Investigation on the properties of Li doped Ni-W oxide film and application for black electrochromic device

Ni-W oxide film within a certain range of Ni/W ratio could be used as anodic electrochromic material. However, with increasing W content, the film has low charge capacity and poor electrochromic properties. In this paper, lithium (Li) was doped into Ni-W oxide film by magnetron sputtering for improving charge capacity in accordance with electrochromic properties. Results show that doping Li could dramatically improve the charge capacity of Ni-W oxide film, which is associated with the doping amount. The charge capacity density of doped film with about 300 nm thickness is more than 30 mC/cm2 by applying a pulse potential with ± 1.2 V, the transmittance in colored state is lower than 10% in the 300–600 nm wavelength range, and transmittance modulation (∆T) is over than 60% in the whole 440–750 nm wavelength range. The Li doped Ni-W oxide film has excellent complementary electrochromic properties to WO3 film. EIS data analysis indicates that doping Li could greatly decrease the resistance of ions movement in the film-electrolyte interface and inside the film, and this is benefit for improving the charge capacity of film. Investigations based on XPS and SEM data also show that upon doping Li, the grains on the surface of film are more smaller, and the chemical surroundings around Ni ions is more likely to be changed. These factors result in that the Li doped Ni-W oxide film exhibits good anodic electrochromic properties. The colored device prepared by using Li doped Ni-W oxide and WO3 films has a balanced light absorption in the whole visible wavelength range, and appears dark brown.