Volume 223, Issue 10-11

Studies of ionic conduction and of tracer diffusion in oxide glasses and of tracer diffusion in metallic glasses mainly from our laboratory and of viscosity data are reviewed. Common aspects of the motion process of ions or atoms in oxide glasses or glassy metals are pointed out.

Measurements of the complex dielectric permittivity in (Li 2 O) x (B 2 O 3 ) 100-x and halide-doped (Li 2 O) x (LiCl) y (B 2 O 3 ) 100-x-y glasses conducted in the nearly constant loss (NCL) regime at low temperatures are reported. Scaling properties reveal that the NCL in (Li 2 O) x (B 2 O 3 ) 100-x is largely ionic in origin for these materials and increases in proportion to the Li 2 O content. However, at low Li 2 O content, the NCL becomes dominated by a non-ionic loss arising from the borate network or other impurity polarizations. Halide doping is seen to produce an additional enhancement of the NCL that is consistent with its known enhancement of the bulk conduction.

Frequency-dependent conductivities are a valuable tool for studying the ion dynamics on different time scales. We present and analyze conductivity spectra of two kinds of solid polyelectrolyte materials, viz. polylectrolyte multilayers (PEM) and polyelectrolyte complexes (PEC). The PEM spectra are taken as a function of relative humidy at ambient temperature. By contrast, the conductivity of different kinds of dried PEC is studied as a function of temperature. For both kinds of material classes we show that the MIGRATION concept developed by Funke and co-workers can be used to describe the experimental spectra over wide ranges in frequency, indicating that forward-backward hopping motions of small ions play a vital role in solid polyelectrolyte materials. Apart from these potentially successful hops, localized motions of charged particles are found to influence the conductivity spectra as well. Based on the shape of our conductivity spectra and their scaling properties, we arrive at important conclusions about the microscopic ion dynamics in PEM and PEC materials.

By the development and investigation of coarse-grained models for ion motion in disordered systems, important progress has been achieved in the past to explain a large variety of ion transport properties in amorphous materials. In this work we discuss recent theoretical advances, which allow one to better understand the nearly constant dielectric loss in disordered solid ionic conductors, the mixed glass former effect in glassy electrolytes, and certain effects of ion motion in polymer electrolytes.

Zusammenfassung We discuss the origins of mixed-former effects in borophosphate glasses of general composition: Li 0.5 BO 1.75 :(1−x)LiPO 3 and 0.4 Na 2 O:0.6 [xB 2 O 3 :(1−x)P 2 O 5 ], and show how these effects relate to changing ion-network interactions. In particular for the latter system, we analyse the permittivity spectra using the MIGRATION concept. The results show that the increase in the calculated “cage length” (spatial extent of localised ion motion) in the mixed former region correlates well with the increased abundance of the BO 4 tetrahedra. We propose that more local cation sites are created and fast electronic bond-switching processes open up these sites to facilitate macroscopic transport. This picture of ion transport independent of network dynamics accounts for all favourable effects observed in mixed former glasses such as increase in ion mobility, rise in T g and improvement in chemical durability.

The structure of two compositional series of ZrF 4 -LaF 3 -BaF 2 -MF (M = Li or Na) glasses has been examined by 19 F magic-angle spinning (MAS)-NMR spectroscopy. From a detailed lineshape analysis, the fractions of corner-bridging (CBF) and non-bridging fluoride (NBF) species were extracted. With increasing LiF (or NaF) content, the fraction of NBF increases significantly, producing an increased spectral contribution of the low-frequency lineshape components. 19 F{ 7 Li} rotational echo double resonance (REDOR) experiments reveal that this part of the MAS-NMR lineshape interacts most closely with the 7 Li nuclei. The compositional dependence of the NBF fractions is discussed in the context of various scenarios, describing the transformation of CBF units into NBF units, and affecting the average coordination number of the Zr ions. The results indicate that the evolution of the NBF fraction cannot be described by a single scenario over the entire composition range.

We have measured third-order conductivity and permittivity spectra of a Li 2 O · Al 2 O 3 · 4 SiO 2 glass and of a 0.15 Na 2 O · 0.85 GeO 2 glass by applying ac electric fields up to 100 kV cm −1 . For the application of these high fields, the glass samples are interfaced with highly conductive liquid electrolyte solutions. In this field range, the electrical response due to ion dynamics in the bulk of a glass sample is weakly nonlinear, while the electrical reponse due to ion transport across the interface between glass sample and liquid electrolyte is strongly nonlinear. For both glasses, the real part of the third-order conductivity is characterised by a change in sign in a frequency range where the diffusive bulk ion dynamics passes over into the subdiffusive bulk ion dynamics. On the other hand, the real part of the third-order permittivity spectra is characterised by a change in sign at lower frequencies where ion transport across the glass / liquid electrolyte interface passes over into the bulk ion transport. We show that these changes in sign can be rationalised by considering an equivalent circuit for a disordered solid ion conductor interfaced with a highly conductive liquid. In this circuit, the bulk and interfacial resistances are assumed to decrease with increasing voltage.

The role of Physical Chemistry of Solids as origin of many fundamental concepts for modern materials science as also the bridge between physics, chemistry and materials engineering is emphasized and exemplified. The model of the electrical double layer, the concept of point defects in crystals and transport phenomena and reactions in the solid state are discussed as examples.

Models of ion transport either consider the complex multi-particle ion interaction in an effective way such as the MIGRATION concept of Funke et al. or consider a single particle hopping scenario in appropriately chosen energy landscapes. Here we first summarize some important aspects of ionic transport, stressing in particular the relevance of interaction effects. Furthermore we review a recently presented schematic model of hopping particles interacting via harmonic forces. This system can be mapped on an effective two- (mean field case) or three-body problem (non-mean field case) and we quantify relaxation effects within linear response. We obtain analytical expressions for the observables, which can be readily compared with those of the MIGRATION concept. General implications of our results to ion transport is also discussed with reference to applicability of the mean field vs. the non mean field scenario for the case of coupled and glassy electrolytes.

Gold doped ruby glasses are classical examples of metal-glass nanocomposites that have been investigated for their striking optical properties. For their multifunctional applications, we have explored the nature of the electrical response of two oxide glasses containing a small amount (<0.1mol%) of gold. Gold-doped lithium borate (LBO) and lanthanum borogermanate (LBGO) glasses are studied using ac conductivity as a function of frequency and temperature in relation to their structure as determined by electron microscopy. For ionically conducting LBO, Au doping produces a noticeable increase of the electrical conductivity. For poorly conducting LBGO, gold doping introduces a dielectric loss peak indicative of dipolar relaxation. The heat treatment of both glasses introduces a new mechanism of dc conduction or dipolar loss, which has about one third the activation energy of the untreated samples. This unexpected behavior is attributed to an ionic-to-electronic conductivity transition in gold doped glasses.

The oxidation of a metal forming a transparent oxide allows it to be followed in situ by optical interference. The layer thickness is determined from the interference pattern. The method can be applied to any tarnishing process as long as the resulting layer stays transparent. Oxidation of copper is followed in air, at T = 230 o C, making use of the three colors of a video camera. In this case a two phase, Cu 2 O|CuO scale is formed on Cu. An analysis of the rate of formation of two layers of two phases is presented. The rate constants are related to a surface reaction rate or to bulk diffusion and conductivity. It is shown that while the total amount of oxide increases with time the layer thickness of one of the phases can temporarily decrease. In the long time limit both layers follow the parabolic law i.e. increase as √time. The method can be applied also to the inverse process of reduction and decrease in the scale thickness.

For solution of the problem of conductivity relaxation and diffusion of ions in ionic conductors with high density of ions, it is essential not to neglect treatment of the effects of many-ion dynamics. This view is shared by the Coupling Model (CM) and the MIGRATION CONCEPT (MC), although the treatment, emphasis and some predictions of the two models are different. Notwithstanding, a basic element is common to both models, namely the primitive relaxation, which performs two important functions. It terminates the caged ion dynamics at short times and initiates the many-ion dynamics at longer times. We demonstrate by experiments and molecular dynamics simulations the existence of the primitive relaxation, and the two functions it performs. The relation of the primitive relaxation time to the conductivity relaxation time predicted by the CM is shown to hold in all cases considered.

Li x CoO 2 and Li x NiO 2 (0.5<x<1) are prototype cathode materials in lithium ion batteries. Both systems show degradation and fatigue during electrochemical cycling. We have performed band structure calculations based on density-functional theory for a series of compounds Li x (Co,Ni)O 2 (0<x<1). The distribution of the transition metals (TM) cobalt and nickel on TM sites as well as the electronic structure of these compounds is investigated with focus on the change of oxidation states of cobalt, nickel and oxygen during lithium de-intercalation. We also study the total energy as a function of the lithium content x, including the vibrational energy E v and the formation energy of lithium vacancies E (V Li ). It is found that E v is small compared to E (V Li ) and that E (V Li ) is increasing with increasing x for all systems.

Results and interpretations of several quasielastic neutron scattering measurements on samples of the isostructural high-temperature phases of Ag 3 PO 4 and Na 3 PO 4 , prominent members of the group of fast cation conducting plastic phases are reported.Time-of-flight experiments reveal details on the phosphate reoriention in Ag 3 PO 4 : We find isotropic rotational diffusion. A consistent interpretation of the data suggests that silver ions are involved in the fast anion dynamics, a behaviour which is in line with the displacement of phosphorus detected in earlier neutron diffraction experiments. The reorientation activation energy of 0.17 eV is in the typical range found also in other ion conducting rotor phases.Na 3 PO 4 was examined in time-of-flight and backscattering experiments, differing considerably in the time scale of the detected molecular dynamics. Anion reorientation, observed on the time scale of picoseconds and activated with 0.18 eV, appears as a predominantly circular diffusion with one P–O axis fixed. Our results suggest the involvement of sodium ions in the anion reorientation, i.e., a correlated motion of both types of ions in the picosecond regime.The much slower sodium jump diffusion in Na 3 PO 4 was found to be thermally activated with 0.74 eV. Moreover, we find clear evidence for the predominance of jumps between tetrahedrally co-ordinated cation sites.Quasielastic neutron scattering reveals a wealth of dynamic details on the two high-temperature phases of Na 3 PO 4 and Ag 3 PO 4 which exhibit considerable differences (e.g., in the geometric details of anion dynamics) in spite of their structural similarities. Both plastic phases, however, give evidence of the involvement of cations in the anion dynamics, i.e., in a correlated reorientational motion of cations and anions on the picosecond time scale.

In the present study the Li diffusivity in nanostructured samples of two glass forming model systems, spodumene (LiAlSi 2 O 6 ) and lithium metaborate (LiBO 2 ), was examined using 7 Li nuclear magnetic resonance (NMR) spin-lattice relaxometry and dc conductivity measurements. The nanostructured samples were prepared by high-energy ball milling of the respective crystalline starting material on the one hand and the corresponding glass on the other hand. The diffusivity of the glass exceeds that of the crystalline sample for both systems. However, when the crystalline samples are mechanically treated by ball milling the diffusivity is enhanced. Nevertheless, the diffusivity of these nanocrystalline samples remains lower than that of the corresponding glass. Surprisingly, when the glassy samples are treated in the same way the diffusivity decreases. After sufficiently long milling times the diffusivity of these nanoglassy samples approaches that of the nanocrystalline samples. This convergence effect seems to be due to structural relaxation processes as is suggested by supplementary infrared spectroscopy and 27 Al, 12 B magic angle spinning NMR measurements.

A method is proposed using mean-field theories to help solve a long standing problem in the study of ionically solid electrolytes. While it has been long known that the ionic conductivity in solid electrolytes is comprised of two terms, the mobility and number of mobile charge carriers, there has been no accurate method developed to determine these two quantities independently. In this paper, we apply a mean-field method based upon low frequency a.c. impedance measurements of the limiting low frequency space charge polarization capacitance that develops as a result of the mobile carrier population diffusing to blocking electrodes. The space charge capacitance that develops is shown to be a simple function of the number of charge carriers and is found to be independent of the d.c. conductivity, but strongly dependent upon temperature. Measurements on two simple but well studied ion conducting glasses, LiPO 3 and NaPO 3 , suggest that the carrier population is thermally activated where only a small fraction of the cations are mobile in the glass. The activation energy for carrier creation in LiPO 3 is larger (49 kJ/mol) than that for NaPO 3 glass (44 kJ/mol) and is in agreement with models of activation energies in ion conducting glasses that associate the creation energy with the cation charge density.

In this chapter, the Solid Electrolyte Interphase (SEI) concept is introduced and the different requirements on the electrolyte and on the SEI formation process in the presence of various anode materials (metallic lithium, graphitic carbons, and lithium storage metals/alloys), in particular, similarities and differences, are highlighted. The difficulties to understand and analyze the SEI characteristics and composition are discussed by examples.