NMR studies of ionic mobility and molecular mobility in polymer electrolytes

Comprehensive Review of Polymer Architecture for All-Solid-State Lithium Rechargeable Batteries

Solid-state batteries are an emerging option for next-generation traction batteries because they are safe and have a high energy density. Accordingly, in polymer research, one of the main goals is to achieve solid polymer electrolytes (SPEs) that could be facilely fabricated into any preferred size of thin films with high ionic conductivity as well as favorable mechanical properties. In particular, in the past two decades, many polymer materials of various structures have been applied to improve the performance of SPEs. In this review, the influences of polymer architecture on the physical and electrochemical properties of an SPE in lithium solid polymer batteries are systematically summarized. The discussion mainly focuses on four principal categories: linear, comb-like, hyper-branched, and crosslinked polymers, which have been widely reported in recent investigations as capable of optimizing the balance between mechanical resistance, ionic conductivity, and electrochemical stability. This paper presents new insights into the design and exploration of novel high-performance SPEs for lithium solid polymer batteries.

Ion transport with charge-protected and non-charge-protected cations using the compensated Arrhenius formalism. Part 2. Relationship between ionic conductivity and diffusion

Temperature-dependent ionic conductivities and cation/anion self-diffusion coefficients are measured for four electrolyte families: TbaTf-linear primary alcohols, LiTf-linear primary alcohols, TbaTf-n-alkyl acetates, and LiTf-n-alkyl acetates. The Nernst-Einstein equation does not adequately describe the data. Instead, the compensated Arrhenius formalism is applied to both conductivity and diffusion data. General trends based on temperature and alkyl chain length are observed when conductivity is plotted against cation or anion diffusion coefficient, but there is no clear pattern to the data. However, plotting conductivity exponential prefactors against those for diffusion results in four distinct curves, one each for the alcohol and acetate families described above. Furthermore, the TbaTf-alcohol and TbaTf-acetate data are "in line" with each other. The conductivity prefactors for the LiTf-alcohol data are smaller than those for the TbaTf data. The LiTf-acetate data have the lowest conductivity prefactors. This trend in prefactors mirrors the observed trend in degree of ionic association for these electrolytes.

Influence of plasticizer type on the properties of polymer electrolytes based on chitosan

Polymer electrolytes were obtained by the casting technique from a solution containing chitosan, hydrochloric acid, and plasticizer such as glycerol, ethylene glycol, and sorbitol. The transparent membranes with good ionic conductivity properties were characterized by impedance and UV-vis spectroscopies, thermal analysis (DSC), and X-ray diffraction. The best ionic conductivity values of 9.5 x 10(-4) S cm(-1) at room temperature and 2.5 x 10(-3) S cm(-1) at 80 degrees C were obtained for the sample containing 59 wt% of glycerol and an equimolar amount of HCl with respect to NH2 groups in chitosan. The temperature dependence of the ionic conductivity exhibits an Arrhenius behavior with activation energy of 16.6 kJ mol(-1). The thermal analysis indicates that both glass transition temperature (-87 degrees C) and crystallinity are low for this electrolyte. The samples with 13 wt% of LiCF3SO3 showed that the ionic conductivity values of 2.2 x 10(-5) S cm(-1) at room temperature and 4 x 10(-4) S cm(-1) at 80 degrees C are predominantly amorphous and showed a low glass transition temperature of about -73 degrees C.

Nonexponential polymer segmental motion in the presence and absence of ions: H2 NMR multitime correlation functions for polymer electrolytes poly(propylene glycol)-LiClO4

The authors measure 2H NMR multitime correlation functions to investigate the segmental motion of poly(propylene glycol) containing various amounts of the salt LiClO4. 2H NMR two-time correlation functions indicate that addition of salt affects not only the time scale of the segmental motion, but also the degree of the nonexponential relaxation behavior. To quantify the origin of the nonexponential segmental motion, the authors analyze 2H NMR three-time correlation functions. In general, nonexponential relaxation can result from homogeneous dynamics, i.e., intrinsic nonexponentiality, and from heterogeneous dynamics, i.e., existence of a distribution of correlation times G(ln tau). For the studied high and low salt concentrations, including neat poly(propylene glycol), the analysis shows that both homogeneous and heterogeneous contributions are important. 2H NMR four-time correlation functions allow the authors to measure the lifetime of the dynamical heterogeneities. For the studied salt concentrations, the rate exchange occurs on the same time scale as the segmental motion, indicating short-lived dynamical heterogeneities. To arrive at these results, the authors reconsider the interpretation of (2)H NMR three-time correlation functions. Results of analytical calculations and computer simulations show that it is necessary to extend the previous way of analysis so as to include effects due to correlated back-and-forth jumps.

Ion and polymer dynamics in polymer electrolytes PPO–LiClO4.II. H2 and Li7 NMR stimulated-echo experiments

We use 2H NMR stimulated-echo spectroscopy to measure two-time correlation functions characterizing the polymer segmental motion in polymer electrolytes PPO-LiClO4 near the glass transition temperature Tg. To investigate effects of the salt on the polymer dynamics, we compare results for different ether oxygen to lithium ratios, namely, 6:1, 15:1, 30:1, and infinity. For all compositions, we find nonexponential correlation functions, which can be described by a Kohlrausch function. The mean correlation times show quantitatively that an increase of the salt concentration results in a strong slowing down of the segmental motion. Consistently, for the high 6:1 salt concentration, a high apparent activation energy Ea=4.1 eV characterizes the temperature dependence of the mean correlation times at Tg<T<or approximately 1.1Tg, while smaller values Ea approximately 2.5 eV are observed for moderate salt contents. The correlation functions are most nonexponential for 15:1 PPO-LiClO4 whereas the stretching is reduced for higher and lower salt concentrations. This finding implies that the local environments of the polymer segments are most diverse for intermediate salt contents, and, hence, the spatial distribution of the salt is most heterogeneous. To study the mechanisms of the segmental reorientation, we exploit that the angular resolution of 2H NMR stimulated-echo experiments depends on the length of the evolution time tp. A similar dependence of the correlation functions on the value of tp in the presence and in the absence of ions indicates that addition of salt hardly affects the reorientational mechanism. For all compositions, mean jump angles of about 15 degrees characterize the segmental reorientation. In addition, comparison of results from 2H and 7Li NMR stimulated-echo experiments suggests a coupling of ion and polymer dynamics in 15:1 PPO-LiClO4.

Ion and polymer dynamics in polymer electrolytes PPO-LiClO4. I. Insights from NMR line-shape analysis

We investigate ion and polymer dynamics in polymer electrolytes PPO-LiClO4 performing 2H and 7Li NMR line-shape analysis. Comparison of temperature dependent 7Li and 2H NMR spectra gives evidence for a coupling of ion and polymer dynamics. 2H NMR spectra for various salt concentrations reveal a strong slowdown of the polymer segmental motion when the salt content is increased. The 2H NMR line shape further indicates that the segmental motion is governed by dynamical heterogeneities. While the width of the distribution of correlation times G(log tau) is moderate for low and high salt content, an extremely broad distribution exists for an intermediate salt concentration of 15:1 PPO-LiClO4. For the latter composition, a weighted superposition of two spectral components, reflecting the fast and the slow polymer segments of the distribution, describes the 2H NMR line shape over a broad temperature range. Analysis of the temperature dependent relative intensity of both spectral components indicates the existence of a continuous rather than a discontinuous distribution G(log tau). Such continuous distribution is consistent with gradual fluctuations of the local salt concentration and, hence, of the local environments of the polymer segments, whereas it is at variance with the existence of large salt-depleted and salt-rich domains featuring fast and slow polymer dynamics, respectively. Finally, for all studied PPO-LiClO4 mixtures, the 2H NMR line shape strongly depends on the echo delay in the applied echo-pulse sequence, indicating that the structural relaxation of the polymer segments involves successive rotational jumps about small angles gamma < 20 degrees .

Electrostatic stabilization of colloids in carbon dioxide: electrophoresis and dielectrophoresis

Over the past decade, steric stabilization has been achieved for a variety of inorganic and organic colloids in supercritical fluid carbon dioxide (scCO2). Herein we demonstrate that colloids may also be stabilized in CO2 by electrostatic forces, despite the ultralow dielectric constant of 1.5. Zeta potentials of micrometer-sized water droplets, measured in a microelectrophoresis cell, reached -70 mV corresponding to a few elementary charges per square micrometer of droplet surface. This degree of charge was sufficient to stabilize water/CO2 emulsions for an hour, even with water volume fractions of 5%. Hydrogen ions partition preferentially, relative to bicarbonate ions, from the emulsion droplets to the cores of surfactant micelles in the diffuse double layer surrounding the droplets. The micelles, formed with a low molecular weight branched hydrocarbon surfactant, prevent ion pairing of the hydrogen counterions to the negatively charged emulsion droplets. Dielectrophoresis of the water droplets at a frequency of 60 Hz leads to chains containing a dozen droplets with lengths of 50 mum. The ability to form electrostatically stabilized colloids in carbon dioxide is particularly useful in practical applications, because steric stabilization in CO2 is often limited by the poor solvation of the stabilizers.

Proton NMR Study of Rouse Dynamics and Ideal Glass Transition Temperature of Poly(ethylene oxide) LiCF3SO3 Complexes

A range of low-molecular-weight linear poly(ethylene oxide) chains with LiCF3SO3 have been examined via the transverse proton NMR relaxation. From the measured relaxation functions it is shown that the chains obey Rouse dynamics. This analysis gives the molecular weight of a Rouse statistical subunit and the fundamental Rouse relaxation time. It was found that varying the temperature and salt concentration had no effect on the size of a Rouse statistical segment and therefore none on the conformation of the chains. The temperature dependence of the fundamental Rouse relaxation time was found to be well modeled by the Vogel-Tamman-Fulcher equation. The NMR-measured correlation times were then successfully used to predict the glass transition temperature as measured by DSC. The main conclusion of this paper is that under the addition of salt the ideal glass transition temperature T0 is found to remain constant or indeed to slightly decrease.