Electrochemically synthesized vanadium oxides as lithium insertion hosts

The electrochemical oxidation of vanadyl cations in aqueous solution leads to a solid deposit on the working electrode, called electrolytic vanadium o xide (e-V2O5). 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 H0.4V2O5.2-delta. nH(2)O with 0.04 < delta < 0.2 and 0 < n < 1.8. These two latter parameters depend on the current d ensity applied during electrodeposition, the duration and the temperature o f a subsequent mild thermal treatment in air. e-V2O5 materials are porous, poorly crystallized layered compounds. At 260 degrees C, they become comple tely anhydrous and undergo a phase transformation into alpha-V2O5. 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 li thium diffusion coefficient. e-V2O5 compounds reversibly intercalate 1.4 co ngruent to Li per formula unit at an average voltage of 2.8 congruent to V/ Li, at a rate of C/50 in the 4-2 V range, and this capacity is maintained d uring several tens of discharge/charge cycles. The electrochemical behavior is slightly dependent on the V-IV content and the crystallization state of the compounds. (C) 1999 Elsevier Science Ltd. All rights reserved.