J. Phys. Chem. C 2010, 114, 20569–20576
20569
Undoing Lithium Ion Association in Ionic Liquids through the Complexation by Oligoethers† Paul M. Bayley,‡ G. H. Lane,§,| L. J. Lyons,⊥ D. R. MacFarlane,§ and M. Forsyth*,‡ Department of Materials Engineering, Monash UniVersity, Clayton, Victoria 3800, Australia, School of Chemistry, Monash UniVersity, Clayton, Victoria 3800, Australia, DiVision of Energy Technology, CSIRO, Clayton, Victoria 3168, Australia, and Department of Chemistry, Grinnell College, Grinnell, Iowa ReceiVed: May 31, 2010; ReVised Manuscript ReceiVed: August 3, 2010
Tetraglyme (TG) and the recently developed trimethylsilyl capped analogue (1NM3) when used as additives in a N-methyl-N-propylpyrrolidinium bis(trifluoromethylsulfonyl) amide [C3mpyr][NTf2]/0.65 M LiNTf2 electrolyte have been shown to dramatically enhance the transport properties of this electrolyte. In fact, at a concentration of 20 mol % tetraglyme (leading to a ratio of ∼1:1 ether molecule per lithium ion), viscosity, conductivity, and the diffusion coefficients of the C3mpyr+ and NTf2- are practically reinstated to the values observed in the absence of lithium, thereby negating the structuring effects of the lithium ion. The 7Li T1 relaxation times also indicate that these additives strongly interact with the lithium ions. Furthermore, although TG has twice the viscosity of 1NM3, the greatest improvement in transport properties was observed for TG. Introduction Ionic liquids (ILs) possess a range of properties that make them particularly well suited as electrolytes for secondary lithium metal batteries, such as negligible vapor pressure, low flammability, wide electrochemical window and reasonably high ionic conductivity, to name a few.1 The two principle factors limiting the commercial use of this unique class of materials as electrolytes are their transport properties and cost. The use of various molecular additives at low concentrations can be beneficial from several perspectives: encouraging the formation of a stable solid electrolyte interphase (SEI),2 improving the transport number of the target ion (e.g., lithium),3 and reducing the overall viscosity of the electrolyte, which can simplify device processing. It is well-known that lithium ions in NTf2--based ILs are strongly coordinated by the oxygen atoms of the anion to form contact ion pairs, triples and higher order clusters.4 It has been demonstrated that certain organic solvents can diminish these aggregates to some extent.5 Pyrrolidinium-based electrolytes have been shown to be promising candidates for lithium battery applications6 with enhanced electrolyte stability as well as formation of suitable SEI layers,7 yet application of these is still limited to low rates of operation due to mass transport limitations. The addition of additives such as the N-methyl-N-(butyl sulfonate) pyrrolidinium zwitterion has been shown to improve the electrochemical performance by almost 100%, primarily due to an enhancement of the lithium ion diffusivity.8 Various molecular additives have also been studied, with some improvements in the lithium ion diffusivity; however, most have been found to also increase ionic association.3,9 Recently, we have shown that 10-20 mol % of low molecular weight diluents can also significantly enhance the lithium ion diffusivity in a [C3mpyr][NTf2]-based electrolyte.3 Moreover, the effective transport number of the lithium †
Part of the “Mark A. Ratner Festschrift”. * Corresponding author. E-mail:
[email protected]. Department of Materials Engineering, Monash University. § School of Chemistry, Monash University. | CSIRO. ⊥ Grinnell College. ‡
ion was enhanced, since the diffusivity of the C3mpyr+ cation remained unchanged. Our previous work showed that the diluent having the highest donor number had the greatest enhancement/ effect on the lithium ion diffusivity, independent of viscosity. Interestingly, the addition of the diluent appears to decrease the overall ionicity (i.e., enhance ion association) in the ionic liquid,3,10 which suggests the persistence of localized IL structures (although these will be dynamic).10 Kim et al. showed that such low additions of diluent were indeed practical for device applications with the desirable low flammability of ionic liquids being predominantly retained.11 Polyether-based electrolytes have been of interest for several decades in terms of their applications in lithium battery devices due to the high solubility of lithium salts in these solvents resulting from strong coordinating/complexing ability.12 Glymes and the oligoether analogues have been investigated in polymer gel electrolytes as well as in model systems and have been shown to strongly interact with lithium ions.12b-f,13 In the case of glymes added to conventional aprotic electrolytes, the lithium ions were found to preferentially interact with the oxygen atoms of the glyme molecule. Such electrolytes showed higher conductivity and lithium cycling efficiency than those without the glyme additive.12d In some cases, ether groups have been covalently attached to ammonium cations in IL systems for improved lithium battery electrolytes.14 More recent work has used mono- to tetraglyme as additives in a [C4mpyr][NTf2]/ LiNTf2 electrolyte, with their use found to decrease the interfacial resistance on a tin electrode.13c In this work, the addition of two oligoethers, tetraglyme (TG) and a trimethylsilyl capped analogue (1NM3), to an ionic liquid electrolyte has been explored to determine the influence of these on the transport and ionicity of these systems. Variable temperature multinuclear NMR (including relaxation, PFG diffusion), conductivity, viscosity, and cyclic voltammetry have been undertaken, and the data have been discussed in terms of changes in ionic association in these electrolytes and improvements in ion transport.
10.1021/jp104957j 2010 American Chemical Society Published on Web 08/24/2010
20570
J. Phys. Chem. C, Vol. 114, No. 48, 2010
Bayley et al.
Figure 1. Chemical structures of the IL [C3mpyr][NTf2] and two diluents.
Method The ionic liquid N-methyl-N-propylpyrrolidinium bis(trifluoro-methylsulfonyl)amide ([C3mpyr][NTf2]) used throughout this work was prepared as described earlier.3 The lithium salt LiNTf2 (3 M Fluororad) was used as received after drying in vacuo at 50 °C for 7 days. TG (Aldrich) was stored with sodium metal for several days, then distilled at reduced pressure with the first and last fractions discarded.15 The clear liquid was then stored over 4 Å molecular sieves (Aldrich) for several days before a Karl Fisher titration indicated a water content below the detection limit. A cyclic voltamogram of TG with 0.5 mol kg-1 LiNTf2 against Li|Li+ conducted in an argon atmosphere with a sweep rate of 20 mV s-1 indicated a wide electrochemical window (∼-0.5-5.5 V) and, hence, reasonable purity. The preparation of the trimethylsilyl analogue 1NM3 is described elsewhere.16 The material was used as received and was stored over 4 Å molecular sieves (Aldrich) for several days before use. Karl Fisher titration indicated a water content below the detection limit. Chemical structures of the IL and diluents are illustrated in Figure 1. All samples were stored and prepared in a nitrogen atmosphere (5 × T1. The temperature of the probe was calibrated with 99.9% methanol (Alrich) packed and sealed in a N2 atmosphere ( [C3mpyr][NTf2] + LiNTf2. This order makes these additives quite unique in that the ionicity ratio is increased, upon their addition, over the [C3mpyr][NTf2] + LiNTf2 sample. Previous studies on the addition of molecular additives to ILs have shown they cause an increase in association (decrease in ionicity ratio).3,9,10,27 The variable temperature 7Li spin-lattice (T1) relaxation times are given in Figure 10 for the lithium-containing samples, showing the change in environment seen by the lithium ions upon the addition of diluent. The T1 data for the 1H and 19F were very similar, regardless of the diluent or lithium salt being present, due to their low concentration with respect to the C3mpyr+ or NTf2- and are excluded from this work. Chemical shift measurements were performed at room temperature on the 7 Li nuclei using 1 M LiCl as an external reference. Although a shift of -1.3 ppm was obtained for the [C3mpyr][NTf2] +
LiNTf2 sample, both diluent-containing samples were ∼-1 ppm. The chemical shift values, although useful, appear to be too insensitive to distinguish the differences between the samples containing these additives. However, the minimum in the T1 is a very sensitive measure of the local chemical environment of the nuclei, with its position on the temperature axis an indication of mobility, whereas its position on the time axis is an indication of the structural environment. Although the minimum was not reached for either of the diluent-containing samples, the important effect of the additive on the lithium ions can still be postulated. Although the 1NM3-containing sample clearly has a much lower temperature minimum (as reflected in the enhancement of the 7Li diffusion coefficient), it appears to be leveling off at about the same relaxation time as the minimum for the [C3mpyr][NTf2] + LiNTf2 sample. This is further indication that the lithium ions are less strongly interacting with the 1NM3 molecule and the resulting enhancement of the physical properties is primarily due to the overall decrease in the viscosity. Conversely, the TG-containing sample appears linear over the measured temperature range without any indication it is leveling off at a minimum. Not only is the mobility of the lithium ions significantly increased in this case but also the data implies a significant change in its environment. The decrease in the value of the T1 at the minimum indicates that a larger electric field gradient exists at the Li+ center in this case. This could reflect a stronger and longer-lived coordination environment in the TG sample arising from the larger number of oxygen atoms available for coordination of the lithium by TG (5 atoms), whereas the 1NM3 has as few as 3 oxygen atoms due to steric hindrance by the trimethylsilyl group. Lithium cycling in both the 1NM3 and TG electrolytes was examined at ambient temperature using cyclic voltammetry. In both instances, the negative limit was set at a point where, on the first scan, the Li+ reduction peak was resolved and a runaway breakdown current was avoided (-4.4 V (vs ref) and -4.3 V (vs ref) for TG- and 1NM3-inclusive samples, respectively). Figure 11a shows the cyclic voltammograms for the TGinclusive electrolyte with 0.65 M LiNTf2. Close inspection of
Undoing Li Ion Association in ILs scan 1 shows that the Li+ reduction peak is resolved, and has a peak current density of 11.9 mA cm-2. Peak Li+ reduction current densities observed in electrolytes that are identical except for the species of diluent (toluene, THF, VC, or EC) have been observed to be significantly lower (
