Ion Transport in Glassy Polymerized Ionic Liquids: Unraveling the Impact of the Molecular Structure (original) (raw)
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Diffusion in ionic liquids: the interplay between molecular structure and dynamics
Soft Matter, 2011
Diffusion in a series of ionic liquids is investigated by a combination of Broadband Dielectric Spectroscopy (BDS) and Pulsed Field Gradient Nuclear Magnetic Resonance (PFG NMR). It is demonstrated that the mean jump lengths increase with the molecular volumes determined from quantum-chemical calculations. This provides a direct means-via Einstein-Smoluchowski relation-to determine the diffusion coefficient by BDS over more than 8 decades unambiguously and in quantitative agreement with PFG NMR measurements. New possibilities in the study of charge transport and dynamic glass transition in ionic liquids are thus opened.
Ion Conduction in Polymerized Ionic Liquids with Different Pendant Groups
Macromolecules, 2015
Polymerized ionic liquids (PolyILs) are promising candidates for energy storage and electrochemical devices applications. Understanding their ionic transport mechanism is the key for designing highly conductive PolyILs. By using broadband dielectric spectroscopy (BDS), rheology, and differential scanning calorimetry (DSC), a systematic study has been carried out to provide a better understanding of the ionic transport mechanism in PolyILs with different pendant groups. The variation of pendant groups results in different dielectric, mechanical, and thermal properties of these PolyILs. The Walden plot analysis shows that the data points for all these PolyILs fall above the ideal Walden line, and the deviation from the ideal line increases upon approaching the glass transition temperature (T g). The conductivity for these PolyILs at their T g s are much higher than the usually reported value ∼10 −15 S/cm for polymer electrolytes, in which the ionic transport is closely coupled to the segmental dynamics. These results indicate a decoupling of ionic conductivity from the segmental relaxation in these materials. The degree of decoupling increases with the increase of the fragility of polymer segmental relaxation. We relate this observation to a decrease in polymer packing efficiency with an increase in fragility.
Molecular dynamics simulation of imidazolium-based ionic liquids. II. Transport coefficients
Journal of Chemical Physics, 2009
A systematic molecular dynamics study is performed to determine the dynamics and transport properties of 12 room-temperature ionic liquids family with 1-alkyl-3-methylimidazolium cation, ͓amim͔ + ͑alkyl= methyl, ethyl, propyl, and butyl͒, with counterions, PF 6 − , NO 3 − , and Cl −. The goal of the work is to provide molecular level understanding of the transport coefficients of these liquids as guidance to experimentalists on choosing anion and cation pairs to match required properties of ionic liquid solvents. In the earlier paper ͑Part I͒, we characterized the dynamics of ionic liquids and provided a detailed comparison of the diffusion coefficients for each ion using the Einstein and Green-Kubo formulas. In this second part, other transport properties of imidazolium salts are calculated, in particular, the electrical conductivity is calculated from the Nernst-Einstein and Green-Kubo formulas. The viscosity is also determined from the Stokes-Einstein relation. The results of the calculated transport coefficients are consistent with the previous computational and experimental studies of imidazolium salts. Generally, the simulations give electrical conductivity lower than experiment while the viscosity estimate is higher than experiment. Within the same cation family, the ionic liquids with the NO 3 − counterion have the highest electrical conductivities: ͓NO 3 ͔ − Ͼ ͓PF 6 ͔ − Ͼ ͓Cl͔ −. The ͓dmim͔͓X͔ series, due to their symmetric cationic structure and good packing and the ͓bmim͔͓X͔ series due to higher inductive van der Waals interactions of ͓bmim͔ + , have the highest viscosities in these ionic liquid series. Our simulations show that the major factors determining the magnitude of the self-diffusion, electrical conductivity, and viscosity are the geometric shape, ion size, and the delocalization of the ionic charge in the anion.
Macromolecules, 2017
Polymerized ionic liquids (poly(ILs)) are considered highly promising for the realization of high-performance and intrinsically safer electrolytes for rechargeable batteries due to their high charge density. However, to date little is known about the ion conduction mechanism for this class of solid polymer electrolytes (SPEs). Herein, we performed an in-depth characterization of a homologous series of 1-alkyl-3-vinylimidazolium bis-(trifluoromethane)sulfonimide-derived homopolymers, i.e., p(C n VIm-TSI) with n = 2, 4, 6, 8, and 10, serving as a model compound family. A particular focus was set on the interplay of the physicochemical properties, nanostructure, and ionic conductivity as well as on the impact of the additional incorporation of a lithium salt, LiTFSI. The results reveal that the nanostructure of these selfassembling poly(ILs) plays a decisive role for the ion conduction mechanism, allowing for a (partial) decoupling of charge transport and segmental relaxation of the polymer backbone.
The Journal of Chemical Physics, 2008
Molecular dynamics simulations are used to study the dynamics and transport properties of 12 room-temperature ionic liquids of the 1-alkyl-3-methylimidazolium ͓amim͔ + ͑alkyl= methyl, ethyl, propyl, and butyl͒ family with PF 6 − , NO 3 − , and Cl − counterions. The explicit atom transferable force field of Canongia Lopes et al. ͓J. Phys. Chem. B 108, 2038 ͑2004͔͒ is used in the simulations. In this first part, the dynamics of the ionic liquids are characterized by studying the mean-square displacement ͑MSD͒ and the velocity autocorrelation function ͑VACF͒ for the centers of mass of the ions at 400 K. Trajectory averaging was employed to evaluate the diffusion coefficients at two temperatures from the linear slope of MSD͑t͒ functions in the range of 150-300 ps and from the integration of the VACF͑t͒ functions at 400 K. Detailed comparisons are made between the diffusion results from the MSD and VACF methods. The diffusion coefficients from the integration of the VACFs are closer to experimental values than the diffusion coefficients calculated from the slope of MSDs. Both methods can show good agreement with experiment in predicting relative trends in the diffusion coefficients and determining the role of the cation and anion structures on the dynamical behavior of this family of ionic liquids. The MSD and self-diffusion of relatively heavier imidazolium cations are larger than those of the lighter anions from the Einstein results, except for the case of ͓bmim͔͓Cl͔. The cationic transference number generally decreases with temperature, in good agreement with experiments. For the same anion, the cationic transference numbers decrease with increasing length of the alkyl chain, and for the same cation, the trends in the cationic transference numbers are ͓NO 3 ͔ − Ͻ ͓Cl͔ − Ͻ ͓PF 6 ͔ − . The trends in the diffusion coefficient in the series of cations with identical anions are ͓emim͔ + Ͼ ͓pmim͔ + Ͼ ͓bmim͔ + and those for anions with identical cations are ͓NO 3 ͔ − Ͼ ͓PF 6 ͔ − Ͼ ͓Cl͔ − . The ͓dmim͔ + has a relatively low diffusion coefficient due to its symmetric structure and good packing in the liquid phase. The major factor for determining the magnitude of the self-diffusion is the geometric shape of the anion of the ionic liquid. Other important factors are the ion size and the charge delocalization in the anion.
Chemistry of Materials, 2017
Polymerized ionic liquids (polyILs), composed mostly of organic ions covalently bonded to the polymer backbone and free counterions, are considered as an ideal electrolytes for various electrochemical devices, including fuel cells, supercapacitors and batteries. Despite large structural diversity of these systems, all of them reveal a universal but poorly understood feature-a charge transport faster than the segmental dynamics. To address this issue, we have studied three novel polymer electrolyte membrane for fuel cells as well as four single-ion conductors including highly conductive siloxane-based polyIL. Our ambient and high pressure studies revealed fundamental differences in the conducting properties of the examined systems. We demonstrate that the proposed methodology is a powerful tool to identify the charge transport mechanism in polyILs in general and thereby contribute to unraveling the microscopic nature of the decoupling phenomenon in these materials.
Ionic transport in glass and polymer : Hierarchical structure
Fundamental aspects of the ionic transport in inorganic glass and organic polymer electrolytes are reviewed. The ion dynamics in the random structures of them can be viewed as a hierarchical dynamic structure in different space-time scales, which is created by the fluctuation freezing during the glass transition. Following a brief histry and recent application of the ionic conductor glasses and polymers, the hierarchical structures of them are explained. Next, some theoretical aspects on the glass transition (strong-fragile, coupling-decoupling, freevolume), ion dynamics in short time scale (generalized
The Journal of Physical Chemistry B, 2013
Polymer electrolytes containing ionic liquid (IL), 2-methyl-1,3dipropylimidazolium dihydrogenphosphate (MDPImH 2 PO 4) have been studied by 1 H solid state NMR and differential thermal analysis (DTA) simultaneously by using a specially designed probe. To the best of our knowledge, this is the first report of its kind for IL based polymer electrolytes. The variation of NMR line width with temperature for the IL and polymer electrolytes shows line narrowing at the glass transition and melting temperature. The onset of long-range ion diffusional motion also takes place at these temperatures and is accompanied by a sudden increase in ionic conductivity value by 2−3 orders of magnitude. The presence of amorphous and crystalline phases in IL-based polymer electrolytes has been observed from X-ray diffraction (XRD) studies, and the amorphous phase is the high conducting phase in these polymer electrolytes. The IL-based polymer electrolytes have been observed to be thermally stable up to 200°C. The results obtained from ion transport studies have also been supported by Fourier transform infrared (FTIR), XRD, and cyclic voltammetry (CV) studies.