Ionic Conduction and Solution Structure in LiPF6 and LiBF4 Propylene

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Cite This: J. Phys. Chem. C 2018, 122, 19438−19446

Ionic Conduction and Solution Structure in LiPF6 and LiBF4 Propylene Carbonate Electrolytes Sunwook Hwang,† Dong-Hui Kim,† Jeong Hee Shin,‡ Jae Eun Jang,‡ Kyoung Ho Ahn,§ Chulhaeng Lee,§ and Hochun Lee*,† Department of Energy Science and Engineering and ‡Department of Information and Communication Engineering, DGIST, Daegu 42988, Republic of Korea § Batteries R&D, LG Chem Ltd., Daejeon 34122, Republic of Korea J. Phys. Chem. C 2018.122:19438-19446. Downloaded from pubs.acs.org by UNIV OF SOUTH DAKOTA on 08/31/18. For personal use only.

†

S Supporting Information *

ABSTRACT: Expanding the performance limit of current Li-ion batteries requires ion−ion and ion−solvent interaction, which governs the ion transport behavior of the electrolytes, to be fully understood as a matter of crucial importance. We herein examine the ionic speciation and conduction behavior of propylene carbonate (PC) electrolytes of 0.1−3.0 M LiPF6 and LiBF4 using Raman spectroscopy, dielectric relaxation spectroscopy (DRS), and pulsed-field gradient NMR (PFG-NMR) spectroscopy. In both LiPF6− PC and LiBF4−PC, free ions and a solvent-shared ion pair (SIP) are dominant species at dilute salt concentrations (2.5 M). The maximum ionic conductivity is 6.2 mS cm−1 at 0.8 M LiPF6−PC and 3.7 mS cm−1 at 0.5 M LiBF4−PC, which is in good agreement with the literature.34,38 The viscosity of LiPF6−PC is higher than that of LiBF4−PC, and the difference becomes more prominent with increasing salt concentration (Figure 1b). High ionic conductivity is usually associated with low viscosity, but this is not the case in this study. This implies that the ionic conductivity of the PC electrolytes is mainly affected by the number of charge carriers, rather than by the solution viscosity. Indeed, the Walden plot of LiBF4−PC displays severe deviation from the ideal KCl line compared to that of LiPF6−PC (Figure 1c), and the deviation is more distinct at high (>1.0 M) LiBF4 concentration. This indicates that the salt association is stronger in LiBF4−PC than in LiPF6−PC.43,44 However, the nature and composition of the electrolyte species and their roles in the ionic conduction are elusive, and they were investigated and are discussed in the following sections. Ionic Speciation. First, the ionic speciation in LiPF6−PC and LiBF4−PC was examined using Raman spectroscopy. The PF6− anion in LiPF6−PC presents two discernible peaks at 741 and 746 cm−1,38,45,46 which are denoted as peak A and peak B,

PC, the intensity of peak A is much higher than that of peak B over the entire salt concentration range, whereas the fraction of peak B steadily increases with LiPF6 concentration (Figure 2a). By contrast, in LiBF4−PC, the intensity of peak A′ is higher than that of peak B′ in the low concentration region ( anion (D−) > Li ion (D+). Notably, LiBF4−PC exhibits higher D values, in particular, DPC and D+, than LiPF6−PC. This manifests that the higher ionic conductivity of LiPF6−PC cannot be attributed to the higher mobility of the ionic species. Figure 6c compares the transference numbers of Li ion (t+ = D+/(D + + D−)) in the two PC electrolytes. With increasing salt concentration, t+ of LiBF4−PC gradually increased from ca. 0.3 and converged to 0.5 at 3.0 M LiBF4. In contrast, t+ of LiPF6−PC remained lower than that of LiBF4−PC and increased only up to 0.45 in 3.0 M LiPF6. This confirms again that the ion association is weaker in LiPF6−PC than in LiBF4−PC. According to the Nernst−Einstein relation (eq 6), the ionic conductivity of the electrolyte solutions can be calculated from

dilute concentration without considering the presence of free ions, and CIP continuously increases with increasing salt concentration.48 Similarly, it was reported that SIPs are dominant in dilute in LiPF6−PC solutions, whereas CIPs are dominant at higher concentration.61 13C NMR study claimed that in PC/DMC (1/1, v/v) solutions, the degrees of ion pairing are similar for various lithium salts including LiPF6 and LiBF4,62 which is contradictory to other studies including this study. Indeed, on the basis of the conductivity measurement and theoretical calculation, Ue et al. reported that the association constant of the lithium salt in PC increases in the following order: Li(CF3SO2)2N < LiAsF6 < and LiPF6 < LiClO4 < LiBF4.63 The disparity seems to be due to the different solvent composition (PC vs PC/DMC), which definitely affects the solution structures via preferential solvation and variation in solvation number.64,65 Solvent Speciation. In the Raman spectra of the PC solvent (Figure S5), the peak at 712 cm−1 is assigned to the ring deformation of free PC, and the peak at 721 cm−1 to the Li-solvating PC.38 The concentration of free PC solvent was obtained from the comparison of fractional Raman intensities of free and Li-coordinating PCs,15,57,58 and its dependency on the salt concentration is compared in Figure 5a. In addition, the solvation number of Li ions was calculated from the ratio of the concentration of Li-solvating PC and the salt concentration (Figure 5b). The detailed description on the calculation of the solvation number is presented in Supporting Information.56−58 As seen in Figure 5a, with increasing salt concentration, the intensity of free PC decreased monotonously in the two PC electrolytes, but the decrease is more distinct in LiPF6−PC than in LiBF4−PC. Accordingly, the solvation number in LiPF6−PC is higher than that in LiBF4− PC for a given salt concentration: 5.1 vs 4.1 at 0.3 M and 2.3 vs 1.5 at 3.0 M in LiPF6−PC and LiBF4−PC, respectively (Figure 5b), which is in good agreement with the previous report.31,47,62,66 These results, together with the Raman data (Figure 2), indicate that the ionic species pertaining to peak A (or peak A′), free ions, SIP, and SSD, are associated with a 19442

DOI: 10.1021/acs.jpcc.8b06035 J. Phys. Chem. C 2018, 122, 19438−19446

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The Journal of Physical Chemistry C

Figure 7. Experimental ionic conductivity (κexp) and the calculated ionic conductivity (κRaman) in (a) LiPF6−PC and (b) LiBF4−PC. (a′) and (b′) are the relative differences of the two conductivities, Δκ = (κexp− κRaman)/κexp × 100.

Figure 6. Self-diffusion coefficients of (a) the Li ion and anions and (b) PC solvent measured by PFG-NMR as a function of salt concentration. (c) Transference number of the Li ion (t+) in LiPF6− PC and LiBF4−PC.

concentrations (≥1.5 M) and becomes as high as −66% at 3.0 M. The origin of this huge disparity can be 2-fold: (1) the diffusion coefficients used in the κRaman calculation are underestimated because the values of D+ and D− derived from PFG-NMR are averaged for all the species present in the electrolyte.68,69 In particular, the underestimation is expected to be more severe at high concentrations where the formation of the bulky and sluggish AGGs is significant. (2) CIPs could participate in the ion conduction via the Grotthuss mechanism, whereby Li ions are delivered through hopping-like behavior.4,28−31 This hopping mechanism is expected to be facilitated as the concentration of CIPs is sufficiently high to ensure proximity among CIPs. Thus, in highly concentrated solutions, the Grotthuss-type conduction by CIPs can be as important as the migratory conduction. Combining the results obtained thus far, the possible ionic species, their Raman or DRS activities, and their role in the ionic conduction were summarized (Table 1). In Table 1, we classify the free ions, SIP, and CIP on the basis of the relative position of the anion in the solvation shell structure of the Li ion: (i) free ions, where the anion is situated outside of the secondary solvation shell; (ii) SIP, where the anion resides in the secondary shell; (iii) CIP, where the anion is at the primary shell, forming direct contact with the Li ion. The SSD and CD were assumed to be the dimeric form of SIP and CIP, respectively, and AGG was assumed to be a higher order composite of the CIPs. We highly encourage the validation of these hypothetical molecular structures of ionic species by other groups, especially by those with expertise in theoretical calculations. The exact value of the static permittivity can significantly affect the accuracy of quantum chemistry calculation.70 Nonetheless, experimental data pertaining to the static permittivity of practical LIB electrolytes are scarce.52,71 We

the diffusion coefficients and concentrations of the species participating in the ionic conduction via the migration (or vehicle) mechanism:27,64,67 z i 2F 2 Di Ciκ (6) RT κ where zi, Dj,Ci ,R, F, and T represent the formal charge, diffusion coefficient, and concentration of the ionic species i, and R, F, and T have their conventional meanings. Figure 7 compares the calculated ionic conductivity (κRaman) with the experimental ionic conductivity (κexp). In the calculation of κRaman, we used the D+ and D− values obtained from PFG-NMR (Figure 5a), and [PF6−]peak A or [BF4−]peak A′ derived from the Raman spectra (Figure 4a,b). The concentration of the Li ion was assumed to be the same as that of the anion. The relative difference in the two conductivities, (Δκ = (κexp − κRaman)/κexp × 100) is also presented in Figure 7b,d, respectively. Remarkably, κRaman shows excellent correspondence with κexp in LiPF6−PC (Figure 7a): Δκ is less than ±5% over the range 0.1−1.5 M, and less than ±15% even above 2.0 M LiPF6 (Figure 7b). This close resemblance between the two conductivities suggests that the ion conduction in LiPF6−PC is governed by the migration of the ionic species pertaining to peak A: free ions, SIP, and SSD. Although it may be rather unexpected that SIP and SSD, the charge-neutral ion pairs, contribute to the ionic conduction, the Li ion and anion involved in SIP and SSD are likely to be loosely bound to each other; hence they can be readily separated by application of an external electric field. In LiBF4− PC, κRaman is also quite similar to κexp at dilute concentrations: Δκ is less than ±6% over 0.3−1.0 M LiBF4 (Figure 7c,d). However, κRaman deviates significantly from κexp at higher κ=

∑

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DOI: 10.1021/acs.jpcc.8b06035 J. Phys. Chem. C 2018, 122, 19438−19446

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The Journal of Physical Chemistry C

Table 1. Possible Ionic Species in LiPF6−PC and LiBF4−PC, Their Activity for Raman and DRS, and Their Contribution to Ionic Conduction

a

Solvent-shared ion pair. bSolvent-shared dimer. cContact ion pair. dContact dimer. eAgglomerate. fMigration. gNonconducting.

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expect the set of static permittivity values presented in this paper to be useful to improve the theoretical simulation results. In addition, our future work will address the composition of other electrolytes more relevant to practical applications including binary solvent systems (e.g., mixtures of ethylene carbonate and linear carbonates), ionic liquids, and post-LIB electrolytes. However, it should be noted that the static permittivity of the electrolyte solutions, determined not only by the fee solvent but also by the dipolar ion pairs, is not a good parameter to assess the solvation of the Li ion.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.8b06035. Raman spectra and dielectric relaxation spectroscopy (DRS) spectra with best-fitted results (Figures S1−S5), calculation of the concentration of free solvent and solvation number and the parameters used for deriving the concentrations of ion pairs using the Cavell equation (Table S1), and derivation of the concentration of ion pair from dielectric strength and the self-diffusion coefficients of ions and solvent in the solutions measured by PFG-NMR (Table S2) (PDF)

4. CONCLUSION This study investigated the electrolyte speciation and ion conduction behaviors of 0.1−3.0 M LiPF6−PC and LiBF4−PC electrolytes. A comprehensive analysis of the Raman and DRS results enabled us to determine the ionic speciation as a function of the Li-salt concentration: (i) at dilute concentrations (1.5 M), CIP could take part in ionic conduction via the Grotthuss mechanism. The new implication that SSD and CIP can contribute to the ionic conduction deserves further validation. This report focused on electrolytes of simple composition (i.e., single solvent), but the analysis of practical LIB electrolytes consisting of a binary or ternary solvent mixture is highly required, the study of which is under way in our group.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +82-53-785-6411. Fax: +82-53-785-6409. E-mail: [email protected]. ORCID

Jae Eun Jang: 0000-0002-8523-1785 Hochun Lee: 0000-0001-9907-5915 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was financially supported by Mid-Career Researcher Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT (MSIT) (NRF-2017R1A2B4004470). H.L. thanks prof. Richard Buchner for his valuable advices for DRS measurements and analysis.

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REFERENCES

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DOI: 10.1021/acs.jpcc.8b06035 J. Phys. Chem. C 2018, 122, 19438−19446