J. Jamnik

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.

Generalised equivalent circuits for mass and charge transport: chemical capacitance and its implications

An exact equivalent circuit including terminal parts, which takes account of electrical and chemical control parameters in a unified way, is derived for a cell with a mixed conductor (or electrolyte) without internal sources or sinks. In one-dimensional problems electrochemical kinetics can be mapped by two-dimensional circuits exhibiting the spatial and the thermodynamic displacement as two independent coordinates. One main advantage of the exact circuits with respect to the underlying differential equations is the ability to simplify the description according to specific situations. As we show in several examples in the second part of the paper, it is straightforward to select the elements relevant for the particular experimental conditions and so to make appropriate approximations. This is most helpful for the description of electrochemical systems, such as fuel cells, membranes, pumps and batteries.

The Importance of Interphase Contacts in Li Ion Electrodes : The Meaning of the High-Frequency Impedance Arc

Li insertion electrodes are made by pressing a mixture of active material and additives on a metallic substrate. Here we estimate how various interphase contacts affect the electrode kinetics. We apply variable external mechanical pressure onto different cathodes and measure their impedance response. Similar experiments are performed on dry composites in contact with: Al or Cu foil, or Ag paste. Most surprisingly, we find that the high-frequency impedance arc is due to the contact impedance between the metal and the electrode material. This is in fundamental contradiction with previous interpretations. We propose an equivalent circuit explaining the observed phenomena.

Interfaces in solid ionic conductors: Equilibrium and small signal picture

Abstract The interface of an ionic conductor to a solid neighbouring phase is considered. First the equilibrium picture is presented. The interface is divided into core and space charge region. It is assumed that the materials parameters, such as defect standard potentials and defect mobilities, behave as step functions. The interaction with an insulator or a parent metal as second phases is briefly discussed. On the basis of the equilibrium picture the interfacial impedance is modelled in a novel manner. The interfacial resistance is derived analytically and an exact method is proposed to calculate the space charge capacitance. In the experimental part a new impedance technique is presented: It is especially sensitive to interfacial responses and allows for scanning of the electrical properties from the boundary to the bulk by changing the frequency.

A powerful electrical network model for the impedance of mixed conductors

Abstract Impedance of mixed conductors contacted with electrodes which are blocking for ions and reversible for electrons (or vice versa) is modelled phenomenologically. A closed form approximation is found which is valid in a very broad materials parameter window. The approximation is verified by the exact solution of Nernst–Planck–Poisson set of equations. The closed form results show that at low frequencies electrostatic as well as chemical relaxation occur. In particular, the closed form expression describes the transition from the 45° Warburg impedance to a 90° rise in the impedance spectra. In the former case the low frequency relaxation time bears information about the chemical diffusion coefficient while in the latter example information about the electrode capacitance is contained.

Impedance spectroscopy of mixed conductors with semi-blocking boundaries

Using generalised circuits consisting of three basic elements (electrochemical resistors, electrical and chemical capacitors), the small signal response of mixed conductors is considered. Simple approximate circuits for polycrystals with grain boundaries that act as obstacles for ions, electrons or both are derived. It is shown that Warburg-type impedance can be induced also by grain boundaries, not only by electrodes. As shown exemplarily, such models are appropriate also for analysis of impedance spectra obtained in studies of mixed conductive cathodes in Li-batteries.

In Situ Monitoring and Quantitative Analysis of Oxygen Diffusion Through Schottky‐Barriers in SrTiO3 Bicrystals

The blocking effect of boundaries in bicrystals on the chemical diffusion of oxygen is monitored in situ. The optical technique that is applied allows precise recording of spatial profiles as a function of time. The low symmetrical boundary (near Σ 13) shows a pronounced chemical resistance reflected by a distinct jump of the diffusion profiles, while the highly symmetrical boundary exhibits no measurable influence. The effects can be quantitatively explained by Schottky barriers and the diffusion profiles precisely modeled with the space‐charge potential as the only unknown parameter. The space‐charge potentials are in good agreement with the values derived from impedance analysis. ©1999 The Electrochemical Society

Phase Boundary Propagation in Large LiFePO4 Single Crystals on Delithiation

Large single crystals of LiFePO(4) have been chemically delithiated. The relevance of chemical oxidation in comparison with electrochemical delithiation is discussed. Analyses of the Li content and profiles were done by electron energy loss spectroscopy and secondary ion mass spectrometry. The propagation of the FePO(4) phase growing on the surface of the large single crystal was followed by in situ optical microscopy as a function of time. The kinetics were evaluated in terms of linear irreversible thermodynamics and found to be characterized by an induction period followed by parabolic growth behavior of the FePO(4) phase indicating transport control. The growth rate was shown to depend on the crystallographic orientation. Scanning electron microscopy images showed cracks and a high porosity of the FePO(4) layer due to the significant changes in the molar volumes. The transport was found to be greatly enhanced by the porosity and crack formation and hence greatly enhanced over pure bulk transport, a result which is supposed to be very relevant for battery research if coarse-grained powder is used.

Mesoscopic electrical conduction in nanocrystalline SrTiO3

Detailed impedance analysis of dense nanocrystalline SrTiO3 ceramics in comparison to microcrystalline samples gives direct and unambiguous evidence of a space charge overlap as a characteristic size effect. Owing to the significant extension of depletion zones for the holes, the bulk impedance signal disappears at about 100 nm grain boundary spacing. In accordance with the increased homogeneity of this mesoscopic situation the remaining signal is characterized by a frequency dispersion of the circuit parameters that is low compared with the microcrystalline samples. Space charge potentials for various sizes are extracted. The conclusions are corroborated by numerical calculations.

Electrochemical kinetics of porous, carbon-decorated LiFePO4 cathodes: separation of wiring effects from solid state diffusion

We try to identify the rate-determining step of electrochemical kinetics of a LiFePO(4)-carbon composite electrode by varying the mass of electrode and, additionally, by varying the charge-discharge current in a wide range. It is shown that the reversible capacity is almost independent of electrode mass at currents lower than ca. 1 C (170 mAh g(-1)). At higher currents, however, the reversible capacity starts to drop significantly. The electrode resistance determined from the corresponding polarization voltage shows inverse proportionality with mass at currents smaller than 1 C. At higher currents the electrode resistance is almost independent of electrode mass. We conclude that at lower currents (below 1 C) the main transport step is related to the active particles themselves (either to incorporation reaction or solid state diffusion of Li). At higher currents the contribution of electronic and ionic transport towards the active particles becomes substantial and should be taken into account when designing high-rate insertion electrodes.