STRESS IN THIN-FILMS OF METAL-OXIDE ELECTRODES FOR INTERCALATION REACTIONS

In this paper the importance of mechanical effects in electrodes durin g electrochemical reactions is stressed. The system under investigatio n was tungsten trioxide WO3. Electrochromic WO3 has been characterized with the laser beam deflection method (LBDM), in order to compare the mechanical effects of H+, Li+ and Na+ intercalation in the host struc ture. Calculation of the electrode stress was accomplished by means of Stoney's formula valid for isotropic thin films. The stress dependenc e on exchanged charge showed an unexpected trend during H+ insertion a nd the occurrence of different coloring-bleaching mechanisms was invok ed. On the other hand Li+ and Na+ intercalation induced analogous stre ss variations when inserted charge ranged between 0 and 25 mC cm(-2). Such phenomenon was associated to comparable electrostatic interaction s between host and guest in both Li+ and Na+ cases. Finally, stress me asurements revealed the not homogeneous distribution of inserted ions when guest diffusivity was considerably lower than insertion rate. The example of NaxWO3 is addressed. (C) 1998 Published by Elsevier Scienc e Ltd. All rights reserved.