Showing papers on "Ionic conductivity published in 1998"

Proton Conductivity in Nafion® 117 and in a Novel Bis[(perfluoroalkyl)sulfonyl]imide Ionomer Membrane

Abstract: A study of proton conductivity in a commercial sample of Nation® 117 and a structurally similar bis[(perfluoroalkyl)sulfonyl]imide ionomer membrane under variable temperature and humidity conditions is reported. The sulfonyl imide ionomer was synthesized using a novel redox‐initiated emulsion copolymerization method, and conductivities were measured using a galvanostatic four‐point‐probe electrochemical impedance spectroscopy technique. Both materials exhibited a strong dependence of conductivity on temperature and humidity, with conductivity in both cases being strongly diminished with decreasing humidity (at constant temperature) and increasing temperature (at constant water partial pressure). The observed behavior is consistent with a "liquid‐like" mechanism of proton conductivity whereby protons are transported as hydrated hydronium ions through water‐filled pores and channels in the ionomer.

Ionic Interactions in Polymeric Electrolytes Based on Low Molecular Weight Poly(ethylene glycol)s

Abstract: It is shown that ionic conductivity of polymeric electrolytes based on low molecular weight amorphous polyglycols can be modified by the addition of α-Al2O3 fillers containing surface groups of the Lewis acid type. An enhancement of conductivity over pure PEG−LiClO4 electrolyte is observed for PEG−α-Al2O3−LiClO4 composite electrolytes containing from 0.5 to 3 mol/kg of the lithium salt. This increase in conductivity is coupled with the lowering of the viscosity of composite electrolytes and increasing chain flexibility when compared to the PEG−LiClO4 system as shown by rheological and DSC experiments. A decrease in the fraction of ionic aggregates is also seen from the FT-IR experiments for composite electrolyte in this salt concentration range. FT-IR studies of the C−O−C stretching mode has shown reduction in the transient cross-link density obtained after the addition of α-Al2O3 in the salt concentration range corresponding to the conductivity enhancement. The phenomena observed are explained in view of...

Composite Polymer Electrolytes with Improved Lithium Metal Electrode Interfacial Properties I. Elechtrochemical Properties of Dry PEO‐LiX Systems

Abstract: Several types of lithium ion conducting polymer electrolytes have been synthesized by hot-pressing homogeneous mixtures of the components, namely, poly(ethylene oxide) (PEO) as the polymer matrix, lithium trifluoromethane sulfonate (LiCF{sub 3}SO{sub 3}), and lithium tetrafluoroborate (LiBF{sub 4}), respectively, as the lithium salt, and lithium gamma-aluminate {gamma}-LiAlO{sub 2}, as a ceramic filler. This preparation procedure avoids any step including liquids so that plasticizer-free, composite polymer electrolytes can be obtained. These electrolyte have enhanced electrochemical properties, such as an ionic conductivity of the order of 10{sup {minus}4} S/cm at 80--90 C and an anodic breakdown voltage higher than 4 V vs. Li. In addition, and most importantly, the combination of the dry feature of the synthesis procedure with the dispersion of the ceramic powder, concurs to provide these composite electrolytes with an exceptionally high stability with the lithium metal electrode. In fact, this electrode cycles in these dry polymer electrolytes with a very high efficiency, i.e., approaching 99%. This in turn suggests the suitability of the electrolytes for the fabrication of improved rechargeable lithium polymer batteries.

A molecular metal with ion-conducting channels

Abstract: Metallic behaviour is well known in charge-transfer complexes that contain stacks of planar, partially oxidized (or reduced) π-conjugated molecules. Electronic conduction occurs in the partially occupied, delocalized π bands formed by intermolecular orbital overlap, and some of these materials exhibit superconductivity1,2. Counter-ions, present to achieve charge neutrality, usually play a passive role, although in some cases they couple to the electronic structure, for example by imposing a new structural periodicity (a superlattice) by orientational ordering1. The development of molecular solids that can simultaneously support the transport of both electrons and ions is important for several fields, including the development of solid-state batteries3,4, electroluminescent devices5 and biomimetic systems6,7. Crown ethers are promising components for such systems, as they provide cavities through which ion motion might occur. Here we report that the charge-transfer salt Li0.6(15-crown-5-ether)[Ni(dmit)2]2·H2O exhibits both electron and ion conductivity: the stacks of the nickel complex (dmit is an organic molecule) provide a pathway for electron conduction, and stacks of the crown ethers provide channels for lithium-ion motion. Evidence for the latter above 250 K is provided by NMR and conductivity studies. We also see evidence for coupling of the electron and ion motions. This compound might serve as a model for the development of other hybrid electronic/ionic conducting materials.

Dopant Substitution and Ion Migration in the LaGaO3-Based Oxygen Ion Conductor

Abstract: Computer-modeling techniques have been used to investigate the defect and transport properties of the LaGaO3-based oxygen ion conductor. We consider a range of cation dopant substitutions with oxygen vacancy compensation. Favorable acceptor dopants (on energetic grounds) are predicted to be Sr at La and Mg at Ga, in accord with experimental work. The formation of dopant−vacancy clusters is examined with the lowest binding energy found for the Sr−vacancy configuration. Oxygen-vacancy migration is along the GaO6 octahedron edge with a curved trajectory and a calculated migration energy of 0.73 eV. We have also examined cation-vacancy transport, which reveals high migration energies (>4 eV). Hole formation from an oxidation process is calculated to be relatively unfavorable, which is compatible with experimental findings that show predominantly ionic conduction in doped LaGaO3. Furthermore, consideration of water incorporation suggests that proton conduction will not be significant in this material.

Sinterability of commercial 8 mol% yttria-stabilized zirconia powders and the effect of sintered density on the ionic conductivity

Abstract: The sintering behaviour of a number of commercially produced 8 mol% yttria-stabilized zirconia powders has been studied. The effect of different sintering regimes on the density and microstructure of the sintered ceramic was determined using density measurements, scanning electron microscopy (SEM) and dilatometry. The chemical homogeneity, particle size and the morphology of the as-received powder were related to the sintering behaviour of the different commercial powders. Powders prepared via a route which involved a spray-drying step sintered more readily than those prepared without a spray-drying step. Plasma-derived powders did not sinter to as high an apparent density as co-precipitated powders. The effect of sample density on the ionic conductivity of sintered YSZ ceramics was studied using a.c. impedance spectroscopy. This technique allowed separation of the bulk and grain-boundary components, enabling clear intepretation of the effects of sample porosity of the conduction pathways. Ceramics prepared from the three different powders achieved a bulk ionic conductivity of ∼16 S cm-1 at 1000 °C for sintered densities of 95% or greater. The results obtained are compared to values reported for a variety of other commercial powders. © 1998 Kluwer Academic Publishers

Preparation and Characterization of Poly(vinyl chloride‐co‐vinyl acetate)‐Based Gel Electrolytes for Li‐Ion Batteries

Abstract: Poly(vinyl chloride) (PVC)-based electrolytes, a class of the most promising polymer electrolytes, are found to suffer from solvent exudation. Two strategies were employed to suppress this shortcoming, one involving the replacement of PVC with poly(vinyl chloride-co-vinyl acetate) (PVCAC) copolymer and the other the direct utilization of solvents for PVC or PVCAC instead of using an auxiliary carrier solvent (e.g., tetrahydrofuran, THF). The thermodynamics of polymer solubility was particularly emphasized in the latter approach. N,N-dimethylformamide (DMF) and N-methyl pyrrolidionone (NMP) are preferred cosolvents of ethylene carbonate (EC) and/or propylene carbonate (PC). The PVCAC-based gel electrolytes prepared were then characterized by ionic conductivity, cyclic voltammetry, and ac impedance data. The results indicate that electrolytes containing NMP/EC mixed solvent exhibit conductivities exceeding 10{sup {minus}3} S/cm whereas the electrolytes containing DMF/EC/PC exhibit conductivities around 10{sup {minus}4} S/cm at room temperature. Moreover, the former category was found to be oxidatively stable up to 4.9 V vs. Li/Li{sup +} and the latter to 4.6 V vs. Li/Li{sup +}. Finally, ac impedance results suggest that the stability of the Li/electrolyte interface needs further improvement, which is a crucial task for most polymer gel electrolytes at present.

Fast Li+ ion conduction in Li2O–(Al2O3 Ga22O3)–TiO2–P2O5 glass–ceramics

Abstract: Fast lithium ionic conducting glass-ceramics have been obtained by heat-treatment of glasses in the systems Li2O–M2O3–TiO2–P2O5 (M = Al and Ga). The glass–ceramics were mainly composed of LiTi2(PO4)3 in which Ti4+ ions were partially replaced by M3+ ions. Considerable enhancement of the conductivity with the substitution of M3+ ions for Ti4+ ions was observed. The maximum conductivity obtained at room temperature was 1.3 × 10−3 S cm−1 for the aluminium system and 9 × 10−4 S cm−1 for the gallium system.

Effect of Aging on Yttria‐Stabilized Zirconia I. A Study of Its Electrochemical Properties

Abstract: The causes of the decrease in electrical conductivity with aging in Y{sub 2}O{sub 3}-stabilized zirconia, an oxygen-ion conductor, were studied. This study was carried out using the dc four-probe technique for measuring electrical conductivity and the activation energy for the migration of oxygen ions. The results show that conductivity decreased with aging below certain temperatures in all specimens. Moreover, it was found that conductivity decreases significantly as the temperature decreases. Samples that were aged at relatively low temperatures exhibited a decrease in conductivity and an increase in activation energy. It was concluded that short range ordering of oxygen ion vacancies toward the zirconium to relax the anisotropy of the lattice distortion is the cause of the decrease in electrical conductivity and the increase in activation energy. When aging was carried out at a relatively high temperature, fully stabilized zirconia showed no change in activation energy and only a slight increase in conductivity. This is because oxygen ion vacancies are in the disordered state and the cubic phase is the only phase at this temperature. Short range ordering of oxygen ion vacancies takes such a long time presumably because these oxygen ion vacancies are still able to move even after aging.more » This was explained using the concept of mean first passage time.« less

Network Polymer Electrolytes with Free Chain Ends as Internal Plasticizer

Abstract: Novel polymer electrolytes based on network polymers with free chain ends have been prepared, and the effects of the free chain ends on the thermal and mechanical properties, the ionic conductivity, and the charge-transfer resistance at the lithium electrode interface have been explored in detail. Terminal hydroxyl groups of poly(ethylene oxide-copropylene oxide) triol (MW 7940) were partly methylated, and the residual hydroxyl groups were esterifited by acrylic acid. The resulting macromonomers were cross-linked by photoirradiation in the presence of an electrolyte salt to produce the network polymer electrolytes. The free chain ends, caused by the methylation, were proved to function as an immobile internal plasticizer, as demonstrated by the decreases in the glass transition temperature and the elastic modulus with increasing number of free chain ends; however, creep-free mechanical strength was maintained due to the cross-linked structure. The introduction of free chain ends not only increased the bulk ionic conductivity but also reduced the charge-transfer resistance. As a result, network polymer electrolytes having a large number of the free chain ends exhibited an ionic conductivity of 10 -3 S cm -1 and a charge-transfer resistance of 20 cm 2 at 80°C when lithium bis(trifluoromethylsulfonyl)imide was used as an electrolyte salt.

Raman Spectroelectrochemistry of a Lithium/Polymer Electrolyte Symmetric Cell

Abstract: 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.

Trivalent Rare Earth Ion Conduction in the Rare Earth Tungstates with the Sc2(WO4)3-Type Structure

Abstract: To realize a trivalent ion conduction in solids, the Sc2(WO4)3-type structure was chosen on the basis of the mobile trivalent ions and the structure which reduces the electrostatic interaction between the framework and the mobile trivalent ionic species as much as possible. The typical conductivity of the rare earth tungstates R2(WO4)3 (R = Sc, Y, and Er−Lu) with the Sc2(WO4)3-type structure was found to be on the order of 10-5 S cm-1 at 600 °C. Among the rare earth tungstates, Sc2(WO4)3 (σ600°C = 6.5 × 10-5 S cm-1, Ea = 44.1 kJ mol-1) was found to be the most suitable size for the ionic conduction with regard to the relation between the mobile ion radius and the lattice size. The rare earth ion conducting characteristics were investigated by means of the rare earth concentration cell measurements and dc electrolyses. The electromotive force measurements with the Sc−Y binary alloy and the yttrium tungsten bronze as the electrodes strongly suggest the possibility of the trivalent ion conduction of rare ear...

Mixed ionic electronic conductor coatings for redox electrodes

Abstract: Disclosed is a redox electrode for a battery cell that has a coating to mitigate plugging by precipitated discharge products. The coating comprises a mixed ionic electronic conductor (MIEC) which is applied to the surface of a redox electrode. The presence of the MIEC coating allows for rapid removal of discharge product precipitates from redox electrodes since it is capable of conducting both electrons and ions. As a result, the chemical action necessary to remove such precipitates may take place on both the electrolyte side of the precipitate and at the precipitate/electrode interface. MIEC coatings in accordance with the present invention may be composed of any suitable material having ionic conductivity for a metal ion in a negative electrode with which the redox electrode is to be paired in a battery cell, and reversible redox capacity. Examples include titanium sulfide (TiS 2 ), iron sulfide (FeS 2 ), and cobalt oxides.

Negative electrode materials for non-aqueous electrolyte secondary batteries and said batteries employing the same materials

Abstract: A material of the negative electrode for the non-aqueous electrolyte secondary battery comprises solid phases A and B. A core is formed by the solid phase A of which lithium absorption and desorption amount resulted from charge and discharge is relatively large. The core is partially or entirely wrapped with the solid phase B of which lithium absorption and disorption amount resulted from charge and discharge is not so much as the solid phase A, however, of which discharge capacity decrease a little resulted from cycles. The solid phase A comprises one of the following materials; lithium, at least one of the elements which is able to alloy with lithium, solid solution including at least one of the above elements being able to alloy with lithium, or an intermetallic compound including at least one of the above elements being able to alloy with lithium. The solid phase B has a different composition, but comprises the same kind of materials except lithium by itself as those of the solid phase A. It is essential that the solid phase B is a mixed conductor having electronic conductivity as well as lithium ionic conductivity. When these materials are used in the negative electrode, the non-aqueous electrolyte second battery featuring a high reliability in the safety aspect, high cycle characteristic, a high capacity and excellent high-rate charge and discharge characteristic can be realized.