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Spectroscopic and Density Functional Theory Characterization of Common Lithium Salt Solvates in Carbonate Electrolytes for Lithium Batteries Navid Chapman, Oleg Borodin, Taeho Yoon, Cao Cuong Nguyen, and Brett L Lucht J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b12234 • Publication Date (Web): 03 Jan 2017 Downloaded from http://pubs.acs.org on January 4, 2017

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The Journal of Physical Chemistry

Spectroscopic and Density Functional Theory Characterization of Common Lithium Salt Solvates in Carbonate Electrolytes for Lithium Batteries Navid Chapman,1 Oleg Borodin,2 Taeho Yoon,1 Cao Cuong Nguyen,1 and Brett L. Lucht*,1

1

Department of Chemistry, University of Rhode Island, Kingston, Rhode Island 02881, USA

2.

Electrochemistry Branch, Sensor and Electron Devices Directorate, Power and Energy Division, U.S. Army Research Laboratory Adelphi, Maryland, 20783, USA e-mail: Brett Lucht: [email protected], Oleg Borodin: [email protected]

1 ACS Paragon Plus Environment


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Abstract The structure and composition of lithium ion solvation spheres of electrolyte solutions composed of common lithium salts (LiTFSI, LiPF6, LiBF4, and LiClO4) dissolved in aprotic polar linear and cyclic carbonate solvents (propylene carbonate (PC) or dimethyl carbonate (DMC)) has been investigated via a combination of FTIR,

13

C NMR spectroscopy and density

functional theory (DFT). Results from the two different spectroscopic methods are in strong agreement with each other and with predictions from quantum chemistry calculations. The coordination of the carbonyl oxygen of the solvents to the lithium cation is observed by IR spectroscopy. The ratio of coordinated to uncoordinated PC and DMC has been used to determine solvent coordination numbers which range from 2 to 5 depending on salt, solvent, and concentration. The relative stability of the lithium-anion solvates were examined using DFT employing the cluster – continuum approach including changes to the intensity and frequency of the IR bands along with the populations of the cis-cis and cis-trans conformers of DMC in the lithium ion solvation shell. Solvent coordination is dependent upon the nature of the salt. Weakly associating salts, LiTFSI, LiPF6, and LiClO4, dissociate to a similar degree with LiPF6 being the most dissociated, while LiBF4 had significantly less dissociation in both solvents. This investigation provides significant insight into the solution structure commonly used LIB electrolytes over wide range of salt concentrations.


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Introduction There is significant need to further improve lithium Ion Batteries (LIBs) due to the increased global demand for the portable energy storage devices.1-3 While the electrolyte is a critical component of the LIB, our current understanding of the solution structures of these electrolytes is incomplete. Further investigation is important since the electrolyte solution structure plays a critical role in the performance of electrochemical systems.4 These fundamental physicochemical properties affect many aspects of performance such as electrochemical stability window, working temperature range, and ionic conductivity.5-11 Determining the solution structure and relative binding affinities of different solvents is also of interest since it would provide insight into electrolyte reduction and solid electrolyte interface (SEI) formation on the on the anode surface.12 The lithium ion solvation structure and aggregation state often determine the electrolyte components that undergo reduction and to generate the SEI.13-16 Salt aggregation can lead to a significant change in the reduction potential of the electrolyte.13, 17-18 Binary mixtures of cyclic and linear carbonates were first implemented by Tarascon and Guyomard to optimize the properties of the electrolyte since the cyclic carbonates have high permittivity and high viscosity while the linear carbonates have low viscosity and low permittivity.19 However, investigations of the solution structures of cyclic and linear carbonates with different lithium salts are limited.11,

20-27

Using LiPF6 as a salt, Seo et al. previously

examined cation-solvent coordination via IR and NMR spectroscopies.11 This study expands on Seo’s work by examining solvation and ion pairing with several different salts. Spectroscopic methods including IR, Raman, and NMR can be used to indirectly observe solvent coordination by detecting changes in the local electronic environment of the ions or solvent molecules.



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In this report, IR and NMR spectroscopy have been used to investigate the cation solvent interactions resulting from lithium salts dissolved in carbonate solvents. Four lithium salts (lithium bis(trifluoromethane)sulfonamide (LiTFSI), LiPF6, LiBF4, LiClO4) and two carbonate solvents (Propylene carbonate (PC) and dimethyl carbonate (DMC)) were investigated over a wide concentration range from a solvent to Li ratio of 20:1 to 4:1.28 Density functional theory (DFT) and the more computationally expensive wave function-based quantum chemistry (QC) methods were also used to examine the relative stability of the lithium solvates and changes in the IR spectra due to complexation of the carbonyl group of the solvent to the Li+ cation. Populations of two most stable DMC conformers in lithium solvates, cis-cis and cis-trans, have also been examined. In this study, previous QC27,

29-36

and simulation20,

24, 29, 37-47

studies of Li+

solvation are built up and utilize the cluster-continuum approach which explicitly considers solvent and anion in the first coordination shell of Li+, while solvent molecules outside of the first solvation shell are described using an implicit solvation model.21, 48

Experimental Section PC was used as-received (MP Biomedicals). DMC was used as-received (Sigma-Aldrich, anhydrous). LiTFSI, LiPF6, LiBF4, and LiClO4 were also used as-received (Sigma-Aldrich). The four lithium salts (LiTFSI, LiPF6, LiBF4, LiClO4) were chosen due to the interest in these salts in lithium ion battery electrolytes. The salts readily dissolve and dissociate in non-aqueous polar aprotic electrolyte solvents, have high ionic mobility, are non-reactive with electrolyte solvents, and have relatively good thermal stability.3, 49-51 The salt-to-solvent ratios (1:20 to 1:4) have been selected to incorporate a full range of salt concentrations with the conventional concentration of ~1 M in the center of the range. Solutions were prepared in an air-free environment and analyzed


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shortly after preparation at room temperature to minimize contamination by water.3, 52 Constant temperature was maintained.4, 20 Samples were prepared in an inert atmosphere (N2) glovebox ( LiTFSI > LiClO4 > LiBF4. This trend is in good agreement with the experimentally observed trends for the Li+ solvation numbers. Importantly, the cluster-continuum DFT calculations not only predict the correct trends in the lithium-anion CIP formation, but also yield a reasonable estimate of the fraction of SSIP in dilute solution. Specifically, application of the Boltzmann factors (eq. 3) suggests that in PC LiBF4 is quite aggregated with only 55% SSIP, while LiClO4 has around 10% CIP. Both LiPF6 and LiTFSI are largely dissociated. DFT calculations also show that both mono-dentate and bidentate LiTFSI binding will be present with the monodentate binding dominating. In contrast to the CIP free energies with the PCM model, application of the SMD model predicts higher 22 ACS Paragon Plus Environment

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energies for CIP formation vs. SSIP. Thus, it will yield more dissociated electrolytes and higher dissociation for LiBF4 than LiClO4. When energies are corrected by subtracting the PBE/631+(d,p)-G4MP2 energy differences calculated using SMD implicit solvent for ion pairs as shown in Table 2, the binding energies for the SMD solvation model start to follow the experimental tends. Another important observation is that when a PC was explicitly included in the first solvation shell of Li+ (Table 4), the difference for the CIP formation reaction from SSIP is much smaller between the SMD and PCM solvation models compared to the differences observed for the CIP formation reaction when all solvent was treated implicitly. Relative stability of the Li+(PC)5 vs Li+(PC)4 solvates were estimated from the PBE/631+G(d,p), PCM(PC) cluster-continuum calculations. Free energy of the Li+(PC)5 solvate was found to be 9 kcal/mol higher than the free energy of the Li+(PC)4 solvate indicating that five PC coordinated Li+ cation SSIP are not expected in the PC-based electrolyte solutions. Interestingly, the energy of the Li+(PC)5 solvate was found to be 0.6 kcal/mol lower than Li+(PC)4 solvate showing that entropic penalty destabilizes higher solvation number complexes in accord with the recent study of the Li+(EC)n-based complexes.73 Thus, DFT calculation support the Li+ solvation numbers from Raman measurements62 at 20:1 EC:Li around four for the strongly dissociating salts (LiTFSI, LiPF6) instead of the higher solvation number of five for the PC-based electrolyte obtained from IR spectra for PC:Li=20 for the same highly dissociating salts.

Table 4. The contact ion pair formation reaction free energy at 298.15K (∆G) for the reaction Li(PC)4+Anion -→

LiAnion(PC)3+PC from PBE/6-31+G(d,p) calculations using PCM and SMD

solvation model with PC parameters. Solvation model: energy

PCM

SMD

SMD

PCM(corr.)

SMD(corr.)a

none, gas-phase

Solvation model:

PCM

PCM

SMD

PCM

SMD

none, gas-phase



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geometry optimization LiTFSI(mono)

2.2

1.4

LiTFSI(bidentate)+3PC

2.8

5.3

LiPF6

4.1

5.4

5.6

LiClO4

1.4

2.3

LiBF4

0.1

3.4

a

3.4

2.6

3.8

-52.9

2.6

4.1

-54.4

2.1

2.1

2.8

-59.5

2.8

0.0

2.7

-62.4

3.2

free energies were corrected using the G4MP2-PBE/6-31+G(d,p) energy differences from Table 2 for

ion pairs in SMD implicit solvent.

Lithium Solvates in Dimethyl Carbonate. Stability of CIP and SSIP Li+ solvates in DMC was

examined

by

calculating

free

energies

for

the

Li(DMCcc)4+Anion

->

LiAnion(PC)3+DMCcc reaction using PBE/6-31+G(d,p) DFT with SMD and PCM solvation models as shown in Table 5 and correspond to geometries shown in Figure 9. The build-in parameters for DiButylEther (ε=3.0473) were utilized to model DMC as implicit solvent. The CIP stability vs. SSIP obtained from PBE/6-31+G(d,p) DFT calculations was also corrected for the differences between PBE and G4MP2 binding free energies in implicit solvent shown in Table 2. We find that while absolute values for the CIP vs. SSIP stability are different for solvates calculated using SMD and PCM solvation models, the trends for the lithium salts are similar. Specifically, LiTFSI and LiPF6 have much smaller CIP formation energies than LiClO4 and LiBF4. LiTFSI monodentate and bidentate binding in CIP are also similar. After correction for the differences between G4MP2 and PBE/6-31+G(d,p), the solvate of LiBF4 has the highest CIP formation energies indicating that it is expected to have the lowest number of DMC in the first solvation shell of Li+ which agrees with IR data shown in Figure 4. Overall, the propensity for CIP formation has the following order: LiBF4 > LiClO4 > LiPF6 > LiTFSI. This agrees with the reverse trend for the Li+ solvation numbers extracted from IR measurements for dilute solutions. High negative free energies for CIP solvates vs. SSIP in DMC indicate that a 24 ACS Paragon Plus Environment

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negligible amount of SSIP is expected even in dilute solutions of these salts, while experiments indicate DMC solvation numbers approaching four in dilute solutions which are indicative of some SSIP formation. At least two factors might influence the solvate stabilities. First, previous work indicated that the quadrupole moment of DMC is expected to significantly contribute to the solvation and might not be adequately accounted for in the implicit solvation model.77 Second, the presence of the high dipole moment DMC(cis-trans) conformer denoted as DMCct could influence the Li+ solvate stabilities.

Figure 9. Optimized contact ion pair geometries for the Li+(DMCcc)3Anion and Li+(DMCcc)4 solvates from PBE/6-31+G(d,p) DFT calculations with PCM(ε=3) implicit solved model surrounding the solvates.



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Table 5. The contact ion pair formation reaction energy (∆E) 0 K and free energy (∆G) at 298.15K for the reaction Li(DMCcc)4+Anion → LiAnion(PC)3+DMCcc from PBE/6-31+G(d,p) calculations using PCM and SMD(ε=3.05) solvation model. SMD ∆G

a

PCM ∆G(corrected)a ∆G

∆G(corrected)a

LiTFSI (monodentate) -24.3

-23.9

-17.3 -16.9

LiTFSI (bidentate)

-23.3

-22.9

-17.6 -17.2

LiPF6

-23.0

-24.5

-16.8 -18.3

LiClO4

-28.7

-28.0

-21.3 -20.6

LiBF4

-28.3

-28.4

-21.5 -21.6

free energies were corrected using the G4MP2-PBE/6-31+G(d,p) energy differences from Table 2 for

ion pairs in SMD implicit solvent.

In order to further explore sensitivity of the solvation energy to the size of the Li+ solvation shell in the cluster – continuum approach, energy and free energy for adding an additional DMCcc was calculated corresponding to the reaction Li+(DMCcc)5 → Li+(DMCcc)4 + DMCcc in implicit solvent as shown in Figure S3. The reaction energy was found to be -2.9 kcal/mol, while reaction free energy is 11.9 kcal/mol. Such a high free energy difference between the Li+(DMCcc)5 and Li+(DMCcc)4 description indicates a high sensitivity of the solvation free energy on the Li+ shell description. Of course, BOMD or force field-based MD simulations with an explicit representation of all solvent would alleviate this drawback but they are either significantly more computationally expensive or rely on the accuracy and adequacy of the force field used in simulations. We have also explored an alternative way to estimate the stability of CIP vs. SSIP solvates in DMC by directly comparing their stability from LiAnion(DMCcc)4 calculations as shown in Figure 10. We stress that while we explored a number of initial configurations for each


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CIP and SSIP solvate and utilized only the most energetically stable configurations for our analysis, the structures are not guaranteed to be global minima. They were confirmed to be local minima by frequency calculations with the exception of the SSIP for LiPF6(DMC)4 shown in Figure 10a which has one imaginary frequency. The later structure has the Li-P distance constrained, otherwise it would converge to the CIP geometry shown in Figure 10b as a result of geometry optimization. The calculated free energies of CIP vs. SSIP are less negative from this analysis than the CIP formation energies listed in Table 5. Note that in the SSIPs in Figure 10, the Li+ and Anion- interact through the first solvation shell of the low dielectric constant DMC solvent, while calculations summarized in Table 5 assume that the Li+(DMC)4 complex is at infinite separation from the anion, which is solvated only via PCM(ε=3) model.



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Figure 10. Optimized geometries, relative energies (∆E) and free energies (∆G) of CIP vs. SSIP for the LiAnion(DMC)4 solvates from PBE/6-31+G(d,p) calculations with PCM(ε=3).

Furthermore, our cluster-continuum calculations assume that the DMC solvent stays in the cis-cis conformation and that the dielectric constant of DMC liquid ε≈3.0 is adequate. 28 ACS Paragon Plus Environment

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Previous MD simulations and experiments suggested that population of the high dipole moment DMC cis-trans conformer increases when DMC is bond to a Li+ cation.64 The appearance of this high dipole moment conformer near Li+ would suggest that the local dielectric constant is expected to be significantly higher than bulk DMC dielectric constant. In order to further characterize populations of DMCcc and DMCct in the first solvation shell of Li+, energies of the LiClO4(DMCcc)m(DMCct)n, n+m=3 solvates were calculated using DFT as shown in Figure 11. Because most salts form CIP in DMC, these solvates are representative complexes for the investigation of the DMCct vs. DMCcc population in the first solvation shell of Li+. The dispersion correction was recently shown to influence the relative stability of the Li+ solvates with high and low dipole moment carbonates,

78

therefore, additional calculations were

performed using PBE+GD3BJ D3 version of Grimme’s dispersion with Becke-Johnson damping. DFT calculations shown in Figure 11 demonstrate that the most energetically favorable solvates have a mixture of DMCcc and DMCct in the first solvation shell of Li+ because the free energies of all solvates as shown in Figure 11 differ by less than 4 kcal/mol. When the Boltzmann factor is applied to estimate the solvate populations based upon free energies of the LiClO4(DMCcc)n(DMCct)m, n+m=3 solvates calculated using PBE-GD3BJ functional, we find that approximately a quarter of the DMC in this CIP solvate is in the DMCct conformation. DFT calculations further support the evidence that a substantial fraction of DMCct is present in the first coordination shell of Li+ as obtained in the neutron diffraction with isotopic substitution experiments on DMC-LiClO4 electrolytes.79 In IR experiments, both DMCcc and DMCct contribute to the same carbonyl band position as discussed below, while yielding different position of the Raman active band around 913 cm-1.



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Figure 11. Optimized geometries and relative free energies for LiClO4(DMCcc)m(DMCct)n, m+n=3 solvates from PBE/6-31+G(d,p), PCM(ε=3) calculations. Numbers in parentheses are from DFT calculations with the GD3BJ dispersion correction.

DFT Study of Carbonyl Shift in IR Spectra in Lithium Solvates We follow our previous studies on other Li+ solvates10,

21, 51, 80

and systematically

investigate the influence of Li+ binding on the PC IR vibrational bands in the (Li+)n(PC)m solvates with n=1,3,4 complexes shown in Figure 12 and the PC3(LiBF4) complexes as shown in Figure 8f. Unlike the previous gas-phase QC study of the influence of Li+ binding on the vibrational signature of the solvent, this work incorporates the solvation effects via both the implicit solvent model and explicit solvents. The Li+ solvate size was systematically increased 30 ACS Paragon Plus Environment

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from 1 PC to 4 PC. Most calculations were performed using B3LYP or M05-2X functionals and a compact 6-31+G(d,p) basis set. A number of additional calculations B3LYP, PBE functionals with the same (6-31+G(d,p)) and much larger aug-cc-pvTz (abbreviated as Tz) basis sets were performed in order to examine the sensitivity of the band shifts and changes of the IR intensity to the choice of density functional and basis set.

Figure 12. Optimized geometries of Li+PC, Li+(PC)3 and Li+(PC)4 from B3LYP/6-31+G(d,p) optimization with PCM(PC).

Shifts of the C=O vibrational band and change of IR intensity of PC upon Li+ complexation from B3LYP calculations are shown in Table 6 for the Li+PC, Li+(PC)3 and Li+(PC)4 clusters surrounding PCM(PC) implicit solvent. Addition of the Li+ cation yields a red shift of the C=O vibrational band by 20-28 cm-1 for the representative larger clusters Li+(PC)3 and Li+(PC)4. This shift is in good agreement with IR spectra. DFT calculations also predict that the Li+ complexation of PC would increase IR intensity of the band by 7-16%. Examination of the LiAnion(PC)3 CIP complexes shown in Figure 10 yielded the C=O band shift between 15 and 25 cm-1, while the intensity of the band increased by 5-8% in PBE/6-31+G(d,p), PCM(PC) calculations.



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Table 6. Shifts (δν) and intensity ratios (I/Ibulk) of the C=O vibrational band of PC upon Li+ complexation from DFT calculations with PCM(PC). PBE/

DFT method

B3LYP/6-31+G(d,p

B3LYP/Tz

Solvate

Li(PC)

Li(PC)3

Li(PC)4

Li(PC)

Li(PC)4

shift δν (cm )

-13

-28

-20

-26

-19, -25

intensity ratio (I/Ibulk)

1.15

1.08

1.09

1.16

1.07,1.07

-1

631+G(d,p)

Analysis of the shifts in the LiAnion(DMC)3 complexes shown in Figure 9 suggests that the carbonyl band shifts by 18-32 cm-1 upon Li+ complexation with the band intensity increasing from 30% to 64% depending on the solvate. A broad range of intensity increases indicates a source of potential uncertainty in extracting the Li+ solvation numbers in DMC from analysis of IR spectra. Moreover, the assumption of the unchanged activity is going to result in overestimation of the number of solvating DMC by 30-60%.

Conclusion Two spectroscopic methods and DFT calculations were utilized to investigate the solvation sphere of the lithium cation. The study was conducted with four lithium salts (LiTFSI, LiPF6, LiBF4, and LiClO4) and two common carbonate solvents (PC and DMC) used in lithiumion battery electrolytes. Solutions were prepared with specific salt-to-solvent mole ratios and the quantities of coordinated and uncoordinated solvent were calculated based on the relative areas of carbonyl IR absorptions characteristic of the coordinated and uncoordinated solvents. Three of the salts, LiTFSI, LiPF6, and LiClO4, are found to readily dissociate in PC at low salt concentrations, while dissociation of LiBF4 occurs to a lesser extent. As the amount of solvent


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per salt is decreased, the solvation numbers steadily decrease to ~3 in the most dissociative salts in PC and to ~2 for LiBF4. The solvation numbers for LiBF4 are significantly lower. At moderate concentrations ionic transport in highly dissociated salts LiTFSI, LiPF6, and to a lesser degree for LiClO4 salt, is expected to occur via diffusion of the dissociated ions, while diffusion of charged clusters is expected for LiBF4 in PC. Due to much smaller solvation numbers in DMC compared to PC, all salts are expected to be strongly aggregating with ion transport occurring via diffusion of charged aggregates or structural diffusion via anion – cation dissociation. Ionic aggregation also has ramifications to electrolyte electrochemical stability, LiPF6 and LiTFSI salt aggregation has been shown to increase electrolyte reduction potential due to anion defluorination upon aggregate reduction,

13, 17

while aggregation of LiBF4 is not expected

increase electrolyte reduction potential because the defluorination of LiBF4 aggregates upon reduction occurs at low potentials lower than solvent reduction.13 The solvation structures determined by IR spectroscopy are supported via

13

C NMR

spectroscopy of the carbonyl resonance of the carbonate solvents as a function of salt concentration. The 13C NMR resonance of the carbonyl carbon shifts upon incorporation of the lithium salt due to solvent coordination and rapid solvent exchange. The change in the chemical shift correlates well with the fraction of coordinated solvent determined by IR spectroscopy providing further support for the solvation numbers. DFT calculations using the cluster-continuum approach yield stability of the Li+ solvates in good agreement with the experimentally observed trends for the Li+ solvation numbers. While both mono- and bi-dentate Li+ coordination to TFSI- were predicted in DFT calculations, the LiPF6, LiBF4 and LiClO4 salts favor the monodendtate binding to Li+. Investigation of the Li+(Anion)(DMC)3 solvates showed that both the DMC(cis-cis) and DMC(cis-trans) conformers



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are expected to be present, with an approximate ratio of 3:1. DFT calculations predict shifts of the C=O bond upon Li+ complexation in reasonable agreement with experimental observations. The ratio of the Li+ coordinated PC C=O band intensity to the PC C=O non-coordinated band intensity is predicted to be 1.08-1.16, while this ratio is significantly higher for DMC, 1.3-1.6.

Supporting Information Optimized geometries of Li-anion and Li-solvent configurations, Thermodynamic values obtained by DFT calculations such as binding energy, reaction energy, and free energy

Acknowledgement The authors gratefully acknowledge funding from Department of Energy Office of Basic Energy Sciences EPSCoR Implementation award (DE–SC0007074)


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Table of Contents Graphic:



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