Nonlinear permittivity spectra of supercooled ionic liquids: Observation of a "hump" in the third-order permittivity spectra and comparison to double-well potential models.

We have measured the third-order permittivity spectra ε33 of a monocationic and of a dicationic liquid close to the glass transition temperature by applying ac electric fields with large amplitudes up to 180 kV/cm. A peak ("hump") in the modulus of ε33 is observed for a mono-cationic liquid after subtraction of the dc contribution from the imaginary part of ε33. We show that the origin of this experimental "hump" is a peak in the imaginary part of ε33, with the peak height strongly increasing with decreasing temperature. Overall, the spectral shape of the third-order permittivity of both ionic liquids is similar to the predictions of a symmetric double well potential model, although this model does not predict a "hump" in the modulus. In contrast, an asymmetric double well potential model predicts a "hump," but the spectral shape of both the real and imaginary part of ε33 deviates significantly from the experimental spectra. These results show that not only the modulus of ε33 but also its phase is an important quantity when comparing experimental results with theoretical predictions.

Anomalous Wien Effects in Supercooled Ionic Liquids.

We have measured conductivity spectra of several supercooled monocationic and dicationic ionic liquids in the nonlinear regime by applying ac electric fields with large amplitudes up to about 180 kV/cm. Thereby, higher harmonic ac currents up to the 7th order were detected. Our results point to the existence of anomalous Wien effects in supercooled ionic liquids. Most ionic liquids studied here exhibit a conductivity-viscosity relation, which is close to the predictions of the Nernst-Einstein and Stokes-Einstein equations, as observed for classical strong electrolytes like KCl. These "strong" ionic liquids show a much stronger nonlinearity of the conductivity than classical strong electrolytes. On the other hand, the conductivity-viscosity relation of the ionic liquid [P_{6,6,6,14}][Cl] points to ion association effects. This "weak" ionic liquid shows a strength of the nonlinear effect, which is comparable to classical weak electrolytes. However, the nonlinearity increases quadratically with the field. We suggest that a theory for explaining these anomalies will have to go beyond the level of Coulomb lattice gas models.

Access to pure and highly volatile hydrochalcogenide ionic liquids.

The reaction of methylcarbonate ionic liquids with H2S or H2Se offers a highly selective synthesis of analytically pure, well-defined and soluble hydrosulphide and hydroselenide organic salts of general interest. Among them, imidazolium hydrochalcogenides show an astonishingly high volatility for cation-aprotic ILs, which allows their quantitative sublimation below 100 °C/10(-2) mbar and actually results in ionic single crystal growth from the gas phase. Vaporisation and decomposition characteristics were investigated by isothermal TGA measurements and DFT calculations.

Nonlinear ion transport in the supercooled ionic liquid 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide: Frequency dependence of third-order and fifth-order conductivity coefficients.

We have carried out nonlinear ion conductivity measurements on the supercooled ionic liquid 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide[C6mim][NTf2] by applying ac electric fields with amplitudes up to about 200 kV/cm. At these field amplitudes, 3ω and 5ω harmonic components in the current response were detected, and the higher-order conductivity coefficients σ3 (1),σ3 (3),σ5 (3), and σ5 (5) were determined. The frequency and temperature dependence of these conductivity coefficients was analyzed in detail. The most important findings were the following: (i) The third-order spectra σ3 (1) and σ3 (3) exhibit very similar values in the dc plateau regime but differ considerably in the dispersive regime. The same was observed for the fifth order spectra σ5 (3) and σ5 (5). (ii) In the dispersive regime, the third-order spectra display a minimum, while the fifth-order spectra display a maximum. (iii) The frequencies of these minima and maxima are thermally activated with the same activation energy as the low-field dc conductivity σ1,dc, whereas the dc values of the higher-order conductivity coefficients, σ3,dc and σ5,dc, are characterized by lower activation energies.

Understanding the ion jelly conductivity mechanism.

The properties of the light flexible device, ion jelly, which combines gelatin with an ionic liquid (IL) were recently reported being promising to develop safe and highly conductive electrolytes. This article aims for the understanding of the ion jelly conductive mechanism using dielectric relaxation spectroscopy (DRS) in the frequency range 10(-1)-10(6) Hz; the study was complemented with differential scanning calorimetry (DSC) and pulsed field gradient nuclear magnetic resonance (PFG NMR) spectroscopy. The room temperature ionic liquid 1-butyl-3-methylimmidazolium dicyanamide (BMIMDCA) used as received (1.9% w/w water content) and with 6.6% (w/w) of water content and two ion jellies with two different ratios BMIMDCA/gelatin/water % (w/w), IJ1 (41.1/46.7/12.2) and IJ3 (67.8/25.6/6.6), have been characterized. A glass transition was detected by DSC for all materials allowing for classifying them as glass formers. For the ionic liquid, it was observed that the glass transition temperature decreases with the increase of water content. While in subsequent calorimetric runs crystallization was observed for BMIMDCA with negligible water content, no crystallization was detected for any of the ion jelly materials upon themal cycling. To the dielectric spectra of all tested materials, both dipolar relaxation and conductivity contribute; at the lowest frequencies, electrode and interfacial polarization highly dominate. Conductivity, which manifests much more intensity relative to dipolar reorientations, strongly evidences subdiffusive ion dynamics at high frequencies. From dielectric measures, transport properties as mobility and diffusion coefficients were extracted. Data treatment was carried out in order to deconvolute the average diffusion coefficients estimated from dielectric data in its individual contributions of cations (D(+)) and anions (D(-)). The D(+) values thus obtained for IJ3, the ion jelly with the highest IL/gelatin ratio, cover a large temperature range up to room temperature and revealed excellent agreement with direct measurements from PFG NMR, obeying to the same VFT equation. For BMIMDCA(6.6%water), which has the same water amount as IJ3, the diffusion coefficients were only estimated from DRS measurements over a limited temperature range; however, a single VFT equation describes both DRS and PFG NMR data. Moreover, it was found that the diffusion coefficients and mobility are similar for the ionic liquid and IJ3, which points to a role of both water and gelatin weakening the contact ion pair, facilitating the translational motion of ions and promoting its dissociation; nevertheless, it is conceivable the existence of a critical composition of gelatin that leads to those properties. The VFT temperature dependence observed for the conductivity was found to be determined by a similar dependence of the mobility. Both conductivity and segmental motion revealed to be correlated as inferred by the relatively low values of the decoupling indexes. The obtained results show that ion jelly could be in fact a very promising material to design novel electrolytes for different electrochemical devices, having a performance close to the IL but presenting an additional stability regarding electrical measurements and resistance against crystallization relative to the bulk ionic liquid.

Bombardment induced ion transport--part II. Experimental potassium ion conductivities in borosilicate glass.

A new experimental approach for measuring the ionic conductivity of solid materials is proposed. The experiment is based on bombarding an ion conducting sample with an alkali ion beam. This generates a well defined surface potential which in turn causes ion transport in the material. The ion transport is measured at the back side of the sample. The viability of the concept is demonstrated by measuring the temperature dependence of the potassium ion conductivity of a potassium borosilicate glass. The activation energy for the potassium transport is 1.04 eV ± 0.06 eV. For comparison, conductivity data obtained by impedance spectroscopy are presented, which support the bombardment induced data.

Bioactivity of electro-thermally poled bioactive silicate glass.

A 45S5 bioactive glass (nominal composition: 46.1 mol.% SiO2, 2.6 mol.% P2O5, 26.9 mol.% CaO, 24.4 mol.% Na2O) was electrothermally poled by applying voltages up to 750 V for 45 min at 200 degrees C, and the thermally stimulated depolarization currents (TSDCs) were recorded. Changes in chemical composition and electrical properties after poling were investigated by TSDC measurements, impedance spectroscopy and scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM/EDX). The poling led to the formation of interfacial layers underneath the surface in contact with the electrodes. Under the positive electrode, the layer was characterized by Na+ ion depletion and by a negative charge density, and the layer was more resistive than the bulk. The influence of poling on the bioactivity was studied by immersion of samples in simulated body fluid (SBF) with subsequent cross-sectional SEM/EDX and X-ray diffraction analysis. It was found that poling leads to morphological changes in the silica-rich layer and to changes in the growth rate of amorphous calcium phosphate and bone-like apatite on the glass surface. The bone-like apatite layer under the positive electrode was slightly thicker than that under the negative electrode.

Enhanced lithium transference numbers in ionic liquid electrolytes.

Ion transport processes in mixtures of N-butyl- N-methyl-pyrrolidinium bis(trifluoromethanesulfonyl)imide (BMP-TFSI) and lithium bis(trifluoromethanesulfonyl)imide (Li-TFSI) were characterized by ac impedance spectroscopy and pulsed field gradient NMR. Molar ratios x = n Li-TFSI/( n Li-TFSI + n BMP-TFSI) up to 0.377 could be achieved without crystallization. From the bulk ionic conductivity and the individual diffusion coefficients of cations and anions we calculate the Haven ratio and the apparent lithium transference number. Although the Haven ratio exhibits typical values for ionic liquid electrolytes, the maximal apparent lithium transference number is higher than found in other recent studies on ionic liquid electrolytes containing lithium ions. On the basis of these results we discuss strategies for further improving the lithium transference number of such electrolytes.

Field-dependent ion transport in disordered solid electrolytes.

We have carried out experimental and theoretical studies of the electric field-dependent ion transport in disordered materials and in disordered potential landscapes, respectively. In our experiments, we work in an electric field range up to 100 kV cm(-1), which is characterised by a weak nonlinear response of the mobile ions. We detect remarkable differences between different ion-conducting glasses regarding the temperature dependence of the nonlinear response. Theoretically, we study one-dimensional hopping models and continuous disordered potential models, respectively. When comparing theoretical and experimental data, we find both analogies and discrepancies.

Cation diffusion and ionic conductivity in soda-lime silicate glasses.

We have studied the mobilities of calcium and sodium ions in silicate glasses of compositions xNa2O (3 - x)CaO x 4SiO2 with x = 0.0, 0.1, 0.3, 1.0 and 3.0 by means of radiotracer diffusion, electrical conductivity measurements, and dynamic mechanical thermal analyses. In glasses containing sodium oxide, the Na+ ions are much more mobile than the Ca2+ ions, and are, therefore, governing the electrical conductivity. In the pure calcium silicate glass, the activation energy of Ca2+ diffusion is higher than the activation energy of the electrical conductivity. This provides strong evidence that the electrical conductivity of this glass is not determined by the migration of Ca2+ ions, but by impurity charge carriers, which are most likely Na+ ions. We sketch the composition-dependent mobilities of Na+ and Ca2+ ions in soda-lime silicate glasses with variable Na2O and CaO content. Our results indicate that the coordination environment of Ca2+ ions remains unchanged when CaO is replaced by Na2O which is consistent with recent results of molecular dynamic simulations. Moreover, our results confirm the formation of dissimilar Na-Ca pairs which lead to a non-random mixing of the cations in the glass. The formation of such pairs was recently deduced from nuclear magnetic resonance spectra of soda-lime silicate glasses.

ac conductivity spectra of alkali tellurite glasses: composition-dependent deviations from the Summerfield scaling.

We report, for the first time, deviations from the Summerfield scaling in the ac conductivity spectra of single ion conducting glasses. In contrast to the extensively studied borate and germanate glasses, the conductivity isotherms of alkali tellurite glasses do not superimpose upon application of the Summerfield scaling. The deviations depend on the alkali oxide content as well as on the type of the alkali ion. Remarkably, our experimental findings differ considerably from theoretical results describing the hopping dynamics of charge carriers in random barrier landscapes.

Ionic conduction in glass: new information on the interrelation between the "Jonscher behavior" and the "nearly constant-loss behavior" from broadband conductivity spectra.

We analyze broadband ac conductivity spectra of various ion conducting glasses with regard to the composition dependence of the "Jonscher behavior" and of the "nearly constant-loss behavior." In the temperature range of our experiments (173 K< or =T< or =573 K), both types of behavior are closely related, indicating that both arise from ion hopping. However, the "nearly constant-loss behavior" observed at temperatures below 100 K seems to be caused by other dynamic processes.

Nonuniversal features of the ac conductivity in ion conducting glasses

Recently, Sidebottom [Phys. Rev. Lett. 82, 3653 (1999)] proposed a new scaling approach for the conductivity spectra of ion conducting glasses. This approach is based on the condition that the shape of the spectra is universal. In this Letter, we show that this condition is generally not fulfilled, but that the shape depends on the glass composition. In single alkali glasses, the frequency dependence of the conductivity varies with the alkali oxide content. Furthermore, the mixing of dissimilar alkali ions leads to pronounced changes in the shape of the conductivity spectra.