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.

Generalized equivalent circuits for mixed conductors: silver sulfide as a model system.

ABSTRACT: The generalized equivalent circuit for Hebb-Wagner polarization in the frequency domain proposed by Jamnik and Maier (J Electrochem Soc 146:4183, 1999) includes the space-charge polarization that was previously neglected. In the present work, using a self-coded Fortran program, the completely generalized equivalent circuit is successfully applied to a mixed conducting silver sulfide with an AgI electrode that suppresses the electronic flow. A whole set of fit parameters, such as geometric capacitance, partial conductivities, chemical capacitance or diffusivity, and the blocking and shunting characteristics of electrodes are independently but self-consistently obtained over a range of silver activities, as controlled by a galvanic cell. The interfacial capacitance was found to be much larger than the diffuse space-charge double-layer capacitance and was thus ascribed to the adsorption capacitance at the core of the interface, which should be connected in parallel with the space-charge double-layer polarization. Two simplified equivalent circuits were shown to be good approximations for the spectra at the extreme low and high silver activity, respectively.

Stabilizers of Particle Size and Morphology: a Road Towards High-Rate Performance Insertion Materials.

It is demonstrated that using a very small amount of ceramic surface stabilizers (not forming coatings) can prevent particle growth and completely preserve their morphology during heating. For the example of titania, a supreme lithium insertion-rate performance is exhibited by such stabilized samples.

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.