Spectroscopic Characterization

Article pubs.acs.org/JPCC

Solvate Structures and Computational/Spectroscopic Characterization of LiPF6 Electrolytes Sang-Don Han,† Sung-Hyun Yun,†,‡ Oleg Borodin,*,§ Daniel M. Seo,† Roger D. Sommer,∥ Victor G. Young, Jr.,⊥ and Wesley A. Henderson*,†,# †

Ionic Liquids & Electrolytes for Energy Technologies (ILEET) Laboratory, Department of Chemical & Biomolecular Engineering, North Carolina State University, Raleigh, North Carolina 27695, United States ‡ School of Environmental Science and Engineering, Gwangju Institute of Science and Technology (GIST), 123 Cheomdan-Gwagiro, Buk-gu, Gwangju 500-712, Republic of Korea § Electrochemistry Branch, U.S. Army Research Laboratory, Adelphi, Maryland 20783, United States ∥ X-ray Structural Facility, Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27695, United States ⊥ X-ray Crystallographic Laboratory, Department of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455, United States # Electrochemical Materials & Systems Group, Energy & Environment Directorate, Pacific Northwest National Laboratory (PNNL), Richland, Washington 99352, United States S Supporting Information *

ABSTRACT: Raman spectroscopy is a powerful method for identifying ion−ion interactions, but only if the vibrational band signatures for the anion coordination modes can be accurately deciphered. The present study characterizes the PF6− anion P−F Raman symmetric stretching vibrational band for evaluating the PF6−···Li+ cation interactions within LiPF6 crystalline solvates to create a characterization tool for liquid electrolytes. To facilitate this, the crystal structures for two new solvates(G3)1:LiPF6 and (DEC)2:LiPF6 with triglyme and diethyl carbonate, respectivelyare reported. DFT calculations for Li-PF6 solvates have been used to aid in the assignments of the spectroscopic signatures. The information obtained from this analysis provides key guidance about the ionic association information which may be obtained from a Raman spectroscopic evaluation of electrolytes containing the LiPF6 salt and aprotic solvents. Of particular note is the overlap of the Raman bands for both solvent-separated ion pair (SSIP) and contact ion pair (CIP) coordination in which the PF6− anions are uncoordinated or coordinated to a single Li+ cation, respectively.

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INTRODUCTION The state-of-the-art Li-ion battery electrolytes which conduct Li+ cations within commercial batteries consist of mixtures of carbonate solvents (i.e., ethylene carbonate (EC) and an acyclic carbonate such as diethyl carbonate (DEC)) and the salt LiPF6. Remarkably, despite the critical role that the LiPF6 serves in both Li-ion and developmental lithium batteries, very little information is available about the solvate structures formed by this salt and the spectroscopic evaluation of PF6−···Li+ cation interactions.1−3 Solvate crystal structures, however, provide detailed insight into the molecular-level solvent···Li+ (solvation) and anion···Li+ (ionic association) interactions and thus serve as useful, but necessarily restricted, models for the diverse range of solvates present in liquid electrolytes. Furthermore, by utilizing these solid-state crystalline lithium salt solvates to generate a Raman spectroscopic characterization tool for evaluating anion···Li+ coordination, detailed information about the liquid-phase electrolyte ionic association interactions may be deduced. Such ionic association interactions are typically classified as either solvent-separated ion pair (SSIP), contact ion pair (CIP), © 2015 American Chemical Society

or aggregate (AGG) coordination depending upon whether the anions form coordinate bonds with zero, one, or more than one Li+ cations, respectively (Chart 1). These may be further differentiated as CIP-I, CIP-II, and CIP-III for a contact ion pair in which the PF6− anion is coordinated to a single Li+ cation through one, two, or three fluorine atoms, respectively. AGG-I, AGG-II, and AGG-III coordination then refers to PF6− anions coordinated to two, three, or more than three Li+ cations, respectively. These are defined in terms of the anion coordination instead of the Li+ cation coordination since the anion coordination modes can be probed using spectroscopic methods, whereas the cation coordination modes cannot. It is important to note, however, that such methods provide insight into, but not the ability to directly discern, the solvates which form and their distribution. What is actually obtained from a Raman spectroscopic analysis of electrolyte anion vibrational bands is the distribution/population of the anion’s modes of Received: January 27, 2015 Revised: March 24, 2015 Published: March 27, 2015 8492

DOI: 10.1021/acs.jpcc.5b00826 J. Phys. Chem. C 2015, 119, 8492−8500

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The Journal of Physical Chemistry C Chart 1. Examples of PF6−···Li+ Cation Coordination: (a) SSIP, (b) CIP-I, (c) CIP-II, (d) AGG-Ia, (e) AGG-Ib, (f) AGG-II, and (g) AGG-IIIa

Chart 2. Structures and Acronyms of the Solvents Used

prior to use using a Mettler Toledo DL39 Karl Fischer coulometer. The materials were handled and stored in a Vacuum Atmospheres inert atmosphere (N2) glovebox ( CIP-I,1,37 but this is not what occurs for the (G3)1:LiPF6 solvate due to steric crowding from the coordinated solvent molecules (Figure S13, Supporting Information). MD simulations for (AN)n−LiPF6 mixtures also indicate that CIP-I coordination predominates in electrolyte solutions for solvates with anions coordinated to a single Li+ cation (with this solvent).5,6 The ion and solvent coordination within the AGG-Ib (DEC)2:LiPF6 solvate is shown in Figure 2b. In this solvate,

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RESULTS AND DISCUSSION Solvate Structures and Li+ Cation Coordination. A limited number of LiPF6 solvate structures have been previously reported, but most of these have rather exotic solvents/ligands.18−26 Exceptions to this include the SSIP (EC)4:LiPF6 (Figure 1b),27 SSIP (SN)2:LiPF6,28 and AGG-Ia (PMDETA) 1:LiPF6 (Figure 1d)29 solvates. The crystal structure of LiPF6 is also known.30 Recently, however, several additional crystalline solvates have been reported including SSIP (AN)6:LiPF6,31 SSIP (AN)5:LiPF6 (Figure 1a),32 SSIP (GBL)4:LiPF6,33 and AGG-Ia (DMC)2:LiPF6 (Figure 1c).34 The present study complements these solvates by reporting the crystal structures for two new solvates in which the PF6− anions 8494


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The Journal of Physical Chemistry C the PF6− anions are coordinated to two Li+ cationsone via a single fluorine atom and another using two fluorine atoms. Each Li+ cation has 5-fold coordination by two carbonyl oxygen atoms from two DEC molecules and three fluorine atoms from two anions. This differs from the AGG-Ia (DMC)2:LiPF6 solvate structure (Figure 1c) in which the Li+ cations have 4fold coordination by two carbonyl oxygen atoms from two DMC molecules and two fluorine atoms from two anions. The differences between the two solvates are due to variations in the overall polymeric chain structures which originate due to the different spatial requirements for the coordinated solvent molecules within the crystalline lattices. Raman Spectroscopic Analysis of Crystalline Solvates. The uncoordinated PF6− anion has octahedral (Oh) symmetry with vibrational modes:1,37,38

oxygens) and uncoordinated anions. 31,40 The known (DMC)2:LiPF6 solvate has AGG-Ia anion coordination.34 For the (DMC)3:LiPF6 solvate, if the Li+ cations have 4-fold coordination, as is found for most of the known solvates with monodentate coordinating solvents, CIP-I anion coordination is most probable if the Li+ cations are coordinated to the three solvent carbonyl oxygen atoms and a single PF6− anion. This would then resemble the coordination found for the (EC)3:LiClO4 crystalline solvate.33 A summary of the anion v1 Raman band positions for the varying PF6−···Li+ cation coordination modes is shown in Figure 5. Two points are especially noteworthy. The first is that the PF6− Raman band found near 747 cm−1 (at 20 °C) does not correspond to CIP solvates, as might be expected from comparable studies for LiClO4 and LiBF4.10−12 Instead, this band is due to AGG-Ia coordination in which the anion is coordinated to two Li+ cations via two anion fluorine atoms (with the AGG-Ib coordination resulting in a band at even lower wavenumber). The second point is that the band for CIPI coordination (i.e., anion coordinated to a single Li+ cation via a single fluorine atom) overlaps with that for SSIP coordination (i.e., uncoordinated anions). Thus, Raman spectroscopy does not enable the SSIP and CIP-I forms of PF6−···Li+ cation coordination to be differentiated from one another. This may, in part, explain the limited number of Raman studies reported for LiPF6-based electrolytes using Raman spectroscopyi.e., no observable peak splitting may have been noted so it may have generally been assumed that the Li+ cations and PF6− anions are fully dissociated, but this is not what MD simulation results have found for (AN)n−LiPF6 mixtures.5,6 Therefore, the results reported in Figure 5 may be used as a tool for the characterization of electrolytes containing LiPF6, but it is evident that there are limitations to the information that may be directly obtained regarding ionic association interactions from such a Raman analysis. DFT Computational Studies. The above experimental study was complemented with a computational evaluation of the Raman band variations. This began with an exploration of the influence of mono- and bidentate coordination of a PF6− anion to a Li+ cation on the PF6− anion Raman spectrum using (AN)3−LiPF6 and (DMC)3−LiPF6 individual solvate complexes as models, as shown in Figure 6. The Li+ cation first coordination shells were explicitly represented by the inclusion of AN or DMC molecules and PF6− anions, while the outer solvation shells were represented using PCM with AN and DMC parameters for the (AN)3−LiPF6 and (DMC)3−LiPF6 clusters, respectively. Note that the inclusion of solvent molecules in the cation coordination shells is necessary as PCM aids in accounting for the polarity of the solvent medium, but does not account for the steric bulk of the solvent molecules or the presence of uncoordinated electron lone-pairs from the solvent, both of which strongly influence how other neighboring solvent molecules and/or anions coordinate a Li+ cation in the restricted (due to the small size of the cation) coordination shell. The positions and activities of the v1 Raman band from the DFT calculations are shown in Table 1. This indicates that both the mono- (CIP-I) and bidentate (CIP-II) coordination modes for the (AN)3−LiPF6 solvate complexes (Figure 6a and 6b) and the monodentate (CIP-I) coordination for the (DMC)3−LiPF6 solvate complex (Figure 6c) have a very small band shift of 1−2 cm−1 from the position for the uncoordinated PF6− anion further confirming that it is challenging to distinguish between the SSIP and CIP modes

Γ = a1g + eg + t1g + t 2g + 3t1u + t 2u

The v1(a1g), v2(eg), and v5(t2g) modes are Raman active; the v3(t1u) and v4(t1u) modes are IR active; and the t1g and t2u modes are inactive.1,37,38 For the Raman spectra of pure LiPF6, the v1 vibration due to the P−F symmetric stretching mode near 770 cm−1 is the most prominent, while the v2 and v5 modes at 570 and 473 cm−1, respectively, are much weaker (Figure 3).

Figure 3. Raman spectrum of LiPF6 (at 20 °C).

Raman spectra were acquired for the crystalline solvates, as well as for the neat LiPF6 salt, as a function of temperature to identify the anion band position variation for specific PF6−···Li+ cation coordination modes. The data for the v1 vibration are shown in Figure 4. The remaining data for the other anion bands are provided in the Supporting Information. In addition to the known solvates, data were also collected for solvates for which the anion coordination can reasonably be surmised. These include the (PN)4:LiPF6, (G2)2:LiPF6, (G1)3:LiPF6, and (DMC)3:LiPF6 solvates. The (PN)4:LiPF6 solvate is believed to have SSIP coordination with the four solvent nitrile groups fully coordinating the Li+ cations, as is found for the (AN)6:LiPF6,31 (AN) 5:LiPF6,32 and (AN)4:LiClO439 solvates. Both the (G2)2:LiPF6 and (G1)3:LiPF6 solvates are also presumed to have SSIP coordination as a large number of (G2)2:LiX and (G1)3:LiX crystalline solvate structures are known, all of which have fully solvated Li+ cations (6-fold coordination by six ether 8495


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Figure 4. Raman spectra vs temperature (°C, shown on the left) of the PF6− anion vibrational bands for the crystalline solvates: (a) SSIP (AN)5:LiPF6, (b) SSIP (EC)4:LiPF6, (c) (speculative SSIP) (G2)2:LiPF6, (d) (speculative SSIP) (G1)3:LiPF6, (e) (speculative SSIP) (PN)4:LiPF6, (f) CIP-I (G3)1:LiPF6, (g) (speculative CIP-I) (DMC)3:LiPF6, (h) AGG-Ia (DMC)2:LiPF6, (i) AGG-Ia (PMDETA)1:LiPF6, (j) AGG-Ib (DEC)2:LiPF6, and (k) AGG-III LiPF6 (pure salt) (the dark lines indicate that a phase transition has occurredi.e., melting).

using this PF6− anion band. A slight red shift (−2 cm−1), however, was observed for the CIP coordination in the (DMC)3−LiPF6 solvate. It is tempting to attribute this band position difference between the CIP-I (DMC)3−LiPF6 and (AN)3−LiPF6 solvates (Table 1) perhaps to the lower dielectric

constant of DMC relative to AN, but this seems to instead be due to the differences in the Li···F−P bond angles (Table 1), as discussed below, with the optimized angle differences for the complexes due to the differences in steric bulk for the three coordinated solvent molecules. 8496


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In order to investigate the influence of treating the solvent in the Li+ cation’s first coordination shell explicitly (i.e., with solvent molecules) vs implicitly (i.e., with the PCM model), DFT calculations were performed on the PF6−···Li+ cation complex shown in Figure 7a using the geometry obtained from

Figure 5. Summary of the PF6− anion Raman band peak positions for the different crystalline solvates (each line is for a separate solvate) (see Supporting Information for the assignments of the data to specific solvates).

Figure 7. PF6−···Li+ cation geometries (a) obtained from the (AN)3− LiPF6 solvate complex (i.e., Figure 6a) and (b), (c), and (d) optimized using M05-2X/6-31+G(d,p) PCM(AN).

the (AN)3−LiPF6 solvate complex (Figure 6a), but using only the PCM(AN) implicit solvation model (no solvent molecules were included). Table 1 shows that the PF6− anion band position shifts slightly from 750.9 cm−1 for the (AN)3−LiPF6 solvate to 749.5 cm−1 for the monodentate PF6−···Li+ cation coordination having the same geometry. This result indicates that replacing the explicit AN solvent molecules with the implicit solvent (PCM) is a reasonable approximation with the PCM results showing a slight red shift as compared to more computationally expensive explicit solvent calculations. Additional DFT calculations were then performed on the bidendate and monodentate PF6−···Li+ cation complexes with C2v and C4v symmetry, respectively, as shown in Figure 7b and 7c using PCM(AN), but without the explicit AN molecules solvating the Li+ cations. Table 1 indicates that for the bidentate PF6−···Li+ cation coordination the PF6− band is only slightly red-shifted as compared to the band for the uncoordinated PF6− anions, while the monodentate configuration with C4v symmetry showed a large blue shift of 10 cm−1 that was not observed in the monodentate (AN)3−LiPF6 and (DMC)3−LiPF6 solvate complexes. In order to understand the origin of this large shift of the PF6− band for the C4v PF6−···Li+ cation coordination, the dependence of the band position on the Li+ cation position about the anion was investigated by constraining the Li···F−P angle and optimizing all other degrees of freedom with the results shown in Figure 8a. A large blue shift was observed for the Li···F−P angles higher than 174°. In the region near 150° a noticeable blue shift up to 5 cm−1 was also observed for some geometries, while other configurations resulted in the PF6− band shift of less than 2 cm−1. These results suggest that, while there is generally a small positive shift of the 750 cm−1 PF6− band upon monodentate coordination to a single Li+ cation, it may be challenging to accurately deconvolute Raman spectra to identify the population of anions with different modes of coordination in the LiPF6 solvates due to this dependence of the band shift on the Li+ cation position for the CIP coordination and the resulting overlap with band positions for both uncoordinated anions (SSIP) and those coordinated to two Li+ cations (AGGI) (Figure 5).

Figure 6. Optimized (a) and (b) (AN)3−LiPF6 and (c) (DMC)3− LiPF6 solvate complexes from M05-2X/6-31+G(d,p) calculations with PCM(AN).

Table 1. PF6− Anion Band Position and Raman Activity Ratio for Solvates from M05-2X/6-31+G(d,p) Calculations Using PCM (AN)3−LiPF6a (monodentate)

(AN)3−LiPF6a (bidentate)

(DMC)3−LiPF6b (monodentate)

ν(CIP) (cm−1) ν(CIP)−ν(SSIP) I(CIP)/I(SSIP) Li···F−P angles

750.9 1.2 0.98 136.8° LiPF6a,d (monodentate)

749.4 −0.3 1.12 103.4°, 102.9° LiPF6a,e (bidentate)

747.3 −2.4 1.12 124.8° LiPF6a,f (monodentate)

ν(CIP) (cm−1) ν(CIP)−ν(SSIP) I(CIP)/I(SSIP) Li···F−P angles

749.5 −0.4 0.96 136.8°

748.6c −1.4 0.98 103.1°, 103.1°

760.3 10.3 0.98 180.0°

a PCM(AN) was used. bPCM(DMC) was used. cOne small imaginary frequency. dWith (AN)3−LiPF6 geometry. eWith C2v symmetry. fWith C4v symmetry.

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solutions,5,6,41,42 as well as within crystalline solvates such as (PMDETA)1:LiPF6 (Figure 1d). The calculated band shifts for the two PF6− anions in the complex in Figure 7d (relative to the uncoordinated PF6− anion) correspond to 3 and 4 cm−1, respectively. These results indicate that AGG coordination does, in general, result in a larger blue shift than for the CIP coordination investigated above, but the difference is not largein reasonable agreement with the experimental Raman analysis of crystalline solvates (Figure 5).

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CONCLUSIONS Two new LiPF 6 solvate crystal structures have been determinedCIP-I (G3)1:LiPF6 and AGG-Ib (DEC)2:LiPF6. These, as well as other known crystalline solvate structures, have been characterized by Raman spectroscopy to aid in providing assignments for the anion Raman bands associated with specific PF6−···Li+ cation coordination modes. This information serves as a general tool for evaluating ionic association interactions within electrolytes containing aprotic solvents and the LiPF6 salt. DFT calculations support the experimental evaluation (i.e., indicate that the anion bands for SSIP and CIP coordination modes may overlap), but also suggest that the orientation of the anion with a coordinated Li+ cation for the CIP mode possibly results in a significant band shift which can also overlap with the AGG-I band position. These results suggest that accurately deciphering the population of anion coordination modes from the deconvolution of a Raman spectrum for an electrolyte solution with aprotic solvents and LiPF6 will be difficult, although, if the experimental study is done in concert with MD simulations, much may still be gleaned about the solution structural interactions.

Figure 8. (a) Shift of the PF6− anion band (750 cm−1) upon Li+ cation coordination for the CIP coordination modes in the LiPF6 solvate complexes as a function of the Li···F−P angle and (b) angular probability p(θ) for the Li···F−P angle obtained from MD simulations of EC-LiPF6 electrolytes at 333 K. A r(Li···F) < 2.4 Å distance criterion was used to define the Li···F bond.

It is therefore important to understand the probability of the angles present for the Li+ cation coordination which may be found in liquid electrolytes. The distribution of the Li···F−P angles was calculated from MD simulations of EC-LiPF6 liquid electrolytes performed using an APPLE&P force field41 at 60 °C as a function of concentration with the results shown in Figure 8b. The angular probability p(θ) was calculated according to the following equation p(θ ) =

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Nθ ∑N

ASSOCIATED CONTENT

S Supporting Information *

Sample preparation details, Raman spectra of LiPF6 solvates, illustrations/crystallographic information for the (G3)1:LiPF6 and (DEC)2:LiPF6 solvates (PDF), and X-ray crystallographic information files for the (G3)1:LiPF6 and (DEC)2:LiPF6 solvates (CIFs). This material is available free of charge via the Internet at http://pubs.acs.org.

θ

where Nθ is the number of the Li···F−P angles with the angle θ. Figure 8b shows a distribution of the angles with two broad peaks present at approximately 100° and 155°. A weak dependence of the Li···F−P angle distribution on salt concentration was observed with the 155° peak increasing and the 100° peak decreasing with increasing salt concentration. Note that the data in Figure 8b include analyses for both CIP and AGG coordination rather than CIP coordination alone, but the simulations with the more dilute concentrations (EO:Li = 30 and 10) have very few anions with AGG coordination. Figure 7b suggests that the peak in Figure 8b near 100° may be due to anions with bidentate coordination to a Li+ cation. Scrutiny of the (G3)1:LiPF6 solvate (Figure 2a) would seem to suggest that the Li···F−P bond is close to 180°, and thus one might expect from Figures 8a and S13 (Supporting Information) that a more sizable shift would be expected than that noted in Figures 4f and S12 (Supporting Information). The crystallographic data for this solvate (Table S3,), however, indicates that this angle is actually 169°, and from Figure 8a, such an angle should only have a modest shift from the position of the uncoordinated anion, thus providing an explanation for the experimental Raman band position for the solvate. An AGG (LiPF6)2 complex geometry was also investigated using DFT calculations, as shown in Figure 7d. Simulations of EC:DMC−LiPF6, oligoether−LiPF6, and AN−LiPF6 mixtures suggest that such an ion cluster is found in electrolyte

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors wish to express their gratitude to the U.S. Department of Energy (DOE) Batteries for Advanced Transportation Technologies (BATT) Program which fully supported the experimental portion of this research under Award Number DE-AC02-05-CH11231. The authors also wish to thank the Department of Chemistry of North Carolina State University and the State of North Carolina for funding the purchase of the Apex2 diffractometer.

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