AN ELECTROCHEMICAL QUARTZ-CRYSTAL MICROBALANCE STUDY OF LITHIUM INSERTION INTO THIN-FILMS OF TUNGSTEN TRIOXIDE .2. EXPERIMENTAL RESULTS ANDCOMPARISON WITH MODEL-CALCULATIONS

Lithium insertion into amorphous thin films of tungsten trioxide (a-WO 3) prepared by thermal vacuum evaporation of WO3 powder has been studi ed experimentally by chronoamperometry, cyclic voltammetry and ac impe dance spectroscopy associated with electrochemical quartz crystal micr obalance (EQCM). During cathodic polarization of the electrode and at short times two antagonistic processes. occur. One is a non faradaic p rocess and is associated with the expulsion of anions from the electro de surface under the effect of the electric field built in the electro lyte when a potential difference is imposed between the electrodes. Th e other one is the faradaic insertion of non-solvated lithium ions int o the oxide. These two processes occur simultaneously. Under applicati on of a cathodic overvoltage the former one leads to a loss of mass of the electrode and the latter one to a gain of mass. These two process es may explain the experimental data. From these data the diffusion co efficient of lithium ions into the oxide is determined and it is shown that it varies as insertion proceeds. A thermodynamic study of lithiu m insertion into a-WO3 is also performed to solve the equations of the theoretical model proposed in Part I of this paper. This thermodynami c study allowed us to determine the repulsive and/or attractive intera ctions between the inserted species and the host matrix as insertion o ccurs and the influence of the electronic term on the potential.