Materials Science and Engineering B 138 (2007) 123–127 Thermodynamic study of WO 3 and WO 3 :Li + thin films C´ esar O. Avellaneda ∗ Laborat´ orio Interdisciplinar de Eletroqu´ ımica e Cer ˆ amica-LIEC, Departamento de Qu´ ımica, Universidade Federal de S˜ ao Carlos, C.P. 676, CEP 13565-905 S˜ ao Carlos, SP, Brazil Received 20 June 2005; received in revised form 21 February 2006 Abstract There is a considerable interest in the research and development of materials and devices that can be used for optical switching of large- scale glazings. Several potential switching technologies are available for glazings, including those based on electrochromic, thermochromic and photochromic phenomena. One of the most promising technologies for optical switching devices is the electrochromism (EC). In order to improve the electrochromic properties of tungsten oxide we have investigated the effect of lithium insertion on the electrochromic behavior of oxide films prepared by the sol–gel process. The kinetics and thermodynamics of electrochemical intercalation of lithium into Li x WO 3 and Li x (WO 3 :Li) films prepared by the sol–gel process were investigated. The standard Gibbs energy for lithium intercalation was calculated. The chemical diffusion coefficients, D, of lithium intercalation into oxide were measured by galvanostatic intermittent titration technique (GITT) as functions of the depth of lithium intercalation. © 2006 Published by Elsevier B.V. Keywords: Electrochromism; Thermodynamic; Kinetics; Sol–gel process 1. Introduction Certain materials, referred as electrochromic materials, are known to change their optical properties in response to the appli- cation of an electric current or an electric potential. This property has been used to produce electrochromic devices, which can be controlled to transmit optical energy selectively. The reaction may be written as WO 3 (colorless) + yM + + ye - ⇔ MyWO 3 (colored) (1) where for each atom injected an electron enters the conduction band of the host oxide and a deep blue coloration develops. At the same time, the electronic conductivity of the oxide rapidly increases [1–3]. The rapid diffusion of atoms in mixed conducting materials is of theoretical interest, as well as practical importance in battery electrode materials, electrochromic display devices, coulome- ters, etc. Using electrochemical methods to measure chemical diffusion coefficients in mixed conducting electrodes, which, combined with the detailed thermodynamic data derived from ∗ Tel.: +55 16 33608214; fax: +55 16 33615215. E-mail address: avellane@dq.ufscar.br. equilibrium cell measurement, has considerably simplified the acquisition and interpretation of the kinetics and thermodynam- ics of the electrochemical insertion of lithium into WO 3 thin films [4]. Since the electrochemical intercalation of lithium is, in gen- eral, limited by the lithium ion diffusion in the oxide elec- trode, the attention of a previous research [5] was focused on the determination of the chemical diffusion coefficient in the electrode material. The interaction between intercalated ions and oxide lattice or between intercalated ions may greatly influence the lithium ion diffusion through the oxide lattice [6]. The primary observation concerning the kinetics of lithium incorporation into WO 3 thin films is that both diffusion and interface kinetics are important. The following considerations are relevant. Firstly, the thermodynamic and kinetic properties of WO 3 thin films are very dependent on the method preparation, and in particular are dependent on the crystallinity degree of the films [7]. The structure of WO 3 host material plays an important role to the thermodynamics and kinetics of lithium intercalation into the oxide. The present paper deals with the kinetics and thermodynam- ics of electrochemical intercalation of lithium into WO 3 and WO 3 :Li thin films. 0921-5107/$ – see front matter © 2006 Published by Elsevier B.V. doi:10.1016/j.mseb.2006.06.006