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J. Phys. Chem. B 2010, 114, 894–903
Structure-Properties Relationships of Lithium Electrolytes Based on Ionic Liquid My Loan Phung Le,†,‡ Fannie Alloin,*,† Pierre Strobel,‡ Jean-Claude Lepreˆtre,† Carlos Pe´rez del Valle,§ and Patrick Judeinstein⊥ LEPMI-Grenoble-INP/UJF/CNRS, BP75, 38402 Saint Martin d’He`res, France, Institut Ne´el, 38054, Grenoble, France, DCM-UJF-CNRS, BP53, 38041 Grenoble, France, and ICMMO, CNRS 8182, Baˆtiment 410, UniVersite´ Paris-Sud, 91405 Orsay Cedex, France ReceiVed: October 15, 2009; ReVised Manuscript ReceiVed: NoVember 23, 2009
Low-melting ionic liquid, IL, based on small aliphatic quaternary ammonium cations ([R1R2R3NR]+, where R1, R2, R3 ) CH3 or C2H5, R ) C3H7, C4H9, C6H13, C8H17, CF3C3H6) and imide anion were prepared and characterized. The physicochemical and electrochemical properties of these ILs, including melting point, glass transition, and degradation temperatures; viscosity; density; ionic conductivity; diffusion coefficient; and electrochemical stability, were determined. Heteronuclear Overhauser NMR spectroscopy experiments were also performed to point out the presence of pair correlation between the different moieties. The LiTFSI addition effect on the IL properties was studied with the same methodology. Some nanoscale organization with segregation of polar and apolar domains was observed. ILs with small alkyl chain length or fluorinated ammonium exhibit very high electrochemical stability in oxidation. 1. Introduction Great interest has been taken in room temperature ionic liquids, ILs, due to their potential as new media for organic, catalytic, and electrochemical reactions.1,2 The use of ILs as electrolytes for various electrochemical devices such as Li-ion batteries,3-5 dye-sensitized solar cells,6,7 and electrochromic devices has been investigated. Given their interesting properties, such as nonvolatility, nonflammability, high thermal stability, wide liquid range and a wide electrochemical window, with very high anodic stability,8 ILs are proposed as electrolytes for highpotential lithium batteries.3 Currently, most studies on ILs are dealing with imidazolium derivatives due to low viscosity and good ionic conductivity. However, the electrochemical stability of imidazolium-based ILs seems insufficient in lithium batteries.9,10 Matsumoto et al.9,10 show that quaternary ammonium imide salts have been the most attractive ILs due to relative better stability of the salt toward lithium metal. Furthermore, Sato et al.11 improved the cyclability of mesocarbon microbead graphitized carbon with the addition of passivating additives in IL-based electrolyte. To obtain low-melting and low-viscosity ILs, the main anion used is the imide one (bis(trifluoromethane sulfonyl)imide, TFSI-). Indeed, this fluorinated anion presents chemical and electrochemical stability, low symmetry, high flexibility, and a weakly coordinating nature. To further explore the potential applications of quaternary ammonium (QA)-based IL, we present in this paper the results of our investigations into their physicochemical properties. Herein, fundamental characterization is reported, including thermal properties, viscosity, molar conductivity, self-diffusion coefficient, and electrochemical stability. Highest occupied molecular orbital values were calculated to explain the evolution of oxidation potential versus the alkyl chain length. The effect * Corresponding author. Fax: 04 76 82 67 77. E-mail: fannie.alloin@ lepmi.inpg.fr. † LEPMI-Grenoble-INP/UJF/CNRS. ‡ Institut Ne´el. § DCM-UJF-CNRS. ⊥ Universite´ Paris-Sud.
of LiTFSI salt addition in the ILs properties (i.e. viscosity, conductivity and diffusion coefficient) was also investigated. Heteronuclear Overhauser NMR spectroscopy experiments (HOESY) were also performed to point out the presence of pair correlation between the different moieties (ammonium, lithium, and anion) and to give insight into local order inside these liquid electrolytes. 2. Experimental section 2.1. Synthesis. Alkyl ammonium bis(trifluoromethane sulfonyl) imide ionic liquid, ILs, were synthesized by metathesis reaction of a freshly prepared halide salt of the ammonium with different alkyl chains (Scheme 1). The details of the synthesis and the NMR characterization of the compounds are given in the Supporting Information. The water content of all the ILs, determined by Karl Fischer titration, was below 20 ppm. The halide ILs were removed using an alumina column. The halide amount, determined by elementary analyses or electrochemical titration, was below 10 ppm. Aluminum titration was carried out by ICP AES (λ ) 396.1 and 309.2 nm; plasma temperature, 1550 °C). Prior analysis, ionic liquid was mineralized in HNO3/H2SO4 mixture at 150 °C, over 3 h. The aluminum amount was lower than 20 ppm, between12 and 18 ppm. The ILs were stored in a glovebox ([H2O] < 3 ppm). 2.2. Measurements. Thermal Measurements. DSC tests were performed using a TA Instrument DSC 2920 CE. Samples of 10 mg were sealed in aluminum pans in a glovebox. Each sample was heated from -120 to 100 °C at a heating rate of 10 °C min-1. The melting temperature, Tm, was taken at the top of the melting peak. The Tg was taken at the midpoint of the transition. Thermogravimetric measurements were carried out with a Netzsch STA409 thermal analyzer. A few milligrams of the sample was heated from room temperature up to 500 at 10 °C/ min under helium flow. The degradation temperature corresponds to a mass loss of 5 wt % (Td).
10.1021/jp9098842 2010 American Chemical Society Published on Web 12/29/2009
Lithium Electrolytes Based on Ionic Liquid
J. Phys. Chem. B, Vol. 114, No. 2, 2010 895
SCHEME 1: Synthesis and Preparation of IL TFSI Salts
The density measurement was performed in the glovebox by weighing a defined volume of ILs using an adapted pipet. All the measurements were performed several times with good accuracy. Viscosity. The viscosity of the ILs was measured on the ARG2 rheometer from a TA Instrument. The ARG2 is a stresscontrolled rheometer, which was equipped with a Peltier temperature-controlled plate and specific chamber to perform experiments under nitrogen in the temperature range -5 to 60 °C. The viscosity of the samples was measured using 40-mmdiameter cylindrical plates with a gap of ∼1000 µm. Raman Spectroscopy. Raman spectra were collected with a Renishaw InVia spectrometer. The light source was a 785 nm laser diode, and the Raman photons were collected on a Peltiercooled CCD detector. The Rayleigh line was suppressed by two dielectric filters. A micro Raman configuration was used. The laser beam was focused at the surface of the sample as a line of about 20 µm in length and 2 µm in width. For temperature measurements, between -50 and 100 °C, a lab-designed cell was used.12 Electrochemical InWestigations. Conductivities were determined by electrochemical impedance spectroscopy using an HP 4192A impedance analyzer in the frequency range 5 Hz-13 MHz. The samples were placed in a dip-type glass cell with two Pt electrodes fixed at a constant distance under argon, and measurements were performed from -10 to 110 °C. The temperature was equilibrated for 1 h before each measurement. The cell constant was determined by using a 0.1 M KCl solution. Cyclic voltammetry (CV) was measured at ambient temperature in the glovebox. The counter electrode was a Pt wire, and the working electrode was a Pt microelectrode with a diameter of 125 µm. The scanning rate used was 5 mV/s. The reference electrode was a Ag wire in AgNO3 10 mM in acetonitrile + 0.1 M tetrabutylammonium. Leaking of the reference solution
was prevented by using an additional compartment containing the IL solution. Potential can be converted to the Li/Li+ scale by adding 3.548 V.13 HOMO Calculations. Highest occupied molecular orbital (HOMO) values were calculated. The geometry of the quaternary ammonium cations was determined and optimized using density functional theory (DFT) methods at the B3LYP/6311G(2d) level of theory. B3LYP is a combination of two methods: B314 and LYP.15 All calculations were performed with the Gaussian03 suite of programs.16 NMR InWestigations. PFG-NMR diffusion measurements were carried out on a 9.4 T Bruker Avance 400 NMR spectrometer equipped with a Bruker 5 mm broadband probe with a z-axis gradient and a temperature controller, allowing experiments to be performed from RT up to 150 °C (stability and accuracy, 0.2 °C). NMR resonance frequencies were 400.13, 376.50, and 155.51 MHz for 1H, 19F, and 7Li nuclei, respectively. The self-diffusion measurements were performed with the pulsed field gradient stimulated echo and LED sequence using two spoil gradients (PFG NMR).17 The magnitude of the pulsed field gradient was varied between 0 and 40 G cm-1; the diffusion time, ∆, between two pulses was fixed at 100 ms; and the gradient pulse duration, δ, was set between 3 and 18 ms, depending on the diffusion coefficient of the mobile species. This allowed us to observe the attenuation of spin echo amplitude over a range of at least 2 decades, leading to good accuracy (
