 In addition to structural changes in cations, it is noteworthy that we also modeled two different family of anions (i.e. [TFSI] and [TfO] as two trifluoromethyl-based anions as well as [PF ] and [BF ] as two of the most common fluorinated anions), and evaluated the effect of anion structure on electrochemical stability. The chemical structure of these cation and anions are shown in Figure 1. Using the calculated redox potentials, the corresponding electrochemical stability windows of 14 different ionic liquids were determined. Tian et al.34 showed that the imidazolium cation dissociation affects the redox potentials of ionic liquid cations. However, in this study, the simple one-electron transfer reaction was considered in the reduction and oxidation of ionic liquid cation and anions as one of the important redox reaction pathways presented in Tian et al.’s work.34 In addition to the thermodynamic cycle method, the oxidation and reduction potentials of these ionic liquid cations and anions were also estimated using the energy of the frontier molecular orbitals based on a


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method presented by Ong et al.16 This approach is based on the premise that the anodic limit is defined by the highest occupied molecular orbital (HOMO) and the cathodic limit is set by the lowest unoccupied molecular orbital (LUMO) of the ion. The evaluated oxidation and reduction potentials by HOMO/LUMO method are compared to the values obtained by the thermodynamic cycle and experimental methods as well. The results provide detailed insight on the effect of chemical structure of the ions on the width of the electrochemical stability window of the ionic liquids.

Figure 1. The chemical structures of ions studied in this work: a) 1-alkyl-3-methylimidazolium ([#$ %&%] ), b) bis(trifluoromethylsulfonyl)imide (['()&] ), c) trifluoromethanesulfonate (['*+] ), d) hexafluorophosphate ([,( ] ) and e) tetrafluoroborate ([-( ] ) are shown. 2. COMPUTATIONAL METHODOLOGY The main goal of the present study is to investigate the effect of the structure of ionic liquid’s cation and anion on the electrochemical stability window of ionic liquid with respect to a Li+/Li reference electrode. For this purpose, the thermodynamic cycle approach30, 7 ACS Paragon Plus Environment

32, 33

was applied


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considering the oxidation and reduction of both cation and anion of ionic liquid with respect to Li+/Li reference electrode (Figure 2 to Figure 4).30

Li (g)

∆Ge(Li)

+

e– (g)

∆Gsolv(Li+)

∆Gvap(Li) Li (s)

Li+ (g)

∆Gref

(0)

Li+ (solv) +

e– (g)

Li+ (g)

e– (g)

(a)

Li (g)

–∆Ge(Li)

–∆Gsolv(Li+)

–∆Gvap(Li) Li (s)

+

–∆Gref

Li+ (solv) +

(0) e– (g)

(b) Figure 2. Thermodynamic cycles for computing the a) oxidation potential and b) reduction potential of the Li+/Li reference. The cycle treats the overall reaction, in which (a) Li is oxidized to Li+ or (b) Li+ is reduced to Li, as a chain of reactions that has the identical change in Gibbs free energy.


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R+ (g)

∆oxG (R+)

–∆Gsolv(R+) R+ (solv)

R2+ (g)

+

∆Gsolv(R2+) ∆oxG (solv)

e– (g) (0)

R2+ (solv) +

e– (g)

R+ (g)

e– (g)

(a)

R (g)

∆redG (R+)

–∆Gsolv(R+)

∆Gsolv(R) R (solv)

+

∆redG (solv)

R+ (solv) +

(0) e– (g)

(b) Figure 3. Thermodynamic cycle for computing the a) oxidation potential and b) reduction potential of an ionic liquid cation is shown.



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∆oxG (A–)

A– (g) –∆Gsolv(A–) A– (solv)

A (g)

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+

e– (g)

∆Gsolv(A) ∆oxG (solv)

(0)

A (solv)

+

e– (g)

A– (g)

+

e– (g)

(a)

∆redG (A–)

A2– (g) ∆Gsolv(A2–) A2– (solv)

–∆Gsolv(A–) ∆redG (solv)

A– (solv) +

(0) e– (g)

(b) Figure 4. Thermodynamic cycle for computing the a) oxidation potential and b) reduction potential of an ionic liquid anion is shown. The oxidation processes are assumed to be a one-electron transfer between ionic liquid cation (R ) or anion (A ) and the lithium electrode, leaving behind a cation with 2+ charge (R ) or a neutral anion (A). On the contrary, the reduction reaction results in the formation of a neutral cation (R) or an anion with 2- charge (A ). For the Li+/Li reference, the change in Gibbs free energy of the oxidation of metallic lithium into Li+ ion, can be calculated using ∆G123 = ∆G456 (Li) + ∆G2 (Li) + ∆G89:4 (Li )

(1)

where ∆G456 (Li) is the free energy of vaporization of metallic lithium and is equal to the tabulated quantity of 118.0 kJ/mol.30 According to the cycles shown in Figure 3, the free energy of oxidation and reduction of the ionic liquid cation in solution can be calculated using Eq. (2a) and Eq. (2b), respectively: 10 ACS Paragon Plus Environment

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∆9; G(solv)(R ) = ∆9; G(g)(R ) + ∆G89:4 (R ) − ∆G89:4 (R )

(2a)

∆12B G(solv)(R ) = ∆12B G(g)(R ) − ∆G89:4 (R ) + ∆G89:4 (R)

(2b)

Using Eq. (3a) and Eq. (3b), the Gibbs free energy change of the oxidation and reduction of cations in the gas phase can be obtained by calculating the Gibbs free energy of the gas phase reactant and product species in the redox reactions shown in Figure 3: ∆9; G(g)(R ) = GCR (g)D + GCe (g)D − G(R (g))

(3a)

∆12B G(g)(R ) = GCR(g)D − GCe (g)D − G(R (g))

(3b)

Similarly, Eq. (4a) and Eq. (4b) can be used to obtain the free energy of oxidation and reduction of the ionic liquid anion in solution according to the thermodynamic cycles shown in Figure 4: ∆9; G(solv)(A ) = ∆9; G(g)(A ) + ∆G89:4 (A) − ∆G89:4 (A )

(4a)

∆12B G(solv)(A ) = ∆12B G(g)(A ) − ∆G89:4 (A ) + ∆G89:4 (A )

(4b)

Moreover, using Eq. (5a) and Eq. (5b), the Gibbs free energy change of the oxidation and reduction of the anions in the gas phase can be obtained by calculating the Gibbs free energy of the gas phase reactant and product species in the redox reactions shown in Figure 4: ∆9; G(g)(A ) = GCA(g)D + GCe (g)D − G(A (g))

(5a)

∆12B G(g)(A ) = GCA (g)D − GCe (g)D − G(A (g))

(5b)

Since the redox potentials of the ionic liquid cation and anion were referenced against a Li+/Li electrode, the free energy of the electron, GCe (g)D, will be cancelled out in the abovementioned equations. As shown in Eq. (6a) and Eq. (6b), the standard redox potentials relative to the Li+/Li reference electrode can be defined using the standard Gibbs free energy change of the corresponding reactions in the solution: 11 ACS Paragon Plus Environment


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∘ E9; [V](vs. Li /Li) = −(−∆9; G(solv)[J] + ∆G123 [J])/F

(6a)

∘ [V](vs. Li /Li) = −(∆12B G(solv)[J] + ∆G123 [J])/F E12B

(6b)

where F is the Faraday’s constant ( = 96485 C/mol). In order to obtain the full electrochemical stability window of the ionic liquid electrolyte, both oxidation and reduction potentials were computed. All DFT calculations were performed by the Gaussian 09 software package.35 The structure of all the chemical species involved in the redox reactions were fully optimized in the gas phase at the MP2 level of theory,36 with a medium level 6-31+G(d,p) basis set that shows good reproducibility in calculating the redox potentials of ion-pairs31,

37, 38

as well as the anodic

stability of fluorine-containing anions.39 The 6-31+G(d,p) is a split-valence double-zeta basis set that considers p-type polarization for hydrogen atoms and d-type polarization and diffuse functions for all other elements, which are important in improving the accuracy of the calculation when considering large soft molecular systems. For comparison, the oxidation potential of [C MIM] in [C MIM] [TFSI] was also calculated with a more diffuse basis set, 6311++G(d,p), at the same level of theory (MP2), which shows less than 1.3 % difference. In order to obtain the zero-point energy and thermal correction to Gibbs free energies, harmonic vibrational frequencies were also computed. The optimized structures of all the chemical species involved in the redox reactions were found to be at global energy minima by ensuring that they have no imaginary frequency. Moreover, self-consistent reaction field (SCRF) methodologies were utilized to obtain the solvation energies of all the species, including Li+ ion, by performing a single-point energy calculation on the gas phase optimized geometry in the presence of an implicitly defined solvent. The polarizable continuum model (PCM),40 which is an approach to implicitly account for dielectric screening in solvents, was implemented. The


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primary inputs to the model are the static dielectric constant of the solvent and the radii of the solvent molecules. For all calculations, the cations and anions in the ionic liquid are surrounded by their corresponding counter-ion in the ionic liquid. For instance, in [C MIM] [TFSI] ionic liquid, [C MIM] cations are surrounded by [TFSI] anions and [TFSI] anions are surrounded by [C MIM] cations. The probe radii of the ions were calculated during structure optimization and the recommended radii for the SCRF calculation provided by the Gaussian software were obtained. The Solvent Accessible Surface (SAS), in which the calculated probe radius of the solvent is added to the unscaled radii of atoms and/or atomic groups, was used to specify the type of molecular surface representing the solute-solvent boundary. The static dielectric constant41 and computed probe radii of the ionic liquid cations and anions are presented in Table 1. Although considering the first solvation shell with explicit solvent molecules results in more accurate solvation energies for small ions such as Li+ ion42-45, keeping with the scope of the current study to screen large number of ionic liquids in a computationally effective manner, we considered the dielectric effect of the solvent molecules using the PCM implicit solvent model. Figure 5 shows the variation of the static dielectric constants of the imidazolium-based ionic liquids considered in this study with respect to the number of carbon atoms in the alkyl side chain, which were calculated based on the internal pressure and cohesive energy density approach in Ref. [41]. In each set of ionic liquids, the difference in the number of carbon atoms in the cation’s alkyl side chain, results in different solvent properties, including static dielectric constant. The plot shows that the static dielectric constant of the imidazolium-based ionic liquids examined here decreases with an increase in the number of carbon atoms in the alkyl side chain. The dependence of the dielectric constant is nearly linear except for the [C MIM] [PF ] ionic liquid.



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Table 1. The ionic probe radii and the dielectric constants of the imidazolium-based ionic liquids studied in this work. The cation and anion radii are the recommended radii for the SCRF calculations provided by the Gaussian software. The dielectric constants are obtained from Ref. [41]. Ionic Liquid

Cation Radius (Å) Anion Radius (Å) Dielectric Constant41

[C MIM] [TFSI]

4.13

4.44

11.5

[C MIM] [TFSI]

4.34

4.44

10.6

[C MIM] [TFSI]

4.57

4.44

9.4

[C MIM] [TFSI]

4.60

4.44

7.9

[C MIM] [TFSI]

4.70

4.44

7.0

[C MIM] [TfO]

4.13

3.89

15.8

[C MIM] [TfO]

4.57

3.89

13.5

[C MIM] [TfO]

4.70

3.89

11.2

[C MIM] [PF ]

4.13

3.58

14.7

[C MIM] [PF ]

4.57

3.58

14.0

[C MIM] [PF ]

4.70

3.58

11.1

[C MIM] [BF ]

4.13

3.37

14.8

[C MIM] [BF ]

4.57

3.37

12.9

[C MIM] [BF ]

4.70

3.37

11.3


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18

Static Dielectric Constant

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TFSI

16

PF6 BF4

14

TfO

12 10 8 6 1

2

3

4

5

6

7

n in [CnMIM]+ Figure 5. Static dielectric constant of the imidazolium-based ionic liquids with different anions.41 For the ionic liquids examined in this work, the static dielectric constant calculated in Ref. [41] decreases with increase in the length of the alkyl side chain of the imidazolium cation. 3. RESULTS AND DISCUSSION 3.1. The oxidation potential of the ionic liquid cation and anion The first step in determining the oxidation potential of the ionic liquid cation and anion is calculating the Gibbs free energy change for the reference reaction that involves the oxidation of lithium. As shown in Eq. (1), the required parameters are the vaporization free energy of Li (∆G456 (Li)), the ionization free energy of Li (∆G2 (Li)) and the solvation free energy of Li+ ions (∆G89:4 (Li )). The term ∆G2 (Li) was evaluated as the difference between free energies of a lithium atom and a Li+ ion in the gas phase. The calculated ionization energy of lithium is 516.4 kJ/mol, which deviates by only 0.73% from the experimentally measured value of 520.2 kJ/mol.46 As mentioned before, the tabulated value of 118.0 kJ/mol was selected for the 15 ACS Paragon Plus Environment


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vaporization energy of lithium metal.30 The free energy change of the reference reaction can be obtained from these values based on Eq. (1). The next step is calculating the oxidation Gibbs free energy of the reactions shown in Figure 3(a) and Figure 4(a). As described by Eq. (2a) and Eq. (4a), the required parameters are the solvation free energy of the ionic liquid cation and anion (∆G89:4 (R ) and ∆G89:4 (A )), the oxidation Gibbs free energy in the gas-phase (∆9; G(g)) for both ions, and the solvation free energy of the ions in their oxidized states (∆G89:4 (R ) and ∆G89:4 (A)). As shown in Eq. (3a) and Eq. (5a), the oxidation Gibbs free energy in the gas phase can be obtained as the difference between the Gibbs free energy of the ion and that of its oxidized counterpart in the gas phase. The standard oxidation potential relative to Li+/Li reference can then be obtained using the relation presented in Eq. (6a). The oxidation potentials of the ionic liquid cations and anions relative to Li+/Li reference, using the methodology described above, are presented in Error! Reference source not found.. As mentioned earlier, the absolute redox potentials provided in this table are based on the one-electron transfer redox reactions without considering ion dissociation and were obtained by employing an implicit solvent model for all ions including Li+ ion in the reference reaction rather than an explicit solvation shell. Therefore, the absolute redox potentials for the individual ions might be different from the values obtained in experimental measurements due to the difference in the reference potential. However, the absolute potentials are valid for qualitative comparison of the oxidative and reductive stability of the ions. Moreover, since the redox potential of all ions are calculated with respect to the same reference electrode, the obtained electrochemical stability windows are expected to agree closely with experimental measured values.


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The data in Error! Reference source not found. indicate that the oxidation potential of 1alkyl-3-methylimidazolium cations increases with increase in the number of carbon atoms in the alkyl side chain. However, the maximum difference between the oxidation potentials of different imidazolium-based cations in the ionic liquids is 0.216 V, which is only 3.6 % of the average value. The HOMO of ionic liquid cations shown in Figure 6 indicates that the HOMO is mostly dominated by the orbitals in the imidazolium ring. The maximum difference between the HOMO energies of different cations in these ionic liquids is only 5.2 % of the average value, which is consistent with the small variation in the oxidation potentials. Cation Ionic Liquid

Anion

ESW

ESW

∘ E9; [V]

∘ E12B [V]

∘ E9; [V]

∘ E12B [V]

[C MIM] [TFSI]

6.072

-3.106

1.125

-7.523

4.232

4.5 (GC)

[C MIM] [TFSI]

6.104

-3.117

1.087

-7.613

4.204

4.3 (GC)

[C MIM] [TFSI]

6.131

-3.131

1.039

-7.723

4.169

4.6 (Pt)

[C MIM] [TFSI]

6.145

-3.140

0.993

-7.807

4.133

-

[C MIM] [TFSI]

6.159

-3.147

0.949

-7.893

4.096

-

[C MIM] [TfO]

5.975

-3.033

1.685

-7.482

4.717

4.1 (Pt)

[C MIM] [TfO]

6.026

-3.052

1.604

-7.703

4.656

-

[C MIM] [TfO]

6.044

-3.061

1.553

-7.815

4.614

-

[C MIM] [BF ]

5.943

-2.955

3.938

-8.307

6.893

-

[C MIM] [BF ]

5.974

-2.973

3.851

-8.538

6.824

6.1 (W)

[C MIM] [BF ]

5.985

-2.980

3.807

-8.637

6.787

-

[C MIM] [PF ]

5.963

-2.990

5.051

-9.635

8.041

-

[C MIM] [PF ]

5.988

-3.005

4.987

-9.818

7.992

> 7.1 (W)

[C MIM] [PF ]

5.996

-3.016

4.926

-9.944

7.942

-

(Cycle Method) (Experimental)6



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Table 2. The calculated values of the oxidation (E_ox^∘) and reduction (E_red^∘) potentials of the cations and anions of the ionic liquids vs. Li+/Li reference electrode are provided. The electrochemical stability windows (ESW) are defined by the oxidation potential of anions and reduction potential of cations. The letters in brackets in the experimental ESW column indicate the working electrode used in the experiment: GC = Glassy Carbon, Pt = Platinum and W = Tungsten.

Figure 6. The HOMO orbitals of a) [# %&%] , b) [# %&%] and c) [# %&%] ionic liquid cations, shown by isosurfaces of the wavefunction with an isovalue of P0.02 (R/STUV )⁄. The color indicates the sign of the wavefunction. In Figure 7, the local value of the electrostatic potential is indicated by color on the isosurface of the electron density for the ionic liquid cations and the corresponding oxidized 2+


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cation. The increase in the electrostatic potential near the imidazolium ring observed in the plot confirms that the oxidation occurs mainly on the imidazolium ring. Therefore, the oxidation potential of 1-alkyl-3-methylimidazolium cations remains almost constant despite the varying length of the alkyl group and the different counter ions since they contain identical imidazolium ring structure.

Figure 7. The electrostatic potential mapped on the electron density isosurface at 4.0 X 10

R/STUV for a) the ionic liquid cations and b) the oxidized cations with 2+ charge. For each ionic liquid anion, the oxidation potential is also nearly constant when varying the length of the alkyl side chain (See Error! Reference source not found.). The largest variation in the oxidation potential is 0.176 V, which is only 6.7 % of the average value of the oxidation potential.



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In Figure 8(a), the oxidation potentials for various anions, averaged over the range of cation pairings, are compared to the average oxidation potential of all imidazolium cations. The corresponding average HOMO energy of each ion is shown in Figure 8(b). During an oxidation reaction, the electron(s) that are at the highest energy level are detached from the molecule or ion. Therefore, the molecules or ions with lower HOMO level are expected to have higher oxidation potential. As shown in Figure 8(a) and Figure 8(b), this expected trend is observed in the ions studied in this work. Among the anions considered in this study, [PF ] is found to be the most stable against oxidation with the highest oxidation potential and lowest HOMO energy level, while [TFSI] has the highest HOMO energy level resulting in the lowest oxidation potential, thereby making it the least stable anion against oxidation. As shown in Figure 9, the HOMO levels of [PF ] and [BF ] are strictly dominated by the orbitals in the highly electronegative fluorine atoms, which lower the HOMO energy level and therefore results in higher oxidation potentials. On the other hand, in both [TfO] and [TFSI] anions, the HOMO are distributed all over the molecule, which increases the HOMO energy level due to the presence of electron-rich regions, i.e. nitrogen atom in the imide group and oxygen atoms in the sulfonyl groups. Therefore, [TfO] and [TFSI] anions are expected to have lower oxidation potentials compared to [PF ] and [BF ] anions. The electrostatic potential mapped on the electron density isosurface of [TfO] and [TFSI] anions and their corresponding oxidized neutral molecules are shown in Figure 10. The isosurfaces for [TfO] and [TFSI] indicate that these ions behave similarly during the oxidation reaction since the trifluoromethyl groups in the resulting neutral molecules of both reactions have higher electrostatic potentials. The oxidation potential of the imidazolium cations ([C MIM] ) is higher than any of the studied anions due to the conjugated-π system in the imidazolium ring, which significantly 20 ACS Paragon Plus Environment

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lowers the HOMO energy level, resulting in the highest oxidation potential. As a result, in all the ionic liquids examined in this study, the oxidation potential of the anions limits the electrochemical stability window of these ionic liquids. 8.0 6.0

E(ox) E(red)

4.0

E(ox) and E(red) (V)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2.0 0.0

[TFSI]–

[TfO]–

[BF 4]–

[PF 6]– [CnMIM]+

-2.0 -4.0 -6.0 -8.0

-10.0 -12.0

(a)



0.3

HOMO/LUMO Energy Level (Hartree)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

HOMO Energy 0.2

LUMO Energy

0.1 [TFSI]–

[TfO]–

[BF 4]–

[PF 6]– [CnMIM]+

0.0 -0.1 -0.2 -0.3 -0.4 -0.5

(b) Figure 8. a) The average oxidation and reduction potentials and b) the respective average HOMO/LUMO energies, of the ionic liquid cations and anions are shown. For anions, the average values were calculated over all possible cation pairings. For the cation, the values were calculated by averaging over all imidazolium cations paired with all anions considered.


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Figure 9. The HOMO orbitals of a) [-( ] , b) [,( ] , c) ['*+] and d) ['()&] ionic liquid anions, shown by isosurfaces of the wavefunction with an isovalue of P0.02 (R/STUV )⁄. The colors indicate the signs of the wavefunction.



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R/STUV for a) ['*+] and b) ['()&] anions and their corresponding oxidized neutral molecules are shown. 3.2. The reduction potential of the ionic liquid cation and anion Determining the reduction potential of the ionic liquid cation and anion requires calculation of the Gibbs free energy change of the reduction reactions shown in Figure 3(b) and Figure 4(b). As evident from Eq. (2b) and Eq. (4b), the required parameters are the solvation free energies (∆G89:4 (R ) and ∆G89:4 (A )), the reduction Gibbs free energies in the gas-phase (∆12B G(g)), and the solvation free energies in the corresponding reduced states (∆G89:4 (R) and ∆G89:4 (A )) for the ionic liquid cation and anion. The reduction Gibbs free energy in the gas-phase can be obtained using Eq. (3b) and Eq. (5b). The standard reduction potential relative to Li+/Li reference can be obtained by adding the Gibbs free energy change for the reference reaction to 24 ACS Paragon Plus Environment

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the Gibbs free energy change for the reduction reaction and dividing the result by the Faraday’s constant according to Eq. (6b). The reduction potentials of the ionic liquid cations and anions relative to Li+/Li reference, obtained using the above-described methodology, are presented in Error! Reference source not found.. The data in Error! Reference source not found. indicate that the reduction potentials of 1alkyl-3-methylimidazolium cations do not vary significantly with changing length of the alkyl side chain. The maximum variation in the reduction potential of imidazolium-based cations in the ionic liquids is 0.19 V, which is only 6.3 % of the average value for all cations considered in this study. The LUMO of imidazolium cations, shown in Figure 11, indicates that the LUMO is mostly dominated by the orbitals in the imidazolium ring. The maximum difference between the LUMO energies of different cations in these ionic liquids is 19.63 kJ/mol, which is only 12.4 % of the average LUMO energy of cations.



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Figure 11. The LUMO orbitals of a) [# %&%] , b) [# %&%] and c) [# %&%] ionic liquid cations, shown by isosurfaces of the wavefunction with an isovalue of P0.02 (R/STUV )⁄. The color indicates the sign of the wavefunction. In Figure 12, the local value of the electrostatic potential is indicated in color on the isosurface of the electron density for the ionic liquid cations and the corresponding reduced neutral molecules. The decrease in the electrostatic potential near the imidazolium ring observed in the plot confirms that the reduction occurs mainly on the imidazolium ring. Therefore, the reduction potential of 1-alkyl-3-methylimidazolium cations remains almost constant despite the varying length of the alkyl group and the different counter ions since they contain identical imidazolium ring structure.


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For each ionic liquid anion, the reduction potential is also nearly constant when varying the length of the alkyl side chain. The maximum difference in the reduction potentials of an anion when coupled with different imidazolium cations is 0.37 V, which is only 4.8 % of its average value.


R/STUV for a) the ionic liquid cations and b) the reduced neutral molecules. In Figure 8(a), the reduction potentials for various anions, averaged over the range of cation pairings, are compared to the average reduction potential of all imidazolium cations. The corresponding average LUMO energy of each ion is shown in Figure 8(b). During a reduction reaction, the electron gained in the process would occupy the lowest available energy level in the molecule or ion. Therefore, molecules or ions with higher LUMO level have more negative reduction potential and are expected to be more stable against reduction. This trend is observed



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for all of the ions studied in this work, as evident in Figure 8(a) and Figure 8(b). Among the anions, [PF ] has the highest LUMO level resulting in the most negative reduction potential. In fact, the presence of fluorine atoms in the structure of the ionic liquid anion (i.e. in [PF ] and [BF ] ) tend to enhance their electrochemical stability against reduction due to their increased LUMO energy level. However, the imidazolium-based cations ([C MIM] ) are less stable than any of the examined anions towards reduction since the LUMO level of the imidazolium cations is even lower than those of [TFSI] and [TfO] , resulting in the least negative reduction potentials among all of the ions examined. As a result, the reduction potentials of the imidazolium cations limit the electrochemical stability window of the ionic liquids examined in this study. 3.3. The electrochemical stability window of the ionic liquids The width of the electrochemical stability window (ESW) of each ion can be calculated from the oxidation and reduction potentials of the constituent cations and anions using Eq. (7): ∘ ∘ ESW = E9; − E12B

(7)

∘ ∘ where E9; and E12B are the oxidation and reduction potentials relative to Li+/Li reference. As

shown in Figure 13(a) and Figure 13(b), the width of the electrochemical stability window of imidazolium-based cations and the anions studied in this work do not vary significantly by changing the length of the alkyl side chain. In fact, the maximum variations in the electrochemical stability window of cations and anions are only 4.5 % and 1.8 %, respectively, relative to the corresponding average values.


Page 29 of 44

9.45

ESW Width (Cation) (V)

TFSI

9.35

PF6 BF4

9.25

TfO

9.15 9.05 8.95 8.85 1

2

3

4

5

6

7

n in [CnMIM]+

(a)

16

ESW Width (Anion) (V)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


15 14 13 12

TFSI PF6 BF4 TfO

11 10 9 8 1

2

3

4

5

n in [CnMIM]+

(b)


6

7


Figure 13. The width of the electrochemical stability window (ESW) of a) imidazolium-based cations and b) anions considered in this study is shown as a function of the number of carbon atoms in the alkyl side chain of the cation. Figure 14 shows the oxidation and reduction potential limits (a) for imidazolium-based cations when paired with different anions and (b) for anions when paired with different imidazolium-based cations, respectively. As previously discussed, the redox potentials of the cations and anions do not vary significantly by changing the length of the alkyl side chain. However, due to different chemistry of the anions, the redox potentials of different anions exhibit a significant variation. Our calculation shows that the [PF ] anion has the most negative reduction potential and the largest oxidation potential and therefore is the most stable anion during the redox reactions.

n in [CnMIM]+

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 44

6 4 2 6 4 2 6 4 2 6 5 4 3 2

[TfO]–

[BF 4]–

[PF 6]–

[TFSI]–

-4

-2

0

2

Ered (V) (Cation)

4

Eox (V) (Cation) (a)


6

8

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


n in [CnMIM]+

Page 31 of 44

6 4 2 6 4 2 6 4 2 6 5 4 3 2

[TfO]–

[BF 4]–

[PF 6]–

[TFSI]–

-12 -10 -8

-6

-4

-2

Ered (V) (Anion)

0

2

4

6

8

Eox (V) (Anion)

(b) Figure 14. The electrochemical stability windows and their limits of a) imidazolium-based cations when paired with different anions and b) anions when paired with different imidazoliumbased cations. The results in Figure 14(a) and Figure 14(b) also indicate that the imidazolium-based cations have the least negative potentials among the ions studied in this work and therefore are the least stable ions towards reduction. Moreover, all the examined anions have lower oxidation potentials compared to the imidazolium cations and hence are less stable towards oxidation. Consequently, as demonstrated in Figure 15, the electrochemical stability window of these ionic liquids are limited by the reduction potential of the cations and the oxidation potential of the anions. Therefore, the corresponding width of the electrochemical stability window for these ionic liquids, [\]^ , are given by: ∘ ∘ (anion) (cation) W\]^ (ionic liquid) = E9; − E12B


(8)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

n in [CnMIM]+


6 4 2 6 4 2 6 4 2 6 5 4 3 2

Page 32 of 44

[TfO]–

[BF4]–

[PF6]–

[TFSI]–

-4 -3 -2 -1

0

1

Ered (V) (Ionic Liquid)

2

3

4

5

6

7

Eox (V) (Ionic Liquid)

Figure 15. The electrochemical stability windows and their limits of the ionic liquids examined are shown. The reduction potential limits are given by the reduction potentials of imidazoliumbased cations, and the oxidation potential limits are given by the oxidation potentials of anions. The widths of the electrochemical stability windows of the ionic liquids shown in Figure 16 suggest that despite the decrease in the ESW width with increase in the length of alkyl side chain, the variation is still small such that the maximum difference in each set is 0.11 V, which is only 2.1 % of its average value. Moreover, [C MIM] [PF ] are the most stable ionic liquids due to the high oxidation potential of [PF ] while [C MIM] [TFSI] are the least stable ones among the ionic liquids examined in this work. Overall, the following trend summarizes the results for the width of electrochemical stability window: ESW[ef ghg]i[jkl]m > ESW[ef ghg]i[okp ]m ≫ ESW[ef ghg]i[r3s]m > ESW[ef ghg]i[rkth]m


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


ESW Width (Ionic Liquid) (V)

Page 33 of 44

8.5 7.5 6.5

TFSI PF6

5.5

BF4 TfO

4.5 3.5 1

2

3

4

5

6

7

n in [CnMIM]+

Figure 16. The width of the electrochemical stability windows (ESW) of the ionic liquids examined in this work. Among these ionic liquids, [#$ %&%] [,( ] have the widest electrochemical stability window while [#$ %&%] ['()&] are the least stable ones in redox reactions. As a result, the electrochemical stability windows of these ionic liquids are greatly affected by the structure of anion. The anions that are highly fluorinated such as [,( ] and [-( ] tend to be electrochemically more stable against reduction and oxidation and therefore have wider ESW when coupled with an ionic liquid cation. As described earlier, such trend is justified by the presence of a highly electronegative atom, i.e. fluorine, in the structure of these ionic liquid anion. The presence of fluorine increases the energy gap between the HOMO and LUMO levels by increasing the LUMO and lowering the HOMO level, which results in more difficult electron transfer during the oxidation and reduction reactions.



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3.4. Comparison with HOMO/LUMO method and experimental data In the work of Ong et al.,16 they benchmarked the electrochemical stability windows calculated via the HOMO/LUMO method against those obtained from a combination of classical molecular dynamics and density function theory calculation, which is much more complicated and computationally expensive. Intermolecular interactions between molecules, which can decrease or increase redox stabilities, are captured in Ong et al.’s method and hence provide better understanding of the dynamics of the system, which is not captured in quantum chemistry calculations on isolated molecules. However, quantum chemistry calculations based on thermodynamic cycle approach provide quicker estimates for screening a large number of ionic liquids based on their electrochemical stability. Ong et al.16 also concluded that the HOMO/LUMO method was sufficiently accurate for qualitative comparison of electrochemical stability windows. In this method, the electrochemical stability can be estimated by the anodic and cathodic limits obtained from the HOMO and LUMO energy levels, respectively.16 With this assumption, the potential of the oxidation and reduction reactions involving electron transfer is obtained from the respective molecular orbital energies according to:16 ∘ E9; = −Eusgs /e

(9)

∘ E12B = −Evwgs /e

(10)

where, Eusgs and Evwgs are the HOMO and LUMO energies computed based on the vertical ionization potentials and electron affinities respectively with implicit solvent corrections, and e is the charge of an electron. Each ion is modeled in the solution by implementing the PCM implicit solvation model. Following the work, the ionization energy (IE) and the electron affinity


Page 35 of 44

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(EA) of each ion are calculated individually in the presence of the continuum solvation model by applying the ∆- Self-Consistent Field (∆ − SCF) approach as shown in Eq. (11) and Eq. (12):16 IE = E(A ) − E(A) = −Eusgs

(11)

EA = E(A) − E(A ) = −Evwgs

(12)

where E(A) is the energy of the ion and E(A ) and E(A ) are the energies of the ion with one additional and one less electron, respectively. Therefore, the ionization energy corresponds to the energy of the HOMO while the electron affinity corresponds to the LUMO energy level. The widths of the electrochemical stability windows of the ionic liquids calculated using the thermodynamic cycle method are compared to the values we obtained following the HOMO/LUMO method and the results are shown in Figure 17. The maximum difference between the values calculated by these two methods is 1.58 V, which is for ionic liquids containing the [TFSI] anion, and the minimum difference is 0.57 V in ionic liquids with the [PF ] anion.



10

ESW Width (Ionic Liquid) (V)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 44

MO Method

[PF 6]–

9

[BF 4]–

8 7 6

[TfO]–

[TFSI]–

5 4 3 2 1 0 2 3 4 5 6 2 4 6 2 4 6 2 4 6

n in [CnMIM]+ Figure 17. The width of the electrochemical stability window of the ionic liquids calculated using the thermodynamic cycle method compared to the values obtained by the HOMO/LUMO method. The calculated ESW of select ionic liquids obtained by the thermodynamic cycle and HOMO/LUMO method are compared to the experimental values from Ref. [6] in Figure 18. The values obtained by the thermodynamic cycle method are in good agreement with the experimental data with errors ranging from 2 % to 15 %. On the other hand, the ESWs obtained by the HOMO/LUMO method are far from the experimental data with errors ranging from 21 % to 50 %. These results are consistent with previous studies by Borodin et al.28 and Tian et al.34 stating that the thermodynamic cycle method provides better predictions for the redox stabilities compared to the HOMO/LUMO method. Despite these errors in the absolute values of the electrochemical stability windows, both thermodynamic cycle and HOMO/LUMO methods predict the observed trends in the experimental measurements i.e. the imidazolium-based ionic 36 ACS Paragon Plus Environment

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liquids with [PF ] anion have the widest ESW while the ones with [TFSI] anion have the smallest ESW. It should be noted that the experimentally measured redox potentials of ionic liquids depend on reference electrodes and current cut-offs.20, 47 Moreover, in some cases, the oxidation and reduction reaction of ionic liquids might be electrochemically irreversible; hence the corresponding oxidation and reduction potentials cannot be measured accurately as in electrochemically reversible organic solvents.47 However, since the thermodynamic cycle method can qualitatively capture the experimental trends, it provides a very useful and fast tool in screening the electrochemical stability of ionic liquids with a reasonable accuracy in quantitative values. As discussed in Ref. [16], the HOMO/LUMO method always overestimates the ESW. This can be understood as follows. The Gibbs free energy change during the oxidation and reduction reactions, required in the HOMO/LUMO method, is estimated from the ionization energy and electron affinity that are calculated using Eq. (11) and Eq. (12), respectively. In the HOMO/LUMO method, the structure of the molecule is fixed during the redox reaction, which results in the reduced computational cost. Therefore, since the energy of the reaction product is higher, the obtained energy change is always greater than the actual energy change of the redox reaction, in which reactant and product molecules or ions possess their respective energetically favorable structures. Thus, the evaluated redox potentials based on HOMO/LUMO method are always greater than the actual values. In contrast, the thermodynamic cycle approach employs the energies from optimized structures for both the reactant and the product, thus reducing the errors greatly. However, the difference between the two methods can be small if the structural change during the redox reaction is small. Specifically, among the anions, [PF ] and [BF ]

have the least structural mutation since, after the electron transfer, all the bond angles remain



almost the same and the B − F and P − F bond lengths decrease by only 1.3 % and 2.5 %, respectively. The comparison shown in Figure 17 confirms this effect, since the difference between the electrochemical stability windows calculated by the HOMO/LUMO and thermodynamic cycle methods in ionic liquids that have [PF ] and [BF ] is only 7.3 % and 10.0 %, respectively, compared to 37.3 % and 30.0 % obtained in ionic liquids with [TFSI] and [TfO] anions.

9 8 7

ESW Width (V)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Cycle Method Experimental HOMO/LUMO Method

6 5 4 3 2 1 0

+ [C MIM]+ [C MIM]+ [C MIM]+ [C MIM]+ [C MIM]+ [C 2MIM] 3 4 4 4 2 C2MIM C3MIM C4MIM C4MIM C4MIM C2MIM – – – – – [TFSI] [TFSI] [TFSI] [PF [BF [TfO] T F SI T F SI T F SI P F 66] BF 44] T fO –

Figure 18. The calculated width of the electrochemical stability windows of select ionic liquids obtained by the thermodynamic cycle and HOMO/LUMO methods compared to the experimental values.6 4. CONCLUSIONS In this study, the oxidation and reduction potentials of the constituent cations and anions of a series of alkyl methylimidazolium-based ionic liquids were calculated. The redox potentials and 38 ACS Paragon Plus Environment

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the corresponding electrochemical stability window of these ionic liquids were obtained using density functional theory following the method of thermodynamic cycles.30, 32, 33 Among the ions considered in this study, the imidazolium-based cations are found to be the least stable towards reduction. Moreover, all the examined anions are less stable towards oxidation compared to the imidazolium cations. Consequently, the electrochemical stability windows of these ionic liquids are limited by the reduction potential of the cations and the oxidation potential of the anions. The results also indicate that the electrochemical stability windows of these ionic liquids are insensitive to the length of alkyl side chain of the imidazolium cations, but greatly affected by the structure of anion. The anions that are highly fluorinated, such as [PF ] and [BF ] , tend to be electrochemically more stable against reduction and oxidation and therefore have wider ESW when coupled with an ionic liquid cation. This phenomenon was explained by the increased LUMO and decreased HOMO energy levels due to the presence of a highly electronegative element i.e. fluorine atoms, in the structure of [PF ] and [BF ] anions, which results in more difficult electron transfer during the oxidation and reduction reactions. These findings will assist greatly in the screening of ionic liquids by reducing the number of ionic liquids to be considered. Furthermore, it is observed that [C MIM] [PF ] are the most stable ionic liquids due to the high oxidation potential of [PF ] while [C MIM] [TFSI] are the least stable among the ionic liquids considered in this study. As shown in pervious studies by Borodin et al.28 and Tian at al.34, by comparing the results to experimental values, we demonstrated that the thermodynamic cycle method provides quantitative predictions of the oxidation and reduction potentials of ionic liquid cations and anions examined in this work. The errors were less than 15 %. In addition, the redox potentials of the ionic liquid cations and anions were also calculated using the energy of the frontier molecular



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orbitals.16 The values obtained by the HOMO/LUMO method were typically overestimated by up to 50 %. The thermodynamic cycle approach is much less computationally expensive than the first principles molecular dynamics and classical molecular dynamics approach and thus can be employed to screen a large number of ionic liquid chemistries efficiently. It can be utilized to populate the electrochemical stability window data within ionic liquid database, which in turn can be used to guide the selection of the ionic liquid electrolytes for a given electrode material. This information is especially essential for the design of high-voltage rechargeable batteries, where the electrochemical stability window is critical. Therefore, this study presents a step towards accelerating the development of non-aqueous batteries. ACKNOWLEDGMENT This work was supported as part of the Joint Center for Energy Storage Research, an Energy Innovation Hub funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences. This research used resources of Washington State University’s High Performance Computing Cluster and University of Michigan’s Advanced Research Computing for carrying out the simulations. REFERENCES 1. Girishkumar, G.; McCloskey, B.; Luntz, A. C.; Swanson, S.; Wilcke, W., Lithium - Air Battery: Promise and Challenges. J. Phys. Chem. Lett. 2010, 1, 2193-2203. 2. Christensen, J.; Albertus, P.; Sanchez-Carrera, R. S.; Lohmann, T.; Kozinsky, B.; Liedtke, R.; Ahmed, J.; Kojic, A., A Critical Review of Li/Air Batteries. J. Electrochem. Soc. 2012, 159, R1-R30. 3. Khaligh, A.; Li, Z., Battery, Ultracapacitor, Fuel Cell, and Hybrid Energy Storage Systems for Electric, Hybrid Electric, Fuel Cell, and Plug-In Hybrid Electric Vehicles: State of the Art. IEEE T. Veh. Technol. 2010, 59, 2806-2814. 4. Kazemiabnavi, S.; Soundararaj, A.; Zamani, H.; Scharf, B.; Thyagarajan, P.; Zhou, X., In ASME 2015 International Mechanical Engineering Congress and Exposition; American Society of Mechanical Engineers: Houston, Texas, USA, 2015; Vol. 6B: Energy. 40 ACS Paragon Plus Environment

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Electrochemical Stability Window of Imidazolium-Based Ionic Liquids

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