Synthesis and electrochemical properties of vanadium oxide aerogels prepared by a freeze-drying process

Vanadium oxide aerogels have received considerable interest as a reversible electrode material for lithium secondary batteries. The aerogel morphology is a critical factor in obtaining the electrochemical properties of these materials. The present paper reports OD the synthesis of V2O5 by freeze-drying, an alternative approach for producing a material with an aerogel-like morphology. In this synthesis, the sublimation of cyclohexane circumvents the capillary pressures which cause pore collapse during the normal solvent removal process. The resulting materials are very porous with specific surface areas as high as 250 m2/g. The electrochemical properties of the freeze-dried material are strongly influenced by both the specific surface area of the solid phase and crystallite size. The controlled heat-treatment of the aerogel produces a ω-LixV2O5-type phase with improved capacity and reversibility. © 2004 The Electrochemical Society. All rights reserved

Electrochemical lithium reactivity with nanotextured anatase-type TiO 2

Anatase TiO2 particles were synthesized by a two-step method consisting of the preparation of a solid precursor through hydrolysis of titanium alkoxide followed by heat treatment at different temperatures under ambient air. This simple method led to a highly porous material with a 200 nm homogenous particle size, while the BET specific surface area and crystallite sizes evolved between 49 m2 g-1 to 223 m2 g-1 and from 17.0 nm to 6.3 nm, respectively. Their electrochemical performances clearly revealed the beneficial influence of the divided texture of anatase-type TiO2 on the reactivity with lithium, namely in terms of reversibility. Furthermore, we showed that the TiO2 texture strongly affects the extent of the solid solution domain. Finally, through a simple chemical titration it was possible to clearly quantify the capacitive/faradaic contributions of the electrochemical reaction. © The Royal Society of Chemistry 2005

Some insights on the use of polyols-based metal alkoxides powders as precursors for tailored metal-oxides particles

In this paper, we describe the synthesis of tailormade cobalt and manganese oxides by a two-step preparation method. The first step consists of precipitating metal alkoxide powders from aliphatic polyalcohol solutions of salts. Their particle size, composition, and morphology can be independently tuned by controlling the precipitation conditions (type of alcohol and concentration). The second step involves thermal decomposition under air, leading to crystalline metal oxides, whose specific surface area and crystallite size can be adjusted by a careful selection of the alcohol used and the annealing temperature. More specifically, we found that the crystallite size of these oxides is determined by the amount of heat released during the combustion of the organic component. Since the size and morphology of the precursors are strictly maintained during the pyrolysis treatment, we can therefore achieve an accurate control of the different aspects of the texture of metal oxide powders. Preliminary results show that this approach can be successfully extended to bimetallic phases

Preparation of nanotextured VO 2[B] from vanadium oxide aerogels

Vanadium oxide aerogels were used as a precursor for preparing nanotextured VO 2[B] by low-temperature heat treatment under vacuum. The VO 2[B] material retains the fibrous morphology and high surface area of the aerogel. Evolution of the VO 2[B] phase, as studied by FTIR and X-ray diffraction, indicates that the local structure of the vanadium oxide aerogel is close to that of VO 2[B], in agreement with the bilayer-type structure previously proposed for vanadium oxide aerogels/xerogels. The electrochemical behavior of VO 2[B] also bears similarity to that of vanadium oxide aerogels. Specific capacities for lithium as high as 500 mA·h/g are obtained for nanocrystalline VO 2[B], and stable electrochemical response is obtained when cycled between 4 and 2.4 V vs Li +/Li 0. © 2006 American Chemical Society

Synthesis and electrochemical properties of nanotextured materials

Evolving from bulk materials towards a smaller scale can affect the electrochemical responses or favor new reactions. The main advantage of high surface area materials is to reduce the diffusion length in the solid thus enabling faster kinetics. In this work we demonstrate how electrochemical properties are strongly influenced by the texture of the materials through three distinct examples. In the first example we demonstrate that in brannerite type materials, Mn(VO 3) 2, the plateau corresponding to the reduction of V 5+ into V 4+ is strongly affected by the small crystallite size in the initial material. Another characteristic of the electrochemical properties of such finely divided materials is the reversibility observed compared to bulk materials. Indeed for lithium insertion in heat treated aerogels, the irreversibility increases with increasing crystallite size. Similarly for proton insertion into niobium oxide, the optical and electrochemical reversibilities are excellent when using mesoporous material. Thus the formation of the nanotextured material opens new opportunities to optimize electrochemical systems, and better understand nanoscale electrochemistry

The electrochemical reduction of Co3O4 in a lithium cell

The first stage of the electrochemical reduction of crystallized Co3O4 spinel in lithium cells was investigated by means of in situ X-ray diffraction. Through the use of tailor-made materials prepared from Co-alkoxide precursors, we observed that the formation of the intermediate LixCo3O4 phase previously evidenced by several authors was highly dependent on the discharge rate, the texture of the active material (i.e., crystallite size, specific surface area), and the cycling temperature. When starting from a highly divided oxide and/or using a low current, we found that this plateau was actually associated with the formation of α-CoO, subsequently leading to metallic cobalt upon further reduction. Alternatively, LixCo3O4 was formed when using materials with a large crystallite size or/and applying a high discharge rate, later on similarly decomposing into divided metal. These findings and the related competition between different reaction paths represent an explanation for numerous electrochemical observations, and for the need of fast insertion in such host materials to stabilize intermediate lithiated compounds. This work illustrates the major influence of the initial texture as well as the temperature on the reactivity of the 3d-metal based oxides recently reinvestigated for their electrochemical performances as negative electrode materials. Also, it emphasizes the implications of the reactive grain size evolution upon cycling