J. Phys. Chem. C 2010, 114, 21775–21785
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Ionic Liquid Electrolyte for Lithium Metal Batteries: Physical, Electrochemical, and Interfacial Studies of N-Methyl-N-butylmorpholinium Bis(fluorosulfonyl)imide George H. Lane,†,‡ Paul M. Bayley,§ Bronya R. Clare,† Adam S. Best,*,‡ Douglas R. MacFarlane,† Maria Forsyth,§ and Anthony F. Hollenkamp‡ School of Chemistry, Department of Materials Engineering, Monash UniVersity, Clayton, Victoria 3800, Australia., CSIRO Energy Technology, BayView AVenue, Clayton, Victoria 3168, Australia ReceiVed: June 15, 2010; ReVised Manuscript ReceiVed: August 6, 2010
The ionic liquid (IL) N-methyl-N-butylmorpholinium bis(fluorosulfonyl)imide (C4mmor FSI) is examined from physical and electrochemical perspectives. Pulsed field gradient NMR spectroscopy shows that ion diffusivities are low compared with similar, non-ethereal ILs. Ionicity values indicate that above room temperature, less than 50% of ions contribute to conductivity. Lithium cycling in symmetrical cells using a C4mmor FSI-based electrolyte is best demonstrated at elevated temperatures. Specific capacities of 130 mAh g-1 are achieved in a Li-LiFePO4 battery at 85 °C. FT-IR spectroscopic investigations of lithium electrodes suggest the presence of alkoxide species in the solid electrolyte interphase (SEI), implying a ring-opening reaction of C4mmor with lithium metal. In contrast, the SEI derived from N-methyl-N-propylpiperidinium FSI lacks the alkoxide signature but shows signs of alkyl unsaturation, and the activation energy for Li+ transport through this SEI is slightly lower than that for the C4mmorderived SEI. Our detailed findings give insight into the capabilities and limitations of rechargeable lithium metal batteries utilizing a C4mmor FSI electrolyte. 1. Introduction A variety of morpholinium ionic liquids (ILs) have been reported in the literature. N-methyl-N-ethylmorpholinium bis(trifluoromethanesulfonyl)imide (C2mmor NTf2) and N-methylN-butylmorpholinium NTf2 (C4mmor NTf2) were synthesized and characterized by Lee et al.1 The melting points of these compounds are reported to be just above room temperature, at 29.2 and 28.7 °C respectively, and the compounds were found to be electrochemically stable within the region -2.5 to 2 V (vs Ag wire).1 The addition of an alcohol functional group to the ethyl group on C2mmor NTf2 did not change the cathodic stability, and a high anodic stability was reported,2 but unfortunately, ionic conductivity was reduced compared with the alcohol group free derivative.1,2 A substantial improvement in anodic stability has been reported for morpholinium ILs when a PF6 or BF4 anion is used, resulting in electrochemical windows >6 V, and oxidative stability beyond 3.5 V (vs Ag wire);3 however, the use of these anions also increases the melting point of the IL. A number of ILs incorporating an aliphatic ether group as a part of the IL cation have been demonstrated in lithium metal batteries,4-8 but no examples exist for cyclic ether cation ILs. For aliphatic ether cation type ILs, LiFePO4 appears to be a suitable cathode material. Using N,N-diethyl-N-methyl-N-2methoxyethylammonium NTf2, Kobayashi et al.7 demonstrated 200 cycles with only ∼6% capacity fade in a lithium metal-LiFePO4 battery. This is a much slower capacity fade compared with when the same IL is used in batteries utilizing different cathodes.4-6,8 * Corresponding author. E-mail:
[email protected]. † School of Chemistry, Monash University. ‡ CSIRO Energy Technology. § Department of Materials Engineering, Monash University.
Because LiFePO4 has a relatively low lithium ion insertion voltage (∼3.4 V vs Li/Li+), the use of the anodic stability promoting anions for morpholinium ILs, PF6 and BF4, is notionally not required in LiFePO4 type batteries. It is therefore logical to use a fluidity-promoting anion such as NTf2 or bis(fluorosulfonyl)imide (FSI). Although fluidity may be raised and the melting point lowered by an agreeable anion such as FSI, conductivity may not be concomitantly improved. A proportionally low conductivity, with respect to fluidity, may result from the failure of individual ions to move as “free” independent entities. Information about the true ionic state of an IL can be obtained by examining the IL’s various physical properties. These properties include viscosity, conductivity, and the self-diffusion of its constituent ions. Analysis of the diffusion data within the context of the Nernst-Einstein equation (eq 1) is a particularly powerful investigative approach. Calculating the molar conductivity (ΛNMR) from diffusion measurements (eq 1) and comparing it to the molar conductivity directly calculated from electrochemical impedance spectroscopy (Λimp) and density measurements gives rise to the ionicity ratio (Λimp/ΛNMR).9 This ratio quantifies the discrepancy between mass and charge transport and can be related to a variety of physicochemical properties and solvent scales.9-14 For IL systems, mass transport is characteristically greater than charge transport, and hence, the ionicity ratio will invariably be less than unity. When the ratio is very small, almost none of the ions are contributing to conductivity, whereas a ratio of unity implies that they all contribute. Perhaps the most important information obtained from this ratio is an indication of the degree of association between the ions, a parameter that we have previously investigated when an IL is doped with a molecular solvent.15 The cyclic ether functionalized cation makes the new IL an attractive subject for a detailed physical characterization.
10.1021/jp1054809 2010 American Chemical Society Published on Web 11/11/2010
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J. Phys. Chem. C, Vol. 114, No. 49, 2010
ΛNMR
NAe2 ) (D+ + D-) kT
Lane et al.
(1)
The reaction of the cyclic ether tetrahydrofuran (THF), a solvent previously studied for use in lithium battery electrolytes, with lithium metal results in the cleaving of the C-O ether bond and the formation of lithium butoxide on the lithium metal surface.16,17 A similar process potentially occurs for ether moieties of IL cations. The products of the reaction of a cyclic ether type cation with lithium metal will create a unique solid electrolyte interphase (SEI)18 on the lithium metal surface, which will affect lithium cycling behavior in that system. Such an SEI may contain species similar to the THF-derived SEI and presents an interesting target for investigation. In this study, we characterize N-methyl-N-butylmorpholinium bis(fluorosulfonyl)imide (C4mmor FSI), the ion structures of which are shown in Figure 1 (together with the non ethereal C3mpip cation). Li FSI is added to C4mmor FSI to form a lithium battery electrolyte, and lithium cycling in the new IL is examined by various means. FT-IR spectroscopy is used to analyze the SEI on lithium electrodes removed from cycled cells, providing valuable information about the chemical groups within the SEI, including the surface effect of the C4mmor ether functionality. 2. Experimental Section 2.1: Synthesis of C4mmor FSI. N-Butyl-N-methylmorpholinium Bromide. 4-Methylmorpholine (31.5 g, 0.312 mol, Aldrich) and 1-bromobutane (41.1 g, 0.300 mol, Aldrich) were stirred at 70 °C with a reflux condenser attached for 24 h. The volatiles were removed in vacuo, and the resulting solid was crystallized from 1-propanol/THF and recrystallized from 1-propanol/n-hexanes. The resulting white solid was dried by azeotropic distillation over toluene to give the final product (47.7 g, 67.1%). N-Butyl-N-methylmorpholium Bis(fluorosulfonyl)imide. NButyl-N-methylmorpholinium bromide (2.44 g, 10.3 mmol) in 30 mL of dry acetonitrile was added to potassium bis(fluorosulfonyl)imide (2.26 g, 10.3 mmol) in 10 mL of dry acetonitrile. The resulting mixture was centrifuged, and the decanted liquid was passed through a column of activated charcoal (SAJ first grade, Aldrich) then a column of acidic alumina (Brockmann I, Aldrich). The volatiles were removed in vacuo to give the colorless liquid product (2.80 g, 80.7%). Varian VISTA ICPOES analysis: K content ) 4.5 ppm, Br content < 50 ppm. 2.2. Pulsed Field Gradient Diffusion NMR. Pulsed-field gradient diffusion measurements were performed on a Bruker 300 MHz Ultrashield with Avance I console utilizing a Diff30 diffusion probe and GREAT60 amplifier. The various constituents of the samples were measured using the 1H and 19F nuclei for the cation and anion, respectively. For each experiment, the recycle delay was set to >5 × T1. For each measurement the gradient pulse (δ) and diffusion time (∆) were optimized, with gradient strengths (g) up to 1700 G cm-1 applied. Due to the conductivity of the samples, the probe was tuned and matched at each temperature with the π/2 pulse checked and optimized, if required, before any experiments were performed. The sample was filled into 5 mm diameter Schott E NMR tubes to a height of 40 mm in an N2 atmosphere (
