Joachim Maier

Nanocrystallinity effects in lithium battery materials

Nanostructured materials offer the possibility to make use of small transport lengths and small separation distances almost like in fluids, but unlike fluids, the higher structural stability of the solid state can be taken advantage of. Recent findings in the field of Li-batteries highlight the potential for room temperature applications. This paper addresses advantages and disadvantages of nanostructured matter with respect to stability, storage capacity, voltage and charging/discharging rates. In this context we discuss a novel interfacial storage mechanism for lithium which, in the mesoscopic case, forms a bridge between batteries and capacitors.

On the Conductivity Mechanism of Nanocrystalline Ceria

Electrical conductivities of Gd-doped (0.15 mol %) and nominally pure nanocrystalline CeO 2-x ceramics (∼30 nm grain size) were measured by impedance spectroscopy in the temperature range of 673-773 K under various oxygen partial pressures (1-10 5 Pa). The ionic and electronic contributions were separated using electrochemical polarization with an electronically blocking electrode, yttria-stabilized zirconia. The results allow for a clear distinction between potential explanations. It is shown that the space charge model (space charge zones with potential of ∼0.3 V resulting in depletion of oxygen vacancies and accumulation of conduction electrons) explains all the experimental features.

Defect chemistry and ion transport in nanostructured materials: Part II. Aspects of nanoionics

Defect chemistry of bulk and interfacial regions is addressed. Special attention is paid to the cases in which the defect chemistry depends on the spacing of interfaces. Such proper size effects occur in the nanoregion and refer to both electrical (space charge overlap) and structural variations. Several examples of nanocrystalline ion conductors are considered (including CeO2 β-AgI:Al2O3, CaF2/BaF2 heterolayers).

Defect chemistry and ionic conductivity in thin films

Abstract The defect chemistry of thin films is considered for the case of ionic conductors. The influence of the finite sample thickness on the electric conductivity parallel to the interface is systematically discussed. Useful expressions for the effective conductivity, in particularly for the important case of large boundary effects, are derived. The determination of the effective conductivity as a function of the sample thickness provides important information about defect chemical and interfacial properties such as electric surface potential, Debye lengths, mobilities, defect concentrations as well as energetic parameters. Conductivity experiments reported in the literature are critically discussed, and the analysis is applied, in particular, to measurements of LiI-films.

Nano-sized mixed conductors (Aspects of nano-ionics. Part III)

This paper deals with the dependence of ionic and electronic conductivities as a function of concentration and spacing of interfaces. It pays particular attention to the alteration of the degree of mixed conductivity on account of a variation in point defect chemistry in space charge regions and in the interfacial core region. Of particular relevance is the size effect on the standard chemical potentials, which can be very different for ions and electrons corresponding to the degree of delocalisation.

Point-defect thermodynamics and size effects

Abstract The introductory part reviews some basic aspects of bulk point-defect thermodynamics. It makes use of the fact that for the purposes under consideration, the real structure can be decomposed in a perfect ground structure and a superimposed defect structure. Then modifications of the point-defect concentration and distribution are considered occurring if interfaces are approached. A simple treatment is possible for the abrupt core-space charge situation in which the standard chemical potentials are assumed to change in a step-like way. Evidence is given that in very many cases this is a reasonable model. There, the adjustment at interfaces occurs solely via space charge regions. ‘Trivial’ size effects are brought about by the changed surface (i.e. core plus space charge layer) to volume ratio. A mesoscale size effect is expected if the width of the space charge layers is no longer small compared with the distance of neighbouring interfaces (Debye-length λ as scaling parameter). In some situations, e.g. if extremely small clusters are treated, distinct deviations in the ground structure also occur, affecting energetic and entropic standard terms. Since such modifications usually decay steeply with increasing interfacial distance ( L ), another scaling parameter (l) defines a further mesoscopic regime. As examples, micro- and nano-sized particles, films, polycrystals and composites are discussed.

Nano-Ionics: Trivial and Non-Trivial Size Effects on Ion Conduction in Solids

Abstract The significance of nano-size effects for ion transport in solids is highlighted both experimentally – by presenting various results of recent investigations – and theoretically – by considering expected size effects using a top-down approach. It is helpful to distinguish between trivial size effects (effects that equally occur at single interfaces but are augmented by the high interfacial density) and true size effects (involving local modifications due to the finite boundary conditions). The latter ones essentially include space charge overlap and structural interference leading to two characteristic sizes with respect to defect chemistry. The contribution also briefly touches upon effects of dimensionality and shape as well as of discreteness of carrier distribution.

Transport in electroceramics: micro- and nano-structural aspects

Abstract Point defects are of paramount importance for electroceramics. They are key structure elements as regards materials functionality; but, in addition, they are also decisive for chemical kinetics, hence for preparation, conditioning, annealing and degradation phenomena. Concentrations and mobilities of these charge carriers are significantly changed at or near interfaces (or more generally higher dimensional defects) giving rise to depletion, accumulation, and inversion layers with respect to ionic and electronic carriers and hence to distinct electrical and chemical effects. It is discussed how these effects can be explained and how such knowledge can be used to design electroceramics purposefully. Examples refer to ionically or mixed conducting oxides and halides. Finally, in nano-structured materials the spacing of interfaces becomes relevant in that local properties can be severely affected. Such size effects do not only lead to confinement effects in the case of electronic carriers but also to anomalies with respect to ion conduction and mass transport. The potential of the nano-regime for electrical and chemical properties of electroceramics is discussed in the framework of a “soft materials science”.

Evaluation of Electrochemical Methods in Solid State Research and Their Generalization for Defects with Variable Charges

The evaluation of instationary and stationary electrochemical techniques (d.c. and a.c. ; electrochemical and chemical polarization ; blocking and non-blocking electrodes ; two or multipoint arrangement) which allow the determination of transport quantities are discussed for mixed conductors, including the limiting cases of purely ionic or purely electronic conductance. Neglecting Onsager-coupling the evaluation formulae (instationary and stationary polarization, Wagner-Hebb-measurements, impedance measurements, concentration cell experiments, permeation technique etc.) are derived under the general aspect of solid state diffusion and system theory for a very general scheme of defect types and are generalized in so far as all ionic defects may change their charges by reacting with electronic defects. Because of the differences of the diffusion coefficients for the differently ionized defects and because of the coupling between electronic and ionic species new effects occur manifesting themselves in additional terms in the evaluation formulae. For the case ofconstant charges an equivalent circuit is given that describes all experiments under investigation.

Complex oxides: high temperature defect chemistry vs. low temperature defect chemistry

The contribution addresses the bridge between high temperature chemistry at which thermodynamic defect equilibrium with neighbouring phases can be assumed, and low temperature physics in which usually only electronic carriers behave reversibly. An optimised quenching procedure is discussed to obtain profile-free reproducible conditions. The conditioning of multinary compounds, typically complex oxides, exhibiting ionic carriers with distinctly different mobilities, requires a multi-step quenching procedure. The treatment of partial equilibria and the connection between quenched and reversible states allows us to understand a variety of “anomalous” phenomena. In this context, some results on mixed conducting, ion conducting and superconducting ceramics are reviewed.