Francesco Nobili

An electrochemical impedance spectroscopic study of the transport properties of LiNi0.75Co0.25O2

Analysis of impedance spectra taken at closely spaced bias potential values on LixNi0.75Co0.25O2 have been interpreted in terms of electronic and ionic transport properties of this electrode material. In the 0.9<x<1 range the material shows semi-conductive properties and the electronic conductivity dominates the transport. For x≤0.9, the properties change into those of a metal-like material in which the ionic conductivity becomes the limiting factor. The transition between these two limiting conditions clearly appears in the impedance spectra sequence. This transition is reversible since the same behaviour is observed during the lithium intercalation process as well as in the reverse lithium deintercalation process.

An electrochemical ac impedance study of LixNi0.75Co0.25O2 intercalation electrode

An EIS study on LixNi0.75Co0.25O2 intercalation cathode has been performed. The spectra have been interpreted on the basis of an equivalent circuit based on a combination of a Voigt-type analog (Li ion migration through surface film and charge transfer) in series with a Warburg type element, an element that takes into account the electronic resistance of the material and a capacitor (Li accumulation).

AC impedance study of a synthetic hydrotalcite-like compound modified electrode in aqueous solution

This paper deals with the electrochemical characterisation of Ni/Al/Cl hydrotalcite modified electrodes. The electrochemical impedance spectroscopy technique has been used in order to study the electronic and ionic conduction, both inside and on the surface of the material. The electronic and ionic contributions have been separated and the behaviour of the respective parameters has been studied as a function of the potential. In order to determine the kinetic limiting step of the overall electrochemical process we performed experiments at different temperatures, and calculated the activation energies of the electron hopping process and ion transport process. In addition we studied the behaviour of the system at different OH concentrations (pH 9.7/12.8) with the aim of clarifying the role of OH ions in the electrochemical process. # 2003 Elsevier Science Ltd. All rights reserved.

Time/Space-Resolved Studies of the Nafion Membrane Hydration Profile in a Running Fuel Cell

2009 WILEY-VCH Verlag Gm The determination of the amount and spatial distribution of water in a polymeric membrane of a proton-exchange-membrane fuel cell (PEMFC) under working conditions is a fundamental task to address in PEMFC technology. Indeed, since proton transfer in such polymeric materials is known to be assisted by water, the fuel-cell (FC) performances depend on the proton-exchange membrane (PEM) hydration degree. However, the hydration degree is influenced not only by the electrochemical conditions the FC is submitted to, but also by many other independent parameters, such as the constriction exerted on the membrane by the other FC components, the electrical current flowing across it, the actual temperature, aging effects, etc, which are hard to take into account in theoretical calculations. In this work, an original method based on very-high-energy synchrotron-radiation X-ray diffraction is applied to carry out the first space/time-resolved measurements of the PEM hydration profile in a running FC. Due to their capability of effectively converting chemical into electrical energy, PEMFCs play a major role in the development of future environmentally friendly hydrogen-based technologies. Indeed, PEMFCs are considered promising candidates for automotive propulsion and for stationary applications. To optimize the performance and lifetime of a PEMFC, one major problem that must be solved is the water management, because the PEM proton conductivity is highly dependent upon its water content. On the other hand, an excess of water is detrimental, as it may produce cathode flooding, and a consequent reduction of gas supply. During the running of the cell, the membrane both absorbs water, which is produced by oxygen reduction at the cathode or carried by the humidified gas stream, and releases it, because of the evaporation induced by the gas flow and by heating occurring under operative conditions. Water transport through the membrane is caused mainly by the electro-osmotic drag of water by protons moving from the anode to the cathode, and by back-diffusion of the water produced at the cathode, towards the anode, as a consequence of the concentration gradients that build up upon operation. In steady conditions, equilibrium among these competitive mechanisms is reached. However, when the operative parameters are changed, complex water dynamics are observed. Several theoretical investigations have been carried out to describe water intake, release, and transport through Nafion membranes. Nevertheless, uncertainties are also present in calculations, due to several experimental effects and constraints, which further complicate the water dynamics and are difficult to model. On the other hand, only a few experimental techniques aimed at measuring the water distribution in the membrane are available, due to the intrinsic difficulty of isolating the signal coming from water molecules inside a relatively thick membrane assembled in a working cell (as required by an in situ investigation). X-ray techniques, small-angle neutron scattering (SANS), magnetic-resonance imaging andmicro-Raman, neutron radiography, electrical-resistance measurements, infrared absorption and fluorescence spectroscopy have been used for this purpose. Unfortunately, if one excludes micro-Raman measurements, all of these techniques exhibit rather low spatial resolutions (if any). Moreover, they have other severe limitations, such as slow response to the hydration-degree variations, weak signals (resulting in poor accuracy), and only indirect dependence on the quantity of interest. Here, we propose an alternative approach, based on veryhigh-energy (about 90 keV) X-ray diffraction (VHEXD), to measure the hydration degree of PEMs in a working cell in situ. The method consists of vertical stratigraphy of the membrane from one electrode to the other, corresponding to ideally ‘‘slicing’’ the membrane itself in a stack of layers. As a result, the time-dependence of the hydration degree in each layer has been determined at the highest accuracy ever achieved, in all the experimental conditions in which PEMFCs may operate, and in the presence of all the concomitant effects mentioned above. To apply the method discussed in the experimental section, preliminary tests were required to identify the right inclination of the cell (parallelism between the PEM plane and the X-ray beam) and the height at which the beam intersects only the membrane. With these tests, spurious contributions from the other components of the cell to the diffraction patterns can be prevented. The parallelism condition was met by taking a sequence of radiographies of the cell during a scan of the rocking angle, carried out to visualize its inner parts. Figure 1a shows the first of these radiographies, collected after the cell was placed in the beam trajectory. The components of the cell around the membrane can be easily distinguished. The

Is the Solid Electrolyte Interphase an Extra-Charge Reservoir in Li-Ion Batteries?

Advanced metal oxide electrodes in Li-ion batteries usually show reversible capacities exceeding the theoretically expected ones. Despite many studies and tentative interpretations, the origin of this extra-capacity is not assessed yet. Lithium storage can be increased through different chemical processes developing in the electrodes during charging cycles. The solid electrolyte interface (SEI), formed already during the first lithium uptake, is usually considered to be a passivation layer preventing the oxidation of the electrodes while not participating in the lithium storage process. In this work, we combine high resolution soft X-ray absorption spectroscopy with tunable probing depth and photoemission spectroscopy to obtain profiles of the surface evolution of a well-known prototype conversion-alloying type mixed metal oxide (carbon coated ZnFe2O4) electrode. We show that a partially reversible layer of alkyl lithium carbonates is formed (∼5-7 nm) at the SEI surface when reaching higher Li storage levels. This layer acts as a Li reservoir and seems to give a significant contribution to the extra-capacity of the electrodes. This result further extends the role of the SEI layer in the functionality of Li-ion batteries.

Electrochemical behavior of superdense ‘LiC2’ prepared by ball-milling

Abstract The electrochemistry of superdense ‘LiC 2 ’ prepared by ball-milling has been investigated in EC-DMC solutions 1 M LiClO 4 . A primary capacity very close to 1115 mAhg −1 per carbon atom was observed during the first deintercalation cycle at constant current. The following intercalation–deintercalation cycles yielded capacity close to the theoretical value of 372 mAhg −1 , typical of natural graphite. Electrochemical ac-impedance spectroscopy demonstrates that a solid electrolyte interface (SEI) is formed spontaneously upon immersion of the electrode in the electrolyte. Due to the complex nature of the compound prepared by ball-milling (a mixture of lithium metal, LiC 3 and LiC 6 ) the mechanism of the first deintercalation is rather complex. It involves the oxidation of lithium metal at about 22 mV versus Li, followed by the decomposition of the superdense phase LiC 3 and of LiC 6 at potentials that corresponds to the normal electrochemical lithium deintercalation from LiC 6 . Lithium metal in ‘LiC 2 ’ easily reacts with nitrogen to yield α-Li 3 N that irreversibly de-intercalates about 1.8±0.1 lithium before decomposing.

Anatase-TiO2 as low-cost and sustainable buffering filler for nanosize Silicon anodes in Lithium-ion batteries

The design of effective supporting matrices to efficiently cycle Si nanoparticles is often difficult to achieve and requires complex preparation strategies. In this work, we present a simple synthesis of low-cost and environmentally benign aAnatase TiO2 nanoparticles as buffering filler for Si nanoparticles (Si@TiO2 ). The average anatase TiO2 crystallite size was approximately 5 nm. A complete structural, morphological, and electrochemical characterization was performed. Electrochemical test results show very good specific capacity values of up to 1000 mAh g-1 and cycling at several specific currents, ranging from 500 to 2000 mA g-1 , demonstrating a very good tolerance to high cycling rates. Postmortem morphological analysis shows very good electrode integrity after 100 cycles at 500 mA g-1 specific current.

A high-voltage lithium-ion battery prepared using a Sn-decorated reduced graphene oxide anode and a LiNi0.5Mn1.5O4 cathode

This paper describes the preparation and characterization of a high-voltage lithium-ion battery based on Sn-decorated reduced graphene oxide and LiNi0.5Mn1.5O4 as the anode and cathode active materials, respectively. The Sn-decorated reduced graphene oxide is prepared using a microwave-assisted hydrothermal synthesis method followed by reduction at high temperature of a mixture of (C6H5)2SnCl2 and graphene oxide. The so-obtained anode material is characterized by thermogravimetric analysis, X-ray diffraction, scanning electron microscopy, and electron diffraction spectroscopy. The LiNi0.5Mn1.5O4 is a commercially available product. The two materials are used to prepare composite electrodes, and their electrochemical properties are investigated by galvanostatic charge/discharge cycles at various current densities in lithium cells. The electrodes are then used to assemble a high-voltage lithium-ion cell, and the cell is tested to evaluate its performance as a function of discharge rate and cycle number.

Fe local structure in Pt-free nitrogen-modified carbon based electrocatalysts: XAFS study

The paper presents a new results on the bonding environment (coordination number and geometry) and on oxidation states of Fe in nitrogen-modified Fe/C composites used as Pt-free catalysts for oxygen reduction in Direct Hydrogen Fuel Cells. Starting from glucose or fructose, two catalysts displaying different electrochemical performance were prepared and studied in the form of pristine powder and thin catalytic layer of electrode by Fe K-edge XAFS spectroscopy. The results show how the Fe local structure varies as a function of different synthesis conditions and how changes in the structural properties of the catalysts are related to fuel cell electrochemical performance increase during a cell activation period.