A comparison of the electrochemical properties of lithium intercalated amorphous and crystalline tungsten oxide

This work investigated the effect of cycling tungsten oxide at low potentials (below 2 V versus Li/Li+). Crystalline and amorphous tungsten oxide thin films were deposited by reactive sputtering in an Ar-O-2 plasma onto various substrates (ITO glass, K-glass, glassy carbon, stainless steel). Crystalline films were obtained by heating the samples at a temperature above 400 degrees C during the deposition or by annealing the samples at 400 degrees C after deposition. The electrochemical behaviour of the films was studied using slow scan cyclic voltammetry and ac impedance spectroscopy. The electrolyte used was a 0.5 M solution of lithium triflate in PC. Normal conditions for the electrochemical study of tungsten oxide films were defined by a potential range of about 2 V versus Li/Li+ to 4.5 V versus Li/Li+. The samples were taken to lower potentials (down to 1.6 V versus Li/Li+). Amorphous tungsten oxide electrodes showed a loss of current and charge during cycling down to 1.6 V versus Li/Li+. Crystalline samples showed a good reproducibility during cycling. No evidence of a damage at the surface of the electrode was shown by the scanning electron microscopy, Spectrophotometric titration of the electrolyte showed no evidence of dissolution of tungsten. Impedance spectroscopy of the amorphous samples showed an increase of the interfacial resistance when the cell was left at 1.6 V versus Li/Li+. This was consistent with a loss of adhesion at the interface between the tungsten oxide and the underlying conductive layer. (C) 1999 Elsevier Science Ltd. All rights reserved.