Molecular Environment and Enhanced Diffusivity of Li+ Ions in Lithium

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Molecular Environment and Enhanced Diffusivity of Liþ Ions in Lithium-Salt-Doped Ionic Liquid Electrolytes Franca Castiglione,† Enzio Ragg,‡ Andrea Mele,*,† Giovanni Battista Appetecchi,§ Maria Montanino,§ and Stefano Passerini|| †

Dipartimento di Chimica, Materiali e Ingegneria Chimica “G. Natta”, Politecnico di Milano, Via L. Mancinelli, 7 I-20131 Milano, a di Milano, Via Celoria, 2 I-20133 Milano, Italy, §ENEA, Italy, ‡Dipartimento di Scienze Molecolari Agroalimentari, Universit Italian Agency for the New Technology, Energy, and the Sustainable Development, UTRINN-IFC, Via Anguillarese 301, unster, Germany I-00123 Roma, Italy, and ||Institute of Physical Chemistry, University of Muenster, Corrensstr. 28/30, D-48149 M€

ABSTRACT Lithium salts dissolved in ionic liquids (ILs) are interesting alternatives to the commonly used electrolytes for Li-ion batteries. In this study, the solution of Li [bis-(trifluoromethanesulfonyl)imide] (LiTFSI) in N-butyl-N-methylpyrrolidinium TFSI (PYR14TFSI) ionic liquid in the 0.1:0.9 molar ratio is studied by heteronuclear NOE and NMR diffusion measurements. The main purpose is to spot on the interions organization and mobility. NOE data support the existence of strongly coordinated Liþ species, whereas variable temperature measurements of the self-diffusion coefficients D show large, selective, and unexpected enhancement of Liþ mobility with T. The measured activation energy for Liþ diffusion is significantly larger than those of TFSI- and PYR14þ. These findings can be related to the mechanism of Liþ diffusion in ILs based on disruption formation of the coordination shells of Liþ with TFSI anions rather than on the Brownian motion of the whole Liþ coordinated species. SECTION Kinetics, Spectroscopy

coordination shell of Liþ in 1-ethyl-2,3-dimethylimidazolium (EMMIþ)PF6/LiPF6 for the occurrence of Li hopping. The conductivity was dramatically influenced by the molecular environment via the ligand exchange rate that, in turn, affected the structure-diffusion mechanism.8 In general, the transport properties of Liþ in LiX-IL systems depend on the size and charge state of the solvation/coordination shell of Liþ. Umecky et al.9 demonstrated that the competitive binding of silica toward TFSI- anions may act as an enhancer of Liþ diffusivity by reducing the size and charge of LiTFSI aggregates, for example, by changing the dominant species from [Li(TFSI)4]3- to [Li(TFSI)2]-, although keeping constant the tetra-coordination of Liþ. The identification of the effective Liþ species present in Li electrolytes is still the object of debate: Less egues et al.10 provided IR and DFT evidence supporting the presence of bidentate coordination leading to [Li(TFSI)2]as major adduct in LiTFSI/alkylmethylimidazolium TFSI electrolytes for salt molar fractions 0.08 < x < 0.2. Concentration dependence on the dominant aggregation number (nþ1) in [Li(TFSI)nþ1]n- adducts present in LITFSI/BMITFSI was also reported on the basis of diffusion measurements.11 In our previous works, we have reported the ionic conductivity and ion diffusion coefficients of neat RTILs without including salts or solvents.12,13 In this Letter,

oom-temperature ionic liquids (RTILs or simply ILs), initially proposed as innovative (or neoteric) solvents beneficial for clean chemical processes,1,2 gained, in the recent years, a more appropriate role as functional advanced materials with an amazingly broad repertoire of applications.3 Challenging electrochemical applications of ionic liquid have recently been reviewed.4 In this scenario, pyrrolidinium-based ILs keep on attracting electrochemists for their ability to dissolve LiX, giving rise to electrolytes of possible innovative uses in Li ion devices, especially batteries.5,6 Along with the applicative interest, LiX-IL mixtures represent interesting systems for gaining fundamental knowledge on the nanostructuration of ILs and to get insight into the state of Liþ ions in such environment. Usually, the lithium salt containing the same anion of IL is preferable because of higher solubility shown with respect to a mixture with dissimilar anions. Borodin et al.7 showed by molecular dynamics simulations that Liþ transport in N,N-dimethyl pyrrolidinium bis(trifluoromethanesulfonyl)imide (PYR11TSFI) doped with LiTFSI is heavily dependent on Liþ coordination with TFSI-. Liþ transport occurred primarily by TFSI- exchange in the first coordination shell (structure-diffusion mechanism), with only minor contribution of the diffusion of Liþ with the whole coordination shell (vehicular mechanism). Because of these discoveries, many studies focused on gaining fundamental knowledge of the molecular environment of Liþ in IL solvents for a rational design of battery electrolytes. Niu et al.8 have recently shown the importance of the stability of the first

R

r 2011 American Chemical Society

Received Date: November 9, 2010 Accepted Date: January 4, 2011 Published on Web Date: January 07, 2011

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DOI: 10.1021/jz101516c |J. Phys. Chem. Lett. 2011, 2, 153–157

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Before any comment on the spectra of Figure 2, it is useful to summarize what was previously obtained for neat compound PYR14TFSI.12 In that case, {1H-19F} HOESY showed selective NOE contacts between the F atoms of the anion and the H(6), H(7), H(5), and H(1) protons belonging to the pyrrolidinium ring and the N-methyl group of the cation, whereas no interaction with the inner protons of the butyl chain was observed. The 0.1 LiTFSI-0.9 PYR14TFSI sample shows a similar {1H-19F} NOE pattern as neat PYR14TFSI, thus indicating that no significant change is brought to the local cation-anion structure by the addition of Liþ cations. Intriguingly, the {1H-7Li} NOE spectrum shows analogous selectivity of the Liþ cations toward the protons of the cation (PYR14þ) butyl chain. In particular, the presence of NOE between Liþ and H(5), H(7), and the remote H(1) should be highlighted. This apparently puzzling result suggests that Liþ ions do have short contacts with PYR14þ despite the presence of a positive charge on both species. Moreover, the NOE pattern exhibited by Liþ cation toward PYR14þ matches the analogue shown by F nuclei of TFSI- toward the same PYR14þ species (F/H(5), F/ H(7), F/H(1); see top contour plot of Figure 3), thus suggesting that Liþ and TFSI- experience similar spatial relationships with respect to PYR14þ. The strong coordination of Liþ with TFSI- is then responsible for the spatial proximity of Liþ and PYR14þ cations irrespective of the presence of Coulomb repulsion, and it is likely to be a consequence of the local structure of the doped IL. The self-diffusion coefficient of both cations and the anion was measured independently by PGSE-NMR experiments in the 7Li, 19F, and 1H frequency domains, respectively. The data processing and the extraction of the self-diffusion coefficients were done as previously reported.12 The average experimental self-diffusion coefficient values at 32 °C are reported in Table 1 together with the values obtained for the neat liquid without dopant. The general trend is D(Liþ) < D(TFSI-) < D(PYR14þ). Such diffusion order is in agreement with other examples taken from the literature in the case of either pyrrolidinium15,16 or imidazolium Liþ-doped ILs series.8,11,17 It is generally admitted that the slower diffusion of Liþ ions compared with that of the bulkier pyrrolidinium ones is a clear indication of the presence of Li-anion aggregates in the liquid, such as [Li(TFSI)n](n-1)-. It is also interesting to note that the diffusion of both PYR14þ and TFSI- is slower in the Li-doped PYR14TFSI compared with the neat liquid (columns 2 and 3 of Table 1). This fact can be ascribed to the increased viscosity only. Indeed, it was demonstrated that the ratio DF/DH can be used as a descriptor of the effect of viscosity on individual ion random motion.17 In the present case, (DF/DH)x=0 = 0.79 (from ref 12) and (DF/DH)x=0.1 = 0.72 (this work). These figures have to be compared with (DF/DH)x=0 = 0.87 and (DF/DH)x=0.105 = 0.79 reported by Dulard et al.17 for the xLiTFSI-(1- x) BMITFSI system, where the similarity of the two ratios was taken as proof of the same influence of viscosity on the diffusion of the individual ions. The dependence of D for all ionic species in the temperature range 300-340 K is shown in Figure 3 in the Arrhenius representation.

Figure 1. Molecular formula, atom numbering of PYR14TFSI ionic components.

we present a study of the local structure and transport properties of the solution xLiTFSI-(1-x)PYR14TFSI with x = 0.1, which is liquid within the explored temperature range (300-340 K).14 The most important achievement is the discovery of the previously unreported enhancement effect of T upon the diffusivity, transference number, and ionic conductivity of Liþ in ILs that opens new scenarios for the design of Li batteries based on IL electrolytes competitive with polymer electrolytes. The research has been carried out by NMR using {1H-7Li}, 1 { H-19F} NOE correlations (HOESY), and pulsed field gradient spin-echo (PGSE) NMR. HOESY experiments provide details, on the atomic level, of cation-anion and cation-cation short contacts of all ionic species in solution. PGSE-NMR allows us to measure the individual self-diffusion coefficients D for Liþ, PYR14þ, and TFSI- species by collecting data in the 7 Li, 1H, and 19F frequency domains, respectively. Eventually, the temperature dependence of D for the observed species is measured and used to obtain the activation energy for the diffusion processes. The data are compared with the activation energy derived from viscosity measurements. The comparison of these data with those previously obtained for neat PYR14TFSI contributes to a deeper understanding of the Li environment and transport phenomena in the LiX-IL system. The molecular formula and atom numbering of PYR14þ and TFSI- are reported in Figure 1. For the sample 0.1 LiTFSI-0.9 PYR14TFSI as well as for neat PYR14TFSI, high-resolution 1H, 19F, and 7Li NMR spectra could be obtained at room temperature, despite the high viscosity of the solution (spectra reported in the Supporting Information). This feature is dramatically different from that reported by Fr€ omling et al.15 for the system 0.377 LiTFSI-0.623 PYR14TFSI, where broad bands without fine structure were observed for all nuclei (line-width 1500, 740, and 360 Hz for 1H, 19F, and 7Li NMR spectra, respectively). For our system, no chemical shift difference induced by LiTFSI dopant is observed in the 1 H NMR spectrum with respect to the undoped reference (Figure S1 in the Supporting Information). The 19F NMR spectrum (Figure S2 in the Supporting Information) displays a singlet at δ -81.5 (reference: external C6F6) with 11 Hz line width. The resonance frequency is shifted 34 Hz upfield with respect to the same signal of the undoped PYR14TFSI. The 7 Li NMR spectrum (Figure S3 in the Supporting Information) consists of a single sharp peak (line width 8 Hz) at 0.45 ppm (reference: external 1 M solution of LiCl in D2O set at 0.00 ppm). {1H-19F} NOE correlation experiments show the dipolar contacts between the protons of PYR14þ and the fluorine nuclei of TFSI-, whereas the {1H-7Li} NOE correlation spectrum gives information on the dipolar interaction between the Liþ and PYR14þ cations. The corresponding spectra are shown in Figure 2.


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Figure 2. Contour plots of {1H-19F} (top) and {1H-7Li} (bottom) HOESY spectra. The horizontal trace is the reference 1H NMR spectrum, whereas the vertical projections show the singlets due to CF3 groups (top) and Liþ species in the 19F and 7Li NMR spectra, respectively. Table 1. Experimental Self-Diffusion Coefficients D (m2 s-1), Ion Transference Number t at 305 K, Activation Energy Ea[D] (kJ/mol) for 0.1 LiTFSI - 0.9 PYR14TFSI and Neat PYR14TFSI ion

Dsola

Dneatb

tsola

PYR14þ 1.7 10-11 2.5 10-11 0.54 Liþ TFSI-

0.9 10-11 0.03 1.2 10-11 2.0 10-11 0.43

tneatb Ea[Dsol]a Ea[Dneat]b 0.53

36

31

0.47

46 37

31

a

This work. Transference number ti is defined as: ti = xiDi/ΣxiDi, with xi = ith ion molar fraction and Di = ith ion diffusion coefficient. Dsol and Dneat are referred to 0.1 LiTFSI-0.9 PYR14TFSI solution and neat PYR14TFSI, respectively. b From ref 12.

(1) A similar relationship is found when comparing the activation energies for the fluidity j (defined as η-1) in the case of the doped and undoped PYR14TFSI. (The corresponding Arrhenius plot is displayed in Figure S4 of the Supporting Information.) The slopes of the linear regressions provide Ea(j) values of 29 and 30 kJ/mol for the undoped and Li-doped PYR14TFSI, respectively. These data confirm that also in the case of a mixture of an IL with a Li salt sharing the same anion, the activation energy for the viscosity is 6 to 7 kJ mol-1 lower than that for the diffusivity of the ions, as already observed for the blends of ILs with the same PYR14þ cation and different (perfluoroalkylsulphonyl)imide anions: we have interpreted this surplus activation energy as an extra barrier for random motion of ions due to nanostructuring.13 Conversely, the measured activation energy for the selfdiffusion of Liþ is significantly higher than that of the

Figure 3. Arrhenius plot showing the temperature dependency of the ion diffusion coefficients in the ionic liquid solution 0.1 LiTFSI-0.9 PYR14TFSI. Regression lines for the neat liquid are also reported for comparison. Legend: black square: D(TFSI-) in Li-doped PYR14TFSI; red circle: D(PYR14þ) in Li-doped PYR14TFSI; pink star: D(Liþ) in Li-doped PYR14TFSI; green square: D(TFSI-) in undoped PYR14TFSI; blue circle: D(PYR14þ) in undoped PYR14TFSI.

Within the explored T range, D can be expressed by eq 1 DðTÞ ¼ D0 expð- Ea ½D=RTÞ PYR14þ

ð1Þ

-

In our sample, both and TFSI show similar Ea[D] values (36 and 37 kJ/mol, respectively). These values are slightly higher than the corresponding 31 kJ/mol found for neat PYR14TFSI.12 The graph of Figure 3 deserves two comments:


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coordination shell of Liþ, hopping of Liþ, formation of a novel coordination shell, and so on. This activation energy is then strictly related to the state of Liþ ions in the solution and to their environment on the atomic level and represents one of the first quantitative and experimental parameter related to the Liþ structurediffusion. In conclusion, the present study demonstrates via heteronuclear NOE that the molecular environment experienced by Liþ and TFSI- ions in the system 0.1 LiTFSI-0.9 PYR14TFSI is similar, thus giving direct evidence of the presence of strongly coordinated lithium-TFSI adducts of general structure [Li(TFSI)n](n-1)-. Diffusion measurements, based on a larger time scale compared with NOE, point out that the motion of Liþ is uncorrelated to those of both components of IL: the activation energy measured for Liþ diffusion is indeed significantly different from the corresponding values related to the PYR14TFSI components. We have interpreted this finding at the light of the “hopping” mechanism for Liþ diffusion, expected to pass through the steps of ligand exchange of coordination shells: the activation parameters presented here represent the overall energy required to pass the activation barrier for structure-diffusion of Liþ ions in ILs. From the point of view of the Li-ion battery technology, the enhanced Liþ diffusivity with temperature in IL electrolytes is amenable to be the starting point for novel and improved prototypes. Although this is a model system for the preliminary investigation of Liþ complexation and transport properties of ILlithium salts mixtures, it is worth noting that similar mixtures, showing much higher conductivities have been proved to be viable for use as electrolytes in lithium ion batteries.19 The experiments were conducted on PYR14TFSI IL synthesized through a procedure developed at ENEA and described in detail elsewhere.20 The water content was measured using the standard Karl Fischer method. The conductivity of neat PYR14TFSI and 0.1 LiTFSI-0.9 PYR14TFSI mixture was determined by a conductivity meter AMEL 160. The viscosity measurements were carried out using a rheometer (HAAKE RheoStress 600) located in the dry room (R.H. < 0.1% at 20 °C). The experimental set up and the measurements were carried out according to a protocol described elsewhere.13 The 1 H and 19F NMR spectra were recorded on a Bruker Avance 500 spectrometer. The 7Li experiments were carried out on a Bruker Avance 600. The NMR samples were prepared in dry room to avoid any contamination. The 0.1LiTFSI-0.9PYR14TFSI mixture was transferred in a 5 mm NMR tube equipped with a coaxial capillary containing DMSO-d6 for locking and immediately flame-sealed. Heteronuclear {1H-19F}HOESY experiments were acquired using the inverse-detected pulse sequence with 512 increments in the t1 dimension with 16 scans for each experiment. Qualitative spectra were acquired with mixing time of 20, 30, and 40 ms. Heteronuclear {1H-7Li}HOESY experiments were acquired with 512 increments in the t1 dimension with 16 scans for each experiment and a mixing time of 30 ms. We measured self-diffusion coefficients by pulsed field gradient spin-echo spectroscopy (PGSE-NMR) by using the bipolar pulse longitudinal eddy current delay (BPPLED) pulse sequence with gradients in the

Figure 4. Plot of Liþ transference number (blue square, left y axis) and Liþ conductivity (red square, right y axis) as a function of the temperature for 0.1 LiTFSI - 0.9 PYR14TFSI. Liþ conductivity is calculated by multiplying the Liþ transference number by the overall conductivity σ of the solution. (t and σ values are reported in the Supporting Information.)

other ionic species, Ea[D(Liþ)] = 46 kJ/mol. This finding indicates that the mechanism of Liþ diffusion is different and independent from the one taking place for PYR14þ and TFSI-. This conclusion has the important consequence of the existence of crossover (c-o) points, that is, temperature values defining the inversion of diffusivity orders of the ions: indeed, the order D(Liþ) > D(TFSI-) can be observed at T > 57 °C, whereas the condition D(Liþ) > (PYR14þ) is expected to occur at 90 °C because it can be extrapolated from the regression lines of the Arrhenius plots. The possible applicative interest of this finding for Li-ion batteries relies on the fact that the contribution of the electrochemically active species, for example, Liþ cation, to the observed conductivity (σ, mS cm-1) grows with temperature faster than those of PYRþ and TFSI-. Actually, in the range 27-67 °C, σ(Liþ) shows a six-fold growth (0.045 to 0.275 mS/cm), whereas σ(PYRþ) and σ(TFSI-) undergo three- and four-fold increases, respectively (full list of transference numbers and ion conductivity values reported in Table T1 of the Supporting Information). The plots of t(Liþ) and Liþ conductivity as function of T are shown in Figure 4. It is worth noting that the enhancement of Liþ diffusivity with temperature is an intrinsic feature of the LiXIL system here considered and can be compared with the enhancement reported by Byrne et al. in LiTFSIPYR13TFSI by action of a zwitterionic additive.18 (2) It is now commonly accepted that the Liþ diffusion occurs mainly through the structure-diffusion mechanism and that the vehicular mechanism of diffusion, namely, the diffusion of [Li(TFSI)n](n-1)- clusters, provides only a minor contribution to the overall diffusivity.7 Under this assumption, the activation energy for the diffusivity of Liþ measured and reported here assumes the significance of the overall energy barrier associated with a rather complex set of phenomena that include disruption of the solvation/


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z direction from a 53 G cm-1 gradient amplifier. The duration of the magnetic field pulse gradients (δ) and the diffusion times (Δ) were optimized for each sample to obtain complete dephasing of the signals with the maximum gradient strength. In each experiment, 16 spectra (1H and 19F) and 32 spectra (7Li) with 16K points were collected. δ values were in the 3-6 ms range, whereas the Δ values were in the 0.3 to 0.6 s range for 1H and 19F and 0.8 to 1 s range for 7Li experiments, respectively. Careful attention to artifacts21 due to convection at long Δ and at high T was paid: all experiments on Liþ frequency domains were repeated by using the sequence proposed by Zhang et al.22 for the removal of convection artifacts. The temperature was set and controlled with an air flow of 535 L h-1 to avoid any temperature fluctuations within the sample. Variable temperature experiments were performed changing the temperature from 300 to 340 K in steps of 5 K. The precision of the measured diffusion coefficient is within 10%.

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SUPPORTING INFORMATION AVAILABLE One-dimensional

NMR spectra (1H, 19F, and 7Li), Arrhenius plot of fluidity, tables of tranference numbers and conductivity at different temperatures, and plot of NMR intensity versus field gradient. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION Corresponding Author:

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*To whom correspondence should be addressed. E-mail: [email protected].

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ACKNOWLEDGMENT We would like to thank Dr. M. Kunze and Dr. S. Jeong for technical assistance in PGSE-NMR measurements. S.P. and E.R. thank European Commission (FP7 project ORION contract no. NMP3-LA-2009_229036) and Cariplo Foundation (project no. 2008.2235), respectively, for financial support. A.M. thanks Prof. Carlo Cavallotti for helpful discussions.

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Molecular Environment and Enhanced Diffusivity of Li+ Ions in Lithium

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