Electrochemically synthesized vanadium oxides as lithium insertion hosts

Abstract

The electrochemical oxidation of vanadyl cations in aqueous solution leads to a solid deposit on the working electrode, called electrolytic vanadium oxide (e-V 2O 5). The electrodeposition reaction occurs in two steps including an oxidation into soluble species followed by a precipitation. Electrodeposited compounds are mixed valence, hydrated vanadic acids. Their chemical formula can be written H 0.4V 2O 5.2− δ · nH 2O with 0.04< δ<0.2 and 0< n<1.8. These two latter parameters depend on the current density applied during electrodeposition, the duration and the temperature of a subsequent mild thermal treatment in air. e-V 2O 5 materials are porous, poorly crystallized layered compounds. At 260°C, they become completely anhydrous and undergo a phase transformation into α-V 2O 5. The electrochemical intercalation of lithium into these compounds shows two main single phase phenomena near 2.6 and 3.1 V/Li. This reduction induces a lengthening of the average vanadium oxygen bond, and a decrease of the lithium diffusion coefficient. e-V 2O 5 compounds reversibly intercalate 1.4≅Li per formula unit at an average voltage of 2.8≅V/Li, at a rate of C/50 in the 4–2 V range, and this capacity is maintained during several tens of discharge/charge cycles. The electrochemical behavior is slightly dependent on the V IV content and the crystallization state of the compounds.