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Über dieses Buch

This book discusses the mechanisms of electric conductivity in various ionic liquid systems (protic, aprotic as well as polymerized ionic liquids). It hence covers the electric properties of ionic liquids and their macromolecular counterpanes, some of the most promising materials for the development of safe electrolytes in modern electrochemical energy devices such as batteries, super-capacitors, fuel cells and dye-sensitized solar cells. Chapter contributions by the experts in the field discuss important findings obtained using broadband dielectric spectroscopy (BDS) and other complementary techniques.
The book is an excellent introduction for readers who are new to the field of dielectric properties of ionic conductors, and a helpful guide for every scientist who wants to investigate the interplay between molecular structure and dynamics in ionic conductors by means of dielectric spectroscopy.



Chapter 1. Introduction to Ionic Liquids

Ionic liquids and polymerized ionic liquids possess a high application potential in synthesis, separation processes, and in processes relating to transport and storage of energy. Therefore, this introduction discusses synthetic ways to obtain ionic liquids as well as selected properties of ionic liquids. Knowledge of chemical reactions occurring during ionic liquid synthesis including purification procedures gives an insight into possible impurities, which may remain after the manufacturing process. The liquid range of ionic liquids with the glass transition temperature or the melting point as lower limit on the one hand and temperatures where weight loss is higher than 0.5 wt% during thermal treatment as possible upper limit is important for both investigation of ionic liquids as well as their application. A brief discussion of selected physical properties, such as viscosity, density, and polarity of ionic liquids should give a first impression about the broad variety of ionic liquid properties that are discussed in more detail in the following chapters. Furthermore, discussion of both polymerization of ionic liquid monomers using different polymerization mechanisms and selected properties of the polymer materials obtained will complete this introduction. The significant increase of the glass transition temperature of polymerized aprotic ionic liquids caused by polymerization of aprotic ionic liquid monomers exhibits differences in the properties between ionic liquids and polymerized ionic liquids .
Veronika Strehmel

Chapter 2. Rotational and Translational Diffusion in Ionic Liquids

Dynamic glass transition and charge transport in a variety of glass-forming aprotic ionic liquids (ILs) are investigated in wide frequency and temperature ranges by means of broadband dielectric spectroscopy (BDS) , pulsed-field gradient nuclear magnetic resonance (PFG NMR), differential scanning calorimetry and dynamic mechanical spectroscopy. On the low-frequency side, the dielectric spectra exhibit electrode polarization effects, while hopping conduction in a disordered matrix dominates the spectra of ionic liquids at higher frequencies. Upon systematic variation of the molecular structure of the ionic liquids, it is observed that the absolute values of dc conductivity and viscosity span more than 11 orders of magnitude with temperature. However, quantitative agreement is found between the characteristic charge transport and the structural α-relaxation rates. These results are discussed in the context of dynamic glass transition-assisted hopping as the underlying mechanism of charge transport in the ionic liquids investigated. In addition, a novel approach to determine diffusion coefficients from dielectric spectra in quantitative agreement with PFG NMR is proposed. This makes it possible to separately determine the effective number densities and mobilities of the charge carriers and the type of their temperature dependence. The observed Vogel–Fulcher–Tammann (VFT) dependence of the dc conductivity is shown to be due to a similar temperature dependence of the mobility while Arrhenius type of thermal activation is found for the number density.
Joshua Sangoro, Tyler Cosby, Friedrich Kremer

Chapter 3. Femto- to Nanosecond Dynamics in Ionic Liquids: From Single Molecules to Collective Motions

The dynamics of room temperature ionic liquids (RTILs) have been intensively studied within the last decades as these are of high relevance for the solvation of solutes in applications of RTILs as reaction media. Broadband dielectric spectroscopy (DS) can readily cover any dynamics ranging from seconds to femtoseconds and is thus a widely applied technique to elucidate RTIL dynamics. As DS probes all dynamics that go along with a change in the macroscopic polarization, DS is excellently suited to study such compounds, where the motions of its charged and dipolar ions inevitably modulate sample polarization. However, interactions in RTILs are not only governed by long-ranged Coulombic forces. Also hydrogen bonding, pi-pi stacking and dispersion forces contribute significantly to the local potential energy landscape, making RTIL dynamics extremely complex. To fully correlate the dynamical information from dielectric spectra to molecular dynamics , the combination of DS with other techniques exploring liquid-state dynamics is advantageous as such a combination allows unraveling the wealth of information present in dielectric spectra and provides detailed molecular level insights. In this chapter we summarize recent advances in understanding the femto- to nanosecond dynamics of RTILs, which could only be obtained using combined experimental and theoretical efforts.
Johannes Hunger, Richard Buchner

Chapter 4. High-Pressure Dielectric Spectroscopy for Studying the Charge Transfer in Ionic Liquids and Solids

In this chapter, the dielectric properties of ionic systems under conditions of high compression are thoroughly discussed. At the beginning the technical details on the high-pressure dielectric measurements are provided. Then we examine the pressure sensitivity of various ionic systems (protic, aprotic ionic liquids, inorganic conductors, as well as ionic polymers) that is reflected in activation volume parameter ΔV and dT g/dP coefficient. On the other hand, the volume dependence of isothermal and isobaric conductivity data collected for a number of ionic materials enable us to separate the contributions of density and thermal effects to the ion dynamics near T g as well as to verify the validity of the thermodynamic scaling concept for these compounds. The next part of this chapter is dedicated to relation between charge transport and structural relaxation in various ionic glass-forming systems. Therein, we show that at σ dc \(\gg\) 10−15 S cm−1 the pressure dependence of dc-conductivity recorded for some protic conductors, inorganic salts and polymerized ionic liquids reveals the characteristic crossover (from VFT to Arrhenius-like behavior) that reflects the time scale separation (so-called decoupling) between charge diffusion and structural/segmental dynamics. Herein, the physical origin of such phenomenon is discussed in the context of charge transport mechanism. We explain in detail the methods used to quantify decoupling phenomenon as well as the physical and chemical factors affecting time scale speciation between charge and mass diffusion. The procedure used to recognize the charge transport mechanism based on the ambient and high-pressure dielectric measurements is also described.
Z. Wojnarowska, M. Paluch

Chapter 5. Glassy Dynamics and Charge Transport in Polymeric Ionic Liquids

While glassy dynamics and (ionic) charge transport coincide in low-molecular weight ionic liquids (ILs), they are widely decoupled in corresponding polymeric systems. This is studied by means of broadband dielectric spectroscopy (BDS) for monovalent and bivalent telechelic polyisobutylene (PIB) carrying the Ionic Liquid (IL)-like cationic headgroup (N,N,N-triethylammonium or 1-methylpyrrolidinium) with Br, NTf2, OTf or pTOS as anions, as well as for neat and polymerized 1-vinyl-3-pentylimidazolium bis-(trifluoromethylsulfonyl) imide (PVIM NTf2). The former shows a wealth of dielectrically active fluctuations in contrast to the latter, where only the dynamic glass transition is observed. Additionally a conductivity relaxation originating from charge transport in the IL-like moieties and a weak electrode polarization caused by the accumulation of mobile charge carriers at the metal interfaces is found. In both systems molecular fluctuations and charge transport are well separated from each other thereby enabling accurate description of the net conductivity within the framework of the effective medium approximation for the PIB-based systems.
Falk Frenzel, Wolfgang H. Binder, Joshua Rume Sangoro, Friedrich Kremer

Chapter 6. Ionic Transport and Dielectric Relaxation in Polymer Electrolytes

Understanding the electrical properties of polymer electrolytes is a fundamentally and practically important problem in electrolyte science. The presence of salt not only poses challenges for the analysis of dielectric spectrum, but also leads to the emergence of new relaxation processes. In addition, the ionic transport mechanism in polymers remains poorly understood. Here we present a brief review of the methods for analysis of the dielectric spectra as well as an account of the phenomenology of ionic transport and dielectric relaxation in polymer electrolytes.
Yangyang Wang

Chapter 7. Electrochemical Double Layers in Ionic Liquids Investigated by Broadband Impedance Spectroscopy and Other Complementary Experimental Techniques

Ionic liquids consist of large asymmetric organic cations and of weakly coordinating anions, i.e., anions with a highly delocalized negative charge. Ionic liquids (IL) exhibit remarkable physicochemical and electrochemical properties, in particular, high thermal stability, low vapor pressure, high ionic conductivity, and broad electrochemical window (Buzzeo and Evans in ChemPhysChem 5:1106, 2004 [1], Endres and Zein El Abedin in Phys Chem Chem Phys 8:2101, 2006 [2], Galiński et al. in Electrochim Acta 8:2101 [3]). By changing the functional groups of cations and by varying the cation/anion combination, the properties of ionic liquids can be adjusted to particular requirements. Thus, ionic liquids are being called designer solvents. They are considered as promising electrolytes for different kinds of electrochemical cells, e.g., for electrosynthesis (Buzzeo and Evans in ChemPhysChem 5:1106, 2004 [1], Hapiot and Lagrost in Chem Rev 108:2238, 2008 [4]), for electroanalysis (Buzzeo and Evans in ChemPhysChem 5:1106, 2004 [1], Baker et al. in Analyst 130:800, 2005 [5]), for electrodeposition of metals (Endres and Zein El Abedin in Phys Chem Chem Phys 8:2101, 2006 [2], Endres et al. in Phys Chem Chem Phys 12:1724, 2010 [6], Endres et al. in Angewandte Chemie (International ed. in English) 42:3428, 2003 [7]), for energy storage in batteries and supercapacitors (Sillars et al. in Phys Chem Chem Phys 14:6094, 2012 [8], Simon and Gogotsi in Nat Mater 7:845, 2008 [9], Lewandowski and Świderska-Mocek in J Power Sources 194:601, 2009 [10], Srour et al. in 200th ECS meeting. ECS, pp 23–28, 2012 [11], for energy conversion in dye-sensitized solar cells (Grätzel in Acc Chem Res 42:1788, 2009 [12]) and for double layer field-effect transistors (Yuan et al. in Adv Funct Mater 19:1046, 2009 [13]). For all these electrochemical applications, the structure and dynamics of the interfacial double layer between ionic liquids and electrode materials plays a crucial role, see for instance Ref. (Endres et al. in Phys Chem Chem Phys 12:1724, 2010 [6]). Since ILs are highly concentrated ionic fluids, the classical Stern model for double layers in diluted electrolytes, in which the ions are treated as point charges, is not applicable. In the case of ionic liquids, the finite volume of the ions, the chemical structure of the ions, and specific interactions of the ions with the electrode surface have to be taken into account. In order to obtain information about the structure and dynamics of the double layer, various experimental techniques have been applied. Broadband impedance spectroscopy in a three-electrode setup yields electrode potential-dependent double layer capacitance values of the electrode | IL interface. Complementary information has been obtained from other techniques, such as scanning tunneling microscopy, atomic force microscopy , surface force apparatus measurements, X-ray reflectivity measurements, surface-enhanced Raman spectroscopy , and sum-frequency generation vibrational spectroscopy . In this chapter, we describe the current level of understanding of the electrode | IL interface based on the combination of the complementary techniques.
Bernhard Roling, Marco Balabajew, Jens Wallauer

Chapter 8. Dielectric Properties of Ionic Liquids at Metal Interfaces: Electrode Polarization, Characteristic Frequencies, Scaling Laws

The electrical and dielectric properties of ionic liquids measured by broadband dielectric spectroscopy are analyzed in detail, in order to determine the characteristic frequencies governing the spectral dependence of electrode polarization effects. A universal behavior is revealed: plotting the characteristic frequencies as a function of the DC-conductivity for a large variety of ionic liquids, single collapsing curves are obtained. This is due to the fact that the charge carriers present in ionic liquids have comparable molecular dimensions. Furthermore, an analytical approach is developed in order to determine, using the dielectric signature of electrode polarization effects, the dielectric properties of ionic liquids at metal interfaces. A new relaxation process taking place in the nanometric interphases formed at the contact with the measurement electrodes is reported. It is assigned to an exchange process between the interphase and the bulk.
A. Serghei, M. Samet, G. Boiteux, A. Kallel

Chapter 9. Decoupling Between Structural and Conductivity Relaxation in Aprotic Ionic Liquids

For the [CnMIm][NTf2] ionic liquids with n = 4, 6, and 8 the dynamic calorimetric glass transition temperature T g,dyn was determined in a wide frequency range from 10−2 to 105 rad s−1. The calorimetric glass transition temperature or vitrification temperature T g from standard DSC with 10 K min−1 cooling rate was determined too. The obtained value for T g in these ionic liquids is in very good agreement with the calorimetric T g,dyn at 100 s relaxation time. The obtained calorimetric data are compared to conductivity and other relaxation data available in the literature. In a relaxation map at short relaxation times (τ < 1 µs), conductivity relaxation and calorimetric relaxation show a similar behavior. However, at low frequencies a significant decoupling between conductivity and calorimetric data is observed. Similar to other ionic conductors, the conductivity relaxation has a weaker temperature dependency than structural relaxation. Interestingly, there is no break in the conductivity data when crossing T g. This is different from many other systems.
Evgeni Shoifet, Sergey P. Verevkin, Christoph Schick


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