Joseph Grondin

Spectroscopic identification of the lithium ion transporting species in LiTFSI-doped ionic liquids.

The solvation of the lithium ion in LiTFSI-doped ionic liquids based on alkyl-substituted imidazolium cations and bis(trifluoromethanesulfonyl)imide anions (TFSI-) was investigated by infrared and Raman spectroscopies. The spectral changes occurring for some TFSI- vibrations sensitive to the lithium coordination were analyzed with the help of DFT calculations. In addition, the vibrations of the lithium ion in its solvating cage were found to produce a broad IR absorption band centered at 374 cm(-1). For low to moderate LiTFSI mole fractions, 0.08 < x < 0.2, the [Li(TFSI)2]- solvating cage was found to involve bidentate coordinations of Li+ with two oxygen atoms of one anion in the trans (C2) conformation and two oxygen atoms of the other anion in the cis (C1) conformation. At higher LiTFSI concentration, up to x = 0.5, the lithium ion-TFSI coordination number progressively becomes less than 2, indicating the possible formation of aggregates.

Lithium solvation in bis(trifluoromethanesulfonyl)imide-based ionic liquids

The lithium solvation in (1 -x)(EMI-TFSI), xLiTFSI ionic liquids where EMI(+) is the 1-ethyl-3-methylimidazolium cation and TFSI(-) the bis(trifluoromethanesulfonyl)imide anion, is shown by Raman spectroscopy to involve essentially [Li(TFSI)(2)](-) anionic clusters for 0 < x < 0.4, but addition of stoichiometric amounts of solvents S such as oligoethers changes the lithium solvation into [Li(S)(m)](+) cationic clusters; the lithium transference number in TFSI-based ionic liquid electrolytes for lithium batteries should thus be strongly improved.

New Interpretation of the CH Stretching Vibrations in Imidazolium-Based Ionic Liquids

In ionic liquids composed of alkyl-substituted imidazolium cations and weakly coordinating anions such as bis(trifluoromethanesulfonyl)imide, (CF(3)SO(2))(2)N(-), the stretching vibrations of the imidazolium CH groups are shown to interact by Fermi resonance with the overtones and the combination of two in-plane ring vibrations. This new assignment, based on isotopic substitutions and anharmonic frequency calculations for gas phase cations, implies that these imidazolium cations do not establish any strong and directional C-H...anion hydrogen bond.

Proton conducting polymer blends and hybrid organic inorganic materials

Abstract Literature results concerning proton conducting polymer electrolytes are reviewed with emphasis on acid doped polymer blends and on derived systems obtained by adding an inorganic filler or a plasticizer or both. In addition, conductivity results are provided on anhydrous polyamide/H 3 PO 4 blends in a wide acid concentration range and on new membranes involving a fluorinated polymer, silica and H 3 PO 4 or H 2 SO 4 aqueous solutions. The room temperature conductivity of 10 −1 S cm −1 reached by these membranes is discussed using in situ Raman spectroscopy measurements of the acid concentration. General trends and prospects are tentatively drawn from the available results.

Raman Spectroelectrochemistry of a Lithium/Polymer Electrolyte Symmetric Cell

A symmetric lithium cell Li/P(EO) 20 LiTFSI/Li has been studied at 80°C by confocal Raman microspectrometry while current densities of up to 0.5 mA cm -2 are passed through the cell. The Raman observation is performed on the edge of the cell along a line of points extending from one electrode to the other at a depth of about 20 μm within the electrolyte. Local salt concentration is measured in the electrolyte as a function of time, current density I, and electrolyte thickness L (90 and 160 μm). When the steady-state regime is reached, linear and symmetric salt concentration gradients are observed. They are proportional to I and L as expected from the theoretical predictions for a binary electrolyte containing a fully dissociated salt with negligible ionic associations. In addition, preliminary results have been obtained concerning the establishment and relaxation of the steady state. From these data, it is shown that salt diffusion coefficient and ionic transport numbers can be determined with a reasonable precision. Confocal Raman microspectrometry can therefore be considered as a new and powerful spectroelectrochemical method to study transport properties in polymer electrolytes.

Proton conduction in poly (acrylamide)-acid blends

Anhydrous mixtures of polyacrylamide (Paam) with orthophosphoric acid H3PO4 present a conductivity better than 10−3 S cm−1 at 300 K for acid concentrations of 1.5 to 2 moles per polymer repeat unit. Infrared spectroscopic studies of these blends indicate that the amide groups of Paam are not protonated by H3PO4. The inter- and intra-chain C=O…H-N interactions are replaced by hydrogen bonds with the acid which produce only a moderate perturbation to the hydrogen-bond network needed for proton migration. This could explain why the high conductivity of molten H3PO4 is less lowered with Paam than with other basic polymers. As Paam·H3PO4 blends are potential electrolytes for electrochemical devices, their thermal and chemical stabilities have been studied. A prototype of smart window using tungsten and iridium oxides as electrochromic electrodes is also described.

Imidazolium-based ionic liquids: quantitative aspects in the far-infrared region.

The optical constants of some imidazolium-based ionic liquids (ILs) are determined in the mid- and far-infrared regions by combining polarized attenuated total reflection (ATR) and transmittance spectra. The internal vibrations of the cations and anions and the interionic vibrations can thus be quantitatively evaluated. A comparison of the far-IR spectral response of several imidazolium derivatives associated with the (CF(3)SO(2))(2)N(-) anion shows that methylation of the more acidic C((2))H imidazolium group does not change the far-IR intensity and hence that the CH...anion hydrogen bonds play a negligible role compared with electrostatic interactions. The calculated spectra of ion-pair dimers reproduce the far-IR density of states better than those of simple ion pairs.

Infrared Spectroscopy of Ionic Liquids: Quantitative Aspects and Determination of Optical Constants

The infrared (IR) spectra of ionic liquids involving 1-butyl-3-methylimidazolium (BMI) and the (CF3SO2)2N-, BF4-, or PF6- anions, recorded in the transmission and attenuated total reflection (ATR) modes, exhibit strong differences in the most intense anion absorption profiles. These distortions come from optical effects and make difficult any quantitative analysis of, for example, the antisymmetric stretching vibrations of the BF4- and PF6- anions. A method is proposed to determine the optical constants, from which any type of experimental spectrum can be simulated. It is then possible to use the anion vibrational bands as spectroscopic probes of the local interactions occurring in the neat ionic liquids and in solutions. This is illustrated by a direct identification of ion pairs and separated ions in the IR spectra of BMI-PF6 solutions.

Supercapacitor using a proton conducting polymer electrolyte

Abstract A carbon-based electrostatic double-layer supercapacitor using a proton conducting electrolyte (Nylon 6–10, 2H3PO4) has been developed and its performances are compared to those of the same device using an aqueous electrolyte (H3PO4, 7.3 M). Impedance spectroscopy has also been applied to the characterization of the supercapacitors in the −5 to 55 °C temperature range.

Spectroscopic study of poly(ethylene oxide)6: LiX complexes (X = PF6, AsF6, SbF6, ClO4)

The infrared and Raman spectra of the poly(ethylene oxide) chains in the complexes P(EO)6–LiX (X = PF6, AsF6, SbF6, ClO4) are very similar although the conformational sequences of these chains are known to be different in the first three complexes. It seems that the leading factors in determining the common vibrational signature are the isostructural character of these complexes and their similar coordination of the lithium ion by two oxygens of one chain and three oxygens of a second chain. The P(EO)6–LiClO4 complex is confirmed to adopt a similar structure as the former three at room temperature even if its degree of crystallinity is lower. In its molten state, an equilibrium between free ions and contact ion-pairs is clearly evidenced by Raman spectroscopy. In the complexes P(EO)3–LiX complexes at 25 °C, PEO exhibits also a unique and specific spectral signature which seems to be characteristic of the P(EO)3–LiSO3CF3 type of structure, with the lithium ion coordinated by three oxygens of a PEO chain and two anions.