Sylvio Indris

Diffusion and ionic conduction in nanocrystalline ceramics

We review case studies of diffusion in nanocrystalline ceramics, i.e. polycrystalline non-metallic materials with average grain sizes typically in the range from 5 to 50 nm. The experimental methods applied are, on the one hand, tracer diffusion or conductivity methods which are sensitive to macroscopic transport, and, on the other hand, NMR techniques which, complementarily to the previous ones, give access to microscopic diffusion parameters like atomic hopping rates and jump barrier heights. In all cases the diffusion properties of the samples, whether single-phase systems or composites, are dominated by their grain boundaries and interfacial regions, respectively. In principle, all experimental techniques allow one to discriminate between contributions to the diffusion from the crystalline grains and those from the interfacial regions. Corresponding examples are presented for SIMS and impedance measurements on oxygen conductors. NMR studies for various nanocrystalline lithium ion conductors reveal that two lithium species with different diffusivities are present. Comparison with the coarse grained counterparts shows that the slower ions are located inside the crystallites and the faster ones in the structurally disordered interfacial regions. Investigations on composite materials exhibit phenomena which can be explained by the percolation of fast diffusion pathways being formed by the interfaces between the two components.

Cycling behaviour of Li/Li4Ti5O12 cells studied by electrochemical impedance spectroscopy

We studied the behaviour of Li/Li(4)Ti(5)O(12) cells by electrochemical impedance spectroscopy to gain insight into the changes at the electrode/electrolyte interfaces during extensive cycling. A simple equivalent-circuit model is able to describe the impedance of the complete battery as a function of both state-of-charge and state-of-degradation. The formation of the solid-electrolyte interface and dendrite growth at the Li metal electrode have a strong influence on the impedance measurements although the battery performance is not significantly affected.

Fast diffusion in nanocrystalline ceramics prepared by ball milling

Nanocrystalline materials can show enhanced diffusivity compared to their microcrystalline counterparts due to the large fraction of atoms or ions located in interfacial regions. In the case of ceramics, resulting properties with potential applications are, e.g., fast ionic conductivity, high mechanical creep rate and increased catalytic activity. Different nanocrystalline ceramic materials were prepared by high-energy ball milling of coarse grained source materials. The samples were characterized by XRD, TEM, BET method and IR spectroscopy. These measurements show that the primary crystallites form larger agglomerates with internal interfaces and that the reduction of the crystallite size is accompanied by a structural degradation of the surface zone. An example is the partial amorphization observed for LiBO2 by IR spectroscopy. The diffusivity and ion conductivity in these materials was studied by NMR relaxation, NMR line shape and impedance spectroscopies. It was possible to discriminate between highly mobile ions in the interfacial regions and immobile ions in the grains. In general diffusion in the nanocrystalline systems was found to be fast compared to that in the corresponding microcrystalline source materials.

Diffusion in amorphous LiNbO3 studied by 7Li NMR — comparison with the nano- and microcrystalline material

Lithium diffusion in amorphous LiNbO3, synthesized by a sol–gel process, was investigated by temperature- and frequency-dependent 7Li NMR experiments. 7Li spin-lattice relaxation rates T−11 and 7Li nuclear magnetic resonance spectra, in particular line shapes and motional narrowing of the central line, were measured. The results are discussed in comparison with those on nano- and microcrystalline LiNbO3 investigated by us earlier. The Li diffusivity in the amorphous material is much higher than in the microcrystalline one and can be described by practically the same diffusion parameters as in nanocrystalline LiNbO3. This suggests that the diffusion pathways in amorphous LiNbO3 and in the interfacial regions of nanocrystalline LiNbO3 have similar structures.

Heterogeneous lithium diffusion in nanocrystalline Li2O:Al2O3 composites

Li diffusion in the nanocrystalline composite system (1 − x)Li2O:xAl2O3 was studied by NMR measurements. Heterogeneous, biexponential 7Li NMR relaxation is found and two Li species with different dynamic behaviour can be discriminated via their individual spin–spin and spin–lattice relaxation times. The heterogeneous dynamics of the two Li species which also shows up in the 7Li NMR spectra reflects the heterogeneous structure of the nanocrystalline composites consisting of crystalline grains and a high volume fraction of interfacial regions. The number fraction of fast Li ions located in the interfacial regions increases with temperature and insulator content x. Accordingly biexponential relaxation becomes more pronounced. From the temperature dependence of the two spin–lattice relaxation rates activation energies of about 0.3 eV for both the fast and the slow Li species are obtained, independent of the composition of the composites.

High-resolution 27Al MAS NMR spectroscopic studies of the response of spinel aluminates to mechanical action

The response of the local structure of various types of spinel aluminates, ZnAl2O4 (normal spinel), MgAl2O4 (partly inverse spinel), and Li0.5Al2.5O4 (fully inverse spinel), to mechanical action through high-energy milling is investigated by means of 27Al MAS NMR. Due to the ability of this nuclear spectroscopic technique to probe the local environment of Al nuclei, valuable quantitative insight into the mechanically induced changes in the spinel structure, such as the local cation disorder and the deformation of the polyhedron geometry, is obtained. It is revealed that, independent of the ionic configuration in the initial oxides, the mechanical action tends to randomize cations over the two non-equivalent cation sublattices provided by the spinel structure. The response of the spinels to mechanical treatment is found to be accompanied by the formation of a non-uniform core–shell nanostructure consisting of an ordered crystallite surrounded by a structurally disordered interface/surface shell region. Based on the comparative NMR studies of the non-treated and mechanically treated spinels, an attempt is made to separate the surface effects from the bulk effects in spinel nanoparticles. The non-equilibrium cation distribution and the deformed polyhedra are found to be confined to the near-surface layers of spinel nanoparticles with the thickness extending up to about 0.7 nm. The cation inversion parameter of the mechanically treated spinel is compared with that of the non-treated material at non-ambient conditions.

Electrochemical insertion of lithium in mechanochemically synthesized Zn2SnO4

We studied the electrochemical insertion of Li in mechanochemically prepared Zn(2)SnO(4). The mechanism of the electrochemical reaction was investigated by using X-ray diffraction, nuclear magnetic resonance spectroscopy, and Mössbauer spectroscopy. Changes in the morphology of the Zn(2)SnO(4) particles were studied by in situ scanning electron microscopy. The results were compared with mixtures of SnO(2) + ZnO and with Zn(2)SnO(4) prepared by conventional solid-state synthesis and showed that the mechanochemically prepared Zn(2)SnO(4) exhibits the best cyclic stability of these samples.

AC and DC Conductivity in Nano- and Microcrystalline Li2O : B2O3 Composites: Experimental Results and Theoretical Models

Abstract We report on impedance measurements of nano- and microcrystalline composites of the Li ion conductor Li2O and the ionic insulator B2O3 as well as their interpretation in the frame of percolation models. In the experimental part, besides the dc conductivity and its dependence on composition and temperature (i.e. its activation energy) also the ac conductivity and its dependence on composition, temperature and frequency (i.e. the conductivity exponent) are presented. Striking differences between the nanocrystalline and the corresponding microcrystalline composites were found. Deviations of the ac from the dc results can be explained by the fact that the experiments probe ion dynamics on different time and thus length scales. In the theoretical part, a continuum percolation model, a brick-layer type bond percolation approach and a Voronoi construction are alternatively used to model the dc behaviour. Based merely on the largely different volume fractions of the interfaces between ionic conductor and insulator grains in the nano- and microcrystalline composites, good overall agreement with the experimental dc results is obtained. The high critical insulator content above which the experimental conductivity vanishes in the nanocrystalline composites suggests the presence of an additional Li diffusion passageway of nanometer length in the interface between nanocrystalline insulator grains.

Electrochemical insertion of Li into nanocrystalline MnFe2O4: a study of the reaction mechanism

The study of the mechanism of Li insertion into nanosized partially inverse spinel MnFe2O4 applying X-ray diffraction, in situ quick X-ray absorption spectroscopy, Mossbauer spectroscopy, high resolution transmission electron microscopy, 7Li MAS NMR, and electrochemical measurements yields a comprehensive picture of the individual steps occurring during Li uptake. At the very early beginning of the reaction Fe3+ on the tetrahedral site is reduced and moves to empty octahedral sites. Increasing the amount of Li to 0.7 per MnFe2O4, further Fe3+ is reduced and Mn2+ residing on the tetrahedral site moves to empty octahedral sites thus forming a defect NaCl-type structure. At least for 2 Li per MnFe2O4 reflections of the spinel disappeared in the X-ray powder pattern and only those of a monoxide are observed. No indications were found for a phase separation and Fe and Mn are homogeneously distributed over the sample. Further Li uptake leads to a stepwise conversion of the material and after insertion of 8 Li/MnFe2O4 only nanosized Mn, Fe, and Li2O are detected. After a capacity loss at the beginning of Li insertion, a constant capacity of about 266 mA h g−1 is reached after 100 cycles discharging–charging the material.

Heterogeneous 7Li NMR relaxation in nanocrystalline Li2O:B2O3 composites

Abstract Nanocrystalline materials can be regarded as heterogeneously structured materials with crystalline grains and disordered interfacial regions. This was verified unambiguously for nanocrystalline Li 2 O using NMR measurements. The heterogeneous structure shows up in the 7 Li NMR line shapes and it results in a heterogeneous dynamic behavior of the Li ions with immobile ions in the grains and mobile ions in the interfacial regions leading to non-exponential recovery of the overall 7 Li magnetization. The two different 7 Li spin reservoirs can be discriminated via their different spin–spin relaxation rates T 2 −1 as well as their different spin-lattice relaxation rates T 1 −1 . The biexponential relaxation behavior becomes more pronounced in the composite material Li 2 O:B 2 O 3 where the volume fraction of interfacial regions and thus the number fraction of fast Li ions is enhanced.