Stress in thin films of metal oxide electrodes for intercalation reactions

Abstract

In this paper the importance of mechanical effects in electrodes during electrochemical reactions is stressed. The system under investigation was tungsten trioxide WO 3. Electrochromic WO 3 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 structure. Calculation of the electrode stress was accomplished by means of Stoney's formula valid for isotropic thin films. The stress dependence on exchanged charge showed an unexpected trend during H + insertion and the occurrence of different coloring–bleaching mechanisms was invoked. On the other hand Li + and Na + intercalation induced analogous stress variations when inserted charge ranged between 0 and 25 mC cm −2. Such phenomenon was associated to comparable electrostatic interactions between host and guest in both Li + and Na + cases. Finally, stress measurements revealed the not homogeneous distribution of inserted ions when guest diffusivity was considerably lower than insertion rate. The example of Na x WO 3 is addressed.