Electrochemical properties of vanadium oxide aerogels

Aerogels are well-known mesoporous materials whose low density and high surface area result from synthesis methods that enable the pore solvent to be removed without collapsing the solid network phase. The interconnected porosity provides both molecular accessibility and rapid mass transport via diffusion, and for these reasons transition metal oxide aerogels are gaining increased interest as intercalation electrode materials for lithium-ion batteries. The present paper reviews recent research on vanadium oxide aerogels that has been directed at establishing both their fundamental properties and unique ways of incorporating these materials into electrode structures. The experiments used to determine the fundamental electrochemical properties of vanadium oxide aerogels involve the use of a sticky-carbon electrode that is designed to both hold the material and serve as the current collector. The results show that these materials combine elements of both capacitor and battery behavior as the materials possess both high specific capacitance (>2000 F/g) and high capacity for lithium incorporation (>450 mAh/g). The unique morphology of the vanadium oxide aerogel has led us to consider alternate electrode structures because traditional methods may compromise the mesoporous, high surface area morphology. One approach has involved the incorporation of single wall carbon nanotubes as the electronically conducting network. The resulting nanocomposites effectively retain the aerogel morphology and exhibit excellent electrochemical properties. These electrodes are especially effective at high discharge rates. Another approach has involved the preparation of aerogel electrodes that possess an inverted opal structure. The fabrication route is based on combining the templating of polystyrene spheres with sol-gel synthesis and leads to materials that possess a hierarchical pore structure. This architecture is also effective at retaining high lithium capacity at high current densities. (C) 2003 Elsevier Science Ltd. All rights reserved.