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J. Phys. Chem. B 2008, 112, 2102-2109
Transport Coefficients, Raman Spectroscopy, and Computer Simulation of Lithium Salt Solutions in an Ionic Liquid Marcelo J. Monteiro,† Fernanda F. C. Bazito,‡ Leonardo J. A. Siqueira,† Mauro C. C. Ribeiro,† and Roberto M. Torresi*,† Instituto de Quı´mica, UniVersidade de Sa˜ o Paulo, CP 26077, 05513-970, Sa˜ o Paulo, SP, Brazil, and Departamento de Cieˆ ncias Exatas e da Terra, UniVersidade Federal de Sa˜ o Paulo, Diadema, SP, Brazil ReceiVed: September 1, 2007; In Final Form: NoVember 15, 2007
Lithium salt solutions of Li(CF3SO2)2N, LiTFSI, in a room-temperature ionic liquid (RTIL), 1-butyl-2,3dimethyl-imidazolium cation, BMMI, and the (CF3SO2)2N-, bis(trifluoromethanesulfonyl)imide anion, [BMMI][TFSI], were prepared in different concentrations. Thermal properties, density, viscosity, ionic conductivity, and self-diffusion coefficients were determined at different temperatures for pure [BMMI][TFSI] and the lithium solutions. Raman spectroscopy measurements and computer simulations were also carried out in order to understand the microscopic origin of the observed changes in transport coefficients. Slopes of Walden plots for conductivity and fluidity, and the ratio between the actual conductivity and the Nernst-Einstein estimate for conductivity, decrease with increasing LiTFSI content. All of these studies indicated the formation of aggregates of different chemical nature, as it is corroborated by the Raman spectra. In addition, molecular dynamics (MD) simulations showed that the coordination of Li+ by oxygen atoms of TFSI anions changes with Li+ concentration producing a remarkable change of the RTIL structure with a concomitant reduction of diffusion coefficients of all species in the solutions.
I. Introduction Rechargeable lithium batteries for portable electronics such as cell phones and laptop computers are under continuous development. Organic solvent-based electrolytes have been widely used in these cells, promoting safety risks due to their flammability and volatility. Thereby, to expand their use in vehicles and other larger equipments, and to make lithium batteries safer, the choice of the electrolyte is crucial and depends on several criteria, such as safety and environmental compatibility. Another motivating reason for alternative solvents is to replace conventional carbon anodes by lithium metal in order to achieve the highest possible energy density. It is worth mentioning that metallic lithium is not chemically compatible with these organic electrolytes, resulting in poor battery performance and formation of dendritic deposits on the metal.1-3 Room-temperature ionic liquids, RTIL, containing 1,3dialkylimidazolium cations and the bis(trifluoromethanesulfonyl) imide anion, (CF3SO2)2N-, TFSI, have been widely tested as solvents in lithium batteries. However, the presence of an acidic proton at the carbon atom C2 of the imidazolium ring (see the schematic structure with atom numbering in Figure 1) precludes their use because of instability toward Li.4-7 Introducing an alkyl group at the C2 position improves electrochemical stability and produces attractive electrolytes for lithium batteries.4,7,8-13 The TFSI anion is usually chosen because RTILs based on this anion have lower viscosity, higher molar conductivity, and higher electrochemical stability. In this work, 1-butyl-2,3-dimethyl-imidazolium bis(trifluoromethanesulfonyl) imide, [BMMI][TFSI], was synthesized and its lithium salt solutions using LiTFSI were also prepared and * Corresponding author. † Universidade de Sa ˜ o Paulo. ‡ Universidade Federal de Sa ˜ o Paulo.
Figure 1. Schematic structure of the 1-butyl-2,3-dimethyl-imidazolium cation, BMMI, and the bis(trifluoromethanesulfonyl) imide anion, TFSI. Atom numbering is shown for further reference.
investigated. Herein, fundamental characterization is reported, including density, viscosity, thermal properties, ionic conductivity, and ionic diffusion coefficients as a function of temperature and Li+ concentration. The interplay between dynamical properties and local structures is emphasized, with the latter being revealed by Raman spectroscopy. A further detailed microscopic picture of dynamical and structural properties of LiTFSI solutions in [BMMI][TFSI] was obtained by performing molecular dynamic simulations of these materials. II. Experimental Section Synthesis of [BMMI][TFSI] followed previous reports.7,14 LiTFSI (Aldrich) was dissolved in [BMMI][TFSI] at different molar concentrations under dry atmosphere at 50 °C. The LiTFSI molar fraction was varied from 0.03 to 0.38. From now on, these solutions will be denoted as [Li][BMMI][TFSI]. In order to avoid water contamination, all samples were prepared, stored, and sealed in an argon atmosphere glovebox Labmaster 130, with H2O and O2 concentration below 1 ppm. Density measurements were performed with a thermoregulated digital densimeter DMA 40 (Anton Paar K. G.) at different temperatures. Viscosity was measured by using a
10.1021/jp077026y CCC: $40.75 © 2008 American Chemical Society Published on Web 01/26/2008
Lithium Salt Solutions in an Ionic Liquid
J. Phys. Chem. B, Vol. 112, No. 7, 2008 2103
Cannon-Fenske (Scott-Gerate) viscometer in a temperaturecontrolled bath within the range 25 < T < 75 °C. Differential scanning calorimetry (DSC) was carried out in a T. A. Instruments Q.10 DSC coupled with T. A. refrigerated cooling system (RCS) interfaced to the Thermal Analyst 2000 software under nitrogen atmosphere. Samples for DSC measurements were sealed in an aluminum pan. The samples were first cooled (20 °C min-1) to -80 °C, kept at this temperature for 1 h, and then heated up to 100 °C at the same 20 °C min-1 rate. Ionic conductivity was measured by the impedance method with the AUTOLAB PGSTAT 30 (Eco Chemie) equipment within the frequency range 0.1-100 kHz. The cell constant was calibrated with KCl standard aqueous solution. Conductivity measurements were performed inside the glovebox. Diffusion coefficients, D, were measured by pulsed gradient spin echo nuclear magnetic resonance (PGSE-NMR) in a Varian INOVA 300 spectrometer equipped with a 5 mm Indirect Detection Probe (22 G cm-1 max), calibrated with water, D ) 2.299 × 10-9 m2 s-1 at 298 K. A stimulated spin-echo pulse sequence, that is, 90°-τ1-90°-τ2-90°-τ1-acquisition, incorporating a gradient pulse in each τ1 period was used. The echo attenuation was fit by the following equation:
Figure 2. Density of ionic liquids: (b) pure [BMMI][TFSI], (0) [Li][BMMI][TFSI] (xLi+ ) 0.24), and (O) [Li][BMMI][TFSI] (xLi+ ) 0.38). Lines are second-order polynomial fit to experimental data.
where g is the gradient strength, Io is the echo amplitude when g tends to zero, γ is the gyromagnetic ratio, δ is the duration of the gradient pulse, and ∆ is the time between gradient pulses. Experiments were conducted with 50 < ∆
