AN ELECTROCHEMICAL QUARTZ-CRYSTAL MICROBALANCE STUDY OF LITHIUM INSERTION INTO THIN-FILMS OF TUNGSTEN TRIOXIDE .1. MODELING OF THE IONIC INSERTION MECHANISM

A theoretical description of the mechanism of lithium insertion into a morphous thin films of tungsten trioxide (a-WO3) prepared by thermal v acuum evaporation of WO3 powder is presented. The model developed is b ased on the experimental results obtained by chronoamperometry and ac impedance spectroscopy associated with electrochemical quartz crystal microbalance (EQCM). The electrode mass change and the current flowing through the electrochemical cell during cathodic polarization are sim ultaneously recorded. As expected, it can be observed that the inserti on process is associated with a gain of mass of the inserted electrode at long times (t > 1 s). On the other hand at short times (t < 1 s) a net loss of mass is observed. The existence of a non faradaic surface process involving the expulsion of anions from the electrode surface and occuring simultaneously with the faradaic insertion of non-solvate d lithium ions into the oxide layer is discussed.