Electrochimica Acta 46 (2001) 2257 – 2262 Electrochemical and optical characterization of RF-sputtered thin films of vanadium – nickel mixed oxides A. Lourenco a , E. Masetti a , F. Decker b, * a ENEA-Casaccia, Lab. Di Film Sottili, Rome, Italy b Dipartimento di Chimica and ‘‘Istituto Nazionale di Fisica della Materia’’, Uniersita ` ‘‘La Sapienza’’, Rome, Italy Received 21 August 2000; received in revised form 10 November 2000 Abstract The optical and electrochemical properties of thin films of a nickel – vanadium mixed oxide have been investigated as a new Li + intercalation compound, to be used as an optically passive counterelectrode in electrochromic switching devices. The thin film samples were prepared by radio frequency (RF) sputtering under different preparation conditions, using targets with different Ni/V ratios and a reactive (20% O 2 ) or, alternatively, non-reactive (pure Ar) sputtering atmosphere. The optical properties of as-grown and Li + inserted – extracted electrodes were determined by computer fitting of the UV – Vis – NIR transmittance and specular reflectance spectra. The electrochemical measure- ments of Li + insertion and extraction consisted of cyclic voltammetry, chronopotentiometry and potentiostatic intermittent titration technique. The samples showed interesting electrochemical and optical properties, a good electrochemical reversibility and ion-storage capacity exceeding 40 mC cm -2 , lasting for several hundreds cycles at a current of 50 A cm -2 . The electrodes displayed an almost passive electrochromic behavior during the electrochem- ical experiments. © 2001 Elsevier Science Ltd. All rights reserved. Keywords: Nickel – vanadium mixed oxides; Electrochromism; Optically passive counterelectrodes; Lithium intercalation; M x V 2 O 5 thin films www.elsevier.nl/locate/electacta 1. Introduction Transition metal oxides have been extensively investi- gated for their use in electrochromic devices (ECD) [1,2]. The majority of the publications in the ECD field refer to electrochromic coloring layers, particularly to thin films of amorphous tungsten oxide, one of the most efficient electrochromic materials that shows a decrease of the transmittance (cathodic elec- trochromism) upon negative charge insertion. The elec- trochromism of WO 3 occurs due to the simultaneous insertion (or intercalation) of an electron and a positive charge-compensating ion, which induces an optical ab- sorption causing the color change [3]. However, in order to give an efficient energy control, a high optical contrast (ratio of bleached to colored transmittance) must be reached by the electrochromic device. Recent emphasis in the development of ECD has resulted in the study of new lithium insertion compounds, which act as a counterelectrode (or charge storage layer) for the WO 3 electrochromic coloring electrode. At the counterelectrode electrochemical oxi- dation and reduction reactions occur, balancing the charge transferred to the WO 3 electrode during its optical switching, and should be at least as reversible as those at the WO 3 , to avoid counterelectrode limitation to the device lifetime. The counterelectrode should also contribute to the optical modulation of the ECD by coloring and bleaching synchronously with WO 3 or, * Corresponding author. E-mail address: franco.decker@uniromal.it (F. Decker). 0013-4686/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved. PII:S0013-4686(01)00385-1