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 potentia ls (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 vario us substrates (ITO glass, K-glass, glassy carbon, stainless steel). Crystal line 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 cycl ic voltammetry and ac impedance spectroscopy. The electrolyte used was a 0. 5 M solution of lithium triflate in PC. Normal conditions for the electroch emical study of tungsten oxide films were defined by a potential range of a bout 2 V versus Li/Li+ to 4.5 V versus Li/Li+. The samples were taken to lo wer potentials (down to 1.6 V versus Li/Li+). Amorphous tungsten oxide elec trodes showed a loss of current and charge during cycling down to 1.6 V ver sus Li/Li+. Crystalline samples showed a good reproducibility during cyclin g. No evidence of a damage at the surface of the electrode was shown by the scanning electron microscopy, Spectrophotometric titration of the electrol yte showed no evidence of dissolution of tungsten. Impedance spectroscopy o f the amorphous samples showed an increase of the interfacial resistance wh en the cell was left at 1.6 V versus Li/Li+. This was consistent with a los s of adhesion at the interface between the tungsten oxide and the underlyin g conductive layer. (C) 1999 Elsevier Science Ltd. All rights reserved.