Surface evolution of Ni–V transparent oxide films upon Li insertion reactions

A detailed picture is reported of the nature of an Li insertion Ni–V mixed-oxide electrode and of its evolution upon prolonged intercalation/deintercalation cycles. A combination of bulk and surface-sensitive techniques is used, including electrochemical analysis (cyclic voltammetry, impedance spectroscopy), atomic force microscopy (AFM) and XPS. The Ni–V oxide film, obtained by r.f. sputtering as a thin film on conductive glass for use as a transparent electrode in electrochromic windows, is an Li insertion material with high charge capacity, very good reversibility and a surface morphology characterized by a low initial roughness. Its surface composition shows an initial Ni enrichment. The nominal Ni–V stoichiometry is recovered already after ten cycles. Extended Li charge–discharge cycles induce progressive changes in the electrode reversibility (capacity fading) and in the reaction kinetics (increase of impedance). The surface evolution consists of the progressive accumulation of an Li carbonate layer, with thickness and roughness increasing with the number of cycles. The constancy of the core level and Auger peaks of Ni and V after 100 cycles excludes major changes in the electronic structure of the host oxide. However, electrochemical and AFM experiments performed after 1000 cycles indicate that the carbonate deposition on the electrode progresses further, therefore the surface layer is unable to yield a permanent electrode passivation and stabilization. Copyright © 2002 John Wiley & Sons, Ltd.