Stress in thin ®lms of metal oxide electrodes for intercalation reactions Danilo Dini and Franco Decker* Chemistry Department, University of Roma, 00185 Roma, Italy (Received in Newcastle 21 January 1998) AbstractÐIn this paper the importance of mechanical eects 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 de¯ection method (LBDM), in order to compare the mechanical eects of H + , Li + and Na + intercalation in the host structure. Calculation of the electrode stress was accom- plished by means of Stoney's formula valid for isotropic thin ®lms. The stress dependence on exchanged charge showed an unexpected trend during H + insertion and the occurrence of dierent coloring±bleaching mechanisms was invoked. On the other hand Li + and Na + intercalation induced analogous stress vari- ations when inserted charge ranged between 0 and 25 mC cm 2 . Such phenomenon was associated to com- parable 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 diusivity was con- siderably lower than insertion rate. The example of Na x WO 3 is addressed. # 1998 Published by Elsevier Science Ltd. All rights reserved Key words: mechanical stress, thin ®lms, laser beam de¯ection method, electrochromism, tungsten trioxide. INTRODUCTION Mechanical eects are known since long to occur in electrodes during reactions [1]. Stress during the anodization of metals [2±4] and semiconductors [5], or during the electrodeposition of coatings [1, 6], are only two examples. Ion insertion in thin ®lm electrodes is a rather recent research ®eld in which interest is growing due to the possible applications in electrochromic windows and micro-batteries [7, 8]. The interest in volume changes and in mechanical property eects of such thin electrodes is due to the primary concern with the deleterious eect that stress causes in ®lms. Moreover, the topic of stress in thin ®lms can be related to the detection of gases using bending beams and to the new concept of arti®cial electrochemical muscles [9a±b, 10]. The electrochemical insertion of protons or Li ions in V-, W- and Ni-oxide electrodes has been shown to be related to a growing compressive (or tensile [11]) stress inside the thin oxide ®lm [11±13]. In such pre- vious papers, however, the stress inside the host matrix has not been correlated with the nature of the speci®c guest cation. Recently, we compared the optical absorption of WO 3 in a series of ``in-situ'' measurements performed during the intercalation of H, Li and Na [14]. In the present work we shall dis- cuss the results of stress measurements in thin ®lms of tungsten trioxide used as electrodes for the inter- calation of H + , Li + , Na + from non-aqueous elec- trolytes. The deformation of the electrode±substrate system, the bulk elastic properties and the electro- static energy of the host solid during the electro- chemical intercalation reaction will be correlated with the measured stress in the various cases. Internal stresses, relaxation eects and their impli- cations for the operation of an electrochemical device based on intercalation will be discussed. EXPERIMENTAL The electrochemical and optical ``in-situ'' measurements were performed using a three-elec- trode Te¯on cell with four ¯at windows for laser- beam measurements. The mechanical stress in the thin ®lm electrodes has been measured by an optical cantilever method, as described in detail elsewhere [15±18]. The probe laser beam for such Electrochimica Acta, Vol. 43, Nos 19±20, pp. 2919±2923, 1998 # 1998 Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain 0013±4686/98 $19.00 + 0.00 PII: S0013-4686(98)00032-2 *Author to whom correspondence should be addressed. 2919