Vacancy dependent electrochromic behaviors of NiOx anodes: As a single layer and in devices

Electrochromic(EC), nonstoichiometric NiOx thin films were made by reactive magnetron sputtering at low oxygen flow ratio(i.e., P=O2/Ar+O2≤10%). The results of optical spectral, x-ray diffraction spectrum, and x-ray photoelectron spectroscopy analyses indicate that the samples are oxygen(nickel)-deficient as P≤%4(≥%6), resulting in sub(over)-stoichiometry films. Spectroelectrochemical measurements show that the EC effect of NiOx in nonaqueous PC−LiClO4 electrolyte is direct correlation with the nickel vacancy concentration in films, while that in the aqueous KOH solution is nearly uninfluenced upon the change in stoichiometry as P≥%4. The films deposited at P=6% exhibit higher coloration efficiency of −25.3cm2C−1, larger ionic diffusion coefficient of −2.84×10−14m2s−1, and broader EC modulation span of 24% in PC−LiClO4 than the other ones. Based upon these values, EC devices featuring a WO3/PMMA−PC−LiClO4/NiOx structure and excellent performances were fabricated. We demonstrated that the nickel anodization should be responsible for the initial “activation” phenomena, which decreases(increases) the number of oxygen(nickel) vacancies. Moreover, the cause of degradation resulting from Li+-ion trapping in the IS layer was also clarified. This work provides a general framework for studying and designing superior EC devices, experimentally as well as theoretically. •NiOx films have been investigated in both KOH and PC−LiClO4 electrolytes.•Nickel vacancies should be responsible for the electrochromic effect.•Nickel anodization should be responsible for the initial “activation'' phenomena in devices.•Device degradation from cation-trapping in NiOx electrochromic anodes.•A general framework for developing superior electrochromic applications.