Malcolm D. Ingram

Impedance and modulus spectroscopy of polycrystalline solid electrolytes

A method of characterising the electrical properties of polycrystalline electrolytes is described which enables grain boundary (intergranular) and bulk (intragranular) impedances to be separated and identified, by reference to an equivalent circuit which contains a series array of parallel RC elements. In the simple case of the “ideal solid electrolyte”, the equivalent circuit contains a single RC element. The impedance and modulus spectra, i.e. plots of Z″ and M″ versus log ω, are simple “Debye” peaks whose peak maxima coincide at an angular frequency ωmax=(τσ)−1, where τσ is the “conductivity relaxation time” and the complex modulus is the inverse complex perimittivity. For real solid electrolytes there is usually a distribution of relaxation times, in which case the maxima in the impedance and modulus spectra no longer coincide. An assignment of peaks in these more complex spectra is possible in principle, since the modulus spectrum effectively suppresses information concerning grain boundary (and electrode) effects. Experimental results are presented for cold-pressed lithium orthosilicate and germanate, and for sintered β-alumina. Some advantages of this new approach are demonstrated by comparison with conventional impedance and admittance plane methods of analysis.

An interpretation of glass chemistry in terms of the optical basicity concept

The basicity of an oxide glass can be measured experimentally from the frequency shifts in the ultra-violet (UV) (s-p) spectra of probe ions such as Pb2+ and can be expressed on the numerical scale of optical basicity Λ (ideally Λ lies between zero and unity). It is possible to relate Λ with (i) the constitution, and (ii) the electronegativity of the cations (e.g. Na+, Si4+, etc.) of the glass, and the relationship allows microscopic optical basicities λ to be assigned to individual oxides and oxy-groups in the glass. These microscopic optical basicities are used for interpreting various aspects of the physics and chemistry of glass including refractivity, network coordination number changes, chemical durability, the glass electrode, UV transparency and the host behaviour of glass towards metal ions generally. Changes in glass basicity in going from one alkali metal oxide to another are also discussed. Finally, the concept of optical basicity, both as an experimentally obtained quantity and as a number calculated from glass constitution and electronegativity, is discussed in relation to the traditional approach to acid-base behaviour in glass.

The dynamic structure model for ion transport in glasses

Abstract A model is developed for ion transport in glass which involves the creation of fluctuating pathways within a dynamically determined structure. Key features include a site memory effect which introduces vacancies appropriate to each kind of mobile ion, and a mismatch energy which emerges whenever an ion attempts to enter a different kind of site. The exploration of this model by numerical methods leads (i) to a power law relationship between ionic conductivity and cation content (now confirmed in the literature) and (ii) to the elucidation of many facets of the mixed alkali effect. It is suggested that this ‘dynamic structure’ model could form the basis for a comprehensive theory of vitreous electrolytes.

Proton conducting sulfon:sulfonamide functionalized materials based on inorganic-organic matrices

A new class of inorganic‐organic protonic polymer electrolyte was developed recently by grafting sulfonic and sulfonamide groups to the inorganic network by the sol‐gel route. It associates the mechanical and thermal resistance of the silica backbone to the chemical reactivity induced by the organic chains grafted to the silica network. The organic chains are slightly acidic proton conductors bearing sulfonic and sulfonamide groups. The polycondensation of alkoxysilanes provides the inorganic silica backbone whereas the organic network is formed from reactive functional groups R% of alkoxysilanes of the type R%Si(OR)3, or by copolymerization of reactive organic monomers with functionalized alkoxysilanes. The synthesis of the resins is completed by organic crosslinking reactions (thermal or UV-curing). The transport of the protons through the solid could be described as a mechanism in which the proton was transferred from a donor (sulfonic group) to a suitable placed acceptor (e.g. sulfonamide group) in the case of a dry material. The conductivity was also studied as a function of relative humidity (r.h.) (wet proton conductors). Here, the proton transport could be described as a vehicular mechanism where the proton rides on a carrier molecule (H3O). Furthermore the conductivity dependence on temperature follows a VTF behavior. By increasing the water content of the membranes up to 16 mass%, the conductivity increases from 10 4 to 6 10 2 Sc m 1 at 70°C. These materials will be developed for thin film batteries. Their mechanical properties, thermal stability and glass transition temperature are discussed in connection with the conductivity results.

Ionic conductivity and glass structure

Abstract The characteristic properties of vitreous electrolytes are summarised for convenience under the three headings of ‘continuity’, ‘variability’ and ‘vulnerability’. These cover respectively the temperature and compositional dependences of conductivity, and non-additive behaviour such as the mixed alkali effect. Some aspects of glass behaviour are apparently ‘universal’ in the sense that they show little sensitivity to changes in short-range structural order. It is argued here that a recently proposed cluster bypass model (in which pathways for ion migration are located within ‘connective tissue’ surrounding ordered microdomains or clusters) provides a straightforward explanation for all these characteristics of vitreous electrolytes. It also provides a framework for more detailed discussions of conduction mechanisms in the context of percolation theory.

Na+-ion conducting glasses

Abstract Na+-ion conduction in glass is discussed with the aim of selecting suitable electrolytes for use in Na/S cells operating around 350°C. From an extensive survey of many glass systems including silicates, borates, etc., it is found that the conductivity increases with increasing optical basicity (Λ), while the activation energy falls toward an apparent limiting value of ca. 50 kJ mol−1. The use of special additives such as NaCl does not lead to any further improvement. Several “optimised ionic conductors” have been identified, where σ ⪢ 10−2 S cm−1 at Tg. These are the best materials for use in battery applications provided they are stable in the cell environment.

Ionic glasses : History and challenges

We review the history of glass technology and glass science and discuss, from a personal point of view, the great challenges in the physics and chemistry of ionic glasses.

Ionic conductivity and the weak electrolyte theory of glass

Abstract Arguments are advanced in support of a weak electrolyte theory of glass which incorporates the paired interstitialcy mechanism recently proposed for s-alumina. The model can account for such heretofore unexplained phenomena as the temperature dependence of diffusion correlation factors, the mixed alkali effect and the anomalously large jump distances obtained from high-field conductivies.

Mixed alkaline-earth effects in ion conducting glasses

Abstract We report ionic mobilities in silicate glasses containing sodium oxide and either one or two alkaline–earth oxides, based on dynamic mechanical thermal analysis (DMTA) and electrical conductivity spectroscopy. In mixed alkaline–earth glasses, the mobilities of alkaline–earth ions are considerably lower than in the corresponding single alkaline–earth glasses. Furthermore, the glass transition temperatures of mixed alkaline–earth glasses are lower than expected from a linear extrapolation of the transition temperatures of the corresponding single alkaline–earth glasses. In order to quantify the degree of decoupling of the mobile ions from the glassy network, we define a new decoupling ratio which makes use of both mechanical and electrical data. We, thereby, show that the effect of mixing divalent ions is qualitatively similar to the effect of mixing monovalent ions. Quantitatively, the mixed alkaline–earth effect is less pronounced than the mixed alkali effect. A possible explanation for this observation is given.

Conductivity anomalies in tungstate-phosphate glasses: evidence for an ion-polaron interaction?

Abstract Conductivity anomalies are observed in mixed ion-polaron conducting glasses in the system Li 2 O: WO 3 : P 2 O 5 . Deep minima in conductivity (reminiscent of the mixed alkali effect) occur when small amounts of Li 2 O are introduced into WO 3 − y : P 2 O 5 glasses. By contrast, glasses melted in air are always deep blue in colour, except when large amounts of Li 2 O are incorporated. It is suggested that negatively charged polarons (effectively the d-electron located on a W 5 + centre) interact strongly with mobile cations to form uncharged diffusing entities. This may be a general phenomenon in mixed ionic electronic conductors where mobilities of ions and electrons are comparable in magnitude.