308 Ionics 9 (2003) Monolithic Electrochromic Devices Using Lithium Ion Conducting Perovskite- Type Oxides V. Thangadurai and W. Weppner Chair for Sensors and Solid State Ionics, Faculty of Engineering, Christian - Albrechts University, Kaiserstrasse 2, D-24143 Kiel, Germany Abstract. The electrical conductivities of several perovskite-type lithium ion conductors in the Li-Sr-Nb-Ta-Ti-O system have been investigated. The Li+-ion conductivities of the Ta-compounds were found to be higher than those of the corresponding Nb-compounds, i.e., Li0.3Sr0.6Ta0.sTi0.503 exhibits a bulk ionic conductivity of 1.7 x 10-~ S/cm at 30 ~ while Li0.3Sr0.6Nb0.sTi0.503 shows a value of 5.4 • 10 -6 S/cm at the same temperature. Substitution of Fe in Li0.3Sr0.6Ta0.sTi0.503 decreases the Li+-ion conductivity slightly. The operation of a monolithic (single element) electrochromic devices was demonstrated using perovskite-type Li0.3Sr0.6B0.sTi0.503 (B = Nb, Ta). The tantalum compound exhibited the largest coloration at the positive electrode side by the application of a voltage of 1.5 V and was bleached under short-circuit conditions at 350 ~ 1. Introduction Electrochromism (EC) is known as a reversible change in the optical properties when a material is electrochemically oxidized or reduced. Presently, there is a strong interest to develop thin film devices for windows, displays and mirrors. Figure 1 shows a schematic representation of the present- day five layer EC devices. It consists sequentially of an electrically conducting transparent layer (commonly ITO), an active electrode made from an electrochromic material, an ionic conductor, a counter electrode and another electrically conductive layer [1]. A large number of materials, both inorganic and organic compounds exhibit such EC upon the application of small voltage changes [2-4]. For the five layer system, coloration and decoloration occur due to simultaneous injection/extraction of electrons and ions into the electrochromic active material by charge fluxes across the electrolyte. In this way, we increase or decrease the chemical potential of the mobile ions (Pro; m = Li) in the EC active layer. Figure 2 shows a schematic representation of the change in the chemical potential of lithium gLi in the case of an electroactive lithium system before and after coloration of the EC working electrode. In general, the coloration accompanying the ion injection reaction into a transition metal oxide (MOy) is a cathodic reduction according to AxMOy (A = H § Li +, Na § ) (1) MOy +xA ++ xe- Fig. 1. Schematic arrangement of the conventional solid-state electrochromic device, which consists of the electrochromic active (EC) layer, e.g., WO 3, liquid, inorganic or polymer solid electrolyte (SE), and a counter electrode (CE). These layers are sequentially placed in-between the two conducting ITO layers.