Electrochromism in lithiated nickel oxide films deposited by rf sputtering

Lithiated nickel oxide films were deposited by rf sputtering from an LiNiO2 target. Different samples were obtained by changing the deposition atmosphere or rf power during deposition, all other parameters remaining constant. The electrochemical/electrochromic performance during lithium extraction/insertion was investigated using aprotic electrolytes, over the whole stability range of the electrolyte. The results showed that all films, independent of the deposition conditions, were electrochemically active. Films deposited under pure Ar atmosphere and high rf power or under Ar + O-2 atmosphere and low power did not present significant electrochromic activity, and the valence band photoelectron spectrum did not show the presence of the Ni 3d line for the as-grown samples. In contrast, samples deposited under Ar + O-2 atmosphere and high power, or pure Ar atmosphere and low power showed a 70% transmittance change, and a stable voltammetric profile after some cycles. For these samples, the valence band photoelectron spectrum clearly presented the Ni 3d line. The electron population at the Ni 3d levels in the as-grown state seems to be responsible for the electrochromic ability. (C) 2001 Elsevier Science Ltd. All rights reserved.