Search

This work highlights a roomerature composition study of the Ag 2O/V2O5/HF(aq) ternary system, leading to the precipitation of either various silver vanadates having Ag/V ratios from 1/2 to 3/1 or the new silver vanadium oxyfluoride compounds Ag 4V2O6F2 and Ag3VO 2F4, and a synthetic procedure that affords nanocrystalline Ag2V4O11 (SVO) at room temperature. The as-precipitated SVO particles exhibit an acicular morphology, 10-15 - 50-200 nm in size, and present a peculiar reactivity vs lithium notably through a Ag+/Li+ displacement reaction that progresses in a reversible fashion. This step forward thus enables the reversible and simultaneous combination of two active redox processes (silver and vanadium), providing a significant enhancement in the cathode gravimetric capacity of 320 mAh/g at C rate and more than 250 mAh/g at 5C. © 2010 American Chemical Society.

New cathode materials will lead to technological advances for implantable cardioverter defibrillators, ICDs, such as reduced size and increased performance of the device. While the industry standard silver vanadium oxide Ag 2V 4On 11 exhibits great chemical/ electrochemical stability, dense silver oxide fluoride materials are advantageous because of high crystal density that can result in an increased capacity above 3 V. This report highlights the reactivity at room temperature between Ag 2O and V 2O 5 in an aqueous HF solution which affords a rapid precipitation of sub-micrometer sized Ag 4V 2O 6F 2 (SVOF), a high capacity Li-battery cathode material. This system opens new and novel synthetic strategies in the design of new oxide fluoride materials. © 2009 American Chemical Society.

Current research in the lithium battery field combines the development of new synthesis methods with the discovery cathode materials that exhibit high energy storage capacity and electrochemical stability. For example, Ag2V4O11, or silver vanadium oxide (SVO), is used commercially as a cathode material in primary lithium batteries for high rate applications, such as those used in implantable cardioverter defibrillators (ICDs). A long-term goal of the medical battery industry is to increase the capacity of the cathode above 3 V while maintaining electrode stability. Low temperature hydrothermal techniques offer new routes to transition metal oxide fluorides, for example the new high silver density material Ag4V2O6F2 (SVOF) which was synthesized recently. The electrochemical behavior of SVOF and the significant impact new materials such as SVOF may have on the future generation of primary lithium batteries for ICDs will be highlighted.

The present paper is in part a continuation of our recent work dealing with the fully reversible Li-driven Cu extrusion/injection of Cu in Cu 2.33V4O11 The peculiar electrochemical property of such a compound was mainly ascribed to both structural considerations (presence of anchoring oxygen, giving flexibility to the [V4O 11] layers) and electronic considerations, such as the existence of a delicate balance between the two Cu+/Cu2+ and V 4+V5+ redox centers. To support such suggestions, numerous compounds were revisited for their electrochemical-structural interplay. Network dimensionality, lattice flexibility, and cationic mobility parameters were addressed by exploring in depth the richness of the Cu-V-O system, while the importance of the donor/acceptor energy levels was checked via the choice of compounds having the right energy match between the two redox centers. As an extensive trial-and-error approach was not realistic, we relied on simple considerations based on the ionization energy of the various d-metals to narrow down metal pairs having redox levels occurring within the same range of energies. Bearing this in mind, it was logical to consider compounds still containing Cu ions but with Nb instead of V or compounds having V and Ag instead of Cu. Regardless of our effort, it turned out that the number of potential candidates worth considering for practical applications is still very limited, in spite of the elegance of this new Li reactivity mechanism.

The electrochemical reactivity of the cathode material Ag4V 2O6F2 (SVOF) versus lithium, with a particular emphasis on the lithium insertion mechanism, was studied by means of the complementary techniques in situ X-ray diffraction, electron paramagnetic resonance, and high-resolution transmisssion electron microscopy. This study confirms the initial reports of a high capacity for SVOF of 148 mAh/g above 3 V and that the reduction of silver above 3 V (vs Li+/Li0) leads to a loss of SVOF crystallinity until it becomes completely amorphous between the third and fourth lithiums inserted. Next, vanadium is reduced between 2.5 and 1.5 V (vs Li+/Li0) for the fifth and sixth lithiums inserted. In addition, the polarization within the cathode is significantly lower for the vanadium reduction than for the silver reduction. The silver metal morphologies consisted of nanoparticles (∼5 nm diameter) and dendrites and were both seen in samples of lithiated SVOF. © 2008 American Chemical Society.

Battery performance depends on many factors amongst which the selection of the appropriate electrode material and the control of the electrode/electrolyte interface upon cycling. In order to address these issues, electrochemists have to design new electrode materials, and 'enter the private life' of a battery. Transmission Electron Microscopy (TEM) is a powerful tool to help scientists to characterize each part of the device. Recent developments in TEM techniques have been proposed and developed, through the Alistore Electron Microscopy platform (AEMP), to allow the observation of Li-ion air sensitive electrode materials. Our team is now going one step further by using High Resolution Electron Energy Loss Spectroscopy (HREELS) and nanoprobe EDS as well as other TEM techniques exhibiting the advantage of high lateral resolution for chemical analysis and for structural determination (by means of electron diffraction and high-resolution imaging). The AEMP holds at the same time the equipment and the skills to run such experiments copyright The Electrochemical Society.

The electrochemical reduction of CuCr2S4 by Li was shown to occur in two steps, involving first a combined Cu extrusion/Li insertion leading to a Li1.75Cu0.25Cr2S 4 phase that, upon further lithiation, undergoes a conversion reaction. Analytical transmission electron microscopy, high resolution electron energy loss spectroscopy and neutron diffraction experiments have been used to elucidate the copper, sulphur and chromium oxidation states during the first electrochemical step. HRTEM shows the growth of copper dendrites during the discharge of the compound and their disappearance upon subsequent re-oxidation, while EDS analysis indicates that all of the copper cannot be extruded from the pristine phase upon discharge, nor be fully reinserted on charging. HR-EELS confirms the following oxidation state Cu+Cr2 3+(S2-)3S- for the pristine material and demonstrates that the insertion of lithium in the sample only corresponds to a change of the sulphur oxidation state whereas the valence of chromium was not modified. This observation was directly confirmed by neutron data that show an increase in the S-S bond length upon lithium insertion. This journal is © The Royal Society of Chemistry.

The electrochemical reactivity of CuTi 2S 4 and CuCr 2S 1 spinel samples toward Li was studied over a wide range of voltages. Their electrochemical behavior can be decomposed into two steps. For less than two inserted lithium, we emphasized a Li-driven electrochemical displacement reaction that led to the extrusion of copper from lithium, leading to the rocksalt-type structure phases Li 2Ti 2S 4 and Li 2-xCu 0.3Cr 2S 4, whereas conversion reactions were observed beyond x = 2. These conversion reactions were shown to have a large rechargeable uptake and release of Li + ions (7 per formula unit) that rapidly fade on cycling. Regarding the Cu extrusion, we demonstrated that this process is reversible for the Ti-based spinel upon oxidation through a chemically assisted electrochemical process. For the Cr-based system, a new phase LiCu 0.7Cr 2S 4 was evidenced to form on oxidation. This difference is explained in terms of band structure considerations and d-sp redox chemistry. Chemical reinjection of Cu into TizS4 forming the single phase Cu xTi 2S 4 (0 < x < 1) from the reaction of Ti 2S 4 powders with an aqueous CuSO 4· nH 2O solution is also reported. It is shown that traces of H 2S coming from the slight chemical instability of Ti 2S 4 in water are necessary for initiating the chemical reinjection reaction to form Cu xTi 2S 4. © 2006 American Chemical Society.

New materials, based on the well-known spinel compound NiMn2O4, have been synthesized and characterized from the magnetic point of view. The manganese cation was partially substituted in the general formula NiMn2-xMexO4 , by nonmagnetic and magnetic elements, such as Me = Ga, Zn, Ni and Cr (0 x 1). Prior to the determination of their magnetic properties, the non-substituted spinel NiMn2O4 was carefully characterized and studied as a function of the oxygen stoichiometry, based on the influence of the annealing atmosphere and quenching rate. The ferrimagnetic character was observed in all samples, with a paramagnetic-to-ferromagnetic transition temperature Tc stabilized at 110 K, and well defined long-range antiferromagnetic interactions at lower temperatures, which depend on the applied field and the substitute concentration.

New materials, based on the well-known spinel compound NiMn2O4, have been synthesized and characterized from the magnetic point of view. The manganese cation was partially substituted in the general formula NiMn2-xMexO4 , by nonmagnetic and magnetic elements, such as Me = Ga, Zn, Ni and Cr (0 x 1). Prior to the determination of their magnetic properties, the non-substituted spinel NiMn2O4 was carefully characterized and studied as a function of the oxygen stoichiometry, based on the influence of the annealing atmosphere and quenching rate. The ferrimagnetic character was observed in all samples, with a paramagnetic-to-ferromagnetic transition temperature Tc stabilized at 110 K, and well defined long-range antiferromagnetic interactions at lower temperatures, which depend on the applied field and the substitute concentration