Appearance of nonuniform electric fields in solid ionic conductors

A computer model is used to study charge transport in ionic materials upon the application of an electrochemical potential gradient. The model incorporates both the diffusion and drift mechanisms, permitting the evaluation of the contribution of each mechanism. Each mechanism is shown to be dominant depending on the material parameters and the experimental conditions. The results obtained are compatible with experimental and theoretical ones as reported in the literature, suggesting that in several cases studying the transport of mobile species in ionic materials upon the application of an electrochemical potential gradient, the existence of an internal field cannot be ignored. Published in Russian in Elektrokhimiya, 2007, Vol. 43, No. 4, pp. 398–401. The text was submitted by the authors in English.     

The influence of the compatibility of plasticizers with polymer ionic conductors on ionic conduction

The influence of the compatibility of plasticizers on the aggregated structure and the ion-conductive behavior of poly(ethylene oxide)/lithium methoxy di (ethyleneoxy) phenylsulfonate (EO₂PSLi) complex were studied using two plasticizers with different polarity, γ-butyrolactone (BL) and 2,5-di(methyl diglycol)-1,4;3,6-dianhydrous sorbitol ether (DGS), a novel pincer-like plasticizer synthesized in this paper. The DGS can effectively destroy the PEO crystalline phase in PEO/EO₂PSLi complex and increase the amorphous area in which Li⁺ cations transport, depending on polymeric segmental movement. However, in BL-plasticized PEO/EO₂PSLi complex, the highly polar BL hardly disrupts PEO crystals and only forms a BL-rich phase, a kind of liquid electrolyte tunnel in which Li⁺ mainly transfers. Received: 2 September 1997 / Accepted: 14 January 1998     

On the necessity for invoking a free-ion-like model for the super ionic conductors

Rice and Roth's free-ion-like model of ionic conduction relates the oscillator frequency, the hopping length, the migration activation energy, and the mass of the jumping ion. They applied their formula to atomic self-diffusion in metals and interpreted the agreement of the calculated oscillator frequencies and the observed Debye frequencies as evidence for the validity and uniqueness of their model. However, an expression of the same form can be derived for a simple harmonic oscillator hopping type model, and therefore the necessity for invoking a free-ion-like model is questionable.     

Dynamics of caged ions in glassy ionic conductors

At sufficiently high frequency and low temperature, the dielectric responses of glassy, crystalline, and molten ionic conductors all invariably exhibit nearly constant loss. This ubiquitous characteristic occurs in the short-time regime when the ions are still caged, indicating that it could be a determining factor of the mobility of the ions in conduction at longer times. An improved understanding of its origin should benefit the research of ion conducting materials for portable energy source as well as the resolution of the fundamental problem of the dynamics of ions. We perform molecular dynamics simulations of glassy lithium metasilicate (Li2SiO3) and find that the length scales of the caged Li+ ions motions are distributed according to a Levy distribution that has a long tail. These results suggest that the nearly constant loss originates from "dynamic anharmonicity" experienced by the moving but caged Li+ ions and provided by the surrounding matrix atoms executing correlated movements. The results pave the way for rigorous treatments of caged ion dynamics by nonlinear Hamiltonian dynamics.     

Practical applications of the chemical strain effect in ionic and mixed conductors

Abstract  The chemical strain effect in solids is the deviation from linear elasticity due to the association and dissociation of point defects. Although to date this effect has been observed and studied only in Ce_0.8Gd_0.2O_1,9, one may expect that it will be found in other ionic and mixed conductors containing a large concentration of point defects. In this work, some practical applications of materials exhibiting the chemical strain effect are discussed. Based on the example of Ce_0.8Gd_0.2O_1,9, mechanical structures built from these materials should exhibit exceptional mechanical stability and are therefore very attractive for use as components of solid oxide fuel cells (SOFC) or other devices subjected to large and frequent temperature variations. The ability of these materials to withstand large strain without accumulating large stress also makes them potentially useful as flexible elements in micro-electromechanical systems (MEMS). Graphical abstract        

Practical applications of the chemical strain effect in ionic and mixed conductors

Abstract  

The chemical strain effect in solids is the deviation from linear elasticity due to the association and dissociation of point defects. Although to date this effect has been observed and studied only in Ce_0.8Gd_0.2O_1,9, one may expect that it will be found in other ionic and mixed conductors containing a large concentration of point defects. In this work, some practical applications of materials exhibiting the chemical strain effect are discussed. Based on the example of Ce_0.8Gd_0.2O_1,9, mechanical structures built from these materials should exhibit exceptional mechanical stability and are therefore very attractive for use as components of solid oxide fuel cells (SOFC) or other devices subjected to large and frequent temperature variations. The ability of these materials to withstand large strain without accumulating large stress also makes them potentially useful as flexible elements in micro-electromechanical systems (MEMS).     

Doping strategies to optimise the oxide ion conductivity in apatite-type ionic conductors

The apatite-type phases, La(9.33+x)(Si/Ge)(6)O(26+3x/2), have recently been attracting considerable interest as potential electrolytes for solid oxide fuel cells. In this paper we report results from a range of doping studies in the Si based systems, aimed at determining the key features required for the optimisation of the conductivities. Systems examined have included alkaline earth doping on the rare earth site, and P, B, Ga, V doping on the Si site. By suitable doping strategies, factors such as the level of cation vacancies and oxygen excess have been investigated. The results show that the oxide ion conductivities of these apatite systems are maximised by the incorporation of either oxygen excess or cation vacancies, with the former producing the best oxide ion conductors. In terms of samples containing cation vacancies, conductivities are enhanced by doping lower valent ions, Ga, B, on the Si site. The presence of higher valent ions on these sites, e.g. P, appears to inhibit the incorporation of excess oxygen within the channels, and so limits the maximum conductivity that can be obtained. Overall the results suggest that the tetrahedral sites play a key role in the conduction properties of these materials, supporting recent modelling studies, which have suggested that these tetrahedra aid in the motion of the oxide ions down the conduction channels by co-operative displacements.     

Vibrational study of some solid protonic superionic conductors

During the last few years superionic protonic conductors roused a great interest because of their potential use in energy storage and conversion systems. They can be used as solid electrolytes in fuel cells, batteries or as constituents of hydrogen monitors, electrochromic displays or capacitors (ref. 1, 2). Vibrational spectroscopy has been shown to be a very useful tool of investigation of these systems and can provide information about the structure of protonic species and the mechanism of structural phase transitions and conductivity. This paper is reporting some spectroscopic results concerning three types of protonic conductors: hydrogen beta alumina, hydrated uranyl phosphate and cesium hydrogen sulphate.     

Preparation and characterization of NASICON type Li+ ionic conductors

The new scandium/aluminium co-doped NASICON phases Li_1?+?x Al _y Sc _x???y Ti_2???x (PO₄)₃ (x?=?0.3, y?=?0,0.1,0.2,0.3) were prepared by mechanical milling followed by annealing of the mixtures at 950 °C. X-ray diffraction of all samples showed the formation of NASICON structure with space group R-3c along with a minor impurity. Rietveld refinement of the X-ray data was performed to identify the structural variation. Doping with Sc³⁺ caused elongation of a- and c- axes for all the compounds when compared with undoped LiTi₂(PO₄)₃. The compound Li_1.3Sc_0.3Ti_1.7(PO₄)₃ showed a maximum of a?=?8.5504(7), c?=?20.986(3) Å at room temperature and exhibited highest coefficient of thermal expansion. The highest ionic conductivity (σ), 7.28×10^?4 S cm^?1 was observed for Li_1.3Sc_0.3Ti_1.7(PO₄)₃, two orders of magnitude higher than for the undoped phase.     

On the influence of ionic strength on the equilibrium constant of copper-murexide interaction

The concentration formation constant for the 1:1 complex between copper(II) and murexide in aqueous solutions has been measured as a function of ionic strength by spectrophotometry at 25 degrees . There is an inverse relationship between the K(c) values and ionic strength. The formation constant at infinite dilution was found to be 6.16 x 10(4). The distance of closest approach for the Cu(2+) ion was estimated as 4.3 +/- 0.3 A.     

Investigation of the stability of Co-doped apatite ionic conductors in NH3

Hydrogen powered solid oxide fuel cells (SOFCs) are of enormous interest as devices for the efficient and clean production of electrical energy. However, a number of problems linked to hydrogen production, storage and transportation are slowing down the larger scale use of SOFCs. Identifying alternative fuel sources to act as intermediate during the transition to the full use of hydrogen is, therefore, of importance. One excellent alternative is ammonia, which is produced on a large scale, is relatively cheap and has the infrastructure for storage and transportation already in place. However, considering that SOFCs operate at temperatures higher than 500 °C, a potential problem is the interaction of gaseous ammonia with the materials in the cathode, anode and solid electrolyte. In this paper, we extend earlier work on high temperature reactions of apatite electrolytes with NH₃ to the transition metal (Co) doped systems, La_9.67Si₅CoO₂₆ and La₁₀(Si/Ge)₅CoO_26.5. A combination of PXRD, TGA and XAFS spectroscopy data showed a better structural stability for the silicate systems. Apatite silicates and germanates not containing transition metals tend to substitute nitride anions for their interstitial oxide anions, when reacted with NH₃ at high temperature and, consequentially, lower the interstitial oxide content. In La_9.67Si₅CoO₂₆ and La₁₀(Si/Ge)₅CoO_26.5 reduction of Co occurs as a competing process, favouring lower levels of nitride-oxide substitution.     

The influence of ionic strength on the interaction of viruses with charged surfaces under environmental conditions

The influence of ionic strength on the electrostatic interaction of viruses with environmentally relevant surfaces was determined for three viruses, MS2, Q beta, and Norwalk. The virus is modeled as a particle comprised of ionizable amino acid residues in a shell surrounding a spherical RNA core of negative charge, these charges being compensated for by a Coulomb screening due to intercalated ions. A second model of the virus involving surface charges only is included for comparison. Surface potential calculations for each of the viruses show excellent agreement with electrophoretic mobility and zeta potential measurements as a function of pH. The environmental surface is modeled as a homogeneous plane held at constant potential with and without a finite region (patch) of opposite potential. The results indicate that the electrostatic interaction between the virus and the oppositely charged patch is significantly influenced by the conditions of ionic strength, pH and size of the patch. Specifically, at pH 7, the Norwalk virus interacts more strongly with the patch than MS2 (approximately 51 vs approximately 9kT) but at pH 5, the Norwalk-surface interaction is negligible while that of MS2 is approximately 5.9kT. The resulting ramifications for the use of MS2 as a surrogate for Norwalk are discussed.     

On the influence of ionic strength on the equilibrium constant of an ion-molecule reaction

Concentration formation constants for the 18-crown-6-sodium ion complex in anhydrous methanol solutions were measured as a function of the ionic strength of the solution. The K_c values remained reasonably constant for I_c ? 0.05 mol dm^?3 At higher ionic strengths the K_c values begin to decrease. The infinite-dilution formation constant was 2.2 × 10⁴.     

Disorder and ionic polarons in solid electrolytes

The role of ion-ion repulsion and ion-phonon coupling in superionic conduction is explored. It is argued that the order-disorder phase transition is not associated with the conductivity discontinuity, but with a higher temperature second order phase transition which has been seen in some superionic conductors and which we predict for others. The specific heat, ion distribution, and conductivity are calculated.     

On the way to high-conductivity single lithium-ion conductors

Solid electrolytes can potentially address three key limitations of the organic electrolytes used in today’s lithium-ion batteries, namely, their flammability, limited electrochemical stability and low cationic transference number. The pioneering works of Wright and Armand, suggesting the use of solid poly(ethylene oxide)-based polymer electrolytes (PE) for lithium batteries, paved the way to the development of solid-state batteries based on PEs. Yet, low cationic mobility–low Li⁺ transference number in polymer materials coupled with sufficiently high room-temperature conductivity remains inaccessible. The current strategies employed for the production of single-ion polymer conductors include designing new lithium salts, bonding of anions with the main polyether chain or incorporating them into the side chains of comb-branched polymers, plasticizing, adding inorganic fillers and anion receptors. Glass and crystalline superionic solids are classical single-ion-conducting electrolytes. However, because of grain boundaries and poor electrode/electrolyte interfacial contacts, achieving electrochemical performance in solid-state batteries comprising polycrystalline inorganic electrolytes, comparable to the existing batteries with liquid electrolytes, is particularly challenging. Quasi-elastic polymer-in-ceramic electrolytes provide good alternatives to the traditional lithium-ion-battery electrolytes and are believed to be the subject of extensive current research. This review provides an account of the advances over the past decade in the development of single-ion-conducting electrolytes and offers some directions and references that may be useful for further investigations.     

Esterification in ionic liquids: the influence of solvent basicity

The second-order rate constant (k2) for the esterification of methoxyacetic acid with benzyl alcohol is reported in a range of ionic and molecular solvents. The solvent effects on esterification rate are examined by using a linear solvation energy relationship based on the Kamlet-Taft solvent scales (alpha, beta, and pi*). It is shown that the hydrogen bond basicity of the solvent is the dominant parameter in determining the esterification rate and that the best rates are achieved in low basicity solvents.     

Lattice dynamics to trigger low temperature oxygen mobility in solid oxide ion conductors

SrFeO(2.5) and SrCoO(2.5) are able to intercalate oxygen in a reversible topotactic redox reaction already at room temperature to form the cubic perovskites Sr(Fe,Co)O(3), while CaFeO(2.5) can only be oxidized under extreme conditions. To explain this significant difference in low temperature oxygen mobility, we investigated the homologous SrFeO(2.5) and CaFeO(2.5) by temperature dependent oxygen isotope exchange as well as by inelastic neutron scattering (INS) studies, combined with ab initio (DFT) molecular dynamical calculations. From (18)O/(16)O isotope exchange experiments we proved free oxygen mobility to be realized in SrFeO(x) already below 600 K. We have also evidence that low temperature oxygen mobility relies on the existence of specific, low energy lattice modes, which trigger and amplify oxygen mobility in solids. We interpret the INS data together with the DFT-based molecular dynamical simulation results on SrFeO(2.5) and CaFeO(2.5) in terms of an enhanced, phonon-assisted, low temperature oxygen diffusion for SrFeO(3-x) as a result of the strongly reduced Fe-O-Fe bond strength of the apical oxygen atoms in the FeO(6) octahedra along the stacking axis. This dynamically triggered phenomenon leads to an easy migration of the oxide ions into the open vacancy channels and vice versa. The decisive impact of lattice dynamics, giving rise to structural instabilities in oxygen deficient perovskites, especially with brownmillerite-type structure, is demonstrated, opening new concepts for the design and tailoring of low temperature oxygen ion conductors.