Roberto Tossici

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.

XAS and electrochemical characterization of lithiated high surface area V2O5 aerogels

Abstract V 2 O 5 aerogel (ARG) has been recently proposed as cathode material for rechargeable lithium batteries. Such a material is amorphous and consists of a highly interconnected solid network with a surface area up to 450 m 2 /g, and a specific pore volume as much as 2.3 cm 3 /g. In a previous paper, it was shown that up to 4 equivalents of lithium per mole of V 2 O 5 aerogel can be inserted by means of chemical or electrochemical lithiation. In the present work, the lithium composition range has been extended. By chemical lithiation (CL) a composition Li 5.8 V 2 O 5 , the highest ever reported for any vanadium oxide host, was achieved. The equilibrium open circuit voltage (OCV)–composition curve of the chemically lithiated aerogel samples showed a wide plateau extending up to 5.8 equivalents of lithium per mole of V 2 O 5 . The surprisingly high OCV has been correlated with the characteristic morphology and structure of the aerogel material by means of X-ray diffraction and absorption and XPS spectroscopies.

Study of amorphous and crystalline Li1+xV3O8 by FTIR, XAS and electrochemical techniques

Abstract Crystalline and amorphous Li 1+ x V 3 O 8 have been compared by electrochemical and spectroscopic investigations. In particular, the behavior of the two materials in cyclic voltammetric experiments at very low scan rate and the Li + diffusion coefficient have been studied. The cyclic voltammetry of crystalline Li 1+ x V 3 O 8 shows the existence of well defined site energies, whilst in the amorphous form the sites have a rather broad energy distribution. Higher D Li + values have been measured in amorphous Li 1+ x V 3 O 8 , this being in agreement with its higher rate capability. The different electrochemical behavior of the two forms have been explained on the basis of structural information obtained by FTIR and X-ray absorption spectroscopy (XAS). These have shown, in particular, that the V-O distances are more homogeneous in the amorphous form and that a partial amorphization occurs in the crystalline form upon Li + intercalation.

XAS and electrochemical characterization of lithium intercalated V2O5 xerogels

Abstract The use of sol-gel processes in the preparation of cathode materials is of growing interest because of their ease and flexibility. The electrochemical properties, e.g. the rate of lithium intercalation, appear to depend on the morphology of the thin-film vanadium oxide xerogels that can be changed by modifying the preparation. In this context, in order to extend the study to bulk materials, xerogel powder samples with surface areas in the range 2–5 m 2 /g have been prepared from pure vanadium pentoxide hydrogels, or in the form of composites, from carbon powder added to hydrogels. The electrochemical properties have been correlated with the morphological and structural changes induced by the presence of carbon using X-ray and XAS spectroscopy.

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).

Electrochemistry of KC8 in lithium-containing electrolytes and its use in lithium-ion cells

The electrochemistry of KC 8 in a lithium-containing ethylene carbonate-dimethylcarbonate electrolyte has been studied. The results show that upon oxidation KC 8 irreversibly releases potassium ions and that during the following cathodic cycle, the residual graphite intercalates lithium reversibly and with fast rate up to a LiC 6 composition. The results also show that a KC 8 electrode can be used in lithium-ion cells in combination with partially lithiated or even with lithium-free cathodes. The maximum capacities (referred to the anode) that may be achieved are 372 and 279 mAh g -1 , respectively.

Electrochemical, ZAS and FTIR study of lithium intercalation in Na1+xV3O8

Abstract Slow scan rate cyclic voltammetry, X-ray absorption (XAS) and FTIR spectroscopy have been used to study the intercalation of lithium in Na 1+ x V 3 O 8 prepared using a high temperature (HT) and a low temperature (LT) synthetic route. All techniques provide evidence for different structural evolution in the two forms as lithium is inserted. The LT form has been found to be able to intercalate more lithium than the HT form (4.5 versus 2.3 moles per unit formula). The ability of the LT form to intercalate more lithium can be related to a limited long range order that is further reduced upon lithiation. The XAS spectra reveal that, during lithium intercalation, a partial loss of long range order also occurs in the HT form. The lithium apparent diffusion coefficients in both forms have also been measured electrochemically and found to be very similar.

Short-range order in solid and liquid KBr probed by EXAFS

Br K-edge EXAFS spectra of solid and liquid KBr have been performed and analysed using advanced techniques for data analysis (GNXAS). Structural results on solid KBr at room temperature and near melting are compared with molecular dynamics (MD) simulations and diffraction data. The first-neighbour distribution is found to be strongly asymmetric even at room temperature, as also shown by MD simulations. This confirms the existence of important anharmonic effects in solid KBr. MD simulations are in agreement with EXAFS data at room temperature. For solid KBr near melting, anharmonic effects become very important. In comparison with MD calculations, the rise of the first-neighbour peak is found to be steeper, while the foot of the first-neighbour distribution is found to be slightly shifted toward longer distances. Experimental short-range partial pair distribution functions and are derived in liquid KBr for the first time. The first peak of the distribution is accurately determined and found to be in good agreement with MD simulations, confirming a slight contraction of the most probable Br - K distance in the liquid phase. The first peak of the distribution, measured with less accuracy, is found to be sharper and slightly shifted to longer distances, lowering the overlap with the first-neighbour K shell.

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.

A High‐Rate Carbon Electrode for Rechargeable Lithium‐Ion Batteries

The preparation and properties of potassium intercalated KC{sub 8} to be used as a starting material for a new type of high-rate electrode for versatile lithium-ion batteries, are presented and discussed. The advantages of this electrode over more conventional negative graphite electrodes are in the faster kinetics of the electrochemical process and in the wider choice of the positive counterelectrode material.